REDUCTION EXTRACTION METHOD FOR DETERMINING THE ENTHALPIES OF DECOMPOSITION REACTIONS OF DIFFERENT MODIFICATIONS OF BORON NITRIDE

Abstract

Pulsed (7–10 sec) reduction extraction in a gas carrier stream has been used for the first time to determine the standard enthalpies and the internal energies of thermal dissociation reactions of various modifications of boron nitride (BN) with and without carbon. The temperature dependences of the weight content of nitrogen extracted from nanosized (4–10 nm) and microsized (˂1; 4–5 µm) BN powders in the range 2100–4000 K with a step of ~200 ± 25 K have been established. Nitrogen extraction from layered BN modifications (graphene and graphite structures) beings at 2400‒2500 K and reaches 100% at 3500 K. The thermal decomposition curve for nano- and micropowders of dense BN modifications (wurtzite and sphalerite structures) is similar to that for layered forms and is shifted toward the high-temperature region by 900 K. The standard enthalpies of decomposition reactions of different BN modifications have been determined: 231 kJ/mole for graphene-like, 249 kJ/mole for graphite-like, 296 kJ/mole for wurtzite-like, and 391 kJ/mole for sphalerite-like.