Dissertations / Theses on the topic 'Antiferroelectric materials'

This thesis examined two-dimensional ferroelectric materials and their applications using density functional theory calculations. The research has revealed several novel applications for 2D ferroelectric materials. It illustrated that ferroelectric materials can be used to modify electronic, photocatalytic and magnetic properties of two-dimensional materials. In addition to exploring applications, new two-dimensional ferroelectric material which exhibits metallic properties was discovered through high through output search. Two-dimensional ferroelectric metals are extremely rare and only handful of materials were ever discovered.

Mesures entre 7 et 300K, dans le domaine 15-400 CM**(-)1 et réalisation des spectres du cristal d'ADP, dans la phase anti-ferroélectrique, en lumière polarisée. Détermination des paramètres des modes de vibration par ajustement au moyen d'un modèle de la fonction diélectrique. Mise en évidence de l'importance des modes de basse fréquence, liés aux mouvements collectifs des protons, dans le mécanisme de la transition de phase

Cette thèse examine les films minces d’oxyde de hafnium (HfO2) pour leur potentiel dans lesnanocondensateurs, répondant aux besoins en miniaturisation et haute performance del’électronique moderne. Le HfO2 est compatible avec la Déposition par Couches Atomiques(ALD), ce qui permet des dépôts minces précis et homogènes, essentiels pour garantir la fiabilitédes dispositifs électroniques. Les films minces sont soumis à différentes techniques de fabricationet de caractérisation pour analyser leur morphologie et leurs propriétés électriques, notamment laconstante diélectrique, la tension de claquage et la capacité de stockage d’énergie. Cette approchepermet de déterminer comment optimiser ces matériaux, à la fois en configurations amorphes eten structures cristallines, pour des performances maximales.Pour les diélectriques amorphes linéaires, HfO2 est combiné avec d’autres oxydes, tels quel’alumine et la silice, dans des structures de nanolaminés et de solutions solides. Ces combinaisonssont conçues pour stabiliser la constante diélectrique et offrir une résistance au claquage,améliorant l'efficacité du stockage d’énergie. La linéarité et la stabilité de ces matériaux amorphesles rendent particulièrement adaptés aux applications nécessitant une capacitance stable.L’étude approfondit aussi les propriétés des diélectriques cristallins non linéaires, dopés avec dusilicium ou de la zircone. Différentes températures de déposition et de recuit révèlent descomportements ferroélectriques et antiferroélectriques, augmentant la densité d’énergie et lastabilité. Cependant, les matériaux ferroélectriques, bien que prometteurs pour des applications àhaute densité, sont sensibles aux variations de tension, ce qui limite leur usage dans les applicationsnécessitant une capacitance constante. Les matériaux antiferroélectriques, en revanche, présententune stabilité accrue face aux variations de tension, mais ils font encore face à des défis d’efficacitéénergétique et de gestion thermique. La recherche souligne la variabilité de la constantediélectrique comme un défi majeur pour l'utilisation de ces matériaux dans des applicationsnécessitant une capacitance stable, comme le filtrage de signaux. Les matériaux nanolaminés etles solutions solides sont privilégiés pour obtenir une capacitance linéaire, mais leur efficacité restelimitée en termes de permittivité. L’exploration des phases non linéaires, cependant, ouvre la voieà des performances accrues dans certaines applications avancées.En conclusion, cette étude apporte un éclairage précieux sur les films minces d’oxyde de hafniumet leur rôle dans les nanocondensateurs, en explorant des solutions d’optimisation pour améliorerles performances diélectriques, notamment par les techniques de fabrication et les compositionsde matériaux. Les matériaux linéaires et non linéaires présentent chacun des avantages distincts,mais des recherches supplémentaires sont nécessaires pour surmonter les défis liés à la durabilité,l’efficacité électrique et la gestion thermique, afin de développer des condensateurs plusperformants pour les technologies électroniques modernes
This research investigates the use of hafnium oxide (HfO2)-based thin films in nanocapacitors, focusing on both their linear and non-linear electrical properties to meet the growing demands of high-performance and miniaturized electronic devices. Starting with the fundamental physics of energy storage capacitors, the investigation highlights the essential characteristics of effective dielectric materials, such as a high dielectric constant and a substantial band gap. Hafnium-based materials are particularly promising due to their compatibility with Atomic Layer Deposition (ALD), which allows for precise and uniform thin-film deposition—crucial for ensuring reliable performance in electronic devices.To understand the potential of these materials, various fabrication and characterization techniques were employed. This includes specific deposition processes to create the thin films and morphological tests to study the physical structure of the capacitors. Electrical testing plays a key role in evaluating critical parameters like dielectric constant, breakdown voltage, and overall energy storage capacity. By analyzing these factors, a comprehensive view of how both linear and non-linear hafnium-based dielectrics perform is provided.When exploring linear, amorphous hafnium-based dielectrics, HfO2 is combined with aluminum oxide and silicon dioxide to enhance dielectric properties. Different configurations, such as nanolaminates and solid solutions, are tested to find the optimal balance. The goal is to achieve materials that maintain a high dielectric constant and resist voltage breakdown, thereby improving their ability to store energy efficiently. On