Search

Processing magnetocaloric materials into magnetic refrigerant particles is an essential issue in developing high-performance magnetic refrigerators. Here, we succeed in stably producing magnetocaloric particles of the promising material HoAl2 by a newly devised method based on electrode induction melting gas atomization process. The particle size range is on the order of submillimeter, which is suitable for practical refrigeration systems. The resulting particles with less contamination have good morphological, magnetic, and magnetocaloric properties: (i) almost spherical shapes with few internal pores, (ii) a sharp ferromagnetic transition around 30 K, and (iii) a large magnetocaloric effect comparable to the bulk counterpart. These features suggest the HoAl$_{2}$ gas-atomized particles have the potential of use as a magnetic refrigerant. The presented method can be applied not only to HoAl2 but also to other brittle magnetocaloric materials with high melting points, facilitating the production of various magnetic refrigerants needed to develop magnetic refrigerators for hydrogen liquefaction.

We fabricate gas-atomized particles by inductively melting electrode rods of HoB2-x (x = -0.3, 0, 0.3, and 1.0) and investigate the effect of non-stoichiometry on the phase fraction, microstructure, and physical properties. Shifting the stoichiometric ratio of the electrode rod to the B (Ho)-rich side increases HoB4 (Ho) phase in the resulting atomized particles. Even if the atomized particles contain 15-20 weight percent (wt.%) of the impurity phase, the influence of which on the physical properties is less severe: the maximum value of the magnetic entropy change is only reduced by 10% compared to HoB2.0 particles. We further find that the ductile Ho phase exists so as to fill the space between the brittle HoB2 phases in the atomized particles, which may be beneficial to the mechanical properties of the particles. Our findings suggest that it would be better to use the Ho-rich electrode rods than the stoichiometric ones to produce HoB2-x particles with more suitable properties as a magnetic refrigerant for magnetic refrigeration systems.

Processing magnetocaloric materials into magnetic refrigerants with appropriate shapes is essential for the development of magnetic refrigeration systems. In this context, the impact of processing on the physical properties of magnetocaloric materials is one of the important issues. Here, we investigate the crystallographic, magnetic, and magnetocaloric properties of gas-atomized particles of the intermetallic compound ErCo$_{2}$, a giant magnetocaloric material for low-temperature applications. The results demonstrate that the physical properties of ErCo$_{2}$ are significantly changed by atomization and subsequent thermal annealing. In the as-atomized particles, the magnetic transition temperature increases from 34 to 56 K and the phase transition changes from first order to second order. The thermal annealing shifts the transition temperature back to the original one and restores the first-order phase transition characteristic. The changes in magnetic properties are closely related to those in crystallographic properties, suggesting the importance of the magneto-structural coupling. The magnetic entropy change $-\Delta S_{M}$ of the particles can be tuned in size, shape, and peak temperature depending on the annealing conditions. The peak value of $-\Delta S_{M}$ varies in the range of 9--33 J kg$^{-1}$ K$^{-1}$ for a magnetic field change of 0--5 T. All the ErCo$_{2}$ atomized particles have magnetocaloric properties comparable or superior to other promising candidates for low-temperature magnetic refrigerants., Comment: Journal of Alloys and Compounds (in press)

The processing of promising magnetocaloric materials into spheres is one of the important issues on developing high-performance magnetic refrigeration systems. In the present study, we achieved in producing spherical particles of a giant magnetocaloric compound HoB2 by a crucible-free gas atomization process, despite its high melting point of 2350 C. The particle size distribution ranges from 100 to 710 micrometers centered at 212-355 micrometers with the highest yield of 14-20wt% of total melted electrode, which is suitable for magnetic refrigeration systems. The majority of the resulting particles are mostly spherical with no contamination during the processing, while unique microstructures are observed on the surface and inside. These spherical particles exhibit sharp magnetic transitions and huge magnetic entropy change of 0.34 J cm^{-3} K^{-1} for a magnetic field change of 5 T at 15.5 K. The high sphericality and the high magnetocaloric performance suggest that the HoB2 gas-atomized particles have good potential as magnetic refrigerants for use in magnetic refrigerators for hydrogen liquefaction., Comment: 6pages(double column), 6 figures

A magnetic refrigerator that makes use of the magneto-caloric effect realizes a highly efficient cooling device. Since the cooling power of magnetic refrigerators depends largely on the strength of the magnetic field, the use of a superconducting magnet is essential. Using magnetic refrigeration, achieving a liquefaction efficiency of larger than 50% is theoretically possible, which is twice that of conventional gas expansion refrigerators. In this study, an active magnetic regenerative refrigerator, one of the magnetic refrigerators using a superconducting solenoid, was built and hydrogen liquefaction was successfully demonstrated.

