Application of high-energy Nd-Fe-B magnets in the magnetic refrigeration

The magnetisation M(T, H) of magnetic refrigerants varies strongly with temperature in the vicinity of their Curie temperatures, thus in realistic magnetic refrigerators employing Nd-Fe-B permanent magnets the isofield process of the Ericsson refrigeration cycle cannot be realized by simply keeping a ferromagnetic working body in the gap of a magnetic circuit made up by two tablet-shaped permanent magnets and an iron yoke. In addition, the operational point of Nd-Fe-B magnets is determined by a temperature-dependent load in the magnetic circuit, i.e. the magnetic circuit gap has a low load (ferromagnetic) below the Curie point T-Curie of the working substance but a high load (paramagnetic) above the T-Curie. To estimate the refrigeration power we consider a pile of equally spaced thin Gadolinium sheets as the working body, which are inserted into (or extracted from) the gap of Nd-Fe-B magnetic circuits. In terms of equivalent reluctance parameters and using iterative calculations, we have determined the average H-field experienced by Gd as a function of temperature for several kinds of Gd sheet pile geometry. The derived adiabatic temperature changes are in good agreement with the experiments. (C) 2000 Elsevier Science B.V. All rights reserved.