Recent progress in application of magnetic regenerator material provides a potentially new technology for the cryogenic field. In the present review, the simple theoretical background of the application is given from the point of view of the magneto-caloric effect. The basic researches on physical properties, mechanical properties and fabrication methods regarding magnetic regenerator materials are introduced. This review describes the development of 4K Gifford-McMahon (GM) refrigerator and other small refrigerators using magnetic regenerator materials, and shows the activities on application of 4K-GM refrigerator to many cryogenic systems, such as superconducting magnet systems and superconducting device systems. Finally, this review points out the wide variety of possibile applications, and discusses some problems awaiting solution at next stage.
Various magnetic regenerator materials, such as Er3Ni, Er3Co, ErNi and ErNi2, are fabricated in the form of a spherical particle by centrifugal atomization. 4K level refrigeration has been achieved by a GM refrigerator using these materials. However, the magnetic regenerator materials are considered brittle, as they are intermetallic compounds. It is important to evaluate the mechanical properties of these materials to confirm their reliability as a regenerator material. In this paper, experimental results of compression and vibration tests for magnetic regenerator materials are described. The technical point of this study is to use spherical particles in a diameter range of 0.18-0.25mm as test samples. The compressive stress of 20MPa was applied to these spherical particles and no fractured spheres were found in the scope of observation. A few spheres were fractured by the compressive stress of at least 50MPa. The morphology of fractured spheres shows brittle fracture. Moreover, no fractured spheres were observed after the vibration test, in which the maximum acceleration was 294m/s2 and the number of vibrations was 1×106, as far as there was no room to stir spherical particles in a test vessel. In practice, the reliability of Er3Ni spheres has been confirmed by a long-run test of 7, 700h in a usual GM refrigerator.
Rare earth-ruthenium intermetallic compounds have been developed for ues as regenerator materials in helium liquefaction Gifford-McMahon refrigerators. These compounds show characteristically lower magnetic transition temperatures than other rare earth intermetallic compounds. The refrigeration capacity of GM refrigerators has been remarkably improved by the use of Ho1·5Er1·5Ru spheres as regenerator materials at the temperature range below 5K. This material posseses a large heat capacity even under magnetic fields of 0.5T in such a temperature range. Spherical powders of these materials were produced by a simple method named the “low pressure jet method, ” which offers good sphericity powders with smooth surface. Nozzle clogging is a serious problem in this method, but we have overcome this problem by reducing gas elements such as oxygen, fluorine, from starting materials.