Rozprawy doktorskie na temat „Lead based Piezoelectrics”

In many commercial devices, there are a number of ways in which energy is wasted or dissipated. This waste energy can, in principle, be harvested by using the correct functional material. In the case of vibrational energy, the logical materials of choice are piezoelectric ceramics. However, all current commercial piezoelectrics contain lead oxide which is classed as a restricted material in environmental legislation. The main contenders for lead free piezoelectrics are based on K0.5Na0.5NbO3 (KNN) and Na0.5Bi0.5TiO3. The former however, has the advantage in that it is compatible with cheap Ni-based internal electrodes and thus it is feasible to manufacture low cost KNN based multilayer devices provided formulations do not deteriorate in the reducing condition required to suppress the formation of NiO. Consequently, KNN based lead free piezoelectric ceramics have been studied from the perspective of optimising their performance for multilayer actuators, potentially for energy harvesting applications. To this end, the defect chemistry of KNN has been investigated under different sintering conditions, dopants (acceptors: Mn2+, Ti4+, Sn4+ in KNN_50/50 ratio; Donor: Sr2+ in KNN_50/50 ratio; Ta5+ as an isovalent in KNN-51/49 ratio; and co-dopants: Bi3+ and Zr4+ in KNN_50/50 ratio) have been incorporated into KNN to enhance the piezoelectric performance and prototype multilayers of 10 and 16 layers with inner Pt electrodes have been fabricated to demonstrate the potential of 0.942KNN-0.058BNZ+ZrO2 for the fabrication of multilayer actuators. This lead free composition has the potential to replace PZT-4 and PZT-8 in piezoelectric devices for room temperature applications. To fabricate the multilayers, a novel Wet-Multilayer-Method (WMM) was also developed to overcome the issues of delamination during firing of MLCCs.

The prime objective of this dissertation is to design, synthesize and characterize lead-free piezoelectric based magnetostrictive components based magnetoelectric (ME) composites that exhibit self-bias characteristics and high amplitude of ME coupling. The secondary goal of this thesis was to lay down the foundation for nanoscale and flexible magnetoelectric devices.
Self-biased ME effect was investigated in lead-free three-phase laminate composites. This effect is characterized by non-zero remanent ME responses at zero magnetic bias field (Hbias). It was revealed that the self-biased ME effects can be observed in three-phase laminate composites consisting of piezoelectric material and two dissimilar magnetostrictive materials. On applying Hbias to the laminates in bending mode configuration, the ME responses were found to exhibit hysteretic behaviors with remanent ME responses. The shape of hysteretic ME response could be controlled by adjusting the magnetic interactions and piezoelectric properties. Further, converse magnetoelectric (CME) responses in bending-mode three-phase laminates exhibited hysteretic behaviors with similar magnitudes during Hbias sweep as it was generated directly by applying ac voltage (Vac) without any external Hbias.
Lead-free (1 - x) [0.948 K0.5Na0.5NbO3 - 0.052 LiSbO3] - x Ni0.8Zn0.2Fe2O4 (KNNLS-NZF) compositions were synthesized for optimizing ME properties of particulate composites. Island-matrix microstructure was developed to improve the magnitude of ME coupling effect by "

overcoming the problems found in conventional particulate composites. The structure lead to improvement of ME coefficient with maximum magnitude of 20.14 mV/cm ae as well as decrease of optimum Hbias of < 500 Oe in the composition of 0.7 KNNLS - 0.3 NZF particulate composites.
Room-temperature ME phase diagram of (1 - x) BaTiO3 - x BiFeO3 materials (BT - x BFO, x = 0.025 - 1.0) was investigated for designing compositions suitable for thin film devices. The BT - x BFO compositions in narrow range of x = 0.71 - 0.8 were found to exhibit good piezoelectric, dielectric and magnetic properties simultaneously. The room temperature ME coefficient was found to be maximum with high magnitude of 0.87 mV/cmOe in the optimized composition of x = 0.725.This composition was found to consist of local monoclinic distortions with average rhombohedral symmetry as confirmed by detailed structural analysis through Raman spectroscopy and atomic pair distribution functions (PDFs).
MnFe2O4 (MFO)-Ni core-shell nanoparticles were synthesized and characterized for developing tunable devices such as memristor. The MFO nanoparticles synthesized by solvothermal method exhibited diameter of 200 nm, mean primary particle size of 15 nm, high saturation magnetization of 74 emu/g and coercivity of 89 Oe. Ni encapsulation on MFO nanoparticles was performed by aqueous ionic coating method. Ni shells with uniform thickness of 1 nm were coated on MFO nanoparticles by this method.
In order to develop future nanoscale dual phase energy harvesters and magnetic field sensors, vertically-aligned piezoelectric nanorods were synthesized. In the initial attempt, Pb(Zr0.52Ti0.48)O3 (PZT) was used to verify the feasibility of developing one dimensional (1D) piezoelectric nanostructures with controlled diameter and height. For the 1D nanostructure, well-ordered anodic aluminum oxide (AAO) templates were prepared by two step aluminum anodizing. The PZT nanorods were synthesized by vacuum infiltration of PZT precursor solutions and exhibited uniform diameter of 90 nm and aspect ratio of 10 with vertical in respect to the Pt-Si substrate. The piezo-response of PZT nanorods showed good magnitude owing to the reduced clamping effect from the substrate.
Attempt towards the development of flexible tunable devices that possess magnetic field sensing and actuation ability was made in the later part of the thesis. The electroactive polymeric actuators in the form of Polypyrrole (PPy) / Au / Polyvinylidene fluoride (PVDF) / Au / Polypyrrole (PPy) were synthesized and the process flow was optimized. Pore size and thickness of PVDF layer was adjusted by changing the solvent, viscosity and drying temperature. Different types of electrolyte solutions were investigated to improve the strain and response time. The actuators exhibited high deflection of 90 % with fast response of 50% deflection per second. Dual-functional structure in the form of  PPy-MFO / Au / PVDF / Au / PPy-MFO was developed by PPy polymerization including MFO nanoparticles via cyclovoltammetric method.
Ph. D.

This thesis is concerned with the fabrication and characterisation of lead free piezocomposites and transducers for use in high frequency medical ultrasound imaging applications. A water based gel casting and micro moulding approach has been developed to fabricate 1-3 composites with a random pillar structure in the lead free and lead based piezoelectric material. High frequency transducers incorporating the random composites as the active components have been fabricated, characterised and demonstrated in real tissue imaging environments. A water based gel casting system has been used incorporating Hydantoin Epoxy resin, amine hardener (Bis (3-aminoproply) amine) and dispersant. Viscosities of the 50BCZT and PZT systems were minimised by the addition of 2.4 and 1 wt% of dispersant respectively. The highest values of piezoelectric and dielectric properties corresponded to 50BCZT samples fabricated with a gel casting slurry incorporating 30 wt% resin and sintered at 1425 °C, with d33 and kp values of 330 pC/N and 0.43, respectively. 1-3 composites were successfully fabricated from the BCZT and PZT bristle block structures and only one resonance peak corresponding to the thickness mode was observed. PZT composites offered generally higher thickness coupling coefficients than 50BCZT composites, where the highest value of 0.78 was measured for samples sintered at temperature 1425 °C. Focused PZT, focused 50BCZT, unfocussed PZT and unfocussed 50BCZT transducers were successfully fabricated using the composites with randomised structure, and have operating frequencies of 35, 40, 50 and 35 MHz respectively.