the other hand, a detailed look into non-linear, crystalline dielectrics examines the effects of doping hafnium oxide with elements like zirconia and silicon. Different deposition and annealing temperatures are assessed for their impact on crystalline structure and polarization behavior, revealing complex ferroelectric and antiferroelectric behaviors that could offer high energy density and stability.The findings suggest that while ferroelectric materials might not be suitable for applications requiring linear capacitance due to their sensitivity to voltage variations, antiferroelectric materials show promise. However, they still face challenges related to electrical efficiency and thermal management. Finding materials that can effectively stabilize voltage variations is crucial, as capacitors are increasingly used to manage these fluctuations in modern electronics.A significant challenge identified is the variability in the dielectric constant, which can limit the use of these materials in applications demanding stable capacitance, such as signal filtering. To address this issue, solid solutions and laminated materials, which provide consistent linear capacitance, are prioritized. Although these materials are effective up to a certain permittivity threshold, exploring non-linear phases opens the door to potentially higher performance under specific conditions.In summary, understanding of HfO2-based thin films and their role in nanocapacitors is advanced by this research. By examining both linear and non-linear dielectric materials, insights into how to optimize fabrication techniques and material compositions to improve dielectric properties are provided. Ongoing research into issues like material endurance, electrical efficiency, and thermal management is essential for developing reliable and high-performing capacitors that meet the evolving demands of modern electronic technologies

Com base em resultados de DSC, análise estrutural por difração de raios X e observações de textura, analisamos os efeitos da mistura do solvente apolar hexadecano com cristais líquidos formados pelas moléculas banana 1,3-fenilenobis[4-(4-tetradecilfeniliminometil)benzoato] (MB14) e 4-cloro-1,3-fenilenobis[4-(4-tetradecilfeniliminometil) benzoato] (MB14Cl). Propusemos um modelo estrutural para explicar as modificações causadas no arranjo molecular da fase B2 pelo acréscimo gradual do solvente. Observamos a diminuição da temperatura de transição entre esta fase e a fase isotrópica, porém a transição entre a fase B2 e a fase a temperaturas mais baixas não sofre alteração significativa. Para concentrações em massa de hexadecano de 45 % no MB14 e de 55 % no MB14Cl, a fase B2 já não é mais observada. Para o MB14Cl, resultados de difração de raios X revelaram que as moléculas de hexadecano penetram entre as camadas esméticas, aumentando a distância intercamada em torno de 3 Å. Acima de 5 % de concentração do solvente, o aumento da distância intercamada satura e ocorre segregação de fases em escala nanométrica. O comportamento da fase B2 sob a ação de um campo elétrico variável também foi analisado para o MB14 puro. Apresentamos um modelo para a linha de base do sinal de corrente de polarização, que leva em conta a não linearidade da condutividade para valores altos de campo aplicado, devido à movimentação iônica na amostra. Para o cálculo da viscosidade, consideramos a não linearidade da constante dielétrica com o campo aplicado, e adaptamos outro modelo, inicialmente utilizado para cristais líquidos ferroelétricos sob a ação de um campo quadrado, para o caso de um cristal líquido antiferroelétrico sob um campo triangular. Quanto aos dois tipos de arranjo molecular da fase B2, o arranjo homoquiral se mostrou bem mais estável que o racêmico, mesmo sob aplicação de campo triangular, quando este último é inicialmente favorecido. O arranjo racêmico se apresentou mais viscoso que o arranjo homoquiral, contrariando nossas previsões.
Based on DSC results, structural analysis by X-ray diffraction and texture observations, we observed the effects of mixing the nonpolar solvent hexadecane with the banana molecules liquid crystals ,3-phenilenebis[4-(4-tetradecilpheniliminometil)benzoate] (MB14) and 4-chloro-1,3-phenilenebis[4-(4-tetradecilpheniliminometil) benzoate] (MB14Cl). We propose a structural model to explain the changes in the molecular ordering of the B2 phase caused by the gradual increase of the solvent. We observed a decreasing of the transition temperature between B2 and isotropic phases, however the transition between B2 and lower temperature phases did not change significantly. For hexadecane concentrations above 45 wt% in MB14 and 55 wt% in MB14Cl, the B2 phase is no longer present. In MB14Cl, X-ray diffraction results showed that the hexadecane molecules penetrate between the smectic layers, increasing the interlayer spacing by about 3 Å. Above 5 wt% of solvent concentration, the increasing of the interlayer spacing saturates, and a phase segregation in nanometric scale occurs. The behavior of the B2 phase under variable electric field was also analysed for the pure MB14. We present a model for the baseline of the polarization current signal, which considers the non-linearity of the conductivity for high values of applied field, due to the presence of ions in the sample. In order to calculate the viscosity, we considered the non-linearity of the dielectric constant with the applied field, and adapted another model, initially used in ferroelectric liquid crystals under rectangular field, for the case of an antiferroelectric liquid crystal under triangular field. Concerning the two kind of molecular ordering in the B2 phase, the homoquiral ordering proved to be far more stable than the racemic, even under triangular field, when the latest is favored. Our measurements resulted in a racemic ordering more viscous than the homoquiral, going against our predictions.

