Focused ultrasound surgery (FUS) is considered as a promising approach for treating cancer and other conditions and is gaining increasing interest. However, the limited availability of experimental ultrasound array sources and multichannel electronics able to drive them hinder the research into FUS system configurations for patient conditions such as breast cancer. The work in this dissertation explored the development of ultrasound arrays for MRI guided FUS, from the point of view of the potential piezoelectric material of choice. Two materials are of particular interests in this work: Binary (x)Pb(Mg1/3Nb2/3) O3 - (1-x)PbTiO3 (PMN-PT) piezocrystal, and newly specialized FUS material, PZ54 ceramic. A characterization methodology was developed to fully characterize the materials of choice, under ambient and extreme conditions relevant to FUS applications. Practicalities of adopting these materials into FUS were studied by using the characterized materials in designing and fabricating FUS arrays. A spherical, faceted array geometry inspired by the geodesic dome structure was proposed and implemented for the first time. Four bespoke devices, each with 96 individual elements, were implemented using PZ26 ceramic, PZ26 composite, PZ54 composite and PMN-PT composite materials, respectively for comparison. The arrays were connected to commercial electronics afterwards, to explore a prototyping route for connecting FUS devices and modular driving systems. It is concluded that PMN-PT piezocrystal and PZ54 ceramic material can offer excellent performance over conventional piezoelectric ceramics, although PMN-PT piezocrystal is sensitive to extreme conditions. The usable range of PMN-PT is suggested to be limited to 60°C in temperature and 10 MPa in pressure. However, PMN-PT piezocrystal could still be a potential alternative to conventional ceramics in FUS application if assisted with sufficient cooling circulation and bias field. The geodesic array geometry is also concluded to be able to achieve good focusing of ultrasound beam. With optimized phase control through multi-channel electronics, the focusing was improved with focusing gain up to about 30; the steering range of focus was explored within a volume of 5 x 5 x 10 mm3 beyond the array’s geometric focus, side lobes were limited to below the level of -9 dB in acoustic intensity. Larger numbers of individual controllable elements and alternative array designs will be explored in future to investigate application such as breast cancer treatment and potential pre-clinical trials.

Relaxor ferroelectric PZN-xPT and PMN-xPT single crystals exhibit extraordinary electromechanical properties. They are under development for applications in sensors, actuators and transducers. The polarization switching and phase transition behavior of PZN-4.5%PT and PMN-32%PT single crystals under external loading has been investigated. Experimental investigation elucidates the polarization switching and phase transition behavior of relaxor ferroelectric crystals at different orientation cuts under combined temperature, electric field and stress loading. These crystals exhibit strong orientation dependence of electromechanical properties, and the applied fields all affect the poling and phase states of the crystals. Based on experimental investigation, crystal variant modeling was developed to compute the piezoelectric properties of multi-domain crystals at different orientation cuts from a set of properties for the single domain. Thermodynamics and work-energy analysis of field induced phase transitions in these single crystals sheds light on the phase transition mechanism of ferroelectric crystals. Fracture behavior of relaxor single crystals under non-uniform electric fields at a partial electrode edge has also been measured and analyzed.

