Pós nanocristalinos de Ba1-xCaxTiO3(0≤ x ≤ 0.30) foram sintetizados pela primeira vez por meio do método dos precursores poliméricos modificados, em temperaturas relativamente baixas (500°C). Para a densificação dos corpos cerâmicos duas técnicas distintas foram utilizadas: a sinterização convencional, utilizando um forno elétrico; e a sinterização a laser, na qual um laser de CO2 foi utilizado como principal fonte de calor. Os corpos cerâmicos sinterizados pelas duas técnicas apresentaram um tamanho médio de grãos em torno de 500 nm, porém uma maior densidade relativa foi obtida nas cerâmicas sinterizadas a laser, chegando a 99%, além de boa transparência, ∼42% a 940 nm para a amostra com 30 mol% de Ca com espessura de 0.5 mm. Também verificamos por meio de simulação numérica, que a equação de condução de calor na aproximação linear, considerando uma fonte de calor com perfil gaussiano, descreve qualitativamente o comportamento observado durante a sinterização a laser no seu estágio final. Por meio da técnica de espectroscopia de impedância foi feita a caracterização dielétrica dos corpos cerâmicos, nos quais três pontos principais foram estudados: i) o efeito do tamanho de grãos; ii) a influência da concentração de Ca; e iii) a influência do método de sinterização. Com a variação do tamanho do grão foram observadas mudanças na intensidade do pico de transição ferro-paraelétrica, na temperatura de Curie e na permissividade relativa à temperatura ambiente. Para concentrações de Ca até 15 mol%, as cerâmicas apresentaram uma transição de Curie estreita semelhante ao BaTiO3,enquanto que para concentrações maiores, uma transição de fase difusa foi observada. A sinterização a laser resultou em corpos cerâmicos com constante dielétrica 30% maior, e menor perda dielétrica do que as cerâmicas sinterizadas convencionalmente. Finalmente, foi proposto um modelo baseando-se na formação de vacâncias de oxigênio para o aumento da condutividade elétrica com a substituição de Ca. / Ba1-xCaxTiO3 (0 ≤ x ≤ 0.30) nanocrystalline powders were successfully synthesized for the first time at relative low temperature (500 °C) by a modified polymeric precursor method. The densification of the ceramics was made by two distinct techniques: conventional sintering using an electric furnace; and by a laser sintering process in which a CO2 laser is used as the main heating source. It was achieved dense ceramics with an average grain size about 500 nm from both techniques. However, the laser sintered ceramics presented a higher relative density (99 %) and a transparency of 42% at 940 nm in the sample with 30 mol% of Ca. By numerical simulation of the thermal conduction equation on the linear approximation case and taking account a thermal source with a gaussian profile, it was possible to describe qualitatively the final stage of the laser sintering process. The dielectric characterization of the ceramic bodies were carried out by the impedance spectroscopy technique and three main effects were studied: i) the grain size effect; ii) the Ca concentration influence; and, iii) the sintering technique influence. Modifying the grain size led to changes in the ferro-paraelectric transition magnitude, on the Curie temperature value and in the relative permittivity at room temperature. For Ca2+ concentrations up to 15 mol% the sintered ceramics presented a sharp Curie transition like BaTiO3,while for higher Ca concentrations a diffuse phase transition was observed. The laser sintered ceramics presented a dielectric constant 30% higher than the conventional sintering and a lower dielectric loss. Finally, we have proposed a model based on the oxygen vacancies formation to explain the conductivity increase with the Ca2+ substitution.

