Yoichi Tokai

High Efficient two-Stage gm Refrigerator with Magnetic Material in the Liquid Helium Temperature Region

This paper describes experimental results obtained from a two-stage Gifford-McMahon (GM) refrigerator which uses a rare earth compound as a 2nd regenerator matrix instead of Pb (lead) matrix. The refrigeration capacity below 10 K for a conventional two-stage GM refrigerator is so small that the lowest temperature achieved is limited to the 8 K level. The purpose of this study is to improve the refrigeration performance in the temperature region below 10 K. The technical point is to use Er3Ni (Erbium-3 Nickel) as a 2nd stage regenerator matrix, because it has much larger volumetric specific heat than Pb below 15 K and has almost the same specific heat as Pb at higher temperature. The reciprocating speed was optimized to improve the refrigeration performance. Refrigeration capacity of 1 W at 6.59 K and no load temperature of 4.50 K were obtained from the Er3Ni regenerator. The refrigeration loss mechanism below 10 K is also discussed.

Specific Heat of a Regenerator Material Er3Ni

Specific heat measurement was performed in the temperature range from 800 mK to 62 K on the regenerator material Er3Ni which is an antiferromagnet with a Neel temperature of 6.1 K and possesses a large volumetric heat capacity even at temperatures above T N. The magnetic contribution to the specific heat has been carefully divided into two parts, one of which is caused by entropy change due to the magnetic ordering and the other a contribution from higher energy level states (Schottky anomaly). The energy levels of the 4f electrons for Er3+ were calculated using the point-charge model, and compared with the experimental data. The entropy change of the Schottky anomaly is large compared with that of magnetic ordering and important for the regenerator material.

Magnetic field influence on er3ni specific heat

Er3Ni specific heat in 0, 1, 3, and 4.3 T magnetic fields was measured. The specific heat curves for 0 T and 1 T have a single peak due to magnetic phase transition, but those for 3 T and 4.3 T have no peak. Er3Ni specific heat decreases with increasing magnetic field around its ordering temperature, but increases in the higher-temperature region. Using the measured specific heat data, regenerator efficiency and cooling power were calculated to estimate the specific heat reduction influence due to a magnetic field on the Er3Ni regenerator performance. The results show that efficiency and cooling power in the practical leaking magnetic field are almost the same as those in 0 T.

Specific Heat of ErNi in Magnetic Fields and Its Performance as Regenerator Material in Gifford-McMahon Refrigerator at Cryogenic Temperatures

Specific heat of the rare earth compound ErNi is measured in magnetic fields up to 5 T in the temperature range 4.2 to 35 K. The specific heat under applied magnetic field of 1 T is larger than that of Er3Ni below 16 K. The numerical simulation of a Gifford-McMahon cycle shows that the refrigeration capacity using an ErNi regenerator is larger than that using one of Er3Ni or lead. It is shown for the first time that the refrigeration capacity can be increased by application of a weak magnetic field to the ErNi regenerator.

New Complex Magnetic Material for Ericsson Magnetic Refrigerator

A complex magnetic material for use in the Ericsson magnetic refrigerator was investigated. In order to refrigerate effectively in the high temperature range above 10K, it is necessary to utilize the Ericsson cycle that includes two kinds of iso-magnetic field process, because lattice entropy for a magnetic refrigerant amounts to the same order of magnitude as that for magnetic entropy. Therefore in the Ericsson magnetic refrigerant, magnetic entropy change ΔSM is required to be constant in the refrigeration range. This property, however, cannot be satisfied by usual ferromagnetic material. In this paper, the complex RAl2 magnetic mixture (where R denotes heavy rare earth atoms) in grain size scale is prepared by shock compression method. Magnetic entropy changes are determined from the specific heat measurement. This magnetic mixture has high density, which is composed of several RAl2 compounds with different Curie temperatures, where each RAl2 is surrounded by gold. From these experimental results, it is concluded that the complex magnetic mixture with gold, such as (ErAl2)0.3 (HoAl2)0.2(Dy0.5Ho0.5Al2)0.5, shows an almost temperature independent magnetic entropy change ΔSM in some temperature range, which is suitable for the Ericsson cycle, satisfying the Carnot principle.