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

Made available in DSpace on 2014-06-11T19:25:33Z (GMT). No. of bitstreams: 0 Previous issue date: 2009-03-13Bitstream added on 2014-06-13T19:53:26Z : No. of bitstreams: 1 felix_aa_me_ilha.pdf: 3988287 bytes, checksum: d87d85fca7e813d5a8e3850d61c9c14e (MD5) / Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP) / O presente trabalho descreve os estudos realizados para a síntese de filmes ferroelétricos ultrafinos de Pb(Mg1/3Nb2/3)O3 (PMN) e o estudo sistemático da cinética de cristalização deste material visando a obtenção de filmes monofásicos. Os filmes foram produzidos a partir da modificação de um método químico baseado em precursores óxidos. A obtenção dos filmes de PMN se deu através de uma técnica de duplo estágio, que possibilita maior controle na supressão de fases pirocloro, e da técnica via Método dos Precursores Óxidos (MPO), através da reação direta dos óxidos. Filmes de PMN foram preparados sobre substratos Si(100) com diferentes números de deposições. Esses filmes foram submetidos a tratamentos térmicos entre 400oC e 800ºC, em forno convencional, sob diferentes condições de preparo. A estrutura dos filmes foi investigada pela técnica de difração de raios-X e refinamentos da estrutura pelo método de Rietveld. Os resultados apresentaram uma coexistência da fase pirocloro Pb1.86Mg0.24Nb1.76O6.5 e da fase perovskita Pb(Mg1/3Nb2/3)O3 (PMN) nos filmes preparados via rota da Columbita e a supressão da fase prirocloro nos filmes preparados pelo método MPO. Um estudo da morfologia por MEV indicou a preparação de filmes com boa homogeinidade e espessura média de 150nm Os resultados obtidos neste projeto indicam que a estabilização da fase perovkita do PMN e a supressão da fase pirocloro está relacionada a uma combinação particular de parâmetros tais como tempo e temperatura de cristalização e a adição de excessos de MgO e PbO a estequiometria. / The present work describes the studies for the synthesis of ultrathin ferroelectric films of Pb(Mg1/3Nb2/3)O3 (PMN) and the systematic study of the crystallization kinetics of this material aimed at obtaining singlephase films. The films were produced from the modification of a chemical method based on oxide precursors. The obtention of the PMN films made by a technique of double stage, that allows greater control in the suppression of pyrochlore phases, and by technique via Oxide Precursors Method(OPM), through a direct reaction of the oxides. PMN films were prepared on Si(100) substrates with different numbers of depositions. These films were subjected to thermal treatments between 400ºC and 800°C, in conventional oven, under different preparation conditions. The structure of the films was investigated by the technique of X-ray diffraction and the structures refinements by the Rietveld method. The results showed a coexistence of pyrochlore phase Pb1.86Mg0.24Nb1.76O6.5 and perovskite phase Pb(Mg1/3Nb2/3)O3(PMN) in the films prepared via Columbita route and suppression of pyrochlore phase in films prepared by the OPM method. A study of morphology by SEM indicated the preparation of films with good homogeneity and medium thickness of 150 nm. The results obtained in this project suggest that the stabilization of PMN phase perovkite and the suppression of the pyrochlore phase is related to a particular combination of parameters such as time and temperature of crystallization and the addition of excess of MgO and PbO in the stoichiometry.

Doctor of Philosophy / Department of Chemistry / Jun Li / Electrical energy storage (EES) is crucial in todays’ society owing to the advances in electric cars, microelectronics, portable electronics and grid storage backup for renewable energy utilization. Lithium ion batteries (LIBs) have dominated the EES market owing to their wide use in portable electronics. Despite the success, low specific capacity and low power rates still need to be addressed to meet the increasing demands. Particularly, the low specific capacity of cathode materials is currently limiting the energy storage capability of LIBs. Vanadium pentoxide (V₂O₅) has been an emerging cathode material owing to its low cost, high electrode potential in lithium-extracted state (up to 4.0 V), and high specific capacities of 294 mAh g⁻¹ (for a 2 Li⁺/V₂O₅ insertion process) and 441 mAh g⁻¹ (for a 3 Li⁺/V₂O₅ insertion process). However, the low electrical conductivities and slow Li⁺ ion diffusion still limit the power rate of V₂O₅. To enhance the power-rate capability we construct two core-shell structures that can achieve stable 2 and 3 Li⁺ insertion at high rates. In the first approach, uniform coaxial V₂O₅ shells are coated onto electrospun carbon nanofiber (CNF) cores via pulsed electrodeposition. The materials analyses confirm that the V₂O₅ shell after 4 hours of thermal annealing at 300 °C is a partially hydrated amorphous structure. SEM and TEM images indicate that the uniform 30 to 50 nm thick V₂O₅ shell forms an intimate interface with the CNF core. Lithium insertion