Magnetic Regenerator Materials Research Articles

Non-magnetic regenerator material of silver oxide for 4 K cryocoolers

In regenerative cryocoolers, magnetic specific heat is often utilized for heat regeneration at cryogenic temperature, such as below 20 K. Magnetic regenerator materials of rare earth compound which have sufficient specific heat enabled cryocooler to reach below 4 K. However, the magnetic noise emitted from the magnetic materials during the refrigeration cycle affects the performance of noise-sensitive devices. In this study, we propose a non-magnetic regenerator material having “optical phonon degree of freedom” even at cryogenic temperatures, in which silver oxide (Ag2O) is selected. Ag2O decomposes at temperature above 410 K in ambient atmosphere, whereas we succeeded in fabricating a sintered Ag2O bulk and small particles of the size of 0.5 mm with high-density more than 90%. Specific heat measurement down to 100 mK was performed for the sintered bulk sample of Ag2O. Below 10 K, a non-Debye behavior was observed, suggesting the contribution from optical phonon modes. Calculation of the phonon density of state (phDOS) performed by the Evolution Strategy algorithm in addition to the first-principles calculation reveals that phDOS has several distinct peaks at low energies, and the low-energy optical phonons contribute to the non-Debye behavior. The low temperature specific heat of Ag2O enhanced by the optical phonons is larger than conventional non-magnetic regenerator materials when compared in terms of specific heat per volume. Moreover, the calculation of cooling performance of cryocooler using Ag2O particles is found to be reached below 4.2 K. This study shows that non-magnetic material with low temperature specific heat enhanced by optical phonon mode can be used as a regenerator material for regenerative cryocoolers.

Investigation on the dynamic adsorption characteristics of activated carbon to Helium-4 for 4-20 K regenerator of cryocoolers

With the advantages of long life and high reliability, cryocoolers gradually substitute cryogenic liquids to become a critical guarantee for the stable operation in space exploration. For cryocoolers, the regenerator is a key component that strongly influences its refrigeration performance. The specific heat of current commonly-used regenerator materials below 20 K is significantly lower than that of the working gas Helium-4, resulting in insufficient cold storage capacity of the regenerator and further limiting the cryocooler’s ability to obtain lower temperatures and larger cooling capacity. In this paper, activated carbon is used to absorb a part of helium in the regenerator to replace the traditional magnetic regenerator materials. By establishing a low-temperature dynamic adsorption model, the dynamic adsorption characteristics of activated carbon to Helium-4 in the temperature range of 4-20 K were studied, and the effect of its microscopic adsorption/desorption heat effect on the performance of the regenerator was investigated.

Specific heat of Cu substituted GdNiSi

The specific heat of magnetic regenerator materials affects the efficiency of cryocoolers below 10 K. GdNiSi with the orthorhombic TiNiSi-type structure shows antiferromagnetic order at 11 K. To tune the magnetic transition temperature below 10 K, Cu substituted GdNiSi alloys were prepared. Powder X-ray diffraction and specific heat measurements were performed to study the effect of Cu substitution on the crystal structure and magnetic transition temperature. Increasing Cu content, the secondary phase which may be the hexagonal BeZrSi-type GdNi1−x Cu x Si is observed. The peak of the specific heat for x ≤ 0.2 was shifted higher temperature (13 K) due to the magnetic transition, though the unit cell volume of the TiNiSi-type structure slightly increased with increasing x. This result possibly originates from the increase of the RKKY interaction for the sinusoidal curve due to an increase of the intersite distance of Gd-ions.