With the ever-increasing global demand for new energy and modern technology, the investigations in advanced ferroelectrics (FE) and antiferroelectric (AFE) ceramics is getting more and more urgent. The phase structure and phase transition behaviors of FE and AFE materials can be influenced by various external stimuli, such as temperature and pressure, and thus offer great possibilities for different applications. With this bearing in mind, several energy related effects in FE/AFE materials are studied in this thesis, including the pyroelectric effect, the explosive energy conversion effect, and the electrocaloric effect. The studies will offer more understanding about FE/AFE materials and pave the way for speeding up the development of FE/AFE materials for real energy harvesting applications. Firstly, the pyroelectric effect of 0.97(0.99Bi0.5Na0.5TiO3-0.01BiAlO3)-0.03K0.5Na0.5NbO3 (BNT-BA-KNN) is investigated. The depolarization temperature of the BNT-BA-KNN ceramics is 118 oC. At room temperature, a large pyroelectric coefficient (p ~3.7*10-8Ccm-2K-1) is achieved. Moreover, the Figure of merits Fi, Fv, and Fd are determined as high as 1.32*10-10m/V, 2.89*10-2m2/C, 1.15*10-5Pa-1/2 at 1 kHz. The temperature dependent study suggests an excellent temperature stability of p and figures of merits during RT~85oC. The high pyroelectric properties and excellent thermal stability of BNT-BA-KNN reveal its promising potential as a lead-free candidate for infrared temperature detector materials. Secondly, the explosive energy conversion effect is studied in the (Ag1-xKx)NbO3 FE materials. Optimized (Ag0.935K0.065)NbO3 (AKN) is demonstrated to show the best explosive energy conversion performance, being able to display a FE-AFE phase transition under a low hydrostatic pressure of 350 MPa. The AKN ceramics also possess a stable high polarization from room temperature to 150 oC. A device has been designed and fabricated and the shock wave experiment shows that the AKN possesses a record-high energy storage density of 5.401 J/g, and enable a pulse current of 22 A within 1.8 microseconds, which is better than that of the commercially employed Pb(Zr,Ti)O3. A detailed study on the structural transition has been conducted using the TEM and in-situ neutron diffraction technique. With increasing hydrostatic pressure, the disappearance of 1/2(301)p, 1/2(321)p and 1/2(341)p reflections and appearance of 1/4(443)p and 1/4(229)p further confirms the pressure driven FE-AFE phase transition. A phenomenological theory is also employed to rationalize the pressure driven FE-AFE phase transition. The explosive energy conversion effect has also been demonstrated in NaNbO3 (NN) ceramics. With increasing pressure to 450 MPa, a sharp FE-AFE transition and depolarization behavior can be observed for NN ceramics. The FE-AFE phase transition was further supported structurally via in-situ neutron diffraction measurement. The polarization of NN can keep stable (~30 uC/cm2) over a wide temperature range of 20-180oC, which adds to its suitability for explosive energy conversion application. The investigation results of AKN and NN under pressure will serve as a guidance for further development of new FE materials and devices for energy conversion application. Finally, the electrocaloric effect of a compositionally modulated (Pb0.97-xLa0.02Bax)(Zr0.58Sn0.29Ti0.13)O3 (PLZST) (x=0, 0.08, 0.09, 0.11) ceramics is investigated. With increasing Ba doping level, the sharp FE-AFE-paraelectric (PE) phase transition sequence with increasing temperature is gradually immerged into a diffused FE-PE phase transition. Moreover, the Curie temperature decreases down to around room temperature. As a result, a large electrocaloric effect (1.2 K) and wide electrocaloric temperature span (112 K), are simultaneously achieved in PLZST with x=0.11. This work provides a novel method to simultaneously realize high electrocaloric effect and large temperature span.

Antiferroelectric (AFE) materials are an important group of functional materials showing unique properties such as double polarization-electric field (P-E) hysteresis loop, and charge release under the pressure and temperature. These performances are strongly connected to the structural phase transitions induced by external conditions. Many investigations were carried out to optimize their properties but the structure-property relationship of such AFE materials still remain unclear. With this bearing in mind, in this thesis, I firstly investigate the crystal structure, domain structure and properties evolution under the external stimuli such as electric-field (E-field), mechanical force, and temperature in the typical PbZrO3-based AFE materials. Secondly, a systematic study was conducted on the doped silver niobate ceramics to understand the impacts of the chemical composition and E-field cycling on these novel lead-free AFE materials. The targeted materials of typical PbZrO3-based samples were selected as La/Nb doped Pb(Zr, Sn, Ti)O3 ternary systems with the composition Pb0.97La0.02(Zr0.56Sn0.33Ti0.11)O3 (PLZST1), Pb0.99(Nb0.02Zr0.73Sn0.21Ti0.04)O3 (PNZST1) and Pb0.99(Nb0.02Zr0.65Sn0.28Ti0.05)O3 (PNZST2). These three compositions are representative, supplying diverse phase transition behaviours for studying. The in situ neutron powder diffraction (NPD) of the PLZST1 material reveals that the pseudo-tetragonal AFE phase is transferred into the rhombohedral FE phase with an application of the sufficient E-field, and recovers after withdrawal of the external field. The resultant average structure change as a function of the E-field is in accordance with the reversible AFE-FE phase transition. However, the ω dependent NPD patterns suggest this process is not fully reversible: in the induced FE state, the strain exhibits an elliptical distribution, which in turn leads to significant preferred orientation in the final AFE state. The formation of this preferred orientation provides an explanation for the properties variation appearing in AFE materials after exposure to the sufficiently high E-field. X-ray diffraction pattern of PNZST1 sample indicates the orthorhombic AFE phase while the result of NPD contradicts this conclusion with a rhombohedral FE phase. After careful characterization of the surface and bulk properties, it is found that the near surface and bulk regions show different phases. Additionally, the surface processing such as polishing and heat-treatment can induce an AFE/FE