Дисертації з теми "CERAMICS DOPED"

Tetragonal polycrystalline zirconia, commonly stabilised with 3 mol% yttria (3Y-TZP), became one of the most interesting ceramics for biomedical applications due to its biocompatibility and high mechanical properties. Among them, its high fracture toughness should be highlighted, which is due to the stress-induced tetragonal-to-monoclinic (t-m) phase transformation near a crack tip. However, the tetragonal grains on the Surface can also spontaneously transform to monoclinic phase in a humid environment, phenomenon known as low-temperature degradation (LTD), which is an important issue for applications in which water is present. Several methods have been proposed to increase the LTD resistance in zirconia, which range from improving the fabrication process in terms of grain size, density or residual stresses, to doping zirconia with other oxides, like magnesia or ceria. Particularly, ceria-stabilised zirconia (Ce-TZP) possesses higher LTD resistance and fracture toughness than 3Y-TZP, but lower fracture strength and hardness, partly because of its larger grain size. The approach proposed in this work consists in the improvement of mechanical properties of Ce-TZP by reducing grain size, without reducing LTD resistance. With this objective, starting from two compositions of Ce-TZP (10 and 12 mol% CeO2), different amounts of CaO and Y2O3 have been added to reduce the grain growth during sintering, and thus increasing the critical t-m transformation stress, and consequently flexural resistance and hardness. On the other hand, as flexural resistance is determined by fracture toughness from small superficial cracks, a novel reproducible methodology to produce superficial micro-notches by means of ultra-short pulsed laser ablation has been developed to measure this property. It has been highlighted that with the developed methodology very sharp cracks are produced, with the size in the order of natural cracks. Results show that small-crack fracture toughness is very different from values measured from methods using large cracks or from the indentation method. This thesis is submitted for the degree of Doctor of Philosophy in the program "Materials Science and Engineering" at the Universitat Politècnica de Catalunya as a compendium of published articles. The research described in this work was carried out by the author during the period from December 2012 to October 2017 under the supervision of Prof. M. Anglada in the Department of Materials Science and Metallurgical Engineering at the Universitat Politècnica de Catalunya, and during 3 research stays during 2015, 2016 and 2017 (406 days in total) in the Department of Materials Science (MTM) at KU Leuven (Belgium) under the supervision of Prof. J. Vleugels. The work described in this dissertation is original, unless otherwise detailed references are provided.
L'òxid de zirconi estabilitzat amb un 3 mol% d'itria és una ceràmica policristal·lina amb estructura tetragonal (denominada freqüentment zircònia, o bé 3Y-TZP) presenta propietats molt interessants per a aplicacions biomèdiques degut a la seva biocompatibilidad i altes propietats mecàniques. Entre elles, destaca la seva alta tenacitat de fractura, la qual és deguda a la transformació de fase tetragonal a monoclínica (t-m) induïda sota tensió al voltant de la punta d'una esquerda. No obstant, els grans tetragonals superficials també poden transformar-se de forma espontània a fase monoclínica en ambients humits, fenomen conegut com a degradació a baixa temperatura (LTD, per les seves sigles en anglès), la qual és un problema important en aplicacions on l'aigua hi és present. Diversos mètodes s'han proposat per tal d'augmentar la resistència a la LTD de la zircònia, des de la millora dels processos de fabricació en termes de tamany de gra, densitat o tensions residuals, fins al dopatge de la zircònia amb altres òxids, com magnèsia o cèria. Particularment, la zircònia estabilitzada amb cèria (Ce-TZP) posseeix una alta resistència a la LTD i major tenacitat de fractura que la 3Y-TZP, però menor resistència a fractura i duresa, en part degut a la seva major grandària de gra. L'estratègia seguida en aquest treball consisteix en la millora de les propietats mecàniques de la Ce-TZP mitjançant la reducció de la grandària de gra, sense disminuir la resistència a la LTD. Amb aquest objectiu, partint de dues composicions de Ce-TZP (10 i 12 mol% de CeO2), s'han afegit diferents quantitats de CeO i de Y2O3 per a reduir el creixement de gra durant la sinterització i així augmentar la tensió crítica per a la transformació t-m, i per tant la resistència a flexió i duresa. Per altra banda, donat que la resistència a flexió ve determinada per la tenacitat de fractura d'esquerdes petites superficials, s'ha desenvolupat una metodologia reproduïble per a produir micro-entalles superficials mitjançant l'ablació làser de puls ultra-curt per així mesurar aquesta propietat. S'ha posat en relleu que amb la metodologia desenvolupada es produeixen esquerdes molt afilades i de dimensions de l'ordre de les esquerdes naturals presents. Els resultats posen de manifest que la tenacitat de fractura per esquerdes petites són molt diferents de les obtingudes per mètodes amb esquerdes grans o a partir del mètode d'indentació. Aquesta tesis és dipositada pel grau de doctor en el programa "Ciència i Enginyeria dels Materials" de la Universitat Politècnica de Catalunya com a compendi de publicacions. La investigació descrita en aquest treball es va desenvolupar per l'autor durant el període de desembre de 2012 a octubre de 2017 sota la supervisió del professor M. Anglada al Departament de Ciència dels Materials i Enginyeria Metal·lúrgica de la Universitat Politècnica de Catalunya, i durant tres estades d'investigació durant 2015, 2016 i 2017 (406 dies en total) al Departament de Ciència de Materials (MTM) de KU Leuven (Bèlgica), sota la supervisió del professor J. Vleugels. El treball descrit en aquesta dissertació és original, llevat quan es proporcionen referències detallades.

The objectives of this research were to investigate the possibility of controlling the '/' phase ratio and morphology in Sialon ceramics. These objectives have been sought by the control of the starting composition, and by post sintering heat treatment. The main emphasis has been on the production of a series of ' and (+') Sialon ceramics with a minimum amount of the glass phase by the pressureless sintering technique and using ytterbium (Yb) as an ' stabilising element. The Yb additions were made via the oxide or the alumino-silicate presynthesised glass; the latter was found to improve the density. The XRD analysis of the as sintered materials revealed ' to be the dominant phase with minor contributions from ' sialon and/or or 12H AIN polytype. Additions of SiO2 or -Si3N4 were made to various materials to assess potential mechanisms for obtaining control over the microstructural development of '/' sialon materials. The addition of silica (SiO2) to sialon with high ' content in Yb system significantly improves the densification and increased the amount of ' phase. The incorporation of -Si3N4 as a seeding agent had a very small effect on the '/' phase ratio and the phase morphology. Further experiments were aimed at optimizing sinterability and sialon microstructure through the introduction of two ' stabilizing cations. Compositions were prepared that contained a combination of light and heavy rare-earth (Yb-Nd and Gd-Nd), and then pressureless sintered and compared with the single cation materials. Materials in the as sintered state were composed of a high ' sialon content with a minor amount of ' sialon and 12H AIN polytype indicating that the heavy rare-earth (which is the principal ' stabilizer) has a dominant effect although EDAX analysis confirmed the presence of both cations (light and heavy) within the ' structure. The research also compared, and developed an understanding of, the thermal stability of '-sialon using single Yb or mixed cations. The Yb single cation '/' materials exhibited excellent stability over a range of temperature (1200 - 1600°C) and for different periods of time up to 168 hrs. The heat treatments result in the crystallisation of the residual phase as a Yb garnet phase which formed at ˜1300°C. The mixed cation '/' materials showed some '-' transformation. The transformation was accompanied by dissolution of RA1O3 (normally crystallized with R=Nd,Gd) and crystallization of melilite.

The influence of the Na/Bi ratio on the electrical properties of nonstoichiometric NBT has been investigated in the Na0.5-xBi0.5+xTiO3+x series. The solid solution limit is very small -0.02 < x < 0.02, but the bulk conductivity has a dramatic change of ~ 4 orders of magnitude at 600 ℃, where it switches from an insulator to an ionic conductor for Na/Bi ≥ 1. Within the solid solution limit, the maximum bulk conductivity is achieved by replacing 1 at% Bi with Na atoms on the A-site (acceptor doping), which generates oxygen vacancies. Furthermore, the limit of oxygen vacancies is identified to be ~ 0.33 at%, which is illustrated by the geometric-independent parameter fmax. In general, ceramics prepared by hand grinding have better control of the nominal starting composition. The bulk conductivity of NBT can also be enhanced by Ga-acceptor doping on the B-site. By decreasing the Na/Bi ratio (donor doping) which compensates for oxygen vacancies, it switches from being an ionic conductor to an insulator. In contrast, by increasing the Na/Bi ratio (acceptor doping) which generates oxygen vacancies, the bulk conductivity remains the same, which is independent of the nominal oxygen content. In fact, the actual composition changes with increasing nominal oxygen vacancies, in order to maintain the actual oxygen content at a certain level. This self-recovery behaviour restricts the enhancement of ionic conductivity in NBT-based ceramics. The influence of B-site acceptor dopants and highly polarised Bi ions on the electrical properties has been investigated in the solid solution of NBT with Bi-based perovskite end members, such as BiGaO3 (BG), BiFeO3 (BF) and Bi(Ni0.5Ti0.5)O3 (BNiT). In general, the ionic conductivity is suppressed with increasing doping level due to the mobile oxygen ions being trapped by the accepter dopants on the B-site. The electronic conductivity is enhanced by transition metal ions, such as Fe3+ and Ni2+. For the BG-NBT series, the solid solution limit is ~ 6%, where the ionic conductivity is eliminated. The actual composition may be influenced by the presence of secondary phases, leading to a slight mismatch of the charge configuration between the A and B-site results in a dramatic change in conductivity. The rhombohedral phase is stabilised by BF-doping, where Tm associated with the permittivity increases from 320 to 445 ℃ when the BF content increases from 0 to 50%. The ionic conductivity is slowly supressed by the BF-doping, where p-type electronic conductivity becomes dominant for samples prepared in air when the BF content is 60%; however, ionic conduction may be dominant when annealed under N2 at 600 ℃. Furthermore, 1 at% Nb-doping for Ti can efficiently suppress the ionic conductivity. For the BNiT-NBT series, the solid solution limit is ~ 60%. Neutron diffraction studies confirm the high temperature tetragonal phase is stabilised at room temperature when the BNiT content is ≥ 40%. The coexistence of rhombohedral and tetragonal phases leads to permittivity peak broadening and a decrease of Tm to a lowest value of 186 ℃ during cooling when the BNiT content is 60%. p-type electronic conductivity becomes dominant when the BNiT content is ≥ 30% for samples processed and measured in air.

