PURPOSE:To obtain a composite calcined compact, composed of hexagonal boron nitride grains with a specific granular shape and aluminum nitride grains, having higher thermal conductivity than that of a conventional boron nitride-aluminum nitride- based compact and excellent in simplified cutting workability, etc. CONSTITUTION:The above-mentioned calcined compact is a boron nitride-aluminum nitride-based composite calcined compact composed of 10-95wt.% hexagonal boron nitride grains in which most thereof have a polygonal or spherical shape and 90-5wt.% aluminum nitride grains. The aforementioned calcined compact is obtained by the following method. That is mixed powder composed of 10-95wt.% hexagonal boron nitride powder having =1mum average grain diameter, 90-5wt.% aluminum nitride powder and 0.1-10wt.% calcining assistant is isostatically formed. The resultant compact is then calcined at 1650-2000 deg.C in a nitrogen-containing nonoxidizing atmosphere or vacuum and subsequently cooled at =20 deg.C/min rate. Ca or a compound thereof, Y and a compound thereof, Al and a compound thereof, B and a compound thereof, P and a compound thereof, Si and a compound thereof, etc., are used as the calcining assistant.
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