Effect of ZnO on Thermo Physical and Structural Properties of Lithium Zinc Silicate Glass-Ceramics

Abstract

Lithium zinc silicate (LZS) glass-ceramics with compositions: (a) Li2O-ZnO-SiO2-Na2OB2O3- P2O5 and (b) Li2O-ZnO-SiO2-K2O-Al2O3-B2O3-P2O5 have been prepared by controlled nucleation and crystallization. The effect of ZnO/(ZnO+SiO2) ratio on various thermo-physical properties was investigated by changing the ratio from 0.1 to 0.31 in the case of (a). Different crystalline phases have been identified by X-ray diffraction studies in glass-ceramics including cristobalite, Li3Zn0.5SiO4 and Li2SiO3. Density ( r) was found to increase from 2.62 to 2.82 gm cm-3 while microhardness (VHN) decreased from 6.56 to 5.79 GPa with increase in ZnO/(ZnO+SiO2) ratio in the glass-ceramics. Average thermal expansion coefficient (TEC) in the temperature range 30 to 450oC increased from 125x10-7 to 185x10-7 /oC. The increase in TEC and decrease in microhardness are thought to be due to the formation of different phases, which in turn influence the rigidity/bonding in the glass-ceramics. A remarkable difference in the microstructure close to interface of the glass-ceramics to Cu seal was seen in both the cases [high ZnO content (a) of ZnO/(ZnO+SiO2) ratio 0.31 and low ZnO content (b) of ZnO/(ZnO+SiO2) ratio 0.024]. Both the microstructures showed globally two contrast phases of bright and dark dispersed in the glass matrix. An interesting dandritic phase observed towards core in the microstructure for the high zinc content is not seen in the microstructure for low zinc content glass-ceramic. The seal withstands a vacuum of ~ 10-6 torr at helium leak rate of 3x10-10 torr litre/sec.