Process for preparation of dense molded bodies of polycrystalline aluminum nitride without use of sintering aids

A process for producing an aluminum nitride molded body wherein the aluminum nitride is present in the form of an essentially single-phase, homogeneous, isotropic microstructure with a grain size of 5 .mu.m maximum, the residual oxygen and the residual carbon being present in the form of a solid solution in the AlN lattice and not detectable as separate phase(s) up to a 2400-times enlargement, having the following properties:

• bending strength (measured according to the 4-point method) at room temperature and up to about 1400.degree. C. of at least 500 N/mm.sup.2, predominantly transcrystalline rupture mode and a heat conductivity at 300 K of at least 150 W/mK. which comprises:
• forming a powder mixture of aluminum nitride and a carboncontaining material in the form of finely divided carbon per se or an organic material carbonizable at temperatures of up to 1,000.degree. C. with formation of carbon;
• compacting the powder mixture of aluminum nitride and the carbon containing material to form a green body;
• heating the green body to a temperature of from 1600.degree. to 1800.degree. C. in a nitrogen atmosphere to form a porous deoxidized green body having a maximum density of 70% TD and a chemical composition of at least 99% by weight aluminum nitride, up to 0.35% by weight residual oxygen, up to 0.35% by weight residual carbon, and up to 0.30% by weight total of metallic impurities (Fe, Si, Ca, Mg);
• encasing said porous, deoxidized green body, under vacuum, in a gas-tight casing;
• isostatically hot pressing the encased green body at a temperature of from about 1700.degree. to 2100.degree. C. while slowly raising the pressure to 100 to 400 MPa by an inert gas used as a pressure transmitting agent to form a molded body having a density of at least 98.8% TD and the same chemical composition as the deoxidized green body;
• removing the molded body from the high pressure autoclave after cooling;
• and stripping the casing from the molded body.