Crystallization Of Silicate Glasses Research Articles

Effects of composition and phase relations on mechanical properties and crystallization of silicate glasses

AbstractCrystallization, mechanical properties, and workability are all important for the commercialization and optimization of silicate glass compositions. However, the inter‐relations of these properties as a function of glass composition have received little investigation. Soda‐lime‐silica glasses with Na2O‐MgO‐CaO‐Al2O3‐SiO2 compositions relevant to commercial glass manufacture were experimentally studied and multiple liquidus temperature and viscosity models were used to complement the experimental results. Liquidus temperatures of the fabricated glasses were measured by the temperature gradient technique, and Rietveld refinements were applied to X‐Ray powder diffraction (XRD) data for devitrified glasses, enabling quantitative determination of the crystalline and amorphous fractions and the nature of the crystals. Structural properties were investigated by Raman spectroscopy. Acoustic echography, micro‐Vicker's indentation, and single‐edge‐notched bend testing methods were used to measure Young's moduli, hardness, and fracture toughness, respectively. It is shown that it is possible to design lower‐melting soda‐lime‐silica glass compositions without compromising their mechanical and crystallization properties. Unlike Young's modulus, brittleness is highly responsive to the composition in soda‐lime‐silica glasses, and notably low brittleness values can be obtained in glasses with compositions in the wollastonite primary phase field: an effect that is more pronounced in the silica primary phase field. The measured bulk crystal fractions of the glasses subjected to devitrification at the lowest possible industrial conditioning temperatures indicate that soda‐lime‐silica glass melts can be conditioned close to their liquidus temperatures within the compositional ranges of the primary phase fields of cristobalite, wollastonite, or their combinations.

Crystallization of neodymium-rich phases in silicate glasses developed for nuclear waste immobilization

Glass-ceramics containing neodymium-rich crystalline phases can be obtained by crystallization of silicate glasses (nucleation + crystal growth heat treatments) or by controlled cooling of melts. Such materials could be envisaged as durable matrices for conditioning minor actinides- and Pu-rich nuclear wastes if the partitioning ratio of the wastes between crystalline phase and residual glass is high (principle of double containment barrier). In radioactive waste forms, Nd would be partially substituted by actinides and neutron absorbers (Gd). In this work, two silicate glass compositions leading to efficient nucleation and crystallization of either zirconolite (Ca 1− x Nd x ZrTi 2− x Al x O 7, x < 1) or apatite (Ca 2Nd 8Si 6O 26) in their bulk were studied as potential waste forms. The effect of the method used to prepare glass-ceramics (controlled cooling from the melt or nucleation + crystal growth from the glass) on both the microstructure and the structure of the neodymium-rich crystalline phase was studied. The highest number of zirconolite or apatite crystals in the bulk was obtained using the nucleation + crystal growth method. However, the percentage of neodymium incorporated in zirconolite crystals remained too small to make realistic the use of such materials for the conditioning of actinides in comparison with more durable bulk ceramics.

Effect of the crystallization of bioactive glass reinforcing agents on the mechanical properties of polymer composites

Polymer composites based on the introduction of bioactive materials into polymer matrices can combine mechanical with bioactive properties suitable for many potential biomedical applications. In this work, glass-ceramics derived from the crystallization of bioactive glasses were tested as reinforcing agents in polymer composites. The kinetics of crystallization of soda-lime silica glasses containing phosphorous was studied to define conditions for obtaining glass-ceramics having different microstructures. This type of glass can be used in biomedical applications due to its ability to bind to bone (bioactive behavior). Evaluation of the mechanical properties of glass-ceramic bodies derived from bioactive glasses has allowed us to determine the relationship between microstructure and properties. A particular microstructure capable of leading to fine grained glass-ceramics with enhanced properties was then reproduced in bioactive glass particles. Thus, bioactive glass particles were crystallized to different levels and used as reinforcing agents in polymer (polysulfone) composites. Mechanical properties of composites were evaluated using a four-point bending test. Glass-ceramic reinforced composites showed higher elastic modulus than glass reinforced composites. Therefore, crystallization of silicate glasses used as reinforcing agents in polymer matrices is a possible way to enhance the mechanical properties of composites. Polymer composites having bioactive glass-ceramic particles display some mechanical properties comparable to human cortical bone.

Structure aspects of the crystallization of silicate glasses

Structure aspects of the crystallization of silicate glasses

A comparative study of ?gels? and oxide mixtures as starting materials for the nucleation and crystallization of silicate glasses

Nucleation and crystallization behaviour of glasses in SiO2-La2O3, SiO2-La2O3-Al2O3 and SiO2-La2O3-ZrO2 systems were investigated using glasses prepared by the fusion of “gels” and the mixtures of oxides in solar/image furnace. Two methods for the preparation of multicomponent homogeneous non-crystalline products in the form of gels were developed. The phase separation, devitrification and micro-hardness of the above glasses were investigated in relation to the starting materials and the composition. The results show that the glasses made from gels are more homogeneous than those made from oxide mixtures. The phase separation characteristics of glasses made from gels are markedly different from those of glasses made from a mixture of oxides. The addition of Al2O3 to the binary SiO2-La2O3 glasses improves the homogeneity but reduces the micro-hardness and the devitrification tendency, whereas the addition of ZrO2 causes a considerable increase in micro-hardness and enhances the devitrification. The rates of nucleation and crystallization of glasses of different compositions made from gels are much higher than those made from the mixture of oxides. The formation of the high temperature crystal form, (Β-La2Si2O7 is more evident with the crystallization of gel-glasses. When the rate of nucleation is low, (in the case of glasses from the mixture of oxides), the curve representing the relation between the micro-hardness and the time of heat-treatment shows a distinct minimum, whereas this minimum is not obtained with the gel-glasses. With most of the gel-glasses, the micro-hardness rises very sharply with the length of heat-treatment. The curve showing the relation between the micro-hardness and the volume fraction of the dispersed crystalline phase also gives a distinct minimum which can be explained on the basis of the fracture mechanism consisting of the processes of crack nucleation and of crack propagation around the dispersed crystalline particles.