The effects of in situ Ti0₂ whisker reinforcement on mechanical and thermal properties of glass-ceramics in the Li₂O-Al₂0₃-P₂0₆-Si0₂ system were investigated. When Ti0₂ whiskers, having an average aspect ratio of 28, are precipitated from the glass-ceramic matrix, (Li_0.4,Ca_0.05)AI(Si_0.75,P_0.5)_04.5, flexural strength is improved from 72 to 134 MPa. Fracture toughness, K_Ic, is increased from the 1.1 to 1.6 MPa·m^1/2 due to crack deflection by the Ti0₂ whiskers. In situ Ti0₂ whisker-reinforced glass-ceramic exhibits rising fracture resistance, K_R, with increasing crack extension. The fracture resistance, K_R, is increased from 1.89 to 2.5 MPa·m^1/2 over the crack extension range range of 40 to 200 μm. The composite shows a narrow failure strength distribution compared to the glass-ceramic without Ti0₂ whisker precipitation. The coefficient of thermal expansion (CTE) changes from -2.8x10⁻⁷/"C to -1.7xl0⁻⁷/°C due to the precipitation of Ti0₂ phase which has a positive CTE (7.3xl0⁻⁶/°C). With the matrix composition, (Li_0.41,Mg_0.035)AI(Si₀0.48</sub>,P_O.52)O₄, a three-fold increase in flexural strength was observed with a Ti0₂ content of 12 wt%. CTE value of the composite increases linearly from a negative to a positive value with increasing Ti0₂ content up to 12 wt%. The in situ composite containing 8-10 wt% Ti0₂ exhibits near zero CTE values up to l000°C. / Ph. D.

Glass-ceramic composites and nanocrystalline glass-ceramics containing barium titanate (BT) or potassium sodium niobate (KNN) ferroelectric phases have been studied, with particular regard to their use as dielectric energy storage materials. Barium borosilicate glass (BBS) was used as a sintering aid for BT and KNN, producing glass-ceramic composites. The temperatures required to achieve densification were reduced from 1400 to 1200 °C for BCZT (Ca, Zr-doped BT) and 1170 to 1050 °C for KBN (Bi, Na, K, Zr-doped KNN) by the use of glass additives. An unexpected observation, found in both BCZT and KBN systems, was the heterogeneous dissolution of dopant elements into the glass, inducing additional anomalies in the relative permittivity-temperature relationships. For BCZT, the orthorhombic-tetragonal phase transformation temperature shifted upwards to ≈ 50 °C, which was attributed to modification of the Ca/Zr ratio by preferential dissolution of Ca into the glass phase. Similarly, for KBN the dopant elements appeared to be leached into the liquid phase during sintering, resulting in relative permittivity-temperature characteristics similar to those of pure KNN. A modified BBS glass having various KNN contents was prepared by the conventional melt-quenching method and then heat-treated to induce crystallisation, producing nanocrystalline glass-ceramics. It is shown that crystallisation of an intermediate barium niobate phase initiates at temperatures in the region of 650 °C; this is subsequently converted into perovskite KNN together with a second phase of Ba3Nb5O15 at temperatures from 700 to 800 °C. The final crystallite size was in the region of 30±7 nm. The highest dielectric energy storage density of 0.134(4) J cm-3 was obtained for a glass-modified BT ceramic at an electric field level of 5 kV mm-1. However, the energy storage efficiency of the BT-based ceramics was relatively poor and they displayed a general tendency for saturation, indicating potentially poor performance at higher field levels. On the other hand, the KNN-based ceramics exhibited slightly lower energy storage density values, up to 0.108(1) J cm-3, but with much improved linearity and energy storage efficiency. Therefore, the latter is considered to be more suitable as energy storage dielectrics. The BBS-KNN glass-ceramics yielded relatively low energy storage density, 0.035(2) J cm-3, but the dielectric linearity and storage efficiency were similar to or better than those of the KNN ceramics, indicating good potential for use as energy storage dielectrics at very high electric field levels as a result of their nanocrystalline microstructures.

