Some properties of lithium aluminium silicate (LAS) glass-ceramics used in glass-ceramic to metal compressive seal for vacuum applications

We report here the preparation of LAS glass-ceramics and some studies on their thermo-physical properties and microstructure, for compressive seals in vacuum applications. Glass of composition 12.6Li₂O-71.7SiO₂-5.1Al₂O₃-4.9K₂O-3.2B₂O₃-2.5P₂O₅ was prepared by the conventional melt quench technique. Based on differential thermal analysis (DTA) data, glass samples were nucleated at 600°C for 2hr and were then crystallized at 800°C for 2-5hr. X-ray diffraction (XRD) spectra showed lithium disilicate to be the major phase. A dwell time of 3hr yielded a sample of good crystallinity. Dilatometric measurements of this sample on a thermo-mechanical analyzer (TMA) measured a thermal expansion coefficient (TEC) of 94.61×10^-7°C^-1. Glass transition temperature (T_g), and dilatometric softening temperature (T_ds) of the sample was recorded as 585°C and 830°C respectively. Considering TEC and T_ds compression type seals were prepared with SS304 (TEC = 172×10^-7°C^-1) housing of length 15mm, outer diameter 30mm and inner diameter 15mm. After pre-heat treatment of the metal components, sealing was carried out under a protective atmosphere of flowing Argon gas. The finished seal was tested for leak tightness on a He leak detector. The seal was capable of withstanding a vacuum of 10^-6 Torr; at a leak rate of 10^-9 Torr lit s^-1. Scanning electron microscopy (SEM) was carried out on LAS before and after fabrication of compressive seal to elucidate the effect of compressive stress and the presence of metal near the interface. We observe a significant difference in microstructure due to compressive stresses of sealing and due to the presence of metal. Energy dispersive analysis of X-rays (EDAX) revealed no interdiffusion of species from glass-ceramic to metal or vice-versa.