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.
A 1.5W-class 4K GM (Gifford-McMahon) cryocooler has been developed. The regenerator material ErNi0·9Co0·1, the cold end of the second cylinder made on copper and intake/exhaust valve having original timing and interval are used in the cryocooler. In this study, four sizes of cylinders and two types of compressor units were prepared and the cooling capacities were measured. As a result, cooling capacity of the first and the second stage depended on the ratio between the first and the second cylinder's volume. When the compressor was fixed and cylinders were changed, the bigger the second stage cooling capacity became and the smaller the first stage cooling capacity became. The maximum cooling capacity of the first stage at 40K was 53.7W, and that of the second stage at 4.2K was 1.74W.
We have investigated the characteristics of some layered structural regenerators (multilayer regenerators) with magnetic regenerator materials using a two-stage GM refrigerator. In this study, we newly used Er0·75Gd0·25Ni which is expected to be placed in the high-temperature part of the second regenerator. To confirm the effect of Er0·75Gd0·25Ni, the heat-exchange efficiency of the regenerator (regenerator efficiency) with Er0·75Gd0·25Ni, Er3Co and Er0·9Yb0·1Ni, which were in the volumetric ratio of x: (0.5-x): 0.5 (0≤x≤0.5) was calculated as a function of x by computer simulation. We found that the regenerator efficiency increased when x (i.e. the amount of Er0·75Gd0·25Ni) was increased and the optimum value of x was -0.25. We then made two kinds of second regenerators: a triplelayer regenerator with Er0·75Gd0·25Ni, Er3Co and Er0·9Yb0·1Ni, which were in the volumetric ratio of 0.25:0.25:0.5, and a doublelayer regenerator with Er3Co and Er0·9Yb0·1Ni, which were in the volumetric ratio of 0.5:0.5. We compared experimentally their refrigeration performances. With the triplelayer regenerator, the lowest temperature was 2.60K at the second stage and the maximum refrigeration capacity at 4.2K was 1.17W. These results were superior to those with the doublelayer regenerator.
The influence of intake/exhaust valve timing on performance of a 4K Gifford-McMahon (GM) cryocooler has been investigated for the optimization of intake and exhaust valves. The 4K-GM cryocooler employed for the present study was a standard two-stage type with a rotary valve for intake/exhaust. The second stage regenerator in the cryocooler had both lead spheres and spherical magnetic regenerator material, ErNi0·9Co0·1, with the latter arranged at the colder end. The experimental results described in this paper show that a reasonably early valve open timing not only produced much greater cooling capacities on both stages, but also was advantageous in terms of cycle speed, no load temperature and input power to the compressor. Under the optimum conditions, the cooling capacities, above 1.10W at 4.2K as well as close to 40W at 40K, were obtained with an input electric power of about 7kW. This paper discusses the results of cooling capacity with P-V diagrams, and indicates that the reduction in pressure drop at the intake and exhaust valves is important for a 4K-GM cryocooler.
Many rare-earth compounds have been studied to attain a large volumetric specific heat in cryogenic temperatures and to aim at developing regenerator materials for a regenerative refrigerator. The magnetic specific heats of these materials are so large that the performance of refrigeration for a Gifford-McMahon refrigerator has been greatly improved and helium temperatures have been attained. In an application for the superconducting magnet, the refrigerator must be operated under the magnetic field. So, magnetic regenerator materials are required to be free from the influences of the magnetic field. In this paper, the magnetic field influences on the regenerator materials Er3Ni, ErNi and Er3Co, are discussed and Ho2Al is proposed as a new regenerator material which is free from the influence of the magnetic field. The magnetic properties of Er3Ni, ErNi and Er3Co were investigated at cryogenic temperatures in a magnetic field of up to 5 Tesla. The magnetic forces were evaluated for these materials under the magnetic field using the magnetic moment values deduced from the magnetization measurements. The magnetic forces for ferromagnetic materials such as the ErNi and Er3Co turned out to be one order of magnitude larger than that for antiferromagnetic material Er3Ni. It was confirmed that the antiferromagnetic material is superior to the ferromagnetic one in case of the exposure to the large magnetic field and large gradient of the field. Further, new antiferromagnetic regenerator materials were searched and Ho2Al was found to have a volumetric specific heat as large as Er3Co in the temperature range from 10 to 16K.
The low temperature specific heats for a series of the ErxAg100-x (x=40, 50, 60 and 70) alloys have been measured over the temperature range 1.6-30K to evaluate their applicability as a regenerator material. A large magnetic specific heat has been observed for these multi-phase samples. It is found to originate from the 4-fold degeneracy in the ground state of the Er 4f-levels. We also found that, in the temperature range of 10-20K, the volumetric specific heats for the x=50-70 samples exceed that for the Er3Ni compound, which is currently known as one of the most efficient regenerator materials. The x=40 sample is superior to Er3Ni below 6K. In addition, we found that the thermal conductivity for the present Er-Ag alloys is 5-10 times higher than that for the Er3Ni below 25K. We are led to conclude that the present Er-Ag alloys are promising as a regenerator material, particularly when a composition-gradient composite regenerator material, in which the Er concentration is varied from 70 to 40 along the direction of the gas flow, is fabricated.