Coordenação de Aperfeiçoamento de Pessoal de Nível Superior / Barium titanate (BaTiO3 - BT) is a ferroelectric material with important technological applications. When partially doped with trivalent cations at the barium sublattice or pentavalent at the titanium sublattice, BaTiO3-based ceramics exhibit a semiconductive character, accompanied by a Positive Temperature Coefficient of Resistivity (PTCR). Several ways of BT modifications have been proposed in literature, as example, the use of different synthesis methods and the doping with different ions. Therefore, in the present work we have as objective the synthesis, sintering and electrical characterization of the rare earth ions (RE = La3+, Y3+, Er3+, Nd3+ e Eu3+) doped barium calcium titanate ceramics (Ba0,77Ca0,227RE0,003TiO3 BCT_RE) in order to study mainly the PTCR behavior. The powder synthesis was done by the polymeric precursor method followed by the sinterization at reducing atmosphere during the heating up ramp and different atmosphere during cooling down ramp in order to investigate the atmosphere and dopant effect on PTCR behavior. The characterization was done using the Differential Thermal Analysis, Thermogravimetry, Scanning Differential Calorimetry, X-ray Powder Diffraction, Scanning Electron Microscopy and Impedance Spectroscopy techniques. The calcined powder at 600ºC/4h presented majority BCT phase and after sintering at 1350ºC/6h, a small amount of Ba6Ti17O40 phase was observed. The sintered ceramics presented a homogeneous grain size distribution but with the presence of a secondary phase at the grain boundary for all compositions, and Curie temperature at about 120ºC. The PTCR characteristics of the ceramics were studied as a function of the sintering atmosphere and dopant type. In summary, all the samples, exception of the Eu doped samples, presented PTCR behavior in all the studied conditions. . / O titanato de bário (BaTiO3 - BT) é um material ferroelétrico com importantes aplicações tecnológicas. Quando dopado com íons trivalentes no sítio do bário ou pentavalentes no sítio do titânio, cerâmicas de BaTiO3 exibem um caráter semicondutor, acompanhado por um coeficiente positivo da resistência com o aumento da temperatura (PTCR). Diversas formas de modificação do BT têm sido sugeridas na literatura, por exemplo, a utilização de diferentes métodos de síntese e a dopagem com diferentes íons. Dessa forma, no presente trabalho tivemos como objetivo a síntese, a sinterização e a caracterização elétrica de cerâmicas de titanato de bário e cálcio (Ba0,77Ca0,227RE0,003TiO3 BCT_RE) dopados com íons terras raras (RE = La3+, Y3+, Er3+, Nd3+ e Eu3+), visando principalmente o estudo do comportamento PTCR. A síntese dos pós foi realizada pelo método dos precursores poliméricos, seguido pela sinterização em atmosfera redutora durante o aquecimento e diferentes atmosferas no resfriamento com o intuito de investigar a influência da atmosfera e do dopante no comportamento PTCR. A caracterização foi feita utilizando as técnicas de Análise Térmica Diferencial, Termogravimetria, Calorimetria Exploratória Diferencial, Difração de Raios X, Microscopia Eletrônica de Varredura e Espectroscopia de Impedância. Os pós calcinados a 600ºC/4h apresentaram fase majoritária BCT e após sinterizados a 1350ºC/6h foi observada a presença da fase Ba6Ti17O40. As cerâmicas sinterizadas apresentaram uma distribuição homogênea de tamanho de grão, porém com a presença de uma segunda fase na região de contorno de grão para todas as composições, com temperatura de Curie em torno de 120ºC. As características PTCR das cerâmicas foram estudadas em função da atmosfera de sinterização e do tipo do dopante. Em resumo, todas as amostras, com exceção das dopadas com Eu, apresentaram comportamento PTCR para todas as condições estudadas.

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.

Composições cerâmicas de CaxSr1-xTi1-yFeyO3-&delta;, x = 0, 0,5 e 1,0, y = 0 e 0,35, foram preparadas por meio de síntese reativa de CaCO3, SrCO3, TiO2 e Fe2O3 e pela técnica dos precursores poliméricos. Os pós-cerâmicos foram avaliados por meio de análise térmica (termogravimétrica e térmica diferencial), difração de raios X e microscopia eletrônica de varredura. Compactos cerâmicos sinterizados foram analisados por difração de raios X, microscopia eletrônica de varredura, microscopia de varredura por sonda e espectroscopia de impedância. A força eletromotriz gerada entre duas faces paralelas de amostras cilíndricas foi monitorada na faixa de temperatura 600 - 1100 oC para pressão parcial de oxigênio de ~50 ppm, utilizando-se uma bomba eletroquímica de oxigênio com transdutores de zircônia estabilizada com ítria. Foram refinadas, por meio de análise de Rietveld as estruturas cristalinas determinadas na análise por difração de raios X: perovskita cúbica (x = 0) e perovskita ortorrômbica (x 0). A condutividade elétrica foi analisada por medidas de espectroscopia de impedância na faixa de freqüências 5 Hz-13 MHz da temperatura ambiente até ~200 C. A deconvolução dos diagramas de impedância [-Z\"() x Z\'()] na faixa de temperaturas 300 < T(K) < 500 mostra dois semicírculos atribuídos às contribuições intragranular (grãos) e intergranular (contornos de grão) à resistividade elétrica. Os compactos sinterizado utilizando pós preparados pela síntese de estado sólido apresentam valores de resistividade intergranular e intragranular maiores que os compactos preparados com pós obtidos pela síntese química. O sinal elétrico (força eletromotriz) gerado sob exposição a oxigênio mostra que esses compostos podem ser utilizados em dispositivos sensores de oxigênio entre 600 e 1100C. Análises topográficas em microscópio de varredura por sonda em superfícies polidas e