Patent GB-726812-A (GB)
Title: Improvements in or relating to silicon nitride materials
IPC: C04B35/591
Filed: 1952-11-11
Abstract: A moulded article of silicon nitride is produced by firing powdered silicon, usually at a temperature between 1,250 and 1,420 DEG  C., in a nitriding atmosphere in the presence of a catalyst consisting of fluorine or a fluorine compound, so as to produce a product containing at least 0.5 per cent. and preferably 0.5 to 5 per cent. of a fluorine compound based on the weight of silicon nitride.  The catalyst is usually incorporated within the mix. Examples of such catalysts are barium fluoride, calcium fluoride, sodium ferric fluoride, potassium ferric fluoride, and cryolite. It may however also be introduced into the nitriding atmosphere if it is suitably volatile.  According to Example I, lens fusion blocks of silicon nitride are produced by firing a mixture of silicon powder, calcium fluoride and dextrin in nitrogen at a temperature of 1,400 DEG C. Specification 15641/09, [Class I], is referred to.ALSO:A material for use as a filter is produced by firing powdered silica, or a mixture of powdered silicon and a granular refractory material, usually at a temperature between 1,250 and 1420 DEG C., in a nitriding atmosphere in the presence of a catalyst consisting of fluorine or a fluorine compound so as to produce a moulded product containing at least 0.5 per cent., and preferably 0.5 to 5 per cent., of a fluorine compound based on the weight of silicon nitride.  The product may be crushed and used in the form of granules or, when a pore-forming material, for example naphthalene or a phenolic resin is present in the mix, it may be used without crushing.  Refractory materials specified are silicon carbide, alumina and mullite.  The catalyst is usually incorporated within the mix.  Examples of such catalysts are barium fluoride, calcium fluoride, sodium ferric fluoride, potassium ferric fluoride and cryolite.  It may also, however, be introduced into the nitriding atmosphere if it is suitably volatile.  Specification 15641/09, [Class I], is referred to.ALSO:A moulded refractory article is produced by firing powdered silicon, or a mixture of powdered silicon and a granular refractory material, usually at a temperature between 1,250 DEG  and 1,420 DEG  C., in a nitriding atmosphere in the presence of a catalyst consisting of fluorine or a fluorine compound so as to produce a product containing at least 0.5 per cent., and preferably 0.5 to 5 per cent., of a fluorine compound based on the weight of silicon nitride.  Refractory materials specified are silicon carbide, alumina and mullite.  The catalyst is usually incorporated within the mix.  Examples of such catalysts are barium fluoride, calcium fluoride, sodium ferric fluoride, potassium ferric fluoride, and cryolite.  It may, however, also be introduced into the nitriding atmosphere if it is suitably volatile.  A number of applications of the product of the invention in which the heat and chemical resistance, hardness and dielectric properties are utilized, is specified.  Porous materials for use as filters, catalysts and insulation material can be made by introducing pore-forming materials, for example, naphthalene or a phenolic resin, into the mix.  The product of the invention may be crushed to produce granules which themselves can be used as insulation material or filtering materially, as a catalyst or as catalyst carrier material.  Such granular material may be bonded by means of sintered metals, vitreous or ceramic bonds to reproduce a moulded article.  Specification 15641/09, [Class I], is referred to.