We report the fabrication of ErAl2 magnetocaloric wires by a powder-in-tube method (PIT) and the evaluation of magnetic entropy change through magnetization measurements. The magnetic entropy change of ErAl2 PIT wires exhibits similar behavior to the bulk counterpart, while its magnitude is reduced by the decrease in the volume fraction of ErAl2 due to the surrounding non-magnetic sheaths. We find that another effect reduces the magnetic entropy change of the ErAl2 PIT wires around the Curie temperature, and discuss its possible origin in terms of a correlation between magnetic properties of ErAl2 and mechanical properties of sheath material., Comment: This is the version of the article before peer review or editing, as submitted by an author to {insert name of Journal}. IOP Publishing Ltd is not responsible for any errors or omissions in this version of the manuscript or any version derived from it. The Version of Record is available online at {https://dx.doi.org/10.1088/1361-6463/ab5c71}

This paper presents the experimental results of 4.2 K cooling power of a Gifford-McMahon (G-M) cryocooler. To improve the cooling power, an original regenerator structure named coaxial pipe regenerator was applied to the second stage regenerator. This structure is that a stainless steel pipe is inserted in the coaxial direction of the second stage regenerator. An effect of the coaxial pipe is considered to rectify the helium flow in the regenerator that leads to an increase in the mass flow rate to the expansion space. Then, an improvement in the cooling power is expected. The second stage regenerator is comprised of three-layer of Pb (warm side), HoCu2 (middle) and Gd2O2S (cold side) spheres of which the volume filling rate is 50, 20 and 30%, respectively. The coaxial pipe is applied to each material layer independently to survey an improvement effect of the cooling power at 4.2 K. These coaxial pipe regenerators were tested with a G-M cryocooler of one-watt model (RDK-408D2, SHI) and a scroll-type compressor (SSC-3700, SUZUKISHOKAN). The three-layer regenerator (without coaxial pipe) obtained the cooling power of 1.70 W. When the coaxial pipe was applied to the Pb layer, the cooling power of 1.79 W was achieved with the electrical input of compressor of 7.06 kW. In contrast, the coaxial pipe of HoCu2 layer and Gd2O2S layer showed no conspicuous improvement. These results prove that the flow condition of helium in the Pb layer significantly affects the 4 K cooling power.

We report the adiabatic temperature change in ErAl2 magnetocaloric wires fabricated by a powder-in-tube (PIT) process. The adiabatic temperature change of the PIT wires is found to be determined by not only the volume fraction of ErAl2 core but also the magnitude relationship between the specific heat of the ErAl2 core and the metal sheath. We propose a quantitative analysis method for calculating the temperature and core volume fraction dependence of adiabatic temperature change in the PIT wire, whose formula is applicable to also various magnetocaloric composites, useful to estimate the magnetocaloric response prior to fabrication., Comment: 14 pages, 5 figures

The magnetocaloric effect is the thermal response of a material to an external magnetic field. This manuscript focuses on the physics and the properties of materials which are commonly used for magnetic refrigeration at cryogenic temperatures. After a brief overview of the magnetocaloric effect and associated thermodynamics, typical requirements on refrigerants are discussed from a standpoint of cooling power density optimization. Finally, a compilation of the most important properties of several common magnetocaloric materials is presented.

A novel filling method of regenerator materials, which we name a co-axial layout , is experimentally investigated. The 2nd stage regenerator of a Gifford–McMahon cryocooler was packed with 50% by volume of the co-axial layout (low temperature side) that consisted of 20% Gd 2 O 2 S and 30% HoCu 2 spheres. The warm temperature side was packed with 50% lead spheres as a single layer. The experimental results show that the cooling power of co-axial layout is almost the same as that of a three-layer layout, 20% Gd 2 O 2 S, 30% HoCu 2 and 50% lead, at temperatures between the ultimate low temperature (3.0 K) and 6 K. Moreover, at the temperatures above 10 K, the co-axial layout has 1.7–2.0 times the cooling power of the three-layer layout. Thus the co-axial layout produces a major improvement in cooling power. In this paper, the performance of five types of material assortments is discussed.