A novel method to hit the limit temperature of Stirling-type cryocooler

The Stirling-type cryocooler with its compact size and high efficiency is always expected to obtain its temperature limit of below 3 K. However, the pressure drop losses caused by high-frequency oscillation create large obstacles for this objective. This paper reports a novel thermal-driven Stirling-type cryocooler to obtain the lowest temperature of a Stirling-type cryocooler. The advantages of a thermal-driven cryocooler (Vuilleumier cryocooler) and pulse tube cryocooler are combined with a new type of cryocooler, called the Vuilleumier gas-coupling pulse tube hybrid cryocooler (VM-PT). A prototype of the VM-PT was recently developed and optimized in our laboratory. By using helium-4 as the working gas and magnetic regenerative materials (HoCu2 and Er3Ni), the lowest temperature of 2.5 K was obtained, which can be regarded as an important breakthrough for the Stirling-type cryocooler to achieve its limit temperature of below 3 K. It can supply >30 mW cooling power at 4.2 K and >500 mW cooling power at 20 K simultaneously. Theoretically, it is feasible to use this VM-PT for cooling the superconducting devices in space applications.

Magneto Caloric Properties of Polycrystalline Gd2O2S for an Adiabatic Demagnetization Refrigerator

Currently, many space missions that use cryogenic equipment are being planned. In particular, high resolution sensors, such as transition edge sensors, require very low operating temperatures, below 100 mK. Adiabatic demagnetization refrigerator (ADR) systems are a useful tool for producing ultra-low temperatures in space because these devices can operate independently of gravity. The magnetic material is one of the most important components with respect to effectiveness of the cooling power. Thus, we could increase the cooling power using a magnetic material that has a large entropy change over the operating temperature range. Polycrystalline Gd2O2S (GOS), which was developed by Numazawa et al, can be used as such as a magnetic regenerator material. Furthermore, GOS has a very large specific heat and a magnetic phase transition temperature of about 5.2 K. These features make GOS suitable for use in the high temperature stage of an ADR. In this study, we measured and evaluated the physical properties of GOS for applications to ADRs.

Magnetocaloric effects and magnetic regenerator performances in metallic glasses

Metallic glasses with functional properties, such as magnetic properties, are promising materials for potential applications and have aroused great interest. Magnetic phase transition is an important feature of metallic glass. The unique effect of the magnetic phase transition can be applied to the field of refrigeration. On the one hand, due to its magnetocaloric effect, the amorphous alloy can be used as a magnetic refrigeration material for magnetic refrigerator. On the other hand, because of its specific heat anomaly the amorphous alloy can be used as a magnetic regenerator material for cryogenic refrigerator. In recent years, the magnetocaloric effects and magnetic regenerator performances of metallic glasses have become hot topics in the field, and opened up possibilities for the functional applications of metallic glasses. In this paper, the principle of magnetocaloric effect and magnetic regenerator performance of metallic glass and its characteristics and application prospect are introduced in detail.

Hydrogenation reaction characteristics and properties of its hydrides for magnetic regenerative material HoCu2

The hydrogenation reaction characteristics and the properties of its hydrides for the magnetic regenerative material HoCu2 (CeCu2-type) of a cryocooler were investigated. The XRD testing reveals that the hydrides of HoCu2 were a mixture of Cu, unknown hydride I, and unknown hydride II. Based on the PCT (pressure−concentration−temperature) curves under different reaction temperatures, the relationships among reaction temperature, equilibrium pressure, and maximum hydrogen absorption capacity were analyzed and discussed. The enthalpy change ΔH and entropy change ΔS as a result of the whole hydrogenation process were also calculated from the PCT curves. The magnetization and volumetric specific heat capacity of the hydride were also measured by SQUID magnetometer and PPMS, respectively.

Experimental study of one-stage VM cryocooler operating below 8 K

The Vuilleumier (VM) refrigerator, known as heat driven refrigerator, is one kind of closed-cycle Stirling type regenerative refrigerator. The VM refrigerator with power being supplied by liquid nitrogen was proposed by Hogen and developed by Zhou, which shows great potential for development below 10K. This paper describes the experimental development of a VM cryocooler operating below 8K, which was achieved by using liquid nitrogen as a heat sink of middle cavity. The regenerator was optimized by using a part of metallic magnetic regenerator material Er3Ni to replace the lead sphere and a no-load temperature of 7.8K was obtained. Then all the lead spheres were replaced by Er0.6Pr0.4 material and a no-load temperature of 7.35K was obtained, which is the lowest temperature for this kind of refrigerator reported so far. The cooling power at 10K is about 500mW with a pressure ratio near 1.6 and a charge pressure of 1.8MPa. Especially, the magnetic material Er0.6Pr0.4 was found to be a potential substitution for the conventional lead.

