History of development of polycrystalline optical spinel in the U.S.

Fabrication of transparent MgAl2O4 spinel through homogenous green compaction by microfluidization and slip casting

“…Various candidate materials such as Al 2 O 3 , AlON and MgAl 2 O 4 have outstanding mechanical and optical characteristics [2], and among them, magnesium aluminate spinel (MgAl 2 O 4 ), which is developed as translucent bulk material by Westinghouse [3], have been investigated as a promising candidate for transparent window and IR dome applications due to its excellent mechanical properties and relatively high IR cut-off wavelength [4]. For highly-transparent MgAl 2 O 4 , its total volume porosity should be lower than tens of ppm, and its nano-size pores (40-70 nm in diameter, which is one-tenth of a visible wavelength) also should be minimized based on the Mie-scattering theory [5].…”

Grain Size-Dependent Hardness of Transparent Magnesium Aluminate Spinel

“…Transparent sintered spinel (MgO· n Al 2 O 3 ) is a major candidate for light‐weight transparent armor 1 . Such cubic polycrystalline ceramics do not encounter birefringent scattering losses as for example sintered translucent corundum (α‐Al 2 O 3 ).…”

Effect of Impurities and LiF Additive in Hot‐Pressed Transparent Magnesium Aluminate Spinel

“…17 In graphitic furnaces, carbon reacts with spinel and forms CO gas at temperatures as low as 500°C 2,4 and aluminum oxy-carbides at temperatures as low as 1250°C, 18 likely influencing chemical evolution, stoichiometry, and oxygen vacancy concentration, com-plicating processing. 7,19,20 Transparent components have since been produced using combinations of pressureless sintering, cold, and hot pressing, cold isostatic pressing (CIP), and hot isostatic pressing (HIP), and also by sparkplasma sintering and fusion casting. 3,9,12,13 Impurities also restrict grain boundary mobility and form scattering second phases, reducing transmittance.…”

“…Spinel is also sensitive to impurities and does not typically sinter to transparency in air, likely due to swelling. 7 Current commercial processing typically involves high-purity starting powders, powder pre-treatment, LiF additive, hot pressing or CIP and sintering, followed by HIP. 1,19 Transparent polycrystalline spinel was first made by General Electric Co. in the 1960s.…”

“…3,9,12,13 Impurities also restrict grain boundary mobility and form scattering second phases, reducing transmittance. 7,[19][20][21][22][23][24][25][26] High-purity, high-surface energy, deagglomerated nanometer-sized powders have also been synthesized. Calcined Al 2 O 3 and MgO powders with LiO 2 and SiO 2 additives were cold pressed and sequentially sintered in H 2 to 1900°C while embedded in spinel powder to maintain stoichiometry.…”