COMPARISON OF THE IMPROVEMENT OF THERMAL-STABILITY OF NDFEB SINTERED MAGNETS - INTRINSIC AND OR MICROSTRUCTURAL

Temperature-dependent magnetic properties of nine NdFeB sintered magne ts with various Co-Nb, Co-V, or Co-Mo additions have been measured up to 175-degrees-C using both closed-loop and open-circuit methods. The irreversible loss of induction, reversible temperature coefficient of induction (alpha), and temperature coefficient of intrinsic coercivity beta have been related to the T(c), the B(r), and the H(ci) at 25 and 175-degrees-C. The irreversible loss of induction is strongly affecte d by the H(ci) at 25-degrees-C and the a has been found to be strongly dependent upon the T(c). Intrinsic coercivity of more than 25 and 7.5 kOe at 25 and 175-degrees-C, respectively, are essential to bring the beta to better than -0.5%/-degrees-C. A magnet with a composition of Nd12Dy3Fe70Co5Nb2B8 has been found to exhibit an outstanding thermal s tability: a B(r) of 8.5 kG (comparable to that of SmCo5 sintered magne t) and a H(ci) of 12.9 kOe when measured at 175-degrees-C. An alpha of -0.10%/-degrees-C and a beta of -0.4%/-degrees-C have been obtained.