Hydrogen impurities in materials influence their properties, including flow strength. ??-Al2O3 single crystals and polycrystalline ceramics were annealed in supercritical water between 850 and 1025??C, under pressures in the range 1500-2000 MPa. A few specimens were further subjected to plastic deformation. Hydrogen penetration was examined using infrared absorption measurements of O-H bond vibrations, which revealed two kinds of hydrogen defects. In single crystals, defects are characterized by sharp O-H absorption bands assigned to interstitial protons. Hydrogen impurities of hydrothermally annealed ceramics and of all hydrothermally deformed specimens are characterized by broad O-H bands assigned to molecular water. The grain boundaries of hydrothermally annealed ceramics are severely damaged. The kinetics of hydrogen penetration is consistent with diffusion data.
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Hydrogen defects in ??-Al2O3 and water weakening of sapphire and alumina ceramics between 600 and 1000??C - I. Infrared characterization of defects