The review of current technologies for measurement of gas velocity in stack flow applications is undertaken and it is shown that the ultrasonic time-of-flight method is the most suitable and offers a number of advantages over alternatives. Weakness of current piezoelectric based transducers are identified as the inability to operate at temperatures above 400 °C due to limitation of piezoelectric materials used, and a case for development of an alternative high temperature material is put forward. A novel and highly enhanced, lead free piezoelectric material, suitable for continuous operation at temperatures above 400 °C has been engineered for ultrasonic gas velocity sensor applications. Structural modification of pure bismuth titanate (Bi4Ti3O12) or BIT compound, through multi-doping at the Ti-site, has been found to enhance piezoelectric properties accompanied with a mild reduction in Curie temperature, Tc. Initially, compounds doped with tungsten and chromium were found to increase the piezoelectric coefficient (d33) from around 5 pC N¯¹ in pure bismuth titanate, to above 20 pC N¯¹ in doped compounds. This increase is attributed to lower conductivity and improved poling conditions. Further increases in d33 (up to 35 pC N¯¹) were then realised through controlled grain growth and reduction in conductivity for niobium, tantalum and antimony doped compounds. The Curie temperature of the material with best properties is found to be 667 °C, which is a slight reduction from 675 °C for pure bismuth titanate ceramic. The enhancements in modified bismuth titanate achieved in present work allow the material to be considered as suitable for high temperature ultrasonic transducer applications. Integration of bismuth titanate material into a working high temperature transducer is then considered and the investigation of suitable, high temperature bonding method is undertaken. It is shown that reactivity of bismuth titanate with the titanium based fillers makes brazing unsuitable as a bonding method between piezo-ceramics and stainless steel. A novel assembly method, using liquid gallium as an electrically conductive bond, and a mechanical restraint for the piezo actuator is then presented as an alternative with the potential to reduce the negative effects of differences in thermal expansion coefficients between constituents of the transducer assembly.

This PhD project is focused on three lead-free ferroelectric solid solutions, which are specifically Na0.5Bi0.5TiO3-KNbO3(NBT-KN), Na0.5Bi0.5TiO3-NaNbO3(NBT-NN) and Na0.5Bi0.5TiO3-BaTiO3(NBT-BT), to evaluate the effects of composition, electric field and temperature on structural and electrical properties. Novel observations of both reversible and irreversible electric field-induced phase switching were made in both NBT-KN and NBT-NN ceramics. The NBT-KN solid solution is the primary focus of this thesis. All compositions were observed to be cubic in the as-sintered, unpoled state. However, a well-defined ferroelectric hysteresis P-E loop was obtained for compositions with low KN contents, indicating that an irreversible phase transition from a weak-polar relaxor ferroelectric (RF) to a long-range ordered metastable ferroelectric (FE) state had occurred during the measurement procedure. Both the unpoled and poled ceramic powders were examined using high resolution synchrotron XRD. For the poled state, a rhombohedral R3c structure was identified for compositions with low KN content, confirming the occurrence of the irreversible electric field-induced structural transformation from cubic to rhombohedral. In contrast, a cubic structure was retained for high KN contents, giving rise to reversible phase switching evidenced by constricted P-E hysteresis loops. Similar behaviour was observed for NBT-NN system. An 'in-situ' electric field poling experiment was conducted using high energy synchrotron XRD. In certain NBT-KN compositions the structural transformation, from cubic to mixed phase cubic+rhombohedral and finally single phase rhombohedral, occurred progressively with increasing cycles of a bipolar electric field. Similar behaviour was observed for NBT-NN compositions having low NN contents. Furthermore, the distributions of domain orientation and lattice strain over a range of orientations relative to the poling direction were determined for NBT-KN, NBT-NN and NBT-BT ceramics exhibiting the rhombohedral phase. By combining the structural information with the results of dielectric and ferroelectric measurements, a phase diagram was constructed to illustrate the influence of temperature and composition on the stability of the metastable ferroelectric and relaxor ferroelectric states for the NBT-KN system. Furthermore, the phase transition temperatures obtained from dielectric measurements were correlated with the ferroelectric and thermal depolarisation characteristics for each of the NBT-KN, NBT-NN and NBT-BT systems.

Doutoramento em Ciência e Engenharia de Materiais
K0.5Na0.5NbO3 (KNN), is the most promising lead free material for substituting lead zirconate titanate (PZT) which is still the market leader used for sensors and actuators. To make KNN a real competitor, it is necessary to understand and to improve its properties. This goal is pursued in the present work via different approaches aiming to study KNN intrinsic properties and then to identify appropriate strategies like doping and texturing for designing better KNN materials for an intended application. Hence, polycrystalline KNN ceramics (undoped, non-stoichiometric; NST and doped), high-quality KNN single crystals and textured KNN based ceramics were successfully synthesized and characterized in this work. Polycrystalline undoped, non-stoichiometric (NST) and Mn doped KNN ceramics were prepared by conventional ceramic processing. Structure, microstructure and electrical properties were measured. It was observed that the window for mono-phasic compositions was very narrow for both NST ceramics and Mn doped ceramics. For NST ceramics the variation of A/B ratio influenced the polarization (P-E) hysteresis loop and better piezoelectric and dielectric responses could be found for small stoichiometry deviations (A/B = 0.97). Regarding Mn doping, as compared to undoped KNN which showed leaky polarization (P-E) hysteresis loops, B-site Mn doped ceramics showed a well saturated, less-leaky hysteresis loop and a significant properties improvement. Impedance spectroscopy was used to assess the role of Mn and a relation between charge transport – defects and ferroelectric response in K0.5Na0.5NbO3 (KNN) and Mn doped KNN ceramics could be established. At room temperature the conduction in KNN which is associated with holes transport is suppressed by Mn doping. Hence Mn addition increases the resistivity of the ceramic, which proved to be very helpful for improving the saturation of the P-E loop. At high temperatures the conduction is dominated by the motion of ionized oxygen vacancies whose concentration increases with Mn doping. Single crystals of potassium sodium niobate (KNN) were grown by a modified high temperature flux method. A boron-modified flux was used to obtain the crystals at a relatively low temperature. XRD, EDS and ICP analysis proved the chemical and crystallographic quality of the crystals. The grown KNN crystals exhibit higher dielectric permittivity (29,100) at the tetragonal-to-cubic phase transition temperature, higher remnant polarization (19.4 μC/cm2) and piezoelectric coefficient (160 pC/N) when compared with the standard KNN ceramics. KNN single crystals domain structure was characterized for the first time by piezoforce response microscopy. It could be observed that <001> - oriented potassium sodium niobate (KNN) single crystals reveal a long range ordered domain pattern of parallel 180° domains with zig-zag 90° domains. From the comparison of KNN Single crystals to ceramics, It is argued that the presence in KNN single crystal (and absence in KNN ceramics) of such a long range order specific domain pattern that is its fingerprint accounts for the improved properties of single crystals. These results have broad implications for the expanded use of KNN materials, by establishing a relation between the domain patterns and the dielectric and ferroelectric response of single crystals and ceramics and by indicating ways of achieving maximised properties in KNN materials. Polarized Raman analysis of ferroelectric potassium sodium niobate (K0.5Na0.5)NbO3 (KNN) single crystals was performed. For the first time, an evidence is provided that supports the assignment of KNN single crystals structure to the monoclinic symmetry at room temperature. Intensities of A′, A″ and mixed A′+A″ phonons have been theoretically calculated and compared with the experimental data in dependence of crystal rotation, which allowed the precise determination of the Raman tensor coefficients for (non-leaking) modes in monoclinic KNN. In relation to the previous literature, this study clarifies that assigning monoclinic phase is more suitable than the orthorhombic one. In addition, this study is the basis for non-destructive assessments of domain distribution by Raman spectroscopy in KNN-based lead-free ferroelectrics with complex structures. Searching a deeper understanding of the electrical behaviour of both KNN single crystal and polycrystalline materials for the sake of designing optimized KNN materials, a comparative study at the level of charge transport and point defects was carried out by impedance spectroscopy. KNN single crystals showed lower conductivity than polycrystals from room temperature up to 200 ºC, but above this temperature polycrystalline KNN displays lower conductivity. The low temperature (T < 200 ºC) behaviour reflects the different processing conditions of both ceramics and single crystals, which account for less defects prone to charge transport in the case of single crystals. As temperature increases (T > 200 ºC) single crystals become more conductive than polycrystalline samples, in which grain boundaries act as barriers to charge transport. For even higher temperatures the conductivity difference between both is increased due to the contribution of ionic conduction in single crystals. Indeed the values of activation energy calculated to the high temperature range (T > 300 ºC) were 1.60 and 0.97 eV, confirming the charge transport due to ionic conduction and ionized oxygen vacancies in single crystals and polycrystalline KNN, respectively. It is suggested that single crystals with low defects content and improved electromechanical properties could be a better choice for room temperature applications, though at high temperatures less conductive ceramics may be the choice, depending on the targeted use. Aiming at engineering the properties of KNN polycrystals towards the performance of single crystals, the preparation and properties study of (001) – oriented (K0.5Na0.5)0.98Li0.02NbO3 (KNNL) ceramics obtained by templated grain growth (TGG) using KNN single crystals as templates was undertaken. The choice of KNN single crystals templates is related with their better properties and to their unique domain structure which were envisaged as a tool for templating better properties in KNN ceramics too. X-ray diffraction analysis revealed for the templated ceramics a monoclinic structure at room temperature and a Lotgering factor (f) of 40% which confirmed texture development. These textured ceramics exhibit a long range ordered domain pattern consisting of 90º and 180º domains, similar to the one observed in the single crystals. Enhanced dielectric (13017 at TC), ferroelectric (2Pr = 42.8 μC/cm2) and piezoelectric (d33 = 280 pC/N) properties are observed for textured KNNL ceramics as compared to the randomly oriented ones. This behaviour is suggested to be due to the long range ordered domain patterns observed in the textured ceramics. The obtained results as compared with the data previously reported on texture KNN based ceramics confirm that superior properties were found due to ordered repeated domain pattern. This study provides an useful approach towards properties improvement of KNN-based piezoelectric ceramics. Overall, the present results bring a significant contribution to the pool of knowledge on the properties of sodium potassium niobate materials: a relation between the domain patterns and di-, ferro-, and piezo-electric response of single crystals and ceramics was demonstrated and ways of engineering maximised properties in KNN materials, for example by texturing were established. This contribution is envisaged to have broad implications for the expanded use of KNN over the alternative lead-based materials.
O niobato de sódio e de potássio, K0.5Na0.5NbO3 (KNN), é o material isento de chumbo mais promissor para substituir o titanato zirconato de chumbo (PZT), que ainda é o líder de mercado utilizado para sensores e actuadores. Para tornar o KNN verdadeiramente competitivo, é necessário compreender e melhorar as suas propriedades. Esse objectivo é perseguido no presente trabalho através de diferentes abordagens, visando o estudo das propriedades intrínsecas do KNN e a subsequente identificação de estratégias apropriadas, como por exemplo a dopagem e a texturização, para desenhar melhores materiais à base de KNN para as aplicações pretendidas. Assim, no presente trabalho, fabricaram-se e caracterizaram-se cerâmicos de KNN dopado e não dopado, de KNN não estequiométrico e de KNN texturizado. Adicionalmente cresceram-se e caracterizaram-se cristais simples de KNN de elevada qualidade. Os cerâmicos de KNN (não dopado, dopado com Mn e não-estequiométrico (NST)) foram preparados pelo método convencional de mistura de óxidos, tendo-se subsequentemente medido as suas propriedades eléctricas e analisadas as respectivas estruturas e microestruturas. No caso dos cerâmicos dopados com Mn bem como no dos cerâmicos NST verificou-se existir uma estreita janela de composição monofásica associada à dopagem e não estequiometria na posição-A. Nos cerâmicos NST a variação da razão (A/B) influencia o ciclo de histerese da polarização ferroeléctrica (P-E), verificandose a obtenção de respostas dieléctricas e piezoeléctricas melhoradas para pequenos desvios da estequiometria (A/B = 0.97). No que se refere ao KNN dopado com Mn, quando comparado com o KNN não dopado cujos ciclos de histerese são não saturados, verificou-se que a dopagem no lugar B conduz a uma curva (P-E) mais saturada e a uma melhoria significativa de propriedades. Usou-se a espectroscopia de impedância para esclarecer o papel do Mn, tendo-se estabelecido uma correlação entre defeitos/transporte de carga e a resposta ferroeléctrica do K0.5Na0.5NbO3 (KNN) e do KNN dopado com Mn. À temperatura ambiente a condução eléctrica no KNN, associada ao transporte por buracos, é minimizada pela dopagem com Mn. A adição de Mn incrementa assim a resistividade do cerâmico, o que permite melhorar a saturação do ciclo (P-E). A temperaturas elevadas a condução passa a ser dominada pela movimento de lacunas de oxigénio ionizadas cuja concentração aumenta com a dopagem com Mn. Preparam-se também cristais simples de KNN recorrendo-se a um método de fluxo de alta temperatura. Usou-se um fluxo modificado com adição de B2O3 para crescer cristais a uma temperatura relativamente baixa. Caracterizou-se a qualidade química e cristalográfica dos