Solid solution magnesium aluminate spinel with the alumina-rich composition MgO·1.2 Al2O3 has been prepared as a transparent polycrystalline ceramic with average in-line transmission at 550 nm of 84.8 ± 2.7% and >82% throughout the visible spectrum. Starting powders are prepared from mixtures of high purity Mg(OH)2 and γ-Al2O3 thoroughly mixed in an aqueous slurry. Water is removed by rotary evaporation. The solids are collected, dried, calcined, mixed with LiF (as a sintering aid), and sieved. The powders are sintered into dense ceramics by hot pressing at 1600°C under vacuum and 20 MPa uniaxial load followed by hot isostatic pressing at 1850°C under 200 MPa Ar. Final grain sizes ranged between 300 and 1000 μm. Samples exhibited flexural strength of 176.8 ± 46.2 MPa; hardness of 12.3 ± 0.2 GPa; and elastic modulus of 292.9 ± 7.5 GPa. Control samples of stoichiometric magnesium aluminate spinel (MgO·Al2O3) were prepared with the same procedure and exhibited comparable values for transmission and physical properties.
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