Made available in DSpace on 2014-06-11T19:25:34Z (GMT). No. of bitstreams: 0 Previous issue date: 2007-04-27Bitstream added on 2014-06-13T20:33:16Z : No. of bitstreams: 1 nahime_bo_me_ilha.pdf: 1294512 bytes, checksum: ffc3048caf6b741e23555fbf3b2fbd5e (MD5) / Na última década, cresceu consideravelmente o interesse pela produção de filmes finos ferroelétricos, em virtude do grande potencial que estes materiais apresentam para a produção de dispositivos de memória ferroelétrica não-voláteis. O titanato de bismuto, Bi4Ti3O12 (BIT) é um ferroelétrico com estruturas de camadas de bismuto, conhecido por apresentar elevada temperatura de Curie (Tc), próxima de 650ºC, e excelentes propriedades de fadiga ferroelétrica. Este trabalho teve como objetivo a síntese de filmes finos de BIT e o estudo das suas propriedades estruturais. Os filmes estudados foram preparados a partir de um método químico, similar ao método Pechini, e depositados sobre substratos de Si(100). Os filmes foram cristalizados em forno convencional entre 500°C e 700°C, por períodos entre 1 e 12 horas. As técnicas de difração de raios-X (DRX) e Espectroscopia no Infravermelho por Transformada Fourier (FT-IR), foram usadas como ferramentas de investigação. Para os filmes tratados termicamente entre 400°C e 700°C, observouse a presença das fases cristalinas Bi4Ti3O12 e Bi2Ti2O7 (paraelétrica). Para os filmes tratados termicamente a temperaturas mais elevadas (700°C) e tempos mais prolongados (10 horas), observou-se uma tendência de desaparecer a fase Bi2Ti2O17. Aliados a estes resultados, o aumento da intensidade do pico (117) do BIT e o decréscimo da sua respectiva largura a meia altura, para os filmes cristalizados a 700°C, sugerem que nestas condições são obtidos filmes mais bem cristalizados. Um estudo usando a técnica FT-IR demonstrou a presença de uma banda de absorção claramente evidenciada em torno de 700 cm-1. Esta banda está associada ao modo stretching da ligação Ti-O e torna-se consideravelmente mais estreita quando a resina polimérica precursora foi tratada termicamente a temperaturas mais elevadas. Este fato também comprovou que temperaturas em torno de 700°C são favoráveis para a... / In the last decade the interest for the production of ferroelectric thin films increased considerably because of the great potential that these materials present for the production of devices of no-volatile ferroelectric memory. The bismuth titanate, Bi4Ti3O12 (BIT), is a ferroelectric with structures of bismuth layers, known by presenting high temperature of Curie (Tc), close of 650ºC, and excellent properties of ferroelectric fatigue. This work had as objectives the synthesis of thin films of BIT and the study of their structural properties. The studied films were prepared starting from a chemical method, similar to the Pechini method, and deposited on substrata of Si(100). The films were crystallized in conventional oven between 500°C and 700°C for periods between 1 and 12 hours. The techniques of X-ray diffraction (DRX) and Fourier Transform Infrared (FT-IR) Spectroscopy were used as investigation tools. It was observed the presence of the crystalline phases Bi4Ti3O12 and Bi2Ti2O7 (paraelectric) for the films termically treated between 400°C and 700°C. To the films termically treated to higher temperatures (700°C) and more lingering times (10 hours) a tendency of the phase Bi2Ti2O7 to disappear was observed. Allied to these results, the increase of the intensity of the pick (117) of the BIT and the decrease of its respective width to half height, for the crystallized films to 700°C suggests that, in these conditions, better crystallized films are obtained. A study using the FT-IR technique demonstrated the presence of an absorption band clearly evidenced around 700 cm-1. This band is associated to the stretching way of the Ti-O connection and becomes considerably narrower when the precursory polimeric resin was termically treated to higher temperatures. This fact also proved that temperatures around 700°C are favorable for obtaining a mono-crystalline phase of Bi4Ti3O12.

