Recently, investigation on magnetic refrigerators, especially in the temperature range below-15K has been considerably progressed. By the noticeable works on the magnetic refrigerators for producing superfluid He and for liquefing He gas, it is verified that those refrigerators have very high efficiency. In this review, the present stage of the investigation and development on the magnetic refrigerators will be briefly introduced and the prospect of those efforts will be discussed. First, the principle of the magnetic refrigerators and important problems necessary for development of those are shown. And then, the several examples of the magnetic refrigerators are shown and given brief consideration in connection with the problems discussed first. Finally, the current of the present investigations on the magnetic refrigeration is introduced briefly.
An important problem in developing magnetic refrigerator is selection of the magnetic refrigerants. The main purposes of this paper are to discuss the physical properties necessary for the magnetic refrigerant, such as the magnetic, thermal and magnetocaloric characters, and to show a brief review on our recent investigation. Magnetic refrigerators can be roughly divided into two groups; one for the Carnot type magnetic refrigerator below 20K and the other for the Ericsson type refrigerator. Therefore, in the present paper, the physical properties requisite for those two kinds of refrigerants are separately discussed. Then, the recent several results are shown. Finally a brief prospect of the magnetic refrigerant is given.
To study magnetic refrigerants useful at low temperatures below 20K, new pseudo-binary garnet single crystals of Gd3(Ga1-xAlx)5O12 with x=0.1, 0.2, 0.3 and 0.4, having about 25mm in diameter and 50mm in length, have been grown using the Czochralski technique. The effective magnetic moments p of the crystals are about 7.9μB, independent of x, and the paramagnetic Curie temperatures θp are-0.60 to -1.11K, showing the minimum at x=0.2. Based on magnetic measurements and adiabatic demagnetization experiments in magnetic fields up to 75 and 60kOe, respectively, the magnetic entropy change ΔSM and the entropy S are estimated to be almost the same for all the crystals.
Two magnetic refrigerators with new heat switches based on the thermosiphon principle were designed and tested, which could produce and preserve superfluid helium. However, these magnetic refrigerators did not satisfy the designed performance. To make clear this problem, the detailed cycle analysis was performed including heat losses due to heat switches. The results show that heat loss relating to heat capacity of liquid helium mainly affects the reduction of experimental cooling power, and is approximately 8 times larger than that of the case taking into account of only unsteady-state conduction heat exchange between magnetic element and liquid helium. It is believed that this large heat loss may be caused by the convection of liquid helium.
The Carnot type magnetic refrigeration in the temperature range from 4.2K to 15K has been studied. This refrigeration system mainly consists of the following components; the magnetic working material, the thermal switches at 4.2K and 15K, and the superconducting pulsed magnet. Gd3Ga5O12 (GGG) single crystal was used as the working material. The entropy diagram and thermal conductivity were obtained. It was concluded that GGG provides large refrigeration capacity (1-3W/mol) in the Carnot cycle. A heat pipe was used as the thermal switch at 4.2K. Thermal conductivity of the heat pipe at 4.2K was as large as that of OFHC copper and switch ratio between 4.2K and 15K was over 103. Some testpieces consist of GGG and the heat pipe were experimented. The difficulty concerning with the thermal resistance between non-metallic material (GGG) and metal (heat pipe) wat solved out by using the specially deviced heat pipe where GGG liquefies directly gaseous helium. The maximum liquefaction efficiency was -70%. Based on these preriminary studies, the Carnot type magnetic refrigerator was constructed. The average liquefaction efficiency for the one-shot demagnetion process was -40%. For continuous operation at 3.1 Tesla between 4.2K and 10K, the refrigeration capacity was 0.34W and the Carnot efficiency was 17%. This efficiency was not taken account of the loss from the superconducting pulsed magnet. This trial magnetic refrigeration system gave the possibility for realizing the small refrigerator with high efficiency at 4.2K.
The requirement for a helium refrigerator with high efficiency and compactness has been growing for cooling superconducting devices. We have developed a reciprocating magnetic refrigerator for liquefying helium from the temperature of 20K region which uses gadolinium-gallium-garnet (GGG) for the magnetic material. The working material (GGG single crystal, 30mm in diameter and 40mm in length) is placed at the end of the each piston, and those two pistons are driven by a pneumatic jack. When the working material is placed in a high-intensity magnetic field (4.5T), the temperature of the GGG rises to 15K. The inner surface of the cylinder is cooled by the auxiliary refrigerator (a G-M refrigerator), and heat is removed through the narrow gap (less than 50μm) filled with gaseous helium between the GGG and the cylinder. When the magnetic field given to the GGG is eliminated by moving the piston, temperature of the GGG goes below 4.2K. Here the refrigeration of liquid helium occurs by condensing the gaseous helium on the surface of the GGG. Technical emphasis was placed on the realization of a high heat exchange rate between the solid magnetic material and gaseous helium. A sufficient heat transfer rate was achieved after several component-level experiments. Sources of inefficiency to the refrigeration capacity has been also discussed. Finally the refrigeration power of 0.95W at 4.2K was achieved at 0.38Hz operation.
Design conditions of the specific refrigerator is discussed, and a proto-type apparatus for the experiment was constructed. Gd disc of 20mm dia. ×10 or 20mm thickness was used and magnetized by step-promoting magnet with Sm-Co compound pole-pieces of (30mm)2×10mm t. It provides magnetic field of 4 to 6 kOe, and gives rise to T≤1.2K adiabatically on the Gd specimen. In order to obtain considerable low temeperatures, 4 or 6 Gd specimens were thermally connected via thermal switch in cascade series. The switch was operated by mechanical contact between high-conducting metal plates. At present, the stationary temperature difference through the cascade series was attained only to 0.46K, probably as due to poor conductance (KON≤0.2W/K) of the thermal switch.