High temperature glass ceramic sealants from R₂O-RO-AI₂O₃-SiO₂

Abstract

Barium-free glass-ceramic sealants for the planar solid oxide fuel cell form the system R₂O-RO-AI₂O₃-SiO₂ (R=Na, K, Mg, Ca) were developed and their properties were investigated. The experimental procedure was divided into two parts. The first part consisted in the characterization and performance of joining tests between glass-ceramics in the system MgO-CaO-Al₂O₃-SiO₂ with additional amounts of Na₂O and K₂O varying from 0 to10 mol%. The second part was devoted to glass-ceramics in the system MgO-CaO-Al₂O₃-SiO₂ with additional amounts of B₂O₃ varying from 0 to 20 wt.%. Both types of glass compositions were prepared by melting at 1450 ºC and 1500 ºC. Their thermal properties including glass transition temperature, crystallization temperature, dilatometric softening temperature, and coefficient of thermal expansion (CTE) were determined. The joins of glass-ceramics and ferritic stainless steel, including AISI430 and Crofer22 APU, were established at 900 ºC, 2 h, thermally treated at 800 ºC for 100 h and observed by SEM/EDX. The stability of the glass-ceramics after long term heat treatment was assessed through the crystallization of the mineral phases and the change in CTE using XRD and dilatometry, respectively. The results in the first part showed that glass-ceramics containing Na₂O and K₂O could be joined with AISI430 stainless steel after thermal treatment at 900 ºC for 2 h. However, they did not tightly bond to YSZ because their CTEs were too high. Long-time exposure at 800 ºC of these glass-ceramics caused a large increase in the CTE, which can be explained by increased formation of nepheline (NaAlSiO₄) and kaliophilite (KAlSiO₄). In the second part, good bonding between glass-ceramics and other components including Crofer 22 APU stainless steel and YSZ was found only in case of glass-ceramics containing at least 5 wt.% B₂O₃. The decrease of CTE is related to the increase of the boron oxide content in glass-ceramics, which leads to an increase in the formation of low-CTE phases diopside and anorthite.