Rozprawy doktorskie: „Ferroelectrics”

1

Ivry, Yachin. "Nano ferroelectrics". Thesis, University of Cambridge, 2011. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.609375.

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2

Tavernor, Andrew. "Modelling relaxor ferroelectrics". Thesis, University of Leeds, 1992. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.305874.

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Huber, J. E. "Ferroelectrics : models and applications". Thesis, University of Cambridge, 1998. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.604713.

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A systematic method for selecting actuators (devices which produce a controlled force or displacement) for a given application is developed. Performance characteristics for several classes of actuator are presented in a graphical form which allows the characteristics of the actuator to be matched to the requirements of tasks. Some conclusions are drawn regarding the suitability of ferroelectric actuators for various tasks, and the opportunity offered by the non-linear, high force and high displacement regime of behaviour in ferroelectrics. A micromechanical constitutive model for the non-linear behaviour of ferroelectric crystals is developed. This model is based on the observation that ferroelectric transformations may be treated as if they were crystal slip systems, which allows conventional crystal plasticity models to be extended to the ferroelectric case. Expressions for the instantaneous tangent properties of a ferroelectric crystal are derived. The behaviour of the constitutive model is explored. The strain and polarization response to calculated for a single crystal subjected to mechanical and electrical loading; the evolution of single crystal yield surfaces is determined. A self-consistent scheme is used in conjunction with the constitutive model to produce estimates of the response of a ferroelectric polycrystal to electrical and mechanical loading. Expressions are derived for self-consistent estimates of the instantaneous tangent properties of a ferroelectric polycrystal. Self-consistent calculations of dielectric hysteresis and "butterfly" hysteresis are compared with experimental measurements made on a commercial Lead Zirconate Titanate ceramic. Predictions of the development of a cornered ferroelectric yield surface under loading are given.

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4

Jung, Dong Jin. "Characterizations of integrated ferroelectrics". Thesis, University of Cambridge, 2006. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.613808.

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Liu, Qida. "Electromechanical creep in ferroelectrics". Thesis, University of Cambridge, 2007. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.613330.

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6

Flores, Suarez Rosaura. "Three-dimensional polarization probing in polymer ferroelectrics, polymer-dispersed liquid crystals, and polymer ferroelectrets". Phd thesis, Universität Potsdam, 2011. http://opus.kobv.de/ubp/volltexte/2012/6017/.

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A key non-destructive technique for analysis, optimization and developing of new functional materials such as sensors, transducers, electro-optical and memory devices is presented. The Thermal-Pulse Tomography (TPT) provides high-resolution three-dimensional images of electric field and polarization distribution in a material. This thermal technique use a pulsed heating by means of focused laser light which is absorbed by opaque electrodes. The diffusion of the heat causes changes in the sample geometry, generating a short-circuit current or change in surface potential, which contains information about the spatial distribution of electric dipoles or space charges. Afterwards, a reconstruction of the internal electric field and polarization distribution in the material is possible via Scale Transformation or Regularization methods. In this way, the TPT was used for the first time to image the inhomogeneous ferroelectric switching in polymer ferroelectric films (candidates to memory devices). The results shows the typical pinning of electric dipoles in the ferroelectric polymer under study and support the previous hypotheses of a ferroelectric reversal at a grain level via nucleation and growth. In order to obtain more information about the impact of the lateral and depth resolution of the thermal techniques, the TPT and its counterpart called Focused Laser Intensity Modulation Method (FLIMM) were implemented in ferroelectric films with grid-shaped electrodes. The results from both techniques, after the data analysis with different regularization and scale methods, are in total agreement. It was also revealed a possible overestimated lateral resolution of the FLIMM and highlights the TPT method as the most efficient and reliable thermal technique. After an improvement in the optics, the Thermal-Pulse Tomography method was implemented in polymer-dispersed liquid crystals (PDLCs) films, which are used in electro-optical applications. The results indicated a possible electrostatic interaction between the COH group in the liquid crystals and the fluorinate atoms of the used ferroelectric matrix. The geometrical parameters of the LC droplets were partially reproduced as they were compared with Scanning Electron Microscopy (SEM) images. For further applications, it is suggested the use of a non-strong-ferroelectric polymer matrix. In an effort to develop new polymerferroelectrets and for optimizing their properties, new multilayer systems were inspected. The results of the TPT method showed the non-uniformity of the internal electric-field distribution in the shaped-macrodipoles and thus suggested the instability of the sample. Further investigation on multilayers ferroelectrets was suggested and the implementation of less conductive polymers layers too.
In dieser Arbeit wird eine zerstörungsfreie Technik zur Analyse, Optimierung, und Entwicklung neuer funktioneller Materialien für Sensoren, Wandler, Speicher und elektrooptische Anwendungen vorgestellt. Die Wärmepuls-Tomographie (engl. Thermal-Pulse Tomography, TPT) liefert dreidimensionale Abbildungen hoher Auflösung von elektrischen Feldern und Polarisationsverteilungen eines Materials. Bei dieser thermischen Methode wird ein fokussierter, gepulster Laserstrahl durch eine undurchsichtige Oberflächenelektrode absorbiert, welche sich dadurch aufheizt. Die einsetzende Wärmediffusion führt – aufgrund der Wärmeausdehnung des Materials – zu Änderungen der Probengeometrie, welche in pyroelektrischen Materialien einen Kurzschlussstrom oder eine Änderung des Oberflächenpotentials zur Folge hat. Diese wiederum enthalten wichtige Informationen über die räumliche Verteilung elektrischer Dipole und Raumladungen im untersuchten Material. Aus dem gemessenen Kurzsschlussstrom kann anschließend das interne elektrische Feld und die Polarisationsverteilung im Material mittels verschiedener Skalentransformations- und Regularisierungsmethoden rekonstruiert werden. Auf diese Weise ermöglichte die TPT-Methode erstmals die Darstellung inhomogener ferroelektrischer Schaltvorgänge in polymeren ferroelektrischen Filmen, welche mögliche Materialien für die Datenspeicherung sind. Die Ergebnisse zeigen eine typische Haftschicht im ferroelektrischen Polymer und unterstützen die Hypothese einer ferroelektrischen Umpolung auf einer der Korngröße äquivalenten Längenskala über Keimbildung und anschließendes Wachstum. Um die Lateral- und Tiefenauflösung zu untersuchen, wurden sowohl die TPT-Methode als auch die äquivalente Methode in der Zeitdomäne (Focused Laser Intensity Modulation Method, FLIMM) auf ferroelektrischen Filme mit Gitterelektroden angewendet. Die Ergebnisse beider Techniken zeigen nach der Datenauswertung mit unterschiedlichen Regularisierungs- und Scale-Methoden eine vollkommene Übereinstimmung. Des Weiteren stellte sich heraus, dass bisherige Untersuchungen der lateralen Auflösung von FLIMM diese möglicherweise überschätzen. Damit behauptet sich TPT als effiziente und verlässliche thermische Methode. Nach einer Optimierung der Optik wurde die TPT-Methode in polymerdispergierten Flüssigkristallen (polymer-dispersed liquid crystals, PDLC), welche in elektrooptischen Anwendungen von Interesse sind, angewendet. Die Ergebnisse deuten auf eine mögliche elektrostatischeWechselwirkung zwischen den COH-Gruppen des Flüssigkristalls und den Fluoratomen der verwendeten ferroelektrischen Matrix hin. Die durch rasterelektronenmikroskopische Aufnahmen (scanning electron microscopy, SEM) gewonnenen geometrischen Parameter der Flüssigkristalltröpfchen konnten mittels TPT reproduziert werden. Für weitere Anwendungen werden schwach ferroelektrische Polymermatrices vorgeschlagen. Im Bestreben neue polymere Ferroelektrete zu entwickeln und deren Eigenschaften zu optimieren, wurden neuartige Mehrschichtsysteme untersucht. Die Ergebnisse aus der TPT-Methode zeigen eine Abweichung der Uniformität der inneren Verteilung des elektrischen Feldes in den geformten Makrodipolen, was auf eine Instabilität der Probe hindeutet. Ebenfalls wurden weitere Untersuchungen an Mehrschicht-Ferroelektreten und die Anwendung von halbleitenden Polymerschichten vorgeschlagen.

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7

Rowley, Stephen Edward. "Quantum phase transitions in ferroelectrics". Thesis, University of Cambridge, 2011. https://www.repository.cam.ac.uk/handle/1810/252224.

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8

Byrne, D. F. "Domain states in nanoscale ferroelectrics". Thesis, Queen's University Belfast, 2012. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.546018.

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9

Nahas, Yousra. "Gauge theory for relaxor ferroelectrics". Phd thesis, Ecole Centrale Paris, 2013. http://tel.archives-ouvertes.fr/tel-01003357.

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Concomitantly with lattice disorder, there is a discrepancy between local and global scales in relaxor ferroelectrics, in that structural distortions occurring at the local scale are not reflected in the average global structure which remains cubic. There is an absence of direct implementation of the local symmetry in the modeling of relaxors, despite its considerable, but often unacknowledged, ability to encode local features. Central to the thesis is an explicit account for local gauge symmetry within the first-principles-derived effective Hamiltonian approach. The thesis thus aims to consider how an extended symmetry allowing independent transformations at different points in space can effectively bridge local features and macroscopical properties. An underlying question the thesis also seeks to answer is whether the disorder-induced non-trivial interplay between local and global scales can be described from a topological point of view

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10

Shieh, Jay. "Ferroelectrics : switching and cyclic behaviour". Thesis, University of Cambridge, 2003. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.619624.

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11

Varlioglu, Mesut. "Mesoscale constitutive behavior of ferroelectrics". [Ames, Iowa : Iowa State University], 2009. http://gateway.proquest.com/openurl?url_ver=Z39.88-2004&rft_val_fmt=info:ofi/fmt:kev:mtx:dissertation&res_dat=xri:pqdiss&rft_dat=xri:pqdiss:3369903.

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12

Cheng, Long. "Relaxor ferroelectrics for neuromorphic computing". Electronic Thesis or Diss., université Paris-Saclay, 2024. http://www.theses.fr/2024UPAST073.

