Role of Zr and Ga additions on the hydrogen process of Nd-Fe-B magnets

International audienceHighly coercive Nd-Fe-B powders for bonded magnets have been successfully produced applying hydrogen processes such as hydrogen decrepitation (HD) or hydrogen-disproportionation-dehydrogenation-recombination (HDDR). XAFS experiments were performed in order to localise the extra elements. Zr addition results in the formation of fine ZrB2 precipitates, whereas Ga substitutes the Fe(4c) site in the Nd2Fe14B structure. Systematic analyses of the hydrogen disproportionation kinetics by thermomagnetic measurements have permitted to point out different regimes of reactivity depending on both the used extra elements and the temperature. It is found that the ZrB2 precipitates act as barriers limiting hydrogen diffusion at the crystallites boundaries and lead to uncompleted disproportionation reactions. Then the Nd2Fe14B remaining particles act as nucleation centres that can be used to induce oriented growth during the recombination step