capacities up to 291 and 429 mAh g⁻¹ are achieved in the voltage ranges of 4.0 – 2.0 V and 4.0 – 1.5 V, respectively, which are in good agreement with the theoretical values for 2 and 3 Li⁺/V₂O₅ insertion. Moreover, after 100 cycles, remarkable retention rates of 97% and 70% are obtained for 2 and 3 Li⁺/V₂O₅ insertion, respectively. In the second approach, we implement a three-dimensional (3D) core-shell structure consisting of coaxial V₂O₅ shells sputter-coated on vertically aligned carbon nanofiber (VACNF) cores. The hydrated amorphous microporous structure in the “as-deposited” V₂O₅ shells and the particulated nano-crystalline V₂O₅ structure formed by thermal annealing are compared. The former provides remarkably high capacity of 360 and 547 mAh g⁻¹ in the voltage range of 4.0 – 2.0 V and 4.0 – 1.5 V, respectively, far exceeding the theoretical values for 2 and 3 Li⁺/V₂O₅ insertion, respectively. After 100 cycles of 3 Li⁺/V₂O₅ insertion/extraction at 0.20 A g⁻¹ (~ C/3), ~ 84% of the initial capacity is retained. After thermal annealing, the core-shell structure presents a capacity of 294 and 390 mAh g⁻¹, matching well with the theoretical values for 2 and 3 Li⁺/V₂O₅ insertion. The annealed sample shows further improved stability, with remarkable capacity retention of ~100% and ~88% for 2 and 3 Li⁺/V₂O₅ insertion/extraction. However, due to the high cost of Li. alternative approaches are currently being pursued for large scale production. Sodium ion batteries (SIB) have been at the forefront of this endeavor. Here we investigate the sodium insertion in the hydrate amorphous V₂O₅ using the VACNF core-shell structure. Electrochemical characterization was carried out in the potential ranges of 3.5 – 1.0, 4.0 – 1.5, and 4.0 – 1.0 (vs Na/Na⁺). An insertion capacity of 196 mAh g-1 is achieved in the potential range of 3.5 – 1.0 V (vs Na/Na⁺) at a rate of 250 mA g⁻¹. When the potential window is shifted upwards to 4.0 – 1.5 V (vs Na/Na⁺) an insertion capacity of 145 mAh g⁻¹ is achieved. Moreover, a coulombic efficiency of ~98% is attained at a rate of 1500 mA g⁻¹. To enhance the energy density of the VACNF-V₂O₅ core-shell structures, the potential window is expanded to 4.0 – 1.0 V (vs Na/Na⁺) which achieved an initial insertion capacity of 277 mAh g⁻¹. The results demonstrate that amorphous V₂O₅ could serve as a cathode material in future SIBs.

Made available in DSpace on 2014-06-11T19:29:06Z (GMT). No. of bitstreams: 0 Previous issue date: 2005Bitstream added on 2014-06-13T20:59:10Z : No. of bitstreams: 1 bruno_jc_me_araiq.pdf: 3796594 bytes, checksum: e4b765c9a1ed1eb7ed9759943001d22b (MD5) / Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES) / Este trabalho propõe o estudo da influência dos métodos de síntese na obtenção da cerâmica (1-x)PMN-xPT, com x = 0,10; 0,28 e 0,35. Foram investigados muitos aspectos envolvidos em cada uma das metodologias de síntese, como quantidade de fases, reatividade superficial, homogeneidade química, densidade, perda de massa e propriedades elétricas. As metodologias de síntese abordam métodos químicos, físicos e mistos, de forma a mostrar suas aplicabilidades nas propriedades de algumas composições cerâmicas do sistema, incluindo x= 0,10, onde a propriedade é relaxora, x= 0,35 com propriedade ferroelétrica, e x= 0,28 com propriedade intermediária entre as duas. Os métodos químicos atuaram favoravelmente na obtenção de pós cerâmicos mais homogêneos e com maior reatividade superficial. Para os métodos mistos, a etapa de inserção do chumbo por via química durante a síntese dos pós de PMN-PT levou a melhores resultados do que a obtenção do pó via reação de estado sólido. Por fim, a associação da composição com a aplicação do material pôde ser estabelecida, sendo demonstrado que as cerâmicas PMN-PT 28 são melhores ferroelétricos e as cerâmicas PMN-PT 35, melhores piezoelétricos. / This work aims at studying the influence of the synthesis method in the attainment of (1-x) PMN-xPT ceramics, with x = 0.10; 0.28 and 0.35. Many aspects related to each one of the synthesis methodologies had been investigated, as phase amount, superficial reactivity, chemical homogeneity, density, weigh loss and electrical properties. The synthesis methodologies concerned chemical, physical and mixed methods, in a manner of showing its applicability in the properties of some ceramic compositions of such system, including x = 0.10, for relaxor property, x = 0.35 for ferroelectric property, and x = 0.28 with an intermediate property between the two. The chemical methods have shown to be very favorable at getting more homogeneous ceramic powders and with higher superficial reactivity. For mixed methods, the stage of lead insertion via chemical method during PMN-PT powder synthesis led to better results if compared to the solid state reaction. Finally, the association between composition and material application could be established, being demonstrated that PMN-PT 28 ceramic exhibits better ferroelectric properties, while PMN-PT 35 ceramic, piezoelectric one.

