To see the other types of publications on this topic, follow the link: Ferroelectric Phase Transitions.
Dissertations / Theses on the topic 'Ferroelectric Phase Transitions'
Create a spot-on reference in APA, MLA, Chicago, Harvard, and other styles
Consult the top 50 dissertations / theses for your research on the topic 'Ferroelectric Phase Transitions.'
Next to every source in the list of references, there is an 'Add to bibliography' button. Press on it, and we will generate automatically the bibliographic reference to the chosen work in the citation style you need: APA, MLA, Harvard, Chicago, Vancouver, etc.
You can also download the full text of the academic publication as pdf and read online its abstract whenever available in the metadata.
Browse dissertations / theses on a wide variety of disciplines and organise your bibliography correctly.
1
Franzbach, Daniel Jason. "Field Induced Phase Transitions in Ferroelectric Materials." Phd thesis, tuprints, 2014. https://tuprints.ulb.tu-darmstadt.de/4134/1/Daniel%20Franzbach%20Field%20Induced%20Phase%20Transitions%20in%20Ferroelectric%20Materials.pdf.
Full textAbstract:
The focus of this dissertation lies in the theoretical description of electrical field- and mechanical stress-induced phase transitions and their influence on the material behavior of ferroelectric single crystals, polycrystals and composite materials. Phase transitions are interesting phenomena that lead to improved properties of the ferroelectric material.
The motivation for this work is presented in the first Chapter. Chapter 2 gives a short introduction on the basic formalism of electrostatics and continuum mechanics, which are the foundations of material models of ferroelectrics. In addition, some fundamentals of crystallography will be discussed to understand the atomistic reason of the ferroelectric effect. In the first part of Chapter 3, a two dimensional Landau type model is presented, which is used to study electrical field-induced tetragonal to orthorhombic phase transitions in arbitrary ferroelectric single crystal materials. The Landau energy landscape was varied to examine the influence of the switching energies and the polarization rotation path on the predicted phase transition field. In the second part, the model is expanded to three dimensions. Landau parameters from literature were used to predict the tetragonal to orthorhombic phase transition behavior of BaTiO3. Large signal measurements on single crystalline BaTiO3 were performed to verify the model and to compare the predictive capabilities of the various Landau potentials. In Chapter 4, the Landau model is further expanded to describe polycrystalline ferroelectrics like Pb(Zr,Ti)O3 under uniaxial compressive strain and electric field. In contrast to micro-mechanical models, the Landau energy model intrinsically considers tetragonal to rhombohedral phase transitions. These induced transitions provide a good explanation for the exceptional switching strain of rhombohedral Pb(Zr,Ti)O3 compositions close to the MPB.
Chapter 5 and 6 elucidate a different type of field-induced phase transition. Novel lead free materials, such as BNT-6BT-2KNN, show exceptional unipolar usable strain values that are larger than Pb(Zr,Ti)O3. Without any applied field these materials show nearly no remanent polarization and strain. Internal mechanisms prohibit the development of long-range interactions between the unit cells, so that the system decays in a disordered nano-domain state. Unfortunately, the fields that are required to induce a phase transition to a polar phase are too high for most applications. A composite structure with a chemical compatible ferroelectric material is used to decrease the required electric field. Two models are proposed to predict the dielectric behavior of a composite from the behavior of both components. In Chapter 5 the composite is replaced by a series configuration of two nonlinear hysteretic capacitors. The model is verified by comparing the results to experimental data from composite samples, and used to identify optimal material parameter combination for future materials. In the second model that is presented in Chapter 6, a two dimensional phase field implementation is expanded by a material model for the high strain material. In contrast to the previous case, this model allows one to study the influence of the microstructure on the composite effect. The model is then applied to test cases to demonstrate its capabilities.
2
Rowley, Stephen Edward. "Quantum phase transitions in ferroelectrics." Thesis, University of Cambridge, 2011. https://www.repository.cam.ac.uk/handle/1810/252224.
Full text
3
Ravel, Bruce D. "Ferroelectric phase transitions in oxide perovskites studied by XAFS /." Thesis, Connect to this title online; UW restricted, 1997. http://hdl.handle.net/1773/9784.
Full text
4
Whittle, Thomas Anthony. "A Structural Investigation of Perovskite and Tungsten Bronze Type Ferroic Materials." Thesis, The University of Sydney, 2015. http://hdl.handle.net/2123/14586.
Full textAbstract:
This thesis set out to investigate lead free ferroic materials with perovskite and tungsten bronze type structures, primarily focussing on the relationship between composition, temperature and crystal structure. A combination of diffraction techniques were employed to investigate the crystal structures. Additionally, other techniques including XANES, SEM, TGA, DSC and ferroic peroperty measurements were also employed to further illuminate these compounds. The first system investigated was the defect perovskite Sr0.8Ti0.6-yZryNb0.4O3, 0.0 ≤ y ≤ 0.6. It was found that neutron powder diffraction data were essential for determining the phase boundary composition. Second order compositional and temperature phase transitions were observed. Increasing the zirconium content increased octahedral tilting and led to higher transition temperatures. Local ordering was determined to be highly probable and it was found that the presence of vacancies extended the range of the high symmetry phase. The second system investigated was the BaxSr3-xTi1-yZryNb4O15, 0.0 ≤ x ≤ 3.0, 0.0 ≤ y ≤ 1.0, tungsten bronze type system. Barium rich compositions were found to adopt a tetragonal structure, while strontium rich compositions adopted an orthorhombic structure. Increasing the zirconium content of samples was seen to make the orthorhombic phase persist further. A large focus was placed on determining the structure of Sr3TiNb4O15 as a model for all orthorhombic compounds. A new structural model was proposed for Sr3TiNb4O15 distinct from those previously published. All orthorhombic compositions were observed to undergo first order phase transitions to the tetragonal structure on heating. The barium and strontium atoms were found to order onto two crystallographically distinct A sites. It was found that the tungsten bronze tolerance factor could be used as a predictive tool for the crystal symmetry of these materials. All compounds in this system for which ferroelectric measurements were performed displayed ferroelectric hysteresis behaviour.
5
Chapman, Brandon D. "The role of disorder in structural phase transitions in perovskite ferroelectrics /." Thesis, Connect to this title online; UW restricted, 2003. http://hdl.handle.net/1773/9692.
Full text
6
Franzbach, Daniel Jason [Verfasser], Jürgen [Akademischer Betreuer] Rödel, and Ralf [Akademischer Betreuer] Müller. "Field Induced Phase Transitions in Ferroelectric Materials / Daniel Jason Franzbach. Betreuer: Jürgen Rödel ; Ralf Müller." Darmstadt : Universitäts- und Landesbibliothek Darmstadt, 2014. http://d-nb.info/1112332642/34.
Full text
7
von, Helden Leonard. "Ferroelectric domains in potassium sodium niobate thin films: impact of epitaxial strain on thermally induced phase transitions." Doctoral thesis, Humboldt-Universität zu Berlin, 2019. http://dx.doi.org/10.18452/20185.
Full textAbstract:
Gegenstand dieser Arbeit ist die experimentelle Untersuchung der Verspannungs-Temperatur-Phasenbeziehungen in epitaktischen KxNa1-xNbO3 Dünnschichten, sowie deren Zusammenhang mit ferro- und piezoelektrischen Eigenschaften. Die präsentierten Ergebnisse ermöglichen es KxNa1-xNbO3 Dünnschichten für neuartige technologische Anwendung zu optimieren.
Zunächst wird eine detaillierte strukturelle Untersuchung der ferroelektrischen Domänenstruktur in epitaktischen K0.7Na0.3NbO3 Schichten auf (110) TbScO3 vorgestellt. Eine Analyse der ferroelektrischen Domänenstruktur mittels lateral aufgelöster Piezoresponse-Kraftmikroskopie (PFM) zeigt vier Arten von Superdomänen. Durch die ergänzende Untersuchung der zweidimensionalen und dreidimensionalen Abbildung des reziproken Raumes mittels hochauflösender Röntgenbeugung (HR-XRD) wird nachgewiesen, dass dieses Domänenmuster mittels monokliner Einheitszellen in einem MC Domänenmodell beschrieben werden kann.
Im Anschluss an die strukturelle Untersuchung wurden die elektromechanischen Eigenschaften der KxNa1-xNbO3 Schichten auf (110) TbScO3untersucht. Mittels Doppelstrahl-Laserinterferometrie (DBLI) wurde ein makroskopischer effektiver piezoelektrischer Koeffizient von bis zu d33,f = 23 pm/V nachgewiesen. Zudem wurden Oberflächenwellen-Experimente (SAW) durchgeführt. Diese zeigten außergewöhnlich hohe Signalstärken.
Um die Temperatur der ferroelektrischen Phasenübergänge gezielt einstellen zu können, wurde der Zusammenhang zwischen epitaktischer Verspannung und der Phasenübergangstemperatur untersucht. Dazu wurden KxNa1-xNbO3 Schichten mit unterschiedlicher Verspannung gewachsen. Die Änderung der Domänenstruktur und der piezoelektrischen Eigenschaften aufgrund von Temperaturänderung wurde in-situ durch temperaturabhängige PFM, HR-XRD und DBLI Messungen untersucht. Die Untersuchung zeigte, dass die Übergangstemperatur des Übergangs von der MC- in die c-Phase mit zunehmender kompressiver Verspannung kontinuierlich um mehr als 400 °C abnahm.The subject of this thesis is the experimental investigation of the strain-temperature-phase relations in epitaxial KxNa1-xNbO3 thin films and their connection to ferro- and piezoelectric properties. This will enable the optimization of KxNa1-xNbO3 layers for novel technological devices. First, a detailed structural investigation of the ferroelectric domain structure in epitaxial K0.7Na0.3NbO3 films on (110) TbScO3 is presented. An analysis of the ferroelectric domain structure with laterally resolved piezoresponse force microscopy (PFM) reveals four types of superdomains. By complementary two-dimensional and three-dimensional high resolution X-ray reciprocal space mapping this domain pattern is proven to be describable by an MC domain structure with monoclinic unit cells. Subsequently to the structural investigation, the electromechanical properties of KxNa1-xNbO3 layers on (110) TbScO3 were investigated. Double beam laser interferometry (DBLI) revealed a macroscopic effective piezoelectric coefficient of up to d33,f = 23 pm/V. Furthermore, surface acoustic wave (SAW) experiments were performed. They exhibited extraordinary signal intensities. In order to be able to selectively tune such phase transition temperatures, the correlation between epitaxial strain and the phase transition temperature was investigated. For this purpose, KxNa1-xNbO3 films with different compressive strain conditions were grown. The change of domain structure and piezoelectric properties upon temperature variation was investigated in-situ by temperature-dependent PFM, HR-XRD and DBLI measurements. The transition temperature between the MC- and c-phase was shown to continuously decrease by more than 400 °C with increasing compressive strain.
8
PACIARONI, MORENO. "A mechanical model for ferroelectric materials: from atomic scale to finite elements." Doctoral thesis, Università Politecnica delle Marche, 2011. http://hdl.handle.net/11566/242132.
Full textAbstract:
I materiali ferroelettrici sono largamente utilizzati in molte applicazioni quali sensori, attuatori e memorie non volatili. Negli ultimi anni la ricerca ha prodotto una miriade di modelli analitici e numerici, sia per la forma cristallina sia per quella amorfa. Gli approcci adottati sono differenti a seconda della scala di lunghezza da cui si osserva il problema: i modelli atomistici descrivono il comportamento intrinseco ed hanno portato ad un grande sviluppo nella chimica dei ferroelettrici, i modelli macroscopici ingegneristici catturano il comportamento statico indipendente dal tempo. Il presente lavoro giace nel mezzo dei due approcci e cerca di fornire un ponte concettuale tra la scala atomica e quella ingegneristica attraverso un modello termodinamico microstrutturale.
Un materiale ferroelettrico esibisce una polarizzazione spontanea al di sotto di un valore critico della temperatura detto temperatura di Curie, a cui è associata anche una deformazione spontanea. La maggior parte dei materiali ferroelettrici presenta una simmetria cristallografica cubica nella fase ad alta temperatura, detta fase paraelettrica, e una simmetria più bassa nella fase ferroelettrica: ciò riflette a scala macroscopica la distorsione del reticolo di lattice legato all'insorgere della polarizzazione.
Non essendo forniti apporti esterni, il materiale si ritroverebbe con uno scompenso di energia sia elettrostatica sia meccanica, per cui tende a minimizzare questi contributi attraverso la formazione di domini a polarizzazione uniforme e di geminati meccanici. Tale processo non prosegue in modo indefinito perché una parte dell'energia si accumula nelle pareti di dominio: queste ultime sono interfacce dello spessore di poche costanti di lattice attraverso cui la polarizzazione cambia rapidamente da un valore a quello opposto. All'equilibrio si ha una polarizzazione netta nulla.
La polarizzazione spontanea può essere riorientata ad alte temperature (comunque inferiori a quella di Curie) per mezzo di forti campi elettrici e il risultato è un materiale piezoelettrico che esibisce un forte accoppiamento elettromeccanico. L'intensità dei campi elettrici dipende da una serie di fattori quali la dimensione dei grani e dei domini e la simmetria del cristallo. Un tipico esempio di materiale ferroelettrico è il titanato-zirconato di piombo Pb(Zr1-xTix)O3, in seguito identificato con l'acronimo PZT. Questa ceramica presenta una fase paraelettrica con simmetria cubica al di sopra dei 650K, mentre al di sotto la fase ferroelettrica può essere associata a diverse simmetrie cristallografiche in ragione della composizione chimica: alti contenuti in titanio favoriscono una simmetria tetragonale, viceversa alti contenuti in zirconio producono una simmetria romboedrale. Intorno al bordo di fase morfotropico, ovvero la linea di separazione tra le diverse simmetrie, è stata scoperta l'esistenza di una fase monoclina, la quale sarebbe all'origine delle eccezionali proprietà del PZT. In effetti per composizioni 0.47 < x < 0.51 al di sotto dei 500K si hanno alti valori della costante dielettrica, la massima rigidezza meccanica e una certa facilità di polarizzazione con molte possibili orientazioni.
Nel presente lavoro la modellazione del comportamento elettromeccanico dei materiali ferroelettrici partirà da un'analogia formale con la Density Functional Theory, la quale viene ad esempio impiegata dai fisici dello stato solido per valutare la struttura di atomi e molecole: in questo modo si stabilisce un collegamento tra modelli atomistici e meccanica dei continui. Sulla base di alcuni risultati di letteratura si formula il problema dell'equilibrio in termini di potenza spesa e il problema di minimo in termini di energia.