Two series of RCo2 compounds, (ErxHo1−x)Co2 and (EryDy1−y)Co2, were investigated as magnetic refrigerants for hydrogen liquefaction. A large magnetocaloric effect (MCE) was observed just above the Curie temperature (Tc) of these compounds which both undergo a first-order magnetic transition. The isothermal entropy change ΔS and the adiabatic temperature change ΔTad of these compounds were larger than those of other Laves-phase materials such as RAl2 and RNi2. However, the temperature range of the large MCE for these compounds was limited. It has been shown that Tc increases almost linearly against the de Gennes factor, and can be controlled by changing the concentration of the rare earth elements while maintaining a first-order transition. By measuring the magnetization and heat capacity, we obtained temperature entropy (T–S) diagrams, which are essential for analyzing the magnetic refrigeration cycle. Both series of compounds showed high potential for use in a regenerative thermal cycle, especially as a combination of several compositions to cover a wide temperature range.

Considering the unignorable factors in practice, a new time independent, 2-dimensional porous media model of room-temperature Active Magnetic Regenerative Refrigeration (AMRR) has been proposed. The 2-D model improved the existing 1-dimensional model by introducing the influence of heat transfer effect though the regenerator wall and conduction for y -axis inside the regenerator. This study compared the previous 1-D model with the 2-D model and concluded that the system can lose 22% of cooling capacity caused by air convection and the conduction loss in y can reach to 10% of cooling capacity. It is concluded that the new model will be useful to predict the performance of room AMRR for more practical conditions.

In order to improve the cooling power at 4.2 K, performance of a Gifford–McMahon cryocooler was investigated. We focus on regenerator materials and structure of 2nd stage regenerator. A ceramics regenerator of Gd 2 O 2 S (GOS) was used in the cold part of the 2nd stage regenerator. To utilize the GOS effectively, we applied a layered structure to the 2nd stage regenerator. It was divided into three parts along the length for the temperature distribution, to which lead (Pb), HoCu 2 and GOS spheres were filled. The cooling power improved to 0.22 W at 4.2 K. The electric power consumption is 1.3 kW. When only Pb and HoCu 2 (without the GOS) are filled, the cooling power is 0.15 W at 4.2 K. The experimental results show that GOS is very effective to increase the cooling power at 4.2 K.

金沢大学理工研究域数物科学系, A model active magnetic regenerator refrigerator (AMR) with the Brayton-like operation cycle was analyzed by numerical cycle simulation in the temperature range between 20 and 77 K. In order to study the performance using magnetic material with various transition temperatures Tc, entropy of magnetic material with second order phase transition was calculated using mean field theory and Debye approximation. The cooling performance is shown to be high when the heat exhaust temperature is close to the transition temperature. It is shown that the optimized operation condition depends on both Tc and operation temperatures. Multi-layered AMR beds were shown to improve the performance of AMR. Multi-stage AMR was also discussed. © 2010.

Superfluid 4He was produced on a small jet plane for the first time using a small GM-refrigerator to condense the liquid and a scroll pump to get the superfluid by evaporation. The surface wave on superfluid under 0.5gE, 0.1gE and 0.05gE, together with 2gE and 1gE, was successfully examined by an optical method utilizing parabolic flight. Here, gE is the gravitational constant on the ground. Assuming that only the fundamental mode was excited as determined by the sample cell width, the resonance peak in the frequency domain was well reproduced by the gravity wave with corresponding gravity constant.

Sample of HoxEr1−xN (x = 0.25, 0.5, 0.75) were synthesized by hot isostatic pressing of alloy blocks in nitrogen gas and their specific heat were measured under zero-field and 5 T. The peak value of the specific heat associated with the ferro–para magnetic transition of HoxEr1−xN was lager than that of Er3Ni which is used commercially as a magnetic regenerator for the Gifford–McMahon refrigerator. Magnetic entropy change ΔS and adiabatic temperature range ΔT were evaluated from entropy curves under zero-field and 5 T. Both the properties were very promising from viewpoints of application of rare-earth mononitride to the magnetic refrigerant and/or regenerator working at cryogenic temperatures around their Curie's temperature.