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Pour surmonter les défis posés par les architectures traditionnelles de von Neumann, l'informatique neuromorphique s'inspire des sciences du cerveau pour créer du matérielécoénergétique adaptable à des tâches complexes. Les memristors, bien que novateurs,rencontrent des problèmes tels que la chaleur de Joule entravant le calcul neuronal à trèsbasse puissance.Pour remédier à cela, nous proposons un mécanisme de memcapacitor -la transition de phase induite par champ électrique. Les memcapacitors, qui expriment les signaux en tension, offrent une consommation d'énergie inférieure aux memristors (basés surle courant). Notre étude sur les matériaux ferroélectriques relaxeur (PMN-28PT, PZN-4.5PT) et le ferroélectrique conventionnel BTO (001) démontre la nature universelle des transitions de phase induites par champ électrique. Des impulsions personnalisées permettent la reproduction de la potentialisation à long terme (LTP), de la dépression à long terme (LTD) et de la plasticité dépendante du temps d'impulsion (STDP).De plus, les ferroélectriques relaxeur présentent un effet dendritique absent dans les contreparties conventionnelles. La mise en œuvre de dendrites PZN-4.5PT dans les réseaux neuronaux améliore la précision (83.44 %), surpassant les réseaux de memristors avec dendrites linéaires (81.84 %) et surpassant de manière significative les réseaux sans dendrites (80.1 %).En fin de compte, nous mettons en œuvre avec succès un memcapacitor relaxeur enutilisant un film mince PMN. Cette structure métal/ferroélectrique/métal/isolant atteint desétats capacitifs de 3 bits par le biais de transitions de phase induites par champ. 8 états memcapacitifs robustes présentent une maintenance cohérente sur plus de 100 secondes et une endurance exceptionnelle dépassant 5×10^5 cycles. Des impulsions sur mesure émulent efficacement LTP, LTD, et permettent l'exploration des fonctionnalités synaptiques dépendantes de la température
To overcome challenges posed by traditional von Neumann architectures, neuromorphic computing draws inspiration from brain science to create energy-efficient hardware adaptable to complex tasks. Memristors, though novel, face issues like Joule heat hindering ultra-low-power neural computing.To address this, we propose a memcapacitor mechanism - the electric-field-induced phase transition. Memcapacitors, expressing signals as voltage, offer lower power consumption than memristors (current-based). Our study on relaxor ferroelectric materials (PMN-28PT, PZN-4.5PT) and conventional ferroelectric BTO (001) demonstrates the universal nature ofelectric-field-induced phase transitions. Customized pulses enable the replication of long-term potentiation (LTP), depression (LTD), and spike-timing-dependent plasticity (STDP).Additionally, relaxor ferroelectrics exhibit a dendrite effect absent in conventional counterparts. Implementing PZN-4.5PT dendrites in neural networks improves accuracy (83.44%), surpassing memristor networks with linear dendrites (81.84%) and significantly outperforming networks without dendrites (80.1%).Ultimately, we successfully implement a relaxor memcapacitor using a PMN thin film.This metal/ferroelectric/metal/insulator structure achieves 3-bit capacitance states through field-induced phase transitions. 8 robust memcapacitive states exhibit consistent maintenance over 100 seconds and exceptional endurance exceeding 5×10^5cycles. Tailored pulses effectively emulate LTP and LTD, and enable the exploration of temperature-dependent synaptic functionalities

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13

Kumara, Cordero Edwards. "Effect of flexoelectricity on the nano-mechanical properties of ferroelectrics". Doctoral thesis, Universitat Autònoma de Barcelona, 2018. http://hdl.handle.net/10803/461587.

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Los materiales ferroeléctricos pueden tener diferentes respuestas electromecánicas, por ejemplo la piezoelectricidad, polarización inducida cuando hay deformación homogénea, y la flexoelectricidad, polarización inducida cuando hay deformación inhomogénea. Dado que la flexoelectricidad está relacionada con los gradientes de deformación, a la nanoescala su efecto es tan o más grande que la piezoelectricidad. La investigación desarrollada en ésta tesis se enfoca en estudiar la interacción entre estas dos propiedades cuando compiten y/o cuando colaboran entre ellas, y de cómo ésta interacción afecta las propiedades mecánicas de los ferroeléctricos. Hasta ahora se ha creído que las propiedades mecánicas son invariantes con respecto a al espacio de inversión, es decir que medirlas en una cara o en la opuesta no debería cambiar su valor. Sin embargo, ésta tesis demuestra que, en presencia de gradientes de deformación, ésta simetría se rompe, ya que tanto las propiedades mecánicas como la respuesta mecánica de los ferroeléctricos depende del signo de su polarización. Éste resultado representa un cambio en la teoría establecida hasta ahora y ofrece un nuevo camino para explorar en la física de fractura de sólidos. Esta tesis está distribuida de la siguiente manera: El capítulo 1 es una introducción a la física de las propiedades mecánicas, la piezoelectricidad y la flexoelectricidad, mientras que el capítulo 2 describe las técnicas experimentales utilizadas para realizar las medidas de las propiedades mecánicas y las respuestas mecánicas requeridas para el proyecto. En el capítulo 3, se midió y analizó las propiedades mecánicas de cristales ferroeléctricos de LiNbO3 con la polarización perpendicular a la superficie y en direcciones opuesta, empleando la técnica de nanoindentación. La inversión de la polarización fue realizada de dos maneras distintas (1) manualmente, es decir, girando el cristal 180º para acceder a la cara opuesta del mismo, y (2) utilizando un cristal periódicamente polarizado, de ésta manera se tuvo acceso a polarizaciones opuestas desde una misma cara. Se observó que, independientemente del método de inversión, todas las propiedades mecánicas son asimétricas con respecto al espacio de inversión. En el capítulo 4, a partir de la ecuación libre de los ferroeléctricos, se desarrolló un modelo para determinar el coeficiente de flexoacoplamiento empleando únicamente las propiedades mecánicas del material. A partir de éste modelo y los datos obtenidos en el capítulo 3, se obtuvo que el valor de dicho coeficiente para LiNbO3 ~ 40 V, un valor más realista que el medido por el método estándar e incluso más cercano al predicho por Kogan y Tagantsev. En el capítulo 5, el objetivo era estudiar el efecto de la flexoelectricidad en la propagación de grietas y la tenacidad de factura en cristales ferroeléctricos de RKTP con la polarización alineada en el plano. Se realizaron grietas paralelas, antiparalelas y perpendiculares a la polarización y Se demostró que la propagación de la grietas esta intrínsecamente relacionado con la dirección de polarización en la que se propaga, ya que la flexoelectricidad disminuye la tenacidad de fractura cuando es paralela a la polarización ferroeléctrica, y por ende las grietas son mas largas. En el capítulo 6, se plantea una posible aplicación como consecuencia de la asimetría en las propiedades mecánicas del capítulo 3: leer la polarización solamente por medios mecánicos. Para probar éste nuevo concepto, se utilizó CRF en el cristal periódicamente polarizado, obteniendo una lectura en concordancia con los resultados del capítulo 3. Además se mostró que al disminuir el volumen ferroeléctrico, es decir con películas delgadas, la resolución de lectura se ve incrementada considerablemente. Finalmente en el capítulo 7 se concluye ésta tesis y plantean las posibles líneas de trabajos futuros.
Ferroelectric materials can present various electromechanical responses. These include electrostriction (strain proportional to the square of the electric field) piezoelectricity (polarization induced by a strain), and flexoelectricity (polarization induced by a strain gradient). Since flexoelectricity is proportional to the strain gradients, and these can grow in inverse proportion to the size, at the nanoscale flexoelectricity can be as big as or greater than piezoelectricity. The research developed in this thesis focuses on studying the interaction between these two properties in ferroelectrics, and specifically on how this interaction affects the mechanical properties of ferroelectrics. Until now it has been believed that the mechanical properties are invariant with respect to space inversion, that is to say that measuring them on one side or on the opposite side of a crystal should not change their value, even when the material in question is non-centrosymmetric (piezoelectric or ferroelectric). However, this thesis shows that, in the presence of strain gradients, mechanical inversion symmetry breaks down: the mechanical response of ferroelectrics depends not just on the orientation but also on the sign of their polarization. This result represents a paradigm shift in the physics of solid state mechanics and fracture physics, and opens up new and interesting functional concepts such as mechanical reading of polarization. This thesis is distributed as follows: Chapter 1 is an introduction to the physics of mechanical properties, piezoelectricity and flexoelectricity, while Chapter 2 describes the experimental techniques used in the project for measuring mechanical and electromechanical properties. Chapter 3 describes the characterization and analysis of the mechanical properties of LiNbO3 ferroelectric crystals with polarization perpendicular to the surface, using the nanoindentation technique. The properties were measured for opposite polarization signs, and the inversion of the polarization was done in two different ways: (1) manually, that is, turning the crystal 180º to access the opposite side of it, and (2) using a periodically polarized crystal, so that opposite polarizations can be accessed on the same face. It was observed that, regardless of the “switching” method, all the mechanical responses are asymmetric with respect to space inversion. The causes and consequences of this discovery are discussed. Chapter 4 derives a free-energy model to quantitatively relate the asymmetry of mechanical responses to the flexocoupling coefficient. Using this model and the experimental results of chapter 3, the flexocoupling coefficient of LiNbO3 was calculated using only the mechanical measurements of the material. The value obtained for LiNbO3 is f ~ 40 V. This is a more realistic value than that measured by the standard electromechanical method, and is close to the theoretical value predicted by the theories of Kogan and Tagantsev. The conclussion of this chapter is that mechanical methods not only allow measuring flexocoupling coefficients, but they are quantiatively advantageous when dealing with polar materials where spurious piezoelectricity can artificially enhance the results obtained by conventional electromechanical means. In Chapter 5, the objective was to study the effect of flexoelectricity on the propagation of cracks and the fracture toughness in ferroelectric crystals with polarization aligned in the plane. The material used for this study was a crystal of Rb-KTiOPO4 (R-KTP) with two antiparallel domains in the plane. Using indentation, sets of cracks were opened in the parallel, antiparallel and perpendicular to the polarization. The results showed unambiguously that the propagation of said cracks is asymmetric and intrinsically related to the direction of polarization: flexoelectricity decreases the fracture tenacity when it is parallel to the ferroelectric polarization, thus yielding longer cracks parallel to the polar direction than antiparallel to it. The term "cracking diode" was coined to denominate this effect. Chapter 6 describes the concept demonstration of one possible application of the asymmetry in mechanical properties reported in Chapter 3: read the sign of ferroelectric polarization by purely mechanical means and in a non-destructive way. To demonstrate this new concept, Contact Resonance Frequency Microscopy was used in the periodically poled crystal, obtaining a reading in accordance with the results of chapter 3, namely, that the contact stiffness of down-polarized domains is higher than that of up-polarized domains. It was also shown that, owing to the inverse size dependence of flexoelectricity, working with films results in a considerable resolution increase.. This demonstrates that, by exploiting the interaction between flexoelectricity and ferroelectricity, it is not only possible to mechanically write a ferroelectric memory, but also to mechanically read it. Finally, in chapter 7 this thesis is concluded with a summary of all the results and their consequences.