phase transition within micrometres of the surface. The in-situ hydrostatic-pressure neutron diffraction proves that the mechanical force helps stabilize the AFE phase of this composition. Therefore, the surface processing induced phase transitions can be attributed to the change of states of residual stress. The in-situ NPD studies of PNZST2 material describe its structural variation as a function of E-field and temperature. Through the mode decomposition approach, the relationships between AFE/FE modes and octahedral rotation mode were systematically investigated. At room temperature, the pristine AFE phase can be poled into the meta-stable FE phase by applying the external E-field. At this stage, both AFE and FE phases consist of modes associated with octahedral rotation and A-site ionic displacements. The temperature-induced phase transition indicates that the octahedral rotation and ionic displacements are weakly coupled in the room-temperature FE phase and decoupled in the high-temperature FE phase. Furthermore, both temperature and E-field-induced phase transitions between the AFE and high-temperature FE phase demonstrate the critical role of coupling between the octahedral rotation and A-site ionic displacements in AFE structure stabilization. The evolution of structure and electrical properties with composition in (1-x)AgNbO3-xLiTaO3 (ANLT100x) (0 ≤ x ≤ 0.09) ceramics have been systematically investigated by diffraction techniques, complemented by dielectric and polarisation measurements. The symmetry mode decomposition and Rietveld refinement of distortive modes were firstly used to analyse the origin of the anti/ferroelectricity observed. The in/out phase octahedral tilting around the a-axis (H2 mode) and the antiparallel ionic displacements (Λ3 mode), present large amplitudes in the pure AgNbO3. These two modes vanish progressively with increasing x and their amplitudes experience a sudden drop when x = 0.053. Accompanied by the disappearance of these two modes, a new phase with R3c symmetry appears and grows with further increasing LiTaO3 content. The composition dependent amplitudes of the primary modes, and R3c phase fractions, lead to a comprehensive understanding of the dielectric and ferroelectric properties affected by LiTaO3. For the composition located around the phase boundary, x = 0.045 and 0.06, FE wake-up effects were detected. The refinement of neutron diffraction patterns after different electric cyclicity describe an increase of ferroelectricity associated with the R3c phase fraction increments i.e., field-cycling-induced phase transition from Pmc21 to R3c. The local probes such as the electron diffraction and piezoresponse microscopy (PFM), show that the in/antiphase octahedral rotation around the <001>p and the local strain state are the decisive factors for this field-cycling-induced phase transition. In summary, the wake-up effects can be regarded as the nucleation and growth of the R3c phase with increasing number of electric cycles.

Metal oxides crystallized in perovskite structure are generally modified in two different ways. According to the general structural formula ABO3, the two ways are A-site modification and B-site modification. The primary significance of perovskite metal oxides rests on their importance in electronic devices. A particular class of perovskites, namely Lead Zirconate or modified Lead Zirconate has received a special attention because of their unique antiferroelectricity and various applications in devices. Among the other modifications, A-site modification of PbZrO3 by La is rare and not much explored. Chapter 1 describes various applications of antiferroelectric thin films along with the synthesis and characterization of pure and La modified PbZrO3, which are relevant to the work presented in this thesis. Sol-gel processing and spin coating technique to deposit solid oxide thin films are well known for their low cost of deposition as well as for their ability to achieve better stoichiometric chemical composition. Common crack formation problem of sol-gel grown films can be prevented by ‘drying control chemical adhesive’ like polyvinylpyrrolidone (PVP). Heat treatment of sol-gel derived thin films is generally determined by TGA and DTA. Crystalline phase of deposited solid thin films is determined by XRD whereas effect of modification can be ascertained by XRD peak assignment and relative crystalline peak shifting. Sol-gel grown film thickness is measured by common cross sectional SEM whereas AFM can detail the surface morphology. Chapter 2 summarizes the deposition and characterization of pure and La modified PbZrO3 thin films. Any nonmetal, which is insulator, is dielectric material and show dielectric dispersion in a frequency domain of low field alternative current. Among the most common feature of dielectric dispersion, Maxwell – Wagner type dispersion is well known. Similar kind of dielectric dispersion, named Maxwell – Wagner like dispersion, can be observed while the equivalent circuit consists of parallel G – C along with a series R. Universal power law of ac conductivity is the deciding factor to distinguish the nature of dispersion. Structural phase transition can be determined by dielectric response and it is widely known as dielectric phase transition. Effect of La modification on dielectric phase transition of PbZrO3 thin films depends on stabilization or destabilization of antiferroelectricity. Maximum dielectric constants of pure and modified PbZrO3 thin films depend on the crystallographic orientations of the growth. Chapter 3 presents dielectric properties of pure and La modified PbZrO3 thin films and these properties are correlated to the stabilization or destabilization of antiferroelectricity, relative integrated intensity of (202)O film orientation and trapped electron charge due to oxygen vacancies. Charge storage property of a capacitor is determined by the polarization of the capacitor on application of electric field whereas field dependent integrated area of polarization on withdrawal of electric field determines the recoverable capacitive energy storage. Among the three kinds of capacitors like linear or paraelectric, ferroelectric and antiferroelectric capacitors, antiferroelectric capacitor is known to be best for their ability to store huge amount of recoverable energy. The recoverable energy in antiferroelectrics can be increased by increasing squareness of the P – E hysteresis