Characterisation of high temperature oxidation of NdFeB magnets

The long term high temperature oxidation properties of a Nd 16.4Fe 75.7B 7.9 commercial sintered magnet were investigated in pure oxygen atmosphere up to 500°C and in air between 350 and 600°C. In pure oxygen atmosphere, three exothermic reactions occur, corresponding to the oxidation of the Nd-rich...

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Bibliographic Details
Published in:Journal of magnetism and magnetic materials 1997-09, Vol.173 (1), p.29-42
Main Authors: Edgley, D.S., Le Breton, J.M., Steyaert, S., Ahmed, F.M., Harris, I.R., Teillet, J.
Format: Article
Language:English
Subjects:
Kinetic parameters
Mössbauer effect
Nd 2Fe 14B
NdFeB magnets
Oxidation
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Summary:The long term high temperature oxidation properties of a Nd 16.4Fe 75.7B 7.9 commercial sintered magnet were investigated in pure oxygen atmosphere up to 500°C and in air between 350 and 600°C. In pure oxygen atmosphere, three exothermic reactions occur, corresponding to the oxidation of the Nd-rich intergranular regions, the Nd 2Fe 14B matrix phase, and the α-Fe phase that forms during the oxidation of Nd 2Fe 14B. In air, the oxidation of Nd 2Fe 14B was investigated further. Instead of the oxidation proceeding along the grain boundaries, the Nd 2Fe 14B matrix dissociates to form an adherent grey surface layer which grows transgranularly into the magnet. This is probably due to a reaction occurring in the Nd-rich regions which prevents fast path oxygen diffusion along the grain boundaries. The main reaction is the dissociation of the Nd 2Fe 14B matrix into α-Fe nanocrystals which contain small precipitates of oxides of Nd. The products of this reaction form the adherent grey layer which grows transgranularly into the magnet. The activation energy and the diffusivity pre-exponential factor for this reaction were found to be 114 kJ mol- −1 and 0.7 mm 2 s −1, respectively. After further oxidation of the dissociated grey layer, α-Fe is oxidised to form αFe 2O 3 and finally, from about 600°C, some of the α-Fe 2O 3 reacts with the small precipitates of oxides of Nd to form FeNdO 3.
ISSN:0304-8853
DOI:10.1016/S0304-8853(97)00189-3