Synthesis of Nd-Fe-B based magnetic materials through the mechanochemical process
Date of Issue2018-12-31
School of Materials Science and Engineering
Rolls Royce Corporate Lab
The limited geographical availability of rare earth elements has resulted in fluctuations in the price of rare earth permanent magnets. The high cost of Nd-Fe-B permanent magnets has limited the range of applications of these magnets. Therefore, a reduction in the use of rare earth elements or a reduction in the processing cost of Nd-Fe-B based magnets is urgently needed. However, most conventional processing methods of rare earth magnets are physical methods which require elemental rare earths as precursors, these elements are much more expensive than rare earth oxides. Developing chemical methods using low cost precursors is necessary to reduce the cost of Nd-Fe-B magnets. The large energy product of nanostructured Nd-Fe-B particles is also of interest. In mechanochemical processing, low cost metal oxides (e.g., Nd2O3, Dy2O3, Fe2O3, CoO and B2O3) are reduced by Ca granules during milling, in the presence of a CaO diluent. The desired tetragonal crystal structure of Nd-Fe-B nanoparticles is observed after annealing. It is a low cost and scalable process to produce high coercivity Nd-Fe-B magnetic nanoparticles. However, only a few studies of the mechanochemical synthesis of Nd-Fe-B magnets have been conducted. Detailed studies of the process have not been carried out. Hence in this work, the role of process parameters, reaction mechanisms and reaction kinetics of mechanochemical processing of Nd-Fe-B based magnets were investigated. The effect of process parameters in the mechanochemical process, e.g., milling time and diluent (CaO) content, were investigated during the synthesis of Nd2(Fe,Co)14B magnetic nanoparticles. The diluent content influenced the formation of Nd2(Fe,Co)14B, with the reduction process delayed for larger CaO content. For smaller CaO content, reduction of precursor oxides occurred during milling. For larger CaO content, the reduction reaction could only be triggered by subsequent annealing. With increasing CaO content, the crystallite size of the synthesized Nd2(Fe,Co)14B magnetic nanoparticles increased from 11 nm to 38 nm, accompanied by an enhancement in coercivity. The highest coercivity value of 8.8 kOe was obtained with 50 wt% of diluent. This is the highest coercivity observed for Nd2(Fe,Co)14B magnetic nanoparticles synthesized by chemical methods. To investigate the reaction mechanism during mechanochemical milling, a study of the phase transformations as a function of milling time for a range of milling speeds was performed. It was found that continuous input of mechanical energy was required for the reaction to reach equilibrium. Three stages were observed during mechanochemical milling: (1) fast amorphization of precursor metal oxides, (2) continuous reduction of metal oxides, (3) steady state. A model was proposed to predict the milling kinetics, there is good agreement between experimental data and the predictions. The milling efficiencies at different milling speeds were calculated by the model; the highest milling efficiency was observed at a milling speed of 550 rpm. Dy alloyed (Nd1-xDyx)2(Fe,Co)14B (x = 0 to 0.6) magnetic nanoparticles were also synthesized to further increase coercivity and thermal stability. The effects of Dy on the structure and magnetic properties of (Nd1-xDyx)2(Fe,Co)14B nanoparticles were investigated. Homogeneous Dy distribution was observed and high room temperature coercivity of 17.5 kOe was obtained at x = 0.5. Improved thermal stability was observed with increasing Dy substitution. The reduced spin reorientation temperature of these particles suggests that they are attractive for cryogenic applications. The coercivity mechanism of these particles was determined and it was shown that the nucleation of reversed domains was the controlling mechanism. These results suggest that mechanochemical synthesis of hard magnets is feasible and cost-effective. Process parameter optimization and modeling can be employed to maximize the properties of Nd-Fe-B based permanent magnets.