High-strength transparent spinel with fine, unimodal grain size

Spinel (MgAl 2 O 4 ) is a good candidate material for transparent armor and IR window applications. Traditionally, transparent spinel has suffered from poor strength and difficult polishing owing to its large, bimodal grain structure. Starting from a spinel nanopowder, spinel ceramics with a grain size of less than 2 microns have been made with better than 80% in-line transmittance at 632 nm wavelength for 3/8" thick samples. A ring-on-ring test has been used to measure biaxial flexural strength on samples machined to 0.8 mm thickness. The average strength was found to exceed 480 MPa.