atacadas termicamente mostram detalhes morfológicos dos grãos, permitindo concluir que compactos sinterizados preparados com pós obtidos pela rota química são menos porosos que os preparados com pós obtidos pela rota convencional de síntese de estado sólido. Estes resultados estão de acordo com os resultados de medidas de espectroscopia de impedância. / CaxSr1-xTi1-yFeyO3-&delta;, x = 0, 0.5 and 1.0, y = 0 and 0.35, ceramic compounds were synthesized by reactive solid state synthesis of CaCO3, SrCO3, TiO2 and Fe2O3, and by the polymeric precursor technique. The ceramic powders were evaluated by thermogravimetry and differential thermal analysis, X-ray diffraction and scanning electron microscopy. Sintered ceramic pellets were analyzed by X-ray diffraction, scanning electron microscopy, scanning probe microscopy and impedance spectroscopy. The electromotive force resulting from the exposing the pellets to partial pressure de oxygen in the ~50 ppm in the 600-1100 oC range was monitored using an experimental setup consisting of an oxygen electrochemical pump with yttria-stabilized zirconia transducer and sensor. Rietveld analysis of the X-ray data allowed for determining the crystalline structures: cubic perovskite (y = 0) and orthorhombic perovskite (y 0). The electrical conductivity was determined by the two probe impedance spectroscopy measurements in the 5 Hz-13 MHz frequency range from room temperature to approximately 200 C. The deconvolution of the [-Z\"() x Z\'()] impedance diagrams in the 300 < T(K) < 500 range shows two semicircles due to intragranular (bulk) and intergranular (grain boundary) contributions to the electrical resistivity. Sintered pellets using powders prepared by the ceramic route present higher inter- and intragranular resistivity values than pellets prepared with chemically synthesized powders. The emf signal under exposure oxygen shows that these compounds may be used in oxygen sensing devices in the 600 - 1100 C range. Scanning probe microscopy topographic analysis of the polished and thermally etched surfaces of the pellets gave details of grain morphology, showing that pellets prepared with powders synthesized by the chemical route are less porous than the ones obtained by the ceramic route. These results are in agreement with the impedance spectroscopy results.

Ce travail de recherche est consacré à l’ingénierie d’un nouveau nanobiohybride à base de nanorubans de titanates pour la médecine régénérative. Dans un premier temps, les nanorubans ont été synthétisés par traitement hydrothermal et leurs caractéristiques morphologiques, structurales et chimiques ont été définies. Une caractérisation fine par différentes techniques de microscopie électronique à transmission a notamment permis de déterminer leur épaisseur; dimension qui n’avait encore jamais été mesurée. Par la suite, les nanorubans de titanates ont été fonctionnalisés par différents PEG hétérobifonctionnels préalablement synthétisés au laboratoire. Ces polymères présentent à l’une de leurs extrémités des groupements fonctionnels spécifiques pouvant se coupler à de nombreuses molécules biologiques. Des peptides de type collagène contenant des sites de reconnaissance cellulaire ont alors été greffés sur ces extrémités. Le nanobiohybride ainsi formé devra permettre l'adhésion et la prolifération des cellules favorisant in fine la cicatrisation et la régénération tissulaire. Pour évaluer les propriétés biologiques du nouveau nanobiohybride, la cytoxicité et le pouvoir agrégeant des nanorubans de titanes ont été déterminés par des tests MTT, réalisés sur deux populations de cellules (cardiomyocytes et fibroblastes) et par des tests d’agrégation plaquettaire (sang humain). Enfin, dans le cas d’une utilisation pour favoriser le processus de cicatrisation, le nouveau nanobiohybride a été formulé sous forme d’un hydrogel d’alginate de sodium permettant une application directe sur les tissus lésés. Pour confirmer l’intérêt de cette formulation galénique, des premiers tests in vivo ont été réalisés / This research work is devoted to new nanohybrid engineering composed of titanate nanoribbons for regenerative medicine. Over a first phase, nanoribbons were synthesized by hydrothermal treatment and their morphological, structural and chemical features were defined. A fine characterization by means of different techniques of transmission electron microscopy mainly enabled to determine their thickness; dimension which had never been measured so far. Subsequently, titanate nanoribbons were functionalized by different home-made heterobifunctional PEG. Those polymers present at one of their extremities specific functional groups being able to couple with numerous biological molecules. Some collagen type peptides containing cellular recognition sites were grafted onto those extremities. The so-formed nanobiohybrid will permit cellular adhesion and proliferation favouring in fine tissue healing and regeneration. To evaluate new nanohybrid biological properties, titanate nanoribbons cytoxicity and aggregating power were determined by MTT tests, performed on two cell populations (fibroblasts and cardiomyocytes) and platelet aggregation tests (human blood). Finally, when used to promote healing process, the new nanobiohybrid was formulated in the form of sodium alginate hydrogel permitting a direct application on damaged tissues. To confirm the interest of this galenic form, initial in vivo tests were realized

