Microwave-Based Chemical Synthesis of Co-Alloyed Nd-Fe-B Hard Magnetic Powders

Modern high-energy-product permanent magnets derive their superior properties from nanoscale interactions, which are a sensitive function of the crystal size. Further property improvements require novel synthesis techniques to control the crystal size. Hence, we report a cost-effective and facile technique, i.e., microwave-based synthesis, to produce Co-alloyed Nd-Fe-B-based magnetic nanostructured powder. Dedicated microwave synthesis equipment with an optical pyrometer attachment was used to synthesize mixed metal oxides, followed by reduction diffusion to produce Nd-Fe-Co-B magnetic nanoparticles. Nanosized mixed oxide powders were formed in a single-step combustion after ignition, which was completed within a few seconds. In the reduction diffusion process, a two-step reaction was observed. Co and Fe oxides were first reduced to Fe and Co as the temperature reached 350°C. Nd oxide was partially reduced to NdH₂ at 350 .C and finally to Nd when the temperature reached 740°C, when the desired Nd₂(Fe,Co)₁₄B phase was also obtained. This reaction sequence matched thermodynamic calculations. Microwave power played a crucial role in controlling combustion temperature, crystal size, and coercivity of the Nd₂(Fe,Co)₁₄B nanoparticles. The coercivity of these nanoparticles increased from 4.6 kOe to 8 kOe as the mean crystal size increased from 20 to 60 nm. The saturation magnetization was 106 emu/g.