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<x≤0.10), temperature, electric field and mechanical-impact by Raman scattering, ferroelectric, piezoelectric measurements, x-ray and neutron powder diffraction methods. The structure of the morphotropic phase boundary (MPB) compositions of this system, which is interesting from the piezoelectric property point of view, has been under controversy for long. While some groups report the structure to be pseudocubic, other groups suggest it to be combination of rhombohedral and tetragonal. A perusal of the literature suggests that the reported nature and composition range of MPB is dependent on the method of synthesis and characterization technique used. In the present study, crystal structure of the NBT-BT solid solution has been investigated at the close interval near the MPB (0.05≤x≤0.10). Though x-ray diffraction study revealed three distinct composition ranges characterizing different structural features in the equilibrium state at room temperature: (i) monoclinic (Cc) + rhombohedral (R3c) for 0≤x≤0.05, (ii) “cubic-like” for 0.06≤x≤0.0675 and (iii) MPB like for 0.07≤x<0.10, Raman and neutron powder diffraction studies revealed identical symmetry for the cubic like and the MPB compositions. Both the cubic like compositions and the MPB compositions exhibit comparatively large d33. In the later part of this chapter this apparent contradiction is resolved by the fact that the cubic like structure transforms irreversibly to MPB after electric poling, a procedure which involves applying high dc electric field (well above the coercive field) to the pellet before carrying out the piezoelectric measurements. The effect of electrical field and mechanical impact has been studied for all the three different composition range, and it was found that electric field and mechanical impact both led to irreversible phase transformation in the same direction, though the transformation with mechanical impact remains incomplete in comparison to electric field. The most pronounced effect was observed for the cubic like compositions 0.06≤x≤0.0675 – they undergo phase separation to rhombohedral and tetragonal phases by electrical and mechanical perturbations. In the non-perturbed state the cubic-like critical compositions mimics features of relaxor ferroelectrics and extremely short coherence length (~ 40-50 Å) of the out-of-phase octahedral tilts. In the poled state this coherence length grows considerably and the system behaves like a normal ferroelectric. This confirmed a strong coupling between the lattice, octahedral tilts and polarization degrees of freedom. Neutron diffraction study of compositions exhibiting cubic-like and the MPB like revealed that the traditional P4bm tetragonal structure model fails to account for the intensity of the superlattice reflections. Thus the tetragonal structure stabilized above room temperature in pure NBT is different from the tetragonal phase observed at room temperature in the NBT-BT system. The results of the effect of mechanical impact and electric field has also been reported in this chapter for the critical composition exhibiting MPB (x=0.07). A detailed structural analysis of the precritical compositions, x≤0.05, revealed coexistence of ferroelectric phases (Cc+R3c) in equilibrium state (annealed specimens). This transforms to single phase (R3c) state after poling. Thus though the precritical (x≤0.05) and critical compositions (0.06≤x<0.10) of NBT-BT exhibits coexistence of ferroelectric phases in the equilibrium state, the fact that the electric poling makes the specimen single phase, R3c, after poling for the precritical compositions and retains the two phase nature of the critical compositions makes the critical compositions exhibit considerably higher piezoelectric response than the precritical compositions. Chapter five is dedicated to phase transition behaviour of the post critical compositions of (1-x)(Na0.5Bi0.5)TiO3–(x)BaTiO3 (0.16≤x≤1) using temperature dependent XRD, dielectric and ferroelectric studies. Though structurally the entire composition range is tetragonal, several notable features were revealed during detailed examination of the structural and dielectric behaviour. This study is also important from the view point that pure BT is a major component of multilayer ceramic capacitors and that an increase in the Curie point would be a welcome step for better temperature stability of the device. NBT does this. The transition temperature increases from 120 ºC for pure BT to 275 ºC for x=0.30 along with simultaneous increase in c/a ratio from 1.009 (pure BT) to 1.02 (x=0.30). Detailed analysis of temperature and frequency dependent dielectric data revealed deviation from Curie-Weiss and suggests a gradual transformation to relaxor-ferroelectric state as the NBT concentration increases in BT. The measure of frequency dispersion ‘γ’ parameter was determined from modified Curie-Weiss law for various compositions in the system. The ferroelectric and piezoelectric properties have also been investigated in detail for this composition range and an attempt has been made to correlate the composition variation of these properties with their structural parameters. This chapter shows a systematic correlation between all physical quantities such as Curie point, piezoelectric coefficient, polarization and tetragonality as a function of composition.