Rutile, TiO2, has drawn significant attention due to its unique properties. In this project, the structural and electrical properties of undoped and Cr/Al/Ga/Zn-doped rutile have been investigated. Undoped rutile is increasingly oxygen-deficient on heating in air above ~700 °C. The weight loss is generally too small for accurate measurement, but the electrical properties of quenched samples provide a sensitive qualitative indicator of oxygen content since their conductivity can vary by many orders of magnitude. The lattice parameters of quenched samples show an unusual dependence on quench temperature and, by implication, on oxygen stoichiometry. Lattice parameters increase with a small oxygen loss, δ; chemical expansion of the lattice occurs and is attributed to increase in average Ti-O bond lengths. At higher δ, lattice parameters start to decrease, giving a chemical contraction effect attributed to partial collapse of columns of edge-sharing TiO6 octahedra in the rutile structure and elimination of oxygen vacancies by crystallographic shear, CS, plane formation. Oxygen-deficient rutile samples quenched from above 700 °C are n-type, whereas samples annealed and measured at 450-500 °C are p-type and believed to be slightly oxygen-rich. Holes are located on oxide ions at or near the sample surface and arise from redox electron transfer between under-bonded surface oxide ions and adsorbed O2 molecules. Samples annealed between 550 and 600 °C show cross-over between n- and p-type behavior. Cr-doped rutile, Ti1-xCrxO2-x/2-δ, samples show a volume expansion at low Cr content, which is associated with the larger size of Cr3+ and the increase in average M-O bond length. At relatively higher Cr content, samples show an unusual volume contraction effect, which is also believed to be attributed to the formation of shear planes. The electrical properties of Cr-doped rutile are very dependent on composition x. Impedance results show that at low dopant concentration (0.001 ≤ x ≤ 0.01), samples are two-phase mixtures. The phase identified by R2C2 is more resistive than R1C1 and has larger activation energy. R2C2 represents a phase with very small Cr3+ dopant concentration, i.e. x = 0.001 ± 0.0005, and R1C1 represents a phase with relatively higher dopant content, i.e. x = 0.008 ± 0.002. In addition, since Cr content of both phases are different, R2C2 may have random point defects, such as oxygen vacancies. With relatively higher dopant content, oxygen vacancies collapse to form shear planes, i.e. defects present in the phase R1C1 may be random shear planes. At low x, Cr-doped rutile samples show mixed conduction, with p-type electronic conduction and oxide ion conduction. The location of holes is thought to be related with O2- ions as well. At higher x, a p-n transition is observed; samples are semiconducting and have small activation energy (~0.2 eV). Such semiconductivity is attributed to intrinsic hopping of charge carriers associated with Cr-Cr or Cr-Ti. Since orbital overlap may occur, charge carriers could hop between Cr3+ and Ti4+ and thus, Cr4+ and Ti3+ are generated and samples become semiconducting. Moreover, if oxygen vacancies collapse to form shear planes, the distance between Cr-Cr or Cr-Ti in face-sharing octahedra in the shear planes are smaller, and hopping of charge carriers becomes easier and the conductivity increases. Moreover, polaron hopping is an alternative description of the amount of semiconductivity. Electrical properties of Al-doped TiO2 are dependent on cooling condition. Samples show a p-n transition with increasing quench temperature. The p-type conductivity is driven by surface absorption of oxygen and associated with O- at the sample surface. Ga-doped TiO2 shows an n-p transition with increasing dopant concentration, whereas such electronic conduction is quite weak. Maybe with Ga3+ doping in TiO2, ionic compensation is the main compensation mechanism. With Zn2+ doping, it is clear that the homogeneity of rutile ceramics improves in some degree. However, Zn-doped samples still show evidence of a constriction resistance or a dipole orientation-related impedance. Zn-doped rutile samples show n-type conduction.

