Les céramiques transparentes élaborées par cristallisation complète du verre constituent une nouvelle famille de matériaux de qualité photonique en compétition avec la technologie des monocristaux pour les applications optiques. Cette approche verrière offre des avantages considérables par rapport aux monocristaux et aux céramiques polycristallines frittées : un coût réduit, la possibilité d’une production à grande échelle, une large gamme de compositions chimiques accessibles, une mise en forme plus souple et un taux de dopage élevé. Dans le cadre de ce travail, nous avons synthétisé des céramiques polycristallines transparentes de Sr3Al2O6 et Sr3Ga2O6 (structures cubiques) par cristallisation complète du verre de même composition. Les verres d'aluminate de strontium (75SrO-25Al2O3) et de gallate de strontium (75SrO-25Ga2O3) sont élaborés par lévitation aérodynamique couplée à un dispositif de chauffage laser. La transparence de la céramique de Sr3Al2O6 obtenue s’explique par son isotropie optique, ses joints de grains très fins et sa structure totalement dense (non poreuse). Nous avons également développé une nouvelle famille de céramiques transparentes Sr1+x/2Al2+xSi2-xO8 (0<x≤0.4) obtenues par cristallisation complète et congruente du verre. La transmission exceptionnelle de plus de 90% dans la gamme visible et proche infrarouge est expliquée grâce à des études microstructurales et structurales. Cette transparence qui atteint la limite théorique est associée à une biréfringence quasi nulle, des joints de grains très fins et une porosité nulle. Les études par RMN prouvent l'existence d'un désordre chimique qui est à l'origine de la valeur pratiquement nulle de la biréfringence calculée par DFT. Ces matériaux céramiques polycristallins évolutifs et hautement transparents sont des candidats prometteurs pour une large gamme d'applications optiques et photoniques dans les gammes IR et visible. Cette étude a donc conduit à de nouvelles céramiques transparentes, avec des valeurs de transmission jamais atteintes jusqu’à présent pour des oxydes. Elle propose également une nouvelle approche pour l'obtention de céramiques transparentes dans le cas de matériaux anisotropes : introduire un désordre chimique contrôlé au sein du matériau afin d’induire une isotropie optique. Ce concept ouvre la voie à de nouvelles compositions, étendant ainsi le domaine des céramiques transparentes et de leurs applications. / Transparent polycrystalline ceramics elaborated by full crystallization from glass are an emerging class of photonicquality materials competing with single crystal technology, especially for optical applications. This approach provides considerable advantages over single crystals and polycrystalline sintered ceramics represented by cost effectiveness, large scale production, wide range of accessible chemical compositions, easy shaping and high doping level hosting structure. In this work, we show the preparation of transparent cubic Sr3Al2O6 and Sr3Ga2O6 polycrystalline ceramics by full crystallization from the parent strontium aluminate (75SrO-25Al2O3) and strontium gallate (75SrO-25Ga2O3) glasses elaborated by aerodynamic levitation coupled to laser heating system. The transparency of the obtained Sr3Al2O6 ceramics is explained by their optical isotropy, thin grain boundaries and highly dense (non-porous) microstructure. We also show a series of novel Sr1+x/2Al2+xSi2-xO8 (0<x≤0.4) oxide compositions leading to highly transparent and readily scalable polycrystalline ceramics to be obtained by full congruent glass crystallization. The outstanding transparency exceeding 90% in the visible and near IR range is explained through different microstructural and structural (average and local) studies. This transparency, reaching the theoretical limit, is associated to the almost null birefringence, thin grain boundaries and non-porosity. NMR experiments prove the existence of chemical disorder which is at the origin of the relatively zero birefringence value calculated by DFT computations. These scalable and highly transparent polycrystalline ceramic materials are promising candidates for a wide range of optical and photonic applications in the IR and visible ranges. This study besides revealing new ceramic compositions with previously unreported transmission values for micro-scale polycrystalline materials, proposes a new approach for obtaining transparent ceramics in anisotropic materials. This approach consists in inducing a controlled chemical disorder within the material in order to induce optical isotropy. It is anticipated that this proposed concept will open the way to different composition candidates to be elaborated as transparent polycrystalline ceramics, thus extending the ceramic technology domain and its applications.