Dans ce travail, nous avons étudié l’effet des contraintes extérieures et des inhomogénéités locales sur les propriétés diélectriques et structurales des cristaux ferroélectriques- relaxeurs à base de plomb PZN-x%PT avec 0%≤x≤12%. Dans une première partie, nous avons déterminé les propriétés diélectriques et structurales du système PZN-6%PT. Pour l’état vierge, ce composé subit la séquence de transition de phase C  T  R, où C, T et R sont, respectivement, les phases cubique, quadratique et rhomboédrique. En appliquant un champ électrique statique, une phase orthorhombique est induite entre les phases T et R. Dans la deuxième partie, nous avons montré la présence d’une anomalie diélectrique à basse température observée sur le PZN-x%PT avec 0%≤x≤12%. Dans ce domaine de température, l’étude structurale ne montre aucune transition de phase. L’ensemble de ces résultats sont interprétés moyennant un modèle basé sur la présence des nano-régions polaires. En troisième partie nous avons déterminé les propriétés diélectriques et piézoélectrique du PZN-12%PT dopé au manganèse dans son état monodomaine. Le dopage affecte, principalement, la permittivité transverse et le coefficient piézoélectrique de cisaillement. Le dopage induit aussi la stabilité de la structure monodomaine et l’effet de mémoire de la microstructure. Ces résultats sont expliqués en utilisant le modèle de symétrie des défauts. Dans la dernière partie, nous nous sommes intéressés à la simulation de l’effet de la présence des dipôles-défaut (dopage) sur les propriétés physiques de BaTiO3. Nous avons mis en évidence l’induction d’un champ électrique interne responsable du décalage du cycle d’hystérésis vers les champs électriques négatifs. / In this work, we studied the effect of external constraints and local inhomogeneities on dielectric and structural properties of the lead-based ferroelectric-relaxor system PZN-x%PT with 0%≤x≤12%. In the first part, we determined phase transformations of [001], [110] and [111] oriented PZN-6%PT single crystals. The un-poled samples undergo C  T  R, phase transitions sequence, where C, T and R, and are rhombohedral, tetragonal and cubic phases, respectively. Under electric field, an intermediate orthorhombic phase is induced between T and R phases. The second part was devoted to the study of the electromechanical and structural properties at cryogenic temperature of the PZN-x%PT system (0%≤x≤12%.). These characterizations show the presence of a dielectric anomaly at low temperatures. In the same temperature range, the structural study does not show any phase transition. These behaviors are interpreted through a model based on the presence of polar nano-regions. In the third part we study the influence of manganese doping on the electromechanical properties of PZN-12%PT single crystal in a single domain state. Doping leads to a decrease of the dielectric transverse permittivity  T 11 and of the shear piezoelectric coefficient d15. This intrinsic effect is discussed by using a volume effect model based on the symmetry conforming principle of point defects. This model explains also the stability of the single domain structure and the memory effect observed during this work. In the last part, we simulated, using molecular dynamics method, the effect of doping (defects-dipoles) on the physical properties of BaTiO3. The introducing of defects-dipoles induces an internal electric field, responsible for the shift of the hysteresis loop.