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14

Seo, Yohan [Verfasser]. "Toughening Mechanisms of Ferroelectrics / Yohan Seo". Aachen : Shaker, 2013. http://d-nb.info/1050343689/34.

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15

Zhou, Donghua. "Multinuclear NMR studies of relaxor ferroelectrics". W&M ScholarWorks, 2003. https://scholarworks.wm.edu/etd/1539623422.

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Multinuclear NMR of 93Nb, 45Sc, and 207Pb has been carried out to study the structure, disorder, and dynamics of a series of important solid solutions: perovskite relaxor ferroelectric materials (1-x) Pb(Mg1/3Nb 2/3)O3-x Pb(Sc1/2Nb1/2)O 3 (PMN-PSN).;93Nb NMR investigations of the local structure and cation order/disorder are presented as a function of PSN concentration, x. The superb fidelity and accuracy of 3QMAS allows us to make clear and consistent assignments of spectral intensities to the 28 possible nearest B-site neighbor (nBn) configurations, (NMg, NSc, NNb), where each number ranges from 0 to 6 and their sum is 6. For most of the 28 possible nBn configurations, isotropic chemical shifts and quadrupole product constants have been extracted from the data. The seven configurations with only larger cations, Mg 2+ and Sc3+ (and no Nb5+) are assigned to the seven observed narrow peaks, whose deconvoluted intensities facilitate quantitative evaluation of, and differentiation between, different models of B-site (chemical) disorder. The "completely random" model is ruled out and the "random site" model is shown to be in qualitative agreement with the NMR experiments. to obtain quantitative agreement with observed NMR intensities, the random site model is slightly modified by including unlike-pair interaction energies.;To date, 45Sc studies have not been as fruitful as 93Nb NMR because the resolution is lower in the 45Sc spectra. The lower resolution of 45Sc spectra is due to a smaller span of isotropic chemical shift (40 ppm for 45Sc vs. 82 ppm for 93Nb) and to the lack of a fortuitous mechanism that simplifies the 93Nb spectra; for 93Nb the overlap of the isotropic chemical shifts of 6-Sc and 6-Nb configurations results in the alignment of all the 28 configurations along only seven quadrupole distribution axes.;Finally we present variable temperature 207Pb static, MAS, and 2D-PASS NMR studies. Strong linear correlations between isotropic and anisotropic chemical shifts show that Pb-O bonds vary from more ionic to more covalent environments. Distributions of Pb-O bond lengthes are also quantitatively described. Such distributions are used to examine two competing models of Pb displacements; the shell model and the unique direction model. Only the latter model is able to reproduce the observed Pb-O distance distribution.

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Le, Van Lich. "Multi-physics Properties in Topologically Nanostructured Ferroelectrics". 京都大学 (Kyoto University), 2016. http://hdl.handle.net/2433/217166.

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Lisauskas, Alvydas. "Electrical Noise in Colossal Magnetoresistors and Ferroelectrics". Doctoral thesis, Stockholm : Tekniska högsk, 2001. http://urn.kb.se/resolve?urn=urn:nbn:se:kth:diva-3217.

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18

McGilly, L. J. "Domain topologies in nanoscale single-crystal ferroelectrics". Thesis, Queen's University Belfast, 2012. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.557960.

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An investigation into domain states' that form spontaneously innanoscale single-crystal structures of BaTi03 and PbZr(042)Ti(o58P3 on cooling through the Curie temperature has been conducted. Through the use of Piezoresponse Force Microscopy (PFM) and Transmission Electron Microscopy (TEM) a hierarchy of structure existing over distinct length scales has been observed revealing large-scale 'superdomains' which are composed of finer-scale 90° stripe 'subdomains'. "The nature of the formation of these superdomains is investigated by considering a higher-level polar ordering to perform roles usually associated with subdomains. In BaTi03 " these superdomains can be considered to possess a resultant polarisation orientated along (110)pseudocubic directions; using PFM to map the resultant polarisations demonstrated that 60°, 90° and 180° superdomain structures can form. Characteristics of thesuperdomains appear to have more in common with an orthorhombic symmetry than with the fine-scale parent tetragonal state. This is rationalised from the orientation of the resultant polarisation and the observed superdomain variants which are generally only seen for orthorhombic systems. Detailed arialysis of the superdomain boundaries reveals that they typically occupy crystallographically defined directions. Complex topologies of domain walls are also seen at superdomain boundaries occasionally including interlinking chains of flux-closure and quadrupole states. For the 180° superdomains composed of a-a subdomains observed in BaTi03 single- crystal free-standing dots, produced by Focused Ion Beam (FIB) milling, their formation is thought to be a response to depolarising fields that should be present in the plane of the structure. This is determined from superdomain scaling adherence to a Landau-Kittel-type relation. A free energy analysis is used to estimate the 180° domain wall energy density to be 20 ± 2 mlm" and to explain the features of the Landau-Kittel-type scaling. Nanoscale structures of wires and dots were produced by means of FIB milling of single-crystal lamellar sheets of PbZr(042)Ti(o58P3' Subsequent investigation through use of TEM revealed complex domain structures that displayed superdomain features. For PbZr(042)Ti(o.58P3 single-crystal nanodots, in the majority of cases, 90° stripe domains were found to form into four distinct 'bundles' or quadrants. Detailed analysis of the dipole orientations in the system was undertaken and led to the conclusion that resultant polarisations, associated with the four quadrant domain bundles, form into a closed loop. This 'polarisation closure' pattern appears to be a highly stable, equilibrium state.

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Potnis, Prashant. "Single crystal ferroelectrics : macroscopic and microscopic studies". Thesis, University of Oxford, 2011. http://ora.ox.ac.uk/objects/uuid:96973376-8596-4fc9-9c53-c58379a766a5.

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The aim of this thesis was to improve the understanding of microstructure in single crystal ferroelectrics. This was achieved through macroscopic testing of Lead Magnesium Niobate – Lead Titanate (PMN-PT) and microscopic observations of Barium Titanate (BT) single crystals. Multi-axial polarization rotation tests on PMN-PT showed a gradual increase in the change in dielectric displacement due to ferroelectric switching as the electric field is applied at increasing angles to the initial polarization direction. A relatively high remnant polarization for loading angle near to 90° suggested that PMN-PT is more polarizable in certain directions. Strains measured in two directions, parallel to the electric field and perpendicular to the electric field, showed a noticeable variation on two opposite faces of the specimen suggesting an effect of local domain configurations on macroscopic behaviour. A micromechanical model gave an insight into the switching systems operating in the crystal during the polarization rotation test. Domain structure in BT was mapped using synchrotron X-ray reflection topography. By making use of the angular separation of the diffracted reflections and specimen rocking, different domain types could be unambiguously identified, along with the relative tilts between adjacent domains. Fine needle domains (width ≈ 10μm) were successfully mapped providing a composite topograph directly comparable with optical micrograph. The domain structure was confirmed using other techniques such as piezoresponse force microscopy and atomic force microscopy/scanning electron microscopy and optical observations on the etched crystal. Results show that combined use of multiple techniques is necessary to gain a consistent interpretation of the microstructure. Finally, domain evolution in BT under compressive mechanical loading was observed in-situ using optical and X-ray diffraction techniques providing a series of images that show ferroelastic transition. The domain configurations influence the switching behaviour and constitutive models that can account for such effects need to be developed. Quantitative and qualitative data presented in this thesis can assist model development and validation.

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Brouwer, William J. "Nuclear magnetic resonance studies of relaxor ferroelectrics". W&M ScholarWorks, 2005. https://scholarworks.wm.edu/etd/1539623478.

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This work is devoted to the study of local order in the ferroelectric PbSc1/2Ta1/2O3 (PST) and relaxor ferroelectric solid solutions (1-x)PbSC2/3W1/3O3-(x)PbTiO 3 (PSW-PT), (1x)PbSC2/3W1/3O3-(x)PbZrO 3 (PSW-PZ). Novel Magic Angle Spinning (MAS) Solid State Nuclear Magnetic Resonance (SS-NMR) experiments, including Multiple Quantum MAS (MQMAS) and Double Quantum Filtered Satellite Transition (DQF-STMAS), have been performed on these materials. A gamma function model is proposed for the distribution of quadrupole coupling constants, based on the Poissonian nature of atomic displacements. Moments for distributions may be subsequently extracted through agreement between experimental spectra and simulations implemented in novel programs. Simple crystal compound scandium oxide provides a reasonable analogue and assists in spectral interpretation. Support is given to the Random Site (RS) model for atomic ordering in Relaxor Ferroelectrics. Based on point charge calculations, significant lead displacement takes place as well as oxygen octahedral tilting. In June 2004, Donghua Zhou proposed an experimental scheme to expedite the interpretation of experimental spectra for materials such as those studied here and a realization with analysis is given.

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Ivanov, Maksim. "Grain size effect on dielectric properties of ferroelectrics and relaxors". Doctoral thesis, Lithuanian Academic Libraries Network (LABT), 2014. http://vddb.library.lt/obj/LT-eLABa-0001:E.02~2014~D_20141230_153112-03380.

