Conversion of polycrystalline Al sub 2 O sub 3 into single-crystal sapphire by abnormal grain growth

In a given batch more than 30%-40% of polycrystalline, MgO-doped Al sub 2 O sub 3 tubes were converted into single crystals of sapphire by abnormal grain growth (AGG) in the solid state at 1880 deg C. Most crystals grew 4-10-cm in length in tubes with wall thicknesses of 1/2 and 3/4 mm and outer diameters of 5 and 7 mm, respectively, and had their c-axes oriented approx =90 deg and 45 deg to the tube axis. Initiation of AGG was associated with low values of bulk MgO concentration near 50 ppm. The unconverted tubes did not develop centimeter-size crystals but instead exhibited millimeter-size grains. The different grain structures in converted and unconverted tubes may be related to nonuniform concentration of MgO in the extruded tubes. The growth front of the migrating crystal boundary was typically nonuniformly shaped, and the interface between the single crystal and the polycrystalline matrix was composed of many 'curved' boundary segments indicative of classical AGG in a single-phase material. The average velocities of many migrating crystal boundaries were quite high and reached approx =1.5 cm/h. The average grain boundary mobility at 1880 deg C was calculated as 2 x 10 exp -10 m exp 3 /(N s), representing the highest value reported so far in Al sub 2 O sub 3 and within a factor of 2.5 of the calculated intrinsic mobility. Under similar experimental conditions sapphire crystals did not grow when a codopant of CaO, La sub 2 O sub 3 , or ZrO sub 2 was added in concentrations of several hundred ppm.