Design of high temperature permanent magnets

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