Orientador: Eudes Borges de Araújo / Banca: Marco Antonio Utrera Martines / Banca: José de los Santos Guerra / Resumo: Na última década, cresceu consideravelmente o interesse pela produção de filmes finos ferroelétricos, em virtude do grande potencial que estes materiais apresentam para a produção de dispositivos de memória ferroelétrica não-voláteis. O titanato de bismuto, Bi4Ti3O12 (BIT) é um ferroelétrico com estruturas de camadas de bismuto, conhecido por apresentar elevada temperatura de Curie (Tc), próxima de 650ºC, e excelentes propriedades de fadiga ferroelétrica. Este trabalho teve como objetivo a síntese de filmes finos de BIT e o estudo das suas propriedades estruturais. Os filmes estudados foram preparados a partir de um método químico, similar ao método Pechini, e depositados sobre substratos de Si(100). Os filmes foram cristalizados em forno convencional entre 500°C e 700°C, por períodos entre 1 e 12 horas. As técnicas de difração de raios-X (DRX) e Espectroscopia no Infravermelho por Transformada Fourier (FT-IR), foram usadas como ferramentas de investigação. Para os filmes tratados termicamente entre 400°C e 700°C, observouse a presença das fases cristalinas Bi4Ti3O12 e Bi2Ti2O7 (paraelétrica). Para os filmes tratados termicamente a temperaturas mais elevadas (700°C) e tempos mais prolongados (10 horas), observou-se uma tendência de desaparecer a fase Bi2Ti2O17. Aliados a estes resultados, o aumento da intensidade do pico (117) do BIT e o decréscimo da sua respectiva largura a meia altura, para os filmes cristalizados a 700°C, sugerem que nestas condições são obtidos filmes mais bem cristalizados. Um estudo usando a técnica FT-IR demonstrou a presença de uma banda de absorção claramente evidenciada em torno de 700 cm-1. Esta banda está associada ao modo "stretching" da ligação Ti-O e torna-se consideravelmente mais estreita quando a resina polimérica precursora foi tratada termicamente a temperaturas mais elevadas. Este fato também comprovou que temperaturas em torno de 700°C são favoráveis para a ... / Abstract: In the last decade the interest for the production of ferroelectric thin films increased considerably because of the great potential that these materials present for the production of devices of no-volatile ferroelectric memory. The bismuth titanate, Bi4Ti3O12 (BIT), is a ferroelectric with structures of bismuth layers, known by presenting high temperature of Curie (Tc), close of 650ºC, and excellent properties of ferroelectric fatigue. This work had as objectives the synthesis of thin films of BIT and the study of their structural properties. The studied films were prepared starting from a chemical method, similar to the Pechini method, and deposited on substrata of Si(100). The films were crystallized in conventional oven between 500°C and 700°C for periods between 1 and 12 hours. The techniques of X-ray diffraction (DRX) and Fourier Transform Infrared (FT-IR) Spectroscopy were used as investigation tools. It was observed the presence of the crystalline phases Bi4Ti3O12 and Bi2Ti2O7 (paraelectric) for the films termically treated between 400°C and 700°C. To the films termically treated to higher temperatures (700°C) and more lingering times (10 hours) a tendency of the phase Bi2Ti2O7 to disappear was observed. Allied to these results, the increase of the intensity of the pick (117) of the BIT and the decrease of its respective width to half height, for the crystallized films to 700°C suggests that, in these conditions, better crystallized films are obtained. A study using the FT-IR technique demonstrated the presence of an absorption band clearly evidenced around 700 cm-1. This band is associated to the stretching way of the Ti-O connection and becomes considerably narrower when the precursory polimeric resin was termically treated to higher temperatures. This fact also proved that temperatures around 700°C are favorable for obtaining a mono-crystalline phase of Bi4Ti3O12. / Mestre

No description available.

