Conversion of Polycrystalline Alumina to Single-Crystal Sapphire by Localized Codoping with Silica

The effect of a localized SiO2 codoping on the conversion of polycrystalline, MgO‐doped Al2O3 tubes to single‐crystal sapphire was investigated. Codoping with SiO2 before sintering intentionally triggered abnormal grain growth, which resulted in the full conversion of tube surfaces to single crystal without adversely affecting densification to a almost pore‐free, translucent state. The degree of surface conversion was strongly dependent on experimental variables, which included furnace temperature and codoping amount. Surface‐converted tubes had excellent physical properties, which included good thermal cycling resistance and optical properties superior to unconverted, polycrystalline Al2O3.