La descrizione della transizione di fase è affidata alla teoria di Landau, di cui si riportano gli aspetti fondamentali. Inoltre viene proposto un approccio originale in cui l'energia libera è sviluppata in serie di potenze a partire da un'opportuna configurazione iniziale: questo permette di descrivere le varie simmetrie di arrivo limitando lo sviluppo al quarto ordine nella variabile che descrive il fenomeno. Per dare una lettura immediata dei risultati, si rappresentano graficamente degli opportuni diagrammi di fase. Essendo la polarizzazione accompagnata da un deformazione spontanea, si analizzerà la compatibilità delle varianti ferroelettriche (le deformazioni omogenee a tratti associate al fenomeno ferroelettrico) all'interfaccia dei domini attraverso la teoria delle deformazioni con discontinuità a salti. Oltre ai classici geminati (o twins) di tipo I, II e composti, verranno presi in considerazione anche twins non generici che possono avere origine per particolari composizioni del materiale calcolando nel caso gli angoli di inclinazione delle pareti dei domini. Infine si provvederà a fornire una traccia sommaria per la minimizzazione del funzionale ferroelettrico sia con approccio analitico ottenendo le equazioni di Eulero-Lagrange sia con il metodo degli elementi finiti. In quest'ultimo caso verranno riportati i risultati ottenuti su un semplice modello monodimensionale.Ferroelectric materials are widely used in many applications such as sensors, actuators and non-volatile memories. In recent years, an ever-increasing theoretical investigation generates many analytical and numerical implementations. Single-crystal specimens may be modeled in an easier way than polycrystalline ones, but the technological process is very hard and expensive, so that both are considered. Different approaches are followed depending on what one focuses on, from the atomic level to the macroscopic engineering one. Atomistic models describe the intrinsic behavior and have led to a great improvements in the chemistry of ferroelectric materials; macroscopic phenomenological models capture the empirical static and time-independent behavior. The present work lies at the interface of two apparently disjointed approaches: it is a thermodynamic and microstructural model that provides a conceptual link between the atomic and the engineering scales. The most important feature of a ferroelectric material is the transition that it undergoes from the paraelectric phase to the ferroelectric one when it is cooled under the Curie temperature: as a result, a spontaneous polarization onsets and it is accompanied by an electric self-field and a spontaneous strain. Most of the ferroelectric materials exhibits a cubic crystallographic point group in the paraelectric phase and a lower symmetry in the ferroelectric phase: within the Cauchy-Born rule this reflects the distortion of the crystal lattice at the macroscopic scale. The associated electrostatic and elastic energies are reduced by means of the formation of domains, i.e. uniformly polarized regions in which electric dipoles are aligned in the same direction. This process does not go on indefinitely, since a certain amount of energy is stored at the so-called domain walls: the latter are thin interfaces (only a few lattice constants) and within them the spontaneous polarization will decrease in magnitude, passing through zero, and increase on the other side with opposite sign. Domain thickness does not approach zero, in fact it has a finite dimension which arises from the competition between the energetic terms. At the equilibrium, ferroelectric materials have a null net polarization. In order to exhibit piezoelectric properties, the spontaneous polarization must be reoriented in the same direction by applying strong electric fields at high temperature below the Curie point: this process is called domain switching or polarization reversal. The magnitude of the electric fields depends on the size of the grains and of the domains and on the symmetry point group. One of the most widely used ferroelectric material is the lead zirconate titanate Pb(Zr1-xTix)O3 (PZT) which shows a cubic point group in the paraelectric phase above 650K, whereas the symmetry in the ferroelectric phase is defined by the composition of the ceramic: titanium-rich compositions favor cubic-to-tetragonal transition and zirconium-rich compositions which favor cubic-to-rhombohedral transition. Close to the morphotropic phase boundary a stable monoclinic phase was recently discovered and it is at the origin of the exceptional properties of the PZT: indeed such compositions are characterized by a very large piezoelectric coupling between electric and mechanical variables related to the presence of a maximum in the dielectric constant, an ease of poling with a large number of reorientable polarization directions and a maximum mechanical compliance. In the present work we would like to establish a conceptual link between the atomistic and the macroscopic continuum models: to this effect, we begin from the Density Functional Theory which is used in the solid state physics to describe the structure of complex atoms, molecules and crystals and their interactions. By a formal analogy we get the equilibrium problem in terms of power expended and the minimum problem in terms of energy. Then we describe the transition from the paraelectric to the ferroelectric phase within the Landau theory: we propose an original approach in which below the transition temperature (and accordingly in a ferroelectric phase) the parameters of the crystallographic potential depends on the composition of the ceramic: then, starting from a suitable reference configuration we attempt to describe all the possible symmetries with a fourth-order expansion of the spontaneous polarization. Because of the formation of domains, different deformations associated with different polarizations may occur within the same grain: using the Hadamard lemma and the Ericksen-Silhavy condition we find all the possible twins arising in PZT and in BaTiO3 and some other non generic non conventional twins which are predicted by our developments from the Landau theory. Finally we propose a three dimensional model for ferroelectric materials with the associated Euler-Lagrange equation, which are integro-differential ones because of the electric field. We also deal with a one dimensional model on which we perform both a stability and a finite element analyses.
9
Kilit, Emel. "Critical Behaviour Of The Thermodynamic Quantities For The Thermotropic And Ferroelectric Liquid Crystals Close To The Phase Transitions." Phd thesis, METU, 2011. http://etd.lib.metu.edu.tr/upload/12612963/index.pdf.
Full textAbstract:
The specific heat Cp has been showed at various temperatures in the literature, which shows a
sharp increase labeled as the lambda-transition at the critical temperature. This transition has been
observed previously among the phases of solid-nematic-isotropic liquid in p-azoxyanisole
(PAA) and anisaldazine (AAD), and among the phases of solid-smectic-cholesteric-isotropic
liquid in cholesteryl myristate (CM). In this thesis work, we analyze the experimental data for
the temperature dependence of Cp and the thermal expansion alpha_p and also pressure dependence
of alpha_p by a power-law formula. From the analysis of pressure dependence of alpha_p, we calculate
the temperature dependencies of specific heat Cp and of the isothermal compressibility kappa_T for
the phase transitions considered in PAA, AAD and CM. Our calculations for the temperature
dependence of the p and kappa_T can be compared with the experimental data when available in
the literature.
Polarization, tilt angle and the dielectric constant have been reported in the literature at various
temperatures close to the solid-smectic C*-smectic A-isotropic liquid transition in the
ferroelectric liquid crystals of A7 and C7. The mean field model with the free energy expanded in terms of the order parameters (polarization and tilt angle) has been reported in the
literature previously. In this thesis work, we apply the mean field model first time by fitting
the expressions derived for the temperature dependence of the polarization, tilt angle and
the dielectric constant to the experimental data for A7 and C7 from the literature. Since the
mean field model studied here describes adequately the observed behaviour of A7 and C7, the
expressions for the temperature dependence of the polarization, tilt angle and the dielectric
constant which we derive, can also be applied to some other ferroelectric liquid crystals to
explain their observed behaviour.
10
Hentati, Mouhamed Amin. "Effets des inhomogénéités locales et des contraintes extérieures sur les propriétés diélectriques et structurales des monocristaux PZN-x%PT." Phd thesis, Châtenay-Malabry, Ecole centrale de Paris, 2013. http://tel.archives-ouvertes.fr/tel-01003354.
Full textAbstract:
Dans ce travail, nous avons étudié l'effet des contraintes extérieures et des inhomogénéités locales sur les propriétés diélectriques et structurales des cristaux ferroélectriques- relaxeurs à base de plomb PZN-x%PT avec 0%≤x≤12%. Dans une première partie, nous avons déterminé les propriétés diélectriques et structurales du système PZN-6%PT. Pour l'état vierge, ce composé subit la séquence de transition de phase C T R, où C, T et R sont, respectivement, les phases cubique, quadratique et rhomboédrique. En appliquant un champ électrique statique, une phase orthorhombique est induite entre les phases T et R. Dans la deuxième partie, nous avons montré la présence d'une anomalie diélectrique à basse température observée sur le PZN-x%PT avec 0%≤x≤12%. Dans ce domaine de température, l'étude structurale ne montre aucune transition de phase. L'ensemble de ces résultats sont interprétés moyennant un modèle basé sur la présence des nano-régions polaires. En troisième partie nous avons déterminé les propriétés diélectriques et piézoélectrique du PZN-12%PT dopé au manganèse dans son état monodomaine. Le dopage affecte, principalement, la permittivité transverse et le coefficient piézoélectrique de cisaillement. Le dopage induit aussi la stabilité de la structure monodomaine et l'effet de mémoire de la microstructure. Ces résultats sont expliqués en utilisant le modèle de symétrie des défauts. Dans la dernière partie, nous nous sommes intéressés à la simulation de l'effet de la présence des dipôles-défaut (dopage) sur les propriétés physiques de BaTiO3. Nous avons mis en évidence l'induction d'un champ électrique interne responsable du décalage du cycle d'hystérésis vers les champs électriques négatifs.
11
Schmidbauer, Martin [Gutachter], Christoph [Gutachter] Koch, and Brice [Gutachter] Gautier. "Ferroelectric domains in potassium sodium niobate thin films: impact of epitaxial strain on thermally induced phase transitions / Gutachter: Martin Schmidbauer, Christoph Koch, Brice Gautier." Berlin : Humboldt-Universität zu Berlin, 2019. http://d-nb.info/120492404X/34.
Full text
12
Lin, Yaochen. "Dispersion de nanoparticules ferroélectriques dans un cristal liquide : élaboration, transitions de phases et propriétés diélectriques." Thesis, Littoral, 2017. http://www.theses.fr/2017DUNK0443/document.
Full textAbstract:
Les cristaux liquides sont des matériaux organiques utilisés pour réaliser des dispositifs électroniques ; avant de les intégrer dans des applications, il est nécessaire d'étudier leurs propriétés physico-chimiques et diélectriques pour optimiser leurs performances. Ce travail de thèse est consacré aux nanocolloïdes obtenus par dispersion de nanoparticules ferroélectriques dans un cristal nématique ; il s'agit d'étudier l'influence des inclusions sur les transitions de phases et sur les propriétés diélectriques de la matrice. L'étude des transitions de phases à l'aide de l'Analyse Enthalpique Différentielle (AED) a mis en évidence l'influence des nanoparticules ferroélectriques ; ceci résulte de deux principaux effets ; la polarisation spontanée des nanoparticules et l'ancrage entre les molécules du cristal liquide et les inclusions. La caractérisation diélectrique a révélé le couplage entre la polarisation macroscopique des inclusions et le champ électrique ; ce couplage se manifeste par une augmentation des températures de transitions de phases par rapport à celles déterminées par l'AED. La compétition entre les effets de la polarisation sous champ électrique et de l'ancrage induit une modification des permittivités (parallèle et perpendiculaire) et de l'anisotropie diélectriques. L'utilisation des nanoparticules fortement polaires sélectionnées a confirmé l'importance de la polarisation macroscopique des nanoparticules pour améliorer les propriétés des nanocolloïdes étudiés ; en effet, de très faibles quantités de ces nanoparticules donnent lieu à des améliorations plus significatives que celles obtenues par les nanoparticules brutesLiquid crystals are organic materials used to make electronic devices ; before using this material in applications, it is necessary to study their physical-chemical and dielectric properties in order to optimize their performance. This study is devoted to the nanocolloids obtained by dispersing ferroelectric nanoparticles in a nematic liquid crystal ; it means an inclusion influences the phase transitions temperatures and the dielectric properties of the host. The phase transitions measured by using Differential scanning calorimetry (DSC) evidenced the ferroelectric nanoparticles influence ; which is attributed two effects : the nanoparticles spontaneous polarization and the anchoring effect between nanoparticles and liquid crystal. The dielectric characterisation revealed the coupling between the macroscopic polarization of the inclusions and the electric field ; this coupling is manafested by an increase of phase transition temperatures compared to those determined by DSC. The competition between the polarization effect under an electric field and the anchoring effect induces a modification of the permittivities (parallel and perpendicular) and the dielectric anisotropy. Using harvested nanoparticles, the study confirmed the importance of the nanoparticles polarization to increase the properties of the studied nanocolloids. In fact, very small quantity of the harvested nanoparticles presents more significant improvements than those obtained with the non-harvested nanoparticles
13
Cherifi, Ryan. "Experimental design of a strong Magneto-Electric coupling system between a ferroelectric and a magnetic phase transition alloy : BaTiO3/FeRh, and theoretical study of the metamagnetic transition of FeRh." Thesis, Paris 6, 2015. http://www.theses.fr/2015PA066309.
Full textAbstract:
Aujourd'hui, la puissance de calcul des processeurs et la capacité de stockage des disques durs tels que conçus dans l'électronique moderne sont limités par la limite thermodynamique aux systèmes finis. Pour garder une vitesse de développement tel que prédit par la loi de Moore, il est donc nécessaire de considérer de nouveaux types d’architecture d’unité de calcul et stockage d’information. Un autre problème réside dans la gestion des pertes de courant par effet Joule, qui deviennent critiques dès lors que l’on atteint de très fortes densités de transistors et bits magnétiques. Notre étude s’inscrit dans ces problématiques, par la conception de nouveaux systèmes à fort couplage magnéto-électrique qui permettrait de contrôler l’information magnétique par l’injection de faibles courants électriques. Notre objectif a été de concevoir un système à fort couplage magnéto-électrique. Il existe des matériaux possédant un couplage entre ordre magnétique et ordre ferroélectrique de façon intrinsèque. Ce type de structures représente une bonne base d’analyse conceptuelle sur la nature d’hybridation des ordres férroiques. Cependant le couplage y est généralement faible, et ne permet pas l’intégration de ces matériaux dans l’électronique moderne.Une autre option consiste à artificiellement générer un couplage magnéto-électrique à travers l’interface entre deux matériaux possédant chacun un des ordres férroiques. Nous avons travaillé essentiellement sur ce type d’hétérostructure binaire, alliant un substrat ferroélectrique type, (BaTiO3) avec, dans un premier temps, un film ultra-mince ferromagnétique type (Fe, Co, FeNi). Nous avons montré la présence d’une signature d’un couplage magnéto-électrique faible à l’interface de ces systèmes. Nous avons ensuite proposé de remplacer le matériau ferromagnétique typique par un film mince de FeRh, un alliage qui possède une transition de phase magnétique d’antiferromagnétique à ferromagnétique juste au-dessus de la température ambiante, qui dépend à la fois de la température, de la pression et du champ magnétique.Nous avons alors réalisé une étude de croissance de FeRh en films ultra-minces. Nous avons pu montrer que l’alliage garde une température de transition bulk et une transition assez abrupte jusqu’à 5nm d’épaisseur. Nous avons ensuite étudié le couplage magnéto-électrique dans le système FeRh(22nm)/BaTiO3 par magnétométrie SQUID sous champ électrique. Nous avons démontré un très fort effet magnéto-électrique induit par contrainte mécanique, possédant une constante de couplage record, α = 1.6 x 10-5 s.m-1, un ordre de grandeur au-dessus des valeurs rapportées dans la littérature.Utilisant notre connaissance du système, nous avons montré l’intérêt conceptuel d’utiliser un matériau à transition de phase dans les architectures novatrices de mémoire, en proposant une description mathématique d’un comportement memristif dans le système FeRh/piézoélectrique.Finalement, l’utilisation pratique de FeRh nous a amené à étudier l’alliage par calculs Ab Initio sous contrainte mécanique et sous injection de charges, pour comprendre plus fondamentalement la nature et les mécanismes de la transitionOne of the most practical concept used in physics and engineering is the concept of triggeror switch, consisting of a means to start a controlled chain of energy transformation.A switch can lead to reversible or irreversible consequences. Technological developmentusually seeks to make use of the former because it allows for repetitive logical tasks. Suchtriggers exist via the coupling between two or more types of energetic transformations.It is formally described by the interaction between two or more distinct fields and theirexpression on a system. Amongst the most studied coupling in material physics, we findelectro-mechanical couplings such as piezoelectricity or ferroelectricity, electro-caloric ormagneto-caloric couplings such as pyroelectricity and pyro-magnetism, magneto-electric,etc. The fundamental and experimental domestication and understanding of these couplingsis usually followed (and very often motivated) by the design of practical applicationin electronics engineering technology
14
PIMENTA, MARCOS ASSUNCAO. "Transitions de phase a haute temperature et conduction ionique dans likso:(4) et composes apparentes." Orléans, 1987. http://www.theses.fr/1987ORLE2045.
