Silicon nitride can be prepared by various processes:
a) direct nitridation of silicon powder EQU 3 Si+2 N.sub.2 .fwdarw.Si.sub.3 N.sub.4 PA0 b) carbothermal reduction EQU 3 SiO.sub.2 +6 C+2 N.sub.2 .fwdarw.Si.sub.3 N.sub.4 +6 CO PA0 c) precipitation of silicon diimide and decomposition EQU SiCl.sub.4 +6 NH.sub.3 .fwdarw.Si(NH).sub.2 +4 NH.sub.4 Cl 3 Si(NH).sub.2 .fwdarw.Si.sub.3 N.sub.4 +2 NH.sub.3 PA0 d) gas phase reaction EQU 3 SiCl.sub.4 +4 NH.sub.3 .fwdarw.Si.sub.3 N.sub.4 +4 HCl PA0 a) gas containing water vapor or PA0 b) water-containing solvents PA0 a) an amorphous silicon nitride obtained by reaction of SiCl.sub.4 and NH.sub.3, followed by heating to about 1000.degree. C. (described in more detail in Patent Application DE-P-4,037,449.1), is used; PA0 b) the amorphous silicon nitride used is a partially hydrolyzed silicon nitride obtained by reaction of SiCl.sub.4 with moist NH.sub.3, followed by heating to about 1000.degree. C. (the reaction with anhydrous ammonia instead of moist ammonia is described in Patent Application DE-P-4,037,449.1); PA0 c) the amorphous silicon nitride is contacted with gas containing water vapor for 0.5 to 30 hours, in particular 1 to 8 hours; PA0 d) the water-containing solvent used is i-propanol containing 0.1 to 10% by volume of water; PA0 e) the gas containing water vapor used is untreated air.
These processes produce crystalline silicon nitride whose particle fineness, expressed by the specific surface area (BET), is usually in the range between 1 and 15 m.sup.2 /g and has a crystallite size of about 70 to 90 nm, determined from X-ray diffraction photographs.