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The aim of doctoral dissertation „Grain Size Effect on Dielectric Properties of Ferroelectrics and Relaxors“ by Maksim Ivanov is to investigate, how grain size of ceramics and powders of a few ferroelectrics and relaxors influences macroscopic dielectric properties. The studied materials are powders of a relaxor PbMg⅓Nb⅔O3 (PMN), ceramics of a relaxor with a spontaneous phase transition PbSc½Nb½O3 (PSN), ceramics of a ferroelectric 0.36BiScO3-0.64PbTiO3, and ceramics of Ba2SnO4, which were compared to a better investigated BaSnO3. Investigations were performed in broad frequency (100 Hz – 55 GHz) and temperature (30 K – 1000 K) ranges. Experimental investigations and modelling showed, that bulk properties of relaxor materials are heavily influenced by polar nanoregions, but they do not fully determine them. Morphology of the material (i.e. grain size of ceramics) determines growth and interactions of the nanoregions, thus influencing bulk properties. Moreover, effective medium approximation can explain evolution of dielectric properties of ferroelectrics and relaxors only if dependence of bulk properties on grain size is known. The most interesting result is, that there exist polar entities in ferroelectrics, which are different from ferroelectric domains and are similar to polar nanoregions in relaxors. Their contribution to dielectric permittivity can be comparable to all other contributions. Dimensions of these entities depend on grain size in accordance with Kittel's law.
Maksimo Ivanovo daktaro disertacijos tema yra “Grūdų dydžio įtaka dielektrinėms feroelektrikų ir relaksorių savybėms”. Šio darbo tikslas yra ištirti, kaip keramikų ar miltelių grūdų dydis įtakoja makroskopiškai stebimas kelių feroelektrikų bei feroelektrinių relaksorių dielektrines savybes. Tirtos medžiagos yra klasikinio relaksoriuas PbMg⅓Nb⅔O3 (PMN) milteliai, relaksoriaus su savaiminiu feroelektriniu faziniu virsmu PbSc½Nb½O3 (PSN) keramikos, feroelektriko 0.36BiScO3-0.64PbTiO3 keramikos, bei Ba2SnO4 keramiką, kuri buvo palyginta su kiek labiau žinoma BaSnO3 keramika. Tyrimai buvo atlikti plačiame dažnių (100 Hz – 55 GHz) bei temperatūrų (30 K – 1000 K) intervaluose. Eksperimentiniai tyrimai bei modeliavimai parodė, kad polinės nanosritys labai stipriai įtakoja tūrines relaksorių savybes, tačiau jų neapsprendžia. Medžiagos morfologija (antai keramikų grūdų dydis) lemia nanosričių augimą bei tarpusavio sąveikas, tokiu būdų įtakojamos tūrinės savybės. Be to, efektyvios terpės aproksimacija gali paaiškinti feroelektrikų ir relaksorių dielektrinių savybių priklausomybę nuo grūdų dydžio tik, jei žinomas sąryšis tarp dydžio ir tūrinių savybių. Įdomiausia yra tai, kad feroelektrikuose yra objektų, kurie nėra feroelektriniai domenai ir yra panašūs į relaksorių polines nanosritis. Jų indėlis į dielektrinę skvarbą gali būti palyginamas su visų kitų procesų (pvz. domenų sienelių ir polinių modų) indėliais. Šių objektų dydis priklauso nuo grūdų dydžio pagal Kittel'io dėsnį.

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Thomas, M. G. "The fabrication and properties of piezoelectric ceramic/polymer composites having 3-3 connectivity". Thesis, University of Leeds, 1991. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.305798.

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Takeishi, Taku. "Low temperature sol gel deposition of lead scandium tantalate (Pb(Scâ†0â†.â†5Taâ†0â†.â†5)Oâ†3) thin films". Thesis, Cranfield University, 1998. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.284868.

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Otto, Tobias. "Local-scale optical properties of single-crystal ferroelectrics". Doctoral thesis, Saechsische Landesbibliothek- Staats- und Universitaetsbibliothek Dresden, 2006. http://nbn-resolving.de/urn:nbn:de:swb:14-1149623191001-10491.

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Das Ziel dieser Arbeit ist die optische Untersuchung von ferroelektrischen Domänen und Domänenwänden auf lokaler Skala. Dafür wurden neuartige nichtinvasive Ansätze entwickelt, die auf der Anwendung optischer Rastersondenmikroskopie basieren. Die untersuchten Schlüsseleigenschaften umfassen den elektrooptischen Effekt für verschiedene Domänenorientierungen und die Brechungindexänderungen an Domänenwänden an Bariumtitanat-Einkristallen. Die lokale Messung der elektrooptischen Eigenschaften wurde mit räumlich stark begrenzten elektrischen Feldern durchgeführt, die mittels elektrisch leitfähigen Spitzen angelegt wurden. Dieser experimentelle Ansatz erlaubt nicht nur die Messung verschiedener elektrooptischer Koeffzienten, sondern auch die Unterscheidung von allen auftretenden, auch antiparallelen, Domänenausrichtungen. Durch Anlegen eines zusätzlichen elektrischen Feldes mittels der gleichen Spitze konnte auch das ferroelektrische Schalten mit dieser optischen Methode untersucht werden. Die Experimente wurden durch eine numerische Modellierung der elektrischen Feldverteilung und der resultierenden elektrooptischen Antwort begleitet. Die Ergebnisse der Modellierung sind dabei in sehr guter Übereinstimmung mit den experimentellen Ergebnissen. Dies erlaubt auch die Trennung von Beiträgen verschiedener elektrooptischer Koeffzienten und den entsprechenden Feldkomponenten. ür die experimentelle Untersuchung von den theoretisch vorhergesagten Brechungsindexprofilen einzelner Domänenwände, wurde die Sensitivität der optischen Sonde auf lokale Änderungen des Brechungsindex mittels Polarisations- und Positionsmodulation erhöht. Obwohl die Abbildung einer einzelnen Domänenwand nicht gelang, konnte damit zumindest eine obere Grenze für den optischen Effekt einer Domänenwand experimentell gewonnen werden, welche verträglich mit den theoretischen Vorhersagen ist
The goal of this thesis is the optical investigation of ferroelectric domains and domain walls at the very local scale. For that, novel noninvasive approaches based on optical scanning probe microscopy are developed. The key properties investigated are the electrooptic effect for different domain orientations and refractive-index changes at single domain walls of barium titanate single crystals. The local probing of the electro-optic response is performed with strongly confined electric fields, applied via a conductive tip. With this approach we can not only probe different electro-optic coeffcients, but also identify all occurring domain orientations, even antiparallel ones. The application of additional bias fields by the same tip is used to investigate ferroelectric switching and domain growth by optical means. The experiments are supported by numerical modelling of the electric-field distribution and the resulting electro-optic response. The modelling shows excellent agreement with the measurements, and allows us to separate the contributions of different electro-optic coeffcients and their associated electric-field components. For the experimental observation of the theoretically predicted refractive-index profiles at single ferroelectric domain walls, polarization and position modulation of the optical probe is used to obtain high sensitivity to local modifications of the refractive index. An upper limit to the optical effect to the optical effect of a single domain wall is deduced from the experiment, which is compatible with the effect predicted by theory

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Stobbs, David Michael. "A multi-nuclear NMR study of titanate ferroelectrics". Thesis, University of Warwick, 2008. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.496942.

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Holsgrove, Kristina. "Transmission electron microscopy study of domains in ferroelectrics". Thesis, Queen's University Belfast, 2017. https://pure.qub.ac.uk/portal/en/theses/transmission-electron-microscopy-study-of-domains-in-ferroelectrics(e83e215a-bcf7-465b-bab9-31442486bb71).html.

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This thesis investigates domain dynamics in one of the most well-known ferroelectric materials – polycrystalline BaTiO₃ ceramics, and one of today’s most promising ferroelectric materials for future device applications – mixed-phase BiFeO₃ thin films. The investigations use primarily TEM techniques accompanied by relevant theory and AFM techniques. The study on polycrystalline BaTiO₃ (FIB lamellae) aims to further understand the link between domains coupling across adjacent grains and to explore the domains’ re-ordering as a function of heating through T_C. Two cases were explored: domains coupling across a single grain boundary, and a more complex case of domains within adjacent grains meeting around a junction (or pore). Analysis using martensite crystallography theory demonstrated that domains sharing a single grain boundary do on average arrange themselves in a compatible and stress-free manner. For the example of grains arranged around a junction, a computational example was created, given the complexity of the case. It was demonstrated that the relaxation of the out-of-plane constraint gives rise to an undetermined set of linear equations which can be solved for compatible domain wall orientations and volume fractions of domains, indicating that groups of adjacent grains can form stress-free domain patterns. STEM in-situ heating cycle experiments, heating and cooling through T_C, showed that the re-configuration of the domain structure (domain density, favourable domain orientations and presence of domain bundles) was directly influenced by the rate and continuous/dis-continuous nature of the performed heating cycles. Furthermore, this material was explored with focus on the functionality of its positive temperature coefficient of resistivity (PTCR) effect. Aberration-corrected STEM and EELS revealed a grain boundary PbTiO₃-like region (~10-15 nm), which was associated with an increased local polarisation in that region. The chemical and electronic heterogeneity of the ceramic was linked to the changes in potential barrier at the grain boundary, theorised by the Heywang-Jonker model. It was inferred that the confined PbTiO₃ rich grain boundary region would have a higher spontaneous polarisation (than BaTiO₃), thus reducing the grain boundary barrier potential further below T_C, augmenting electronic transport and enhancing the magnitude of resistivity jump at T_C, and so justifying the optimised PTCR effect exhibited by this ceramic. For the study on phase reversibility in BiFeO₃ thin films, the native polymorphs, known as T and R, were initially identified. The thermal activation phase transformation was investigated by STEM in-situ heating cycle experiments; showing a lateral growth of the highly-strained T phase above 400°C. Additionally, an AFM tip was used to locally apply electric field and stress, demonstrating reversible switching between the native mixed-phase and a pure T phase state. Energy-based effective Hamiltonian simulations verified phase competition under the application of electric field and stress, comparable to experimental data. The stress-written phase boundaries (R’/T’) were investigated via c-AFM showing enhanced conductivity. TEM analysis of cross-sectional lamellae from pre-written AFM regions revealed that the stress-written R’ and T’ polymorphs differ in structure from the native polymorphs and, the R’/T’ boundaries have higher in-plane strain gradients compared to the native R/T boundaries, rationalising the enhanced conductivity as a strain mediated effect.

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Edwards, David. "Microstructural dynamics induced by nanoscale stress in ferroelectrics". Thesis, Queen's University Belfast, 2018. https://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.766283.

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Renuka, Balakrishna Ananya. "Application of a phase-field model to ferroelectrics". Thesis, University of Oxford, 2016. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.728788.

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Manzo, Michele. "Engineering ferroelectric domains and charge transport by proton exchange in lithium niobate". Doctoral thesis, KTH, Kvantelektronik och -optik, QEO, 2015. http://urn.kb.se/resolve?urn=urn:nbn:se:kth:diva-162269.

