Milan Hubáček

A contribution to the crystallochemistry of boron nitride

Abstract A reaction mechanism of formation and growth of hexagonal boron nitride planar macromoleukules during reductive nitridation of oxidic boron compounds has been proposed. This model was verified by a comparison of computed values of oxygen contents with experimental results and a good agreement has been found. The growth of boron nitride crystallites is suggested to be of a condensation character and the termination of it is seen as a reaction of hydroxyllic and hydrogen radicals in periphery areas of the macromolecules. By this approach an occurrence of a certain amount of oxygen in samples of hexagonal boron nitride treated even at high temperatures can be elucidated.

The effect of copper on the crystallization of hexagonal boron nitride

The crystallization process of hexagonal boron nitride in the presence of copper has been investigated. The positive effect of copper on the crystallinity of boron nitride was observed in the three studied systems of: nitrided boron, nitrided boron–carbon, and previously prepared turbostratic boron nitride. However, the presence of copper hindered the formation of boron carbonitride and produced graphite and boron nitride phases separately. Poor crystallinity was found as a conditio sine qua non for the existence of such a compound. Well-crystallized boron nitride had a very low spacing parameter 0.3328 nm and a regular hexagonal shape.

COPPER-BORON NITRIDE INTERACTION IN HOT-PRESSED CERAMICS

Ceramics were prepared by hot pressing of chemical mixtures of turbostratic boron nitride with copper. A positive effect of copper on boron nitride crystallization and densification has been found. Preferred orientation of boron nitride grains has been revealed in such ceramics, being opposite to general experience: the c-axis of boron nitride crystallites was arranged preferably within a plane perpendicular to the direction of uniaxial pressure. Such an arrangement is interpreted as a result of the uniaxial compression of the boron nitride grains extensively growing within the c crystallographic axis. {copyright} {ital 1997 Materials Research Society.}

Kinetics of deoxidizing of hexagonal boron nitride preparations

Abstract Kinetic equations were established to describe the removal of oxygen-containing phases from hexagonal boron nitride. The diffusion characteristics of the deoxidizing process are discussed. The refining of turbostratic boron nitride powder by reductive chlorination is selective for boron oxide. Fusion extraction by carbon involves removal of oxinitridically bound oxygen at a rate proportional to the temperature of extraction. Diffusion through the ceramic skeleton is the controlling process of boron oxide evaporation.