Design of high temperature permanent magnets

There has been a demand for permanent magnet materials which can be used at up to 450-500 degrees C. Normal Sm2TM17 magnets have a coercivity H-ci of similar to 10 kOe at room temperature, At 150 degrees C, the coercivity is decreased by about 50%, The maximum operating temperature (MOT) of these magnets is 150-200 degrees C. By increasing the H-ci to 30 kOe, the MOT is increased to about 300 degrees C, Further increases in MOT are anticipated with further increases in coercivity. However, the degree of increase in a MOT will be small as long as the same temperature coefficient of coercivity (beta) is maintained, resulting in MOT that is far below the required operating temperature. Therefore we studied the effect of beta when they have the same H-ci at room temperature. This study indicates that the MOT is more effectively increased by reducing the beta than increasing the H-ci. For example, a 1:7 magnet having a beta of -0.13%/degrees C and a H-ci of 18.6 kOe starts an irreversible loss at 550 degrees C, while a 2:17 magnet having a beta of -0.29%/degrees C and a H-ci of 28.5 kOe starts an irreversible loss at 350 degrees C, Therefore, high temperature permanent magnets (MOT>500 degrees C) are possible when the beta is lower than -0.15%/degrees C with moderately high coercivity (similar to 20 kOe). (C) 1997 American Institute of Physics.