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Ferroelectrics are dielectric materials possessing a switchable spontaneous polarization, which have attracted a growing interest for a broad variety of applications such as ferroelectric lithography, artificial photosynthesis, random and dynamic access memories (FeRAMs and DRAM), but also for the fabrication of devices for nonlinear optics, etc. All the aforementioned applications rely on the control of the ferroelectric domains arrangement, or the charge distribution and transport. In this regard, the main prerequisite is the engineering of the spontaneous polarization, obtained by reversing its orientation or locally inhibiting it. In the latter case, the interface created by the spatial discontinuity of the spontaneous polarization generates local charge accumulation, which can be used to extend the capabilities of ferroelectric materials. This thesis shows how engineering the spontaneous polarization in lithium niobate (LN) by means of proton exchange (PE), a temperature-activated ion exchange process, can be used to develop novel approaches for ferroelectric domain structuring, as well as fabrication of self-assembled nanostructures and control of ionic/electronic transport in this crystal. In particular, it is shown how the electrostatic charge at PE:LN junctions lying below the crystal surface can effectively counteract lateral domain broadening, which in standard electric field poling hampers the fabrication of ferroelectric gratings for Quasi-Phase Matching with periods shorter than 10 μm. By using such an approach, ferroelectric gratings with periods as small as ~ 8 μm are fabricated and characterized for efficient nonlinear optical applications. The viability of the approach for the fabrication of denser gratings is also investigated. The charge distribution at PE:LN junctions lying on the crystal surface is modelled and used to drive the deposition of self-assembled nanowires by means of silver photoreduction. Such a novel approach for PE lithography is characterized for different experimental conditions. The results highlight a marked influence of the orientation of the spontaneous polarization, the deposition times, as well as the reactants concentrations and the doping of the substrate with MgO. Based on the fact that proton exchange locally reduces the spontaneous polarization, a quick and non-destructive method for imaging PE regions in lithium niobate with nanoscale resolution is also developed by using Piezoresponse Force Microscopy. Moreover the relative reduction of the piezoelectric d33 coefficient associated to PE is estimated in lithium niobate substrates with and without MgO-doping. Finally, by using advanced Scanning Probe Microscopy techniques, the features of charge transport in PE regions are further investigated with nanoscale resolution. A strong unipolar response is found and interpreted in light of ionic-electronic motion coupling due to the interplay of interstitial protons in the PE regions, nanoscale electrochemical reactions at the tip-surface interface, and rectifying metal-PE junctions.

QC 20150325

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Suwardi, Ady. "Vertically-aligned oxide nanocomposite films for improved ferroelectrics and ferromagnetics". Thesis, University of Cambridge, 2018. https://www.repository.cam.ac.uk/handle/1810/274373.

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In this work, I start by introducing a relatively recently innovated thin film architecture which offers a new direction in strain control, the vertically aligned nanocomposite (VAN). I first present the literature in the field, explaining the advantages and unique novel properties stemming from VAN structures. Next, I introduce the work I did to examine the unique strain states of Ba0.6Sr0.4TiO3–Sm2O3 VAN structures. It was found that the strain states in the functional Ba0.6Sr0.4TiO3 phase are unconventional compared to those in planar thin films. 3-dimensional strain was found to be acting on the Ba0.6Sr0.4TiO3 phase in the VAN structure. The origin of the strain was explained using a simple model which takes into account thermal expansion mismatch as well as lattice mismatch and elastic coefficients. The ferroelectric properties of the films were presented in relation to the observed strain states. I next present the work I did on the influence of strain on the magnetic properties in VAN film of Sm0.34Sr0.66MnO3–Sm2O3. Ferromagnetism was achieved in an otherwise antiferromagnetic Sm0.34Sr0.66MnO3. The effect was explained by a strain induced transition from super-exchange to double exchange coupling in the material. Last but not least, the potential of scalability of VAN films was explored by using sputtering to grow VAN structures instead of the commonly-used PLD growth. BaTiO3–Sm2O3 was used as a primary study material due to its well reported VAN properties. Preliminary results showing indications of a VAN structure. Some basic physical property characterization is also presented and compared to the properties of PLD-grown films in the literature. Limitations and challenges that arise due to the fundamental differences between sputtering and PLD are also described.

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Zang, Jiadong [Verfasser]. "High-temperature dielectrics based on relaxor ferroelectrics / Jiadong Zang". Aachen : Shaker, 2014. http://d-nb.info/1053904207/34.

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Hervoches, Charles H. "Structural studies of Aurivillius phase ferroelectrics and related materials". Thesis, University of St Andrews, 2002. http://hdl.handle.net/10023/12926.

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The research carried out for this thesis has concentrated on Aurivillius bismuth oxide materials and the closely related Sillen and Bipox bismuth oxyhalides. X-ray and neutron powder diffraction techniques have been used to characterise precisely their structure and revealed several important features. The unexpected presence of a double phase transition scheme has been discovered for the Aurivillius phases Sr0.85Bi2.1Ta2O9 and SrBi4Ti4O15, both materials present the particularity of having an even number of perovskite layers n (n = 2 and n = 4 for Sr0.85Bi2.1Ta2O9 and SrBi4Ti4O15 respectively). The first transition from orthorhombic space group A21am to Amam, occurs at Tc (375deg.C and 550deg.C for Sr0.85Bi2.1Ta2O9 and SrBi4Ti4O15 respectively). The second transition from orthorhombic Amam to tetragonal I4/mmm, occurs at higher temperature (550deg.C and 650deg.C for Sr0.85Bi2.1Ta2O9 and SrBi4Ti4O15 respectively). These phase transitions have been understood in terms of specific octahedral tilt and displacive modes. In contrast, the n = 2 phase SrBi2Nb2O9 and the n = 3 phase Bi4Ti3O12 are shown to undergo a single-step transition from orthorhombic space group A21am to tetragonal I4/mmm at Tc = 440deg.C for SrBi2Nb2O9, and from orthorhombic space group B2cb to tetragonal I4/mmm at Tc = 675deg.C for Bi4Ti3O12. The atomic disorder of the Bi and A cations in the fluorite and perovskite sites of the Aurivillius phases has been demonstrated and thoroughly studied for the solid solutions Bi4-xSrxTi3-xNbxO12 and Bi4-xLaxTi3O12, and "size-matching" between the fluorite and the perovskite site has been found to be the key to understanding this disorder phenomenon. In the Sillen family, we extended the members of the Bi2MO4Cl group, where M = Lanthanide to the three following compounds: Bi2ErO4Cl, Bi2YbO4Cl, and Bi2LuO4Cl. We have demonstrated that in the Bi2MO4Cl group, the ionic radius of M must be greater than that of Sc3+ (r (Sc3+) = 0.87 Å for CN = 8). Another new compound with a novel ordering scheme for this family has been studied, viz. Bi5TeO8.5I2, crystallising in the orthorhombic space group Cmm2. Te4+ is shown to adopt only one of the three available M sites, thus inducing a polar structure. In the Bipox family, the crystal structures of two materials exhibiting ferroelectric properties, Bi4NbO8Cl and Bi4TaO8Cl have been determined to be orthorhombic, space group P21cn.

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Xu, Jie. "The low temperature synthesis, characterization and properties of ferroelectrics". Diss., Georgia Institute of Technology, 2000. http://hdl.handle.net/1853/26239.

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Rao, Weifeng. "Computer Modeling and Simulation of Morphotropic Phase Boundary Ferroelectrics". Diss., Virginia Tech, 2009. http://hdl.handle.net/10919/28493.

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Phase field modeling and simulation is employed to study the underlying mechanism of enhancing electromechanical properties in single crystals and polycrystals of perovskite-type ferroelectrics around the morphotropic phase boundary (MPB). The findings include: (I) Coherent phase decomposition near MPB in PZT is investigated. It reveals characteristic multidomain microstructures, where nanoscale lamellar domains of tetragonal and rhombohedral phases coexist with well-defined crystallographic orientation relationships and produce coherent diffraction effects. (II) A bridging domain mechanism for explaining the phase coexistence observed around MPBs is presented. It shows that minor domains of metastable phase spontaneously coexist with and bridge major domains of stable phase to reduce total system free energy, which explains the enhanced piezoelectric response around MPBs. (III) We demonstrate a grain size- and composition-dependent behavior of phase coexistence around the MPBs in polycrystals of ferroelectric solid solutions. It shows that grain boundaries impose internal mechanical and electric boundary conditions, which give rise to the grain size effect of phase coexistence, that is, the width of phase coexistence composition range increases with decreasing grain sizes. (IV) The domain size effect is explained by the domain wall broadening mechanism. It shows that, under electric field applied along the nonpolar axis, without domain wall motion, the domain wall broadens and serves as embryo of field-induced new phase, producing large reversible strain free from hysteresis. (V) The control mechanisms of domain configurations and sizes in crystallographically engineered ferroelectric single crystals are investigated. It reveals that highest domain wall densities are obtained with intermediate magnitude of electric field applied along non-polar axis of ferroelectric crystals. (VI) The domain-dependent internal electric field associated with the short-range ordering of charged point defects is demonstrated to stabilize engineered domain microstructure. The internal electric field strength is estimated, which is in agreement with the magnitude evaluated from available experimental data. (VII) The poling-induced piezoelectric anisotropy in untextured ferroelectric ceramics is investigated. It is found that the maximum piezoelectric response in the poled ceramics is obtained along a macroscopic nonpolar direction; and extrinsic contributions from preferred domain wall motions play a dominant role in piezoelectric anisotropy and enhancement in macroscopic nonpolar direction. (VIII) Stress effects on domain microstructure are investigated for the MPB-based ferroelectric polycrystals. It shows that stress alone cannot pole the sample, but can be utilized to reduce the strength of poling electric field. (IX) The effects of compressions on hysteresis loops and domain microstructures of MPB-based ferroelectric polycrystals are investigated. It shows that longitudinal piezoelectric coefficient can be enhanced by compressions, with the best value found when compression is about to initiate the depolarization process.
Ph. D.

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Zhang, Rongjing Ravichandran G. "Mechanical characterization of thin films with application to ferroelectrics /". Diss., Pasadena, Calif. : Caltech, 2006. http://resolver.caltech.edu/CaltechETD:etd-01312006-170959.

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Flores, Suarez Rosaura [Verfasser], i Reimund [Akademischer Betreuer] Gerhard. "Three-dimensional polarization probing in polymer ferroelectrics, polymer-dispersed liquid crystals, and polymer ferroelectrets / Rosaura Flores Suarez. Betreuer: Reimund Gerhard". Potsdam : Universitätsbibliothek der Universität Potsdam, 2012. http://d-nb.info/1023989239/34.