Borate glasses and glass-nano/microcrystal composite fabrication and investigations into their physical properties, have been interesting from their multifunctionalities view point. Certain borate structural units possess high hyperpolarizabilities and give rise to high nonlinear optical effects. High refractive index materials are important for photonic applications. Heavy metal oxide (Bi2O3) containing compounds have high refractive indices. Glasses embedded with wide band-gap semiconducting oxide crystals such as TiO2 received much attention due to their easy processing, stability and promising physical properties. Though TiO2 is used as nucleating agent to fabricate glass-ceramics of various phases, crystallization of TiO2 in glass matrices is difficult and the data are scarce in the literature. Therefore it was worth attempting to find glass compositions in which one can obtain TiO2 crystallization in large volume fractions. Towards this TiO2 crystallization was accomplished in BaO-TiO2-B2O3 glass matrix over wide composition ranges by tuning the concentration of BaO-TiO2 content in B2O3 network. The physical properties of these glasses of various compositions and glass-nanocrystal composites of TiO2 phase (anatase) were investigated. Interestingly BaO-TiO2-B2O3 glasses found to be hydrophobic in nature. The results obtained in the present research work are classified into five chapters apart from the Introduction, Materials and Methods chapters. Chapter 1 constitutes preface to oxide glasses, principles of glass formation and structural criteria followed by crystallization kinetics. In addition, principles of dielectric, optical and mechanical phenomena in glasses are discussed, since the present thesis focuses on the aforesaid physical properties. This chapter concludes with scope of the present thesis. Chapter 2 includes the detailed description concerning the fabrication techniques of materials under study and various characterization methods that have been employed at various stages of the present research work. The principles and experimental tools adopted for the structural and microstructural studies of materials were illustrated. Measurement techniques and experimental setup used to study physical parameters such as dielectric, optical, mechanical etc. were elaborated. Chapter 3 comprises structural, dielectric, electrical transport characteristics and optical studies of mixed alkali borate glasses in the 0.5Li2O-0.5M2O-2B2O3 (M=Li, Na and K) system. Transparent glasses in the Li2O-2B2O3 (LBO), 0.5Li2O-0.5Na2O-2B2O3 (LNBO) and 0.5Li2O-0.5K2O-2B2O3 (LKBO) were fabricated via the conventional melt quenching technique. Amorphous and glassy nature of the samples was confirmed via the X-ray powder diffraction and the differential scanning calorimetry, respectively. LKBO glass was found to have high thermal stability than that of LBO and LNBO. The frequency and temperature dependent characteristics of the dielectric relaxation and the electrical conductivity were investigated in the 100 Hz - 10 MHz frequency range. The relaxation and conductivity were rationalized using impedance and modulus formalism. Imaginary part of the electric modulus spectra was modelled using an approximate solution of Kohlrausch-Williams-Watts relation. The stretching exponent, β, was found to be temperature independent for LNBO glasses. Activation energies for conduction and relaxation process were calculated using the Arrhenius relation. The activation energy was found to be higher (1.25eV) for LKBO glasses than that of the other glass systems under study. This is attributed to the mixed cation effect. It has wide optical transmission window and optical band gap. Urbach energies were calculated for all these glasses. LBO, LNBO and LKBO glass compositions were found to crystallize in Li2B4O7, LiNaB4O7 and LiKB4O7 phases respectively upon heat treatment at appropriate temperatures. Transparent glass-micro crystal composites of LiKB4O7 were fabricated from LKBO glasses and found to be SHG active. BaO-TiO2-B2O3 Chapter 4 delineates the evolution of nanocrystalline TiO2 phase (Anatase) in BaO-TiO2-B2O3 (BTBO) glasses. Transparent colourless glasses in the ternary system were fabricated via conventional melt-quenching technique. The glasses with certain molar concentrations of BaO and TiO2 upon heat treatment at appropriate temperatures yielded nanocrystalline phase of TiO2 associated with the crystallite size in the 5-15 nm range. Nanocrystallized glasses exhibited high refractive index (no=2.15) at λ=543nm. These glasses were found to be hydrophobic in nature associated with the contact angle of 90o. These high index glass nanocrystal composites would be of potential interest for optical device applications. Crystallization kinetics of anatase phase in BTBO glasses were studied using non-isothermal Differential Scanning Calorimetry (DSC) at three different heating rates (10, 20 & 30 K/min). Scanning Electron Microscopy (SEM) carried out on heat treated (at 920 K) glasses confirmed bulk nucleation and three-dimensional growth. Johnson-Mehl-Avrami model could not be applied for this system suggesting considerable overlap of the nucleation and growth involving complex transformation process. However, modified Kissinger and Ozawa models were used to calculate the effective activation energy associated with anatase crystallization. The kinetic exponent n was found to be temperature dependent indicating the change in the