The use of hydrogen in the production and characterization of NdFeB magnets

Abstract

A combination of hydrogen decrepitation (HD) and jet milling (JM) has been used to produce powder for the processing of permanent magnets. The procedure has proved to be very successful for both NdFeB (Neomax) alloys and the NdDyFeNbB high coercivity alloys. The magnets produced by the HD/JM process showed excellent coercivities when sintered between 980 and 1040 °C. At higher temperatures, excessive grain growth reduced the coercivity significantly. The hydrogen absorption/desorption (HAD) behaviors of alloys based on Nd16Fe76B8 and the stoichiometric phase Nd2Fe14B have been examined in an attempt to relate these effects to the microstructures of these alloys. The HAD processes were studied by microbalance, differential thermal analysis (DTA), differential scanning calorimetry (DSC), and mass spectroscopy, and these techniques showed that the absorption process in the Nd16Fe76B8 alloy are multistage and occur readily at room temperature. The Nd2Fe14B alloy, on the other hand, could not be activated at room temperature and hydrogen absorption could only be achieved at elevated temperatures. The mass spectroscopy/vacuum degassing studies showed a two-stage desorption process whereby hydrogen was first desorbed from the matrix (Nd2Fe14B) phase and then from the phase(s) at the grain boundaries.