Mestrado em Ciência e Engenharia de Materiais
Cerâmicas ferroeléctricas do tipo relaxador têm despertado um renovado interesse científico, devido às suas muitas aplicações em tecnologia: memórias dinâmicas de acesso aleatório, condensadores “bypass”, dispositivos com aplicações na gama das microondas, sensores piroeléctricos de baixa temperatura, etc. Estas aplicações estão associadas a problemas conceptuais, relacionados com a distribuição de agregados polares e suas interacções, e também com o efeito mediador das matrizes em que os agregados estão inseridos. Este trabalho tem como objectivo principal o estudo das propriedades polares de cerâmicas em matrizes altamente polarizáveis. Entre os materiais fortemente polarizáveis, os paraeléctricos quânticos são os que mais se destacam. São muito sensíveis a defeitos, impurezas, tensões e campos eléctricos. Um dos mais interessantes é o paraeléctrico quântico Titanato de Estrôncio (SrTiO3), que apresenta uma transição de fase paraantiferrodistorsiva a ~ 110 K e que para concentrações adequadas de dopantes podem ser induzidos estados ferroeléctricos ou do tipo relaxador, geralmente localizados a baixas temperaturas. Estes estados podem corresponder à activação de novas bandas Raman de primeira ordem, a valores elevados de constante dieléctrica e a diferentes processos de relaxação polares. Assim, este trabalho está focado no estudo comparativo da rede, das propriedades dieléctricas e de relaxação polar em cerâmicas de SrTiO3 dopadas com Ítrio, Lantânio e Manganês, designados pelos sistemas Sr1- 1.5xLaxTiO3 (x=0.0133, 0.0533 and 0.13), Sr1-1.5xYxTiO3 (x=0.005 and 0.01), Sr1- xMnxTiO3 (x=0.01, 0.02 and 0.05) and SrTi1-xMnyO3 (y=0.01 and 0.05). Estes sistemas foram sintetizados pelo método convencional. A sua microestrutura e a estrutura cristalográfica foram analisadas através de difracção de raios X, microscopia electrónica de varrimento e transmissão, assim como por espectroscopia de energia dispersiva. Estudos detalhados de dinâmica de rede e de processos polares foram realizados utilizando as técnicas de espectroscopia Raman e de Correntes Termicamente Estimuladas (CTE), num amplo intervalo de temperaturas variando entre 10 K e a temperatura ambiente. Os resultados experimentais foram analisados através de modelos teóricos e discutidos com base na estequiometria química, características dos iões e a sua localização na rede. Pretende-se, a partir do conjunto de resultados obtidos, compreender melhor as propriedades do Titanato de Estrôncio. Embora, a dinâmica da rede do Sr1- 1.5xLaxTiO3 (x=0.0133, 0.0533 and 0.0833) ter sido já estudada, o efeito da dopagem de Ítrio é ainda desconhecido. No presente estudo, propomos analisar o espectro Raman obtido em ambos os sistemas e diferentes concentrações, com o objectivo de clarificar o comportamento da rede do SrTiO3 aquando a dopagem por iões aliovalentes. Os resultados experimentais indicam que a substituição do ião de Estrôncio por iões de Lantânio e Ítrio, dá origem a novas bandas nos espectros Raman, devido à perda do centro de inversão originado pelo efeito de dopagem do Titanato de Estrôncio. Este facto permite o acesso ao comportamento das bandas activas em infravermelho, sendo assim possível obter informações relativas à existência de estados polares a baixas temperaturas. O amortecimento parcial do modo polar TO1 obtido em todos os sistemas estudados aponta para a ausência de uma fase ferroelétrica a baixas temperaturas, confirmando resultados publicados anteriormente. O desvio da temperatura de transição para-antiferrodistorsiva para temperaturas mais elevadas nos sistemas dopados com Ítrio e Lantânio mostra que Ta é fortemente dependente do factor de tolerância da estrutura. Apesar das mudanças observadas em Ta nos sistemas dopado de SrTiO3, o comportamento em temperatura do modo mole associado à transição de fase antiferrodistorsiva A1g, pode ser descrito em termos de uma lei de potência semelhante à lei utilizada em trabalhos previamente realizados no sistema de SrTiO3 não dopado. Observam-se ainda alguns modos de baixa frequência, que por não poderem ser identificados com nenhum modo da rede, podem efectivamente estar associados a efeitos de desordem originados pela presença dos iões dopantes nestes sistemas. Vários trabalhos focaram-se já no estudo do SrTiO3 dopado, contudo os processos de relaxação polar observados em alguns destes sistemas não foram ainda completamente analisados. Assim, os processos de relaxação polar observados nas cerâmicas de SrTiO3 dopadas com Lantânio,Ítrio e Manganês foram estudados a partir de medidas de Currentes Termicamente Estimuladas (CTE) na gama de temperatura 10-300 K. Os resultados experimentais obtidos foram analisados através de modelos de relaxação dipolar e cargas espaciais, com o objectivo de determinar a natureza dos processos de relaxação. Estes resultados revelam a existência de diferentes processos de relaxação localizados na gama de temperaturas estudada. Com base em resultados previamente publicados, os mecanismos de relaxação detectados por CTE nos sistemas estudados e que ocorrem a baixas temperaturas foram associados a mecanismos de origem dipolar, enquanto que aqueles que se observam a altas temperaturas foram atribuídos à deslocação de cargas espaciais. Os resultados obtidos nos sistemas em que o ião dopante substitui o catião no lugar A ou B da rede apontam para a existência de mecanismos de relaxação polares muito distintos. Considerando apenas os processos de tipo dipolar observados a baixas temperaturas, foi possível identificar os correspondentes mecanismos de relaxação e concluir que estes dependem principalmente do raio iónico do ião dopante e do nodo da rede onde a substituição ocorre. Esta técnica (CTE) não convencional tem revelado ser de grande interesse como complemento de resultados previamente obtidos por espectroscopia dieléctrica. Esta dissertação é constituída por quatro principais capítulos. O capítulo 1 inclui uma revisão da literatura sobre materiais relacionados com SrTiO3 e concluí com os objectivos deste trabalho. No capítulo 2 são apresentados resultados referentes à dinâmica da rede dos sistemas Sr1-1.5xLaxTiO3 e Sr1- 1.5xYxTiO3 obtidos por espectroscopia Raman. Ainda neste capítulo, é introduzido o fenómeno da difusão Raman e apresentada um estudo comparativo da transição de fase para-antiferodistorsiva e a natureza paraléctrica quântica destes dois sistemas. Uma técnica distinta foi também utilizada na identificação dos mecanismos de relaxação em cerâmicas de SrTiO3 dopadas: Correntes Termicamente Estimuladas (CTE). No capítulo 3, este método eficaz é descrito e aplicado no estudo das cerâmicas de SrTiO3 dopadas com Lantânio, Ítrio e Manganês. Neste capítulo é desenvolvida uma discussão geral que relaciona estes resultados com os resultados apresentados na literatura. A análise teórica dos resultados experimentais permitirá uma melhor compreensão dos processos de relaxação observados a partir da determinação dos parâmetros característicos, tais como a energia de activação e tempo de relaxação a temperatura infinita. Esta informação proporcionará a identificação dos mecanismos subjacentes ao comportamento de relaxador anteriormente observado. Finalmente, os resultados obtidos serão resumidos no capítulo 4, bem como apresentadas algumas sugestões de trabalho futuro. ABSTRACT: Ferroelectric ceramics and relaxors have awaken a renewed scientific interest, due to their many applications in technology: dynamic random access memories (DRAMs), bypass capacitors, tuneable microwave devices, low temperature pyroelectric sensors, etc. These applications are associated with conceptual problems, which deal with the distribution of polar aggregates and their interactions, and also with the mediator effects of the matrices in which the aggregates are inserted. This work is centred in the study of the properties of polar ceramics in highly polarizable matrices. Among the highly polarizable materials the quantum paraelectrics stand out. Their electrical properties are very sensitive to defects, impurities, stresses, and applied electric fields. One of the most interesting quantum paraelectrics is Strontium Titanate (SrTiO3), which presents a paraantiferrodistortive phase transition at ~110 K and atomic lattice substitutions can induce ferroelectric or relaxor states, commonly located at low temperatures. These may yield the activation of Raman first order bands, high dielectric constant values and distinct polar relaxation processes. Consequently this work is focused on the comparative study of the lattice, dielectric and polar relaxation properties of SrTiO3 ceramics doped with Yttrium, Lanthanum, and Manganese ions, namely Sr1-1.5xLaxTiO3 (x=0.0133, 0.0533 and 0.13), Sr1-1.5xYxTiO3 (x=0.005 and 0.01), Sr1-xMnxTiO3 (x=0.01, 0.02 and 0.05) and SrTi1-xMnyO3 (y=0.01 and 0.05) systems. These systems were synthesised by the conventional mixed oxide method. Their crystallographic and micro structures were analysed through X-ray diffraction, scanning and transmission electron microscopy, together with energy dispersive spectroscopy methods. Detailed studies of both lattice dynamics and polar relaxations processes have been carried out through Raman and Thermally Stimulated Currents techniques, in a wide interval of temperatures ranging between 10 K and room temperature. The experimental results have been analysed through comprehensive theoretical models, and discussed on the basis of charge and chemical stoichiometry, ion characteristics, and site occupancy. With the ensemble of these results it is expected to provide a better understanding of the physical properties of Strontium Titanate. Although, lattice dynamics of Sr1-1.5xLaxTiO3 (x=0.0133, 0.0533 and 0.0833) have already been reported, the Yttrium doping effect is still unknown. In the present work we propose to analyze Raman spectra for both systems and different concentrations in order to clarify the aliovalent doping behaviour on SrTiO3 lattice. The obtained results show that Lanthanum and Yttrium substitution gives rise to new features in the Raman spectra, due to the loss of the center of inversion originated by doping SrTiO3 ceramics. Those features enable us to have also access to the behavior of infrared active bands, which provides additional information regarding the existence of polar states at low temperatures. The absence of the softening of the TO1 polar mode in all the systems studied supports the non-existence of a ferroelectric phase at low temperatures, which has been reported previously. The shift of the para-antiferrodistortive transition temperature (Ta) towards higher temperatures on both Lanthanum, and Yttrium doped systems clearly evidences that Ta is strongly dependent on the structural tolerance factor. Despite the observed changes in Ta in doped SrTiO3, the temperature behaviour of the antiferrodistortive soft mode A1g can be described in terms of a power law as it has been previously reported for pure SrTiO3. Moreover some low frequency modes, which could not be assigned to other lattice modes, may be apparently associated with disorder effects, stemming from the presence of dopants ions. Although there are several works addressed to the study of doped SrTiO3 systems, the polar relaxation processes observed in some of these systems have not been fully investigated. Therefore, polar relaxation processes in La-, Y- and Mn- doped SrTiO3 ceramics were studied by undertaking Thermally Stimulated Currents (TSC) measurements from room temperature to 10 K. The experimental results were analyzed by using dipolar and space-charge relaxation models in order to determine the nature of the relaxation processes involved. The results reveal the existence of different relaxation processes within the studied range of temperatures. Whereas at low temperatures (18-80 K), relaxation mechanisms of dipolar type were disclosed, space-charge relaxation processes could be identified at higher temperatures (150-300 K) confirming the previous dielectric results. In addition differences in the relaxation processes are observed for different substitute site of the lattice. Regarding dipolar relaxation processes observed at low temperatures, we were able to identify the corresponding mechanisms, which mainly depend on the ionic size, charge and site occupancy. This non conventional technique (TSC technique) has revealed to be a powerful technique to complement the results previously obtained by dielectric spectroscopy. This thesis will comprise four main chapters. Chapter one includes a survey of literature addressing the background of SrTiO3-related materials and concludes with the aims and objectives of this work. Chapter 2 disclose the lattice dynamics of Sr1-1.5xLaxTiO3 and Sr1-1.5xYxTiO3 systems revealed by Raman spectroscopy. Here, Raman diffusion theory is briefly introduced as well as the experimental set-up used in this work. This chapter concludes with a comparison between both systems regarding the para-antiferrodistortive phase transition and quantum paraelectricity features. A different technique was used to identify relaxation mechanisms in doped SrTiO3: Thermally Stimulated Depolarization Currents (TSDC) technique. In Chapter 3, this powerful method is reviewed and applied to La-, Y- and Mn- doped SrTiO3 ceramics. This chapter outlines the general discussion and links the results to the literature survey from the Chapter 1. The theoretical analysis of the experimental data should enable us to gain a better understanding of the relaxation processes through the knowledge of their characteristic parameters such as the activation energy, and relaxation time at infinite temperature. This information will allow determining which mechanisms are underlying the relaxation behaviour previously reported. Finally, a summary of the results obtained for the studied systems is presented in Chapter 4, which ends with a few concluding remarks and directions for further work.

A study of alumina (AI203 ) and magnesium aluminate spinel (MgAb04) was undertaken with the aim of investigating the changes in properties and microstructural characteristics upon doping with specific rare earth elements. Microscopic imaging and analysis of RE doped polycrystalline oxide ceramics has shown convincing evidence for monolayer segregation of RE cations to grain boundaries. State of the art aberration corrected scanning transmission electron microscopy (SuperSTEM I Daresbury Laboratories) has shown monolayer segregation to grain boundaries, and atomic resolution parallel electron energy loss spectroscopy has confirmed the presence of the RE cation at the grain boundary position. The region affected by segregation has been shown to extend no further than one monolayer from the centre of the grain boundary with RE cations occupying matrix cation boundary sites. The effect of RE dopants on the powder processing and sintering of high purity commercial grade precursor powders was investigated. Differences were found between doped alumina and spinel in the sintering whereby the alumina grain growth was restricted by grain boundary mobility such that the grain size was reduced for a given sintering temperature. The grain size of spinel was unaffected by sintering temperature. Differences in the fracture behaviour between doped alumina and spinel was found. The alumina samples manifested a change from trans-granular fracture to inter-granular fracture due to the addition of RE dopants. Spinel did not show such an effect. Alumina was shown to posess an approximate Hall-Petch relationship between hardness and grain size for both doped and undoped samples, such that sub-micron grain size samples posessed high hardness. Optical characterisation has shown the potential for the use of fine grained RE doped alumina and spinel samples for hard window applications. A reduction in the grain size of alumina to below 1 μm leads to a change in the scattering mechanism, thus reducing low angle scatter and birefringence due to the refractive index mismatch. The benefits to optical properties are in addition to the benefits in mechanical properties of a submicron grain structure.