<p>Bismuth titanate Bi₄Ti₃O₁₂, is one of the bismuth based layered ferroelectric materials that is a candidate for replacing the lead based ferroelectric materials in for instance non-volatile ferroelectric random access memories (FRAM). This is due to the fact that the bismuth based ferroelectrics consists of pseudo perovskite units sandwiched in between bismuth oxide layers, which gives them a better fatigue nature.</p><p>In this thesis thin films of Bi₄Ti₃O₁₂have been deposited by chemical vapour deposition (CVD) using the metal iodides, BiI₃ and TiI₄ as precursors. Films grown on MgO(001) substrates were found to grow epitaxially. The electrical properties were determined for films grown on Pt-coated silicon and good properties such as a high dielectric constant (ε) of 200, low <i>tan</i> δ of 0.018, a remnant polarisation (<i>P</i>_r) of 5.3 μC/cm² and coercive field (E_c) as high as 150 kV/cm were obtained. Thin films in the Bi-Ti-O system were also deposited by atomic layer deposition (ALD) using metalorganic precursors.</p><p>In addition to the ternary bismuth titanates, films in the binary oxide systems <i>i.e.</i> bismuth oxides and titanium oxides were deposited. Epitaxial TiO₂ films were deposited both by CVD and ALD using TiI₄ as precursor. The rutile films deposited by ALD were found to grow epitaxially down to a temperature of at least 375 ¢ªC on α-A1₂O₃(0 1 2) substrates. The TiO₂ ALD process was also studied <i>in-situ</i> by QCM. Different bismuth oxides were deposited by halide-CVD using BiI₃ as precursor on MgO(0 0 1) and SrTiO₃(0 0 1) substrates and the results were summarised in an experimental CVD stability diagram. The Bi₂O_2.33 phase was found to grow epitaxially on both substrates.</p>

Bismuth titanate Bi4Ti3O12, is one of the bismuth based layered ferroelectric materials that is a candidate for replacing the lead based ferroelectric materials in for instance non-volatile ferroelectric random access memories (FRAM). This is due to the fact that the bismuth based ferroelectrics consists of pseudo perovskite units sandwiched in between bismuth oxide layers, which gives them a better fatigue nature. In this thesis thin films of Bi4Ti3O12 have been deposited by chemical vapour deposition (CVD) using the metal iodides, BiI3 and TiI4 as precursors. Films grown on MgO(001) substrates were found to grow epitaxially. The electrical properties were determined for films grown on Pt-coated silicon and good properties such as a high dielectric constant (ε) of 200, low tan δ of 0.018, a remnant polarisation (Pr) of 5.3 μC/cm2 and coercive field (Ec) as high as 150 kV/cm were obtained. Thin films in the Bi-Ti-O system were also deposited by atomic layer deposition (ALD) using metalorganic precursors. In addition to the ternary bismuth titanates, films in the binary oxide systems i.e. bismuth oxides and titanium oxides were deposited. Epitaxial TiO2 films were deposited both by CVD and ALD using TiI4 as precursor. The rutile films deposited by ALD were found to grow epitaxially down to a temperature of at least 375 ¢ªC on α-A12O3(0 1 2) substrates. The TiO2 ALD process was also studied in-situ by QCM. Different bismuth oxides were deposited by halide-CVD using BiI3 as precursor on MgO(0 0 1) and SrTiO3(0 0 1) substrates and the results were summarised in an experimental CVD stability diagram. The Bi2O2.33 phase was found to grow epitaxially on both substrates.

Ce travail de thèse porte sur l’étude et la réalisation d'une nouvelle génération de capteurs ultrasonore dédiés à la caractérisation des gaz de fission. Plus généralement, ces études concernent le développement de l’instrumentation du réacteur d’essai des matériaux Jules Horowitz (RJH), visant entre autre à effectuer le contrôle in situ de la composition du gaz libéré afin d’optimiser la durée de vie du combustible et le taux de combustion. La température de fonctionnement de ce nouveau réacteur devant se situer dans la plage entre 200 °C à 400 °C, la principale problématique concerne donc le développement d’un matériau piézoélectrique, capable de fonctionner dans la plage de température requise, et son intégration à un dispositif de détection.Nous proposons l’utilisation du sodium titanate de bismuth (NBT) développé par la méthode de la sérigraphie. Dans le but d'optimiser les conditions de fabrication des matériaux, plusieurs versions de matériaux piézoélectriques ont été produites au cours de cette thèse. Chacun des matériaux a été caractérisé (paramètres morphologiques, chimiques, électriques, diélectriques, piézoélectriques et électromécaniques) et des tests en fonction de la température ont été conduits. Après avoir démontré une bonne répétabilité dans la production du matériau, le protocole de fabrication des capteurs a été déterminé et un prototype de capteur ultrasonore a été réalisé.Ces capteurs ont été fabriqués par dépôt du matériau actif sur un substrat d'alumine. Après caractérisation des propriétés des capteurs, des essais ont montré une sensibilité acoustique importante à température ambiante. De plus, la possibilité d'une détection de gaz sur une gamme de pression de 50 à 70 bars a été démontrée par l'intégration d’un capteur dans une enceinte. Pour vérifier la possibilité d'application du capteur à la détection de gaz