Conversion of Polycrystalline Al₂O₃ into Single‐Crystal Sapphire by Abnormal Grain Growth

Abstract

In a given batch more than 30%-40% of polycrystalline, MgO-doped Al 2 O 3 tubes were converted into single crystals of sapphire by abnormal grain growth (AGG) in the solid state at 1880°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 ϳ 90°a nd 45°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 ϳ1.5 cm/h. The average grain boundary mobility at 1880°C was calculated as 2 ؋ 10 ؊10 m 3 /(N⅐s), representing the highest value reported so far in Al 2 O 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 2 O 3 , or ZrO 2 was added in concentrations of several hundred ppm.

Growth of single crystalline seeds into polycrystalline strontium titanate: Anisotropy of the mobility, intrinsic drag effects and kinetic shape of grain boundaries

“…The growth constant of single crystals was estimated as 1.6 Â 10 À15 -3.7 Â 10 À15 m 2 /s (Table 3), which is in the same order as those reported for exaggerated grain growth [18]; thus the single crystals grow exaggerated in the current study. Exaggerated growth of single crystalline seeds in a polycrystalline matrix was also observed for alumina [69,70], PMN-PT [71] and barium titanate [72][73][74]. In strontium titanate the exaggerated growth of single crystalline seeds was reported as well [34][35][36], but the polycrystalline matrix was not dense and no data were given for the mobility.…”

CSL grain boundary distribution in alumina and zirconia ceramics

“…This may be explained by the anisotropy of physical parameters that influence grain growth, such as GB mobility. Prism faces (parallel to the [0 0 0 1] axis) tend to move faster (along the ½2;1;1; 0 axis) than the basal planes (along the [0 0 0 1] axis) [36]. Therefore, the surface ratio S prism /S basal decreases with increasing grain size, whereas the number fraction ratio f prism /f basal increases.…”

Transparent Nanometric Cubic and Tetragonal Zirconia Obtained by High‐Pressure Pulsed Electric Current Sintering

“…[13] Furthermore, large grain size was traditionally considered beneficial in transparent ceramics since the reduction in the number of grain boundaries, particularly for nonisotropic structures, was expected to produce better transparency. [14] However, recent analysis provide evidence that residual porosity is much more important than grain boundaries in defining the transparency, even in materials that possess anisotropic optical properties, such as alumina. [11,12] The scattering efficiency for spherical pores, however, decreases dramatically when the size is reduced to the nanometric range.…”