Nd-Fe-B Permanent Magnets: Hydrogen Absorption/Desorption Studies (HADS) on Nd₁₆Fe₇₆B₈ and Nd₂Fe₁₄B

Abstract

The hydrogen absorption/desorption (HAD) behaviours of alloys based on Nd15Fe77B8 (Neomax composition) 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) and mass spectroscopy and these showed that the absorption process in the Nd16Fe76B8 alloy is multi-stage and occurs 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. This confirmed the crucial role of the grain boundary phase(s) in the activation process of the Nd16Fe76B8 alloy. 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 grain boundaries. The mass spectroscopy peaks indicated that almost equal amounts of hydrogen were desorbed from the matrix and the grain boundary phases.