cristais por análise de DRX, EDS e ICP. Os cristais obtidos exibiram propriedades com valores elevados, designadamente uma permitividade dieléctrica de 29,100 à temperatura de transição da fase tetragonal para fase cúbica, uma polarização remanescente 19,4 μC/cm2 e um coeficiente piezoeléctrico de 160 pC/N, valores estes superiores aos dos cerâmicos convencionais de KNN. Usou-se pela primeira vez a microscopia de força piezoeléctrica para caracterizar a estrutura de domínios dos monocristais de KNN. Foi possível observar que os cristais simples de KNN orientados segundo <001>, evidenciaram um padrão de estrutura de domínios, com domínios de 180º dispostos paralelamente e domínios de 90º dispostos em zig-zag. Com base na comparação entre cristais e cerâmicos de KNN é possível sustentar-se que a presença nos cristais simples de um tal padrão de domínios ordenados com longo alcance, ausente nos cerâmicos, é responsável pelas propriedades melhoradas dos cristais simples de KNN. Espera-se que os presentes resultados, ao estabelecerem uma relação entre o padrão de estrutura de domínios, uma espécie de impressão digital, e a resposta dielétrica e ferroelétrica dos cristais simples e ao indicarem vias para se atingirem propriedades maximizadas em materiais de KNN, venham a ter fortes implicações na expansão do uso dos materiais de KNN. Caracterizaram-se também os monocristais ferroeléctricos de KNN por espectroscopia de Raman, obtendo-se pela primeira vez evidências que permitem a atribuir a estrutura cristalina de simetria monoclínica ao KNN. As intensidades dos fonões A′ , A" e A' + A" foram calculadas teoricamente e comparadas com os dados experimentais em função da rotação de cristal, o que permitiu a determinação precisa dos coeficientes do tensor Raman para modos (non-leaking) em KNN monoclínico. No contexto da literatura este estudo confirma que a atribuição da simetria monoclínica é mais adequada do que a ortorrômbica. Este estudo constitui ainda uma base para a avaliação não-destrutiva da distribuição de domínios por espectroscopia Raman em materiais ferroelétricos isentos de chumbo, à base de KNN e com estruturas complexas. Procurando aprofundar a compreensão do comportamento eléctrico dos cristais simples e dos cerâmicos de KNN, com o objectivo de desenhar materiais com propriedades optimizadas, realizou-se um estudo comparativo ao nível de defeitos e transporte de carga, usando-se para tal a espectroscopia de impedância. Os monocristais apresentam menor condutividade do que os materiais policristalinos homólogos para temperaturas até 200 ºC ao passo que, acima desta temperatura, são os materiais policristalinos quem apresenta menor condutividade. O comportamento de baixa temperatura (T < 200 ºC) reflecte as diferentes condições de processamento dos cerâmicos e dos cristais, que são responsáveis pelo menor teor de defeitos transportadores de carga no caso dos cristais simples. À medida que a temperatura aumenta, (T > 200 ºC) os monocristais tornam-se agora mais condutores do que as amostras policristalinas nas quais as fronteiras de grão actuam como barreiras ao transporte de carga eléctrica. Para temperaturas ainda mais elevadas a diferença de condutividade entre cristais e cerâmicos é incrementada devido à contribuição da condução iónica nos cristais. Efectivamente, para a gama de temperatura elevada (T > 300 ºC),calcularam-se valores da energia de activação de 1,60 e 0,97 eV que confirmam um transporte de carga associado a condução iónica e a lacunas de oxigénio ionizadas, em cristais simples e em cerâmicos, respectivamente. Sugere-se assim que, dependendo da aplicação em em vista, os cristais, com baixo teor de defeitos e propriedades electromecânicas melhoradas serão uma escolha indicada para aplicações a temperaturas próximas da temperatura ambiente ao passo que, para altas temperaturas, os cerâmicos, menos condutores, serão a opção mais indicada. Com o objectivo de desenhar as propriedades dos materiais policristalinos de KNN na mira de um desempenho semelhante ao dos cristais simples, prepararam-se e estudaram-se as propriedades de cerâmicos de (K0.5Na0.5)0.98Li0.02NbO3 (KNNL) com orientação (00l), usando cristais simples de KNN como partículas modelo para produzir cerâmicos texturizados por crescimento de grão modelado ( do inglês “template grain growth”). A escolha dos cristais simples como partículas modelo baseou-se no facto destas possuírem boas propriedades, aqui usadas como ferramenta indutora de melhores propriedades nos cerâmicos de KNN. A análise DRX revelou que os cerâmicos preparados com partículas modelo evidenciavam uma estrutura monoclínica à temperatura ambiente e um fator de Lotgering (f) de 40 %, o que confirma o desenvolvimento de textura cristalográfica. Estes cerâmicos texturizados apresentam um padrão de domínios ordenado com longo alcance que consiste em domínios de 90º e de 180º, semelhante ao observado nos cristais simples. Observaram-se valores elevados de constante dieléctrica (13017 na transição de fase C/T), de polarização ferroelétrica (2Pr = 42,8 μC/cm2) e de coeficiente piezoelétrico (d33 = 280 pC/N ) nos cerâmicos KNNL texturizados, quando comparados com os cerâmicos não orientados. Sugerese que esta resposta eléctrica se deve ao padrão de domínioordenados, observado nas amostras texturizadas. Os resultados obtidos, quando comparados com dados anteriormente reportados para cerâmicos de KNN texturizados confirmam a superioridade das propriedades obtidas, que se atribui à estrutura de domíneos observada. Este estudo fornece uma abordagem que pode ser de grande utilidade para a melhoria das propriedades dos cerâmicos piezoelétricos à base de KNN. Globalmente considerados, os presentes resultados configuram um importante contributo para o conjunto dos conhecimentos sobre as propriedades do niobato de sódio e de potássio: demonstrou-se que existe uma relação entre o padrão de estrutura de domínios e a resposta dieléctrica, ferroeléctrica e piezoeléctrica de cristais simples e de cerâmicos de KNN e apontou-se uma via para a melhoria das propriedades dos cerâmicos através da texturização. Prevê-se assim que este contributo tenha um impacto significativo na viabilização do uso generalizado do KNN em detrimento dos materiais à base de chumbo.

Lead-based piezoelectric materials, such as PbZrxTi1-xO3 (PZT), have attracted considerable attention and have been widely used in actuators, sensors and transducers due to their excellent electric properties. However, considering the toxicity of lead and its oxides, environmentally friendly lead-free piezoelectric materials are attracting more attention as potential replacements for PZT. Among them, Bi0.5Na0.5TiO3 (BNT)-based materials exhibit good electrical properties and electromechanical coupling response. In this work, the 0.97Bi0.5Na0.5TiO3-0.03BiAlO3 (BNTBA) thin films (~120 nm thickness) were successfully prepared using the pulsed laser deposition (PLD) method on Pt/TiO2/SiO2/Si substrates. The effects of substrate temperature, oxygen pressure, laser repetition rate, and post-annealing treatment were investigated. X-ray diffraction (XRD) and scanning electron microscope (SEM) are used to study the structure of the films and the ferroelectric and dielectric properties are measured. The results show that it is necessary to introduce excess sodium and bismuth to compensate for their evaporation in further thermal treatment. The values of remnant polarization increase from 8.7 μC/cm2 to 12.3 μC/cm2 with the introduction BiAlO3. The dielectric constant increases from 600-550 to 710-600 and the dielectric loss increases from 4.2% to 6.7% at higher frequency when the oxygen pressure increases from 20 Pa to 30 Pa.