The optical properties of transparent chromium-doped glass-ceramics with the chemical composition similar to that of cunyite (Cr 4+ :Ca 2 GeO 4 ) crystal were investigated. Room temperature absorption, DTA, XRD as well as fluorescence were measured. Parent (as quenched) glass does not exhibit any fluorescence; after the heat treatment Cr 4+ fluorescence band appears at 1280 nm, very similar to the emission of Cr 4+ :Ca 2 GeO 4 bulk crystals. Optical properties of nanocrystals formed in the glass-ceramics. The size of crystallites is below 600nm. glass-ceramics we concluded that the nanocrystal phase is a modified Ca 2 GeO 4 structure. In order to increase the fluorescence and synthesis more small crystals, we doped TiO 2 and ZrO 2 as nucleating agents, glass does not exhibit any fluorescence when glass-ceramics does not heat treatment ; after the heat treatment Cr 4+ fluorescence band appears at 1280 nm too. The crystals formed in glass media after heat-treatment have spherulitic shape with the size up to 300-500nm, and there are more nanocrystals distribute the surface, especially doped TiO 2 we found the more smaller crystals in it. We expect that crystallites with the size below 100 nm may be formed by adjustment of initial glass composition with nucleating agents and optimization of heat-treatment procedure parameters.

No description available.

The study of surface and interface structure of thin film devices is becoming increasingly important in industrial applications today. In this thesis, the technique of grazing incidence x-ray scattering (GEXS) is developed and its validity examined for many different materials. In addition, the technique of Born wave analysis introduced by Lagally has been extended, showing that in certain cases surface roughness can be obtained without full simulations of the data. GEXS measurements performed on float glass revealed that the density of the tin and air sides were 2.7 ± 1 and 2.3 ± 1 g/cc respectively across the entire ribbon. For all samples studied a surface layer on the order of a few nanometers was in evidence. When similar measurements were made on container glass it was seen to be layer free. In addition, excellent agreement was found between the roughness deduced from the simulations and the Born wave analysis. In all samples, the surface roughness deduced from the diffuse scatter was less than that from the specular, indicating that a vertical density gradient was present. When x-ray scattering measurements were performed on CVD layers deposited on glass it was noted that two sets of Yoneda wings existed in the diffuse scatter. From these it was concluded that lateral density variations existed in the SnO(_2) layer which were on the order of 0.5mm in diameter. Furthermore, excellent specular simulations were obtained. These indicated that the control of both layer thickness and interface roughness between samples was on the Angstrom level. Specular and diffuse x-ray scattering measurements were performed on Cu/Co multilayers grown on sapphire using a Nb buffer. Here the effect of anomalous dispersion was employed in order to highlight the scattering from the Cu/Co interfaces. From these data Born wave analysis was used to characterise the roughness of the samples. It was found that little change took place for anneals of 45 minutes at temperatures up to 300 C. In addition, it would appear that the measured OMR bore no relationship to the component of uncorrelated roughness within the samples. GIXS measurements perfumed on Cu/Co multilayers grown on Si, using a Cu(_3)Si buffer, revealed an asymmetry in the diffuse scatter, which was attributed to terracing within the layers. Simulations were obtained to the data using a slight modification of the fractal model for several scattering vectors with a single set of parameters. From this, a model of step bunching has been proposed which allows the diffuse scatter to be described using such a fractal surface. Diffuse x-ray scattering studies performed on Al(_2)O(_3), which had been ground in different ways, indicted that the surface density as measured from the Yoneda wings followed the bearing area as measured from optical techniques. Comparisons performed between these results and those from optical microscopy and surface acoustic wave techniques allowed a model to be proposed which was consistent with all the findings. Finally, GIXS measurements were performed on GaAs which had been subjected to various polishing techniques. Simulations revealed that a nanometer scale surface layer was present on all samples. Born wave analysis was successfully applied to the diffuse scatter and the effect of surface layers on the deduced roughness was investigated. In addition, symmetric and asymmetric triple axis diffraction studies revealed that the distribution of strains was independent of the type of polishing used whereas the tilts were not. A model has been proposed in which the surface consists of unstrained mosaic blocks separated by cracks.

The thermal diffusivity and conductivity of several ceramic composites were determined experimentally to observe the effect of temperature, heat treatment and orientation on these properties. The thermal conductivity and diffusivity of SiC whisker reinforced Ba-osumilite glass-ceramic exceeded that of SiC fiber reinforced osumilite due to higher thermal conductivity and diffusivity of the whiskers. An anisotropy was observed in thermal conductivity of the whisker composite due to whisker orientation normal to hot-pressing direction. Thermal conductivity of carbon fiber reinforced borosilicate glass parallel to the fiber plane was about twice that of the normal orientation and of the matrix. Heating the composite to above 600°C resulted in permanent decrease in thermal conductivity due to void formation by relaxation of the elastically bent fibers by viscous flow of the matrix. Thermal conductivity of iniaxial carbon fiber reinforced lithia-aluminosilicate glass-ceramic was over ten times higher parallel to the fibers than perpendicular. Permanent decrease of thermal conductivity normal to the fibers was due to matrix cracking caused by relief of internal stresses due to matrix crystallization and thermal gradients during fabrication. Modification of the original theories of Maxwell and Rayleigh permitted derivation of expressions for thermal conductivity of composites of a continuous matrix with dilute concentrations of spherical and cylindrical geometries and of parallel flat plates with thermal resistance at the interfaces. / M.S.

