Kinetic study of the mechanochemical synthesis of Nd₂(Fe,Co)₁₄B hard magnetic nanoparticles

Mechanochemical processing is an environmentally friendly, low temperature and low cost technique to synthesize hard magnetic nanoparticles. However, it is challenging to control, monitor and optimize this process. Hence, a kinetic study of mechanochemically processed Nd₂(Fe,Co)₁₄B hard magnetic nanoparticles was carried out. Nd₂(Fe,Co)₁₄B hard magnetic nanoparticles were synthesized through mechanochemical milling of Nd₂O₃, Fe₂O₃, CoO and B₂O₃ in the presence of Ca. This process was monitored by measuring CaO content at different milling times. The process was found to consist of 3 stages: (i) fast amorphization of precursor oxides, (ii) reduction of precursor oxides, leading to the formation of the desired phase and (iii) steady-state. The kinetics of this process were modeled and compared to experiment. It was found that the maximum milling efficiency was achieved at a ball milling speed of 550 rpm. The synthesized Nd₂(Fe,Co)₁₄B particles exhibited coercivity values of up to 9.4 kOe; the coercivity increased with increasing crystallite size and lattice parameter.