Blybaserade piezoelektriska material, såsom PbZrxTi1-xO3 (PZT), har väckt stor uppmärksamhet och har använts i stor utsträckning på grund av deras utmärkta elektriska egenskaper. Men med tanke på toxiciteten hos bly och dess oxider lockar miljövänliga blyfria piezoelektriska material mer uppmärksamhet från forskare som potentiella utbyten för PZT. Bland dem uppvisar Bi0.5Na0.5TiO3 (BNT) -baserade material bra elektriska egenskaper och elektromekanisk kopplingssvar. I detta arbete framställdes 0,97Bi0.5Na0.5TiO3-0.03BiAlO3 (BNTBA) tunna filmer (~ 120 nm tjocklek) med användning av pulserad laseravsättningsmetod på Pt / TiO2 / SiO2 / Si-substrat. Effekterna av substrattemperatur, syretryck, laserrepetitionshastighet och efterglödande behandling undersöktes. Röntgendiffraktions (XRD) och skanningelektronmikroskop (SEM) används för att studera filmens struktur och de ferroelektriska och dielektriska egenskaperna mäts. Resultaten visar att det är nödvändigt att införa överskott av natrium och vismut för att kompensera för deras avdunstning vid vidare termisk behandling. Värdena för återstående polarisation ökar från 8,7 μC / cm2 till 12,3 μC / cm2 med introduktionen BiAlO3. Den dielektriska konstanten ökar från 600-550 till 710-600 och den dielektriska förlusten ökar från 4,2% till 6,7% vid högre frekvens när syretrycket ökar från 20 Pa till 30 Pa.

Ferroelectrics are important class of functional materials that finds application in transducers, actuators, memory devices, electro-optics and position sensing devices. Until recently, the high performance piezoelectrics were based on lead-based alloys such as Pb(Zr,Ti)O3(PZT), Pb(Mg1/3Nb2/3)O3-PbTiO3(PMN-PT) and Pb(Zn1/3Nb2/3)O3-PbTiO3(PZN-PT). However, the presence of ~60% of toxic lead(Pb) in PZT based commercial piezoelectric materials has raised serious health and environmental concerns and legislation are being framed world wide to free industrial products from toxic elements. As a consequence, there is now greater emphasis on research on lead free electronic materials. BaTiO3, (Na0.5Bi0.5)TiO3 and KNbO3 are the well known lead-free ferroelectrics compounds which can be used for designing a lead free materials with improved piezoelectric properties. It is interesting to note that among pure ferroelectric compounds, BaTiO3 shows the highest piezoelectric response with the longitudinal piezoelectric coefficient (d33) ~190 pC/N and transverse piezoelectric coefficient (d31) ~ -79 pC/N in ceramic form. It also shows a significant larger relative permittivity ~ 1700 as compared to other ferroelectric compounds. BaTiO3 is therefore an attractive compound for design of lead-free piezoelectric material by suitable chemical substitutions. Apart from the general objective to design lead-free ferroelectric compositions with the enhanced piezoelectric response, it still remains an interesting question as to what makes BaTiO3 exhibit far superior dielectric and piezoelectric properties at room temperature. Davis et al (J. Appl. Phys, 101, 054112, 2007) have highlighted that the large piezoelectric response of BaTiO3 is related to considerably large dielectric anisotropy (χ11/χ33) leading to enhancement in the shear piezoelectric coefficient d15. The mechanism contributing to large d15 is associated with the polarization rotation and field-induced low symmetry phases. In order to gain better insight, in the present work, we looked for a structural basis to understand the anomalously large electromechanical properties exhibited by BaTiO3 ceramic. With this in view we have carried out a detailed high resolution synchrotron XRD and aberration corrected TEM study on the polycrystalline BaTiO3 specimen. A careful analysis of the higher order weak Brag reflections with d-values less than 0.6 Å (e.g . 600 reflection) in the synchrotron XRD pattern, which otherwise are not accessible in normal laboratory XRD and neutron diffraction patterns, revealed the presence of a subtle monoclinic distortion (β ~ 90.03°) coexisting with the conventional tetragonal phase. Additionally, we also verified the presence of orthorhombic regions of very small coherence length in BaTiO3 at room temperature. In this work, we also developed a new “powder poling technique” that enabled us to apply very strong electric field to free ferroelectric crystallites and capture the nature of the electric field induced phase transformation in this system. Using this technique we have shown the evidence of increased phase fraction of the orthorhombic phase after poling confirming that BaTiO3 at room temperature have the tendency for polarization switching. These interesting results helps in providing a rational explanation for the anomalous piezoelectric properties exhibited by BaTiO3 ceramics at room temperature. In the next step, we use this understanding to improve the piezoelectric property of BaTiO3 by suitable chemical substitutions. The presence of subtle monoclinic distortion and small orthorhombic regions at room temperature was interpreted as the precursor effect associated with the ferroelectric (tetragonal) -ferroelectric (orthorhombic) phase transition which occurs near 0 °C in pure BaTiO3. It was therefore anticipated that those chemical substitutions which can raise the tetragonal-orthorhombic transition temperature towards room temperature would further enhance the system’s response. A survey of literature revealed that chemical substitution of Zr, Hf and Sn in BaTiO3 increases both the tetragonal-orthrohombic and also the orthorhombic-rhombohedral transition temperatures from below room temperature. Accordingly, in this work, we carried out a detailed structure property correlation study on Zr, Hf and Sn modified BaTiO3 ceramics. The success of this modest approach was proved when we found that d33 increased to ~425 pC/N by only 4% of Sn substitution. Likewise, in Zr and Hf modified BaTiO3 d33 increased to ~ 350 pC/N by just 2% of substitution. We also found that very dilute substitution of Zr(1%) and Sn(1-3%) increased the electric field-strain response to double (0.2%) at field strength of ~ 40 kV/cm, a value double in comparison to what could be achieved in pure BaTiO3 (0.1%). Using our innovative powder poling technique we found that the composition exhibiting large piezostrain response exhibit a large fraction of field induced orthorhombic phase as compared to the rest of the compositions. The mechanism for such large piezoelectric strain response is explained on the basis of the polarization rotation model. Having succesfuly exploited the results of the powder poling technique towards understanding the mechanism associated with high piezoelectric response in pure and modified BaTiO3, we employed this innovative technique to understand the mechanism associated with the anomalous piezoelectric response on a high performance soft-PZT (La modified Pb(Zr, Ti)O3) exhibiting longitudinal piezoelectric coefficient of ~ 650 pC/N. We carried out a comparative structural study of the structural states of this system in the annealed (equilibrium) and the powder poled state using high resolution synchrotron XRD. Rietveld analysis revealed that the poling of the free crystallites led to increase in the monoclinic fraction and also a considerable increase in the tetragonality of the coexisting tetragonal phase. The results were interpreted in terms of the twin mechanism of polarization rotation and polarization extension with electric field. We also found that the relative permittivity of the pellet increases with increasing the poling field. This was suggestive of the decrease in the coherence length of polarization on poling, a phenomenon which seems counterintuitive at the outset. The powder poling technique also offered a unique opportunity to study separately the effect of electric field and stress with regard to the nature of phase transformation exhibited by ferroelectrics exhibiting morphotropic phase boundary (MPB). For the first time we could prove that moderate stress brings about the same phase