Ceramics in the solid solution series, Ag(NbxTal-x)03 were fabricated to study their phase transitions and functional properties. As Ta concentration increased, the onset temperature of the M-phase transitions decreased in agreement with the Pawelcyck phase diagram. Electron diffraction and Raman spectroscopy revealed that the reduction in onset temperature may be attributed to a decrease in the correlation length and magnitude of cation displacements as the less polarisable Ta replaced the highly polarisable Nb ion in the B06 octahedra. For compositions with x = 0.75, a previously unreported phase transition was observed at ~ 3 l5K. Further studies were also performed on Li doped AgNb03, AgNbl/2 Ta 1/203 and AgTa03 compositions to determine whether their functional properties could be modified or enhanced. For > 0.05% Li doped AgNb03, the structure transformed from orthorhombic, Pbcm (√2ap,√2ap,4ap) to a new structure with a unit cell, √2ap√2ap,6ap, where ap is the pseudocubic perovskite lattice parameter. The new cell may be envisaged as arising from a complex modulated tilt system in which blocks of 3 in- phase tilted octahedra are rotated in antiphase with respect to each other around the c- axis. The onset of this transition resulted in asymmetry of the M3-M2 transition but there was no discernible improvement in ferroelectric properties at room temperature. For Li doped AgNbl/2 Ta1/2O3, the correlation length of cation displacements and to a lesser extent octahedral tilting decreased according to electron diffraction and Raman spectroscopy as Li concentration increased. The decrease in correlation length manifested itself as a highly perturbed domain structure at the solubility limit (~ 15 mol% Li) and resulted in convergence of the M-phase transitions to give a single broad dielectric response at the solubility limit. For Li doped AgTaO3, there were no apparent changes in the structure at room temperature. However, temperature dependent dielectric measurements clearly indicated that Li substitution had induced the onset of a ferroelectric phase transition in contrast to the incipient behaviour of AgTaO3. The temperature of the ferroelectric transitions increased with increase in Li concentration up to the solubility limit (~ 15 mol %.). The onset of the ferroelectric transition was also observed by Raman spectroscopy but further work is required to determine its precise nature.

Rare earth RE3+ doped fluorophosphates glasses and glass ceramics are among the most promising candidates for high efficiency laser generation in the near-infrared spectral region. Glass ceramics are polycrystalline materials of fine microstructure that are produced by the controlled crystallization (devitrification) of a glass. By developing fluorophosphate base glasses with appropriate compositions and by controlling crystal nucleation and growth in them, glass ceramics with special properties can be fabricated combining the advantages of fluorides (low phonon energy, low refractive indexes, extensive optical window, lower hygroscopicity) and oxides (high chemical and mechanical stability and high dopant solubility), resulting in enhancement of the RE3+ emissive properties. In this study, we present the synthesis by melting/quenting and structural/spectroscopic investigation of new glasses and glass ceramics with composition 25BaF225SrF2(30-x)Al(PO3)3xAlF3(20-z)YF3: zREF3, where x = 15, 20 or 25, RE = Er3+ an/or Yb3+ and Nd3+. A detailed structural investigation of a series of this glasses has been conducted, using Raman, solid-state nuclear magnetic resonance (NMR) and electron paramagnetic resonance (EPR) spectroscopies.
Vidros e vitrocerâmicas fluorofosfatos dopados com íons terras raras (TR3+) estão entre os candidatos mais promissores para a geração de laser de alta eficiência na região espectral do infravermelho próximo. As vitrocerâmicas são materiais policristalinos com microestrutura bem definida obtida a partir da cristalização controlada do vidro base. Desenvolvendo vidros base de fluorofosfato com composições apropriadas e controlando a nucleação e crescimento de cristais, vitrocerâmicas com propriedades especiais podem ser fabricadas combinando as vantagens dos fluoretos (baixa energia de fônons, baixos índices de refração, janela ótica extensa, baixa higroscopicidade) e óxidos (alta estabilidade química e mecânica e alta solubilidade dopante), resultando no aumento das propriedades emissoras dos íons TR3+. Neste estudo, apresentamos a síntese por fusão/resfriamento e investigação estrutural/espectroscópica de novos vidros e vitrocerâmicas com composição 25BaF225SrF2(30-x)Al(PO3)3xAlF3(20-z)YF3: zREF3, onde x = 15, 20 ou 25, RE = Er3+ an / ou Yb3+ e Nd3+. Uma investigação estrutural detalhada de uma série destes vidros foi conduzida utilizando espectroscopias Raman, de ressonância magnética nuclear de estado sólido (RMN) e de ressonância paramagnética eletrônica (EPR).

Increasing operating temperatures of gas engines is an effective approach to further improve thermal efficiency. But the ceramic top-coat (conventional 7-8 wt% yttria-stabilised zirconia, 7-8YSZ) in thermal barrier coatings (TBCs) cannot afford application temperatures above 1250 oC, the main reason of which is the accelerated phase transformation, due to quick cation diffusion. In addition, 7-8YSZ has relatively large thermal conductivity, small coefficient of thermal expansion (CTE) and large oxygen ion diffusivity. The relatively small CTE could create the mismatch between ceramic top-coat and metallic bond coat in thermal barrier coatings (TBCs), which can generate strain or stress at the interface to cause the spallation of TBCs. The oxygen atoms for the formation of thermally grown oxides (TGO) in TBCs come from ceramic top-coat, but the thickness of TGO cannot exceed 10 μm in order to avoid the spallation of TBCs. Therefore, it is extremely important to find new ceramic candidates to replace 7-8YSZ and to be used at elevated temperatures. According to these critical criteria of ceramic top-coat, three ceramic candidates were chosen and studied, which are stannate with pyrochlore crystal structure, erbia-based oxides stabilised zirconia and yttrium aluminium garnet. All candidates were synthesised using a sol-gel method. Crystalline phases of all as-synthesised powders were characterised by X-ray diffraction (XRD). Thermal conductivity, coefficient of thermal expansion (CTE), phase stability, chemical compatibility were also studied. Furthermore, molecular dynamics (MD) simulations were used to calculate thermal conductivity and CTE, and to analyse and explain experimental results. Pyrochlore has excellent phase stability and relatively low thermal conductivity, but relatively small CTE. From the consideration of crystal structure and ion radius, La3+ doped Yb2Sn2O7 ceramics with pyrochlore crystal structures are investigated as one TBCs candidate material. As La3+ and Yb3+ ions have the largest radius difference in lanthanide group, La3+ ions are expected to produce significant disorders by replacing Yb3+ ions in cation layers of Yb2Sn2O7. Both experimental and computational XRD data and phase analysis are carried out and good agreement has been obtained. The lattice constants of solid solution (LaxYb1-x)2Sn2O7 (x=0.3, 0.5, 0.7) increase linearly when the content of La3+ is increased. The thermal properties (thermal conductivity and coefficients of thermal expansion) have been compared with the traditional 8 wt% yttria-stabilised zirconia (8YSZ) and La2Zr2O7 (LZ). La3+ doped Yb2Sn2O7 exhibits lower thermal conductivities than un-doped stannates and thermal conductivity deceases with the increase of doping concentration, but it reaches a minimum value when x=0.5 then increases again. Thus among (LaxYb1-x) 2Sn2O7 compositions, (La0.5Yb0.5)2Sn2O7 exhibits the lowest thermal conductivity (0.851 W•m-1•K-1 at room temperature), which is much lower than that of 8YSZ (1.353 W•m-1•K-1). Moreover, it possesses a high coefficient of thermal expansion (13.530×10-6 K-1 at 950 oC). In addition, the simulation results obtained from the MD simulations, indicate that the variation of thermal conductivity with increasing doping concentration in solid solution (LaxYb1-x) 2Sn2O7 (x=0.3, 0.5, 0.7) is independent of external effects, like porosity, grain size and grain boundary. Stabilised zirconia is still one of the most popular TBCs top-coat systems, but new stabilisers are still under-seeking. For stabilised zirconia, to improve thermal phase stability and further reduce thermal conductivity are the most urgent challenges. Erbia-based rare earth oxides stabilised zirconia is studied at 1400 oC. XRD patterns confirmed that erbia-stabilised zirconia have the non-transformable phase. Thermal conductivity decreases and thermal phase stability is improved with the increase of the erbia content. Mono- or bi-rare earth oxides doped erbia-stabilised zirconia is studied to further reduce thermal conductivity due to creating cluster defects. The influences on thermal phase stability produced by different dopants are also analysed. The chemical formulas of Mono- or bi-rare earth oxides doped erbia stabilised zirconia are Er0.05R0.05Zr0.9O1.95 (R=Yb, Gd and Dy) and Er0.04R0.04 R*0.04Zr0.88O1.94 (R+R*=Yb+Dy, Yb+Gd and Dy+Gd), respectively. Thermal conductivities of Er0.05R0.05Zr0.9O1.95 (R=Yb, Gd and Dy) and Er0.04R0.04 R*0.04Zr0.88O1.94 (R+R*=Yb+Dy, Yb+Gd and Dy+Gd) are lower than that of conventional 8YSZ and also they exhibit comparable CTEs to 8YSZ. Importantly, it is found that the thermal phase stability is dependent on the tetragonality, c/a√2 ratio. When the c/a√2 ratio decreases from 1.010 to 1.006 for mono- or bi-rare earth oxides doped erbia, the thermal phase stability is better. Yttrium aluminium garnet (YAG) is also an attractive candidate as thermal barrier material, due to its relatively low thermal conductivity, low oxygen diffusivity and good phase stability at elevated temperatures. YAG has a complex crystal structure, in which Y3+ ions locate in dodecahedra and Al3+ ions in octahedra and tetrahedra. Replacing the host cations with rare earth elements can cause the distortion of polyhedrons which influences the thermal properties of YAG. Because the space inside the octahedra are relatively small, Yb3+ ions which have the smallest ionic radial sizes in the lanthanide series, have been selected and doped on dodecahedral and octahedral sites to investigate the effects on thermal conductivity and thermal expansion. The variation of lattice constant indicates that Yb3+ ions are located on the dodecahedra or octahedra. In addition, when Yb3+ ions replace Al3+ ions on octahedral sites, thermal conductivity at room temperature is dramatically reduced and the coefficient of thermal expansion is over 10×10-6 K-1 at high temperatures, which results from the expansion of octahedra due to Yb3+ ions with much larger radii compared with the host cations (Al3+ ions). On the contrary, replacing Y3+ ions with Yb3+ ions in dodecahedra, thermal conductivity gradually reduces but the coefficient of thermal expansion is smaller, due to the relatively small ionic radii of Yb3+ ions causing the contraction of the dodecahedra. Additionally, it is found that the largest decrease in thermal conductivity results from phonon scattering caused by the large atomic mass difference rather than the relatively small difference in ionic radius.