Thesis (Ph. D.)--Virginia Polytechnic Institute and State University, 1993. / Vita. Abstract. Includes bibliographical references (leaves 170-175). Also available via the Internet.

Glass-ceramics are of growing interest due to their enhanced properties compared to the base glasses. More specifically, the control of microstructures is a major challenge as the properties of glass-ceramics are the direct consequences of microstructures. Microstructures can be modified by forming specific crystal phases or by using a prior amorphous phase separation before crystallization. The PhD thesis objectives are to demonstrate that the properties of silicate glasses can be enhanced by controlling their microstructure genesis with composition and thermal process parameters. More specifically, two systems were studied and compared: the BaO-TiO2-SiO2 system and the soda-lime silica Na2O-CaO-SiO2 used industrially. Both systems exhibit a large zone of immiscibility allowing the study of the influence of phase separation on crystallization.<p>The first system BaO-TiO2-SiO2 has gathered interest from the interesting properties of fresnoite (Ba2TiSi2O8): piezo and pyroelectricity, second harmonic generation and blue/white photoluminescence. Many studies on the stoichiometric composition were conducted to understand and improve those promising properties. However, it was recently suggested that the photoluminescence can be improved with composition exhibiting phase separation. This indicates that the photoluminescence intensity can be improved through a microstructural control. The possible role of a prior amorphous phase separation on the subsequent crystallization has been however the topic of vigorous debates over the last decades and has not yet been clarified, especially regarding the role of the interfaces created by the phase separation. In this PhD, the effect of phase separation on fresnoite crystallization was studied. This had to pass through the calculation of the liquid-liquid immiscibility in the phase diagram in order to select suitable compositions to compare in a systematic study. The systematic study concludes to a surface crystallization mechanism for all non- stoichiometric compositions and shows no influence between amorphous droplets and matrix crystallization. This study was also completed with the investigation of the effect of composition (i.e. SiO2-excess), annealing temperature and prior heat treatment, i.e. heating rate, cooling rate or a prior isothermal step before annealing. It is shown that specific microstructures are obtained depending on the process parameters. Finally, selected compositions and heat treatment show how photoluminescence intensity can be improved by a microstructural control. The highest intensity is obtained with a high crystallization fraction and a maximization of the number of interfaces.<p>The results obtained in the study of the BaO-TiO2-SiO2 system are extended to the soda-lime-silica system in order to study the effect of phase separation on crystallization. It is shown that cristobalite forma- tion from the surface cannot be avoided and that the involved composition shift inhibits phase separation. It is consequently difficult to observe an interplay. Those studies lead to a general discussion about the criteria allowing to observe an interplay between phase separation and crystallization in oxide glasses. / Doctorat en Sciences de l'ingénieur / info:eu-repo/semantics/nonPublished

