High temperature thermal expansion of BaAl2Si2O8, CaAl2Si2O8, and Ca2Al2SiO7 studied by high-temperature X-ray diffraction (HT-XRD)

• High temperature seals are obtained by sintering and crystallization of a glass.
• For joining of materials with high CTE, barium and calcium silicate are advantageous.
• Barium and calcium aluminosilicates if crystallized from glass are less favourable.
• The crystallization of CaAl2Si2O8 should be avoided.

Sealing of components for high temperature applications with coefficients of linear thermal expansion (CTE) > 10·10−6 K−1 can be achieved by glasses from which crystalline phases with high CTE are precipitated. Many sealing glasses also contain further components as e.g. aluminium and hence, not only the desired phase is crystallized, but also additional phases. For this purpose, high temperature XRD was performed in order to determine the CTE of BaAl2Si2O8, CaAl2Si2O8, and Ca2Al2SiO7. In the case of BaAl2Si2O8 and Ca2AlSi2O7 the CTEs were 7.8·10−6/K and 7.9·10−6/K, respectively. In the case of CaAl2Si2O8 the CTE is 4.4·10−6/K. Especially the formation of the latter phase should be avoided for a sealing material of high temperature fuel cells. Sintered specimens of the respective compounds were also characterized by dilatometry.

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