dans des environnements hostiles (haute température), un modèle théorique basé sur les propriétés électromécaniques et les équations d’adaptation d’impédance a été introduit. Il a été démontré théoriquement que le capteur est capable d'effectuer des mesures de gaz de la température ambiante jusqu’à 350 °C. / This Ph.D. thesis is dedicated to the development of a new generation of ultrasonic sensors devoted to fission gas characterization. More generally, these studies concern the development of instrumentation for the Jules Horowitz material testing reactor (JHR) aiming to perform in-situ control of the released gas composition for optimization of burn-up rate and fuel rod lifetime. The operation temperature of this new reactor will be in the range of 200 °C - 400 °C. Hence, the main problem concerns the development of piezoelectric material, able to operate in the required temperature range, and its integration with a sensor device.To resolve this problem, we propose to use the sodium bismuth titanate (NBT) ceramic material developed by the screen-printing technique. Several versions of piezoelectric materials were produced during this research, with the purpose of optimizing material manufacturing conditions Each material was characterized (morphological, chemical, electric, dielectric, piezoelectric and electromechanical parameters) and “tests as a function of temperature” were carried out. After demonstrating repeatability in material fabrication, the protocol for NBT sensor production was determined and a prototype of the ultrasonic sensor was fabricated.The sensor was fabricated by deposition of an active material onto an alumina substrate. After characterization of sensor properties, acoustic tests showed a high sensitivity of measurements at ambient temperature (25 °C). Furthermore, by integration of sensors into a pressurized enclosure the possibility of gas detection in the range from 50 to 70 bars was demonstrated. To verify the sensor’s applicability to gas detection under harsh temperature environment, a theoretical model based on electromechanical properties and impedance matching equations was introduced. It was theoretically demonstrated that the sensor is able to perform gas measurements from ambient temperature up to 350 °C.

Made available in DSpace on 2014-07-29T15:07:08Z (GMT). No. of bitstreams: 1 Mestrado_Thiago_Martins_Amaral.pdf: 3609467 bytes, checksum: ece5cabfa428decf86242ad3624c5852 (MD5) Previous issue date: 2011-02-16 / This work presents the systematic results of studies concerning the sintering of the Bi12TiO20 (BTO) aiming the production of transparent ceramic samples. All that because the BTO is a material that has great scientific and technologic visibility due to its optical, electro and electro-optical properties, but still little explored as ceramic. The sintering starting material was obtained by solid state reaction between Bi2O3 e TiO2; the synthesized BTO was grinded in ball milling until sub micrometric sizes. The conditions for preparing green samples were also investigated, allowing samples with green relative density between 58% and 62%, with uniaxial and isostatic pressing techniques. The study of the sintering process involved different sintering programs, under different atmospheres conditions and additive utilization. Optimized conditions were established and the limiting factors discussed. The quality of the obtained ceramics was assessed by their final relative density, phase homogeneity, microstructure and optical properties (transmission, optical activity and electro-optical performance). Samples with relative density above 99,9% and 50% of the monocrystal´s optical transmission in the 633 nm wavelength and equal optical activity were obtained. Nevertheless, the ceramics still show scattering centers and none liquid electro-optical effect was observed. / Neste trabalho são apresentados resultados de estudos sistemáticos referentes à sinterização do Bi12TiO20 (BTO) visando a obtenção de amostras cerâmicas transparentes. Isso porque o BTO é um material de grande interesse científico e tecnológico devido às suas propriedades ópticas, elétricas e eletroópticas, mas ainda pouco explorado como cerâmica. O material de partida para sinterização foi obtido por reação no estado sólido entre Bi2O3 e TiO2; o BTO assim sintetizado foi moído em moinho de bolas até tamanhos submicrométricos. As condições de preparação de pastilhas a verde foram investigadas, permitindo a obtenção de corpos com densidade relativa a verde, entre 58% e 62%, com uso de prensagem uniaxial e isostática. O estudo do processo de sinterização envolveu avaliar diferentes rampas de aquecimento, atmosfera de sinterização e utilização de aditivos. Condições otimizadas foram estabelecidas e os fatores limitantes discutidos. As cerâmicas foram avaliadas com respeito à densidade relativa final, homogeneidade de fase, microestrutura e propriedades ópticas (transmissividade, atividade óptica e modulação eletro-óptica). Amostras com densidade relativa maior que 99,9%, transmitância óptica de até 50% daquela do monocristal em 633 nm e atividade óptica idêntica foram obtidas. No entanto, as cerâmicas ainda apresentam centros espalhadores e nenhum efeito líquido de modulação eletro-óptica foi observado.