Full textAbstract:
Etude par reflexion ir, diffusion brillouin et la mesure des conductivites electriques sur likso::(4), linaso::(4) et linh::(4)so::(4). Identification et analyse des differentes transitions; mise en evidence d'une relation etroite entre la mobilite cationique et les mouvements de rotationd es groupes sulfate. Observation d'une transition ordre-desordre a 435**(o)c, avec phse intermediaire a surstructure de basse symetrie, dans le cas de likso::(4)
15
Sharma, Henu. "Theoretical studies of PbTiO3 and SrTiO3 under uniaxial mechanical constraints combining firstprinciples calculations and phenomenological Landau theory." Thesis, Grenoble, 2014. http://www.theses.fr/2014GRENI078.
Full textAbstract:
Dans cette thèse, nous présentons des études théoriques de matériaux pérovskites sous con-trainte mécanique uniaxiale en combinant les calculs de premier principe DFT ainsi quela théorie phénoménologique de type Landau. Les pérovskites ABO3 forment une classetrès importante de matériaux fonctionnels, qui peuvent présenter un large éventail de pro-priétés (e.g., supraconductivité, magnétisme, ferroélectricité, multiferroïcité, transitionsmétal-isolant. . . ) grâce aux petites distorsions d’ une même structure prototype cubique.Bien que ces composés aient été largement étudiés expérimentalement et théoriquement, ilreste encore des questions importantes et non résolues concernant les effets de contraintesuniaxiales. Au cours de ces dernières années, l’ ingénierie de contrainte a été décrite commeune approche originale pour ajuster les propriétés ferroélectriques pérovskites ABO3. Alorsque les effets de tension épitaxié-biaxiale et pression la hydrostatique, sont plutôt bien com-pris dans cette classe de matériaux, très peu est connu en ce qui concerne l’ effet des con-traintes mécaniques uniaxiales. Notre étude est motivée par ce manque de compréhensionactuelle de l’ effet de tension et compression uniaxiale, qui a été jusqu’à présent presquetotalement négligé. Deux composés prototypes sont étudiés dans le détail: PbTiO3 etSrTiO3. Après une introduction générale sur les composés ABO3 et les calculs techniques(ab initio et modèle phénoménologique de Landau), nous avons étudié l’ effet de contraintesmécaniques sur ces matériaux dans notre thèse.PbTiO3 est un composé ferroélectrique prototypique et également l’ un des composantsmère de la solution solide Pb(Zr,Ti)O3 (PZT), qui est le piézoélectrique le plus largementutilisé dans des applications. Pour PbTiO3, nous avons montré que indépendammentde la contrainte mécanique uniaxiale appliquée, le système conserve un état fondamentalpurement ferroélectrique avec la polarisation alignée, soit le long de la direction de lacontrainte (en phase FEz) ou bien le long d’ un des axes pseudo-cubique, qui lui estperpendiculaire (phase de FEx). Cela contraste avec les cas de contraintes mécaniquesisotropes ou bi-axial, pour qui de nouvelles phases combinant des modes ferroélectriqueset antiferrodistortives ont déjà été décrites. Sous contrainte uniaxiale, PbTiO3 passe d’unétat fondamental FEx sous compression à un état fondamental FEz en tension au-delà d’une tension critique !czz! +1%. Sous contrainte uniaxiale, PbTiO3 présente soit un étatfondamental FEx sous compression ("zz < 0) ou un état fondamental de FEz sous tension("zz > 0). Cependant, ici, un brusque saut des paramètres structuraux est prévu sousdes contraintes de compression et de traction à des valeurs critiques "zz! +2 GPa et −8GPa. Ce comportement semble similaire à celui pré-prédit sous pression isotrope négativeet pourrait se révéler utile en pratique pour améliorer la réponse piézoélectrique dans lesnano-composants.Le deuxième composé intéressant est SrTiO3. Il a été largement étudié au cours desdernières décennies, en raison de ses propriétés exceptionnelles à basse température. Dansce travail, nous avons élargi nos précédentes études de PbTiO3, en explorant théorique-ment les effets de pression sur la perovskite SrTiO3, combinant les premiers principes decalculs et un modèle phénoménologique de type Landau. Nous avons discuté de l’évolutiondes fréquences des phonons de SrTiO3 des trois cas de contraintes isotrope, uniaxial ettensions biaxiaux en utilisant les calculs de premier principe. Nous confirmons des travauxexpérimentaux précédents sur SrTiO3 que ça soit en contrainte épitaxiée ou sous pressionhydrostatique. Enfin, nous avons calculé de diagramme de phase de SrTiO3 sous contrainteuniaxiale, obtenue à partir de la théorie de Landau que nous avons comparé aux calculsde premier principeIn the present thesis we present theoretical studies of perovskite compounds under uniax-ial mechanical constraints combining first-principles DFT calculations approach and phe-nomenological Landau theory. ABO3 perovskites form a very important class of functionalmaterials that can exhibit a broad range of properties (e.g., superconductivity, magnetism,ferroelectricity, multiferroism, metal-insulator transitions. . . ) within small distortions ofthe same simple prototype cubic structure. Though these compounds have been exten-sively studied both experimentally and computationally, there are still unresolved issuesregarding the effect of pressure. In recent years, strain engineering has reported to bean original approach to tune the ferroelectric properties of perovskite ABO3 compounds.While the effect of epitaxial biaxial strain and hydrostatic strain is rather well understoodin this class of materials, very little is yet known regarding the effect of uniaxial mechanicalconstraints. Our study is motivated by the little existing understanding of the effect ofuniaxial strain and stress, that has been up to now almost totally neglected. Two proto-type compounds are studied in detail: PbTiO3 and SrTiO3. After a general introductionon ABO3 compounds and calculations techniques (ab initio and phenomenological Landaumodel), we studied the effect of mechanical constraints in these compounds in our thesis.PbTiO3 is a prototypical ferroelectric compound and also one of the parent components ofthe Pb(Zr,Ti)O3 solid solution (PZT), which is the most widely used piezoelectrics. ForPbTiO3, we have shown that irrespectively of the uniaxial mechanical constraint applied,the system keeps a purely ferroelectric ground-state, with the polarization aligned eitheralong the constraint direction (FEz phase) or along one of the pseudocubic axis perpen-dicular to it (FEx phase). This contrasts with the case of isotropic or biaxial mechanicalconstraints for which novel phases combining ferroelectric and antiferrodistortive motionshave been previously reported. Under uniaxial strain, PbTiO3 switches from a FEx groundstate under compressive strain to FEz ground-state under tensile strain, beyond a critical strain !czz! +1%. Under uniaxial stress, PbTiO3 exhibits either a FEx ground state undercompression ("zz < 0) or a FEz ground state under tension ("zz > 0). Here, however, anabrupt jump of the structural parameters is also predicted under both compressive andtensile stresses at critical values "zz! +2 GPa and −8 GPa. This behavior appears similarto that predicted under negative isotropic pressure and might reveal practically useful toenhance the piezoelectric response in nanodevices.The second compound of interest is SrTiO3. It has been widely studied in the past decadesdue to its unusual properties at low temperature. In this work, we have extended ourprevious investigations on PbTiO3 by exploring theoretically the pressure effects on per-ovskite SrTiO3 combining the first-principles calculations and a phenomenological Landaumodel. We have discussed the evolution of phonon frequencies of SrTiO3 with the threeisotropic, uniaxial and biaxial strains using first-principles calculations. We also reproducethe previous work done in SrTiO3 with epitaxial strain and hydrostatic strain. Finally,we have calculated the phase diagram of SrTiO3 under uniaxial strain, as obtained fromLandau theory and discussed how it compares with the first-principles calculations
16
Dos, Santos Adenilson Oliveira. "Difração de raios-X de n-feixes na caracterização estrutural de monocristais sob a ação de temperatura e campo elétrico externo." [s.n.], 2006. http://repositorio.unicamp.br/jspui/handle/REPOSIP/278132.
Full textAbstract:
Orientador: Lisandro Pavie CardosoTese (doutorado) - Universidade Estadual de Campinas, Instituto de Fisica Gleb Wataghin
Made available in DSpace on 2018-08-06T02:28:50Z (GMT). No. of bitstreams: 1 DosSantos_AdenilsonOliveira_D.pdf: 4430176 bytes, checksum: 30089a1aff00bfd64635f23601dd3039 (MD5) Previous issue date: 2006
Resumo: Como primeira contribuição deste trabalho, as varreduras Renninger (VR) da difração múltipla de raios-X foram empregadas no estudo da transição de fase estrutural do Sal de Rochelle (monoclínica-ortorrômbica) induzida por temperatura. Devido ao caráter tridimensional e sensibilidade dessa técnica acompanhamos as deformações na célula unitária com a temperatura, através do deslocamento dos picos secundários com maior sensibilidade. Foi possível determinar os parâmetros de rede e simular cada VR com o programa UMWEG. Os picos secundários de 4-feixes ( 0 0 0 ) ( 0 0 10 )( 1 0 9 )(1 0 1 ) e (0 0 0 )( 0 0 10 )( 2 3 0 )(2 3 10 ) da fase ortorrômbica foram medidos na VR para 24ºC (Tc). Os coeficientes de expansão térmica do sal de Rochelle também foram obtidos, e estão em bom acordo com a literatura. Como segunda contribuição, usamos a VR na determinação precisa dos parâmetros de rede dos materiais magnetocalóricos PrAl2, NdAl2e PrNi5. Implementamos uma rotina baseada na simulação da VR com o programa UMWEG, através da qual escolhe-se o comprimento de onda adequado para a medida de picos secundários muito sensíveis à distorção na célula unitária. Na aplicação no caso do PrAl2 usamos o caso de 4-feixes ( 0 0 0 )( 6 0 0 )( 1 3 7 )( 7 3 7 ) com 2.... =6.663(3)º e obtivemos com grande precisão a=8,03332(7)Å. O efeito de polimento mecânico na superfície dessa amostra foi analisado pelo mapeamento das reflexões de superfície (BSD) e o comportamento mosaico do cristal foi evidenciado. Nas outras medidas, como utilizamos a mesma geometria da estação XRD1, não foi possível obter a melhor condição para a rotina apresentada, mas foram obtidos para o NdAl2 (a=7,9972(5)Å) e PrNi5 (a=4,9590(8)Å e c=3,9794(5)Å) com boa precisão. Outra contribuição foi o estudo do efeito de campo elétrico no cristal orgânico MBANP através do monitoramento por curvas de rocking da reflexão ( 10 0 0 ). Observou-se uma histerese "asa de borboleta", ainda não observada em cristais orgânicos, e sem modelo para cristais monoclínicos. Cálculos usando mecânica quântica para moléculas isoladas de MBANP mostram que as principais características da forma de histerese podem ser explicadas em termos de mudanças induzidas pelo campo elétrico nos perfis de carga e na geometria de moléculas isoladas de MBANP
Abstract: As the first contribution of this work, the Renninger scan (RS) of the X-ray multiple diffraction were used in the study of the Rochelle salt structural phase transition (monoclinic-orthorhombic) induced by temperature. Due to its three-dimensional feature and sensitivity of the technique was possible to follow the unit cell deformations with temperature, through the angular shifts of the most sensitive secondary peaks. We were able to determine the lattice parameters as well as to simulate each RS by using the UMWEG program. The ( 0 0 0 )( 0 0 10 )( 1 0 9 )(1 0 1 ) and ( 0 0 0 )( 0 0 10 )( 2 3 0 )(2 3 10 ) 4-beam peaks for the orthorhombic phase were measured in the RS at 24ºC (Tc). Rochelle salt thermal expansion coefficients were also obtained in good agreement with literature values. As the second contribution, we have also used the RS in the precise lattice parameter determination of PrAl2 , NdAl2 and PrNi5 magnetocaloric materials. We have implemented a routine based on the RS simulation (UMWEG program), through which, one can choose the adequate wavelength to measure the most sensitive secondary peaks to the unit cell variations. The application of thid method in the case of PrAl 2, has allowed to measure the ( 0 0 0 )( 6 0 0 )( 1 3 7 )( 7 3 7 ) 4- beam case that presents 2.... =6.663(3)º and hence, to determine a=8.03332(7)Å, with high precision. PrAl2surface polishing effect was also analyzed by the secondary surface peak (BSD) mapping, through which, the crystal mosaic behavior was exhibited. For the other crystals, the same LNLS geometry was used and the best condition to applying the routine could not be obtained, however good precision lattice parameters were obtained for NdAl2 (a=7.9972(5)Å) and PrNi5 (a= 4.9590(8)Å and c= 3.9794(5)Å). Another contribution to the study of the electric field application in the MBANP organic crystal through the monitoring of ( 10 0 0 ) rocking curves was performed. It was observed a (butterfly wing) hysteresis, not yet observed for organic crystals and with no model suggested for monoclinic crystals. Quantum mechanical calculations on isolated MBANP molecules show that the main features of the hysteresis shape can be explained in terms of field-induced changes in the charge profiles and geometry of isolated MBANP molecules
Doutorado
Física da Matéria Condensada
Doutor em Ciências
17
Chu, Fan. "The ferroelectric phase transition in complex perovskite relaxors /." [S.l.] : [s.n.], 1994. http://library.epfl.ch/theses/?nr=1248.
Full text
18
Qu, Weiguo. "In-situ TEM investigation of the phase transitions in perovskite ferroelectrics." [Ames, Iowa : Iowa State University], 2008.
Find full text
19
Liu, Tieqi. "Electromechanical Behavior of Relaxor Ferroelectric Crystals." Diss., Georgia Institute of Technology, 2004. http://hdl.handle.net/1853/4881.
Full textAbstract:
Relaxor ferroelectric PZN-xPT and PMN-xPT single crystals exhibit extraordinary electromechanical properties. They are under development for applications in sensors, actuators and transducers. The polarization switching and phase transition behavior of PZN-4.5%PT and PMN-32%PT single crystals under external loading has been investigated. Experimental investigation elucidates the polarization switching and phase transition behavior of relaxor ferroelectric crystals at different orientation cuts under combined temperature, electric field and stress loading. These crystals exhibit strong orientation dependence of electromechanical properties, and the applied fields all affect the poling and phase states of the crystals. Based on experimental investigation, crystal variant modeling was developed to compute the piezoelectric properties of multi-domain crystals at different orientation cuts from a set of properties for the single domain. Thermodynamics and work-energy analysis of field induced phase transitions in these single crystals sheds light on the phase transition mechanism of ferroelectric crystals. Fracture behavior of relaxor single crystals under non-uniform electric fields at a partial electrode edge has also been measured and analyzed.
20
Al-Zein, Ali. "Order and Disorder of Relaxor and Ferroelectric Materials : structural and Vibrational Studies." Thesis, Montpellier 2, 2010. http://www.theses.fr/2010MON20078/document.
