Abstract: A process for the liquid phase sintering of silicon carbide, comprising forming a shaped, consolidated powder body which, not allowing for binder, comprises a powder mixture containing at least 75 wt % silicon carbide and from 1 to 25 wt % (calculated as Al.sub.2 O.sub.3) of a powder comprising a source of aluminum selected from alumina, precursors for alumina and mixtures thereof. The body is heated in a non-oxidising atmosphere to a sintering temperature of from 1500.degree. C. to 2300.degree. C. to form a liquid phase and a resultant liquid phase sintered body. In said heating step, the body is heated in the presence of a source of magnesium which is distinct from the source of aluminum and comprises at least one of magnesia, precursors for magnesia, magnesium vapour and combinations thereof, whereby said liquid phase produces secondary oxide constituent.

Abstract: A method of forming a dense ceramic product comprised of silicon nitride and silicon oxynitride or aluminum substituted derivative of either in which silica is subjected to a controlled partial carbothermal reduction process in the presence of nitrogen to produce residual silica and silicon nitride, fabricating the resulting material into the desired shape and heating in a nitrogen atmopshere at a temperature of from 1400.degree. C. to 2000.degree. C. with or without pressure to form the product. The silica may conveniently be derived from pyrolysed rice hulls. The product may also comprise silicon carbide, which may be derived from the carbothermal reduction, and a glassy phase.

Abstract: A silicon aluminium oxynitride product having the crystal lattice structure of silicon oxynitride but of expanded cell dimensions is produced by sintering a powder mixture of 50 to 60 wt % of Si.sub.3 N.sub.4, 22 to 35 wt % SiO.sub.2, 1 to 20 wt % Al.sub.2 O.sub.3 and 1 to 15 wt % of at least one oxide of Y, Li, Mg, Ca, Sc, Ce or another rare earth element, the molar ratio of SiO.sub.2 to Si.sub.3 N.sub.4 being greater than 1:1 and the percentages being based on the total weight of the ingredients. Sintering is effected at 1600.degree. to 1900.degree. C. in a non-oxidizing atmosphere so that the Al.sub.2 O.sub.3, said at least one oxide and part of the SiO.sub.2 in the mixture react to form a liquid phase from which silicon aluminium oxynitride having an expanded silicon oxynitride crystal lattice is crystallized. The sintered material is cooled to form a product containing said silicon aluminium oxynitride and at least one grain boundary phase containing silicon and the metal(s) of said at least one oxide.