Hydroxyapatite (HA) based bioceramics have been developed to treat bone defects for the last 30 years. Doping HA with elements is a common approach to increase mechanical strength, biocompatibility and osteointegrity. Bone morphogenetic protein (BMP)-containing bioceramic composites enhance osteointegrity and induce bone formation. Strontium (Sr) is currently used to treat osteoporosis clinically as this element inhibits bone resorption and stimulates bone formation. In this study, HA was doped with silicon (Si), Sr, BMP-2 and evaluated in cortical bone defect healing. Ceramics were produced and tested mechanically after characterization. Sr release from ceramics was assessed. Ceramics were further evaluated in in vitro and in vivo conditions. X-ray diffraction analysis results of HA were in line with the literature and Sr-Si-HA ceramics showed similar intensities with HA. Ceramics had 36.9 to 41.6% porosity. Compression strength of Sr1000-Si-HA ceramics was 117.5 MPa which was more than that of the other groups. Consistent Sr release was observed in the Sr1000-Si-HA and the Sr250-Si-HA groups. Sr1000-Si-HA and Sr250-Si-HA ceramics showed higher cellular proliferation rates than the other groups in vitro. BMP addition increased alkaline phosphatase activities and DNA amounts. BMP-Sr-Si-HA group presented higher (0.304±
0.02 g/cm2) bone mineral density values than the other groups 4 weeks after implantation however differences between groups were not significant in vivo. Sr-Si-HA and BMP-Sr-Si-HA composites stimulated new bone formation at cortical bone defects of tibia according to micro computerized-tomography and histological results. Findings of this study promote future research on Sr containing bioceramics in treatment of orthopedic problems.

Donor doped barium titanate ceramics are well known for their Positive Temperature Coefficient of Resistance (PTCR) characteristic above the crystallographic transition temperature, T° '130°C, where the material changes from the ferroelectric state to paraelectric. The shape and magnitude of the PTCR characteristic are known to be dependent on the composition and preparation of the ceramic, the presence of impurities, particularly donor dopant concentration and acceptor ions, and the sintering conditions. Thirty years ago Heywang proposed a model based on the presence of two-dimensional resistive grain boundary layers consisting of discrete electron traps located in energy between the conduction and valence bands, to explain the PTCR effect. Donor doped barium titanate samples were prepared in a number of different ways: the variation of donor concentration, the addition of impurity acceptor ions, reduction of the sintering temperature and variation of the sintering atmosphere. These samples were investigated by examining their microstructure and their electric and dielectric properties, both at room temperature and above the transition. Theoretical analysis of the experimental results, based on the Heywang model, was then performed to investigate the effects of preparation on the grain boundary layer characteristics. Resistivity - temperature measurements were carried out to find the effect of composition and sintering conditions on the PTCR characteristic and capacitance - temperature measurements demonstrated the effects of donor and acceptor incorporation on the dielectric properties of barium titanate. Grain boundary and grain bulk resistance were separated by means of a. c. impedance methods at room temperature, where the effects of composition and sintering on each were observed. Finally, current - voltage measurements between TT and the resistivity maximum were made for samples containing different donor concentrations, to examine the current conduction mechanism. Detailed analysis of the electric and dielectric measurements permitted the effects of composition and sintering on the grain boundary layer characteristics to be determined. Acceptor state densities were estimated using the resistivity - temperature measurements and capacitance - temperature results, between TT and the resistivity maximum. Resistivity - temperature measurements above the maximum enabled acceptor energies to be estimated. Analysis of the dielectric properties showed that neither the composition nor sintering atmosphere affected the dielectric properties of the grain boundary layers, which were found to obey the Curie-Weiss law above the transition temperature in the same way as the grain bulk. The observed effects of the changes in the preparative conditions to the electric and dielectric properties were explained in terms of the Heywang model and microstructural development, resulting from modifications to the grain boundary layers. The conduction mechanism was examined by means of current - voltage measurements above the transition temperature and below the resistivity maximum. In contrast to the prediction of Heywang. this was found to be predominantly diffusion limited.

Injectable biomaterials that are able to set in situ provide clinicians a valuable tool in treating patients with clinically relevant tissue defects, as these can be administered via minimally invasive procedures and able to fill complex-shaped tissue defects. Injectable biomaterials also allow for novel fabrication techniques that aid in developing complex structures that mimic biological tissue. However, clinically-used injectable biomaterials that are currently available possess critical and inherent shortcomings that are difficult to address. The aim of this thesis was to develop and characterize new synthetic injectable biomaterials to address some of these shortcomings. To achieve this aim, the works in this thesis combined two distinct strategies: (1) utilize injectable material platforms not currently established for clinical use, but proven to be biocompatible and exhibit promising properties for filling musculoskeletal tissue defects; and (2) further improve the physicochemical and biological properties of these injectable materials, by incorporating bioactive doped calcium silicate ceramics. In particular, this thesis focused on the development and characterization of three such novel injectable biomaterials: (1) thermoplastic polycaprolactone composited with baghdadite (Ca3ZrSi2O9); (2) strontium-hardystonite (Sr-Ca2ZnSi2O7, Sr-HT)-phosphate cement; and (3) self-setting polyvinyl alcohol hydrogel composited with Sr-HT. All the synthetic injectable materials developed in this thesis possess sets of simultaneous ideal physicochemical and biological properties that address the limitations of injectable biomaterials currently established in the clinical setting. This thesis demonstrates that the physicochemical and biological properties of various injectable material platforms can be improved significantly by incorporating DCSC such as baghdadite and Sr-HT in order to render these materials ideal for filling various musculoskeletal tissue defects.

Recent theoretical calculations have predicted that ZnO doped with Mn exhibits room-temperature ferromagnetism. This has attracted intensive interest, because of its prospective applications in spintronic devices. However, experimental studies on Mndoped ZnO materials have revealed that their magnetic properties vary from the ferromagnetic through antiferromagnetic and spin-glass to the paramagnetic. This makes the question of the origin of ferromagnetism in Mn-ZnO materials become more complicated.

Kyoto University (京都大学)
0048
新制・課程博士
博士(人間・環境学)
甲第15469号
人博第499号
新制||人||122(附属図書館)
21||人博||499(吉田南総合図書館)
27947
京都大学大学院人間・環境学研究科相関環境学専攻
(主査)教授 田部 勢津久, 教授 杉山 雅人, 准教授 木下 俊哉
学位規則第4条第1項該当

In recent years, zirconia ceramics have been used in many applications. Some of them include tool forming due to its high strength and fracture toughness, thermal barrier coatings on turbines engines due to its low thermal conductivity, as an electrolyte in the elaboration of Solid Oxide Fuel Cells (SOFC) for its high ionic conductivity, and in the manufacture of dental implants for its biocompatibility and its aesthetic appearance. Biomedical zirconia is mainly tetragonal zirconia doped with 3 mol% of yttria (3Y-TZP). However, it is sensitive to the hydrothermal degradation phenomenon. This is the reason for investigating the processing and mechanical properties of ceria doped zirconia-based ceramics since their high aging resistance is well established. Ce-TZP ceramics present low strength due to their large grain size. Many studies have been devoted to finding a way to reduce grain size. Several methods have been proposed, such as the addition of a stabilizer, or a second phase, the improvement of powder processing, or the modification of the sintering conditions. Following the above ideas, this thesis aims to reduce the grain size in 10Ce-TZP ceramics (10 mol% CeO2), with the aim to increase the strength by three different strategies. The first strategy has been manufacturing composites with alumina (a-Al2O3) addition by colloidal processing. The second strategy has been to use calcium oxide (CaO) as a co-stabilizer and adding alumina (a-Al2O3) as a second phase. The third strategy was directed to improve processing by combining ball milling, slip casting, and subsequent powder grinding. In all these studies microstructural characterization, and mechanical has been performed. The hydrothermal degradation test was also measured to evaluate the resistance of the elaborated composites to aging. The results obtained in the two first strategies showed a reduction in the grain size, which led to an increase in hardness and strength. However, in the third strategy, the grain size was not reduced as much as in the previous strategies, which allowed to obtain a moderate fracture toughness and a slight increase in hardness and strength compared to the 10Ce-TZP base material. A high hydrothermal degradation resistance was achieved for all the elaborated composites in all the strategies.
En los últimos años, las cerámicas de circona se han utilizado en muchas aplicaciones. Algunos de ellos incluyen el conformado de herramientas por su alta resistencia y tenacidad a la fractura, recubrimientos de barrera térmica en motores de turbinas por su baja conductividad térmica, como electrolito en la elaboración de Pilas de Combustible de Óxido Sólido (SOFC) por su alta conductividad iónica, y en la fabricación de implantes dentales por su biocompatibilidad y su apariencia estética. La circona biomédica es principalmente circona tetragonal dopada con un 3% molar de itria (3Y-TZP). Sin embargo, es sensible al fenómeno de degradación hidrotérmica. Esta es la razón para investigar el procesamiento y las propiedades mecánicas de la circona dopada con ceria, debido a su alta resistencia al envejecimiento. Sin embargo, la circona dopada con ceria presenta baja resistencia mecánica debido a su gran tamaño de grano. Se han dedicado muchos estudios a encontrar una forma de reducir el tamaño de grano. Se han propuesto varios métodos, como la adición de un estabilizador, o una segunda fase, la mejora del procesamiento del polvo o la modificación de las condiciones de sinterización. Siguiendo las ideas anteriores, esta tesis tiene como objetivo reducir el tamaño de grano en cerámicas 10Ce-TZP (10% mol de CeO2), con el objetivo de aumentar la resistencia mediante tres estrategias diferentes. La primera estrategia ha sido la fabricación de compuestos con adición de alúmina (a-Al2O3) mediante procesamiento coloidal. La segunda estrategia ha sido utilizar óxido de calcio (CaO) como co estabilizador y añadir alúmina (a-Al2O3) como segunda fase. La tercera estrategia dirigida a mejorar el procesamiento mediante la combinación de molienda de bolas, moldeo por barbotina y posterior molienda de polvo. En todos estos estudios se ha realizado una caracterización microestructural, y mecánica. También se ha medido la degradación hidrotérmica para evaluar la resistencia de los compuestos elaborados al envejecimiento. Los resultados obtenidos en las dos primeras estrategias mostraron una reducción del tamaño de grano, lo que condujo a un aumento de la dureza y la resistencia. Sin embargo, en la tercera estrategia, el tamaño de grano no se redujo tanto como en las estrategias anteriores, lo que permitió obtener una tenacidad a la fractura moderada y un ligero aumento de dureza y resistencia en comparación con el material base 10Ce-TZP. Se logró una alta resistencia a la degradación hidrotérmica para todos los compuestos elaborados en todas las estrategias.
Ciència i enginyeria de materials