A new numerical simulation of a pneumatic driven GM refrigerator is developed. To accurately predict the refrigeration capacity of a pneumatic driven GM refrigerator operating at close to 4K, full one-dimensional governing equations for expander are solved using the finite difference method considering variable physical properties of helium and regenerator materials. Furthermore, the numerical model also includes the compressor unit to estimate its influence of pressure oscillation as well as its performance. In this paper, the calculation results of a two-stage pneumatic driven 4K-GM refrigerator, with magnetic materials in the 2nd-stage, are presented briefly. The predicted refrigeration capacity at 4K and temperature fields are in good accordance with both our experimental results and with the experimental data from the literature.
A two-stage GM cryocooler is commonly used in many cryogenic systems, such as a cryopump and an MRI. The lowest temperature for a conventional GM cryocooler, however, was limited to about 10K. Recently, magnetic regenerator materials, which have larger heat capacity below 10K than a conventional regenerator material of Pb, enable the GM refrigerator to achieve the liquid helium temperature level. In this temperature range, helium in the GM cryocooler cannot be regarded as an ideal gas. The purpose of this study is to investigate a regenerator, which is operated with non-ideal helium gas. The experimental results of the regenerator performance in a two-stage Gifford-McMahon (GM) cryocooler are described in this paper. Temperatures and pressure in the regenerator were measured and mass flow rate in the regenerator was calculated from these data. The temperature profile, temperature fluctuation and mass flow rate in the regenerator are compared between ideal gas operation and non-ideal gas operation. For ideal gas operation, the temperature profile was smooth and the temperature fluctuations at each positions are almost equal. For non-ideal gas operation, a steep temperature gradient and large temperature fluctuation, which almost covered the temperature difference of the regenerator were observed from the hot end to the middle of the regenerator. From the middle to the cold end, however, both the temperature gradient and the temperature fluctuation were very small. The mass flow rate in the regenerator for ideal gas operation decreased from the hot end to the cold end. For non-ideal operation, the mass flow rate increased. The difference between the regenerator for ideal gas operation and that for non-ideal gas operation was shown clearly.
Recenty, regenerative refrigerators such as the Gifford-McMahon (GM) refrigerator achieve liquid helium temperature level by using magnetic regenerator materials, which have larger heat capacity below 10K than a conventional regenerator material of Pb. The purpose of this study is to investigate the heat characteristic of the regenerator, which is operated with liquid helium temperature level. In the present paper, we describe the experimental results of the regenerator performance in a two-stage GM refrigerator. Temperatures and pressure in the regenerator were measured and the stored heat in the regenerator was calculated from these data. The stored heat was divided between that stored in regenerator material and that stored in helium gas. The stored heat is compared between ideal gas operation and non-ideal gas operation. The regenerator efficiency was calculated from the stored heat. For ideal gas operation, the stored heat was equal at each position; however, for non-ideal gas operation, it was slight from the middle of the regenerator to the cold end. The multi-layered regenerator constructed using several magnetic materials which have different peaks of the specific heat increases the stored heat in the part in which heat capacity was increased, and achieves higher regenerator efficiency. Regenerator efficiency is higher at lower flow rates, and at higher the hot end temperatures of the regenerator. Therefore, the best condition for cryocooler operation changes with the temperature range in which the regenerator is used.
A Joule-Thomson (JT) expansion refrigerator with a Gifford-McMahon (GM) precooler which had magnetic regenerator was installed in a dilution refrigerator for use in long-term experiments without the necessity of helium transfer. A helium circulation system was constructed in which the boiled-off gas was precooled by the GM refrigerator with strong cooling power and liquefied by a JT loop heat exchanger. The dilution refrigerator was operated for two months without liquid helium transfer. The cooling power of the JT refrigerator with magnetic regenerator material Er0·7Ho0·3Ni was about 3.5W and was about 17% larger than that with lead. Then, the performance and stability of the system was much improved with magnetic regenerator material compared with Pb because of the increase in cooling power. The system was able to have some margin for extra boil-off using magnetic regenerator material. The only maintenance needed was supplying of liquid nitrogen to the trap and cleaning it every four days. An automatic refilling system was made for liquid nitrogen trap. A vibration isolation system effectively reduced heat leakage caused by vibration of the GM refrigerator. Nuclear demagnetization was possible with this system and the nuclear stage was cooled down to sub mK temperature. This system is applicable to various low temperature apparatus as well as dilution refrigerator because of little modification of the cryostat.