Effect of Cathodic Hydrogen Evolution on the Coercivity and Thermal Stability of Sintered NdFeB Magnets

Abstract

The effect of cathodic hydrogen evolution on coercivity and thermal stability was investigated in sintered NdFeB magnets. The magnetic properties, phase structure and morphology were studied by SQUID-VSM, DSC, XRD, TEM and SEM. After cathodic hydrogen evolution, the Hcj decreases from 1034.8 kA•m-1 to 963.16 kA•m-1. The temperature coefficient α declines from-0.253%•ºC-1 to-0.3229%•ºC-1 , and the β declines from-0.7518%•ºC-1 to-0.7738%•ºC-1. A mechanism was proposed to explain the results. In the process of cathodic hydrogen evolution, some of the generated hydrogen atoms react with the Nd2Fe14B phase and Nd-rich phase, forming Nd2Fe14BHx and NdHx, respectively. The formation of NdHx causes a volume expansion, which results in intergranular cracks and stress. X-ray diffraction and morphology characterization confirms the presence of these defects. These defects would significantly promote the nucleation of reverse magnetic domains, and further decline the coercivity and thermal stability of magnets.