Experimental Investigation of Cooling Capacity of 4K GM Cryocoolers in Magnetic Fields

4K GM cryocoolers are inevitably exposed to the magnetic field inMRI systems. The cooling capacity of a 4K GM cryocooler is strongly dependent on the heat capacity of the magnetic regenerator materials, such as HoCu2, Er3Ni and Gd2O2S(GOS). In order to clarify the effect of the magnetic field on acryocooler's performance, we measured the cooling capacity of Sumitomo Heavy Industries, Ltd. (SHI) 1W 4K GM cryocoolers in magnetic fields up to 2.0 T. It is found that the impact of a magnetic field on the cooling capacity with a HoCu2/GOS hybrid regenerator is much smaller than that with a HoCu2 regenerator.

Hydrogenation Induced Change in Structures, Magnetic Properties and Specific Heats of Magnetic Regenerative Material ErNi and ErNi<sub>2</sub>

High specific heats of the magnetic regenerative material, at the temperatures lower than 20K, is crucial for a regenerative cryocooler to reach a liquid-helium temperature. The hydrogenation of the magnetic regenerative materials ErNi and ErNi2 may change their structures, magnetic properties and specific heats, which will be investigated in this paper. XRD patterns show that crystalline and amorphous phases can both be formed in the hydrogenation at 293K. The insertion of hydrogen atoms can lead to a larger specific heat, measured by a physical property measurement system (PPMS), in some higher temperature ranges. But the peak values of specific heat of the hydrides are lower than those of their parent compounds below 15K, which indicates that the idea of regenerative material hydrogenation should be left out in the efforts of regenerator performance enhancement. [doi:10.2320/matertrans.M2012276]

Hydrogenation-induced variation in crystal structure and heat capacity of magnetic regenerative material Er 3Ni

In order to analyze the influence of hydrogenation on magnetic regenerative material Er 3Ni, the structures of Er 3Ni and its hydrides Er 3NiH x are tested by X-ray diffraction (XRD), and their magnetic properties and heat capacities are measured by a magnetic property measurement system (MPMS) and by a physical property measurement system (PPMS), respectively. The results show that different crystal structures can be formed by the hydrogenation at 293 K, while the Er 3NiH x does not go through magnetic transition above 2 K. The Er 3NiH x exhibits different specific heat features with that of original compound Er 3Ni, which could be attributed to the absence of magnetic transition and other effects caused by hydrogenation. The insertion of hydrogen atoms can lead to a larger specific heat than that of Er 3Ni in some certain temperature range.

Temperature and magnetic field induced multiple magnetic transitions inDyAg2

The magnetic properties of the rare-earth intermetallic compoundDyAg2 are studied in detail with the help of magnetization and heat capacity measurements.It is shown that the multiple magnetic phase transitions can be induced inDyAg2 both by temperature and magnetic field. The detailed magnetic phase diagram ofDyAg2 is determined experimentally. It was already known thatDyAg2 undergoes an incommensurate to commensurate antiferromagnetic phase transition close to10 K. The present experimental results highlight the first order nature of this phase transition,and show that this transition can be induced by magnetic field as well. It is further shown thatanother isothermal magnetic field induced transition or metamagnetic transition exhibited byDyAg2 atstill lower temperatures is also of first order nature. The multiple magnetic phase transitions inDyAg2 give rise to large peaks in the temperature dependence of the heat capacitybelow 17 K, which indicates its potential as a magnetic regenerator material forcryocooler related applications. In addition it is found that because of the presenceof the temperature and field induced magnetic phase transitions, and becauseof short range magnetic correlations deep inside the paramagnetic regime,DyAg2 exhibits a fairly large magnetocaloric effect over a wide temperature window, e.g., between10 and 60 K.