Ferroelectric ceramics are very promising materials for a variety of piezoelectric applications such as high permittivity dielectrics, piezoelectric sensors, piezoelectric/electrostrictive transducers, actuators, electro-optic devices, etc. Among the commercially viable ferroelectric ceramics, the lead-zircon ate-titivate Pb(Zr1-xTix)O3 (PZT) based ceramics have dominated the market due to their superior piezoelectric and dielectric property along with other advantages like high electromechanical coupling, ease of processing and low cost. However, the toxicity of lead based materials, and its volatility at processing temperatures is a serious health and environmental concern. Several legislations against lead-based products have been passed all over the world in order to encourage identification of alternative lead-free materials for these applications. As a consequence, there has been a remarkable surge in efforts by researchers on identifying lead-free alternatives for piezoelectric applications. A larger emphasis has been placed on perovskite based ceramics since in addition to possessing excellent properties, their relatively simple structure facilitates understanding structure-property relationships which are important for developing novel high performance materials. Na1/2Bi1/2TiO3 (NBT) and its solid solutions are one of the leading classes of perovskite ceramics, which show promising ferroelectric, piezoelectric and dielectric property thereby having the potential to replace PZT based ferroelectrics. The parent compound NBT is ferroelectric with large ferroelectric polarization (~40 C/cm2), promising piezoelectric properties with 0.08% maximum strain and longitudinal piezoelectric coefficient (d33) ~ 80 pC/N. Though NBT was discovered nearly six decades ago, a clear understanding of its structure remained elusive for a long time since different characterization techniques yielded contradicting reports on its structure and nature of phase transformation. However, rapid advances in characterization techniques in recent years have led to uncovering of new results, thereby shedding light on the true structure of NBT. X-ray and neutron diffraction studies in the past have shown that NBT exhibits rhombohedral (R3c) structure at room temperature, which undergoes a gradual transformation into tetragonal (P4bm) structure at ~230oC. However, recent characterization of both single crystal and powder of NBT using high resolution x-ray diffraction showed that the room temperature structure is not purely rhombohedral and the structure can be better modeled with a monoclinic (Cc) structure. In contrast to x-ray and neutron diffraction, electron diffraction studies have shown evidence for the presence of planar disorders, corresponding to in-phase octahedral tilts in the sample which cannot be accounted for by either R3c or Cc structure. In addition, EXAFS, x-ray and neutron total scattering studies, diffuse scattering studies, etc. have shown that the structural parameters obtained from bulk diffraction techniques are significantly different from the local structure of the material. Similar ambiguities have been observed even in NBT based solid solutions like BaTiO3, K1/2Bi1/2TiO3, etc. which show enhanced properties at the morphotropic phase boundary (MPB). A major breakthrough in understanding the structural complexity involved in NBT based solid solutions was achieved when it was found that the structure of the MPB compositions were sensitive to electric field. This led to solving the mystery behind the appearance of cubic-like phase at some of the MPB compositions where the application of electric-field resulted in the transformation of the structure into a co-existence of rhombohedral and tetragonal phases. Observation of an electric-field-induced structural transition at the MPB compositions was expected, because the MPB signifies instability in the system and even a minor external force can significantly influence the system. However, we have found that the structure of even pure NBT is significantly influenced by electric field and the framework of this thesis is based on this particularly important result. The intrinsic tendency of the electric field to affect the structure of NBT is a major factor which needs to be considered when studying similar phase transitions in the MPB compositions of NBT-substituted systems also. This was not taken into account by other research groups, and they assumed that the instability associated with the MPB was the only major factor involved in the electric-field induced transitions. A simple but highly effective strategy of grinding the electrically poled pellet into fine powder and then collecting x-ray diffraction patterns, facilitated elimination of preferred orientation in the sample. Thus, structural analysis by Rietveld refinement was possible even on the poled sample, which has not been carried out by any other groups on any ferroelectric ceramics so far. The initial part of the thesis deals with addressing the structural complexity of pure NBT, wherein the effect of electric field on the bulk structure as well as the local structure was studied in great detail. Later on, similar concepts are used to investigate BaTiO3 and K1/2Bi1/2TiO3 substituted NBT also. The first chapter of the thesis provides a brief introduction to the field of ferroelectrics, perovskite structure and their phase transition. An exposure to concepts like relaxor ferroelectrics, morphotrophic phase boundary, octahedral tilting, etc. has been