Abstract The main objective of this thesis was to research and develop a sensitive characterization method for dielectric powdery substances which could be utilized in various industrial and research fields. With modern electromagnetic simulation tools and the presented experimental measurements, the characterization of dielectric powders using an indirectly coupled open-ended coaxial cavity resonator operating in TEM mode at 4.5 GHz was found to have potential. The modelling and the experimental measurement results of commonly used dielectric powders revealed that, from the nine classical mixing rules, it was possible to derive reliable inclusion permittivity values when using the properties of the perturbed resonator and the Bruggeman symmetric and the Looyenga mixing theories. In addition, the determination accuracy of the inclusion permittivity was found to be greatly improved with the correction factor included into the previously presented permittivity equation of the quarter wave coaxial resonator. Determination of the dielectric losses was found to be reliable when using the differences between the perturbed and unperturbed resonator and the general mixing model equation. The sensitivity of the characterization method was researched with both humidity exposed and surfactant treated modified SiO2, Al2O3 and ZrO2 powders. Experimental results showed that the effect of adsorbed water on the particles was most pronounced when measuring the dielectric losses of the inclusions. Furthermore, a clear correlation with the theory of the general mixing model was found. Thus, in addition to the changed dielectric properties between dry and humidity exposed powders, it was also possible to determine the moisture content with reasonable accuracy. This correlated well with the traditional, mass based, determination. Stearic acid coating of the particles induced only a small change in inclusion permittivity but a notable change in dielectric losses. Unlike the cases with the larger particle sized SiO2 and Al2O3, the dielectric loss of ZrO2 inclusions, with stearic acid coating, was decreased significantly from 6.2 × 10-3 to 3.8 × 10-3. When characterizing magnesium and calcium titanate composite powders, with CaTiO3 molar ratios of 0.0, 0.02, 0.05 and 0.1, the method was found to have good sensitivity and accuracy. Additions of CaTiO3 exhibited a clear increase in net inclusion permittivities from 13.4 up to 14.9 and in dielectric loss tangents from 7.1 × 10-3 up to 8.5 × 10-3. In addition, a good correlation was found in the theoretical determination of the CaTiO3 molar ratios using the resonator measurement results and the general mixing model. The characterization method was proved to be capable of measuring the dielectric properties and detecting even very slight changes in the powders. Thus, the method could be utilized in various types of powdery material characterization, for example, in the analysis and quality control of different composite powders. / Tiivistelmä Väitöstyön päätavoitteena oli tutkia ja kehittää herkkä jauhemaisten eristemateriaalien karakterisointimenetelmä, jota voitaisiin hyödyntää usealla teollisuuden ja tutkimuksen alueella. Hyödyntäen nykyaikaisia sähkömagnetiikan simulointityökaluja sekä kokeellisia mittauksia, havaittiin TEM -moodissa 4,5 GHz taajuudella toimivan avoimen epäsuorasti kytketyn koaksiaalionteloresonaattorin soveltuvan karakterisoimaan eristejauheita. Sähkömagneettinenmallinnus yhdessä yleisesti käytettyjen eristejauheiden kokeellisten mittausten kanssa osoitti, että partikkelin permittiivisyysarvot voitiin johtaa luotettavasti. Tällöin käytettiin näytemateriaalilla täytetyn resonaattorin ominaisuuksia, sekä yhdeksästä vertaillusta klassisesta sekoitusyhtälöstä Bruggeman symmetristä ja Looyengan sekoitusteorioita. Tämän lisäksi tarkkuutta voitiin parantaa huomattavasti lisäämällä korjauskerroin aikaisemmin julkaistuun neljännesaalto-koaksiaaliresonaattorin permittiivisyyden määrittävään yhtälöön. Eristehäviöiden määrittäminen havaittiin luotettavaksi, kun käytettiin mitattavalla materiaalilla muutetun ja tyhjän resonaattorin ominaisuuksien eroja, sekä general mixing model -yhtälöä. Karakterisointimenetelmän herkkyys tutkittiin ilmankosteudelle altistetuilla, sekä pintakäsittelyllä muutetuilla SiO2, Al2O3 ja ZrO2 -jauheilla. Mittaustulokset osoittivat, että partikkelin pinnalle adsorpoituneen veden vaikutus oli kaikkein korostunein, kun mitattiin partikkelien eristehäviöitä. Tämän lisäksi havaittiin myös selkeä korrelaatio general mixing model -teorian kanssa. Näin ollen muuttuneiden eristeominaisuuksien määritysten lisäksi voitiin määrittää jauheen kosteussisältö riittävällä tarkkuudella. Tulokset korreloivat hyvin perinteisen massaeroihin perustuvan määrittämisen kanssa. Partikkelien pinnoitus steariinihapolla aiheutti ainoastaan pienen muutoksen permittiivisyyteen mutta merkittävän muutoksen eristehäviöihin. Toisin kuin suuremman partikkelikoon SiO2:lla ja Al2O3:lla ZrO2 -partikkelin eristehäviö laski huomattavasti steariinihappopinnoituksen jälkeen, 6,2 × 10-3:sta 3,8 × 10-3:een. Karakterisoitaessa magnesium- ja kalsiumtitanaattikomposiittijauheita käyttämällä CaTiO3 -moolisuhteita 0,0, 0,02, 0,05 ja 0,1 menetelmällä havaittiin olevan hyvä herkkyys ja tarkkuus mitattavan materiaalin muutoksille. CaTiO3 -määrän lisäykset aiheuttivat selkeän nousun partikkelien kokonaispermittiivisyyksissä, joka kasvoi 13,4:sta 14,9:ään, sekä häviötangentissa, joka kasvoi 7,1 × 10-3:sta aina 8,5 × 10-3:een. Tämän lisäksi resonaattorin mitattuihin ominaisuuksiin sekä general mixing model -yhtälöön perustuvan teoreettisen määrityksen havaittin korreloivan hyvin CaTiO3 -moolisuhteisiin. Karakterisointimenetelmän osoitettiin soveltuvan eristeominaisuuksien mittaukseen sekä havaitsemaan jopa erittäin pienet muutokset jauheiden eristeominaisuuksissa. Menetelmää voitaisiin hyödyntää usean tyyppisissä jauhemateriaalien ominaisuuksien määrityksissä, kuten esimerkiksi erilaisten komposiittijauheiden analysoinnissa ja laaduntarkkailussa.

