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Conversion of Polycrystalline Al 2 O 3 into Single‐Crystal Sapphire by Abnormal Grain Growth
Author(s) -
Scott Curtis,
Kaliszewski Mary,
Greskovich Charles,
Levinson Lionel
Publication year - 2002
Publication title -
journal of the american ceramic society
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.9
H-Index - 196
eISSN - 1551-2916
pISSN - 0002-7820
DOI - 10.1111/j.1151-2916.2002.tb00257.x
Subject(s) - crystallite , sapphire , materials science , grain boundary , crystal (programming language) , crystallography , single crystal , doping , mineralogy , crystal growth , analytical chemistry (journal) , composite material , optics , microstructure , chemistry , metallurgy , optoelectronics , laser , physics , chromatography , computer science , programming language
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 ∼ 90° and 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 ∼1.5 cm/h. The average grain boundary mobility at 1880°C was calculated as 2 × 10 −10 m 3 /(N·s), 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.