Low Temperature Synthesis of Phase Pure MoAlB Powder in Molten NaCl.

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.

Low-Temperature Molten Salt Synthesis and the Characterisation of Submicron-Sized Al8B4C7 Powder.

Submicron-sized (~200 nm) aluminium boron carbide (Al8B4C7) particles were synthesised from Al, B4C and carbon black raw materials in a molten NaCl-based salt at a relatively low temperature. The effects of the salt type/assembly and the firing temperature on the synthesis process were examined, and the relevant reaction mechanisms discussed. The molten salt played an important role in the Al8B4C7 formation process. By using a combined salt of 95%NaCl + 5%NaF, an effective liquid reaction medium was formed, greatly facilitating the Al8B4C7 formation. As a result, essentially phase-pure Al8B4C7 was obtained after 6 h of firing at 1250 °C. This temperature was 350-550 °C lower than that required by the conventional direct reaction and thermal reduction methods.

Pore Architectures and Mechanical Properties of Porous α-SiAlON Ceramics Fabricated via Unidirectional Freeze Casting Based on Camphene-Templating.

Porous α-SiAlON ceramics were fabricated using the camphene-based unidirectional freeze casting method, in which a gradient porous structure was formed as a result of the decreased solidification velocity in the freezing direction. Microstructure, porosity and pore size distribution of different parts of as-prepared samples were examined and compared, and correlated with their mechanical properties. For a given solid loading, the overall pore size and porosity of the top part were greater than those of the bottom part. Interestingly, despite its higher porosity, the flexural strength and fracture toughness of the top part were both higher than those of the bottom part, suggesting that apart from porosity, pore morphology and size affected mechanical properties of as-prepared porous α-SiAlON ceramics.