Visible Light Transparency for Polycrystalline Ceramics of MgO•2Al2O3 and MgO•2.5Al2O3 Spinel Solid Solutions

Magnesium aluminate spinel solid solutions with the alumina-rich compositions MgO2Al(2)O(3) and MgO2.5Al(2)O(3) have been prepared as polycrystalline ceramics with average in-line transmissions at 550nm of 85.5 +/- 0.3% and 80.9 +/- 0.4%, respectively. Starting powders are prepared from combinations of high purity Mg(OH)(2) and -Al2O3 thoroughly mixed in an aqueous slurry, and the solids are collected, dried, calcined, mixed with LiF sintering aid, and sieved. The optimum amount of LiF added varies with the alumina composition of the spinel solid solution. The powders are sintered into dense ceramics by hot pressing at 1600 degrees C under vacuum and 20MPa uniaxial load followed by hot isostatic pressing at 1850 degrees C under 200MPa in Ar. Both compositions exhibit exaggerated grain growth with average sizes well over 500m. Knoop hardness measurements are 11.2 +/- 0.3GPa for MgO2Al(2)O(3) and 11.0 +/- 0.4GPa for MgO2.5Al(2)O(3).