transformation as the electric field. Since stress/electric driven phase transformation has a close relationship with the anomalous piezoelectric behavior in MPB based systems, we have indicated that the equivalence of the stress and electric field with regard to the structural changes in MPB could be linked with the thermodynamic equivalence of the direct (stress as stimulus) and converse (electric field as stimulus) piezoelectric coefficients. The thesis is divided into five chapters. The first chapter of the thesis provides a brief introduction to the basic concepts related to ferroelectrics and the perovskite structure. A brief exposure to the conventional high performance lead based piezoelectrics and then on lead free piezoelectrics is presented. A detailed overview of the existing models to explain the phenomenon of enhanced piezoelectric response is discussed. The details of the experimental procedures and the characterization techniques used have been given in Chapter 2. The third chapter deals with understanding the mechanism of high piezoelectric response of BaTiO3 in comparison to other compounds available. The piezoelectric response of BaTiO3 (d33 ~ 190 pC/N) is relatively large compared to other compounds such as (Na0.5Bi0.5) TiO3 (d33 ~ 66 pC/N) and KNbO3 (d33 ~ 57 pC/N). In order to understand the origin of high piezoelectric response in BaTiO3, initially Rietveld refinement was carried out on laboratory x-ray and high resolution neutron powder diffraction data using a tetragonal P4mm model. Neutron diffraction data at a higher angles (120°<2θ<145°) shows the inadequacy of the single phase tetragonal model. Thus to resolve the issue more accurately, high resolution synchrotron powder diffraction was data collected at ESRF, France. The excellent signal-to-noise ratio data even at the higher angle clearly reveals additional reflections along with tetragonal reflections like {600}C. The careful structural analysis along with a novel powder poling technique developed to understand the electric field effect on powder specimen reveals that, at room temperature the unpoled state of BaTiO3 is characterized by the coexistence of metastable monoclinic Pm and orthorhombic Amm2 phases along with the tetragonal P4mm phase and the application of a strong electric field (poled state) leads to switching of polarization direction from [001]C direction towards [101]C direction. The fourth chapter deals with the development of lead free BaTiO3-based high performance piezoceramics. Three lead free BaTiO3 based solid solutions Ba(Ti1-xZrx)O3, Ba(Ti1-xSnx)O3 and Ba(Ti1-xHfx)O3 respectively were investigated. The addition of these elements improved the piezoelectric response drastically, longitudinal piezoelectric coefficient (d33) increased from ~190 pC/N for pure BaTiO3 to a maximum of ~355 pC/N in Zr substitution, to ~425 pC/N in Sn substitution and to ~360 pC/N in Hf substitution. These d33 values are more than the existing value of hard PZT (d33 ~225 pC/N) and nearly competing to soft PZT (d33 ~500 pC/N). To understand the origin of this high piezoelectric response in these systems, structural characterization was carried out using x-ray and neutron powder diffraction techniques. Structural refinement suggested that the composition exhibiting high piezoelectric response contains a coexistence of phases. Coexistence of phases is either tetragonal (P4mm) and orthorhombic (Amm2) or orthorhombic (Amm2) and rhombohedral (R3m) phases respectively. Stabilization of two phases becomes possible because of i) first order nature of phase transitions which ensures coexistence of phases persists ii) possible increase in the temperature interval corresponding to the phase coexistence due to random elastic strain induced by substitution of bigger sized cations (Sn+4 or Zr+4 or Hf+4), iii) critical temperatures associated with phase transitions approaching near room temperature with increasing cation concentration. A very dilute concentration of substitutional elements (1% in Zr and 1-3% in Sn) has doubled (0.2%) the electric field- strain response in comparison to 0.1% of BaTiO3 at field strength of ~ 40 kV/cm. In-situ/ex-situ electric field dependent structural study revealed a one-to-one correspondence between the high strain response and enhanced propensity for polarization switching. In Chapter 5 we have made use of the innovative powder poling technique to understand the mechanism associated with high piezoelectric response in a high performance conventional soft-piezoceramic, La modified PZT (PLZT) exhibiting longitudinal piezoelectric coefficient d33 ~ 650 pC/N. Comparative structural studies of annealed and poled powder specimens revealed an increased fraction of the monoclinic phase after poling. The chapter reports the phenomenon of increase in the dielectric constant after poling of the ceramic thereby suggesting a counterintuitive phenomenon of decrease in polarization coherence on application of electric field. We have used the results to comment on the plausibility of the two contending theories regarding anomalous piezoelectric behavior: adaptive phase theory vis-à-vis polarization rotation via a genuine monoclinic phase. Summary and Scope for further work is given in chapter 6. v

Advanced piezoelectric ceramics are widely used as piezoelectric sensors, transducers, actuators, etc. Among the commercially viable piezoelectric systems, lead zirconate titanate, PbZr1-XTiXO3 (PZT) has dominated the market due to its superior properties and cost effective reproducibility. PZT exhibits enhanced piezoelectric properties in the vicinity of morphotropic phase boundary (MPB). The mechanism(s) associated with the anomalous piezoelectric response in the MPB systems is still a subject of considerable debate. The two contending theories in this regard are based on (i) field induced polarization rotation in single domain crystal and (ii) formation of a high density of low energy domain walls. Diffraction experiments in-situ with electric field can, in principle, give direct information about domain reorientation, lattice strain and phase transformation (if any) in ferroelectric systems. However, the combined effect of preferred orientation and severe overlapping of Bragg peaks in diffraction pattern of MPB compositions makes structural analysis very challenging. As a consequence, domain reorientation studies have mostly been reported for compositions away from the critical MPB, i.e., in the single phase regime. So far it has not been established how the domain switching and phase transformation influence each other in the core MPB compositions of piezoelectric alloys. In the present dissertation, we have addressed this issue in an important lead based piezoelectric alloy system (X)BiScO3- (1-X)PbTiO3 (BSPT). In spite of their superior piezoelectric performance, lead based systems have major drawback because of its toxicity and environmental concerns. Several legislations have been passed all over the globe to encourage the development of lead free alternatives. Lead free piezoelectrics belong to three major categories - BaTiO3 (BT) based, Na0.5Bi0.5TiO3 (NBT) based and K0.5Na0.5NbO3 (KNN) based. Among them, NBT based systems have been most widely investigated due to their promising piezoelectric properties –mostly with regard to their large electrostrain response. In this thesis, we have attempted to understand the underlying mechanisms associated with the large piezoelectric response in the lead-based MPB piezoelectric BiScO3-PbTiO3 and also in one of NBT-based lead-free piezoelectric. In the process, we investigated the effect of grain size in influencing the global structure, piezoelectric properties and electrical conduction behaviour of the parent compound NBT. For the first time, in this thesis, we have used rare-earth photoluminescence as a tool to probe the nature of local symmetry in the non-ergodic relaxor ferroelectric state. The thesis comprise of several novel results and ideas which has been systematically detailed