This work has examined the influence of grain size in sintered Sm2(Co,Fe,Cu,Zr)17-type permanent magnets on the growth of an internal oxidation zone (IOZ). Two magnets of similar composition but with average grain sizes of 46 μm and 170 μm respectively, were exposed to temperatures of 500 °C in air for times up to 1000 h. Overall, the smaller grained magnet showed a reduced growth rate of the IOZ (∼39.01 μm2h−1) compared to the larger grained magnet (∼48.95 μm2h−1) and a smaller irredeemable loss in Br (3.11%) after 1000 h. However, growth rate of the IOZ is shown to depend on the growth direction, in respect to the crystallographic c-axis of the matrix phase. The smaller grain size was shown to be most effective at retarding the growth of IOZ under surfaces parallel to the c-axis. It is proposed therefore, that a smaller grained sintered magnet may be the microstructure of choice for use in demanding, high temperature applications, where a protective coating is not always a possibility, such as a high-performance electrical machine with a surface mount PM rotor. Orientation of the exposed face must also be considered to minimise the resultant magnetic induction loss caused by oxidation.
- samarium cobalt
- Permanent magnets