Lasers which operate at both high average power and energy are in demand for a wide range of applications such as materials processing, directed energy and EUV generation. Presented in this dissertation is a high-power 1 ?m ytterbium-based hybrid laser system with temporally tailored pulse shaping capability and up to 62 mJ pulses, with the expectation the system can scale to higher pulse energies. This hybrid system consists of a low power fiber seed and pre-amplifier, and a solid state thin-disk regenerative amplifier. This system has been designed to generate high power temporally tailored pulses on the nanosecond time scale. Temporal tailoring and spectral control are performed in the low power fiber portion of the system with the high pulse energy being generated in the regenerative amplifier. The seed system consists of a 1030 nm fiber-coupled diode, which is transmitted through a Mach-Zehnder-type modulator in order to temporally vary the pulse shape. Typical pulses are 20-30 ns in duration and have energies of ~0.2 nJ from the modulator. These are amplified in a fiber pre-amplifier stage to ~100 nJ before being used to seed the free-space Yb:YAG thin-disk regenerative amplifier. Output pulses have maximum demonstrated pulse energies of 62 mJ with 20 ns pulse after ~250 passes in the cavity. The effects of thermal distortion in laser and passive optical materials are also. Generally the development of high power and high energy lasers is limited by thermal management strategies, as thermally-induced distortions can degrade laser performance and potentially cause catastrophic damage. Novel materials, such as optical ceramics, can be used to mitigate thermal distortions; however, thorough analysis is required to optimize their fabrication and minimize thermal distortions. Using a Shack-Hartmann wavefront sensor (SHWFS), it is possible to analyze the distortion induced in passive and doped optical elements by high power lasers. For example, the thin-disk used in the regenerative amplifier is examined in-situ during CW operation (up to 2 kW CW pump power). Additionally, passive oxide-based optical materials and Yb:YAG optical ceramics are also examined by pumping at 2 and 1 ?m respectively to induce thermal distortions which are analyzed with the SHWFS. This method has been developed as a diagnostic for the relative assessment of material quality, and to grade differences in ceramic laser materials associated with differences in manufacturing processes and/or the presence of impurities. In summation, this dissertation presents a high energy 1 ?m laser system which is novel in its combination of energy level and temporal tailoring, and an analysis of thermal distortions relevant to the development of high power laser systems.
Ph.D.
Doctorate
Optics and Photonics
Optics and Photonics
Optics

Les céramiques conductrices à base de ZnO suscitent un important intérêt pour leurs applications comme varistances, capteurs de gaz, électrodes transparentes, dispositifs piézoélectriques, électro-optiques ou thermoélectriques. Le dopage de ZnO confère aux céramiques formées des propriétés électriques et optiques remarquables caractérisées par une transparence optique modulée, des énergies de liaison élevées pour les excitons, et des conductivités électriques atteignant 0.1 MS.m-1. La grande conductivité de ZnO est intimement liée aux éléments dopants, à la microstructure des céramiques et aux conditions de synthèse et traitements. Les joints de grains, la structure cristalline, le désordre structural et les défauts électroniques contribuent au comportement électrique des matériaux. Le présent travail de thèse s'inscrit dans ce contexte et porte sur la mise en œuvre de méthodes de fabrication de céramiques à base de ZnO co-dopées par des éléments (Al, Ti, Mg) et l'étude de leurs caractéristiques physiques incluant la structure cristalline, la microstructure et le comportement électrique. Ainsi, des études exhaustives ont été menées par des méthodes structurales (RX, Raman), microscopies (MET, MEB) et de résonance magnétique (RPE, RMN) sensible à l'ordre local et aux défauts électroniques actifs. La conductivité est ajustée par la nature des éléments dopants, l'atmosphère de frittage et les méthodes de fabrication par solutions solides ou par frittage flash (SPS). La corrélation "Préparation - Structure - Conductivité" a été établie pour la réalisation de céramiques à base de ZnO avec de fortes conductivités ouvrant la voie à des applications technologiques potentielles
ZnO based ceramics with appropriate doping elements show excellent electrical and optical properties such as high exciton binding energies, a modulated optical transparency and high electrical conductivities. Therefore, ZnO based conducting ceramics have been extensively investigated in the aim of their application as resistors, visitors, gas sensors, transparent electrodes, solar cell windows, piezoelectric, electro-optical and thermoelectric devices. The high conductivity of ZnO ceramics up to 0.1MS·m-1 is closely related to the doping elements along with the ceramic microstructure and the processing conditions with particular effects of grain boundaries, crystalline structure and structural disorder within the ceramics. Thus, the present thesis is devoted to the fabrication by sintering under defined conditions (sintering atmospheres, processes) of ZnO based ceramics co-doped by (Al, Ti, Mg) , the investigations and deep analysis of their related properties including crystalline structure, micro-structure and the electrical behavior. Exhaustive studies were developed on the doped ceramics by using structural methods (XRD, Raman), microscopy (TEM, SEM) and magnetic resonance (EPR, NMR) probing the local order and electronic active defects. The conductivity is adjusted by the nature of the structure influenced by the doping elements, the sintering atmosphere, and the sintering method. The correlation "Preparation - Structure - Conductivity" has been established to pave the way for the potential technological applications of highly conducting ZnO-based ceramics

博士
國立中山大學
材料與光電科學學系研究所
100
It is particularly intriguing in the role of the solid-state dopants, which are not only responsible for the semiconductivity, but also the dielectric properties of BaTiO3 ceramics, e.g. CaO, MgO, Y2O3, CoO and MnO2 for EIA-X7R characteristics. We have chosen to investigate three important processing parameters, oxygen partial pressure (pO2) for sintering, and two alkali-earth-metal oxides, i.e. CaO and MgO for solid-state additives in order to study how microstructure and dielectric properties are determined by them. They are used in MLCC industry for the effect in attaining X7R characteristics and protecting against or improving for dc degradation. Apart from establishing the temperature-dependent dielectric properties, i.e. temperature-coefficient of capacitance (TCC), for both qualitative and quantitative analysis, crystalline phases in sintered ceramics of tetragonal mixed with cubic, orthorhombic and rhombohedral phases are studied using XRD and Raman spectroscopy. For microscopic studies, SEM and TEM techniques, e.g. CBED and LACBED, combined with EDS are used to study phases both in core-shell. We will examine and confirm the fidelity of whether core-shell grains are induced by chemical inhomogeneity, and more importantly, if the diffuse phase transition is caused by such microstructure with direct observations, as previous studies in perovskite-related ceramics, and crystalline phase determination for these grains. It is also an objective that we investigate why and how, by what mechanism, the chemically similar alkali-earth metal oxides should impart completely different (and indeed opposite) effect in protecting against dc degradation. Keywords: MLCC, dc degradation, DPT, Raman spectroscopy, electron microscopy.

碩士
國立臺南大學
材料科學系碩士班
96
This research is composed of two parts. First, the ceramics of Ag1-xLixNbO3（ALN） solid solution were sintered for Li concentration range 0≦x≦0.1. We investigate the phase transition behavior and dielectric properties of Li-doped with different concentration by using XRD、SEM、DSC、dielectric constant and Raman scattering. As a result, with increasing lithium contents, the phase transition of M1-M2 was shifted to lower temperature while the phase transition of M3-O1 was shifted to higher temperature. The orthorhombic symmetry was changed to tetragonal symmetry when x is greater than 0.05. The maximum value of the dielectric constant at x＝0.05 was 400, and the dielectric loss value was controlled below 5%. Second, the ceramics of Ag1-yBiy/3NbO3 solid solution were sintered for Bi concentration range 0≦y≦0.15. Using the same analytic instruments, we investigate the phase transition behavior and dielectric properties of Bi-doping with different concentration. Room and high temperature phase transition were shifted to lower temperature by Bi-doping. Phase transition points were shifted to become fast and the phase structure was changed toward tetragonal symmetry after y>0.15. In addition, the value of dielectric loss was controlled below 0.2% from 250K to 450K.

