A new method of preparing NdB₆ ultra-fine powders

Combustion synthesis method was used to prepare NdB₆ ultra-fine powders with B₂O₃, Nd₂O₃ and Mg powders as the raw materials. The basic thermodynamic data of NdB₆ were estimated. The standard formation heat of NdB₆ is −357.48 kJ·mol⁻¹. The values of the heat capacity and the standard entropy are 96.87 and 86.60 J·K⁻¹·mol⁻¹, respectively. The adiabatic temperature of the reaction is 2726 K, which is higher than the thermodynamic criterion of 1800 K. This indicates that the combustion synthesis reaction of the B₂O₃–Nd₂O₃–Mg system could spontaneously take place by itself to generate NdB₆. The NdB₆ powders were characterized by X-ray diffractometer (XRD), scanning electron microscopy (SEM) and differential scanning calorimetry–thermogravimetry (DSC–TG). The results indicate that the combustion products consist of NdB₆, MgO, and a few Mg₃B₂O₆ and Nd₂B₂O₆. The leached products consist of single NdB₆ phase, and its purity is 98.6 wt%. When the sample preparation pressure is 20 MPa, the average particle size of NdB₆ powders is less than 500 nm. The antioxidant ability of NdB₆ is very strong, which is oxidized step by step. The apparent activation energies of the oxidation reactions are 986.14 and 313.83 kJ·mol⁻¹, respectively. In addition, the reaction orders are 4.10 and 3.75, respectively.