Full textAbstract:
Parmi les matériaux piézo-électriques, les pérovskites ferroélectriques à base de plomb sont connus pour avoir les meilleurs coefficients piézo-électriques et couplage électromécanique. Ils sont largement utilisés dans diverses applications industrielles et technologiques. Les "ferroélectrique relaxeurs" appartiennent à cette famille. Leur structure est caractérisée par la présence de nanorégions polaires orientées de façon aléatoire. Dans cette thèse, nous nous sommes intéressés à l'étude des propriétés structurales et dynamiques de matériaux ferroélectriques et relaxeurs tels que PbTiO3, PbZr0.52Ti0.48O3, PbMg1/3Nb2/3O3 (PMN), PbZn1/3Nb2/3O3, et PbMg1/3Ta2/3O3 (PMT). La structure à longue et courte portée a été étudiée par diffraction de neutrons et spectroscopie d'absorption des rayons X (XAFS), alors que la spectroscopie hyper-Raman (HR) est utilisée pour sonder les vibrations. L'analyse de la structure locale de matériaux pérovskites complexes AB'B''O3 montre que la pression diminue le désordre statique des gros cations occupant le site B, tandis que le champ électrique appliqué a un effet opposé. Cette distortion induite sous champ pourrait être à l'origine des forts coefficient piézoélectrique dans ces matériaux. La diffusion HR dans PMN et PMT a permis d'observer pour la première fois le "mode mou" responsable de la dépendance en température de la constante diélectrique. L'analyse des règles de sélection et la description en modes propres des vibrations actives en HR, permet de rendre compte de l'implication de chaque atome dans le comportement structural en température des ferroélectriques relaxeursAmong piezoelectric materials, lead-based ferroelectric perovskites are known to have the largest piezoelectric coefficients and electromechanical coupling. They are widely used in dfferent industrial and technological applications. The so-called "relaxors" belong to this family. Their structure is characterized by the presence of randomly oriented polar nanoregions. In this thesis, we are interested in studying the structural and dynamical properties of prototypical ferroelectric materials and relaxors such as PbTiO3, PbZr0.52Ti0.48O3, PbMg1/3Nb2/3O3 (PMN), PbZn1/3Nb2/3O3, and PbMg1/3Ta2/3O3 (PMT). The long and short range structure has been investigated by neutron diffraction and X-ray absorption fine structure (XAFS), while hyper-Raman scattering (HRS) is used to probe the vibrations. The local structure analysis of complex perovskite materials AB'B''O3 shows that pressure reduces the static disorder of the large cation occupying the B-site, while an applied electric field has an opposite effect. This field-induced distortion might relate to the large piezoelectric coefficient in such materials. HRS in PMN and PMT allows the first observation of the "primary" soft mode responsible for the temperature dependence of the dielectric constant. The selection rule analysis reveals the nature of the HRS active vibrational bands and enables us to get insights about the involvement of each atom in the structural modifications upon temperature
21
Labeeb, Abd El-Hamied Ahmad Mohammad. "The rotational viscosity and field-induced transitions in the intermediate phases of ferroelectric liquid crystals." Thesis, University of Manchester, 2012. https://www.research.manchester.ac.uk/portal/en/theses/the-rotational-viscosity-and-fieldinduced-transitions-in-the-intermediate-phases-of-ferroelectric-liquid-crystals(ab186cd9-0c25-4b31-83db-15ec9cfdaeab).html.
Full text
22
Wang, Ge. "Phase switching behaviour in lead-free Na0.5Bi0.5TiO3-based ceramics." Thesis, University of Manchester, 2017. https://www.research.manchester.ac.uk/portal/en/theses/phase-switching-behaviour-in-leadfree-na05bi05tio3based-ceramics(267b315d-3757-4865-9f88-5eeed76d61c4).html.
Full textAbstract:
This PhD project is focused on three lead-free ferroelectric solid solutions, which are specifically Na0.5Bi0.5TiO3-KNbO3(NBT-KN), Na0.5Bi0.5TiO3-NaNbO3(NBT-NN) and Na0.5Bi0.5TiO3-BaTiO3(NBT-BT), to evaluate the effects of composition, electric field and temperature on structural and electrical properties. Novel observations of both reversible and irreversible electric field-induced phase switching were made in both NBT-KN and NBT-NN ceramics. The NBT-KN solid solution is the primary focus of this thesis. All compositions were observed to be cubic in the as-sintered, unpoled state. However, a well-defined ferroelectric hysteresis P-E loop was obtained for compositions with low KN contents, indicating that an irreversible phase transition from a weak-polar relaxor ferroelectric (RF) to a long-range ordered metastable ferroelectric (FE) state had occurred during the measurement procedure. Both the unpoled and poled ceramic powders were examined using high resolution synchrotron XRD. For the poled state, a rhombohedral R3c structure was identified for compositions with low KN content, confirming the occurrence of the irreversible electric field-induced structural transformation from cubic to rhombohedral. In contrast, a cubic structure was retained for high KN contents, giving rise to reversible phase switching evidenced by constricted P-E hysteresis loops. Similar behaviour was observed for NBT-NN system. An 'in-situ' electric field poling experiment was conducted using high energy synchrotron XRD. In certain NBT-KN compositions the structural transformation, from cubic to mixed phase cubic+rhombohedral and finally single phase rhombohedral, occurred progressively with increasing cycles of a bipolar electric field. Similar behaviour was observed for NBT-NN compositions having low NN contents. Furthermore, the distributions of domain orientation and lattice strain over a range of orientations relative to the poling direction were determined for NBT-KN, NBT-NN and NBT-BT ceramics exhibiting the rhombohedral phase. By combining the structural information with the results of dielectric and ferroelectric measurements, a phase diagram was constructed to illustrate the influence of temperature and composition on the stability of the metastable ferroelectric and relaxor ferroelectric states for the NBT-KN system. Furthermore, the phase transition temperatures obtained from dielectric measurements were correlated with the ferroelectric and thermal depolarisation characteristics for each of the NBT-KN, NBT-NN and NBT-BT systems.
23
Džiaugys, Andrius. "Influence of impurities on dielectric properties of ferroelectric and superionic crystals." Doctoral thesis, Lithuanian Academic Libraries Network (LABT), 2011. http://vddb.laba.lt/obj/LT-eLABa-0001:E.02~2011~D_20110628_134612-56944.
Full textAbstract:
Nowadays the ferroelectrics containing of several feroically active sublattices are very attractive, because interactions between these sublattices can caused novel phenomena. Antiferroelectrics, ferrielectrics and multiferoics belong to these materials. In this work new crystalline materials MNP2X6 (M = Cu, Ag; N=In, Cr, Bi; X=S, Se) were investigated, which have ferrielectric and multiferoic properties. The dielectric and electric properties of above mentioned materials have been investigated by broadband dielectric spectroscopy methods, which allows to analyze the collective processes related to order – disorder and displacive phase transitions, ions migration and freezing of dipoles (glassy state) in wide temperature (25 K - 500 K) and frequency (10-5 HZ - 3 GHz) ranges. By substitution or doping it becomes possible to tailor the ferroelectric materials to different properties. In this work is determined that the substitution of 10% Cu ions by Ag ions shifts the phase transition temperature of CuInP2S6 crystal toward lower temperatures while the addition of In ions shifts the phase transitions temperature toward the higher ones. The phase transition temperature difference is about 50 K for mentioned crystals. If the ferroelectric crystal CuInP2S6 is mixed with the antiferroelectric CuCrP2S6 the dipole glass phase occupies the middle of the phase diagram. The distribution of relaxation times has been calculated from the broadband dielectric spectra of dipolar glasses. The... [to full text]Šiai dienai ypač populiarūs ferroelektrikai susidedantys iš kelių feroiškai aktyvių subgardelių, kurių persitvarkymas fazinio virsmo temperatūroje atskleidžia naujų, dar neaprašytų reiškinių. Prie šių medžiagų priskiriami antiferoelektrikai, ferielektrikai ir multiferoikai. Šiame darbe buvo tiriama nauja medžiagų šeimos MNP2X6 (M = Cu, Ag; N=In, Cr, Bi; X=S, Se ), kurios pasižymi ferielektrinėmis bei multiferoinėmis savybėmis, ir kurių dielektrines ir elektrines savybes galima efektyviai keisti įvedant priemaišas. Minėtų medžiagų dielektrinės ir elektrinės savybės buvo tiriamos dielektrinės spektroskopijos metodais, kurie leidžia tirti kristalų kolektyvinius reiškinius susijusius su tvarkos – netvarkos bei poslinkio tipo faziniais virsmais, jonų migracija bei dipolių užšalimu (stiklėjimu) plačiame dažnių (10-5 Hz iki 3 GHz) bei temperatūrų (25 K iki 500 K) intervaluose. Įvedus 10% Ag jonų vietoj Cu jonų ferielektriniame kristale CuInP2S6 fazinio virsmo temperatūra pasislenka į žemesnias temperatūras, o padidinus indžio koncentraciją fazinio virsmo temperatūra pasislenka į aukštesnes temperatūras. Minėtų kristalų fazinių virsmų temperatūrų skirtumas 50 K. Sumaišius skirtingomis proporcijomis feroelektriką (CuInP2S6) su antiferoelektriku (CuCrP2S6) stebima dipolinio stiklo fazė. Iš dielektrinių matavimų stiklo fazėje buvo paskaičiuota relaksacijos trukmių pasiskirstymo funkcija, kurios aprašymas dvigubos potencialinės duobės modeliu leido susieti mikroskopinius kristalo... [toliau žr. visą tekstą]
24
Plonka, Rafael. "Impact of the interface on the paraelectric-to-ferroelectric phase transition in epitaxial BaSrTiO_tn3 thin film capacitors." Jülich Forschungszentrum, Zentralbibliothek, 2007. http://d-nb.info/1000127257/34.
Full text
25
Reis, Idalci Cruvinel dos [UNESP]. "Estudo das propriedades físicas de sistemas ferroelétricos com estruturas tipo Aurivillius." Universidade Estadual Paulista (UNESP), 2015. http://hdl.handle.net/11449/133940.
Full textAbstract:
Submitted by IDALCI CRUVINEL DOS REIS null (idalcireis@yahoo.com.br) on 2016-01-27T19:12:31Z
No. of bitstreams: 1
Tese_Idalci Cruvinel dos Reis.pdf: 11534800 bytes, checksum: bebcb1750beb5d5066bead83c37b196a (MD5)Approved for entry into archive by Juliano Benedito Ferreira (julianoferreira@reitoria.unesp.br) on 2016-01-28T18:47:45Z (GMT) No. of bitstreams: 1 reis_ic_dr_ilha.pdf: 11534800 bytes, checksum: bebcb1750beb5d5066bead83c37b196a (MD5)
Made available in DSpace on 2016-01-28T18:47:45Z (GMT). No. of bitstreams: 1 reis_ic_dr_ilha.pdf: 11534800 bytes, checksum: bebcb1750beb5d5066bead83c37b196a (MD5) Previous issue date: 2015-12-18
Fundação de Amparo à Pesquisa do Estado de Goiás (FAPEG)
Neste trabalho, as características estruturais, microestruturais, ferroelétricas e dielétricas de materiais ferroelétricos com estruturas laminares de bismuto (BLSFs), conhecidos como Aurivillius, foram investigadas. Os sistemas objetos de estudo foram compostos bicamadas (SrBi2Nb2O9, SrBi2Ta2O9, BaBi2Nb2O9) e tricamadas (Bi4Ti3O12), onde foi considerada a dopagem do sítio A da estrutura com lantânio (La3+). As propriedades físicas, portanto, foram investigadas considerando a influência do La3+ para várias concentrações. As cerâmicas foram preparadas pelo método de reação de estado sólido com sinterização em temperaturas acima de 1000 °C, para todos os casos. Para o estudo das propriedades estruturais e microestruturais, foram utilizadas as técnicas de difração de raios-x, Espectroscopia Raman, Microscopia Eletrônica de Varredura (MEV) e Espectrometria de Dispersão de Energia (EDS). As propriedades ferroelétricas e dielétricas foram investigadas a partir da dependência da polarização com o campo elétrico e resposta dielétrica com temperatura, respectivamente. Fases puras foram verificadas para todos os sistemas e analisadas com refinamento pelo método de Reitveld. Os resultados apresentaram indícios de alterações nas estruturas das cerâmicas, o que pode ser associado à influência do lantânio na estrutura. Essas alterações foram confirmadas nas medidas de MEV, onde foi possível observar um aumento significativo dos tamanhos médios dos grãos com aumento do dopante para os sistemas bicamadas e uma redução para o sistema tri-camadas. As medidas de Raman identificaram os modos ativos característicos dos sistemas Aurivillius para todos os sistemas estudados, com influência do conteúdo de lantânio na estrutura. As propriedades ferroelétricas foram confirmadas para todos os casos com uma pequena contribuição da condutividade, que pode ser a causa da obtenção de ciclos de histerese aquém da saturação para alguns casos. Em particular, o sistema SrBi2Ta2O9 revelou ciclos bem característicos e definidos, enquanto o sistema BaBi2Nb2O9 mostrou o comportamento esperado para os ferroelétricos relaxores. Picos bem definidos foram observados na resposta dielétrica para todos os casos, observando uma forte influência do conteúdo de dopante nos parâmetros dielétricos. As características da transição de fases revelam comportamentos associados tanto a um ferroelétrico normal quanto ao comportamento típico de relaxores. Este efeito foi discutido detalhadamente para cada caso. É importante destacar que, para todos os casos, baixos valores de perdas dielétricas foram obtidos, muito abaixo dos reportados na literatura para sistemas Aurivillius. Diante destes resultados, os sistemas aqui estudados são fortes candidatos para aplicações tecnológicas.
In this work the structural, microstructural, ferroelectric and dielectric characteristics of ferroelectric materials with bismuth layer structures (BLSFs), known as Aurivillius, were investigated. The studied materials were based on bi-layers (SrBi2Nb2O9, SrBi2Ta2O9, BaBi2Nb2O9) and tri-layer (Bi4Ti3O12) systems, where the lanthanum (La3+) doping in the A-site of the structure has been considered. The physical properties where, therefore, investigated considering the influence of La3+ at various concentrations. The ceramics were prepared by the solid state reaction method for temperatures above 1000 °C for all the cases. For the investigation of the structural and microstructural properties the x-ray diffraction technique, as well as Raman spectroscopy, Scanning Electron Microscopy (SEM) and Energy Dispersive of Spectrometry (EDS) were used. The ferroelectric and dielectric properties were investigated from the electric field dependence of the polarization and dielectric response with temperature, respectively. Pure phases were found for all the systems and analyzed with structural refinement by the Reitveld’s method. The results showed changes in the structure of the ceramics, which can be associated with the influence of the lanthanum content in the structure. These changes were confirmed in the SEM measurements, where a significant increase in average grain size with the increase of the doping content for bi-layer systems, as well as a reduction in average grain size for the tri-layer system, has been observed. The Raman measurements showed the characteristic bands of the Aurivillius systems, with influence of the lanthanum content in the structure, for all the cases. The ferroelectric properties were confirmed in all cases, with a small contribution of the conductivity, which can be the cause for the observation of unsaturated loops in some cases. Particularly, the SrBi2Ta2O9 system showed very well defined hysteresis loops and the BaBi2Nb2O9 system showed the expected behavior for relaxor ferroelectrics. On the other hand, well defined peaks were observed in the dielectric response for all cases, showing a strong influence of the dopant content in the dielectric parameters. The phase transition characteristics revealed behaviors associated with both normal and relaxor ferroelectrics. This effect has been discussed in details for all the cases. Very low values of the dielectric losses were found, when compared to those reported in the literature for typical Aurivillius systems. This result makes the materials studied here promissory systems for practical applications.
FAPEG: 201310267000048
26
Behrendt, Karsten. "Der Einfluss von Defekten auf das Schaltverhalten ferroelektrisch modulierter Substanzen." Doctoral thesis, Niedersächsische Staats- und Universitätsbibliothek Göttingen, 2015. http://hdl.handle.net/11858/00-1735-0000-0028-873E-C.
Full text
27
Leist, Jeannis-Nicos, Jakob Sidoruk, Holger Gibhardt, Klaudia Hradil, Martin Meven, and Götz Eckold. "Domain redistribution and ferroelectric phase transition in SrTiO 3 under the influence of an electric field and mechanical stress." Universitätsbibliothek Leipzig, 2015. http://nbn-resolving.de/urn:nbn:de:bsz:15-qucosa-187976.
Full text
28
Leist, Jeannis-Nicos, Jakob Sidoruk, Holger Gibhardt, Klaudia Hradil, Martin Meven, and Götz Eckold. "Domain redistribution and ferroelectric phase transition in SrTiO 3 under the influence of an electric field and mechanical stress." Diffusion fundamentals 12 (2010) 69, 2010. https://ul.qucosa.de/id/qucosa%3A13899.
Full text
29
Peräntie, J. (Jani). "Electric-field-induced dielectric and caloric effects in relaxor ferroelectrics." Doctoral thesis, Oulun yliopisto, 2014. http://urn.fi/urn:isbn:9789526204406.