Die Realstruktur des perowskitischen Modellsystems SrTiO3, welches in der Raumgruppe Pm-3m kristallisiert, wird durch die Sauerstoffvakanz als wichtigstem Defekt dominiert. Durch Temperaturbehandlung unter reduzierenden Bedingungen können Sauerstoffvakanzen in die Kristallstruktur eingebracht werden. Aufgrund ihrer positiven Ladung relativ zum Kristallgitter bewegen sie sich im elektrischen Feld entlang des TiO6-Oktaedernetzwerkes. Die Elektroformierung folgt dabei einem Arrheniuszusammenhang, wobei sowohl die Aktivierungsenergie als auch die Mobilität eine deutliche Abhängigkeit von der Kristallorientierung zeigen. Die Umverteilung der Sauerstoffvakanzen führt zu lokalen reversiblen Strukturänderungen, welche die Ausbildung einer neuen migrationsinduzierten feldstabilisierten polaren (MFP) Phase verursachen. In Abhängigkeit von der elektrischen Feldstärke zeichnet sie sich strukturell durch eine tetragonale Verzerrung der ursprünglich kubischen Elementarzelle aus und geht mit dem Verlust der Inversionssymmetrie einher. Die Polarisation in der Struktur wird durch die erwiesene Pyroelektrizität bestätigt und gilt als Grundlage, um anhand eines kristallographischen Symmetrieabstieges die Herleitung der Raumgruppe P4mm zu ermöglichen. Der durch die Migration hervorgerufene intrinsische Defektkonzentrationsgradient ruft eine elektromotorische Kraft hervor, deren Verwendung in einem elektrochemischen Energiespeicher experimentell nachgewiesen wurde. Diese neuen Funktionalitäten sind durch die Anwendung defektchemischer und kristallphysikalischer Konzepte infolge gezielter Materialmodifizierung unter Einfluss von Temperatur und elektrischem Feld zu verstehen. / The real structure of the perovskite-type model system SrTiO3, crystallizing in space group Pm-3m, is dominated by oxygen vacancies as most important defects. They are introduced in the crystal structure through heat-treatment under reducing conditions. Because of their positive charge relative to the crystal lattice, oxygen vacancies move in an electric field along the TiO6 octahedron network. This electroformation process follows an Arrhenius behavior. Both the activation energy and the mobility show an obvious dependence on the crystal orientation. Redistribution of oxygen defects causes local reversible structural changes, which involve the formation of a migration-induced field-stabilized polar (MFP) phase. In dependence on the electric field strength, this is structurally marked by a tetragonal distortion of the original cubic unit cell and accompanied by a loss of inversion symmetry. The polarisation in the crystal structure is confirmed by the proven pyroelectricity and serves for the argumentation to derive the space group P4mm by means of a crystallographic symmetry descent. The migration-induced intrinsic concentration gradient of oxygen vacancies leads to an electromotive force, whose application as electrochemical energy storage was proven experimentally. These new functionalities are explainable using defect chemistry and crystal physics in consequence of specific material modifications under the influence of temperature and external electric fields.