in four comprehensive chapters (Chapter 3 to Chapter 6). The thesis comprise of seven chapters. Chapter 1 summarizes some of the basic concepts related to the work presented in this thesis. The experimental techniques and characterizations are discussed in Chapter 2. Chapter 3 deals with a comparative in-situ electric field dependent high energy synchrotron x-ray diffraction (XRD) study on a critical MPB (x=0.3725) and a close by non-MPB (x=0.40) compositions of (X)BiScO3- (1-X)PbTiO3.We show that the rhombohedral ferroelectric-ferroelastic domain reorientation fraction is considerably reduced in the MPB composition as compared to the non-MPB composition. The reduction in the rhombohedral domain reorientation is also accompanied by a corresponding anomalous reduction in the field-induced rhombohedral lattice strain in the MPB along the nonpolar direction. The MPB composition however shows electric-field-induced rhombohedral to tetragonal phase transformation, the fraction of which follows the same hysteretic trend as the lattice strain and domain reorientation fraction. The non-MPB composition, on the other hand does not show field induced phase transformation. We use these results to propose that the field-induced structural transformation is most likely to be the dominant mechanism responsible for the larger macroscopic piezoelectric response in the critical MPB composition of this piezoelectric system. We also found a strong correlation between lattice strain and phase transformation with the field induced domain reorientation in the MPB composition. Although the magnitudes of the changes are very small, we could demonstrate that these phenomena and their coupling occur even in the subcoercive field regime. Chapter 4 to 6 detail the results of the investigation carried out on pure and modified NBT. In Chapter 4, we have reported an extensive study of the effect of grain size on the global structure and piezoelectric response of pure NBT prepared via spark plasma sintering. The grain size was varied in the range 0.2 – 40 µm by changing the annealing temperature. We found that the reported global monoclinic (CC) distortion of NBT collapses for grains size below ~2.5 microns even while it retains its non-ergodic relaxor state. This dramatic change in the global structure on such a large grain size confirms that the characteristic length scale responsible to alter the global structure in this system is mesoscopic in nature. We argue that the perceived difference in the global structure of NBT on size reduction is because of the change in nature of the assemblage of the mesoscopic structural heterogeneity (in-phase and out-of-phase octahedral tilts) inherently present in the system. Further, while we observed a dramatic decrease in the piezoelectric and ferroelectric properties of bulk ceramic with grains in the submicron range, we used an innovative powder poling technique to demonstrate unambiguously that the lack of piezoelectric response is not because of the submicron grains loosing ferroelectric character, but rather because of the inability of the grains to transform to a long ranged rhombohedral ferroelectric state on application of electric (poling) field due to clamping effect and/or increased incoherence of grain boundaries. In Chapter 5, we have demonstrated that the highly efficient nature of the photoluminescence emission of rare earth ions, in conjunction with the sensitivity of the Stark manifolds to the local symmetry, can be utilized to examine the nature of the local structure in the non-ergodic relaxor ferroelectric state of 0.94Na0.5Bi0.5TiO3-0.06BaTiO3. This composition exhibits a cubic like phase on the global structure and there is no unanimity in literature on its structural state on the local length scale. We used Eu and Er as two rare earths to examine the local symmetry in the unpoled and poled states of this system. A series of Eu and Er modified compositions were synthesized as per the nominal formula 0.94Na0.5Bi0.5-x(Eu/Er)xTiO3– 0.06BaTiO3 and a systematic structural, microstructural, piezoelectric and dielectric measurements was carried out. Surprisingly, we found that the number of Stark manifolds in the PL spectra of cubic-like phase is more as compared to that in the field stabilized rhombohedral (R3c) phase. This proves that the local symmetry on the A-site of what appears to be globally more symmetric cubic phase is lower than that in the field stabilized rhombohedral (R3c) phase. Chapter 6 details the results of an extensive investigation to understand the structural mechanism associated with the large electrostrain reported in the lead free system (0.94-x)Na0.5Bi0.5TiO3-0.06BaTiO3-xK0.5Na0.5NbO3 (0.0≤x≤0.025). We show that in the unpoled state, while XRD data suggest cubic like structure, neutron powder diffraction data shows weak superlattice peaks. A systematic structural analysis led us to propose a long period structural modulation of the type √2 x √2 x 16, contradicting the earlier proposal of coexistence of rhombohedral (R3c) and tetragonal (P4bm) phases. We show that the large electrostrain is associated with field induced transformation of the long period modulated structure to rhombohedral (R3c) ferroelectric phase. We also demonstrated that if the field induced R3c phase is partly retained when the field is reduced to zero in the previous cycle, the electrostrain is substantially reduced in the next cycle. The reduced large electrostrain could however be reproduced after heating the specimen above depolarization temperature (~75 oC). Chapter 7 summarizes the essential results of this thesis and also suggests prospects for further research.

博士
國立成功大學
資源工程學系碩博士班
97
Lead-based Pb(Zr,Ti)O3 (PZT) piezoelectric ceramics with perovskite structure are widely used as actuators, sensors and micro-electro-mechanical devices owing to their superior dielectric properties. However, toxic lead oxide may evaporate during the heating process due to their high vapor pressure which is risky to human health. Therefore, to develop new environment-friendly materials to replace PZT-based materials has become one of the most attractive topics. In this study, (Bi0.5Na0.5)TiO3 (BNT) lead-free solid solution system was choosen as studied material to investigate the extrinsic and intrincis factors on dielectric/piezoelectric/ferroelectric properties and find out the relation between crystal structure-microstructure-dielectric properties. For extrinsic factor, this study focuses on the microstructure-dielectric properties relation via fabricating dense ceramics with different grain size. The averge grain size of dense ceramics is from 0.28 to 1.81 um. The results show that their dielectric properties performance goes down while grain size decreases, and that is due to their domain switching ability decreases as grain size decreases. This phenomenon significantly affects its properties while the grain size of ceramic is below 0.8 um. For intrinsic factors, this study focuses on establishing the crystal structure-dielectric properties relation. There are three main results. First, the displacement of ions and lattice constant can be calculated via Rietveld method. According to the results, we find the phase transformation is caused from the increase in the distance between cations and anions (dc-a) and the variation in ferroelectric properties is related to dc-a, lattice constant, and the fraction of composed phases. Second, the relation between MPB (Morphotropic Phase Boundary) composition and tolerance factor has been found in this study and two material systems were designed to prove it. Third, by comparing some different MPB compositions, the variation in dielectric/piezoelectric constant is found to proportional to cell volume. Finally, a comprehensive discussion between crystal structure, microstructure, and dielectric/ piezoelectric/ferroelectric properties was made in this study to find out some evidences for developing new lead-free dielectric/ piezoelectric/ferroelectric ceramics.

