Low Temperature Synthesis of Phase Pure MoAlB Powder in Molten NaCl

Abstract

MoAlB fine powders were prepared in molten NaCl from Al, B and Mo powders. The effects of key parameters affecting the synthesis process and phase morphology were examined and the underpinning mechanisms proposed. MoAlB product particles exhibited different shapes/sizes, as follows: spherical grains (1~3 μm), plate-like particles (<5 μm in diameter) and columnar crystals with lengths up to 20 μm and diameters up to 5 μm, resultant from different reaction processes. Phase pure MoAlB was synthesised under the following optimal conditions: use of 140% excess Al and 6 h of firing at 1000 °C. This temperature was at least 100 °C lower than required by other methods/techniques previously reported. At the synthesis condition, Mo first reacted with Al and B, forming Al8Mo3 and MoB, respectively, which further reacted with excess Al to form Al-rich Al–Mo phases and MoAlB. The Al-rich Al–Mo phases further reacted with the residual B, forming additional MoAlB. The molten NaCl played an important role in accelerating the overall synthesis process.