Full textAbstract:
Abstract In this thesis, dielectric and thermal behaviours due to the application of an electric field were studied in relaxor ferroelectric (1−x)Pb(Mg1/3Nb2/3)O3-xPbTiO3 (PMN-PT) and (1−x)Pb(Zn1/3Nb2/3)O3-xPbTiO3 (PZN-PT) systems of great technological importance. Special attention was given to the behaviour of the electric-field-induced phase transitions and electrocaloric effect, which are closely related to the existing and potential applications. Reactive sintering or columbite methods were used to fabricate polycrystalline PMN-PT ceramics with various compositions (x=0−0.3). In addition, commercial PMN-PT single crystals with composition close to the morphotropic phase boundary region were used. A studied PZN-PT crystal composition was grown by solution gradient cooling technique. Materials were mainly studied by means of dielectric and direct temperature measurements. The electrocaloric effect observed in a ceramic PMN-PT system was found to show distinct maximum values close to the thermal depolarization temperatures with low electric fields. The temperature range and magnitude of the electrocaloric effect was significantly expanded to high temperatures with increasing electric fields due to the contribution of polar nanoregions. The maximum electrocaloric temperature change was in the range of 0.77−1.55 °C under an electric field of 50 kV/cm. In addition, temperature change measurements on depoled PMN-0.13PT ceramics demonstrated that the electrocaloric effect is accompanied with an irreversible part below its depolarization temperature due to hysteresis loss and a possible phase transition type response related to the evolution of the macroscopic polarization. An electric field application to the <001> and <011> directions in PMN-PT crystals was found to cause distinct anomalies in the dielectric and temperature change responses. These anomalies were attributed to the complex polarization rotation routes and different phase stability regions in the electric-field-temperature phase diagrams of PMN-PT. Furthermore, measurements on PMN-PT crystals provided the first direct indications of a temporarily reversed electrocaloric effect with an increasing electric field. In addition, the measured electrocaloric trends in PZN-PT crystal were reproduced by a simple lattice model and mean-field approximation around the transition temperature. This demonstrated that the electrocaloric effect is driven mainly by the dipolar entropy loweringTiivistelmä Tässä työssä tutkittiin dielektristen ominaisuuksien ja lämpötilan käyttäytymistä teknologisesti merkittävissä (1−x)Pb(Mg1/3Nb2/3)O3-xPbTiO3 (PMN-PT) ja (1−x)Pb(Zn1/3Nb2/3)O3-xPbTiO3 (PZN-PT) ferrosähköisissä relaksorimateriaaleissa sähkökentän vaikutuksen alaisena. Tutkimuksen erityishuomion kohteena olivat sähköisesti indusoidut faasimuutokset sekä sähkökalorinen ilmiö, jotka liittyvät läheisesti nykyisiin sekä tulevaisuuden sovellutuksiin. Monikiteisiä PMN-PT keraamikoostumuksia (x=0−0,3) valmistettiin sekä reaktiivisella sintrauksella että kolumbiittimenetelmällä. Lisäksi tutkimuksessa käytettiin kaupallisia PMN-PT erilliskiteitä, joiden koostumus on lähellä morfotrooppista faasirajaa. Työssä käytetty PZN-PT erilliskide kasvatettiin jäähdyttämällä korkean lämpötilan liuoksesta. Materiaaleja tutkittiin pääosin lämpötilan ja dielektristen ominaisuuksien mittauksilla. Kun PMN-PT keraamisysteemiin kohdistettiin alhainen sähkökenttä, sähkökalorisen ilmiön selkeä maksimiarvo havaittiin lähellä materiaalin termistä depolarisaatiolämpötilaa. Suuremmilla sähkökentän arvoilla sähkökalorinen ilmiö voimistui ja sen lämpötila-alue laajeni korkeampiin lämpötiloihin polaaristen nanoalueiden kytkeytymisen vuoksi. Sähkökalorisen lämpötilamuutoksen maksimi vaihteli välillä 0,77−1,55 °C sähkökentän arvolla 50 kV/cm. Lisäksi lämpötilamittaukset depoolatulle PMN-0,13PT koostumukselle osoittivat, että sähkökalorisen ilmiön ohella materiaalissa esiintyy makroskooppisen polarisaation muodostumiseen liittyvä palautumaton lämpöenergia depolarisaatiolämpötilaa pienemmissä lämpötiloissa hystereesihäviön ja mahdollisen faasimuutoksen vaikutuksesta. PMN-PT erilliskiteiden dielektrisyys- ja lämpötilavasteessa havaittiin selkeitä muutoksia sähkökentän vaikuttaessa <001> ja <011> kidesuuntiin. Nämä muutokset ovat selitettävissä PMN-PT:n polarisaation kompleksisten rotaatiosuuntien ja erityyppisten sähkökenttä-lämpötila -faasidiagrammien stabiilisuusalueiden avulla. PMN-PT kiteiden mittauksissa havaittiin myös ensimmäinen suora osoitus väliaikaisesti käänteisestä sähkökalorisesta ilmiöstä sähkökentän kasvaessa. Lisäksi mitatut PZN-PT erilliskiteen sähkökaloriset ominaisuudet transitiolämpötilan läheisyydessä pystyttiin pääpiirteittäin mallintamaan käyttämällä yksinkertaista hilamallia ja keskimääräisen kentän approksimaatiota. Mallinnuksen mukaan sähkökalorinen ilmiö aiheutuu pääasiassa sähköisesti indusoidusta dipolientropian alenemisesta
30
Cheng, Juan. "Light scattering study on phase transition and micro-heterogeneity in relaxor ferroelectrics (1-x)Pb(Mg₁/₃Nb₂/₃)O₃-xPbTiO₃ and (1-x)Pb(Zn₁/₃Nb2₂/₃)O₃-xPbTiO₃ /." access full-text access abstract and table of contents, 2009. http://libweb.cityu.edu.hk/cgi-bin/ezdb/thesis.pl?phd-ap-b23749179f.pdf.
Full textAbstract:
Thesis (Ph.D.)--City University of Hong Kong, 2009."Submitted to Department of Physics and Material Sciences in partial fulfillment of the requirements for the degree of Doctor of Philosophy." Includes bibliographical references (leaves 140-167)
31
Džiaugys, Andrius. "Priemaišų įtaka feroelektrinių ir superjoninių kristalų dielektrinėms savybėms." Doctoral thesis, Lithuanian Academic Libraries Network (LABT), 2011. http://vddb.laba.lt/obj/LT-eLABa-0001:E.02~2011~D_20110628_134724-71267.
Full textAbstract:
Šiai dienai ypač populiarūs ferroelektrikai susidedantys iš kelių feroiškai aktyvių subgardelių, kurių persitvarkymas fazinio virsmo temperatūroje atskleidžia naujų, dar neaprašytų reiškinių. Prie šių medžiagų priskiriami antiferoelektrikai, ferielektrikai ir multiferoikai. Šiame darbe buvo tiriama nauja medžiagų šeimos MNP2X6 (M = Cu, Ag; N=In, Cr, Bi; X=S, Se ), kurios pasižymi ferielektrinėmis bei multiferoinėmis savybėmis, ir kurių dielektrines ir elektrines savybes galima efektyviai keisti įvedant priemaišas. Minėtų medžiagų dielektrinės ir elektrinės savybės buvo tiriamos dielektrinės spektroskopijos metodais, kurie leidžia tirti kristalų kolektyvinius reiškinius susijusius su tvarkos – netvarkos bei poslinkio tipo faziniais virsmais, jonų migracija bei dipolių užšalimu (stiklėjimu) plačiame dažnių (10-5 Hz iki 3 GHz) bei temperatūrų (25 K iki 500 K) intervaluose. Įvedus 10% Ag jonų vietoj Cu jonų ferielektriniame kristale CuInP2S6 fazinio virsmo temperatūra pasislenka į žemesnias temperatūras, o padidinus indžio koncentraciją fazinio virsmo temperatūra pasislenka į aukštesnes temperatūras. Minėtų kristalų fazinių virsmų temperatūrų skirtumas 50 K. Sumaišius skirtingomis proporcijomis feroelektriką (CuInP2S6) su antiferoelektriku (CuCrP2S6) stebima dipolinio stiklo fazė. Iš dielektrinių matavimų stiklo fazėje buvo paskaičiuota relaksacijos trukmių pasiskirstymo funkcija, kurios aprašymas dvigubos potencialinės duobės modeliu leido susieti mikroskopinius kristalo... [toliau žr. visą tekstą]Nowadays the ferroelectrics containing of several feroically active sublattices are very attractive, because interactions between these sublattices can caused novel phenomena. Antiferroelectrics, ferrielectrics and multiferoics belong to these materials. In this work new crystalline materials MNP2X6 (M = Cu, Ag; N=In, Cr, Bi; X=S, Se) were investigated, which have ferrielectric and multiferoic properties. The dielectric and electric properties of above mentioned materials have been investigated by broadband dielectric spectroscopy methods, which allows to analyze the collective processes related to order – disorder and displacive phase transitions, ions migration and freezing of dipoles (glassy state) in wide temperature (25 K - 500 K) and frequency (10-5 HZ - 3 GHz) ranges. By substitution or doping it becomes possible to tailor the ferroelectric materials to different properties. In this work is determined that the substitution of 10% Cu ions by Ag ions shifts the phase transition temperature of CuInP2S6 crystal toward lower temperatures while the addition of In ions shifts the phase transitions temperature toward the higher ones. The phase transition temperature difference is about 50 K for mentioned crystals. If the ferroelectric crystal CuInP2S6 is mixed with the antiferroelectric CuCrP2S6 the dipole glass phase occupies the middle of the phase diagram. The distribution of relaxation times has been calculated from the broadband dielectric spectra of dipolar glasses. The... [to full text]
32
Schader, Florian Herbert [Verfasser], Kyle G. [Akademischer Betreuer] Webber, and Bai-Xiang [Akademischer Betreuer] Xu. "Mechanical Stability of the Electromechanical Properties and Phase Transitions in Lead-Containing and Lead-Free Ferroelectrics / Florian Herbert Schader. Betreuer: Kyle G. Webber ; Bai-Xiang Xu." Darmstadt : Universitäts- und Landesbibliothek Darmstadt, 2016. http://d-nb.info/1112332995/34.
Full text
33
Roukos, Roy. "Transitions de phases dans des oxydes complexes de structure pérovskite : cas du système (1-x)Na0,5Bi0,5TiO3 - xCaTiO3." Thesis, Dijon, 2015. http://www.theses.fr/2015DIJOS020/document.
Full textAbstract:
Les solutions solides (1-x)Na0,5Bi0,5TiO3 (NBT) – xCaTiO3 (CT) ont été étudiées par diffraction des rayons X, spectroscopie Raman, microscopie électronique à balayage, spectroscopie d’impédance et DSC. Ce sont des matériaux présentant la structure cristalline pérovskite. L’étude révèle la complexité mais aussi la richesse des phénomènes physiques dans cette famille de composés : les séquences des transitions de phases, l’influence du dopant Ca2+ sur les propriétés physico-chimiques du matériau, la relation étroite entre propriétés diélectriques et caractéristiques structurales. Des solutions solides (1-x)NBT – xCT, avec 0 ≤ x ≤ 1,00, ont été synthétisées par voie solide classique puis frittées selon une procédure spécifique dans un milieu confiné pour éviter toute perte de sodium et de bismuth. Les caractéristiques cristallines des solides obtenus imposent clairement de distinguer trois domaines suivant les valeurs de x. En effet, pour les valeurs croissantes de x et à la température ambiante, on observe un premier domaine (Région I, pour x ≤ 0,07) dans lequel le solide obtenu est une solution solide de structure cristalline, de groupe d’espace R3c, identique à celle de NBT pur. Pour les valeurs les plus élevées de x (Région II, pour x ≥ 0,15), le solide obtenu est une solution solide de structure cristalline, de groupe d’espace Pnma, identique à celle de CT pur. Enfin, entre ces deux domaines (Région III, 0,09 ≤ x ≤ 0,13), les solides obtenus sont biphasés, R3c + Pnma, en se limitant aux appellations des groupes d’espacé des phases formées. Dans la région I, lors du chauffage, la séquence des transitions de phases R3c → P4bm → Pm3m est mise en évidence; les températures des transitions se déplacent vers les plus basses températures quand la concentration en Ca2+ augmente. Les solides sont ferroélectriques à l’ambiante puis développent un caractère relaxeur, par coexistence de deux phases, avec l’augmentation de la température. Dans la région II, les solides révèlent un comportement relaxeur dès l’ambiante. Une transition de phase diffuse au sein de la phase orthorhombique Pnma est toutefois mise en évidence ; le solide passe d’un état relaxeur à un état paraélectrique tout en conservant, a priori, la même structure cristalline. Le phénomène de relaxation dans ces composés est expliqué par la formation de micros ou nanorégions polaires. La région III, quant à elle, est caractérisée par l’apparition d’une hystérésis thermique mise en évidence pour la première fois ; elle est expliquée par la relation entre la microstructure cristalline et les propriétés diélectriques. Enfin, l’ensemble de nos résultats a été regroupé dans un diagramme de phase original en composition et en températureThe solid solutions (1-x)Na0,5Bi0,5TiO3 (NBT) – xCaTiO3 (CT) were studied by X-ray diffraction, Raman spectroscopy, scanning electron microscopy, impedance spectroscopy and DSC. These materials have a perovskite crystalline structure. This study reveals not only the complexity but also the richness of physical phenomena in these compounds: phases transitions sequences, the Ca2+ effect on the physical-chemistry properties and the relation between dielectric properties and crystalline structure. Thereby, (1-x)NBT – xCT solid solutions (0 ≤ x ≤ 1.00) were synthesized by chemical solid route, then they were sintered by a particular procedure in order to avoid sodium and bismuth volatilization. The solid crystalline characteristics obtained prove clearly the necessity to distinguish three fields as a function of x values. First of all, for increasing x at room temperature, there is a first region so called region I (x ≤ 0.07), wherein the crystalline structure of solid solutions obtained has a space group R3c identical to that of pure NBT. For the highest values of x, (Region II, x ≥ 0.15), the solid obtained has a space group Pnma, identical to that of pure CT. Finally, between these two regions, (0.09 ≤ x ≤ 0.13), the solid solutions obtained are biphasic, R3c + Pnma, limited to appellations of the space groups formed phases. In region I, upon heating, phase transition sequence R3c → R3c + Pnma → Pnma was determined; the corresponding transition temperatures move to low values with increasing Ca2+ concentration. These solids are ferroelectric at room temperature and then develop a relaxor character, by coexistence of two phases, with increasing temperature. In region II, these solids reveal a relaxor behavior at room temperature. However, a diffuse phase transition within the orthorhombic phase Pnma has been identified; the solid changes from relaxor to paraelectric while maintaining the same crystal structure. This phenomenon was explained by the formation of micro or nano-polar regions. Region III, demonstrated for the first time, is characterized by thermal hysteresis, and explained by the relation between crystalline microstructure and dielectric properties. Finally, all our results were assembled in an original phase diagram as a function of concentration of Ca2+ dopant and temperature
34
Nóbrega, Jaldair Araújo e. "Síntese e Caraterização de MgxPb(1-x)TiO3." Universidade Federal de São Carlos, 2015. https://repositorio.ufscar.br/handle/ufscar/9373.