Lead-free piezoelectric materials have received increasing attention in the last decade, driven by environmental issues and health concerns. Of considerable interest is the (K,Na)NbO3 (KNN)-based system, which possesses a relatively high Curie temperature and good piezoelectric properties. Abundant publications on KNN-based polycrystalline ceramics increased the interest in studying their single-crystalline form, based on two major concerns. The first concern refers to the negative role of grain interactions on the electromechanical response. The second concern deals with domain engineering. The relationship between external electric field direction, crystallographic orientation, and spontaneous polarization vectors for a specific structure can be more readily established in single crystals and thus offers a pathway for an in-depth understanding of fundamental mechanism and potential applications. The exciting enhancement of both piezoelectric and ferroelectric response in lead-based single crystals also encourages the further exploration of KNN-based piezoelectric crystals, as they possess the same perovskite structure. The main goal of this thesis is to find possible approaches for improved electromechanical properties in KNN-based piezoelectric single crystals. In Chapter 2, the current development of KNN-based single crystals as piezoelectrics is reviewed, following a short introduction of fundamental knowledge on piezoelectrics and ferroelectrics. Both submerged-seed solution growth and top-seeded solution growth techniques were employed to produce single crystals, as described detailed in Chapter 3. Emphasis is subsequently placed on issues of the crystal growth process, effective methods to enhance electrical properties, and crystallographic orientation-dependent electrical properties in Li-, Ta-, and/or Sb-substituted KNN single crystals. The main conclusions from the crystal growth aspect are presented in Chapter 4 and can be summarized as follows: (i) For individual elements, segregation coefficients highly rely on the initial concentration in the liquid solution. The systematic discussion in this work contributes to future composition design in KNN-based crystals. (ii) A competition between elements occupied on the same lattice site was found. (iii) The very low Li segregation coefficient in the KNN matrix is importantly responsible for the occurrence of a secondary phase with the tetragonal tungsten bronze structure. (iv) The observed optically-cloudy regions in as-grown KNN-based single crystals decrease the electrical response and can be reduced by thermal treatment with slow cooling. In the second part of the thesis we used three approaches to enhance the piezoelectric and ferroelectric behavior of KNN-based single crystals, which is shown in Chapter 5. Chemical substitution with Ta or Sb ions indicates that enhanced electromechanical response is achieved when the orthorhombic-tetragonal phase transition temperature is in the proximity of room temperature, as previously reported for polycrystalline ceramics. Thermal treatment in pure O2 atmosphere resulted in a twofold increase of the piezoelectric coefficient and ferroelectric parameters (maximum and remanent polarization) of a (K,Na,Li)(Ta,Nb,Sb)O3 single crystal. The up-to-date highest room-temperature piezoelectric coefficient in annealed KNN-based single crystals of 732 pC/N was obtained, which is attributed to the lower defect concentration after the thermal treatment. The third approach, doping with a small amount of transition metal Mn ions in (K,Na,Li)(Ta,Nb)O3 single crystals, is also presented. Orientation dependence of electromechanical properties in Chapter 6 indicates that high maximum polarization, remanent polarization, coercive field, maximum strain, and negative strain were observed when the electric field was applied along one of the spontaneous polarization vectors in both tetragonal and orthorhombic phases. This is related to the effect of polarization rotation under different electric field directions.

Ferroelectric materials, especially the polycrystalline ceramics, are very promising material for a variety of applications such as high permittivity dielectrics, ferroelectric memories, piezoelectric sensors, piezoelectric/electrostrictive transducers, electrooptic devices and PTC thermistors. Among the ferroelectric based piezoelectric ceramics the lead–zirconate-titanate Pb(Zr1-xTix)O3 (PZT) have dominated transducer and actuator market due to its excellent piezoelectric and dielectric properties, high electromechanical coupling, large piezoelectric anisotropy, ease of processing and low cost. However, the toxicity of lead based compounds has raised serious environmental concerns and therefore has compelled the researchers to look for new lead free alternatives with good piezoelectric and ferroelectric properties. (Na0.5Bi0.5)TiO3 (NBT) and its solid solution is one of the leading lead free piezoceramic ceramics due to their interesting ferroelectric, piezoelectric, electromechanical and dielectric property. The parent compound NBT is a ferroelectric with a moderately high Curie temperature (~250 oC), large ferroelectric polarization (~40µC/cm2) polarization, promising piezoelectric properties with 0.08% strain and longitudinal piezoelectric coefficient (d33) ~ 80 pC/N. X-ray and neutron diffraction studies in the past have shown that NBT exhibits rhombohedral (R3c) at room temperature. Neutron diffraction studies have suggested that NBT undergo a gradual rhombohedral to tetragonal (P4bm) transformation in a temperature region 200-320 ºC. Though the structure and phase transition behavior of NBT has been extensively investigated for over six decades now, this subject has again become debatable in recent few years, with some group reporting formation of orthorhombic phase above room temperature and another group suggesting monoclinic distortion at room temperature using high resolution x-ray diffraction technique. Interestingly the intermediate orthorhombic instability, reported by electron diffraction studies, has never been captured by neutron diffraction method though neutron diffraction is an equally powerful tool for studying (oxygen) octahedral tilts in perovskites. Needless to mention, the understanding of the subtle structural distortions have great significance with regard to the determination of the structure-piezoelectric property correlations in NBT based piezoceramics. The present thesis deals with such subtle structural issues in great detail. The systems investigated in the thesis are Ca and Ba modified NBT. While the Ca modified system was chosen to understand the subtle orthorhombic instability that has been reported above room temperature (only) by detailed electron diffraction work, Ba-modified NBT is the most investigated among the NBT-derived piezoelectric material systems and this thesis attempts to address some of the very complex nature of the structure-piezoelectric property correlation of this system. The first chapter of the thesis provides a brief introduction to the field of ferroelectrics, perovskite structure and their phase transition. A brief exposure to the conventional lead based relaxor ferroelectric and piezoelectric material is provided. A detailed overview of the existing knowledge related to room temperature structure of NBT and its phase transition studies with temperature has been discussed in the later part of this chapter. The second chapter includes various the experimental techniques that have been employed to synthesis and characterize the specimens under investigation. The third chapter deals with the phase transition behaviour of Ca modified NBT as a function of composition and temperature in the dilute concentration region. This work was carried out with the view to obtain a better understanding and compliment the intrinsic high temperature orthorhombic instability in NBT reported by electron diffraction technique. Interestingly, inspite of the fact that neutron diffraction method is a very sensitive tool for investigating subtle change in the nature of octahedral tilt in oxide perovskites, the intermediate orthorhombic distortion proposed by the electron diffraction studies has so far never been captured in any of the neutron diffraction studies. In this work we have verified the genuineness of the intrinsic instability with regard to the non-polar orthorhombic structure using neutron powder diffraction by adopting a special strategy which helped in capturing the characteristic signatures (the superlattice reflections) of the orthorhombic phase in the neutron powder diffraction patterns. It was found that small fraction of Ca-substitution (8-10 mol %) was good enough to amplify the magnitude of the orthorhombic (Pbnm) distortion, without altering the sequence of the structural evolution with temperature of the parent compound (NBT) itself, and stabilizing it at the global length scale at lower temperatures than pure NBT. This chapter presents the innovative approach that was used to extract reliable information about the very complex phase transition behaviour, involving coexistence of the various similar looking but crystallographically different phases in different temperature regimes by Rietveld analysis of temperature dependent neutron powder diffraction pattern in conjunction with temperature dependent dielectric and ferroelectric characterization of the specimens. The detailed study revealed the following sequence of structural evolution with temperature: Cc+Pbnm →Pbnm + P4/mbm → P4/mbm →Pm3 m. The fourth chapter gives a detail account of the structure-property correlations and the phase transition behaviour of (1-x)(Na0.5Bi0.5)TiO3 – (x)BaTiO3 (0≤x≤0.10), the most important solid solution series with NBT as reported in the literature. The phase transformation behaviour of this system has been investigated as a function of composition (0