Application of hydrogen vibration milling in the processing of NdFeB and (Nd,Pr)FeB permanent magnets

A Hydrogen Vibration Mill (HVM) was designed, constructed and then used to produce Nd-Fe-B powders. The powders were processed to give fully; dense, s intered magnets. When exposed to hydrogen, Nd-Fe-B material undergoes decre pitation, splitting bulk material into highly friable particles, suitable f or milling. The supply of hydrogen to the mill was from a LaNi5 store. The mill construction allowed in situ hydrogenation and milling at 0.5-5 ba r H-2. This occurred in a stainless steel chamber of 1000 cm(3) capacity wi th carbon chrome milling balls. The amplitude of vibration was 2-4 mm at 45 Hz. The in situ decrepitation and dry milling facilitated a low oxygen con tamination, which is beneficial to sintering and magnetic properties. A mil ling time of 3 h yielded powders of 7 mu m mean diameter, suitable for prod ucing magnets. The best properties obtained were an energy product of 305 k Jm(-3), with coercivity of 704 kAm(-1). (C) 1999 International Association for Hydrogen Energy. Published by Elsevier Science Ltd. All rights reserved .