Full textAbstract:
Submitted by Aelson Maciera (aelsoncm@terra.com.br) on 2018-02-01T16:29:26Z
No. of bitstreams: 1
TeseJAN.pdf: 5398076 bytes, checksum: 23f1bcf63802cdec3d1b5caf6731eca8 (MD5)Approved for entry into archive by Ronildo Prado (bco.producao.intelectual@gmail.com) on 2018-02-01T19:07:53Z (GMT) No. of bitstreams: 1 TeseJAN.pdf: 5398076 bytes, checksum: 23f1bcf63802cdec3d1b5caf6731eca8 (MD5)
Approved for entry into archive by Ronildo Prado (bco.producao.intelectual@gmail.com) on 2018-02-01T19:08:13Z (GMT) No. of bitstreams: 1 TeseJAN.pdf: 5398076 bytes, checksum: 23f1bcf63802cdec3d1b5caf6731eca8 (MD5)
Made available in DSpace on 2018-02-01T19:11:28Z (GMT). No. of bitstreams: 1 TeseJAN.pdf: 5398076 bytes, checksum: 23f1bcf63802cdec3d1b5caf6731eca8 (MD5) Previous issue date: 2015-08-14
Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES)
We report the investigation of structural, optical and electrical properties of the MgxPb(1- x)TiO3 ceramic system produced by mechanosynthesis. The solid state reaction manufacturing method has been proved to be an efficient alternative to manufacture this type of material. By using Raman spectroscopy techniques it was possible observe the effects induced by pressure, from 0 up to 14 GPa, and by temperature from 10 up to 723 K for the samples with x ranging from 0.2 to 1.0. Complementary characterization techniques were also used, including X-ray Diffraction (XRD), Scanning Electron Microscopy (SEM), Optical Absorption Spectroscopy and Electrical Impedance. XRD enabled the study of structural changes related to the increase in Mg concentrations. Raman analysis allowed the study of vibrational modes, in particular the transversely optical mode E(1TO) of lower frequency in order to characterize the phase transition from tetragonal-ferroelectric to cubic-paraelectric phases. From these results it was possible to construct a phase diagram that relates Mg concentration, temperature and hydrostatic pressure, yielding determine critical temperature (Tc) and critical pressure (Pc) in which phase transition occurs for each sample. Permittivity dielectric measurements were performed for each material, through which it was possible to compare the effects of Mg ratios in the modification of the values of these properties. Finally, a study of optical absorption as a function of Mg concentration was conducted, evidencing changes in gap energies related to the increase of Mg incorporation. The lowest values, 2.86 and 2.9 eV correspond to samples with concentrations of 0.4 and 0.2, respectively. Thus, from a technological point of view, these characteristics are interesting for applications in devices operating in the microwave region.
Neste trabalho foram investigados os efeitos estruturais, ópticos e elétricos do sistema cerâmico MgxPb(1-x)TiO3 produzido por mecano-síntese. O método de fabricação de reação de estado sólido mostrou-se uma alternativa eficiente de fabricação do material. Utilizandose de técnicas de espectroscopia Raman, foram observados os efeitos da pressão, de 0 a 14 GPa, e temperatura, de 10 a 723 K, para as amostras com x variando de 0,2 a 1,0. Também foram usadas técnicas complementares de caracterização entre elas a Difração de Raio-X (DRX), Microscopia Eletrônica de Varredura (MEV), Absorção Óptica e de Espectroscopia de Impedância Elétrica. A técnica de DRX possibilitou o estudo da alteração estrutural relacionada ao aumento das concentrações de Mg. As análises Raman permitiram o estudo dos modos vibracionais, em especial o modo transversal óptico E(1TO) de mais baixa frequência, a fim de caracterizar a transição de fase, da fase tetragonal ferroelétrica para a cúbica paraelétrica. A partir desses resultados foi possível construir um diagrama de fase relativo à concentração de Mg, com a temperatura e pressão hidrostática, obtendo-se temperatura crítica (Tc) e pressão crítica (Pc) nas quais ocorrem transição de fase para cada amostra. Foram realizadas medidas de permissividades dielétricas para cada material, por meio das quais foi possível a comparação dos efeitos das proporções Mg na alteração dos valores dessas propriedades. Por fim, foi realizado um estudo de absorção óptica em função da concentração de Mg, onde mostrou-se que há alteração do gap relacionada ao aumento da concentração de Mg. Os valores mais baixos, 2,86 e 2,9 eV correspondem a amostras com concentração de 0,4 e 0,2, respectivamente. Assim, do ponto de vista tecnológico, as características encontradas mostram-se úteis para dispositivos que operam na região das micro-ondas.
35
Kvitek, Zdenek. "Etude de la transition ferroélectrique-ferroélastique du KD2PO4 : forme du front de phase en fonction du gradient thermique." Université Joseph Fourier (Grenoble), 1995. http://www.theses.fr/1995GRE10046.
Full textAbstract:
La coexistence de phase paraelectrique-ferroelectrique (et ferroelastique) du kd#2po#4 a ete etudiee en fonction du gradient thermique. Dans cette transition tetragonale-orthorhombique, la forme du front de phase varie en fonction de l'orientation et de l'intensite du gradient thermique. Les fronts quasiplans ou en forme de toits (sections en zigzag) sont etudies experimentalement et expliques a l'aide d'un bilan energetique ou l'energie mecanique est calculee par la theorie de khachaturyan
36
Plonka, Rafael [Verfasser]. "Impact of the interface on the paraelectric-to-ferroelectric phase transition in epitaxial BaSrTiO_tn3 thin film capacitors / Rafael Plonka. [Forschungszentrum Jülich in der Helmholtz-Gemeinschaft, Institut für Festkörperforschung (IFF), Elektronische Materialien (IFF-6)]." Jülich : Forschungszentrum, Zentralbibliothek, 2007. http://d-nb.info/1000127257/34.
Full text
37
Barbosa, Quiroga David Antonio. "Processamento e propriedades do sistema ferroelétrico livre de chumbo (Bi, Na)TiO3 (Bi, K)TiO3 BaTiO3." Universidade Federal de São Carlos, 2015. https://repositorio.ufscar.br/handle/ufscar/5075.
Full textAbstract:
Made available in DSpace on 2016-06-02T20:16:54Z (GMT). No. of bitstreams: 1
6698.pdf: 13275782 bytes, checksum: 2c4bbd5a5c838552f7cbf82ce709e116 (MD5)
Previous issue date: 2015-03-09Financiadora de Estudos e Projetos
In this work, the lead-free ceramic powders of xBi0,5Na0,5TiO3 - (0,7186 - 0,7143x) Bi0,5K0,5TiO3 - (2814 - 2857x) BaTiO with x = 0,8200; 0,8625; 0,8792, 0,9126 and 0,9300 (BNBK1000x) were prepared by the solid state reaction method, followed by conventional densification. The ceramic bodies showed high density, which was higher than 95% in obtained samples. For the all ceramics were performed structural, micro structural, electric and anelastic characterizations. The X-ray diffraction (XRD) analysis indicated the formation of the complex perovskite type crystaline structure for all compositions analyzed without the presence of spurious phases. By the structural refinement by the Rietveld method of XRD data and by Raman spectroscopy were observed that for BNBK912 and BNBK930 compositions, at room temperature, the predominant symmetry is rhombohedral (R3c), while the BNBK820 composition exhibits a tetragonal crystalline symmetry (P4mm). For the BNBK879 and BNBK826 compositions presented a mixture of phases, possibly with rhombohedral and tetragonal symmetry, reaveling the morphotropic phase boundary (MPB) of this system. The microstructure of BNBK1000x ceramics was investigated by scanning electron microscopy (SEM), where the morphology grains with irregular sizes and shapes, where the increased levels of K+ and Ba2+ ions suppressed the growth of the grains. The characterizations by Raman spectroscopy at room temperature showed broad Raman modes, due to the chemical and/or structural disorder related to the substitution of elements Bi and Na for Ba and K. The ferroelectric characterizations at room temperature of the BNBK1000x ceramics showed that all compositions studied are ferroelectric. Through the comparison among the measurements of electrical impedance and mechanical spectroscopy, was possible to identify the different structural and electric phase transitions that were employed in the construction of a pseudodiagram of phases for the BNBK1000x compositions.
Neste trabalho, os pós-cerâmicos livres de chumbo xBi0,5Na0,5TiO3 - (0,7186 - 0,7143x) Bi0,5K0,5TiO3 - (2814 - 2857x) BaTiO com x = 0,8200; 0,8625; 0,8792, 0,9126 e 0,9300 (BNBK1000x) foram obtidos através do método de reação de estado sólido, seguido por densificação convencional. Os corpos cerâmicos obtidos apresentaram elevada densificação, sendo maior que 95% nas amostras produzidas. Para todas as cerâmicas foram realizadas caracterizações estruturais, microestruturais, elétricas e anelásticas. As análises por difração de raios-X (DRX) indicaram a formação da estrutura cristalina tipo perovkista complexa para todas as composições analisadas, sem a presença de fases espúrias. O refinamento estrutural, pelo método de Rietveld, dos resultados de DRX e os resultados de espectroscopia Raman apontam que para as composições BNBK930 e BNBK912 a simetria predominante em temperatura ambiente é romboédrica (R3c), enquanto que a composição BNBK820 apresenta a simetria cristalina tetragonal (P4mm). Já as composições BNBK879 e BNBK862 apresentaram uma mistura de fases, possivelmente com simetrias romboédrica e tetragonal, evidenciando o contorno de fase morfotrópico (CFM) deste sistema. A microestrutura das cerâmicas de BNBK1000x foi investigada por microscopia eletrônica de varredura (MEV), onde a morfologia apresentada pelas diferentes composições estudadas possuíam grãos com tamanhos e formatos irregulares, sendo que o aumento dos teores dos íons de K+ e Ba2+ inibiram o crescimento dos grãos. A caracterização por espectroscopia Raman, em temperatura ambiente, apresentou modos Raman amplos, mostrando um elevado grau de desordem química e/ou estrutural devido à substituição dos elementos Bi e Na por Ba e K. As caracterizações ferroelétricas, também em temperatura ambiente das cerâmicas de BNBK1000x mostraram que todas as composições estudadas possuem propriedades ferroelétricas. Através da comparação entre as medidas de impedância elétrica e anelástica, levando em consideração as caracterizações estruturais e ferroelétricas com temperatura, foi possível identificar diferentes transições de fase estruturais e elétricas, que foram empregadas na construção de um pseudo-diagrama de fases para as composições de BNBKx.
38
Si, Ahmed Fariza. "Elaboration et caractérisation de matériaux de type pérovskite, à faible teneur en plomb ou dépourvus de plomb." Thesis, Bourgogne Franche-Comté, 2017. http://www.theses.fr/2017UBFCK005.
Full textAbstract:
La plupart des matériaux ferroélectriques actuellement utilisés pour les applications industrielles sont des céramiques à base de plomb. De tels composés sont nocifs à l’environnement en raison de la toxicité et de la volatilité de l’oxyde de plomb durant le processus de préparation de ces matériaux. Dans le cadre du respect de l'environnement, la recherche de céramiques sans plomb ou de faible teneur en plomb ayant des propriétés équivalentes est devenue l'une des principales alternatives aux céramiques à base de plomb. C’est dans cette perspective que se situe le présent travail, focalisé sur l’investigation de matériaux de formules : Ba1-xPbx(Ti1-yZry)O3 (2.5% ≤ à x ≤ 10%) et Ba1-xEr2x/3Ti1-yZryO3. La synthèse par voie solide a été utilisée pour l’élaboration des matériaux. Les techniques expérimentales utilisées sont l’analyse thermogravimétrique, la dilatomètrie, la diffraction des rayons X, la microscopie électronique à balayage couplée à un système de microanalyse EDX et la spectrométrie d’impédance. L’utilisation de ces diverses techniques nous a permis d’optimiser les conditions d’élaboration et d’obtenir des céramiques denses de densité supérieures à 90%. Par ailleurs, les paramètres structuraux ont été affinés par la méthode de Rietveld puis discutés. En outre, ces céramiques possèdent des propriétés ferroélectriques classiques et/ou relaxeurs dont certaines ont des caractéristiques diélectriques très intéressantes au voisinage de la température ambiante. De telles compositions dépourvues de plomb ou contenant très peu de plomb permettent de limiter considérablement la pollution de l’environnement. Grâce à leurs bonnes performances diélectriques, ils pourraient être des candidats potentiels pour remplacer les matériaux à base de plomb actuellement usagés dans divers appareillages électroniquesMost of ferroelectric materials currently used in the industrial applications are lead-based ceramics. Such compounds are harmful to the environment due to the toxicity and volatility of lead oxide during the preparation process of these materials. Therefore, environmental friendly lead-free ceramics or low-lead content materials with equivalent properties has become one of the main topics as an alternative to lead-based ceramics. In this view, the present work focuses on investigating materials with formulas :Ba1-xPbx(Ti1-yZry)O3 (2.5% ≤ x ≤ 10%) and Ba1-xEr2x/3(Ti1-yZry)O3.Solid state route was used for materials elaboration. The experimental techniques used are thermogravimetric analysis, dilatometry, X-ray diffraction, scanning electron microscopy coupled to an EDX microanalysis system and impedance spectrometry. The use of these various processes allowed us to optimize the synthesis conditions and to obtain dense ceramics with density superior to 90%. In addition, the structural parameters were refined by the Rietveld method and then discussed. Furthermore, these ceramics exhibited normal and/or relaxor ferroelectric properties then some of them have very interesting dielectric characteristics in the vicinity of room temperature. Such lead-free ceramics or low-lead content materials reduce significantly the environmental pollution. Due to their good dielectric performances, they could be potential candidates for replacing lead-based materials currently used in various electronic equipments
39
Crossley, Samuel. "Electrocaloric materials and devices." Thesis, University of Cambridge, 2013. https://www.repository.cam.ac.uk/handle/1810/245063.
Full textAbstract:
The temperature and/or entropy of electrically polarisable materials can be altered by changing electric field E. Research into this electrocaloric (EC) effect has focussed on increasing the size of the EC effects, with the long-term aim of building a cooler with an EC material at its heart. Materials and experimental methods are briefly reviewed. A ‘resetting’ indirect route to isothermal entropy change ∆S for hysteretic first-order transitions is described. An indirect route to adiabatic temperature change ∆T, without the need for field-resolved heat capacity data, is also described. Three temperature controllers were built: a cryogenic probe for 77-420 K with ∼5 mK resolution, a high-temperature stage with vacuum enclosure for 295-700 K with ∼15 mK resolution, and a low-temperature stage for 120-400 K with electrical access via micropositioners. Automation enables dense datasets to be compiled. Single crystals of inorganic salts (NH4)2SO4 , KNO3 and NaNO2 were obtained. Applying 380 kV cm−1 across (NH4)2SO4 , it was found that |∆S| ∼ 20 J K−1 kg−1 and |∆T | ∼ 4 K, using the indirect method near the Curie temperature TC = 223 K. Without the ‘resetting’ indirect method, |∆S| ∼ 45 J K−1 kg−1 would have been spuriously found. Preliminary indirect measurements on KNO3 and NaNO2 give |∆S| ∼ 75 J K−1 kg−1 for ∆E ∼ 31 kV cm−1 near TC = 400 K and |∆S| ∼ 14 J K−1 kg−1 for ∆E ∼ 15 kV cm−1 near TC = 435 K, respectively. A cation-ordered PbSc0.5Ta0.5O3 ceramic showing a nominally first-order transition at 295 K was obtained. The Clausius-Clapeyron phase diagram is revealed via indirect measurements where |∆S| ∼ 3.25 J K−1 kg−1 and |∆T | ∼ 2 K, and direct measurements where |∆T | ∼ 2 K. Clamped samples show broadening of the field-induced transition. Epitaxial, ∼64 nm-thick SrTiO3 films were grown by pulsed laser deposition on NdGaO3 (001) substrates with a La0.67Sr0.33MnO3 bottom electrode. The indirect method gives |∆S| ∼ 8 J K−1 kg−1 and |∆T | ∼ 3.5 K near 180 K with |∆E| = 780 kV cm−1. Finite element modelling (FEM) was used to optimise the geometry of multilayered capacitors (MLCs) for EC cooling. Intrinsic cooling powers of 25.9 kW kg−1 are predicted for an optimised MLC based on PVDF-TrFE with Ag electrodes.
