EVIDENCE OF A QUANTITATIVE RELATIONSHIP BETWEEN THE DEGREE OF HYDROGEN INTERCALATION AND THE COERCIVITY OF THE 2 PERMANENT-MAGNET ALLOYS ND2FE14B AND ND2FE11CO3B

Nd2Fe14BHx micron-size loose particles have been synthetised through t he well known oxide reduction-diffusion (ORD) technique. When excess C a is used as reducing agent, an exothermic reaction is observed by was hing under water, with the production of native hydrogen that diffuses in the interstitial sites of the alloy. Depending on the experimental conditions, different contents of hydrogen intercalation have been ob served, with x ranging from x approximately 0 to x approximately 4-5. Cell parameters were observed quickly increasing with hydrogen uptake, resulting in a drastic and regular decrease of the coercivity. In ord er to understand the relation between the coercivity and the content o f hydrogen uptake, 2 to 4 mum size powders of a highly coercive Nd2Fe1 4B industrial alloy have been submitted to a native hydrogen environme nt obtained through different chemical conditions. A very reliable rel ationship has been established between the cell parameters (and thus t he content or interstitial hydrogen) and the coercivity of the alloy p articles. The degradation of the coercivity of industrial magnets subm itted to moisture corrosion could well result from the formation of lo w coercive layers -with a high hydrogen content- at the surface of gra ins, resulting in the possibility to initiate reverse nucleus domains.