A study of microstructure-coercivity relationship in sintered Nd-Fe-B magnets

Abstract

A sintered Nd–Fe–B magnet has recently become a hot issue in applications to HEVs and EVs for driving motors. These driving motors require high magnetic properties in order to minimize thermal magnetic degradation. It is necessary to obtain high coercivity in order to overcome the thermal magnetic degradation of sintered Nd–Fe–B magnets for driving motors (HEVs and EVs). Generally, the Nd–Fe–B sintered magnets are fabricated via liquid-phase sintering of an Nd-rich phase in the temperature range of 1,000–1,100°C to achieve full densification. These conditions were difficult to suppress propagation and nucleation of reverse domains because of demagnetizing parameter easily occurred. The demagnetizing parameters are determined mainly by the sizes and shapes of grains, the grain boundary, and defects in the areas of domain nucleation. Thus, the changes in microstructure (interfacial control and grain refinement) that are induced during the sintering process can greatly affect the nucleation-type coercivity mechanism and magnetic properties of a Nd-Fe-B sintered magnet. This study therefore looks at how the nucleation-type coercivity mechanism is influenced by changes to (1) grain boundaries (interfacial control) and (2) grains (grain refinement) during sintering with a view to developing new technologies to increase the coercivity of Nd-Fe-B sintered magnets. (1) The interfacial control study was demonstrated by inducing a homogeneous distribution of Nd-rich phases during sintering. This study employed a “cyclic sintering” to achieve higher coercivity in Nd–Fe–B sintered magnets. The increase in the coercivity and thermal stability of the Nd-Fe-B sintered magnet will be discussed from a microstructural point of view. (2) The grain refinement study employed a “modified refractory-metaladdition method” to achieve higher coercivity in Nd–Fe–B sintered magnets. To control the microstructure of the secondary phase effectively, a metal organic compound of the refractory metal (Mo) was coated on the surfaces of the particles of an Nd–Fe–B powder. This study discusses the resulting changes in the magnetic properties of the Nd–Fe–B sintered magnets from a microstructural perspective.