40
Herklotz, Andreas. "Einfluss reversibler epitaktischer Dehnung auf die ferroische Ordnung dünner Schichten." Doctoral thesis, Saechsische Landesbibliothek- Staats- und Universitaetsbibliothek Dresden, 2012. http://nbn-resolving.de/urn:nbn:de:bsz:14-qucosa-87097.
Full textAbstract:
In dieser Arbeit werden die Auswirkungen epitaktischer Dehnung auf die Eigenschaften ferromagnetischer und ferroelektrischer Perowskitschichten untersucht. Dazu wird der biaxiale Dehnungszustand einer Schicht reversibel verändert, indem einkristalline piezoelektrische Pb(Mg1/3Nb2/3)0.72Ti0.28O3 (001) Substrate (PMN-PT) verwendet werden. Ergänzt werden die Messungen mit dieser “dynamischen” Methode durch Untersuchungen an statisch gedehnten Schichten, gewachsen auf LaAlxSc1-xO3-Pufferschichten mit gezielt abgestimmter Gitterfehlpassung. Drei verschiedene Materialsysteme werden studiert: die ferromagnetischen Oxide La0.8Sr0.2CoO3 und SrRuO3 und das ferroelektrische Pb(Zr,Ti)O3. Für La0.8Sr0.2CoO3 wird ein dehnungsinduzierter Übergang von der bekannten ferromagnetischen Phase zu einer magnetisch weniger geordneten, spinglasartigen Phase nachgewiesen. Es ergeben sich keine Hinweise auf eine Beeinflussung des Co-Spinzustandes. In epitaktischen SrRuO3-Schichten bewirkt eine Zugdehnung einen strukturellen Phasenübergang von der orthorhombischen Bulk-Phase zu einer out-of-plane orientierten tetragonalen Phase. Die leichte Richtung liegt in der Ebene. Reversible Dehnungsmessungen zeigen einen deutlichen Einfluss auf die ferromagnetische Ordnungstemperatur und deuten auf eine geringe Veränderung des magnetischen Moments hin. Der Dehnungseffekt auf die elektrischen Transporteigenschaften wird bestimmt. Pb(Zr,Ti)O3 wird als ferroelektrisches Standardmaterial genutzt, um erstmalig den Einfluss biaxialer Dehnung auf das ferroelektrische Schaltverhalten dünner Schichten zu untersuchen. Für kleine elektrische Felder zeigen die Messungen das typische Verhalten einer gepinnten Domänenwandbewegung. Hier wird der Schaltvorgang unter Piezokompression stark beschleunigt. Werden an die elektrischen Kontakte größere elektrische Felder angelegt, geht die Domänenwandbewegung in das Depinning-Regime über. Die Schaltkinetik wird in diesem Bereich unter Piezokompression leicht verlangsamtIn this work, the effect of epitaxial strain on the properties of ferromagnetic and ferroelectric perovskite thin films is studied. Single-crystalline piezoelectric Pb(Mg1/3Nb2/3)0.72Ti0.28O3 (001) substrates (PMN-PT) are utilized to reversibly change the biaxial strain state of the films. The measurements performed by this “dynamic” approach are complemented by studying statically strained films grown on LaAlxSc1-xO3 buffer layers with deliberately tuned lattice misfit. Three different material systems are investigated: the ferromagnetic oxides La0.8Sr0.2CoO3 and SrRuO3 and the ferroelectric compound Pb(Zr,Ti)O3. In case of La0.8Sr0.2CoO3 a strain-induced transition from the known ferromagnetic phase to a magnetically less ordered spinglas-like phase is observed. No indications for an effect on the Co spin state are found. In epitaxial SrRuO3 films tensile strain is causing a structural phase transition from the bulk-like orthorhombic structure to an out-of-plane oriented tetragonal phase. The magnetic easy axis is in the film plane. Reversible strain experiments show a significant effect on the ferromagnetic ordering temperature and point to a small change of the magnetic moment. The strain effect on the electric transport properties is also determined. Pb(Zr,Ti)O3 as a standard ferroelectric material is used to study the influence of biaxial strain on the ferroelectric switching behaviour of thin films for the first time. At small electric fields the measurements reveal the typical signs of creep-like domain wall motion caused by wall pinning. In this regime the switching process is accelerated strongly under piezo-compression. For higher electric fields a transition of the domain wall motion to the depinning regime is observed. Here, the switching kinetics is slowed down moderately by compressive strain
41
Lu, Teng. "Structure and Property Evolution Induced by the Phase Transitions in Several Antiferroelectric Materials." Phd thesis, 2017. http://hdl.handle.net/1885/144426.
Full textAbstract:
Antiferroelectric (AFE) materials are an important group of
functional materials showing unique properties such as double
polarization-electric field (P-E) hysteresis loop, and charge
release under the pressure and temperature. These performances
are strongly connected to the structural phase transitions
induced by external conditions. Many investigations were carried
out to optimize their properties but the structure-property
relationship of such AFE materials still remain unclear. With
this bearing in mind, in this thesis, I firstly investigate the
crystal structure, domain structure and properties evolution
under the external stimuli such as electric-field (E-field),
mechanical force, and temperature in the typical PbZrO3-based AFE
materials. Secondly, a systematic study was conducted on the
doped silver niobate ceramics to understand the impacts of the
chemical composition and E-field cycling on these novel lead-free
AFE materials.
The targeted materials of typical PbZrO3-based samples were
selected as La/Nb doped Pb(Zr, Sn, Ti)O3 ternary systems with the
composition Pb0.97La0.02(Zr0.56Sn0.33Ti0.11)O3 (PLZST1),
Pb0.99(Nb0.02Zr0.73Sn0.21Ti0.04)O3 (PNZST1) and
Pb0.99(Nb0.02Zr0.65Sn0.28Ti0.05)O3 (PNZST2). These three
compositions are representative, supplying diverse phase
transition behaviours for studying. The in situ neutron powder
diffraction (NPD) of the PLZST1 material reveals that the
pseudo-tetragonal AFE phase is transferred into the rhombohedral
FE phase with an application of the sufficient E-field, and
recovers after withdrawal of the external field. The resultant
average structure change as a function of the E-field is in
accordance with the reversible AFE-FE phase transition. However,
the ω dependent NPD patterns suggest this process is not fully
reversible: in the induced FE state, the strain exhibits an
elliptical distribution, which in turn leads to significant
preferred orientation in the final AFE state. The formation of
this preferred orientation provides an explanation for the
properties variation appearing in AFE materials after exposure to
the sufficiently high E-field.
X-ray diffraction pattern of PNZST1 sample indicates the
orthorhombic AFE phase while the result of NPD contradicts this
conclusion with a rhombohedral FE phase. After careful
characterization of the surface and bulk properties, it is found
that the near surface and bulk regions show different phases.
Additionally, the surface processing such as polishing and
heat-treatment can induce an AFE/FE phase transition within
micrometres of the surface. The in-situ hydrostatic-pressure
neutron diffraction proves that the mechanical force helps
stabilize the AFE phase of this composition. Therefore, the
surface processing induced phase transitions can be attributed to
the change of states of residual stress.
The in-situ NPD studies of PNZST2 material describe its
structural variation as a function of E-field and temperature.
Through the mode decomposition approach, the relationships
between AFE/FE modes and octahedral rotation mode were
systematically investigated. At room temperature, the pristine
AFE phase can be poled into the meta-stable FE phase by applying
the external E-field. At this stage, both AFE and FE phases
consist of modes associated with octahedral rotation and A-site
ionic displacements. The temperature-induced phase transition
indicates that the octahedral rotation and ionic displacements
are weakly coupled in the room-temperature FE phase and decoupled
in the high-temperature FE phase. Furthermore, both temperature
and E-field-induced phase transitions between the AFE and
high-temperature FE phase demonstrate the critical role of
coupling between the octahedral rotation and A-site ionic
displacements in AFE structure stabilization.
The evolution of structure and electrical properties with
composition in (1-x)AgNbO3-xLiTaO3 (ANLT100x) (0 ≤ x ≤ 0.09)
ceramics have been systematically investigated by diffraction
techniques, complemented by dielectric and polarisation
measurements. The symmetry mode decomposition and Rietveld
refinement of distortive modes were firstly used to analyse the
origin of the anti/ferroelectricity observed. The in/out phase
octahedral tilting around the a-axis (H2 mode) and the
antiparallel ionic displacements (Λ3 mode), present large
amplitudes in the pure AgNbO3. These two modes vanish
progressively with increasing x and their amplitudes experience a
sudden drop when x = 0.053. Accompanied by the disappearance of
these two modes, a new phase with R3c symmetry appears and grows
with further increasing LiTaO3 content. The composition dependent
amplitudes of the primary modes, and R3c phase fractions, lead to
a comprehensive understanding of the dielectric and ferroelectric
properties affected by LiTaO3. For the composition located around
the phase boundary, x = 0.045 and 0.06, FE wake-up effects were
detected. The refinement of neutron diffraction patterns after
different electric cyclicity describe an increase of
ferroelectricity associated with the R3c phase fraction
increments i.e., field-cycling-induced phase transition from
Pmc21 to R3c. The local probes such as the electron diffraction
and piezoresponse microscopy (PFM), show that the in/antiphase
octahedral rotation around the <001>p and the local strain state
are the decisive factors for this field-cycling-induced phase
transition. In summary, the wake-up effects can be regarded as
the nucleation and growth of the R3c phase with increasing number
of electric cycles.
42
Lee, Jiun-Ting. "Phase transitions and diffusion of colloid-polymer systems and thin ferroelectric films for device applications." Thesis, 2000. http://hdl.handle.net/1911/19527.
Full textAbstract:
This thesis is divided into two independent parts: the first part deals with the phase transitions and diffusion of colloid-polymer systems, while the second part concerns the growth of thin ferroelectric films, their physical and chemical characterizations, and theoretical predictions for surface-acoustic-wave (SAW) propagation.
In the first part, the phase transitions of colloid-polymer systems are studied by experiment and theory in both two and three dimensions. For a colloid-polymer system consisting of polystyrene spheres in aqueous solutions of hydroxyethylcellulose, the transitions are examined experimentally using enhanced videomicroscopy with particle resolution, and the results are compared with statistical mechanical predictions based on the Percus-Yevick and mean-field approximations. The Brownian motion of colloidal particles in polymer solutions is also investigated for the system of colloid (polystyrene)-polymer (polyethyleneoxide), and the diffusivity is determined as a function of polymer concentration.
In the second part, thin films of the ferroelectric lithium niobate are deposited by radio-frequency magnetron sputtering on diamond-coated silicon substrates for high-frequency SAW-device applications. The SAW velocity and electromechanical coupling coefficient are predicted theoretically for various film orientations and thicknesses. Moreover, thin films of the ferroelectric barium titanate are deposited on silicon substrates for random-access-memory applications, and their crystal and electrical properties are investigated.
43
Schiemer, Jason Alexander. "Average and local structure, phase transitions and physical properties of key lead-free ferroelectric materials." Phd thesis, 2012. http://hdl.handle.net/1885/149880.
Full textAbstract:
This thesis presents a comprehensive structural and temperature-dependent properties study of two key lead-free ferroelectric families of materials: the bismuth ferrites and the alkaline niobates. Doped bismuth ferrites are of interest not only as lead-free ferroelectric materials but also as multi-ferroic materials. Both alkaline earth doped Bi^{III}{u2081}-xA^{II}xFe^{III}O{u2083}-x/{u2082} and rare earth doped Bi^{III}{u2081}-xLn^{III}xFe^{III}O{u2083} systems are investigated. An incommensurately modulated, perovskite-related, solid solution is found to be characteristic of both the Bi^{III}{u2081}-xA^{II}xFe^{III}O{u2083}-x/{u2082}, A = Sr and Ca systems. Contrary to previous reports, a tetragonal distortion is found across the phase diagram. Despite prior expectations, piezoresponse force microscopy suggests that BCFO is non-piezoelectric at room temperature. Through transmission electron microscopy studies, Mossbauer spectroscopy as well as temperature dependent electrical and mechanical examination, the incommensurate modulation in these systems is attributed to oxygen vacancy and associated alkaline earth ordering. A characteristic relaxation in the dielectric spectroscopy results is assigned to oxygen vacancy hopping while the material as a whole is found to be semiconducting.
Materials of composition Bi^{III}{u2081}-xLn^{III}xFe^{III}O{u2083}, Ln = Nd and Sm, are synthesised by both solid state and sol-gel synthesis techniques. Magnetization and electrical measurements show that BSFO 10 and BNFO 10 are simultaneously ferroelectric and ferromagnetic. Correlated temperature dependent magnetic, electrical and mechanical behaviours were investigated, with similar low temperature magnetic and mechanical anomalies in both materials. Neutron diffraction is used to investigate the Neel temperatures of these materials.
Finally, the K^{I}xNa^{I}{u2081}-xNb^{V}O{u2083} (KNN x) system is systematically investigated in two ways. Firstly, the KNN system is examined with regard to its ferroelectric and piezoelectric properties, primarily near x = 35% (KNN 35), with and without y% Ta(V) doping (KNNT 35/y%). Ta doping of KNN 35 is found to improve piezoelectric behaviour, decrease grain size, decrease elastic stiffness and to lower high temperature phase transitions. The KNN 35 region of the phase diagram is found to be remarkably similar to the more often characterised K{u2080}.{u2085}Na{u2080}.{u2085}NbO{u2083}{u03B6} region. The reported morphotropic phase boundary related property enhancements in the KNN x system are also discussed in light of the results obtained.
Secondly, the KNN x system is examined with regard to structural disorder across the entire phase diagram. Electron diffraction shows transverse polarized planes of diffuse intensity perpendicular to the parent perovskite [010] axis across the KNN x phase diagram at room temperature, demonstrating 1-D ferroelectric disorder. Additionally, 1-D rods of diffuse intensity of the type G +/- {u03BE}, 1/2, 1/2]p* and G +/- [1/2, 1/2, {u03BE}]p* (G a parent perovskite sub-structure reflection) are also observed, demonstrating octahedral tilt disorder at all compositions. Bond valence sum calculations suggest that this octahedral tilt disorder is due to the crystal chemical conflict between the larger K ions and the smaller Na ions occupying the perovskite A site. The reported unit cells and space groups of the phases in KNN x are re-examined in light of the inherent ferroic and octahedral tilt disorder characteristic of this system. The thesis is concluded with a summary of the key findings and some propositions for further work in these systems. -- provided by Candidate.
44
Li, Chen-Liang, and 李振良. "Behavior of Field-Induced Strain and Ferroelectric/ Antiferroelectric Phase Transitions in Lead Zirconate Titanate Stannate System." Thesis, 1997. http://ndltd.ncl.edu.tw/handle/21339131023772881095.