loop, applicable electric field, polarization or by the all possible combinations of them. Chapter 4 describes the correlation of relative integrated intensity of (202)O [RI(202)O] with critical applied electric field of P – E saturation to provide enhanced squareness of the hysteresis loops. This chapter also describes the variation of charge and recoverable energy storage properties with respect to RI(202)O. Like magnetocaloric effect, electrocaloric effect is capable to alter the temperature of a system by adiabatic polarization or depolarization. From the Maxwell’s relation of thermodynamics, assuming, (∂p ) = (∂s )electrocaloric effect can be calculated from temperature dependent polarization value of a paraelectric, ferroelectric or an antiferroelectric. Chapter 5 presents the electrocaloric effect of pure and La modified PbZrO3 thin films. Summary of present study and discussion have been delineated in Chapter 6 along with the future work which can give more insight into the understanding of antiferroelectric PbZrO3 thin films with respect to Pb and Zr site modification and with respect to different electrodes. (For formulas pl see the pdf file of the thesis)

Metal oxides crystallized in perovskite structure are generally modified in two different ways. According to the general structural formula ABO3, the two ways are A-site modification and B-site modification. The primary significance of perovskite metal oxides rests on their importance in electronic devices. A particular class of perovskites, namely Lead Zirconate or modified Lead Zirconate has received a special attention because of their unique antiferroelectricity and various applications in devices. Among the other modifications, A-site modification of PbZrO3 by La is rare and not much explored. Chapter 1 describes various applications of antiferroelectric thin films along with the synthesis and characterization of pure and La modified PbZrO3, which are relevant to the work presented in this thesis. Sol-gel processing and spin coating technique to deposit solid oxide thin films are well known for their low cost of deposition as well as for their ability to achieve better stoichiometric chemical composition. Common crack formation problem of sol-gel grown films can be prevented by ‘drying control chemical adhesive’ like polyvinylpyrrolidone (PVP). Heat treatment of sol-gel derived thin films is generally determined by TGA and DTA. Crystalline phase of deposited solid thin films is determined by XRD whereas effect of modification can be ascertained by XRD peak assignment and relative crystalline peak shifting. Sol-gel grown film thickness is measured by common cross sectional SEM whereas AFM can detail the surface morphology. Chapter 2 summarizes the deposition and characterization of pure and La modified PbZrO3 thin films. Any nonmetal, which is insulator, is dielectric material and show dielectric dispersion in a frequency domain of low field alternative current. Among the most common feature of dielectric dispersion, Maxwell – Wagner type dispersion is well known. Similar kind of dielectric dispersion, named Maxwell – Wagner like dispersion, can be observed while the equivalent circuit consists of parallel G – C along with a series R. Universal power law of ac conductivity is the deciding factor to distinguish the nature of dispersion. Structural phase transition can be determined by dielectric response and it is widely known as dielectric phase transition. Effect of La modification on dielectric phase transition of PbZrO3 thin films depends on stabilization or destabilization of antiferroelectricity. Maximum dielectric constants of pure and modified PbZrO3 thin films depend on the crystallographic orientations of the growth. Chapter 3 presents dielectric properties of pure and La modified PbZrO3 thin films and these properties are correlated to the stabilization or destabilization of antiferroelectricity, relative integrated intensity of (202)O film orientation and trapped electron charge due to oxygen vacancies. Charge storage property of a capacitor is determined by the polarization of the capacitor on application of electric field whereas field dependent integrated area of polarization on withdrawal of electric field determines the recoverable capacitive energy storage. Among the three kinds of capacitors like linear or paraelectric, ferroelectric and antiferroelectric capacitors, antiferroelectric capacitor is known to be best for their ability to store huge amount of recoverable energy. The recoverable energy in antiferroelectrics can be increased by increasing squareness of the P – E hysteresis loop, applicable electric field, polarization or by the all possible combinations of them. Chapter 4 describes the correlation of relative integrated intensity of (202)O [RI(202)O] with critical applied electric field of P – E saturation to provide enhanced squareness of the hysteresis loops. This chapter also describes the variation of charge and recoverable energy storage properties with respect to RI(202)O. Like magnetocaloric effect, electrocaloric effect is capable to alter the temperature of a system by adiabatic polarization or depolarization. From the Maxwell’s relation of thermodynamics, assuming, (∂p ) = (∂s )electrocaloric effect can be calculated from temperature dependent polarization value of a paraelectric, ferroelectric or an antiferroelectric. Chapter 5 presents the electrocaloric effect of pure and La modified PbZrO3 thin films. Summary of present study and discussion have been delineated in Chapter 6 along with the future work which can give more insight into the understanding of antiferroelectric PbZrO3 thin films with respect to Pb and Zr site modification and with respect to different electrodes. (For formulas pl see the pdf file of the thesis)

碩士
大同大學
化學工程研究所
90