The first chapter introduces the basic principles governing the phenomenon like ferroelectricity, piezoelectricity and pyroelectricity, which influences the material properties for its device applications. An effort is made to examine the present status of material issues, measurement techniques and applications pertaining to the lead based PZT type systems. This chapter also highlights the objectives and the scope of work. The second chapter deals with the various basic experimental techniques and principles adopted for the synthesis and characterizations of materials which include phase and quantitative analysis by X-ray diffraction, density measurements, microstructures by scanning electron microscopy, electrical properties such as dielectric permittivity, dielectric loss, and piezoelectricity by impedance analyzer and piezometer etc. The materials were synthesized via two step solid state reaction by adopting a low temperature calcinations route. Further, hot processing was employed for densification and better control of microstructure of the ceramics. In the third chapter PZT1-x –PZNx (x=0, 0.1, 0.2 & 0.3) compositions prepared by the single step low temperature calcination method have been described. It is seen that the pyrochlore free perovskite phase could be obtained up to x=0.2 compositions. The effect of additives like Li and Mn on the structure, sinterability, microstructure, density and dielectric properties has been investigated. The improvement in densification and ferroelectric properties were observed for Li addition favor tetragonal phase while Mn addition compositions were inclined to pseudocubic phase. Further, the addition of Mn led to the significant decrease in Tc than the parent compositions compared to Li added compositions. In the fourth chapter, the X-ray diffraction data on pbzrx Ti1-x O3 (PZT) for x=0.48 to 0.52 are presented. High resolution x-ray studies for composition x=0.5 show the MPB which consists of monoclinic Zr rich studies and tetragonal Ti rich phase at room temperature. The refined structural parameters for MPB compositions have been obtained using least square Rietveld refinement program, FULLPROF 2006. The evolutions of lattice parameters of the system were also studied with respect to the temperature. The phase transformation in the system has been analyzed by x-ray diffraction pattern and dielectric measurements. The monoclinic phase transforms to tetragonal phase at 270oC after which the tetragonal phase transforms to paraelectric cubic phase at 370DoC. Dielectric properties show signature of the phase transformation. Hence, it is concluded to pole the MPB samples below 270o C to gain the advantage of increased ease of polarization reorientation for monoclinic phase. The fifth chapter deals with the systematic structural investigation on PZT1-y-PNZy (PZT-PNZ) and PZT1-y-PMNy (PZT-PMN) systems. In this chapter, an effort has been made to determine quantitatively the MPB phase contents and variation in Zr/Ti ratio of PZT-PZN and PZT-PMN systems. High resolution XRD data has been used for quantitative phase analysis using FULLPROF 2006. The correlation between the width of MPB and grain size has also been discussed for these systems. It is found that the addition of PMN and PZN to PZT system shifts the MPB towards pbZrO3 (PZ). The MPB can be regained by tuning the Zr/Ti ratio in the system. Further, there exists an inverse relation between the grain size and coexistence region in the system. It is seen that the MPB range is from x=0.48 to 0.58 and x=0.44 to 0.58 for 10% and 20% PZN concentration respectively. Similar trend has been obtained for the PZT-PMN system. The MPB ranges from x=0.46 to 0.53 and x=0.42 to 0.50 for 10% and 20% PMN respectively. The broadening of coexistence width is attributed to the lower grain size of our samples synthesized by adopting low temperature calcinations route. The sixth chapter deals with the hot pressing technique employed (adopting low temperature calcinations) for the synthesis of various PZT-PMN compositions with an intention of obtaining highly dense piezoceramics with fine, homogeneous and uniform microstructure. It also describes the dielectric, pyroelecrtic and pi ezoelectric properties were enhanced by hot processing technique. Li and Mn addition further improved the properties of the system. The seventh chapter investigates various nominal compositions of PZT-(Li, Nb) compositions based on certain assumptions. The attempt was made to introduce Li at A site and B site of ABO3 perovskite lattice. The ball milled, calcined powders were densified at<1000oC using hot pressing technique to prevent Li and Pb loss. High density ceramics have been studied for structural, dielectric, piezoelectric and pyroelectric properties. Through the clear cut evidence for the identification of Li site in the PZT system could not be established but the system which were synthesized under the assumption that Li substitutes A-site of the perovskite, favored the tetragonal phase and led to the enhancement in the dielectric, pyroelectric and piezoelectric properties. Further, their transition temperature was higher compared to the compositions where Li was tried to substitute B-site, which makes them promising candidates for transducer applications. The key finding in this thesis has been carried out by the candidate as part of the ph. D. programme. She hopes that this would constitute a worthwhile contribution towards the understanding of the behavior of lead based perovskites and in tailoring the properties of these ceramics towards device applications by the introduction of suitable additives in the system.

The following investigations have been carried out in this thesis 1)Cordierite is already well known for its low thermal expansion behaviour. Chemical substitutions at various octahedral and tetrahedral sites have been done and their thermal expansion characteristics have been studied Synthesis of cordierite in more reactive environment provided by AlF3 used as sintering aid has been attempted 2) Diffuse ferroelectric phase transition of lead based perovskite materials leads to low expansion region. Solid solutions of lead iron niobate with lead titanate is investigated to increase the structural distortion and see it this low expansion region can be extended to wider temperature Preparation of materials with higher tetragonal distortion In PbTi03- BlFeO3 system is undertaken to study the thermal expansion anisotropy. 3) Composites between lead iron niobate(+(x) and lead titanate (-(x below Tc) has been undertaken to prepare low expansion hulk over a wide temperature range 4) Acoustic emission has been employed as a tool to detect the microcracking in solid solutions between PFN1-x, PTx, and PT1-x, ,BFx, It is hoped to understand relation between magnitude of lattice distortion transition temperature and microcracking in ceramics of the class of materials.

