Mechanism of "Solid-State" Single-Crystal Conversion in Alumina

Single crystals of Al₂O₃ were reproducibly grown from an MgO-doped polycrystalline precursor. The single crystals were grown through controlled abnormal grain growth at temperatures between 1670° and 1945°C. It was observed that CaO impurities segregated to the boundary between the single crystal and the polycrystalline region, and formed a wetting intergranular film. This type of film is required to produce the highly mobile grain boundaries that facilitate single-crystal conversion. The measured grain boundary mobilities correspond reasonably well with the mobilities calculated from data for a grain boundary containing a film with properties of the bulk glass, although some deviation from bulk behavior is indicated by the difference in activation energy. The grain boundaries are the most highly mobile alumina grain boundaries measured to date. This suggests that extrinsic effects produce the highest grain boundary mobility, rather than intrinsic behavior, which has conventionally been assumed to be the fastest.