Grain Boundary Diffusion

Commonly up to 8 wt.% of heavy rare earths (HRE such as Dy and Tb) are required to compensate the deterioration in coercivity of Nd-Fe-B magnets at elevated operation temperatures. Because of the high price volatility and the forecasted long-term criticality of these elements this amount is a considerable cost and supply risk in the production process of permanent magnets. Therefore, the reduction of HRE without losing or even with improving the performance is one main subject of permanent magnet research. A very promising approach to achieve this goal is the grain boundary diffusion process (GBDP) which we investigate in our work group.

During the process HRE are deposited on the surface of the magnet, which is subsequently subjected to a heat treatment at moderate temperatures for several hours. This way the HRE diffuses into the magnet over the grain boundaries (see schematic). The attainable increase in coercivity can be as high as 35% without a significant loss of remanence, as shown in the diagram. With the GBDP the amount of needed HRE can be reduced by an order of magnitude [1].