Oxidation of Fe-R-B powders during preparation of permanent magnets

Differential scanning calorimetry (DSC) has been employed to study the changes which occur in the powders used to prepare Fe-R-B permanent magnets upon heating. Three exothermic reactions are observed upon heating the powders. The correlation of DSC data with the magnetic properties of Fe-R-B powders shows that these reactions cause a significant deterioration of the hard magnetic properties of the resulting magnets. The most probable explanation of these exothermic reactions is the reaction between chemisorbed oxygen and rare-earth metals to form bulk oxides. It appears that there is a critical oxygen concentration associated with a critical particle size which limits the development of hard magnetic properties.