The present research work mainly focuses on fabrication of compositionally modulated multilayers of (l−x) Pb(Mgi/3N2/3)O3 - x PbTiO3 (PMNPT) through multi target pulsed laser ablation technique. Heterostructures like compositionally varying multilayers; multilayers with graded interface and a ferroelectric [PbTiO3 (PT)] and relaxor (PMN) superlattices of different periodicities were fabricated. Role of artificially enhanced chemical heterogeneity and strain on enhancement of physical property was studied. Dimensional dependent ferroelectric and antiferroelectric type of polarization behavior was observed in the case of both compositionally varying multilayers and the superlattice structures fabricated. The dimensional dependence of various ferroelectric interactions like long-range, short-range and interfacial coupling among the layers was studied. The phase transition behavior and dielectric studies were carried out on these heterostructures. An artificial superlattice of a relaxor ferroelectric with a ferromagnetic layer was also fabricated for magnetoelectric applications. Chapter 1 provides a brief introduction to ferroelectric (FE) heterostructures, their technological applications and the fundamental physics involved in ferroelectric heterostructures. Initially an introduction to the technological importance and advantages of ferroelectric heterostructures is provided. A brief introduction to relaxor ferroelectrics and their characteristic structural features are discussed. A brief review of the ferroelectric heterostructures both from fundamental science and technological point of view is provided. Finally the specific objectives of the current research are outlined. Chapter 2 deals with the various experimental studies carried out in this research work. It gives the details of the experimental set up and the basic operation principles of various structural and physical characterizations of the materials prepared. A brief explanation of material fabrication, structural, micro structural and physical property measurements is discussed. Chapter 3 addresses the problem of phase formation of PMNPT over platinum substrates and the role of the template over the phase formation, micro structural evolution and polarization behavior. The surface modifications of bare Pt under the processing conditions used to fabricate PMNPT was also studied. An intermediate roughening mechanism was observed. The role of LSCO over the micro structural evolution of PMNPT, the minimum thickness of LSCO required for phase formation of PMNPT, role of LSCO on phase formation and its effect on the polarization behavior of PMNPT of constant thickness are discussed. Chapter 4 deals with fabrication of different types of relaxor based heterostructures studied in this work. Three different types of PMNPT based heterostructures was fabricated using a multi target laser ablation chamber. The first type of heterostructure is a compositionally modulated multilayer thin film with four different compositions of (1-x) PMN - x PT (x = 0.0, 0.1, 0.2, 0.3 at.%) and is represented as PMNPT multilayer (ML) further in this thesis. PMNPT ML with different individual layer thickness was fabricated (30, 40, 60, 80, 100 and 120 nm). The second type of heterostructure is the PMNPT ML of same dimensions, but associated with a post deposition annealing to achieve a graded interface between the multilayers present and will be named as PMNPT graded or simply graded, further in this thesis. The third type of heterostructure is an artificial superlattice of a simple relaxor ferroelectric (PMN) and a normal ferroelectric (PT), which will be named as PMN-PT superlattice (SL) further in this thesis. The crystallinity, micro structural features and the nature of the interface present in the fabricated heterostructures were studied using various experimental techniques. Chapter 5 deals with the FE studies of compositionally modulated PMNPT ML thin films and PMNPT graded thin films. The ML with individual layer thickness of 120nm exhibited a clear FE behavior but with a reduced remnant polarization and reduced non linear behavior in capacitance - voltage (C-V) characteristics. But on varying the dimensions of the individual layers (30, 40, 60, 80, 100 and 120nm) a large dielectric tunability of around 74% was observed at lOOnm. The polarization behavior of these ML exhibited an interesting size dependent polarization behavior. A FE behavior was observed at low dimensions of 40 and 30nm. An AFE type of loop was observed at 60 and 80nm of individual layer thickness and at lOOnm it showed a clear paraelectric kind of behavior both in polarization hysteresis (P-E) and C-V studies. Graded films exhibited clear FE behavior at all dimensions fabricated and hence the role of interface in developing a critical polarization behavior in the case of ML was confirmed. Apart from the fundamental physics these ML and graded films permits the tunability of their physical properties on just varying the individual layer thickness. The dimensional dependence of dielectric tunability of ML and graded films were studied and it was found that in the case of a ML the dielectric tunability was high at lOOnm individual layer thickness and at 40nm in the case of a graded film. Thus the interfacial strain, interfacial coupling and chemical heterogeneity give an opportunity to engineer the physical property depending on the requirements. Chapter 6 deals with ferroelectric studies (P-E, C-V) of PMN-PT superlattice structures with different periodicities. The dimensional range in which, the interfacial coupling dominates the overall polarization behavior of the system was analyzed. A dimensional dependent FE and AFE behavior was observed in the PMN-PT SL structures. The dimensional dependent tunability of physical properties was achieved. The different interactions like short range, long range and the interfacial coupling and their dimensional dependent behavior was studied. The dimensional dependent tunability of the P-E and C-V behavior