This thesis presents experimental and related theoretical studies of pyrochlore titanate oxides, boron nitride nanotubes, and multiferroic bismuth ferrite. We have investigated these systems at high pressures and at low temperatures using Raman spectroscopy. Below, we furnish a synoptic presentation of our work on these three systems. In Chapter 1, we introduce the systems studied in this thesis, viz. pyrochlores, boron nitride nanotubes, and multiferroic BiFeO3, with a review of the literature pertaining to their structural, electronic, vibrational, and mechanical properties. We also bring out our interests in these systems. Chapter 2 includes a brief description of the theory of Raman scattering and infrared absorption. This is followed by a short account of the experimental setups used for Raman and infrared measurements. We also present the technical details of high pressure technique including the alignment of diamond anvil cells, gasket preparation, calibration of the pressure, etc. Chapter 3 furnishes the results of our pressure-and temperature-dependent studies of pyrochlore oxides which has been divided into eight different parts. In recent years, magnetic and thermodynamic properties of pyrochlores have received a lot of attention. However, not much work has been reported to address the quasiparticle excitations, e.g., phonons and crystal-field excitations in these materials. A material that shows exotic magnetic behavior and high degree of degenerate ground states can be expected to have low-lying excitations with possible couplings with phonons, thereby, finger-printing various novel properties of the system. Raman and infrared absorption spectroscopies can, therefore, be used to comprehend the novel role of phonons and their role in various phenomena of frustrated magnetic pyrochlores. Recently, there have been reports on various novel properties of these systems; for example, Raman and absorption studies [Phys. Rev. B 77, 214310 (2008)] have revealed a loss of inversion symmetry in Tb2Ti2O7 at low temperatures which has been suggested as the key reason for this frustrated magnet to remain in spin-liquid state down to 70 mK. Powder neutron-diffraction experiments [Nature 420, 54 (2002)] have shown that an application of isostatic pressure of about 8.6 GPa in spin-liquid Tb2Ti2O7 induces a long-range magnetic order of the Tb3+ spins coexisting with the spin-liquid phase ascribing this transition to the breakdown of the delicate balance among the various fundamental interactions. Moreover, Raman and x-ray studies have shown that Tb2Ti2O7,Sm2Ti2O7,and Gd2Ti2O7 undergo a structural transition followed by an irreversible amorphization at very high pressures (~ 40 GPa or above) [Appl. Phys. Lett. 88, 031903 (2006)]. In this chapter, therefore, we present our temperature-and pressure-dependent Raman studies of A2Ti2O7 pyrochlores, where ‘A’ is a trivalent rare-earth element (A = Sm, Gd,Tb, Dy,Ho, Er,Yb, and Lu; and also Y). Since all the group theoretically predicted Raman modes of this cubic lattice are due to oxygen vibrations only, in Part (A), we revisit the phonon assignments of pyrochlore titanates by performing Raman measurements on the O16 /O18 − isotope based Dy2Ti2O7 and Lu2Ti2O7 and find that the vibrations with frequencies below 250 cm−1 do not involve oxygen atoms. Our results lead to a reassignment of the pyrochlore Raman phonons thus proposing that the mode with frequency ~ 200 cm−1, which has earlier been known as an F2g phonon due to oxygen vibration, is a vibration of Ti4+ ions. Moreover, we have performed lattice dynamical calculations using Shell model that help us to assign the Raman phonons. In Part (B), we have explored the temperature dependence of the Raman phonons of spin-ice Dy2Ti2O7 and compared with the results of two non-magnetic pyrochlores, Lu2Ti2O7 and Y2Ti2O7. Our results reveal anomalous red-shift of some of the phonons in both magnetic and non-magnetic pyrochlores as the temperature is lowered. The phonon anomalies can not be understood in terms of spin-phonon and crystal field transition-phonon couplings, thus attributing them to phonon-phonon anharmonic interactions. We also find that the anomaly of the disorder activated Ti4+ Raman vibration (~ 200 cm−1) is unusually high compared to other phonons due to the large vibrational amplitudes of Ti4+-ions rendered by the vacant Wyckoff sites in their neighborhood. Later, we have quantified the anharmonicity in Dy2Ti2O7. We have extended our studies on spin-ice compound Dy2Ti2O7 by performing simultaneous pressure-and temperature-dependent Raman measurements, presented in Part (C). We show that a new Raman mode appears at low temperatures below TC ~ 110 K, suggesting a structural transition, also supported by our x-ray measurements. There are reports [Phys. Rev. B 77, 214310 (2008), Phys.Rev.B 