Resumo: Vitrocerâmicas transparentes baseadas em novas composições de calco-halogenetos dopadas com íons de terras raras foram produzidas com o objetivo de gerar luz no visível. A adição de diferentes halogenetos alcalinos em vidros do sistema Ga2S3-GeS2 foi estudada com a finalidade de aumentar a sua janela de transparência no ultravioleta. Vidros no sistema Ga2S3- GeS2-CsCl totalmente transparentes no visível (400-750 nm) até a região do infravermelho médio (11,5μm) foram obtidos. Vitrocerâmicas transparentes e homogêneas foram em seguida preparadas a partir de vidros variando a concentração de CsCl no sistema Ga2S3-GeS2-CsCl. As condições dos tratamentos térmicos (temperaturas e tempos) foram estabelecidas permitindo o controle dos processos de nucleação e crescimento dos cristalitos dentro da matriz vítrea. Nanocristalitos uniformemente distribuídos e com tamanho homogêneo foram obtidos e confirmados por microscopia eletrônica de varredura (MEV) e microscopia eletrônica de transmissão (MET). Estudos estruturais foram realizadas nas vitrocerâmicas utilizando-se as técnicas de difração de raios X e resonância magnetica nuclear do sólido do 133Cs e 71Ga. Foi evidenciado o papel do agente de nucleação do gálio neste material, com a cristalização da fase Ga2S3. Um aumento da resistência mecânica foi também observado nestes materiais em comparação aos vidros de base. Em seguida, vitrocerâmicas dopadas com íons de terras raras (praseodímio Pr3+ e neodímio Nd3+) foram sintetizadas no sistema Ga2S3-GeS2-CsCl. A luminescência dos íons Pr3+ e Nd3+ no visível foi estudada em função dos tratamentos térmicos aplicados aos materiais. Foi observado que não houve a incorporação dos íons Pr3+na matriz. Por outro lado, um aumento da fluorescência do íon Nd3+ foi observado, sugerindo uma incorporação parcial destes íons dentro da fase cristalina... (Resumo completo, clicar acesso eletrônico abaixo) / Abstract: The present work deals with the study of new chalco-halide glass-ceramics doped with rare earth ions for visible luminescence. The effect of the addition of different alkali-halide into glasses belonging to the Ga2S3-GeS2 system has been studied with the aim to extend their transparency into the ultraviolet region. Glasses transparent over the visible range (400-750 nm) up to the mid infrared (11,5 μm) have been obtained in the Ga2S3-GeS2-CsCl system. Transparent and homogeneous glass-ceramics have been prepared from several base glasses varying the CsCl content in the Ga2S3-GeS2-CsCl system. Nanocrystals with homogeneous size and uniformly distributed in the amorphous matrix have been generated with reproducibility by appropriate heat-treatment. A structural study of the crystallization has been realized using X-ray diffraction and solid state nuclear magnetic resonance performed on the 71Ga and 133Cs nuclei. The nucleating agent character of gallium has been shown in the glassy matrix with the crystallization of the Ga2S3 phase. An improved mechanical behaviour has also been observed in the prepared composite materials. Rare-earth ion (praseodymium Pr3+ and neodymium Nd3+) doped glass-ceramics have been synthesized in the Ga2S3-GeS2-CsCl system. The visible luminescence from Pr3+ and Nd3+ ions has been studied as a function of heat-treatment conditions. It appeared from experiments that Pr3+ ions are not incorporated into the crystals contrary to the Nd3+ ions from which an enhanced luminescence was observed, suggesting their partial integration into the crystalline phase. Finally, silver and Pr3+ doped glasses have been synthesized in the Ga2S3-GeS2 system. Metallic silver nanoparticles were generated by heat-treatment at 370°C and characterized by transmission electronic microscopy. The influence of the NPs on the Pr3+ ions luminescence properties was studied by performing frequency... (Complete abstract click electronic access below) / Résumé: Les travaux présentés dans ce manuscrit concernent l'étude de nouvelles vitrocéramiques transparentes à base de chalco-halogénures dopées par des ions de terres pour la luminescence dans le visible. L'addition de différents halogénures d'alcalins à des verres du système Ga2S3-GeS2 a été étudiée avec l'objectif d'étendre leur transparence vers l'ultraviolet. Des verres du système Ga2S3- GeS2-CsCl complètement transparents dans le visible (400-750 nm) jusqu'au moyen infrarouge (11,5 μm) ont été obtenus. Des vitrocéramiques transparentes et homogènes ont ensuite été préparées de manière contrôlée à partir de plusieurs verres à différentes teneurs en CsCl dans le système Ga2S3-GeS2-CsCl. Des nanocristallites de taille homogène et uniformément dispersées dans la matrice amorphe sont formées de manière reproductible par des traitements thermiques appropriés. Une étude d'un point de vue structural de la cristallisation a été réalisée par diffraction de rayons X et résonance magnétique nucléaire à l'état solide sur les noyaux 71Ga et 133Cs. Le rôle d'agent de nucléation du gallium a été mis en évidence dans cette matrice avec la cristallisation d'une phase Ga2S3. Un meilleur comportement mécanique a également été observé pour les matériaux composites préparés. Des vitrocéramiques dopées par des ions de terres rares (praséodyme Pr3+ et néodyme Nd3+) ont été synthétisées dans le système Ga2S3-GeS2-CsCl. L'évolution de la luminescence dans le visible des ions Pr3+ et Nd3+ a été étudiée en fonction des conditions de traitement thermique. Au vu des résultats obtenus, il semble que l'ion Pr3+ ne soit pas incorporé dans les cristallites. Une augmentation de la luminescence de l'ion Nd3+ a en revanche été constatée, suggérant une incorporation partielle de ces ions dans une phase cristalline... (Résumé complet accès électronique ci - dessous) / Orientador: Bruno Bureau / Coorientador: Younés Messaddeq / Coorientador: Marcel Poulain / Banca: Cid Bartolomeu de Araújo / Banca: Brigitte Boulard / Banca: X. H. Zhang / Doutor

No description available.