Abstract The purpose of this research work was an attempt to elucidate the structure-property correlation in the chiral amide smectic liquid crystals, because the exciting results have been obtained this field of chiral system　in our laboratory. Therefore, two series of chiral materials DPmPBNPA(m=9~13) and DEmPBNPA(m=10~15) were designed for the synthesis and investigation of the stricture-property relationship in the chiral amide liquid crystal system. The target compounds was modified independently by the length of the alkyl chain length (m) and swallow-tailed length (n) for the study. Chiral amide liquid crystals were derived by using chiral (S)-2-(6-methoxy-2-naphthyl)propionic acid with N,N’-dialky amines. Mesophases and their corresponding transition temperatures of the materials were identified by the polarizing microscopic textures, DSC carlorimetry and electro-optical measurements. The results showed that most of the chiral amide materials exhibited phase : chiral smectic A (SmA*), antiferroelectric (SmCA*) phases. SmCA* phase was further characterized by the schlieren textures, switching behaviors, dielectric constants and eletro-optical response. Compounds DPmPBNPA(m=9~11) exhibited monotropic mesophases of the SmA* and SmCA* phases in cooling process. The variation of alkyl chain length (m) showed that the temperature range of SmA* phase became smaller, but the temperature range of SmCA* phase became larger as the number of alkyl chain length increased. Compounds DPmPBNPA with higher alkyl chain length m=12,13 had no mesophase observed either in heating or cooling process, therefore, they are not mesomorphic compounds. Compounds DEmPBNPA(m=10~15), with the exception of compounds DEmPBNPA(m=12,13) exhibited enantiotropic SmA* and SmCA* phases, while the rest of compounds displayed monotropic SmA* and SmCA* phases. The temperature range of the SmCA* phase decreased as the alkyl chain of compounds DEmPBNPA increased. Physical properties of these two series of chiral amide materials in SmCA* phase were characterized by spontaneous polarization, dielectric constant, switching current and electro-optical responses. The maximum magnitudes of spontaneous polarization (Ps values) for the chiral amide materials were obtained in the range of 80~115 nC cm-2. The electro-optical response demonstrated that material DEmPBNPA (m=12) displayed thresholdless V-shaped switching in antiferroelectric(SmCA*) phase while other compounds of these two series of materials displayed typical characteristic of double hysteresis in antiferroelectric(SmCA*) phase. It is strongly demonstrated that V-shaped switching properties for these materials are critically dependent on the alkyl chain length; the effect of nonchiral alkyl chain length. The analogous V-shaped switching was reported by Inui et al. in some antiferroelectric liquid crystal mixtures. Thus, binary mixtures of two chiral materials; compound DEmPBNPA(m=12) and chiral swallow-tailed ester material EP10PBNP possessing V-shaped switching property were studied in order to investigate the miscibility of these two compounds, more particularly, the effect of mixtures on the electro-optical responses; the thresholdless, V-shaped switching properties. The different between two materials DEmPBNPA(m=12) and EP10PBNP is that the former appeared monotropic SmCA* phase and the later has enantiotropic SmCA* phase in heating and cooling processes. EP10PBNP displayed a phase sequence of BPII-N*-TGBA*-SmA*-SmCA*, but as the component ratio of DEmPBNPA(m=12) greater than 9wt%, the BPII phase was disappeared and as the component ratio of DEmPBNPA(m=12) greater than 75wt%, the TGBA* phase was disappeared. The temperature range of SmCA* phase in component ratio D50/E50 was the largest. The eletro-optical responses in the SmCA* phase of the binary mixtures exhibited V-shaped switching in the cell with 5μm thickness. Ours results demonstrate two materials with V-shaped switching responses are mixable. In particular, we like to emphasize that the magnitudes of spontaneous polarization have no any effect on the appearance of thresholdless, V-shaped switching property.

博士
大同大學
化學工程學系(所)
95
The primary of this research work was an attempt to elucidate the correlation between structure and property in the chiral smectic liquid crystals. Thus, two new chiral moieties, (S)-2-(metylsulfanyl)ethyl 2-hydroxypropanoate prepared by reacting (S)-lactic acid with 2-methylsulfanyl-1-ethanol, and (S)-[1-methyl-2-(2’-methylsulfanylethoxy)]ethanol prepared by reacting (S)-propylene oxide with 2-methylsulfanyl-1-ethanol under basic condition, were designed and synthesized, and three series of chiral liquid crystal compounds were subsequently prepared for the mesomorphic and physical studies. The target compounds were modified independently by (i) the non-chiral peripheral methylene chain length ‘m’ and (ii) the lateral substitution group in the core. In first series of chiral materials, MSEmBCPP(m=8-12), all materials displayed enantiotropic mesophases of the SmA*, and SmC* phases. The maximum magnitude of spontaneous polarization (Ps) for materials was around 85nC/cm2. The electro-optical response in the series of chiral materials in the SmC* phase displayed typical ferroelectric hysteresis loop or hysteresis-free, V-shaped switching property upon various applied frequencies and temperatures. In second series of chiral materials, MMSEEmCB(m=8-12), the formation of smectic phases are significantly depended of the achiral alkyl chain length. For example, material at m=7 displayed SmA*, SmCA* and SmX phases, while materials at m=8-12 display an additional SmC* phase but at m=10-12 the SmCA* phase and at m=11-12 the SmX* were suppressed. The maximum Ps value was observed to be 37.76 nC cm-2, and the maximum tilt angle reached 29.5o. In the third series of materials, the results obtained from MMSEEmBC2F(m=7–10) with lateral 2-fluoro substitution and MMSEEmBC2F(m=7–10) with 3-fluoro show that all materials display SmA* and SmC* phases. The antiferroelectric SmCA* phase only occurred in two materials, MMSEE7BC2F and MMSEE8BC3F. The results also indicate that lateral fluoro-substituent in the phenyl ring of the materials could depress the formation of higher ordered smectic phases, especially the SmCA* and SmX* phases, lowering the transition temperatures, melting points, clearing points. The measured Ps values, as compared fluoro-substituted materials to non-substtuted materials, have the order of MSEmBCPP > MMSEE7BC2F > MMSEE8BC3F, suggesting that fluoro-substituents depress the Ps values in which 3-fluoro-substituent has more effect than 2-fluoro-substituent. In conclusion, the results indicated that the chiral materials derived from (S)-2-(metylsulfanyl)ethyl 2-hydroxypropanoate are favorable for the formation of the wide temperature range of the ferroelectric phase, while that from (S)-1-methyl-2-(2’-methylsulphanylethoxy)ethanol are favorable for the formation of antiferroelectic phase. Fluoro-substituents could decrease the transition temperatures but suppress the formation of antiferroelectric phase.

