Des simulations numériques conduites sur un ferroélectrique, le Titanate de Baryum, permettent d'extraire les températures où se produisent les transitions pour chacune des composantes et elles utilisent le plus souvent des conditions limites périodiques. Cependant, il est possible de modifier la simulation de manière à rendre compte d'une particule isolée, et les conditions limites électriques de type court-circuit ou bien circuit ouvert affectent alors le résultat. Réduire la taille accorde encore davantage d'importance à de tels effets de surface par rapport aux effets de volume. Expérimentalement, la formation de nanocubes creux ou de nanotores a été observéelors de synthèses solvothermales. De possibles mécanismes de morphogénèse sont évoqués. Dans ce travail, les simulations ont ensuite été conduites sur des particules nanométriques, dont les formes avaient été obtenues expérimentalement. Pour des cubes creux, il a été constaté que la taille de l'inclusion vide modifiait la valeur du coefficient d'écrantage critique à partir duquel le comportement de la polarisation passe d'une configuration de type court-circuit à une configuration de type circuit fermé. Pour des nanotores aux basses températures, une modification du rapport entre petit et grand rayon donne lieu à des configurations possédant un moment toroïdal seul ou accompagné d'un moment hypertoroïdal voire d'oscillations de ce dernier. / Numerical simulations performed on a ferroelectric, Barium Titanate, yield all the transition temperatures under periodic boundary conditions. However, the same simulation can be modified to model an isolated particle under short-circuit or opencircuit electrical boundary conditions. Reducing size makes these surface effects even more important with regard to volume effects.From an experimental point of view, solvothermal synthesis of hollow nanocubesand nanotori is reported and various morphogenesis mechanisms are listed.In this work, simulations were performed on nanometric particles that had experimentally obtained shapes. In hollow cubes, it was shownthat the hole size changed the numerical value of the critical screening coefficient at which the system changes its behavior from aconfiguration that is short-circuit like to another one that is open-circuit like. For nanotori at low temperatures, a modification of the ratio between torus minor and major radius gives either configurations with only toroidalmoment or configurations in which it coexists with homogeneous or oscillating hypertoroidal moment.

The thermoelectric properties of lanthanum-doped strontium titanate (LSTO) were investigated for high temperature applications. Ceramics with the formulation La(2x/3)Sr(1-x)TiO(3-delta) (x = 0.1, 0.3, 0.5, 0.7, 0.9) were produced using the conventional mixed oxide route. After 18 hours of milling the powders were calcined at 1373 K for 4 hours and sintered at 1733 K (± 180 K/hour) for 4 hours in air. SEM, XRD, and TEM techniques were employed to characterise the microstructure of the ceramics as well as density measurements. Subsequently the electrical conductivity, Seebeck coefficient, and thermal conductivity in order to determine the thermoelectric figure of merit of the ceramics. Key thermal conductivity results were further investigated using a computational approach. This production method resulted in high quality, high density (> 97 %) ceramics that were mostly single phase determined by XRD with a Pm3m space group, with the exception of the x = 0.9 ceramic that had a Cmmm space group. SEM imaging confirmed this finding and revealed a core-shell structure in x = 0.1 and x = 0.3 ceramics whereby the core was La-rich/Sr-deficient. Thermal conductivity of the ceramics decreased with increasing La content. This was investigated further computationally employing the Green-Kubo method. It was established that the decrease in thermal conductivity was due to phonon-scattering from A-site vacancies, and not from the stabilisation of oxygen vacancies as suggested elsewhere. The electrical properties were dramatically improved through sintering the LSTO ceramics in a 5 % H2 95 % Ar atmosphere. This led to a reduction of Ti4+ to Ti3+. The maximum electrical conductivity increased to 789 S.cm-1 resulting in a power factor of 0.0013 W.m-1 K-2 at 477 K for x = 0.3. A zT of 0.27 at 870 K for x = 0.5 was obtained due to a lower thermal conductivity. The ceramic LSTO x50H was subsequently doped with excess lanthanum; A maximum of 3 % excess was able to be fully incorporated into the lattice. This resulted in a further increase of the electrical conductivity to 875 S.cm-1 at 377 K. A decrease in the lattice thermal conductivity of ∼ 1 W.m-1 K-1 was also achieved due to the oxygen vacancies that were introduced as a result of the reducing sintering conditions. Overall a zT 0.27 at 1016 K was obtained for 3 % excess lanthanum. The ceramic LSTO x50H was also doped with niobium and vanadium. Reduction in the lattice parameter from the La substitution inhibited the niobium from fullyincorporating into the matrix. This was not the case for vanadium that resulted in an electrical conductivity of 144 S.cm-1 , Seebeck coefficient of -106 µV.K-1 , and thermal conductivity of 2.08 W.m-1 K-1 at 308 K, resulting in an overall maximum zT of 0.08 at 1070 K.

