Crystallization behaviour of Li 2O [sbnd]ZnO [sbnd]SiO 2 glass–ceramics system

Abstract

We have studied the crystallization behaviour in two types of lithium zinc silicate (LZS) glasses: (a) LZSL composition (wt.%): Li 2O, 12.65; ZnO, 1.85; SiO 2, 74.4; Al 2O 3, 3.8; K 2O, 2.95; P 2O 5, 3.15; B 2O 3, 1.2 (low ZnO); and (b) LZSH composition (wt.%): Li 2O, 8.9; ZnO, 24.03; SiO 2, 53.7; Na 2O, 5.42; P 2O 5, 2.95; B 2O 3, 5 (high ZnO). The glass has been prepared by usual melt–quenched technique. The crystallization was carried out following two different procedures: (i) cooling schedule (CS) and (ii) heating schedule (HS). The nucleation and crystallization temperatures were determined by DTA studies. In case of LZSL sample, the crystallization was carried out in the temperature range of 580–780 °C, whereas in case of LZSH it was done in the temperature range of 570 and 720 °C. Crystalline phases were identified using powder X-ray diffraction technique. Significant differences in the formation of crystalline phases and their relative ratios were observed between heating and cooling schedules. For LZSL, formation of Li 2SiO 3 phase along with small fraction of cristobalite phase was seen when CS was followed. On the other hand, mixed phase having small fraction of lithium zinc silicate in major cristobalite phase was obtained for heating schedule. However, in the case of LZSH, formation of lithium zinc silicate as major crystalline phase along with cristobalite phase is seen when HS was followed. For LZSL, the average thermal expansion coefficient (TEC) values were found to be around 178 × 10 −7 and 114 × 10 −7 °C −1, for CS and HS, respectively, and for LZSH, TEC was found out to be ≈185 × 10 −7 °C −1 for HS. LZS glass–ceramics-to-Cu as well as to-SS-321 seals withstanding a vacuum of 10 −6 Torr with leak rate 10 −9 Torr l/s were fabricated using both compositions. Glass–ceramics-to-metal interface was studied using electron probe micro analysis (EPMA) to understand the mechanism of sealing.