The kinetics of oxidation of Nd-Fe-B powders

The kinetics of oxidation of 2.5 mu m Nd-Fe-B powder are studied at temperatures 20, 80, 100 and 115 degrees C. The powdered alloy compromises 62.5 wt.% Fe, 35 wt.% Nd and 1.3 wt.% B. The percentage of weight gain is measured and compared with X-ray spectra recorded after various periods of oxidation. Two stages of oxidation, with powder-like and exponential weight gain reaching a plateau, are distinguished at all temperatures. The effect of changes in phase composition on ferromagnetic properties due to oxidation are examined by a magnetic balance. On the basis of reported measurements, it is concluded that two processes contribute to oxidation of Nd-Fe-B in air, i.e., relatively quick, total oxidation of intergranular Nd and oxidation of Nd/sub 2/Fe/sub 14/B ( Phi ) which stops at 12% level. The recognition of three products of the oxidation at X-ray spectra, Nd/sub 2/O/sub 3/, NdFeO/sub 3/ and Fe/sub 3/O/sub 4/, makes it possible to deduce a final phase composition. >