Active and passive magnetic regenerators in gas/magnetic refrigerators

Abstract

Abstract Rare earth materials have found three types of applications in refrigeration technology. One is as passive regenerators in gas cycle refrigerators; the second is as working materials in non-regenerative or in externally regenerated/recuperated magnetic refrigerator cycles; the third is as the combined refrigerant and regenerator in active magnetic regenerative refrigerators. Below 10 K, conventional regenerators in regenerative cryogenic refrigerators lose their effectiveness because the thermal mass of regenerator materials decreases compared with that of the helium gas convected in these cycles. The high specific heat of antiferromagnetic or ferromagnetic ordering transitions below 10 K was suggested for increasing the thermal mass in regenerators over two decades ago. However, it was not until 1987 that Japanese workers made a breakthrough in regenerative refrigerator performance below 10 K with new passive magnetic regenerator materials and by simultaneous modifications of the cycle operation. These new materials are intermetallic rare earth compounds with ordering temperatures below 10 K. The rapid progress that has recently occurred in this area is summarized in this paper. Only a few new references on the application of rare earth materials in Carnot or externally regenerated magnetic refrigerators exist because it has proven difficult to make practical, economical refrigerators using these cycles. Cryogenic magnetic refrigerators using regenerative cycles with rare-earth ferromagnets as the working materials are less established than gas cycle refrigerators. However, there has been significant progress on the active magnetic regenerator (AMR) cycle that combines the magnetic refrigerant and the regenerator in the same material. Demand for improved refrigerants in engineering prototypes using the AMR cycle is driving the search for new ferromagnetic materials with large magnetocaloric effects. Attractive candidates are predominately rare earth elements, alloys and compounds. This paper summarizes past progress and identifies areas where new materials are being sought.