Process for densifying polycrystalline articles

TLDR: In this paper, a polycrystalline article is densified to provide either a nonporous body or a body with controlled interconnected porosity, and the sintering aid can then be leached out at elevated temperature, further densifying the compact to form a substantially non porous body.

Abstract: A polycrystalline article is densified to provide either a nonporous body or a body with controlled interconnected porosity. A mixture of fine powders of the polycrystalline material and a sintering aid is compacted and outgassed under reduced pressure. The outgassed compact is then subjected to a permeation anneal step in which it is heated in a closed chamber to a temperature sufficient to form a liquid of the sintering aid, but under pressure conditions which inhibit evaporation of the sintering aid. The sintering aid can then be leached out to provide a densified article having interconnected porosity. Alternatively, the sintering aid can be leached out at elevated temperature, further densifying the compact to form a substantially nonporous body. Alternatively, the sintering aid can be removed by subjecting the densified article to an evaporation anneal step in which the article is heated to evaporate the sintering aid, further densifying the compact to form a substantially nonporous article. Apparatus is provided containing interconnected sections to accomplish the foregoing process. When applied to magnesium fluoride, a transparent polycrystalline body is obtained as a new article of manufacture which is substantially uniformly transparent to infrared radiation throughout the entire range of 0.7-8 microns.