碩士
大同大學
化學工程研究所
88
Two series of achiral swallow-tailed materials; 2-propylpentyl 4-(4'-alkyloxybiphenyl-4-carbonyloxy) benzoates, PPmBCB(m=8~12), and 2-ethylbutyl 4-(4'-alkyloxybiphenyl-4-carbonyloxy)benzoates, EBmBCB (m=8~12) were designed and synthesized for the investigation of the structure-property relationship of the mesophases. The results obtained from the study of these two series achiral swallow-tailed materials (PPmBCB and EBmBCB) showed that all materials have enantiotropic SA and Scalt phases. The Scalt phase was confirmed by the first order characterization of SA-Scalt transition in the DSC thermograms and by the schlieren texture showing two-brush and four-brush singularities. Both series of materials showed that the clearing point gradually decreases and the thermal stability of SA phase has no significant variation as the terminal alkyl chain, m, is varied. The thermal stability of SCalt phases is relatively larger for compounds PP10BCB and EB11BCB, while the rest of materials are nearly the same. The study of binary mixtures were conducted by mixing PP10BCB and EB11BCB separately with the standard antiferroelectric liquid crystal (S)-4-[(1-methylheptyloxy)-carbonyl]pheny 4′-octyloxy-4-biphenyl-carboxylate, (S)-MHPOBC, by various weight percentages to observed the influence of mesophases and physical properties. The results showed that the binary mixture of (S)-MHPOBC as a host doped with achiral materials (PP10BCB and EB11BCB) displayed a phase sequence of SA-SC*-SCA*, while that of the achiral materials (PP10BCB and EB11BCB) as host doped with (S)-MHPOBC displayed a phase sequence of SA-SCA*. The temperature range of SC* phase for p15/m85 (15wt% PP10BCB doped with 85wt% (S)-MHPOBC), is larger than that for p5/m95 (5wt% PP10BCB doped with 95wt% (S)-MHPOBC), and the SC* phase disappears in the mixture p85/m15 (85wt% PP10BCB doped with 15wt% (S)-MHPOBC). It seems that ferroelectric phase in (S)-MHPOBC can be induced to form antiferroelectric phase by doping with specific ratio of hosts, PP10BCB and EB11BCB. The electro-optical responses in the SCA* phase of the mixture p15/m85 and p5/m95 show double hysteresis switching behavior, whereas, that of host achiral materials p85/m15 show V-shaped switching in the cell with 5μm thickness. Similar results were also found in the binary mixtures of e15/m85 and e85/m15 correspondingly. The study of electro-optical response for the binary mixture indicated that the electro-optical response critically depended on the frequency, temperature and thickness of cell.

博士
大同大學
化學工程學系(所)
95
ABSTRACT AFLCD display with such good characteristics as: wide screen, high resolution, excellent image quality and full color have been developed. However, so far, display manufacturers have been unable to commercialize AFLC display devices despite continuous efforts. It seems that the appearance of the SmCA* phase and structure-property relationship of AFLC materials are still understood well enough to optimize the material parameters. Thus, in order to ascertain the structure-property correlations and the practical application in electro-optical services, in this work, we attempt to elucidate the structure-property correlation in the chiral smectic liquid crystal. A homologous series of chiral materials derived from (S)-propylene oxide has been successfully synthesized and the structures-property relationship investigated in the chiral liquid crystal system. The target compounds were modified independently by the (i) the structure of the non-fluorunated, fluorinated alkoxy chain length and chiral swallow-tailed structure (ii) lateral halogen substituents in the core and (iii) different core structure of the molecules of the chiral molecules for the study. Chiral non-fluorinated materials, I(m, n; m=8-12, n=0-3) displayed enantiotropic mesophases of the SmA* and SmC* phases; however, chiral semi-fluorinated materials, II(m, n; m=8-12, n=1-3), exhibited SmA*, SmC* and SmCA* phases. These results demonstrate that the presence of the semi-fluorinated alkyl chain in the chiral tail of the material induces an antiferroelectric SmCA* phase. Chiral swallow-tailed materials, III(m, p; p=0, m=8-12), displayed enantiotropic mesophases of the SmA*, SmC* and SmCA* phases. The materials increasing methylene chain lengths n, III(m, p; m=8-12, p=1), the SmCA* was disappeared and only exhibited SmA* and SmC* phase. In addition, the antiferroelectric liquid crystal materials, II(m, n) and III(m, 0), have higher Ps and θ values than ferroelectric liquid crystal materials, I(m, n) and III(m, 1). The electro-optical response in the series of chiral materials, I(10, 2) and III(10, 1), in the SmC* phase displayed V-shaped switching property; however, in the series of chiral materials, II(10, 2) and III(10, 0), in the SmCA* phase displayed typical antiferroelectric tri-stable state of double hysteresis loop switching behavior . Chiral semi-fluorinated materials, IV(m, X, Y) and V(m, X, Y), were synthesized and investigating the influence of lateral halogen substituents in the core. The SmCA* phase was generally occurred in the alkyl chain length from m=8 to m=10 of the materials. The materials with lateral substitutents in the core could depress the formation of mesophase, lowering the