provided. Then, a detailed overview of the existing literature related to the structure of NBT and its phase transition studies with temperature has been discussed. The details of the experimental procedures, characterization techniques used, and some theory behind these techniques have been provided in chapter 2. The third chapter deals with the room temperature structural characterization of pure NBT using techniques like x-ray diffraction, neutron diffraction, electron diffraction and EXAFS analysis. The results of these structural characterizations are also complemented with first-principles calculation study of the ground state structure of NBT, dielectric and ferroelectric characterization, and ageing studies. While x-ray and neutron diffraction clearly established electric-field induced structural transition from a monoclinic (Cc) to rhombohedral (R3c) structural transition, first principles calculation showed that the monoclinic phase is not stable and hence cannot be the ground state structure of NBT. Also, the possibility of the monoclinic features appearing in the x-ray diffraction solely due to micro structural effects by nano-sized domains was discussed. Comparison of electron diffraction of poled and unpoled samples of NBT showed that the in-phase tilted regions were greatly suppressed in the poled samples. Even HRTEM images showed that the unpoled samples had a very high concentration of strain heterogeneity in them, which was absent in the poled samples. This gave a direct evidence of correlation between observation of monoclinic phase and the presence of in-phase tilted regions in the unpoled samples. It was proposed that the strain caused by these in-phase tilted disorders caused distortion in the original rhombohedral lattice thereby making the structure appear monoclinic. Application of electric field causes the in-phase octahedral tilt disorders to vanish, thereby even the monoclinic features observed in the XRD also disappear. The fourth chapter discusses the consequences of poling on the high temperature phase transition behavior of NBT. We have used temperature dependent x-ray and neutron diffraction, Raman spectroscopy and EXAFS analysis whose results were correlated with the anomalies observed in temperature dependent dielectric and polarization studies. We found that the poled sample shows a sharp anomaly at the depolarization temperature (Td) in all the characterization techniques used, in contrast to a diffuse or negligible effect seen in the unpoled sample. The depolarization temperature was found to be associated with introduction of structural disorder in the sample in the form of in-phase octahedral tilts. This also gave rise to a normal to relaxor ferroelectric transition at Td, and this relaxor behavior persisted even after cooling the sample to room temperature. An intermediate cubiclike phase was observed from x-ray diffraction at around 300C wherein the rhombohedral phase vanishes and the tetragonal phase begins to appear. Even single crystal study of NBT in the past showed sudden disappearance of the domains at 300C, even though they were visible at both above and below this temperature. This effect was called isotropization, and was postulated to arise due to extremely small domains which made the system isotropic. However, our neutron diffraction pattern showed that in-phase tilted superlattice reflections persisted at this temperature which meant that the structure was not truly cubic at this temperature. Further, a neutron diffraction study at higher temperatures showed that the in-phase tilted superlattice reflections persisted even above the cubic phase transition temperature, in corroboration with similar reports from high temperature electron diffraction. Chapter five deals with the BaTiO3 substituted NBT system, which has gained tremendous interest in the last decade as a viable piezoelectric ceramic for commercial applications. Though a large number of groups have already carried out exhaustive studies on this system, most of the work concentrated mainly on the MPB compositions which showed enhanced piezoelectric properties. In this chapter, we highlight some important findings in the pre-MPB and post-MPB compositions. Using room-temperature and high temperature x-ray diffraction, we show that there exists a subtle compositional phase boundary at x = 0.03, which disappears upon poling the sample. While the monoclinic phase in pure NBT becomes cubiclike at this composition, the depolarization temperature (Td) also slightly increases up to this composition and then decreases upon further Ba substitution. Similar studies were also carried out with compositions containing slightly excess bismuth in them (0.1 mol %), whose purpose was to negate the effects of Bi-vaporization during sintering. It was found that while the compositional phase boundary remained essentially unchanged, there was a decrease in Td for all the compositions. It was also realized that the addition of excess bismuth improved the overall piezoelectric property of the system. Studies on higher compositions of Ba in the post-MPB regions further revealed two additional compositional phase boundaries. A criticality in the coercive field and spontaneous tetragonal strain was observed at x = 0.2, which was found to be associated with crossover from a long-period modulated tetragonal phase (for x < 0.2) to a no modulated tetragonal phase (for x > 0.2). Near the BT rich end (x ~ 0.7), the system exhibits a crossover from normal to a diffuse/relaxor