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Shi, Yuping, i 史玉平. "On polarization physics and electrocaloric effect in normal and relaxor ferroelectrics". Thesis, The University of Hong Kong (Pokfulam, Hong Kong), 2012. http://hub.hku.hk/bib/B49617989.

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Switchable polar properties of ferroelectric and multiferroic nanostructures are ideal to further diversify applications of mainstream semiconductors. Recent breakthroughs in Scanning Probe Microscopy (SPM) have enabled tailoring of polar domain structures at the nanoscale, which is critical to fabricate polarization-based devices. However, highly inhomogeneous electric fields of biased SPM-tips complicate polarization physics in ferroelectrics and multiferroics. Also, typical diffused phase transition in relaxor bulks originates from coupled inhomogeneities of intrinsic polar nanoregions (PNRs). In this thesis, anisotropic and time-dependent mechanisms were developed to study SPM-tip poled polarization switching in ferroelectric and multiferroic thinfilms. Moreover, frequency-related PNR thermodynamics and its effect on electrocaloric effect of locally disordered relaxors were modeled. Firstly, a three dimensional model was established to clarify tip-poling effect on ferroelectric domain nucleation and growth. The concept of “domain shape invariance” was confirmed through constant aspect ratio obtained for conic ferroelectric nucleus. This domain aspect ratio was found to abruptly decrease under the depolarization effect, saturating domain radius. Further increasing tipvoltage could drive longitudinal breakdown of already reverted domains throughout film thickness. Subsequently, tip-activated evolution of domain wall width in ferroelectric and multiferroic thinfilms was studied via extended Kittle’s law, which included anisotropic and dynamic effects arising from tip-fields. Our calculation results showed that wall width in LiNbO3 varied slightly in an initial stage, followed by a drastic change. This wall variation corresponded to three varying regions of coercive field. Besides, we highlighted three polarization switching modes in BaTiO3 - absence, activation and nonactivation mode. Importantly, distinct switching modes, i.e., breakdown mode of 71° domain switching and activation mode of 180°/109° switching, were revealed to fundamentally control filmorientation dependent multipolarization switching sequence in BiFeO3. Thirdly, Pauli’s mater theory was utilized to bridge microscopic evolution of PNRs and characteristic properties of Pb(Mg1/3Nb2/3)O3 (PMN) relaxors. Temperature dispersion and frequency dependence of PMN dielectric susceptibility were related to nonlinear PNR dynamics over a broad temperature interval. We could not validate PNR-volume predictions of percolation theory above the freezing temperature, but suggest a gradual saturation of PNR volume at lower temperatures. Besides, observed deviations of relaxor permittivity from the Curie-Weiss law were attributed to thermal effects on PNR dynamics and resultant polarization rotations. Furthermore, time-dependent PNR dynamics was proposed to study strong frequency dependence of typical relaxor behaviors. It was implied that frequency effect on PNR coercive field was governed by classic Merz’s-switching, leading to suitability of Vogel-Fulcher law for relaxors bulks. Last but not least, above-mentioned framework for PMN relaxors was incorporated with Landau-Ginzburg-Devonshire thermodynamics and Maxwell relation to better understand recently observed giant electrocaloric (EC) effect of relaxor thinfilms, which is promising for solid-state refrigeration. Three contributions were found to dominate relaxor EC response: temperature-dependent dielectric dispersion, inverse pyroelectric effect and thermally enhanced dielectric stiffness. We emphasized that the EC material with larger dielectric stiffness and smaller correlation length could extend its enormous EC response above Curie temperature. Finally, potential approaches, e.g., by manipulating shape, volume and density of PNRs, were suggested to engineer the EC enhancement in relaxor nanostructures.
published_or_final_version
Mechanical Engineering
Master
Master of Philosophy

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Stringfellow, Stephen Bailey. "The processing and dielectric properties of Pb(Mgâ†1â†/â†3Nbâ†2â†/â†3)Oâ†3 based ceramics". Thesis, University of Leeds, 1990. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.277601.

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39

Li, Zheng. "Multiferrocity in bismuth layer structured materials". Thesis, Queen Mary, University of London, 2016. http://qmro.qmul.ac.uk/xmlui/handle/123456789/23218.

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Multiferroics (MF) have attracted much research attention due to the coexistence of ferroelectric and magnetic ordering as well as magnetoelectric (ME) coupling. At present there are very few room temperature single phase MF except BiFeO3. Multiferroic properties of Aurivillius compound Bi5FeTi3O15 were reported at 80 K. The at 80 K. The at 80 K. The at 80 K. The at 80 K. The magnetization of Bi5FeTi3O15 was significantly improved by substituting parts of Fe cations by Co cations. Bi5FeTi3O15 showed ferromagnetic order above room temperature. The magnetic cations Fe/Co in B-site contribute to the both ferroelectric and ferromagnetic properties, which could possibly induce strong magnetoelectric effect. Aurivillius materials are layered structured materials with formula (Bi (Bi2O2)2+ (A m-1BmO3m+1 )2-. The polarization of Aurivillius materials is mainly in a-b plane . High dense and textured ceramics were fabricated by a two-step spark plasma sintering (SPS) method to improve the polarization of ceramics. The multiferroic properties of of Aurivillius materials with different octahedral layers (m=2, 3, 4 and 5) were investigated. All these materials showed ferroelectric and ferromagnetic order at room temperature except Bi 3Nb 1.1251.1251.1251.1251.125Fe 0.1250.1250.1250.1250.125Co 0.1250.1250.1250.1250.125Ti 0.750.750.750.75O9 (m = 2). (m = 2). (m = 2). (m = 2). (m = 2). Bi 3.253.253.253.25La 0.750.750.750.75Nb 0.250.250.250.25Fe 0.1250.1250.1250.1250.125Co 0.1250.1250.1250.1250.125Ti 2.52.52.5O12 (m = 3) was identified to be single phase. (m = 3) was identified to be single phase. Although a small amount of secondary phase (CoFe (CoFe2O4/Co /Co2FeOFeOFeO4) were found in Bi 4.254.254.254.25La 0.750.750.750.75Fe 0.50.50.5Co 0.50.50.5Ti 3O15 (m = 4) and Bi 5.25.25.25La 0.750.750.750.75FeCoTiFeCoTiFeCoTiFeCoTi FeCoTi3O18 (m = 5), ), the intrinsic multiferroicity of the main Aurivillius phase was confirmed by the magnetic controlled ferroelectric domain switching. Clear ME couplings were observed in these materials.

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40

Scholz, Joseph Robert Lanagan Michael Thomas Randall Clive. "Aging rates in PZT ferroelectrics with mixed acceptor-donor dopants". [University Park, Pa.] : Pennsylvania State University, 2009. http://etda.libraries.psu.edu/theses/approved/WorldWideIndex/ETD-4432/index.html.

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41

Hong, Liang. "On nanoferroelectric domain structures and distributions of defects in ferroelectrics". Click to view the E-thesis via HKUTO, 2010. http://sunzi.lib.hku.hk/hkuto/record/B44138763.

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42

Peräntie, J. (Jani). "Electric-field-induced dielectric and caloric effects in relaxor ferroelectrics". Doctoral thesis, Oulun yliopisto, 2014. http://urn.fi/urn:isbn:9789526204406.

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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 lowering
Tiivistelmä 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

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43

Hirohashi, Junji. "Characterization of domain switching and optical damage properties in ferroelectrics". Doctoral thesis, Stockholm, 2006. http://urn.kb.se/resolve?urn=urn:nbn:se:kth:diva-4100.

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44

Zang, Yongyuan. "Investigation of novel ferroelectrics and their electrical and optical applicaitons". Thesis, McGill University, 2013. http://digitool.Library.McGill.CA:80/R/?func=dbin-jump-full&object_id=114469.

