Rare earth permanent magnets for high temperature applications

We have studied extensively the influence of Cu and Fe content and the transition metal to rare earth ratio z on the magnetic properties and microstructure of Sm(CobalCuxFeyZr0.033)z magnets with x ranging from 0.048 to 0.128, y from 0 to 0.244, and z from 6.5 to 9.1. High Cu, low Fe and low z were found to contribute to high coercivity at high temperatures. Based on the knowledge we obtained from our studies, we designed a new Sm-Co magnet with a coercivity of 10 kOe at 773 K. This newly developed magnet has a temperature coefficient of coercivity β(RT-773 K) of -0.03%/ C, which is about seven times smaller than the commercial 2:17 magnets. TEM studies showed that both a low ratio z and high Cu content lead to small cell size, which we believe is responsible for the high coercivity at high temperatures. The density of lamella phase does not change significantly with the ratio z. This leads us to believe that the density of lamella phase does not have a direct influence on the intrinsic coercivity. More Cu is needed in the low ratio z magnets to get a high coercivity, and this may be due to smaller cell size and thus large percentage of Sm(Co1-xCux)5 cell boundary phase. In the high Cu content magnets the coercivity is saturated. (orig.)