2, Nanyang Technological University, Singapore, , Singapore
With increasing demand for high performance permanent magnets in energy generation and conversion systems, the quest to develop novel processing routes to produce such materials has become urgent. Nd-Fe-B based permanent magnets, due to their superior magnetic properties, have attracted intensive attention. In this alloy, cobalt substitution of iron can increase the Curie temperature. However, conventional physical processing methods are associated with high cost, and chemical synthesis methods have limited scalability. Hence, we report a cost-effective and scalable mechanochemical process for synthesis of high coercivity Nd2(Fe,Co)14B nanoparticles. Nd2(Fe,Co)14B nanoparticles with a size of 40 – 150 nm and coercivity up to 8.8 kOe was obtained by milling Nd2O3, Fe2O3, CoO and B2O3 in the presence of Ca and CaO diluent, followed by annealing and removal of the by-product. The formation mechanism was investigated for the first time to understand the process and to facilitate process parameter optimization. The formation mechanism changed with increasing CaO diluent content, the average crystal size of the Nd2(Fe,Co)14B nanoparticles also increased, resulted in an enhancement in coercivity values. The reduction kinetics of the mechanochemcial process were also studied by determining the change in the concentration of reactants and products as a function of milling time. It was found that unlike self-propagating reactions, this reduction reaction during milling requires continuous input of mechanical energy to reach a steady state. The impact energy of the milling event activates the chemical reactions and was found to play a role analogous to that of thermal energy in the reduction-diffusion process. Our experimental data was found to fit well with a model of the kinetics of the mechanochemical process.