was observed both in symmetric and asymmetric SL structures. Chapter 7 deals with the relaxor behavior of the fabricated PMNPT ML, graded and PMN-PT SL structures. The dielectric phase transition of a PMNPT ML exhibited local maxima in the real part of dielectric constant with temperature. The local maxima correspond to the temperature regime at which, the individual layer dielectric maxima dominates the phase transition behavior of the ML structure. In the case of graded films an averaged behavior of all the compositions, with an enhanced diffusivity was observed. All the characteristic features of a relaxor ferroelectric were observed in the phase transition behavior of a graded thin film. The dielectric maxima exhibited a Vogel-Fulcher type of behavior with frequency, A similar averaged behavior was observed in the phase transition behavior of PMNPT ML at low dimensions (< 40 nm) of the individual layer. The dielectric phase transition behavior of PMN-PT SL structures of different periodicities was studied. No characteristic of a relaxor ferroelectric was observed for the periodicities in the range of 10 to 50 nm. At 60 nm periodicity the individual layer dominance was observed in the phase transition behavior of the SL structure. The phase transition behavior was found to be insensitive to the interfacial coupling in both the PMNPT ML and PMN-PT SL. Chapter 8 deals with the dielectric response, impedance spectroscopy and the DC leakage characteristics of the relaxor heterostructures. All the relaxor heterostructures fabricated, exhibited low frequency dispersion, similar to that of the Jonscher's universal type of relaxation behavior. The anomalous dispersion common of a relaxor ferroelectric was observed in the imaginary dielectric constant at high frequencies. A.multi debye type of relaxation behavior was observed in the impedance analysis and the relaxation time was found to obey Vogel-Fulcher type of relation with temperature. The leakage current of all the heterostructures were found to be few orders less than the homogeneous single layer thin films. A space charge limited conduction was observed in all the heterostructures fabricated. Chapter 9 deals with an attempt of realizing the magnetoelectric effect in an artificial superlattice structure consisting ferromagnetic [Lao.6Sro.4Mn03 (LSMO)] and ferroelectric (PMNPT 70-30) layers. Both symmetric and asymmetric SL structures were fabricated and the asymmetric SL exhibited both room temperature ferromagnetic and ferroelectric behavior. A weak influence of magnetic field over the polarization behavior was observed. The magnetic behavior and its influence over electrical behavior were found to be dominated by the interface and were confirmed from the Maxwell-Wagner type of relaxation. Chapter 10 gives the summary and conclusions of the present study and also discusses about the future work that could give more insight into the understanding of the relaxor heterostructures.

Electron emission from ferroelectric materials is initiated by a variation of the spontaneous polarization. It is the main focus of this work to develop ferroelectric cathodes, which are characterized by a significantly decreased excitation voltage required to initiate the electron emission process. Particular attention is paid to the impact of the polarization on the emission process. Two materials are investigated. Firstly, relaxor ferroelectric lead magnesium niobate - lead titanate (PMN-PT) single crystals are chosen because of their low intrinsic coercive field. Electron emission current densities up to 5 · 10^(−5) A/cm² are achieved for excitation voltages of 160 V. A strong enhancement of the emission current is revealed for the onset of a complete polarization reversal. Secondly, lead zirconate titanate (PZT) thin films are investigated. A new method to prepare top electrodes with sub-micrometer sized, regularly patterned apertures is introduced and a stable electron emission signal is measured from these structures for switching voltages < 20 V. Furthermore, a detailed analysis of the polarization switching process in the PMN-PT samples is given, revealing a spatial rotation of the polarization vector into crystallographic easy axes, as well as the nucleation of reversed nano-domains. Both processes are initiated at field strengths well below the coercive field. The dynamics of the polarization reversal are correlated to the electron emission measurements, thus making it possible to optimize the efficiency of the investigated cathodes. / Die Ursache für Elektronenemission aus ferroelektrischen Materialien ist eine Veränderung des Zustandes der spontanen Polarisation. Gegenstand der vorliegenden Arbeit ist eine Verringerung der dafür nötigen Anregungsspannung, wobei besonderes Augenmerk auf die Rolle der ferroelektrischen Polarisation innerhalb des Emissionsprozesses gelegt wird. Es werden zwei verschiedene Materialien untersucht. Das Relaxor-Ferroelektrikum Bleimagnesiumniobat - Bleititanat (PMN-PT) wurde aufgrund seines geringen Koerzitivfeldes ausgewählt. Es konnten Emissionsstromdichten von bis zu 5·10^(−5) A/cm² bei einer Anregungsspannung von 160 V erreicht werden. Bei Einsetzen eines vollständigen Umschaltens der Polarisation wurde eine deutliche Verstärkung des Emissionsstromes festgestellt. Desweiteren werden Untersuchungen an Bleizirkoniumtitanat (PZT) Dünnfilmen gezeigt. Eine neue Methode, eine Elektrode mit periodisch angeordneten Aperturen im Submikrometerbereich zu präparieren, wird vorgestellt. Diese Strukturen liefern ein stabiles Emissionssignal für Anregungsspannungen < 20 V. Eine detailierte Analyse des Schaltverhaltens der Polarisation der PMN-PT Proben zeigt sowohl eine Rotation des Polarisationsvektors als auch eine Nukleation umgeschaltener Nanodomänen. Beide Prozesse starten bei Feldstärken unterhalb des Koerzitivfeldes. Die ermittelte Zeitabhängigkeit des Schaltprozesses erlaubt Rückschlüsse auf den Emissionsprozess und erlaubt es, die Effizienz der untersuchten Kathoden weiter zu optimieren.

