Combustion synthesis of boron nitride via magnesium reduction using additives

This study is aimed at enhancing the product yield in combustion synthesis of h-BN using Mg reduction of B2O3 as the boron source under a low N2 pressure by using additives. The reactant powders and the additives were placed in perforated aluminum containers without pressing. Due to a loose and highly porous structure of the powder stack, the surrounding N2 can penetrate easily into it and the N2 generated by NaN3 or C3H6N6 escapes easily, addition of these two additives only increases slightly the product yield in the low content region but decreases the product yield in the high content region due to decreasing temperature. Addition of inert particles (i.e., MgO or BN) increases the product yield only when the temperature is higher than the melting point of boron, under which coalescence of molten boron is suppressed due to capillary spreading of the molten boron on the particles. When the temperature is lower than the melting point of boron, addition of the inert particles decreases the product yield because of their cooling effect. NH4X (X=Cl or Br) was found the most effective in enhancing the product yield because it creates an easier route for the nitridation of boron by first converting boron to BXx, which then reacts with N2 under the reduction of H2. A product yield of 67% was achieved by simultaneous addition of NH4Cl and BN under a N2 pressure of 1.6 MPa.