Quantification of oxygen concentration on the purification of Nd metals by Solid-state electrotransport

Neodymium used for permanent magnets was purified through the Solid-state electrotransport (SSE) process that reduced the oxide concentration. Interstitial and transition-metal impurities such as Fe, Al, Ti, Cr, Ni, O, N in the Nd specimen were analyzed via inductively coupled plasma mass spectrometry and time-of-flight secondary ion mass spectrometry. The results revealed that the concentration of oxygen was significantly reduced (by up to 1000 ppm) during the purification. In addition, oxygen atoms in the Nd specimen migrated to each end of the specimen, toward the electrode (anode and cathode). The high temperature by high voltage test condition (> 1.2 V) was sufficient for a phase transformation from the hexagonal close-packed to the body-centeredcubic crystal structure. After the SSE process, the cross-sectional morphology and hardness were examined via field emission scanning electron microscopy and measured with a Vickers hardness tester, respectively. The concentration of oxygen atoms in the Nd specimen was significantly reduced (to 42 ppm), and an optimal condition for purification was determined.