crystallization mechanism. This is attributed to the high entropy fusion of anatase phase, fast crystallization rate and nano dimension of the anatase phase. Chapter 5 illustrates structural changes that occur in the x(BaO-TiO2)-B2O3 (x=0.25, 0.5, 0.75 &1 mol.) system on increasing the x apart from the details concerning some physical property correlations. Thermal stability and glass forming ability as determined by Differential Thermal Analysis (DTA) were found to increase with increasing BaO-TiO2 (BT) content. However, there was no noticeable change in the glass transition temperature (Tg). This was attributed to the active participation of TiO2 in the network formation especially at higher BT contents via the conversion of the TiO6 structural units into TiO4 units which increased the connectivity and resulted in an increase in crystallization temperature. Dielectric and optical properties at room temperature were studied for all the glasses under investigation. Interestingly, these glasses were found to be hydrophobic. The results obtained were correlated with different structural units present in the glass and their connectivity. These glasses exhibited low loss (tan δ≈0.002), frequency (10 kHz- 10 MHz) and temperature independent (or very weak temperature response) flat-dielectric response. Crossover temperature was encountered between flat response and Jonscher’s universal response. The cross-over temperature and cross-over energy barrier from flat dielectric response to Jonscher’s response was deduced for all the glasses in the present investigation. Electric modulus formalism was invoked to rationalize the relaxation phenomena. The observed dielectric response and conduction process in these glasses were attributed to the local vibration and switching of non-bridging oxygen ions in their potential cage and hopping over distributed energy barriers above the crossover temperature. Chapter 6 depicts the dielectric and mechanical properties of glasses embedded with TiO2 nanocrystals. BaO-TiO2-B2O3 glasses on subjecting to appropriate heat treatment temperature yielded TiO2 nano crystalline anatase phase. NMR studies carried out on the as-quenched glasses facilitated the estimation of fraction of tetrahedral and trigonal borate units. Poisson’s ratio and Young’s modulus were evaluated through theoretical expressions proposed by Makishima and Mackenzie. Nano-indentation and micro-indentation studies were carried out on the as-quenched glasses and glass-nanocrystal composites to examine mechanical characteristics. Estimated and indentation Young’s modulus of glasses were found to be in reasonable agreement. Hardness and Young’s modulus increased with increasing fraction of nano crystallites whereas fracture toughness was found to depend strongly on surface conditions. The results were corroborated by the structural units and particulates present in these glasses. Dielectric constant increased with increasing volume fraction of the nanocrystals which was rationalized via mixture rule. Chapter 7 describes the dielectric properties, electrical conduction and electric relaxation phenomena in 2Bi2O3-B2O3 (BBO) glasses followed by thier linear and nonlinear optical characteristics. Glasses in BBO system were obtained via melt-quenching technique. X-ray diffraction and differential scanning calorimetry were used to study the structural characteristics. Dielectric studies carried out on these glasses revealed near constant loss (NCL) response in the 1 kHz to 1 MHz frequency range at moderately high temperatures (300-450 K) accompanied by relatively low loss (tan δ=0.006, at 1 kHz & 300 K) and high dielectric constant (ε' =37, at 1 kHz & 300 K). The variation in AC conductivity with temperature at different frequencies showed a cross over from NCL response characterized by local ion vibration within the potential well to universal Jonscher’s power law dependence triggered by ion hopping between potential wells or cages. Thermal activation energy for single potential well was found to be 0.48±0.05 eV from cross over points. Ionic conduction and relaxation processes were rationalized by modulus formalism. The promising dielectric properties (relatively high ε' and low tan δ) of the BBO glasses were attributed to high density (93 % of its crystalline counterpart), high polarizability and low mobility associated with heavy metal cations, Bi3+. Optical band gap obtained for BBO glasses was found to be 2.6 eV. The refractive index measured for these glasses was 2.25±0.05 at λ=543 nm. Nonlinear refraction and absorption studies were carried out on BBO glasses using z-scan technique at λ=532 nm of 10 ns pulse width. The nonlinear refractive index obtained was n2=12.1x10-14 cm2/W and two-photon absorption coefficient was β=15.2 cm/GW. The n2 and β values of the BBO glasses were higher than that reported for high index bismuth based oxide glass systems in the literature. These were attributed to the high density, high linear refractive index, low band gap and two-photon absorption associated with these glasses. The electronic origin of large nonlinearities was discussed based on bond-orbital theory. Thesis ends with summary and conclusions followed by prospective views, though each chapter comprises conclusions associated with complete list of references. Patent, publications and conference proceedings that are listed below are largely based on the studies conducted as a part of the research work reported in the present thesis.