Synthesis and Characterization of Alkaline Earth Oxide Added Borosilicate Glasses for High Temperature Sealant Application

Abstract

BaO-CaO-Al2O3-SiO2 (BCAS) glass and their derivatives have gained extreme importance for their high endurance to elevated temperatures and being suitable for various high temperature applications. Two glass systems, BaO-CaO-B2O3-SiO2-Al2O3 (BCBSA) and another without Al2O3 (BCBS) were synthesized using melt quenching technique in the present work. Addition of ZnO and MgO as flux helped in melting them at 1300 ᴼC which is much lower than the usual melting temperature of these glasses. Density of the quenched glasses was measured by Archimedes method and structural bond vibrations were confirmed through FTIR. UV Visible spectroscopy was used to determine band gap energy and confirm the insulating nature of the synthesized glasses. Annealed glass powders were isothermally heated at 700 ᴼC, 800 ᴼC for 50 hours and at 900 ᴼC for 50 and 100 hours duration in air to observe their devitrification behaviour, which were then analysed by X-ray diffraction to identify the developed phases. The growth of detrimental monocelsian phase could be avoided by removing Al2O3 from the composition. The BCBS glasses with low BaO concentrations were found to be most promising for high temperature sealing. Reducing BaO concentrations improved the coefficient of thermal expansion (CTE) and other characteristic temperatures of glasses. Compacted glass pellets show maximum shrinkage at 700 °C. Dilatometric analysis conducted on these glass compacts showing maximum shrinkage, exhibited CTE more than the bulk glasses. Glasses with low BaO concentrations exhibited excellent mechanical properties. The optimized glass composition 00B was screen printed and sandwiched between two Crofer22APU plates forming Crofer22APU – Glass – Crofer22APU (CGC) sandwiches and heat treated for repeated thermal cycles from RT to 800 °C. Microscopic images of the CGC interface, XRD analysis of the internal surface of the broken sandwich and DC resistivity measurements of the glass under sandwiched conditions between two Crofer22APU plates, at RT and at elevated temperatures up to 800 °C were carried out, to determine its compatibility with Crofer22APU at high temperatures. The CGC sandwiches showed good adherence and compatible structural and thermal properties after repeated thermal cycles. It also exhibited sufficiently high specific resistance at room and elevated temperatures.