Magnetic transitions and thermomagnetic properties of GdCu6

Abstract We report results of dc magnetization and specific heat studies focusing on the paramagnetic to antiferromagnetic transition in GdCu6. These results clearly reveal the evidences of multiple magnetic transitions in GdCu6. In addition, a marked thermomagnetic irreversibility is observed in the temperature dependence of magnetization in low ( 10 kOe ) applied magnetic fields. Nature of the magnetic response changes with the increase in applied magnetic field in the temperature regime around the paramagnetic–antiferromagnetic transition temperature and also well inside the antiferromagnetic state. Experimentally measured specific heat in GdCu6 is quite large in the temperature regime below 20 K, which indicates to the potential of GdCu6 as a magnetic regenerator material for cryocooler related applications. Isothermal magnetic entropy change estimated from the results of magnetization and specific heat measurements shows a change in sign at the antiferromagnetic ordering temperature.

Characterization of a low frequency magnetic noise from a two-stage pulse tube cryocooler

Magnetic noise of a two-stage pulse tube cryocooler (PT) was measured by a fundamental mode orthogonal fluxgate magnetometer and by a LTS Double Relaxation Oscillation SQUID (DROS) first-order planar gradiometer. The magnetometer was installed in a dewar made of aluminum at 12 cm distance from a section containing magnetic regenerative materials of the second pulse tube. The magnetic noise spectrum showed a clear peak at 1.8 Hz, which is the fundamental frequency of the He gas pumping rate. The 1.8 Hz magnetic noise registered a peak, during the cooling down process, when the second cold-stage temperature was around 12 K, which is well correlated with the 1.8 Hz variation of the temperature of the second cold stage. Hence, we attributed the main source of this magnetic noise to the temperature variation of the magnetic moments resulting from magnetic regenerative materials, Er 3Ni and HoCu 2, in the presence of background static magnetic fields. We have also pointed out that the superconducting magnetic shield of lead sheets reduced the low frequency magnetic noise generated from the magnetic regenerative materials. With this arrangement, the magnetic noise amplitude measured with the LTS DROS gradiometer, mounted at 7 cm horizontal distance from the magnetic regenerative materials, in the optimum condition, was lower than 500 pT peak-to-peak, whereas the noise level without lead shielding was higher than the dynamic range of DROS instrumentations which was around ± 10 nT .

Hydrogen induced structural and magnetic transformations in magnetic regenerator materials ErNi n ( n=1, 2) and HoCu 2

The effect of hydrogenation on the structures and magnetic properties of magnetic regenerators HoCu 2 (CeCu 2-type), ErNi 2 (MgCu 2-type) and ErNi (FeB-type) has been investigated. All these compounds can form crystalline hydrides which remain in the structure of the original compound. In the case of ErNi 2, hydrogenation induces volume expansion up to 13% compared with the parent compound. The magnetic moment and the Curie temperature of the crystalline hydrides decreases as the hydrogen content increases. In the case of ErNi and HoCu 2, there is a little change in the lattice parameters and magnetic properties of the crystalline hydrides compared with original compounds. Amorphous hydrides are also observed after the hydrogenation of ErNi 2 and HoCu 2 compounds.

Performance of Ceramic Magnetic Regenerator Materials for Sub-2 K Cryocoolers

A new ceramic magnetic regenerator material GdVO/sub 4/ has been fabricated and tested. The new regenerator material has an antiferromagnetic transition at 2.2 K with a large heat capacity more than 0.8 J/cm/sup 3/K. The cooling tests with a 0.1 W-class GM cryocooler have been done with /sup 3/He gas as a working fluid. The 2nd regenerator consisted of Pb, HoCu/sub 2/, Gd/sub 2/OS, GdAlO/sub 3/ and GdVO/sub 4/. The configuration was adjusted by partially replacing the GdAlO/sub 3/ with the GdVO/sub 4/. A cooling capacity at 1.8 K for the 2nd regenerator was increased by about 10% and the minimum cooling temperature was obtainable at 1.4 K. The cooling test results show that the high heat capacity magnetic materials are useful to provide both the higher cooling capacity and the lower minimum temperature, but more precise adjustments are needed to optimize the configuration of the regenerator.

