Key powder characteristics in sintered optical ceramics

There are two principal reasons why powder properties are highly critical in the fabrication of optical ceramics such as polycrystalline Al2O3 (PCA), ALON, MgA2O4 and Y2O3. The first is that all but a few hundred ppm of the starting porosity must be eliminated in the sintering process, which required not only an excellent powder, but superb control over the entire sintering process. Second, powder imperfections usually result in visually apparent defects that cause rejection. The key powder properties that must be controlled to achieve pore elimination are crystallite morphology, crystallite size or surface area, and agglomerates. Visually apparent defects are principally controlled through chemical purification and hard agglomerate reduction. If the system provides good grain growth control during sintering, there is a surprising degree of latitude in these variables. However, once the limits are exceeded, reject rates increase, and the process goes out of control. This paper illustrates the importance of these key variables principally through microstructural analysis