79, 214437 (2009)] in the literature where the new mode in Dy2Ti2O7 at low temperatures has been assigned to a crystal field transition. Here, we put forward evidences that suggest that the “new” mode is a phonon and not a crystal field transition. Moreover, the TC is found to depend on pressure with a positive coefficient. In Part (D), we have presented our results of temperature-and pressure-dependent Raman and x-ray measurements of spin-frustrated pyrochlores Gd2Ti2O7, Tb2Ti2O7,and Yb2Ti2O7. Here, we have estimated the quasiharmonic and anharmonic contributions to the anomalous change in phonon frequencies with temperature. Moreover, we find that Gd2Ti2O7 and Tb2Ti2O7 undergo a subtle structural transition at a pressure of ~ 9 GPa which is absent in Yb2Ti2O7. The implication of this structural transition in the context of a long-range magnetically ordered state coexisting with the spin-liquid phase in Tb2Ti2O7 at high pressure (8.6 GPa) and low temperature (1.5 K), observed by Mirebeau et al. [Nature 420, 54 (2002)], has been discussed. As we have established in the previous parts that the anomalous behavior of pyrochlore phonons is due to phonon-phonon anharmonic interactions, we have tuned the anharmonicity in the first pyrochlore of the A2Ti2O7 series, i.e., Sm2Ti2O7,by replacing Ti4+-ions with bigger Zr4+-ions, presented in Part (E). Our results suggest that the phonon anomalies have a very strong dependence on the ionic size and mass of the transition element (i.e., the B4+-ion in A2B2O7 pyrochlores). We have also observed signatures of coupling between a phonon and crystal-field transitions in Sm2Ti2O7. In Part (F), we have studied spin-ice compound Ho2Ti2O7 and compared the phonon anomalies with the stuffed spin-ice compounds, Ho2+xTi2−xO7−x/2 by stuffing Ho3+ ions into the sites of Ti4+ with appropriate oxygen stoichiometry. We find that as more and more Ho3+-ions are stuffed, there is an increase in the structural disorder of the pyrochlore lattice and the phonon anomalies gradually disappear with increasing Ho3+-ions. Moreover, a coupling between phonon and crystal field transition has also been observed. In Part (G), we have examined the temperature dependence of phonons of “dynamical spin-ice” compound Pr2Sn2O7 and compared with its non-pyrochlore (monoclinic) counterpart Pr2Ti2O7. Our results conclude that the anomalous behavior of phonons is an intrinsic property of pyrochlore structure having inherent vacant sites. We also find a coupling between phonon and crystal-field transitions in Pr2Sn2O7. In the last part of this chapter, Part (H), we present our Raman studies of Er2Ti2O7. Here, we show that in addition to the anomalous phonons, there are modes that originate from photoluminescence transitions and some of these luminescence lines show anomalous temperature dependence which have been understood using the theory of optical dephasing in crystals, developed by Hsu and Skinner [J. Chem. Phys. 81, 1604 (1984)]. Temperature dependence of a few Raman modes and photoluminescence bands suggest a phase transition at 130 K. In Chapter 4, we furnish our pressure-dependent Raman studies of boron nitride multi-walled nanotubes (BNNT) and hexagonal boron nitride (h-BN) and compare the results with those of their carbon counterparts. Using Raman spectroscopy, we show that BNNT undergo an irreversible transition at ~ 12 GPa while the carbon counterpart, multi-walled carbon nanotubes, show a similar transition at a much higher pressure of ~ 51 GPa. In sharp contrast, the layered form of both the systems (i.e. h-BN and graphite) undergo a hexagonal to wurtzite phase at nearly similar pressure (~ 13 GPa of h-BN and ~ 15 GPa for graphite). A molecular dynamical simulation on boron nitride single-walled nanotubes has also been undertaken that suggests that the polar nature of the B−N bonds may be responsible for the irreversibility of the pressure-induced transformations. It is interesting to see that in hexagonal phase both the systems have almost similar mechanical property, but once they are rolled up to make nanotubes, the property becomes quite different. Chapter 5 presents the temperature dependence of the Raman modes of multiferroic thin films of BiFeO3 and Bi0.7Tb0.2La0.1O3. Though there have been several Raman investigations of BiFeO3 in literature, here we emphasize the observation of unusually intense second order Raman phonons. Our results have motivated Waghmare et al. to suggest a theoretical model to explain the anomalously large second order Raman tensor of BiFeO3 in terms of an incipient metal-insulator transition. In Chapter 6, we summarize our findings on the three different systems, namely, pyrochlores, boron nitride nanotubes, and BiFeO3 and highlight a few possible experiments that may be undertaken in future to have a better understanding of these systems.