ZnO based materials have attracted much attention both in science and in technology, due to their rich properties and wide applications. The study of the sintering and electrical properties of ZnO based materials is important for their applications, such as the varistor application. The present work is concerned with the study of the sintering and electrical properties of Mn-doped and Aldoped ZnO ceramics, since these dopants play important roles in ZnO varistors. In this work, samples of undoped, Mn-doped, and Al-doped ZnO ceramics were prepared via conventional ceramic processes, and the dopants were incorporated using ethanol solutions of Mn(NO3)2.4H2O and Al(NO3)3.9H2O. The solid solution and second phases in Al-doped and Mn-doped ZnO ceramics were investigated by XRD, SEM with EDS, and TEM with EDS techniques, in order to understand their effects on the sintering and electrical properties of ZnO. The results showed that, for Al-doped ZnO, a second phase of spinel ZnAl2O4 forms at ~800?C and is stable at higher temperatures. The solid solubility of Al in ZnO was determined to be around 600 atomic ppm at 1200?C. For Mn-doped ZnO, a second phase of cubic ZnMnO3 is formed at temperatures ? 450?C and transforms into another second phase of tetragonal ZnMn2O4 at ~1100?C. The second phases gradually dissolve into the ZnO phase and are no longer detected at 1200?C in the samples with Mn contents ? 1.2 mol%. The densification and grain growth of ZnO doped with Al from 0.08 to 1.2 mol% were investigated during isothermal sintering between 1100? and 1400?C. The Al dopant significantly inhibits the grain growth of ZnO and increases the grain growth exponent from 3, for undoped ZnO, to 4 – 6, for Aldoped ZnO. The grain growth activation energy is also changed from ~200 kJ/mol, for undoped ZnO, to ~480 kJ/mol, for Al-doped ZnO. The microstructural analysis showed that a ZnAl2O4 spinel phase exists as a second phase and that the spinel particles exert an effective drag (pinning) on the migration of ZnO grain boundaries. The analysis of the Al doping effect on the densification rate provides evidence that the driving force for densification is reduced by the second phase particles, too. A mechanism of pore-surface drag (pinning) on densification equivalent to the observed grain-boundary drag (pinning) on grain growth was proposed. [...]
Os materiais cerâmicos baseados em ZnO têm sido objecto de estudo intenso devido às suas propriedades encontrarem larga aplicação tecnológica. O estudo da sinterização e das propriedades eléctricas do ZnO dopado é relevante pelas suas potenciais aplicações, nomeadamente em cerâmicos de resistência variável com a tensão (varistores). Neste contexto, o presente trabalho debruçou-se sobre os sistemas binários Mn-ZnO e Al-ZnO, já que ambos os dopantes são utilizados na formulação química daqueles materiais. Prepararam-se amostras de ZnO puro e dopado com manganês e alumínio pela via convencional de preparação de cerâmicos, usando para a incorporação dos dopantes soluções etanólicas de Mn(NO3)2.4H2O e Al(NO3)3.9H2O. Para clarificar os efeitos de solução sólida e de segundas fases na sinterização e nas propriedades eléctricas do ZnO, investigaram-se os aspectos microestruturais e cristalográficos das amostras dopadas com as técnicas de XRD e SEM e TEM com EDS acoplado. Os resultados mostraram que no ZnO dopado com Al forma-se uma segunda fase de espinela, ZnAl2O4, a ~800ºC, que é estável a temperaturas mais elevadas. Determinou-se um limite de solução sólida de cerca de 600 ppm em Al no ZnO a 1200ºC. No ZnO dopado com Mn forma-se uma segunda fase cúbica de ZnMnO3 a temperaturas iguais ou inferiores a 450ºC e que se transforma noutra segunda fase tetragonal de ZnMn2O4 a ~1100ºC. As segundas fases dissolvem-se gradualmente na fase de ZnO e a 1200ºC já não se detectam segundas fases nas amostras com teores de manganês iguais ou inferiores a 1,2 mol%. Investigou-se a densificação e o crescimento de grão do ZnO dopado com Al, de 0,08 a 1,2 mol%, durante a sinterização isotérmica entre 1100º e 1400ºC. A dopagem com Al inibe significativamente o crescimento de grão do ZnO e aumenta o expoente do crescimento de grão de 3, no ZnO puro, para 4- 6 no ZnO dopado com Al. A energia de activação do crescimento de grão também varia de ~200 kJ/mol, no ZnO puro, para ~480 kJ/mol no ZnO dopado com Al. A análise microestrutural mostrou que as partículas de espinela, ZnAl2O4, exercem uma travagem efectiva na migração das fronteiras de grão no ZnO dopado com Al. A análise do efeito da dopagem com Al na velocidade de densificação evidenciou também a redução da força motriz para a densificação pelas partículas da segunda fase de espinela. Foi proposto um mecanismo de travagem do movimento da superfície dos poros na densificação, equivalente ao efeito de arrastamento-travagem exercido pelas partículas de segunda fase nas fronteiras de grão, durante o crescimento de grão.
Mestrado em Engenharia de Materiais

碩士
輔仁大學
物理學系碩士班
102
This work study BiFeO3-100x% (Bi0.5Na0.5)TiO3 (x=0.005,0.05) multiferroic ceramics and their photovoltaic responses, structure, dielectric permittivity, conductivity, magnetic properties, ferroelectric, and X-ray absorption spectroscopy. Open-circuit voltage, short-circuit current, and the I-V curve without illumination had been measured, using P-N junction model to do the data fitting. Ferroelectric (Bi0.5Na0.5)TiO3 doping maybe can do the high E-field poling in order to increase the conversion efficiency. Use XRD & SEM to analyze the structure, α angle, and lattice parameter at plane (110), and to compare the grain sizes due to different NBT compounds. The temperature point closes to the Nèel temperature (TN), suggested the magnetoelectric coupling. Conductivity can be calculate by the imagine part of dielectric permittivity. The enhanced ferromagnetism in BFO-5%BNT may due to variation of oxidation valences of Fe ion (Fe3+=>Fe4+) and/or the angle changing of Fe-O-Fe, we use the X-ray absorption spectroscopy to determine.

Bi3.25La0.75Ti3O12 (BLT) was prepared by the conventional solid state route as well as the chemical route. Calcination of the sample prepared through the solid state route was done at 7500C for 4 hours and of the sample prepared through the chemical route at 6000C for 4 hours. XRD of the samples was done in which BLT layered perovskite phase was confirmed. The samples were sintered using the conventional sintering method at 11500C for 4 hours and that of the sample prepared by chemical route was done at 11500C for 2 hours. Density measurement was done to confirm final densification after sintering. SEM images of the samples were taken which showed dense microstructure and uniform grain size. Comparative study of dielectric properties of the samples prepared by both solid state and chemical route were performed. PE loop measurements were done to confirm the ferroelectric nature of the sample.

碩士
國立臺南大學
材料科學系碩士班
98
SrTiO3 is an excellent candidate system in lead-free ceramic materials because of its a wide range of applications in integrated microelectronics. Recently, there has been an increasing interest in strontium titanate materials for microwave tunable applications, such as frequency-tunable resonators and filters operating in the GHz range. In this work, the Sr1-1.5xBixTiO3 and Sr1-xMgxTiO3 ceramics have been prepared by solid-state reaction method. Effect of the bisumth and magnesium content on the crystal structure and dielectric properties of ceramic were studied. This work is composed of two parts. First, the ceramics of Sr1-1.5xBixTiO3 solid solution were sintered for bismuth concentration range 0≦x≦0.1. could be successful synthesized in this work, and investigated the phase transion and dielectric properties of Bi-doped with different concentraction by using X-ray diffraction, SEM, DSC, dielectric constant and Raman scattering. As a result, the x-ray diffraction and Raman scatting profiles of Sr1-1.5xBixTiO3 are characteristic of the coexistence of cubic and orthorhombic, with increasing bismuth contents at x≧0.05. The formation of solid solution is restricted to x≦0.02；The maximum value of dielectric constant at x=0.05 was 1200. The phase transition point shifts to high temperature as bismuth content increases. Second, the x-ray diffraction and Raman scatting profiles of Sr1-xMgxTiO3 are characteristic of the coexistence of cubic and hexagonal, with increasing magnesium contents at x≧0.1. The formation of solid solution is restricted to x≦0.1. Mg doping itself does not induce ferroelectricity or a relaxor-like behavior in strontium titanate.

碩士
國立臺南大學
材料科學系碩士班
99
Na2Ti6O13 is an excellent candidate system as lead-free dielectric materials at present years because it will not cause the problem of lead pollution. In this study, Na(2-x)KxTi6O13 and lithium sodium titanate ceramics have been prepared by solid-state reaction method. Effects of the potassium and lithium content on the dielectric properties of ceramics were studied. This study is composed of two parts. First, Na(2-x)KxTi6O13 had successful synthesized. The phase transition and dielectric properties was investigated by X-ray diffraction, Raman scattering, SEM , impedance analyzer and density measurements. The crystal maintains monoclinic structure for different compositions. The peak belongs to K2Ti6O13 starts to appear at x=1.2 from Raman scattering results. Second, lithium sodium ceramics had synthesized in this research, dielectric properties measure results show that the samples with Li/Na molar ratio 1:4 have high dielectric constant.

碩士
國立臺南大學
材料科學系碩士班
97
(Bi0.5Na0.5)TiO3 is an excellent candidate system in lead-free piezoelectric materials system at present because of its no lead pollution. In this research, the [Bi0.5(Na1-xKx)0.5]TiO3 and [Bi0.5(Na1-xLix)0.5]TiO3 ceramics have been prepared by solid-state reaction method. Effect of the potassium and lithium content on the crystal structure and dielectric properties of ceramics were studied. This research is composed of two parts. First, [Bi0.5(Na1-xKx)0.5]TiO3 could be successful synthesized in this research, and the phase transition and dielectric properties was investigated by X-ray diffraction, Raman scattering, SEM and impedance analyzer. X-ray diffraction and Raman scattering results show that increasing of potassium content makes the crystal structure change from cubic to tetragonal. The transition point shifts to high temperature as potassium content increases. Second, [Bi0.5(Na1-xLix)0.5]TiO3 could be successful synthesized in this research, X-ray diffraction results show that two phases mix together between x=0.3 and x=0.9. The transition point shifts to high temperature as the lithium content increases, and the dielectric peak becomes broaden.

