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

The magnetisation M(T, H) of magnetic refrigerants varies strongly with tem perature in the vicinity of their Curie temperatures, thus in realistic mag netic refrigerators employing Nd-Fe-B permanent magnets the isofield proces s 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 tw o tablet-shaped permanent magnets and an iron yoke. In addition, the operat ional point of Nd-Fe-B magnets is determined by a temperature-dependent loa d in the magnetic circuit, i.e. the magnetic circuit gap has a low load (fe rromagnetic) below the Curie point T-Curie of the working substance but a h igh load (paramagnetic) above the T-Curie. To estimate the refrigeration po wer we consider a pile of equally spaced thin Gadolinium sheets as the work ing body, which are inserted into (or extracted from) the gap of Nd-Fe-B ma gnetic circuits. In terms of equivalent reluctance parameters and using ite rative 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.