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

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 H_cj 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 Nd₂Fe₁₄BH_x and NdH_x, respectively. The formation of NdH_x 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.