GB717555A - Improvements in or relating to shaped silicon nitride bodies and their manufacture - Google Patents

Abstract

A shaped body of silicon nitride is made by forming into the desired shape silicon metal which has been reduced to a mesh size of say U.S.S. 180 or smaller, firing in an atmosphere of nitrogen, ammonia gas, or in a substantially non-carbonaceous, non-oxidising atmosphere under such conditions that complete nitriding is effected, with the production of silicon nitride having the formula Si3N4, the electrical resistivity of the body being over 1 megohm per c.c. at 25 DEG C. Finely divided silicon nitride may be mixed with the silion raw material, and pore-forming materials, preferably those removed by volatilization during drying and/or firing, may be included, e.g. powdered or granular naphthalene, or organic resinous material such as phenolic resins. More rapid nitriding is obtained if commercial silicon or pure silicon containing up to 5 per cent. iron preferably 3/4 -1 per cent. is used, and if the silicon is of a size U.S.S. 200 mesh and finer down to 10-20m . Any usual moulding technique may be used and the firing is effected at 1250-1420 DEG C., preferably 1300-1400 DEG C., although temperature above 1420 DEG C. may be used in the later stages. The firing atmosphere may be nitrogen, commercial nitrogen, annealing hydrogen (93 per cent. N2: 7 per cent. H2), or ammonia. The article is exposed to nitrogen on all sides during firing. Briquettes of silicon may be nitrated by the process and the resulting silicon nitride mechanically shaped. The articles produced are refractory, have a low coefficient of expansion, a madulus of rupture 9,000-20,000 lbs. per sq. in. and are resistant to acids and aqueous alkalis, and many applications of these articles are exemplified. In examples: (1) lens fusion blocks are made by mixing commercial silicon, 200 mesh and finer, with 1 1/2 per cent. dextrin as temporary binder, moistening with water, moulding under 1,400 lbs. per sq. in., drying and heating in a muffle furnace in nitrogen with the temperature gradually increasing to 1,400 DEG C., maintaining at 1,400 DEG C. for 7 hrs. and cooling in nitrogen; (2) crucibles, combustion boats, and the like are made by mixing silicon with 2 per cent. bentonite, and sufficient water for slip casting, pouring the slip into a plastic mould, drying the moulded body, and firing as in (1).