Performance improvement of a two-stage GM cryocooler by use of Er(Ni 0.075Co 0.925) 2 magnetic regenerator material

A newly developed magnetic regenerator material Er(Ni 0.075Co 0.925) 2, having high specific heat at temperatures from 10 to 20 K, has been tested inside the second regenerator of a two-stage GM cryocooler. Different working conditions have been examined: four Er(Ni 0.075Co 0.925) 2/Pb material ratios, three displacer reciprocating speeds, and two cryocooler stroke lengths. In the best working conditions, the experimental results show a cooling power improvement up to 15% over the whole temperature range from 10 to 20 K compared to that of the same GM cryocooler employing only lead.

２１世紀へ伝えたい超伝導応用技術 ５ 冷凍機伝導冷却式超電導磁石の開発 スイッチポンで超電導，より使い易い超電導磁石を目指して

This paper describes the development of a conductive-cooled superconducting magnet that uses no cryogen, such as liquid helium or liquid nitrogen. First, the history of the development is introduced. A conductive-cooled superconducting magnet was realized in the late 1980s for the first time and was commercialized about 10 years ago. The technical points and construction for the magnet are explained. The important points were a 4K-GM cryocooler and an HTS superconducting current lead. A conventional GM cryocooler had not achieved 4K level, but by using magnetic regenerator material, we realized refrigeration at liquid helium temperature. An HTS current lead was used as a current lead between a thermal shield and a superconducting coil. Heat leakage by a superconducting current lead is less than 1/10th by a conventional copper current lead. Heat leakage to the 4K level was then dramatically reduced, and the total thermal load at 4K level becomes low enough for a 4K-GM cryocooler. In a conductive-cooled superconducting magnet, a superconducting coil is directly cooled by the second stage of a 4K-GM cryocooler at the 4K level via a good thermal conductive pass. The coil is surrounded by a thermal shield, which is cooled by the first cooling stage at around 50K. A conductive-cooled superconducting magnet has such features as simple operation, small size, and easy access to a magnetic field. Several companies have commercialized a conductive-cooled magnet, and the magnets have been applied not only to research, but also to industrial use, such as MRI, silicon crystal growth, and magnetic separation. The latest R & D and future aspects for a conductive-cooled superconducting magnet are also described.

A computational model for two-stage 4K-pulse tube cooler: Part I. Theoretical model and numerical method

A new mixed Eulerian-Lagrangian computational model for simulating and visualizing the internal processes and the variations of dynamic parameters of a two-stage pulse tube cooler (PTC) operating at 4 K-temperature region has been developed. We use the Lagrangian method, a set of moving grids, to follow the exact tracks of gas particles as they move with pressure oscillation in the pulse tube to avoid any numerical false diffusion. The Eulerian approach, a set of fixed computational grids, is used to simulate the variations of dynamic parameters in the regenerator. A variety of physical factors, such as real thermal properties of helium, multi-layered magnetic regenerative materials, pressure drop and heat transfer in the regenerator, and heat exchangers, are taken into account in this model. The present modeling is very effective for visualizing the internal physical processes in 4 K-pulse tube coolers.

Performance improvement of a pulse tube cooler below 4 K by use of GdAlO 3 regenerator material

The cooling power and coefficient of performance of a two-stage pulse tube cooler below 4 K have been increased greatly by using the newly developed ceramic magnetic regenerative material GdAlO 3 (GAP). Cooling powers of 200 mW at 2.8 K, 300 mW at 3.13 K, and 400 mW at 3.70 K have been achieved with a compressor input power of about 4.8 kW. The results show that the cooling power near 3.0 K increases by 150% compared to that of the same pulse tube cooler (PTC) employing only conventional HoCu 2 and ErNi regenerator materials.