Magnetic Properties of Nd-Fe-Co-V-B System Bulk Exchange-spring Magnets Formed by Spark Plasma Sintering Method

摘要

Bulk exchange-spring magnets of Nd-Fe-Co-V-B alloys were prepared by spark plasma sintering (SPS) method using rapidly quenched ribbon powders. The effects of SPS process conditions on the magnetic properties and structures were investigated. The SPS method is a type of solid compression sintering similar to hot-pressing (HP) and can be performed at lower temperatures with a shorter holding time. In SPS method, the sintering promotion factor is self heat generation by electric discharges between particles at an early stage of on-off DC pulse application. Bulk exchange-spring magnets reached approximately full density at a temperature higher than 650 deg C for 5 min and had magnetically isotropic properties regardless of the direction in which pressure was applied during sintering. They indicated the spring back phenomenon peculiar to exchange-spring magnets and consisted of multiphase of the Nd_9Fe_14B type and the alphe-Fe type. Grain growth did not occur between the starting ribbon powders, but fine crystals were observed inner them with an increase in the sintering temperature. Optimum preparation conditions and magnetic properties were as follows: composition, Nd_9Fe_76Co_8V_1B_6; surface velocity of the roller during melt spinning for starting ribbons, V_s=20 m/s; SPS conditions, at 675 deg C for 1 min under 980 MPa; magnetic and mechanical properties; Br=1.04 T (10.4 kG), H_cJ=502 kA/m (6.27 kOe), (BH)_max=126 kJ/m~3 (15.7 MGOe), H_k/H_cJ x 100 = 30 percent, spring back ratio=40 percent and irreversible flux loss = -4 percent after 100 deg C for 1 h (Pc=2), D=7.70 Mg/m~3 and compressive strength=1.1 GPA.