The composition dependent variation of ferroelectric domain structure in lead zirconate titanate (Pb(Zr0.52Ti0.48)O3) ceramics have been investigated within the morphotropic phase boundary (MPB). Tetragonal phase in sintered samples were identified via X-ray diffractometry (XRD). Representative microstructures of ferroelectric domains were examined using scanning electron microscopy (SEM). £\-boundaries, £_- boundaries, and £k-boundaries were analyzed from the contrast of extreme fringe patterns by transmission electron microscopy (TEM). Twin planes for 90o domains lie in {011) and for 180o domains lie in {100) and {220) were determined by selected area diffraction patterns (SADP). Traditional contrast analysis was adopted for determining displacement vectors (R). 90o domains with R = £`[011] and 180o domains with R = n[001]. Convergent beam electron diffraction (CBED) was performed to identify crystalline phases of different domain configurations. By examined the symmetry along the Z = [100], [110], and [111] zone axis, both £_-boundaries and £k-boundaries are tetragonal phase.

The area of advanced gate dielectrics has gained considerable attention recently, and there are significant leakage current and reliability concerns for oxy-nitride in this regime. So it¡¦s an important business to use alternate high-k dielectrics instead of oxy-nitride. Titanium dieoxide shows a high dielectric constant for dielectric applications. Besides, strontium can create additional oxygen vacancies that can enhance dielectric constant. In this study, we prepared SrTiO3 film by liquid phase deposition which is a novel material considered to have high dielectric constant. From several characteristic measurements, we found that SrTiO3 with exhibiting higher dielectric constant and well interface state which is very promising candidates to instead of titanium dieoxide. The physical and chemical properties of SrTiO3 films by means of several measuring instruments, including Fourier transform infrared spectrometer (FTIR), secondary ion spectrometer (SIMS), and X-Ray diffractometer (XRD). An Al / SrTiO3 / Si metal-oxide-semiconductor (MOS) capacitor structure was used for the electrical measurements. To improve the electrical properties, we investigated the characteristics of SrTiO3 films after annealing in oxygen, nitrous oxide, and nitrogen ambient. Including the variations of thickness, structure, dielectric constant, and leakage current were discussed in this work.

Thesis (Ph. D.)--Oregon Graduate Center, 1988.

