Combustion synthesis of boron nitride via magnesium reduction using additives

Abstract This study is aimed at enhancing the product yield in combustion synthesis of h-BN using Mg reduction of B 2 O 3 as the boron source under a low N 2 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 N 2 can penetrate easily into it and the N 2 generated by NaN 3 or C 3 H 6 N 6 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. NH 4 X (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 BX x , which then reacts with N 2 under the reduction of H 2 . A product yield of 67% was achieved by simultaneous addition of NH 4 Cl and BN under a N 2 pressure of 1.6 MPa.