Near‐zero thermal expansion transparent lithium aluminosilicate glass‐ceramic by microwave hybrid heat treatment

Abstract

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).

Crystallization and sintering studies on an anomalous Li2O‐Al2O3‐SiO2 glass for making tunable thermal expansion ceramic

“…However, these morphologies features are not comparable to the several microstructures presented for bulk glass derived LEGCs, which are presented in our previous works. 33,50 High thermal stability of the present LAS glass could be why high CTE was recorded for samples S8 and S9. The absence of zinc in the composition is expected to precipitate only low PTE crystals whose CTE is sufficient to achieve near-zero thermal expansion GC.…”

Toward controllable material removal of glass–ceramic surface for low‐deformation machining

“…The volumes of scratch before and after 400 • C annealing were V O-400 and V A-400 , respectively. The proportion of densification-induced deformation M D can be expressed as 𝑀 𝐷 = (𝑉 𝑂−400 − 𝑉 𝐴−400 ) ∕𝑉 𝑂−400 (4) After annealing at 500 • C (1.1 Tg), all deformations caused by residual stress and densification were recovered, and the remaining volume that cannot be recovered belongs to the permanent removal caused by shear stress. The volumes of scratch before and after 500 • C annealing were V O-500 and V A-500 , respectively.…”

“…From the calculated volume deformation in Figure 4 and formulas (3)(4)(5), the ratios corresponding to three deformation forms as a function of normal load and sliding velocity are plotted in Figure 5, respectively. As applied load increased from 4 to 12 mN, the densification ratio M D gradually decreased, whereas shear flow ratio M S and residual stress ratio M R increased consistently.…”

“…has been extensively used in the production of aerospace mirror substrates, lenses of various optical devices, and so on. 3,4 Especially for lithium aluminosilicate (LAS) glassceramic, because of its ultrahigh uniform refractive index and optical homogeneity, it is employed in high-tech fields which require high precision, 5 such as wafer positioning as a new generation of lithography lens. 6,7 This puts forward higher requirements for the flatness of processed surface.…”

Preparation study for the low thermal expansion spodumene/mullite composites

“…Chen et al 17 prepared transparent glassceramics with a low CTE by using spodumene as the main raw material and adding TiO 2 and ZrO 2 as nucleating agents. Venkateswaran et al 18 synthesized transparent lithium aluminosilicate glass-ceramics with near-zero thermal expansion via a microwave mixed heat treatment.…”