A new class of Sm-TM magnets for operating temperatures up to 550°C

Abstract

A breakthrough has been made for magnets to be used at high temperatures. A new class of Sm(Co/sub w/Fe/sub v/Cu/sub x/Zr/sub y/)/sub z/ type magnets with linear demagnetization curves up to 550/spl deg/C has been developed. A new symbol, T/sub M/, is introduced, which is defined as the maximum temperature at which the induction demagnetization curve of a magnet is a straight line. A magnet in this class can be made with its own unique T/sub M/. There is a direct relationship between the optimum Co content and T/sub M/. An equation has been established to relate the Co content and the T/sub M/. Using the equation, magnets can be provided with the best combination of highest (BH)/sub max/ and a linear demagnetization curve for any application with a specified operating temperature. These magnets have high resistance to thermal demagnetization because of their low temperature coefficients of /sub i/H/sub c/ which result in a higher /sub i/H/sub c/ at high temperature. Thermal stability of these magnets at 300-550/spl deg/C has been studied, showing that the loss due to metallurgical changes is <2% for new magnets at 550/spl deg/C for 360 hr. Experiment confirms that coated magnets can be expected to have considerable life expectancy and relatively low magnetic losses at the intended T/sub M/. Experiment also shows that, at high temperatures, the magnetic pinning strength in the magnets increases as the T/sub M/ increases. TEM microstructures for some magnets have been studied. Magnets with higher T/sub M/ have smaller cell sizes and larger volume of cell boundaries than conventional 2:17 magnets. It is believed that the large volume of cell boundaries plays a role in the high resistance to thermal demagnetization.