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The lithium aluminosilicate system (LAS) is being explored for almost seven decades due to its anomalous and attractive properties (especially low/negative thermal expansion and fast ion conduction) and its commercial significance in consumer as well as strategic sectors. This review introduces current commercial applications of LAS systems, necessary background science, structural features of different LAS crystal systems, including their polymorphs and solid-solutions, and the origin of unusual properties. Significant emphasis is provided on processing transparent, nanocrystalline, low thermal expansion glass-ceramic (LEGC), the role of chemical constituents and additives, the effect of heat-treatment, and microstructural evolution while processing LEGC. Detailed discussions are provided on the following areas: LAS matrix composites, chemical strengthening of glass and glass-ceramic, low temperature co-fired ceramics (LTCC) and associated joining technologies (hydrolysis catalysis bonding, anodic bonding, brazing, etc.) that would further extend the application portfolio of LAS system.

The material of choice for space applications which demand very high dimensional stability is lithium aluminosilicate (LAS) based Ultra Low thermal Expansion Glass-Ceramic (ULEGC). Generally, the controlled crystallization process recommended for the processing of transparent ULEGC involves a long soaking duration to achieve the required crystal number density. This paper brings out the process optimization procedure adopted for realizing transparent and nanocrystalline ULEGC from conventionally processed LAS glass using microwaveassisted (hybrid) crystallization. The experimental strategy involves two stages (i) identification of the optimum crystallization temperature (T c ) under a microwave field (ii) optimization of a microwave-assisted crystallization process to achieve near zero Coefficient of Thermal Expansion (CTE).. Optimum heat-treatment schedules for nucleation and crystallization under a microwave environment were found to be 720°C/24 hours and 775°C/0.3 hours, respectively. The optimized heat-treatment condition revealed the efficacy of the microwave hybrid heating, by producing nanocrystalline (35-50 nm) and transparent (>82%) ULEGC having a thermal expansion of À0.03 9 10 À6 K (0°C to 50°C).

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The material with dimensional and thermal stability manifested their importance in widespread applications from kitchen to cosmos. 1,2 Lithium aluminosilicate (LAS) glassceramic is one of such material attracted research as well as industrial community due to their properties namely low coefficient of thermal expansion (CTE), thermal shock resistance, chemical durability, and mechanical strength. [2][3][4] Attracted by their thermal properties, LAS glass-ceramics are studied for dielectric properties. 5,6 The process of making low expansion glass-ceramic (LEGC) relies on precipitating high-quartz type LAS based negative thermal expansion (NTE) crystals uniformly throughout the glass matrix. 7 These stuffed derivatives of quartz structure proposed by Buerger 8 and Li et al. 9 are comparable with magnesium aluminosilicate based -cordierite system. 10 Mg 2+ ions occupy octahedral coordination sites, and Zn 2+ ion occupies tetrahedral sites in high-quartz type crystals. Zn 2+ substitution in the LAS crystals system can result in very high NTE behavior whereas Mg substitution results in low positive thermal expansion (PTE). Mg substitution up to 20% allows the precipitation of high-quartz type s.s. 11 The addition of MgO with CaO in LAS glass system was reported to reduce the crystallization temperature. 12 Alekseeva et al. 13 reported that the addition of PbO, MgO, BaO in LAS glass improves the precipitation of quartz solid solution and ZnO, Na 2 O, and K 2 O addition accelerates the crystallization process. BaO addition in LAS glass reported yielding better dielectric properties. 14 In LAS GC precipitating high-quartz

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Mica containing glass-ceramic is prepared by crystallizing magnesium alumino silicate glass between 600°C and 1100°C without the use of any additives or nucleating agents. Effects of ceramization heat treatment conditions on phase constitution and microstructures are analyzed using XRD and HRSEM respectively. Developed glass-ceramics have been analyzed for its microhardness and machinability. Optimum heat treatment condition for achieving uniformly distributed fluorophlogopite phase essential for machinability, and at the same time, magnesium aluminum silicate phase for increasing the hardness, is presented.