transition temperature, melting point and Ps values but increasing θ values. Comparing with the materials of different atom substitution, it is seen that the materials with lateral chloro substituents in the core of molecular was most decreasing transition temperature and Ps values. The materials with halogens substuted at 2-position on the ring of the molecule were sterically shielded so that possessed higher transition temperature than that materials at 3-position; however, the materials with halogens substuted at 2-position on the ring of the molecules also enhanced Ps and θ values. In chiral semi-fluorinated materials, VI(m) and VII(m), the SmCA* phase was also occurred in the alkyl chain length from m=8 to m=10 of the materials. Comparing with different core structure of three series materials, II(m, 2), VI(m), VII(m), it can be found the materials, III(m, 2), with PhPhCOOPh as the core structure of the molecule were favorable for the formation of antiferroelectric SmCA* phase; however, the materials have highest Ps values in those materials. In chiral materials, VIII(m)-XIII(m) which derived firm cinnamatic acid can be found two ring core materials only exhibit ferroelectric SmC* phase; however, three ring core materials favor the formation of antiferroelectric SmCA* phase. Changing the core structure of molecule exhibited some sub-phases such as SmCα* and SmCγ* phases. Binary mixture study shows that the antiferroelectric mixtures with ambient temperature can be obtained by two antiferroelectric liquid crystals. The response of the optical transmission obtained from the antiferroelectric mixture of 80% V(8, H, Cl) and 20% XIV(10) in SSFLC cell by driving at 60Hz and 26V square wave at 25℃ show that the raise time is 586.7 μs and fall time is 109.5μs. In conclusion, the chiral swallow-tailed and semi-fluorinated alcohol groups can be prepared by the optically pure (S)-propylene oxide treating with semi-fluorinated or swallow-tailed alcohols. These chiral alcohols are then developed large amount of antiferroelectric liquid crystals, and subsequently, the structure-property correlation for the formation of antiferroelectric liquid crystals is established. The establishment of this correction will be beneficial for the future design and synthesis of more antiferroelectric liquid crystals. A large amount of antiferroelectric liquid crystals developed in the present work demonstrates that there is a potential for the preparation of suitable antiferroelectric mixtures for the display devices.

碩士
大同工學院
化學工程研究所
87
The purpose of this research work was an attempt to elucidate the structure-property correlation in the chiral smectic liquid crystals, because the exciting results have been obtained this field of chiral system. A homologous series of chiral materials HNP(p,n,2) was designed for the synthesis and investigation of the structure-property relationship in the chiral liquid crystal system. The target compounds was modified independently by the length of the alkyl chain p and n methylene group for the study. Compounds HNP(p,0,2; p=8~14) have the Iso-N*-TGBA-SA-SCA*-Cr. transitions. The study of dielectric property, switching behavior and electro-optical response demonstrates that these chiral materials in the antiferroelectric SCA* phase possess thresholdless antiferroelectricity. The variations of peripheral chain length, p, in the series of compounds HNP(p,0,2) have no remarkable effect on spontaneous polarization, dielectric property and switching behavior in the SCA* phase. The moderate peripheral chain length of HNP(p,0,2) favors the formation of good V-shaped switching behavior in the thresholdless SCA* phase. Compounds HNP(p,1,2; p=9~14) mostly have the Iso-SA-SCA*-Cr. transitions. In the compounds with non-chiral peripheral chain length p=11~ 12, a N* phase and a TGBA phase with the temperature range of 0.1~0.2℃were detected. The study of dielectric property, switching behavior and electro-optical response demonstrates that these chiral materials in the antiferroelectric SCA* phase possess thresholdless antiferroelectricity. The variations of peripheral chain length, p, in the series of compounds HNP(p,1,2) have no remarkable effect on spontaneous polarization, dielectric property and switching behavior in the SCA* phase. The moderate peripheral chain length of HNP(p,1,2) favors the formation of good V-shaped switching behavior in the thresholdless SCA* phase. Investigating the effect of additional methylene group between carboxyl group and swallow-tailed group on the switching behavior, spontaneous polarization, dielectric properties, and electro-optical responses for these compounds also indicate that there has no remarkable difference. In conclusion, the results from above indicate that of HNP(p,0,2) and HNP(p,1,2) possess antiferroelectric phase, SCA* phase. Good V-shaped switching behavior achieved in the SCA* phase, demonstrating the existence of thresholdless antiferroelectricity. It also indicates additional methylene group (n=1) between carboxyl group and swallow-tailed group has no remarkable effect on V-shaped switching behavior. The moderate peripheral chain length of HNP(p,0,2) and HNP(p,1,2) favors the formation of good V-shaped switching behavior in the thresholdless SCA* phase.