Full textAbstract:
碩士國立台灣工業技術學院
機械工程技術研究所
85
ABSTRACT In this thesis, the ferroelectric to antiferroelectric phase transformation of Pb0.99Nb0.02[(Zr1- xSnx)1-yTiy]0.98O3 (PNZST) as a function of temperature and field as well as the annealing effect on material properties were investigated by measuring P-E curves, dielectric constants, field-induced strains and remanent polarizations as function of frequency.The PNZST powders were prepared by solution coating method.Samples were sintered at 1200 ℃for 2 hours and heat treatedat 1250℃for 2~60 hours using double-crucible method. Polarization curves were measured with various frequenciesby modified Sawyer-Tower circuit, and dielectric constants were studied using a gain-phase analyzer. Field-induced strains as a function of temperature were measured by means of a triangular laser measuring system with an oil bath. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) were employed in microstructural analysis. Experimental results show that samples can get larger remanent polarization(Pr) by appropriate heat treatments, but Pr decreased with increasing frequency and decreasing Ti content. Dielectric constant maximum that occurs before multiple cell shows significant increase by suitable heat treatment and increasing Ti content. Dielectric relaxation disappeared at ferroelectric to antiferroelectric transformation. Field-induced strains are temperature dependent and the strains that use the same field strength of 30kV/cm for 43/7/2 specimens treated at 1250 ℃for 20 hours can produce a maximum value of 0.22﹪around the ferroelectric to antiferroelectric transition temperature (TF/A) (at 77℃). When the temperature increased over TF/A, the field- induced strain still showed a pronounced value of 0.16﹪, and can achieve higher by increasing the field. The P-E curves of 43/7/2 show complete ferroelectric behavior, but the specimens contain incommensurate phase from the results of microstructural investigations, and the P-E curves can change to antiferroelectric by increasing frequency, indicating that the transition between the ferroelectric commensurate and the antiferroelectric incommensurate phase is relatively easy, and the incommensuration is the origin of the second-order transition in this material. Ferroelectric to antiferroelectric phase transition which shows similar behavior of the soft-mode produces a maximum strain during transition. When the specimen is over transformation temperature and in the antiferroelectric imcommensurate phase range, high strain is produced by field- induced AFE/FE transformation with a decoupling of the polarization of the modulated polar structure.
45
Maier, Bernd Jens [Verfasser]. "Phase transitions in advanced relaxor-ferroelectric materials with a perovskite-type structure / vorgelegt von Bernd Jens Maier." 2010. http://d-nb.info/1003355994/34.
Full text
46
Yeh, Chien-Hung, and 葉建宏. "Successive Phase Transitions in Relaxor Ferroelectric PMN-PT and PZN-PT Single Crystals by Brillouin Light Scattering." Thesis, 2000. http://ndltd.ncl.edu.tw/handle/49729987007008618697.
Full textAbstract:
碩士輔仁大學
物理學系
88
Abstract The longitudinal Brillouin back-scattering spectra and dielectric permittivities have been measured as a function of temperature in relaxor ferroelectric crystal (Pb(Mg1/3Nb2/3)O3) 1-x(PbTiO3)x (PMN-xPT：x=0.24, 0.32, 0.31 and 0.33), and (Pb(Zn1/3Nb2/3)O3)1-x(PbTiO3)x (PZN-xPT：x=0.085 and 0.095). These crystals have successive phase transitions (beside PMN- 24%PT). A sharp ferroelectric phase transition which is associated with a minimum in phonon frequency and an abrupt change in polarization, was observed in the high tempre- ature region (above 400 K). As temperature decreases, a diffuse phase transition which is associated with a broad frequency dependent dielectric spectrum and a gradual evolution in polarization. In addition, the nature of the thermal hysteresis for the dielectric permittivity confirms that these transition are first-order (high-temperature phase transition) and diffuse first-order (low-temperature diffuse phase transition), respectively.
47
Chih-Hung, Chen, and 陳志洪. "Phase Transitions and Electric-Field Effects in Relaxor—Based Ferroelectric Single Crystals (PbZn1/3Nb2/3O3)1-x(PbTiO3)x." Thesis, 2002. http://ndltd.ncl.edu.tw/handle/00722933631130246761.
Full text
48
Chen, Horng-Ying, and 陳虹穎. "Phase transition in ferroelectric liquid crystal." Thesis, 2003. http://ndltd.ncl.edu.tw/handle/01028338628113076240.
Full textAbstract:
碩士國立中央大學
物理研究所
91
Abstract High-resolution heat-capacity measurements have been conducted near the Sm-A to Sm-C transition in free-standing films of liquid-crystal compound C7. We find that the profiles of associated heat-capacity anomalies are strongly thickness-dependent and exhibit a novel crossover behavior in reduced dimensions. This result is similar to the observed in MDW1397 liquid-crystal films. Furthermore, heat-capacity measurements have provided the direct confirmation of surface transitions from Sm-C immediately to the Sm-G phase in C7. The molecular interactions are found to be Thermal-Casimir force.
49
Liou, Sheng-Cheng, and 劉信成. "Electrical properties and Ferroelectric Phase Transition of Donor-Doped Barium Titanate." Thesis, 1993. http://ndltd.ncl.edu.tw/handle/40249470827692241316.
Full textAbstract:
碩士大同工學院
材料科學(工程)研究所
81
PTC鈦酸鋇之商品化應用已超過三十年,但是仍無法合理解釋為什麼室溫電 阻係數會隨著添加量而改變 以往研究各種摻雜施體的含量約在0.3％摩耳 比時室溫電阻值最低,遠離0.3％摩耳比時室溫電阻率均增加,相對使ρ／ ρ 值減少本研究為瞭解添加 Sm,Nd 和 Pr 三 室溫電阻率差異較大的元 素(Fig. 1) 之電導機構, 改變不同的含量測試電阻率,晶格常數, 介電 常數及飽和極化值等隨溫度變化的改變以了解晶界上電子能障減少或消失 的原因另外,為瞭解不同取代型式對鈦酸鋇半導體之電導機構的影響,實驗 中亦添加Nb對其做初步的探討由實驗結果中顯示添加Sm,Nd 和 Pr 其室溫 電阻率未見有甚大的差異此結果與以往報告有所不同(Fig.1) 並且由X- ray 繞射結果可知所添加之元素其室溫電阻率與在正方相的c/a值有密切 的關係而添加高含量的元素其居禮溫度有降低的趨勢,此結果有必要進一 步了解 Donor-doped semiconducting BaTiO has a remarkable property of positive temperature coefficient of resistivity(PTC). Although PTC BaTiO has been in use for more than 30 years,it is difficult even today to explain the resistivity at room temperature as a function of doping content. In past studies, the addition of 0.3 at％ of donor dopes to BaTiO a dramatic decrease in the resistivity is observed at room temperature. Beyond this optimum doped, the room temperature resistivity will increase and the amount of increase depends on the kind of donors. The purpose of this study is to understand the conduction mechamism of Sm, Nd and Pr doped BaTiO which show large difference of room temperature resistivity.(Fig.1)We measured the variations of resistivity, lattice constant and saturation polarization of various amount donor - doped BaTiO. The measurementswill be under different temperature ranges.From the resultsofthose measurements willbe hopefully to understand the causes ofthediminished or decreased energy barrier at grain boundaries. In order to understand effect of different substitution type on conduction mechanism of semiconducting BaTiO, Nb is also used to as donor. From the results of ρ-T of Sm, Nd, and Pr doped BaTiO, We found that there is no much room temperature resistivity difference among those donors. This is results is different from that of O.Saburi. By examing X-ray diffraction results, the room temperature resistivity of donor-doped BaTiO is believed to be close-related to c/a ratio of tetragonal phase. Also, high room temperature resistivity for higher doping concentration samples is may be to shifting the Curie temperature to lower temperature. Although this result has to be examined in more detail.
50
Amorim, Carlos de Oliveira. "Experimental and modeling studies of magnetoelectric multiferroic heterostructured materials." Doctoral thesis, 2019. http://hdl.handle.net/10773/27751.
Full textAbstract:
Multiferroic materials are a very exotic type of materials which present simultaneously two or more ferroic properties. Magnetoelectric multiferroics, in particular, are a very prominent class of materials, mainly due to their outstanding foreseen applications such as magnetic sensors, energy harvester/conversion devices, and high efficiency memories. However, intrinsic magnetoelectric materials are quite rare and do not have, yet, the adequate properties to the everyday applications. One of the reasons for this to occur is due to the requirements for magnetism and ferroelectricity in matter being a priori contradictory, since the former needs unfilled dn orbitals, while the latter favours d0 orbitals. Nevertheless, extrinsic magnetoelectric multiferroics do not suffer from this problem because they do not share the same phase, hence being a very promising approach to engineer adequate magnetoelectric multiferroics. This thesis focus on the study of Fe and BaTiO3 systems as a means of achieving novel magnetoelectric effects. It is shown that a peculiar type of BaTiO3:Fe auto-composite presents an ordered magnetic behaviour, despite the concentration of Fe being as low as 113 atomic ppm. The Fe magnetization displays two abrupt changes in its spontaneous value, one with M/M ≈ 32% and the other with M/M ≈ 14%. These magnetic transitions are correlated the BaTiO3 orthorhombic↔tetragonal
and tetragonal↔cubic ferroelectric phase transitions. This magnetoelectric auto-composite was the motivation to resort to Density Functional Theory (DFT) modeling as a means to discover the microscopic mechanism(s) behind such a strong magnetoelectric effect. The study of an iron monolayer placed upon several possible BaTiO3 unit cells lead to the discovery of several interfaces with abrupt changes in their spontaneous magnetization, either through the enhancement and reduction of the Fe magnetic moments, or through the change between antiferromagnetic and ferromagnetic order of the Fe monolayer.
However, the highlight of these DFT studies lies in the discovery of a particular kind of interfaces, namely in the BTO221_2ndFe and BTO99_2ndFe
supercells, where there is a High-Spin–Low-Spin state transition which can quench completely the atomic magnetic moment of each of Fe atom, depending on the local crystal field felt by the Fe atoms. Based on this specific effect, where it is possible to turn on and off the magnetic moments of the Fe atoms, a magnetoelectric multiferroic device was proposed. Knowing the importance of the crystal field for the High-Spin–Low-Spin state transition, a thorough study regarding the Electric Field Gradient (EFG) of each possible BaTiO3 site was performed, resorting to a combined study of DFT and Perturbed Angular Correlations (PAC) spectroscopy. In this study, it was concluded that the PAC spectroscopy is not the most adequate hyperfine technique to be used in a quantitative study of the BaTiO3/Fe interfaces EFG tensor, due to the non-negligible effects of the radioactive probe on the BaTiO3 matrix. Finally, the deposition of BTO/Fe heterostructures on LaAlO3, MgO, Al2O3 and SrTiO3 substrates using RF-Sputtering, and the Molecular Beam Epitaxy (MBE) deposition of Fe layers on BaTiO3 cut at the (100), (110) and (111) planes were performed as an attempt to recreate the interfaces with the most appealing magnetoelectric effects predicted in the DFT modeling. The thin films deposited using sputtering showed the growth of many Fe, Ba-Ti-O and Fe-Ti-O oxides depending strongly on their substrate, as well as in the deposition and annealing conditions. Still no magnetoelectric coupling was observed in such thin films. On the other hand the Fe thin films deposited on BaTiO3 substrates showed large magnetoelectric couplings between the BaTiO3 ferroelectric phase transitions and the magnetization of the Fe layers (similarly to what happened in the BaTiO3:Fe auto-composite). The magnitude of this magnetoelectric couplings is strongly correlated with the BTO interface where the Fe was deposited, showing a huge change in spontaneous magnetization and coercivity for the rhombohedral↔orthorhombic ferroelectric phase transition up to M/M ≈ 148% and HC/HC ≈ 183% respectively for the (110) case.Materiais Multiferróicos são um tipo de materiais bastante exótico que apresentam simultaneamente dois ou mais tipos de propriedades ferróicas. Multiferróicos magnetoelétricos, em particular, são uma classe de materiais muito proeminente, principalmente devido às suas espantosas aplicações tecnológicas, tais como sensores magnéticos, dispositivos de conversão/colheita de energia, e memórias the alta eficiência. Todavia, materiais magnetoelétricos intrínsecos são verdadeiramente raros e ainda não possuem propriedades adequadas ao uso do dia-a-dia. Uma das razões para que isto aconteça prende-se com o facto dos requisitos para existência de magnetismo e ferroeletricidade na matéria serem a priori contraditórios, uma vez que enquanto os primeiros necessitam de orbitais dn semipreenchidas, os últimos tendem a favorecer orbitais d0. Porém, Multiferróicos magnetoelétricos extrínsecos não sofrem desta limitação pois não partilham a mesma fase sendo portanto uma abordagem promissora para a construção de um bom Multiferróico magnetoelétrico. Esta tese focar-se-á no estudo de sistemas contendo Fe e BaTiO3 como meio de se alcançarem novos efeitos magnetoelétricos. Um auto-compósito de BaTiO3:Fe é apresentado, que apesar da sua diminuta concentração de Fe (apenas 113 ppm atómicas), ainda assim apresenta um comportamento magnético ordenado. A magnetização do Fe apresenta duas variações bruscas no seu valor espontâneo, uma com M/M ≈ 32% e outra com M/M ≈ 14%. Estas transições magnéticas estão correlacionadas com as transições de fase ferroelétricas do BaTiO3 (ortorrômbica↔tetragonal e tetragonal↔cúbica). Este auto-compósito magnetoelétrico foi a motivação par ao uso da Teoria de Densidade Funcional (DFT) como meio para descobrir os mecanismos microscópicos por trás deste acoplamento magnetoelétrico tão intenso. O estudo de uma mono-camada de Fe colocada sobre várias células unitárias de BaTiO3 levaram à descoberta de várias interfaces com mudanças abruptas na sua magnetização espontânea, ora através do aumento ou diminuição dos momentos magnéticos do Fe, ora através da mudança entre a natureza antiferromagnética ou ferromagnética da camada de Fe. Contudo, o destaque dos estudos de DFT reside na descoberta de um tipo particular de interfaces onde ocorre uma transição de estado High-Spin–Low-Spin que consegue colapsar completamente o momento magnético atómico dos átomos de Fe, dependendo do campo cristalino local sentido por esses mesmos átomos. Baseado neste efeito, um dispositivo Multiferróico magnetoelétrico foi proposto. Sabendo a importância do campo cristalino para as transições de estado High-Spin–Low-Spin state, um estudo minucioso foi feito relativo ao gradiente de campo elétrico (EFG) nos sítios possíveis do BaTiO3, usando um estudo combinado entre Correlações Angulares Perturbadas (PAC) e DFT. Neste estudo, concluiu-se que PAC não é uma técnica hiperfina adequada para o estudo quantitativo do tensor EFG de interfaces de BaTiO3/Fe, dados os efeitos não desprezáveis das sondas radioativas na matriz de BaTiO3. Finalmente, foi feita a deposição de Heteroestruturas de BTO/Fe em substratos de LaAlO3, MgO, Al2O3 e SrTiO3 usando RF-Sputtering, assim como deposição de camadas de Fe em substratos de BaTiO3 cortados nos planos (100), (110) e (111) planes, usando Molecular Beam Epitaxy (MBE), numa tentativa de recrear as interfaces com efeitos magnetoelétricos mais apelativos, previstos pela modelação DFT. Os filmes finos depositados por sputtering mostraram o crescimento de múltiplos óxidos de Fe, Ba-Ti-O e Fe-Ti-O dependendo fortemente do substrato onde foram crescidos, assim como das condições de deposição e tratamentos térmicos. Porém, nenhum efeito magnetoelétrico foi observado nestes filmes. Por outro lado, os filmes depositados nos substratos de BaTiO3 mostraram grandes acoplamentos magnetoelétricos entre as fases ferroelétricas do BTO e a magnetização das camadas de Fe (à semelhança do que aconteceu no auto-compósito de BaTiO3:Fe). A ordem de grandeza destes acoplamentos está fortemente correlacionada com a interface do BTO onde o Fe foi depositado, apresentando uma enorme variação na magnetização espontânea e na coercividade para o caso da transição romboédrica↔ortorrômbica, até M/M ≈ 148% e HC/HC ≈ 183% respetivamente para o caso da orientação (110).
Tese realizada com apoio financeiro da FCT através da bolsa SFRH/BD/93336/2013.
Programa Doutoral em Física