Elevated Temperature Properties of Sintered Magnets of (Nd,Dy)-Fe-B Modified with Cobalt and Aluminum
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Elevated Temperature Properties of Sintered Magnets of (Nd,Dy)-Fe-B Modified with Cobalt and Aluminum Y. Xiao, H.F. Mildrum, K.J. Strnat and A.E. Ray School of Engineering, University of Dayton Dayton, Ohio 45469
ABSTRACT The effect of small aluminum additions on the temperature dependence of remanence and intrinsic coercivity in Co- and Dy-containing Nd-Fe-B was studied. Sintered magnets were prepared and demagnetization curves measured at temperatures between -50 to +200*C. Curie temperatures and irreversible flux losses in open circuit were determined. Al increases the coercivity while decreasing remanence, energy product and Curie temperature. Other unfavorable side effects are the increase in temperature coefficients of Br and, especially, MHc. Substitution of Al is not beneficial for magnets used at elevated temperature. INTRODUCTION The rapid decline on heating of their remanence, Br, the energy product, (BH)max, and especially of the intrinsic coercivity, MHc, limits the appli0 cation of ternary Nd-Fe-B magnets to below 100-150 C. To enable such magnets to operate at higher temperatures, different alloy modifications have been tried. Design engineers commonly describe the utility of magnets at elevated temperatures by two parameters: the temperature coefficient, c4 (Br), of the remanent induction, and the irreversible flux losses in open circuit during a heating-cooling cycle. The losses must be tied to a specific operating point, e.g. by specifying the unit permeance, p=-Bd/Hd. The large value of IoLlfor Nd-Fe-B magnets is due to the low Curie temperature, Tc, and the rapid decrease in Nd moment with increasing temperature, T. Hence, two approaches might result in a reduction ofJO-4: raising the Curie temperature of the main phase of the magnet, or partially substituting heavy rare earths (HRE) for Nd. Cobalt is an effective alloying element for raising Tc. [1,2,3] Substitution of 18 to 20 at% Fe by Co enhances Tc to above 500'C and thus lowers O . However, this is at the expense of the intrinsic coercivity. A lower MHc, in turn, leads to an increase of the irreversible loss. The irreversible losses are generally inversely related to the coercivity; and since MHc usually decreases with increasing operating temperatures, the losses are indirectly related to the temperature coefficient of coercivity, A (MHc). The physical parameters affecting the temperature dependence of the coercivity are not well known. One common practical approach to lowering the irreversible losses is simply to increase the intrinsic coercivity at room temperature. Then, even if MHc is reduced by the same factor as in the absence of Dy on heating, a higher absolute value is left at T, and the flux loss is lower. A modest dysprosium addition can nearly double MHc of a Nd-Fe-Co-B magnet and bring the irreversible 0 loss to below 5% at 200 C [4]. However, the use of the expensive Dy significantly raises the cost of magnets, especially at higher dysprosium contents. Aluminum has also been found effective for enhancing the coercivity in bo
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