Mechanism for Solid State Crystal Conversion
Abstract
A MgO-depletion mechanism is proposed for the Solid State Crystal Conversion (SSCC) phenomenon in MgO-doped A12O3, where sapphire crystals grew up to 5 cm in length by classical abnormal grain growth in microstructures with average grain sizes of 10-45 microns. When the MgO concentration decreased (via evaporation) at temperatures between ~1750 and 1900 deg C and reached a critical concentration (C(sub MgO)-Crit.) of ~ 60ppm, an abnormal grain suddenly 'nucleated' and migrated through the polycrystalline matrix at velocities 1000-fold higher. Above C(sub MgO)-Crit., charged Mg(2+) ions and oxygen vacancies apparently interacted strongly with grain boundaries. Near C(sub MgO)-Crit. the sudden, 1000-fold rise in grain boundary diffusion indicated with grain boundary/solute interaction. This phenomenon was related to possibly: (a) charge compensation of Mg(2+) by Si(4+)-impurity and/or; and (b) a 'non-blocking' phenomenon, whereby Mg(2+) ions no longer impede grain growth by occupying kink sites on grain boundary steps. Large single crystals grew with irregular growth fronts moving at velocities of 4 microns per second (1.5 cm/h) with their c-axis oriented primarily ~ 90 deg or 45 deg to the tube axis. TEM lattice images of growth fronts showed clean crystal boundaries with no liquid phase or film.
Document Details
- Document Type
- Technical Report
- Publication Date
- Dec 30, 2000
- Accession Number
- ADA391732
Entities
People
- Charles D. Greskovich
- James A. Brewer