Mechanochemical synthesis of high coercivity Nd₂(Fe,Co)₁₄B magnetic particles

With increasing demand for magnets in energy conversion systems, the quest for the development and understanding of novel processing routes to produce permanent magnets has become urgent. We report a novel mechanochemical process for the synthesis of Nd₂(Fe,Co)₁₄B magnetic particles with a high coercivity of 12.4 kOe. This process involves the reduction of neodymium oxide, iron oxide, cobalt oxide and boron anhydride in the presence of a calcium reducing agent and a CaO diluent. The formation mechanism of Nd₂(Fe,Co)₁₄B changed with increasing CaO content, and the average crystal size of the Nd₂(Fe,Co)₁₄B particles also increased, resulting in an increase in the coercivity values. The reaction mechanism during milling was revealed through a study of the phase transformations 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.