Processing low thermal expansion glass-ceramic from lithium aluminosilicate (LAS) glass (PhD)

Abstract

The material with dimensional and thermal stability manifested their importance in widespread applications in kitchen to cosmos. The material of choice for applications which demand very high dimensional stability is lithium aluminosilicate (LAS) based low thermal expansion glass ceramic. This doctoral thesis work explored the possibility of realizing an ultra-low expansion, transparent glass-ceramic (GC) for its potential use in space applications. Chapter 1 introduces the field of research, background motivation and objective of the thesis. Literature Review presented in Chapter 2 is targeted to discuss the structural features of different LAS crystal system including their polymorphs and solid-solutions, the origin of unusual properties, the role of chemical constituents and additives, significant results of thermo-analytical studies, current commercial applications, recent trends, emerging technologies and future research perspectives. This review offers adequate fundamental, and recent progress in the LAS system with significant emphasise on processing low thermal expansion glass-ceramic (LEGC), ceramic matrix composite, low temperature co-fired ceramics and associated technologies. Chapter 3 brings out the process optimisation procedure adopted for realising transparent and nanocrystalline ultra-low thermal expansion glass-ceramic using microwave-assisted (hybrid)crystallisation of LAS glass realised from conventional melt quenching route. The experimental strategy involved two stages (i) identification of the optimum crystallisation temperature (Tc) under a microwave field and (ii) optimisation of microwave-assisted crystallisation process to achieve near-zero coefficient of thermal expansion (CTE). Optimum heat treatment schedules for nucleation and crystallisation under a microwave environment were found to be 720 °C/ 24 h and 775 °C/0.3 h respectively. The optimised heat treatment condition revealed the efficacy of the microwave hybrid heating, by producing nanocrystalline (35-50 nm) and transparent (>82%) ultra-low thermal expansion glass-ceramic (ULEGC) having a linear coefficient of thermal expansion of 0.03 ×10 6 oC 1 (0-50 °C). In Chapter 4, crystallisation parameters of the LAS glass composition were studied using non isothermal DSC and thermoanalytical (TA) methods. Available sites for nucleation has to reach a saturated condition, that is a primary validity criterion for employing conventional TA methods. The activation energy of crystallisation for a thermally stable LAS composition was determined after the prenucleation process, and it was found to be 371 ± 14 kJ/mol. A heat treatment programme for controlled crystallisation process was designed to result in a transparent (>80%), nanocrystalline, low expansion (CTE: 0.31×10 6 oC 1 between 60 to +60 oC) GC. Crystal growth at 775 °C was determined to be in the range 2.56 3.53 ×10 11 m/s and viscosity of glass near the growth front was predicted to vary between 1.28 ×105 and 2.82 ×105 N. s. m 2. In Chapter 5, the LAS glass compositions with P2O5 content varying between 0 6.8 mol% were prepared through the conventional melt-quenching route. From high-temperature dilation results, it was found that different amount of P2O5 in the LAS glass greatly influences phase transformation characteristics, the softening and melting points. Two LAS glass systems, namely 3.1 mol% of P2O5 (P3.1) and without P2O5 (P0) were considered further towards making low expansion GC using bulk and sintering route due to their contrary thermal behaviour. The optimum nucleation temperature for P0 and P3.1 glass system was determined to be 640 and 700 oC, respectively using the Marotta method. Effect of heat-treatment temperature on the thermal expansion behaviours of the LAS GC was explained in detail. Negative thermal expansion (NTE) and low expansion GCs were produced from bulk and sintering route. Transparent quartz s.s. based ultra-low thermal expansion (0.04 ×10 6 °C 1 between 60 and 400 °C) GC was produced. Chapter 6 presents the crystallisation behaviour of unconventional LAS (1: 1.2: 7) composition having MgO, BaO, K2O, and ZrO2. Crystallisation parameters were determined using thermo analytical models based on Differential Scanning Calorimetry (DSC). The activation energy of crystallisation, E, and frequency factor, were calculated to be 354.40 kJ mol 1 K 1 and 1.63 1015, respectively. Effect of sintering temperature on density, phase constitution, thermal expansion, and microstructure are reported herein. The temperature range between 1373 K and 1473 K was found to be the optimum window for sintering the glass particles. GC with CTE matching the Fe-Ne superalloy is reported herein. Considering the LAS glass system sinterability, efforts were made towards sintering alumina with LAS glass as a sintering aid. The 5 wt.%. LAS/Al2O3 composite was prepared with density: 3.6 g/cm3, relative permittivity 10.5, and dielectric loss tangent 2.45 10 3. This composition was found to be a potential CTE compensator material while processing tailorable CTE ceramic or polymer-based composites. Limitation of the present work, summary and future scope of work are presented in Chapter 7.