This thesis deals with the investigations on the dielectric properties of polycrystalline ceramics having uncommon substitutions in barium titanate and other related phases of BaTiO3-CaTiO3, MgTiO3-CaTiO3 and MgTiO3-BaTiO3 systems. After presenting a brief introduction on the ceramic materials studied in terms of their crystal structures, electrical properties, nonstoichiometry and microstructural characteristics. The thesis describes the synthesis of the ceramics and the methodology of different techniques utilized in characterizing the samples. Barium calcium titanate was synthesized through novel wet chemical techniques and the dielectric properties of calcium substituted barium titanate do not reveal multi-site occupancy whereas they are predominantly influenced by the A/B cationic ratio. The role of transition metals of the 3d series from vanadium (Z=23) to zinc (Z=30) in modifying the crystallographic phase content, microstructure and the dielectric properties of BaTiO3 ceramics containing 10 at% impurities were studied. All the transition metals brought about the phase conversion to hexagonal BaTiO3, although no systematics could be arrived at relating the hexagonal content to the 3d electronic configuration of the impurities. The relaxor dielectrics arising from the titanate solid solution with uncommon substitution and its interconversion to normal ferroelectrics is studied. The effects of cationic substitutions of iron and niobium for titanium in BaTiO3 pervoskite lattice in crystal symmetry and dielectric properties were investigated. The above dielectric characteristics are comparable in a converse way to those of the well known Pb(Mg1/3Nb2/3)O3-PbTiO3 system wherein the relaxor behaviour occurs within the lower lead titanate compositional limits. The modification in -T characteristics of positive temperature coefficient in resistance (PTCR) by the addition of segregative additives such as B2O3, Al2O3 etc in BaTiO3 and its conversion to grain boundary layer capacitance is studied. The presence of Al-related hole centers at the grain boundary regions resulted in charge redistribution across the modified phase transition temperatures due to symmetry-related vibronic interactions, which result in broad PTCR characteristics extending to higher temperatures. The processing of high permittivity ceramics by the manipulation of microstructural features in semiconducting BaTiO3 is studied wherein the grain boundary layer effect superimposed with the contributions from the barrier layers formed during electroding related to microstructure is proposed to be responsible for the unusual high permittivity in semiconducting BaTiO3. The influence of Mg2+ as a substituent in modifying the crystallographic phase contents, microstructure and the dielectric properties of (Ba1-xMgx)TiO3 ceramics, (x ranging from zero to 1.0 ) is studied. The results point to the dual occupancy of Mg2+ both in A and B sublattice and the role of oxygen vacancy as well as (Ti3+ –VO) defects in stabilization of hexagonal phase to lower temperatures. The microwave dielectrics of the BaMg6Ti6O19 phase formed in the compositional range of x=0.4 to 0.7 was investigated for suitable application in microwave dielectrics. Extensive miscibility between the ilmenite-type MgTiO3 and perovskite-type CaTiO3 over a wide compositional range is brought about by the simultaneous equivalent substitution of Al3+ and La3+. The resulting Mg1-(x+y)CaxLay)(Ti1-yAly)O3 ceramics exhibit improved microwave dielectric properties by way of high permittivity, low TCK and high quality factor. The elemental distribution reveals the complexity in the Mg/Ca distribution and its correlation with the solid state miscibility as well as dielectric properties. Microwave dielectric property of Mg4Al2Ti9O25 which is detected as secondary phase is studied in detail.