Resumo: Titanato de cálcio possui estrutura tipo perovisquita com interessantes propriedades elétricas para aplicações tecnológicas, como termistores, capacitores multicamadas e memória DRAM. Para a indústria cerâmica e eletrônica, pesquisas com perovisquita tem aumentado sua importância devido a propriedade fotoluminescente observada nesses materiais. Neste trabalho, o titanato de cálcio puro e dopado com escândio nas concentrações 0,25;0,5 e 1,0% mol foi sintetizada pelo método dos precursores poliméricos. Este método é baseado na quelação dos cátions por ácido cítrico em etileno glicol. Os pós foram calcinados variando a temperatura de 350º a 850°C. Os resultados da análise térmica mostraram que a obtenção de pós amorfos com baixo teor de matéria orgânica está diretamente relacionado à ordem estrutural da estrutura perovisquita. Os pós amorfos e cristalinos puro e dopado com Sc foram caracterizados por termogravimétria e pela técnica da calorimetria exploratória diferencial (TG/DSC), espectroscopia de infra vermelho (IR), Raman, espectroscopia UV-visível, microscopia eletrônica de transmissão (TEM), fotoluminescência e difração de raio X, onde foi usado refinamento pelo Método de Rietveld. A adição de Sc causou um decréscimo da cela unitária e causou interferência na distribuição dos cátions nos sítios octaédricos formando vacâncias de cálcio e conseqüentemente aumentando a desordem. Medidas de fotoluminescência desses materiais mostraram uma intensa banda larga atribuída à desordem do CT e CTSc. / Abstract: Calcium titanate presents perovskite type structure with interesting electrical properties for technological applications, such as thermisters, multilayer capacitors and DRAM memories. For the ceramic and electronic industries, research with perovskite has increased in importance due to the photoluminescent properties observed in these materials. In this work pure and Sc doped CaTiO3 at concentrations of 0.25, 0.5 and 1% molar was synthesized through the method of polymeric precursors . This method is based on the chelation of cations through citric acid in ethylene glycol solvent. The materials were calcined at temperatures varying from 350 to 850°C. The experimental results obtained through thermal analysis showed that the condition to obtaining amorphous powder with low content of organic matter is directly related to structural order of perovskite structure. The crystalline and amorphous pure and Sc doped powder was characterized through termogravimetric and diferential scaning calorimetry technique (TG/DSC), infrared spectroscopy (IR), Raman, UVvisible spectroscopy, transmission electron microscopy (TEM), photoluminescence and x-ray diffraction were used, refinement through method of Rietveld. Addition of Sc caused a decrease in the unitary cell and caused interference in the distribution of the cations in octahedral sites forming calcium vacancies, and consequently increasing the disorder. The Photoluminescent s of these materials showed an intense large band, attributed to the disorder of CT e CTSc. / Orientador: Carlos de Oliveira Paiva Santos / Coorientador: Elson Longo da Silva / Banca: Nelson Ramos Stradiotto / Banca: Maria Aparecida Zaghete Bertochi / Banca: Antonio Carlos Hernandes / Banca: Fabio Furlan Ferreira / Doutor

The theory of ferroelectricity, and in particular the ferroelectric behavior of barium titanate, are discussed and qualitative agreement is obtained with experimental results. Limitations of the theory are pointed out. Experimental procedure used to manufacture samples of polycrystaline barium titanate ceramic, and to investigate their ferroelectric properties is outlined. The phenomena of permanent polarization and hysteresis are investigated in the ferroelectric temperature range, and are discussed. An electric equivalent of the Barkhausen effect is observed. / Science, Faculty of / Physics and Astronomy, Department of / Graduate

No description available.

<div>Understanding grain growth in dielectric ceramics is essential to controlling the electrical and mechanical properties necessary to produce ceramic capacitors and sensors. The effect of alloying barium titanate with strontium titanate on the equilibrium crystal shape was investigated in order to determine possible impacts on grain growth. The equilibrium crystal shape was studied through three experimental methods to identify possible changes in grain boundary energy or anisotropy with changing composition.</div><div>The first method was by imaging intergranular pores to observe faceting behavior and relative interfacial energies. Intergranular pores were reconstructed to determine the relative surface energies of the identified facets. The second method was to perform atomic force microscopy on surface facets to collect topography data. The topography data was combined with orientation data obtained by EBSD analysis from the same region, and used to calculate the normal vector of the surface facets. These datasets were plotted in a stereographic projection to study the faceting anisotropy. The third method involved collecting EBSD orientation data and images of surface faceting behavior. The surface faceting behavior of each grain was categorized by type of facet and plotted on a stereographic projection at the corresponding orientation. This allowed for the analysis of faceting transitions and the differentiation of faceted and continuous regions of the equilibrium crystal shape. The analysis of faceting behavior across compositions has implications on grain growth of the barium titanate/strontium titanate system.</div>

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