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Ferroelectrics are considered essential components in a wide spectrum of electrical and optical applications, including but not limited to ferroelectric random access memory (FeRAM), piezoelectric transducers, and optical waveguides and modulators, owing to their remarkable ferroelectric, piezoelectric, and electro-optic properties. The research presented in this thesis provides a comprehensive investigation, including theoretical principles, fabrication technology, performance optimization, and electrical/optical applications, of two innovative ferroelectric materials, namely, Nd-doped bismuth titanate (BNdT) and bismuth ferrite (BFO). We start by describing the fabrication procedure, and then study the dependence of ferroelectric properties on various fabrication parameters. By varying the post annealing temperature and tuning the elemental constituents, ferroelectrics with optimized performance parameters are demonstrated in our work. Then, we further our study into the optical characterization, and provide a systematic investigation of BNdT optical properties for the first time. Optical parameters including the optical transmittance, refractive index, extinction coefficient, and optical band gap energy, are examined and their correlation with different fabrication variables and microstructure properties is studied both experimentally and theoretically. In the third part of this thesis, by introducing the optimized BNdT ferroelectric thin film as a seeding layer for the BFO, we report the first room temperature (RT) ferroelectric measurement of pure BFO.We further advance our study by investigating the photovoltaic nature of multiferroic BFO, and report enhanced photovoltaic performance of the BFO material in a graphene/polycrystalline BFO/Pt heterojunction structure for the first time. The unique properties of the graphene electrode lead to a short circuit current density of 61 μA/cm2 and an open circuit voltage of 0.52 V in the heterojunction. These values are much higher than the results previously reported in the literature. A theoretical model that takes into consideration the ferroelectric polarization, interface states, and energy band bending effect, is constructed to describe the ferroelectric photovoltaic effect and carrier transport behaviour for the first time. Key photovoltaic parameters, such as conversion efficiency, illumination intensity response, ON/OFF characteristics, minority carrier lifetime, and external quantum efficiency (EQE), are investigated through both theory and measurements. Finally, to improve the existing photovoltaic performance of the graphene/polycrystalline BFO/Pt heterojunction, two independent chemical and physical routines, are processed and compared. It can be seen that the photocurrent density exhibits a significant improvement from 61 μA/cm2 to 8.67 mA/cm2 (~150 fold) after HNO3 treatment, while a considerable enhancement of ~5 fold is seen with QDs filling/sensitizing. An optical application of a photosensitive detector and an electrical application of a tunable switch are also demonstrated as two interesting applications of the BFO photovoltaic effect. Research in the field of novel ferroelectrics continues to thrive, and a number of interesting theoretical and application studies are currently underway. The results achieved in this thesis may provide a useful understanding of the principles and properties of two such ferroelectrics, and can serve as assistance in future research perspectives.
Les matériaux ferroélectriques sont des composantes essentielles pour plusieurs applications en électriques et en optique comme les mémoires vives ferroélectriques (FeRAM), les transducteurs piézoélectriques et les guides d'onde et modulateurs optique, dues à leur propriétés ferroélectrique, piézoélectrique et électo-optique remarquables. La recherche présentée dans cette thèse fournie une étude compréhensive de deux nouveaux matériaux ferroélectriques, soit le bismuth de titane dopé au néodyme (BNdT) et la ferrite de bismuth (BFO). Cette étude inclue les principes théoriques, les technologies de fabrication, l'optimisation des performances et les applications électriques et optiques. Nous débutons avec la fabrication des matériaux, et nous étudions la dépendance des propriétés ferroélectriques sur différents paramètres de fabrications. En modulant différemment la température de post-hybridation et en accordant le constituant élémentaire, nous démontrons des matériaux ferroélectriques ayant des paramètres de performance optimisés, tel une large polarisation rémanente (39.6 μC/cm2), une faible fatigue de dégradation (<1.5%), une faible fuite de densité (5×10-7 A/cm2 at 5 V DC), etc. Ensuite nous approfondissons notre recherche dans la caractérisation du BNdT et fournissons une étude systématique, en première, des propriétés optique du BNd. Nous examinons les paramètres optiques tels la transmittance optique, l'index de réfraction, les coefficients d'extinction et l'énergie de gap de bande et nous étudions expérimentalement et théoriquement leur corrélation avec différents paramètres de fabrication et de microstructure. Dans la troisième partie de cette thèse, en introduisant le film mince optimisé de BNdT ferroélectrique comme couche nourrissante du matériel BFO, nous rapportons la première mesure ferroélectrique à la température pièce du matériel BFO pure. Un champ ferroélectrique induit dans la structure multicouche BFO/BNdT est adressé pour expliquer une telle amélioration du phénomène ferroélectrique.Nous faisons progresser notre étude en examinant la nature photovoltaïque de la BFO multiferroïque et signalons pour la première fois une performance améliorée photovoltaïque du matériel BFO dans une hétérojonction de graphène / BFO polycristalline / Pt. Les propriétés uniques de l'électrode de graphène mènent à un courant de court-circuit de la densité de 61μA/cm2 et un circuit ouvert du voltage de 0,52 V à l'hétérojonction. Un modèle théorique qui prend en considération la polarisation ferroélectrique, les états d'interface et l'effet de flexion de la bande d'énergie est, pour la première fois, construit pour décrire l'effet photovoltaïque ferroélectrique et le comportement de transport des porteurs. Enfin, le traitement avec HNO3 et le remplissage / la sensibilisation des points quantiques (PQ) de CdSe, comme deux routines indépendantes chimiques et physiques, sont traités et comparés pour améliorer la performance photovoltaïque existante de l'hétérojonction de graphène / BFO polycristalline / Pt. Il peut être observé que la densité du photocourant montre un accroissement significatif à partir de 61 μA/cm2 à 8,67 mA/cm2 (~ 150 fois) après le traitement avec HNO3, tandis qu'un accroissement considérable de la ~5 fois est observé avec le remplissage / la sensibilisation des points quantiques. Une application optique d'un détecteur photosensible et une application électrique d'un interrupteur accordable se également démontrent comme deux utilisations intéressantes de l'effet photovoltaïque BFO.La recherche continue à prospérer dans le domaine des nouveaux matériaux ferroélectriques de BNDT et de BFO; un certain nombre d'études théoriques et sur l'application intéressantes sont actuellement en cours. Les résultats obtenus dans cette thèse-ci peuvent fournir une compréhension utile des principes et des propriétés des deux matériaux ferroélectriques et peuvent aider à la recherche en perspective à l'avenir.

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45

Villaurrutia, Arenas Rafael. "Microstructure, nanostructure, and local crystallography in perovskite ferroelectrics and antiferroelectrics". Thesis, University of Glasgow, 2010. http://theses.gla.ac.uk/2362/.

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Selected area and Kikuchi diraction patterns, traditional bright eld and dark eld imaging techniques in electron microscopy as well as high resolution TEM and STEM techniques, together with electron backscattered electron diraction technique have been used to study the domain structures, local crystallography and atomic structures in PZT-based materials. Reliable EBSD mapping of 90 degrees domains in a tetragonal Pb(Zrx; Ti1-x)O3 with x = 0.5 ferroelectric perovskite has been achieved for the rst time, together with reliable automated orientation determination from TEM-Kikuchi patterns. This has been used to assess the local crystallography of domains by determining misorientation angles at 90 degrees domain boundaries and thus local c/a ratios. In most cases, a good agreement is found between local c/a ratios and global measurements by X-ray diraction, but some clear discrepancies have also been found suggesting that real local variations are present, perhaps as a consequence of compositional inhomogeneities. The details of the domain structure of the incommensurate antiferroelectric struc- ture in La-doped zirconium-rich lead zirconate titanate have been revealed in detail for the rst time. The structure is dominated by 60 degrees domain boundaries close to {101} planes of the primitive perovskite cell; and tilts of the perovskite sublattice of about 0.5 degrees are also noted at such boundaries consistent with a small tetragonal distortion of the primitive cell. Within each domain a streaked nanostructure is revealed under weak diraction conditions perpendicular to the long b-axis of the incommensurate supercell, which appears to be a consequence of planar faulting perpendicular to this b-axis. 90 degrees domain boundaries are also observed but are less frequent than 60 degrees boundaries and in con- trast to previous reports, these often have rather curved and irregular boundary planes. The atomic arrangement of these 90 degrees boundaries was studied by aberration corrected HRSTEM. Dierent stackings and periodicities were identied.

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46

Conduit, C. J. "Collective phenomena in correlated semiconductors, degenerate Fermi gases, and ferroelectrics". Thesis, University of Cambridge, 2009. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.597887.

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Degenerate semiconductors: Assuming that there is large number of degenerate conduction band minima provides a useful route to developing an exact analytical treatment of semiconductors. The new formalism, which was also verified computationally, gives an exact expression for the total electron energy, and provides convenient access to the electron dynamical response. Ultracold atomic gases: These are a new tool that offers investigators an exquisite level of control over a many-body systems. Firstly we show how an atomic gas could be used to unravel a long-standing mystery about textured superconductors, secondary we explore the properties of collective models, and thirdly we investigate a novel form of ferromagnetism. Critical phenomena in correlated quantum systems: As the temperature falls thermal excitations give way to quantum fluctuations. These can couple leading to unexpected phases; firstly we search for a putative textured phase that could pre-empt the first order ferromagnetic transition, and secondly predict a metaelectric phase transition in ferroelectrics.

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47

Kowalewsky, Olga Bhattacharya Kaushik. "Theory of complex lattice quasicontinuum and its application to ferroelectrics /". Diss., Pasadena, Calif. : California Institute of Technology, 2005. http://resolver.caltech.edu/CaltechETD:etd-12202004-182638.

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48

Fina, Martínez Ignasi. "Ferroelectricity and magnetoelectric coupling in magnetic ferroelectrics and artificial multiferroic heterostructures". Doctoral thesis, Universitat de Barcelona, 2012. http://hdl.handle.net/10803/81981.

