Design of high temperature permanent magnets

There has been a demand for permanent magnet materials which can be used at up to 450–500 °C. Normal Sm2TM17 magnets have a coercivity Hci of ∼10 kOe at room temperature. At 150 °C, the coercivity is decreased by about 50%. The maximum operating temperature (MOT) of these magnets is 150–200 °C. By increasing the Hci to 30 kOe, the MOT is increased to about 300 °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%/°C and a Hci of 18.6 kOe starts an irreversible loss at 550 °C, while a 2:17 magnet having a β of −0.29%/°C and a Hci of 28.5 kOe starts an irreversible loss at 350 °C. Therefore, high temperature permanent magnets (MOT>500 °C) are possible when the β is lower than −0.15%/°C with moderately high coercivity (∼20 kOe).