Phase formation and microstructure during laser sintering and crystallization of a 4.2 MgO·5.0 ZnO·44.1 CaO·26.7 Al₂O₃·20.0 SiO₂ glass

In order to produce housings for high-temperature applications, alumina is a highly advantageous material because it has a high chemical durability and withstands high temperatures. If alumina is to be sealed, materials are necessary which have an adapted coefficient of thermal expansion (8.6 × 10⁻⁶ K⁻¹). If temperature-sensitive components have to be encapsulated, a rapid laser sealing process is highly advantageous. This process requires a glass which can rapidly be crystallized. In this paper, a glass powder with the composition 4.2 MgO·5.0 ZnO·44.1·CaO·26.7 Al₂O₃·20.0 SiO₂ was sintered and subsequently crystallized using a CO₂-laser. As crystalline phases, predominantly a solid solution of akermanite and gehlenite (AGSS) was formed and as phases with minor concentrations Al₂O₃, spinel/gahnite solid solution and ZnO. The AGSS grains have sizes of approximately 5 µm, and Mg and Zn are enriched at the grain boundaries. After sealing at temperatures of 985 and 1135 °C, a similar microstructure and similar grain sizes were observed. The AGSS seems to nucleate at the glass/Al₂O₃ interface but also in the bulk. The AGSS and all other phases do not show a preferred orientation. The resulting coefficients of thermal expansion fit well to that of Al₂O₃.