ferroelectric transition with increasing Na1/2Bi1/2 substitution. The onset of relaxor state by Na1/2Bi1/2 substitution on the Ba-site, was shown to increase the spontaneous tetragonal strain in the system. This was because of the enhancement in the covalent character of the A-O bond by virtue of the Bi+3 6s2 lone pair effect, and it also led to a sudden increase in the tetragonal-to-cubic transition temperature. This was in contrast to other chemical modifications of BT reported in the past (like Zr, Sn, Sr, etc.) where the relaxor state is accompanied by a weakening of the ferroelectric distortion and a decrease in the critical temperature. Finally, chapter six covers the effect of electric field induced phase transition in K1/2Bi1/2TiO3 substituted NBT. Visual observation showed that while the compositions (x < 0.2) behaved similar to pure NBT, wherein the structure became purely rhombohedral upon poling, the effect of electric field was negligible in the post-MPB compositions (x > 0.2). In addition, the peaks in the annealed samples were considerably overlapping which made identifying the structural transitions at the MPB compositions difficult using Rietveld analysis. However, comparison of the FWHM of the {200}pc peaks of compositions x < 0.2 showed that the FWHM began to increase suddenly for x > 0.15 indicating emergence of the tetragonal phase. Also, all the compositions showed an increase in the {200}pc peak FWHM by 0.03 after poling. The depolarization temperature showed only slight variation in the pre-MPB compositions, but showed a minimum at the MPB compositions. Temperature dependent dielectric studies further showed that for the post-MPB compositions, the system remains relaxor even after poling.

The present research work mainly focuses on the fabrication and characterization of single and multilayer thin films based on Bismuth Vanadate (BVO) and Bismuth Titanate (BTO). The multi-target laser ablation technique was used to fabricate single layer thin films of BVO, BVN and BTO; and multilayers composed of BVO and BTO in different structures. The fabricated thin films exhibited dense microstructure and a sharp interface with the substrate. The lattice strain, surface roughness and grain size could be varied as functions of composition and individual layer thickness in different structure fabricated. The optical properties were studied by spectroscopic ellipsometry and optical transmission spectra. The various models that were used for ellipsometric data analysis gave an excellent fitting to the experimental data. The optical constants were determined through multilayer analyses of the films. The band gap of these films was studied by spectroscopic ellipsometry and optical transmission. The optical studies carried out on BVO-BTO bilayer indicated the presence of an interfacial layer in between the BVO and BTO layer, whose refractive index was different from that of the individual layers and is attributed to different nature of the interfacial layer. The ferroelectric nature of BVO films was confirmed by P-E hysteresis loop studies under different applied fields and at various probing frequencies. The same was corroborated via the C-V measurements of these BVO films which exhibited butterfly shaped C-V characteristics. Fatigue studies in these films indicated that the switchable polarization is essentially constant through 105 cycles, after which it starts increasing probably due to the ionic conduction in BVO thin films. The dielectric response of undoped and Nb doped BVO as well as BVBT ML thin films were studied over a wide range of temperatures. The BVO films exhibited remarkable dielectric dispersion at low frequencies especially in the high temperature regime. Further, the frequency and temperature dependence of the dielectric, impedance, modulus and conductivity spectra of these films were investigated in detail. The ac conductivity was found to obey well the double power law in case of ML, indicating the different contributions to the conductivity, the low frequency conductivity being due to the short range translational hopping and the high frequency conductivity is due to the localized or reorientational hopping motion. DC leakage conduction in BVO, BVN and BVBT ML thin films was studied over a wide range of temperatures and applied electric fields. The experimental data were analyzed in light of different models to investigate the dc conduction mechanism in these films which were broadly classified into electrode limited and bulk limited conduction processes. In the case of BVO thin films the dc leakage current exhibited an ohmic nature at low electric fields followed by an onset of the space charge limited conduction (>1). Further in case of BVN films, three distinct regions were observed in I-V characteristics signifying different types of conduction processes in these films. In case of BVBT ML thin films, bulk limited PF mechanism was found to determine the conduction behavior at moderate electric fields. At higher electric fields, a trap filled region was observed which was followed by SCL conduction at higher fields. Therefore the present observation indicates the presence of more than one bulk limited conduction process in BVBT ML thin films. BVO thin films exhibiting good structure and dense morphology were successfully prepared on p-type Si by chemical solution decomposition technique. The C-V characteristics were evaluated for Au/BVO/Si MFS structure which showed a typical high frequency feature of a conventional MFIS structure.