SYNTHESIS AND CHARACTERIZATION OF ND2FE14B-25 VOL PERCENT FE NANOCOMPOSITE MAGNETS

Synthesis of nanocomposite two-phase permanent magnet materials by mec hanical milling requires optimization of the volume fractions of the t wo phases, milling time, heat treatment temperature, and time. In the present study, optimization of milling time, heat treatment temperatur e, and time have been carried out by fixing, respectively, the volume fractions of hard and soft phases at 75 and 25. Prior to the synthesis of nanocomposite magnets, the effect of milling on the magnetic prope rties of the hard phase was studied. The results on milling of hard ph ase have shown the lower and upper limits of milling time to be 108 an d 144 ks, respectively. Milling for 144 ks and heat treatment at 823 K for 1.8 ks was found to result in nanocomposite magnets with the best properties. The microstructural characterization has shown clearly th at when the heat treatment temperature is higher, grain growth takes p lace and consequently the magnetic properties of the nanocomposites de teriorate. Compositional analysis using transmission electron microsco py has shown grains richer in Fe and Nd. The rationalization of magnet ic properties shows that to improve the energy product, coercivity nee ds to be improved. Alloying addition of Cr and Cu was found to improve coercivity and consequently the energy product. Finally, it was shown that by using finer Fe particles one can reduce the time required for milling and avoid many adverse effects resulting from long time milli ng. (C) 1998 American Institute of Physics. [S0021-8979(98)09301-3].