Glass-based sealants for joining α to β’’-Al2O3 in Na–Zn batteries

This study deals with the design and the characterisation of glass-based materials for the hermetic sealing of ceramic components (α-to β’’-Al2O3) in solid-electrolyte cells operating at approximately 300 °C. A silica-based commercially available glass and a newly developed silica-free glass, potentially resistant to corrosion by molten sodium salts, are reviewed and discussed. 25 wt% alumina powder is incorporated into both formulations. The sintering behaviour of the glass systems and their wetting characteristics on alumina are evaluated at the selected sealing temperatures: 900 °C for 40 min (for the silica-based system) and 700 °C for 1 h (for the silica-free system). The viscosity of both glass systems at these temperatures, which is approximately 105 Poise, is deemed suitable for facilitating proper glass flow and bonding with the alumina components. Furthermore, the coefficient of thermal expansion (CTE) of the potential sealant candidates is investigated. The results demonstrate good compatibility between the glass-based systems (CTE ≈ 7–7.6· 10−6 K−1 at temperatures ranging from 100 to 300 °C) and α- and β’’-Al2O3 (CTE ≈ 7.3· 10−6 K−1 within the same temperature range). The microstructural analysis conducted using field-emission scanning electron microscopy (FE-SEM) confirms good thermo-mechanical matching between the glass-based sealants and the alumina ceramic components and that no reaction occurs during the sealing between alumina powder and the glass matrix.