A comparison of hot-pressing, rate-controlled sintering, and microwave sintering of magnesium aluminate spinel for optical applications

There are several crystalline materials that transmit electromagnetic radiation in the visible and IR portion of the spectrum, (from 0.4 microns to 5 microns). At this time, single-crystal sapphire, aluminum oxynitride (ALON), and spinel show promise for applications, including advanced electromagnetic windows and transparent armor. These applications require materials with high strength, hardness, and the ability to withstand high temperatures. Because of lower processing temperatures and shorter processing times, it is reasonable to assume that spinel should ultimately be less costly to produce than ALON or sapphire. Despite many attempts to commercialize spinel, it is not available today as an optical material due to difficulties in reliably obtaining the desired transparency. To help develop a commercial source for transparent spinel, the U.S. Army Research Laboratory (ARL) and Ceramic Composites Inc. of Annapolis have signed a Cooperative Research and Development Agreement (CRDA), on the "Development and Dual-Use Assessment of Transparent Spinel". The advent of commercially available, highly pure spinel powders should lead to improvements in processing spinel to transparency. This investigation compares the advantages and limitations of hot-pressing, microwave sintering, and rate-controlled sintering and compares the limited property data available from each of these fabrication techniques.