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Multiferroic materials are those materials in which more than one ferroic order coexist. The most technologically appealing multiferroic materials are those showing ferromagnetism and ferroelectricity. Coupling between the mentioned ferroic orders, called magnetoelectric coupling, can yield to new interesting functional applications. In spintronics this coupling would result in the possibility of building magnetic memories controlled by electric field, or transistors where charge is contact-less controlled by a magnetic field. The ultimate goal of the present thesis is to explore the control of the ferroelectric polarization and dielectric properties by magnetic field in thin films. To that purpose dielectric, ferroelectric and magnetoelectric characterization methods of different multiferroic materials have been developed and used. Two big groups of multiferroic materials can be found. On one hand, single-phase multiferroics are those that intrinsically display multiferroicity. On the other hand, multiferroic composites are those where multiferroicity results from the mixture of two different materials that display ferroelectric and ferromagnetic order separately. Single-phase multiferroics can be divided in two subgroups: those where ferroic orders have different sources and those, called magnetic ferroelectrics, where magnetic order induces ferroelectricity and, consequently, larger magnetoelectric coupling is expected. The single-phase multiferroic material studied in the present thesis is YMnO3 in its orthorhombic phase, and it belongs to the magnetic ferroelectrics family. Even though it shows collinear magnetic order in bulk, we will show that cycloidal order in thin film form can be stabilized, giving rise to the capability of controlling the ferroelectric polarization by magnetic field in a reversible manner. Multiferroic composite thin films can be built mainly in two different architectures: vertical (ferromagnetic/ferroelectric columns embedded in a ferroelectric/ferromagnetic matrix) and horizontal (multilayered structures alternating ferromagnetic and ferroelectric materials). Here we compare both, using a ferroelectric perovskite (BaTiO3) and a ferromagnetic spinel (CoFe2O4). We will show that horizontal heterostructures display better ferroelectric properties and larger magnetoelectric coupling, compared to vertical heterostructures, where leakage current is a limiting parameter. The control of dielectric/ferroelectric properties under appropriate heterostructure configuration (in horizontal heterostructures) or deposition conditions (in vertical heterostructures) has been also achieved.
Els materials multiferroics són aquells materials en què coexisteix més d'un ordre ferroic. D'aquests els més interessants són els que presenten ferromagnetisme i ferroelectricitat. La presencia d'acoblament entre aquests dos ordres ferroics, anomenat acoblament magnetoelèctric, obre un nou camp d'aplicacions. En spintrònica, aquest acoblament significaria poder construir memòries magnètiques controlades mitjançant camp elèctric, o transistors on la càrrega es controlaria mitjançant camp magnètic. L'objectiu final d'aquesta tesi és explorar el control de la polarització ferroelèctrica mitjançant camp magnètic en capes fines. Amb aquesta finalitat, s'han utilitzat mètodes de caracterització dielèctrica, ferroelèctrica i magnetoelèctrica en diferents materials multiferroics en capa fina. Existeixen dos grans grups de materials multiferroics. D'una banda, els materials de fase única són aquells que presenten multiferroïcitat de manera intrínseca. D'altra banda, els multiferroics de fase mixta són aquells en els quals la multiferroïcitat és resultat de la barreja de dos materials diferents que presenten ordre ferroelèctric i ferromagnétic per separat. Els materials de fase única es poden dividir en dos subgrups: aquells en què els ordres ferroics tenen diferent origen i aquells, anomenats ferroelèctrics magnètics, en què l'ordre magnètic indueix ferroelectricitat i, de manera conseqüent, s'espera un major acoblament magnetoelèctric. El material multiferroic de fase única estudiat en la present tesi és la o-YMnO(3) en la seva fase ortoròmbica que pertany a la família dels ferroelèctrics magnètics. Tot i que presenta ordre magnètic col.lineal en forma màssica, mostrarem que es pot estabilitzar l'odre cicloïdal en capa fina, permetent el control de la polarització ferroelèctrica mitjançant camp magnètic de manera reversible. Els multiferroics de fase mixta en capa fina es poden créixer utilitzant principalment dues arquitectures diferents: vertical (les columnes ferromagnètiques/ferroelèctriques en una matriu ferroelèctrica/ferromagnètica) i horitzontal (estructures multicapa alternant materials ferromagnètics i ferroelèctrics). Aquí comparem aquestes dues arquitectures, utilitzant una perovskita ferroelèctrica (BaTiO(3)) i una espinela ferromagnètica (CoFe(2)O(4)). Demostrarem que les heteroestructures horitzontals presenten millors propietats ferroelèctriques i un major acoblament magnetoelèctric comparades amb les heteroestructures verticals, en les quals el corrent de pèrdues sembla ser un paràmetre limitant. També s'han aconseguit controlar les propietats dielèctriques/ferroelèctriques mitjançant la modificació de la configuració en les heteroestructures horitzontals o mitjançant la modificació de les condicions de dipòsit en heteroestructures verticals.
Los materiales multiferroicos son aquellos en los que coexiste más de un orden ferroico. DE estos los más interesantes son los que presentan ferromagnetismo y ferroelectricidad. Su acoplamiento, llamado acoplamiento magnetoeléctrico, puede permitir la aplicación de nuevas funcionalidades en el campo de la tecnología. En espintrónica, este acoplamiento significará poder construir memorias magnéticas controladas mediante campo eléctrico, o transistores donde la carga se controlará mediante campo magnético. El objetivo final de esta tesis es explorar el control de la polarización ferroeléctrica mediante campo magnético en capas finas. Con este fin, se han utilizado métodos de caracterización dieléctrica, ferroeléctrica y magnetoeléctrica en diferentes materiales multiferroicos en capa fina. Existen dos grandes grupos de materiales multiferroicos. Por un lado, los materiales de fase única son aquellos que presentan multiferroicidad de forma intrínseca. Por otro lado, los multiferroicos de fase mixta son aquellos en los cuales la multiferroicidad es el resultado de la mezcla de dos materiales diferentes que presentan orden ferroeléctrico y ferromagnético por separado. Los materiales de fase única se pueden dividir en dos subgrupos: aquellos en los que los órdenes ferroicos tienen diferente origen y aquellos llamados ferroeléctricos magnéticos en los que el orden magnético induce ferroelectricidad y, por consiguiente, se espera un mayor acoplamiento magnetoeléctrico. El material multiferroico de fase única que se ha estudiado en esta tesis es la o-YMnO(3) en su fase ortorrómbica y pertenece a la familia de los ferroeléctricos magnéticos. Aunque presenta orden magnético colineal en forma másica, mostraremos que se puede estabilizar el orden cicloidal en capa fina, permitiendo el control de la polarización ferroeléctrica mediante campo magnético de forma reversible. Los multiferroicos de fase mixta en capa fina se pueden crecer utilizando principalmente dos arquitecturas diferentes: vertical (las columnas ferromagnéticas/ferroeléctricas en una matriz ferroeléctrica/ferromagnética) y horizontal (estructuras multicapa alternando materiales ferromagnéticos y ferroeléctricos). Aquí comparamos ambas, utilizando una perovskita ferroeléctrica (BaTiO(3)) y una espinela ferromagnética (CoFe(2)O(4)). Demostraremos que las heteroestructuras horizontales presentan mejores propiedades ferroeléctricas y un mayor acoplamiento magnetoeléctrico comparadas con las heteroestructuras verticales, en las cuales la corriente de pérdidas parece ser un parámetro limitante. También se han conseguido controlar las propiedades dieléctricas/ferroeléctricas mediante el cambio de configuración en heteroestructuras horizontales o mediante el cambio de las condiciones de depósito en heteroestructuras verticales.

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49

Gao, Zhipeng. "Perovskite-like layered structure A₂B₂O₇ ferroelectrics and solid solutions". Thesis, Queen Mary, University of London, 2013. http://qmro.qmul.ac.uk/xmlui/handle/123456789/15025.

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In this project, the ferroelectric materials Pr2Ti2O7, La2Ti2O7, Sr2Nb2O7, La2-xCexTi2O7 (x=0.15, 0.25, 0.35), Nd2-xCexTi2O7 (x=0.05, 0.25, 0.5, 0.75) and Sr2-xBaxNb2O7 (x=0.1, 0.2, 0.3, 0.4, 0.5) were investigated. They have a provskite-like layered structure (PLS), and are well known for their super-high Curie points (>1200 ËC). Their ceramics were fabricated using Spark Plasma Sintering. For Pr2Ti2O7, single phase, dense and textured ceramics were prepared. The Curie point is greater than 1560 Â°C which is the highest known Curie Point so far for ferroelectric materials. Pr2Ti2O7 was shown for the first time to be ferroelectric because it showed piezoelectric activity after poling. For the La2-xCexTi2O7 solid solution system, the ferroelectric and dielectric properties of cerium (Ce) substituted La2Ti2O7 (LTO) were investigated. The solubility limit of Ce in La2-xCexTi2O7 was found to be between 0.35 and 0.5 supported by XRD results. The a-, b- and c-axes of the unit cell decrease with increasing Ce substitution. The Curie points (Tc) of La2-xCexTi2O7 (x=0, 0.15, 0.25, 0.35) also decreases. The dielectric constant and loss increase with increasing Ce substitution. Electrical resistivity decreases due to Ce substitution. Cerium can increase the d33 of La2Ti2O7. The highest d33 was 3.9 Â± 0.1pC/N for La1.85Ce0.15Ti2O7. In the Nd2-xCexTi2O7 system, the cell volume increases from Nd2Ti2O7 to Nd1.25Ce0.75Ti2O7 and the Curie point (Tc) decreases with Ce increase. For the Sr2-xBaxNb2O7 solid solution system, the effect of Ba substitution on the structure and ferroelectric properties of Sr2-xBaxNb2O7 (x<1.0) was investigated. The a-, b-, c- axes and cell volume increase with Ba addition because Ba2+ is a relatively large ion. A atomic displace move model was developed to explain the spontaneous lattice strain, spontaneous polarization and Curie point change in the orthorhombic phase (Cmc21) with increasing Ba substitution. The critical point of Sr2-xBaxNb2O7 solid solution (x < 0.6) was determined by XRD and was supported by the XPS spectra of Ba 2p and O 1s. Textured ceramics of Sr2-xBaxNb2O7 compounds were prepared using the spark plasma sintering technique and the piezoelectric activity can be improved by Ba substitution, which increases the domain switch mobility. The highest d33 was measured as 3.6Â±0.1pC/N for Sr1.8Ba0.2Nb2O7. The thermal depoling behaviors of La2Ti2O7, and Sr2Nb2O7 were investigated due to their relatively high d33 piezoelectric constant and high Curie point. Both of them have a high resistance to thermal depoling, especially La2Ti2O7. Ginzburg - Landau theory was used to explain their behavior. The electric resistivity degradation of Sr2Nb2O7 was studied at different temperatures, and it was found to be stable below 800 Â°C.

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Chapman, Jacob Bernard John. "Improving the functional control of ferroelectrics using insights from atomistic modelling". Thesis, University College London (University of London), 2018. http://discovery.ucl.ac.uk/10056036/.

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Lead zirconate titanate is a ferroelectric material of considerable interest with a wide range of technological applications. It has been the subject of many experimental and theoretical studies yet there are a number of unsolved questions preventing further miniaturisation and optimisation of this and other ferroelectric materials. Exotic ultra-dense domain morphologies, as an example, offer an exciting avenue for the development of novel nanoelectronics. In this work, large scale molecular dynamics is used to construct a strain-temperature phase diagram of the domain morphology of PbTiO3 ultrathin films. By sampling a wide range of strain values over a temperature range up to the Curie temperature, it is found that epitaxial strain induces the formation of a variety of closure- and in-plane domain morphologies. The local strain and ferroelectric-antiferrodistortive coupling at the film surface vary for the strain mediated transition sequence and this could offer a route for experimental observation of the morphologies. Remarkably, a new nanobubble domain morphology is identified that is stable in the high-temperature regime for compressively strained PbTiO3. It is demonstrated that the formation mechanism of the nanobubble domains morphology is related to the wandering of flux closure domain walls, which is characterised using the hypertoroidal moment. Molecular dynamics calculations, supplemented with electrical measurements from collaborators, are used to provide insight into the microscopic switching properties of near-morphotropic PZT. The simulations and experiments exhibit qualitatively similar hysteretic behaviour of the polarisation at different temperatures, showing widening of the Polarisation - Electric field hysteresis loops, and the decrease of the coercive field towards high temperatures. Remarkably, polarisation switching at low temperatures is shown to occur via a polarisation rotation and growth mechanism that is fundamentally different from the high temperature switching, where nucleation is rate limiting. Analysis of B-cation contributions show that nucleation and switching are facilitated by Zr centred unit cells and, by extension, Ti centred unit cells in Zr-rich environments. Ti-rich clusters in morphotropic PZT, at low temperature, are observed to have suppressed ferroelectric displacements which may incorrectly be perceived as ferroelectrically inactive `dead-layers'. Finally, fundamental insight into the microscopic mechanisms of the ageing processes are provided. From simulations of the prototypical ferroelectric material PbTiO3, it is demonstrated that experimentally observed ageing phenomena can be reproduced from intrinsic interactions of defect-dipoles related to dopant-vacancy associates, even in the absence of extrinsic effects. Variation of the dopant concentration is shown to modify the material's hysteretic response, identifying a universal method to reduce loss and tune the electromechanical properties of inexpensive ceramics for efficient technologies.

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