碩士
大同大學
材料工程學系(所)
92
Mullite glass-ceramics doped Cr3+ to be possible to apply to being possible to adjust the laser and the solar concentrators. This study is SiO2-Al2O3-B2O3-ZnO-K2O in the fixed parent glass the ingredient, doped different content of the Cr2O3, after the suitable heat treatment, produces nano- size mullite crystallization pellet in the glass. Cr produces matter of the Luminescence, but Cr also possibly affects the nucleation and the crystal behavior, then influence microstructure and properties. Therefore changes content of the Cr2O3, discusses whether can further promote the properties. When DTA analysis, along with Cr2O3 by 0 increase to 0.5, Tp can reduce advances again, but when 0.25, Tp has the minimum value. In the XRD analysis, the increase content of the Cr2O3, second the stages heat treatments after nucleation - growth, forms the glass-ceramics, the main crystallization phase is the mullite. In the UV- visible and the infrared transmission analysis, is worthy of looking at increases Cr2O3 the glass -ceramics to have the absorption phenomenon in the visible, but not obviously absorbs in the near-infrared. In the Photoluminescence analysis, increases 0.1 Cr2O3 the mullite glass ceramics, increases along with nucleation temperature and the nucleation time, the crystal size changes small and the crystallized amount increases. Then increases along with crystallization temperature and crystallized amount, the emission intensity is bigger.

碩士
國立清華大學
材料科學(工程)研究所
82
Yan等人曾做出(Nb,Ba)-doped的TiO2變阻器，其非線性指數α，約3-4。 本研究藉由添加Al到(Nb,Ba,Si)-doped的TiO2，探討不同的 Al添加量及 燒結溫度，對於其電性的影響。結果發現，隨著適量Al的添加，變阻特性 變佳。(0.25% Nb,1% Ba,0.3% Si,0.15% Al)-doped的TiO2於1250℃燒結2 小時，可得到α值為7的變阻器。而隨著Al添加，電子傳導的熱活化能提 高，空乏區變厚，電容值下降，而散逸因子在低頻區則降低很多，但晶粒 半導化則變差。故過多的Al添加(0.2%以上)則變阻性不復存在。 @　 低 價位的Fe3+其離子半徑與Ti4+相近，易溶入晶粒中，抵消一些半導化。微 量的(約0.01%到0.025%)Fe3+添加，能使(0.25% Nb, 1% Ba, 0.3% Si, 0.15% Al)-doped的TiO2於1300℃燒結2小時後，熱活化能由 0.37eV提升 至0.4eV附近，而空乏區厚度亦稍增加故α值可從7提升至9。隨著Fe3+添 加量的增加散逸因子弛緩峰向低頻移動，晶粒半導化的程度變差，過量的 Fe3+(0.05%以上)將使變阻特性變得不明顯，甚至消失。

Polymer derived ceramics (PDC) is an important class of ceramics that are fabricated from the inert atmosphere pyrolysis of Si- containing precursors. Various Si- based ceramics including silicon oxycarbide (SiCO), silicon carbonitride (SiCN), silicon carbide (SiC), silicon borocarbonitride (SiBCN) etc. can be prepared from various Si- containing precursors. These ceramics are characterized by their high specific strength, oxidation resistance, enhanced creep resistance and other functional properties. One of the most important developments in the ceramic system is the possibility of using these materials as coatings for high temperature structural ceramics. Although oxidation resistance of PDCs are good, they need to be fabricated in the multilayer structures that includes phase stability with various other metal oxides and carbides. The present work focuses on transition metal modified Polymer derived ceramics (PDC). Poly (methyl) silsesquioxane was used as the preceramic polymer in the process. Various transition metal elements having similar atomic size and different valence such as Al and Ti were introduced in the SiCO structure to form modified PDC. The doping amount in the structure is varied to study effect of amount of transition metal doping at the molecular level on phase evolution at different temperatures Phase evolution and thermal behavior of the modified PDCs at different high temperatures are studied.

碩士
國立中山大學
電機工程學系研究所
90
Microwave dielectric resonators (DRs) are being widely used in microwave telecommunication devices owing to their excellent characteristics of suitable dielectric constant, good temperature stability, and low dielectric loss. In this study, the crystalline phase and the microwave dielectric properties of the (1-x)Al2O3 - xTiO2 (x=0.08, 0.12, 0.16) compositions with 2wt%, 4wt%, 6wt%, and 8wt% MgO-CaO-Al2O3-SiO2 (MCAS) glass addition have been investigated. By combining the material Al2O3 with negative temperature coefficient of the resonant frequency (τf = -55 ppm/℃) and the material TiO2 with positive τf value (τf = +450 ppm/℃), it is desired to produce the ceramics with τf ~0 ppm/℃. The MCAS is used as liquid-phase sintering aid to lower down the sintering temperature. In the MCAS-doped (1-x)Al2O3 - xTiO2 system, the Al2TiO5 phase starts to appear at about 1250℃, and then the crystalline intensity of Al2TiO5 phase increases with the increase of sintering temperatures and MCAS glass content, until the temperatures that TiO2 is consumed. As the sintering temperature increases, the maximum dielectric constants and Q×f values can be obtained at 1250℃, and the τf values shift from positive to negative. The optimum τf value of —0.6 ppm/℃ exists in the 88mol%Al2O3 - 12mol%TiO2 composition with 2wt% MCAS addition and sintering temperature of 1300℃. The MCAS content, TiO2 content, and sintering temperature will result in the variation of microwave dielectric properties. In this study, MCAS-doped (1-x)Al2O3 - xTiO2 system exhibits the microwave dielectric properties of： εr=7~9.5, Q×f=6500~11000, and τf = -60 to +40ppm/℃. By adjusting the MCAS content, TiO2 content, and sintering temperatures, ceramics with good microwave properties can be obtained in the MCAS-doped (1-x)Al2O3 - xTiO2 system.

碩士
國立臺南大學
材料科學系碩士班
99
This research is composed of two parts. First, the ceramics of AgNbO3 sintered for 1000℃,1050℃,1100℃ and 1200℃ are investigated by XRD、SEM、DSC、dielectric constant and Raman scattering to evaluate. Second, the phase transition behavior ceramics of Ag1-xLixNb0.9Ta0.1O3 solid solution were sintered for Li concentration range 0≦x≦0.15. We investigate the phase transition behavior and dielectric properties of Li-doped with different concentration by using XRD、SEM、DSC、dielectric constant and Raman scattering. As a result, we can find the powder have excellent perovskite structure,with increasing lithium contents the vibrational mode is increase. M3-O1 and O2-T phase transition temperature is increase. The maximum value of the dielectric constant at x＝0.1 was 1273, and the dielectric loss value was controlled below 0.008.

碩士
國立成功大學
資源工程學系
106
In this study, calcium-doped barium titanate (BCT) powders were used as the raw materials to study different amounts of acceptor doping effects on the microstructure, crystal structure, and electrical properties of BCT. The A/B ratio of (Ba0.92Ca0.08)Ti1.005O3 was set for Ti-excess to prevent Ca doping onto the B-site. Powder of (Ba0.92Ca0.08)Ti1.005O3 was calcined at 1100°C which temperature can make the powder no second phase. Based on the Rietveld analysis results, Ca in the BCT mainly entered A-site to replace Ba. To simulating the MLCC sintering condition, the BCT ceramics should be sintered in reduction atmosphere to prevent the Ni electrode oxidation, and we also add different amounts of MnCO3 as acceptor to prevent the ceramic samples from semiconduction. The grain sizes after sintering are approximately 0.36 μm which fits the MLCC condition. In this study, We use two RQ equivalent circuit model to fit the impedance spectroscopy data, and the results meet the Curie-Weiss behavior. According to the impedance spectroscopy data, the grain and grain boundary conductivity of acceptor-doped BCT ceramics is determined by Vo ̈ and e' respectively. With the increase of the amount of addition, the grain and the grain boundary conductivity tends to increase. The similar grain and grain boundary conductivity of BCT ceramics are due to maintain the electric neutrality. While the depletion layer and grain boundary barrier height are tend to decrease as the amounts of acceptor increase. The comprehensive conclusions show that the amounts of acceptor (MnCO3) has a certain influence on the electrical analysis calculated by the impedance spectroscopy analysis.

碩士
崑山科技大學
電子工程研究所
102
Thermoelectric materials gain much attentions in energy saving and environmental issues recently. They can capture electric power from waste heat or transfer electricity to cryogenic function. Their apparent thermoelectric properties or the so-called figure of merit Z can be improved by lowering the thermal conductivity of the materials. This leads us to find the Sr2Nb2O7 ceramic with low thermal conductivity. We try to dope the ceramics to increase their electric conductivity, which maybe even turn into semiconducting, while keeping their low thermal conductivity simultaneously. Therefore, excellent thermoelectric ceramics with high figure of merit Z were explored in this study. Fabrication and thermoelectric properties of La- and Ti-doped Sr2Nb2O7 ceramics was conducted in this study. The Sr2Nb2O7 ceramics was prepared by reaction sintering process, which is similar to conventional oxide route process but deleting the calcination process. The effects of sintering temperatures and soaking times on the microstructures and thermoelectric properties were investigated. The results show that no second phase exhibits in La and Ti doped Sr2Nb2O7 ceramics. The bulk density of La-doped and Ti-doped Sr2Nb2O7 ceramics increases with increasing sintering temperature up to 1350℃ and 1400℃ respectively, and remains nearly the same with further increasing temperature. Surfaces of the Sr2Nb2O7 ceramics show significant grain growth with increasing sintering temperature, but La-doping and Ti-doping seem to suppress grain growth relatively, which result in large number of crystal grains and grain boundaries. The huge grain boundaries are expected to affect conductivities and heat resistance of the ceramics.