The effect of transition metal doping in YB66 has been studied with respect to increase its thermal conductivity. Single crystals of YB66 doped with 4th, 5th and 6th group transition metals were grown by floating zone method. A detailed structure analysis suggested that an existing special B—B pair at the (1/4, 1/4, 1/4) site of YB66 could be attributed to a phonon scattering center that causes the amorphous-like low thermal conductivity. It was found that 5th and 6th group transition metals of V, Cr, Nb and Mo substitute the B—B pair. By contrast 4th group transition metals such as Ti and Zr were found to enter another (x, x, x) site with x ∼ 0.135. Thermal conductivity of both Nb-doped and undoped YB66 was measured in the range 4 K ≤ T ≤ 150 K. Nb doping increased thermal conductivity of YB66 with a factor of about 2.

We have synthesized Gd x Tb 1− x N and Tb x Ho 1− x N ( x = 0, 0.25, 0.5, 0.75 and 1) by the carbothermic reduction performed in a nitrogen gas stream. GdN and TbN have been completely miscible with each other as well as TbN and HoN. As x was changed from 0 to 1, the Curie temperature increased monotonously from 18.5 to 43.8 K for Tb x Ho 1− x N and from 43.8 to 61.2 K for Gd x Tb 1− x N. The magnetocaloric effects have been evaluated by calculating the magnetic entropy changes from the magnetization data sets measured at different applied fields and temperatures. In any composition x , the magnetocaloric effects of Gd x Tb 1− x N and Tb x Ho 1− x N are larger than those of Gd x Dy 1− x N. Therefore, Gd x Tb 1− x N and Tb x Ho 1− x N are promising magnetic refrigerant materials for hydrogen liquefying system working below liquid nitrogen temperature.

The rare earth mononitride, ErN, has been synthesized by the carbothermic reduction and hot isostatic press methods. The magnetocaloric effect of ErN has been evaluated by calculating the magnetic entropy changes, ΔS, from the magnetization data sets and from the heat capacity measured at various temperatures and applied fields. The two results are in good agreement with each other. The ΔS value of ErN is the highest at 7.5 K and higher than that of ErNi2 reported as the candidate material for the magnetic refrigerant of cryogenic technology. Heat capacity curve against temperature of ErN has a peak at 4.4 K at zero-field. The peak value of the heat capacity of ErN is 507 kJ K−1 m−3. This value is larger than those of Er3Ni used commercially as a magnetic regenerator for the Gifford–McMahon refrigerator. The present results indicate that ErN is a promising material for the magnetic refrigerant and regenerator of cooling systems working above the boiling temperature of helium.

Nb-doped YB 66 single crystals were grown by the floating-zone method using a 4 ellipsoidal mirror-type xenon lamp image furnace. Nb-doping is expected to improve the performance of YB 66 soft X-ray monochromators. Maximum site occupancy of Nb achieved was 95%. Full-width at half-maximum of the rocking curve measured for CuK α 10 0 0 reflection achieved 40–50 arcsec. Thermal conductivity of the crystal with the maximum site occupancy increased about two times that of undoped YB 66 crystals.

Superconducting shielding is a powerful technique for shielding detectors. However, it is sensitive to details of geometry. This paper uses the geometry of the shielding of the submillimeter and far infrared experiment (SAFIRE) as a basis for study. Shielding tests are performed on three different geometries of niobium - disk, ring and tube. Attempts to model the superconducting shielding are also made and compared with measurements.

Magnetic material with first order phase transition has been studied from the view point of magnetic refrigerant. Gd/sub 5/(Si/sub x/Ge/sub 1-x/)/sub 4/ has field induced metamagnetic transition, whose temperature can be varied by changing Si concentration. It was shown that suitable transition temperature for hydrogen magnetic refrigeration can be obtained. Magnetization was measured as functions of temperature (M-T) and magnetic field (M-H). Magnetic entropy change has been calculated from the magnetization of M-T and M-H using Maxwell relations. This compound showed the hysteresis in M-H so that we obtained magnetic entropy changes in both the magnetization and demagnetization process. The magnitude of entropy change agreed with each other but the temperature range of large entropy change had a small difference. Entropy changes of this compound obtained from M-T and M-H were in reasonable agreement. The entropy change of the compound with x 0.25.

It is well know that some of rare earth compounds that show giant magnetocaloric effect are utilized for adiabatic magnetic refrigeration (ADR) at cryogenic temperatures. Recent development of materials science, however, suggested practical possibility of magnetic refrigeration at higher temperatures using Active Magnetic Regenerative Refrigeration (AMRR). A project WE-NET plans to make use of AMRR to liquefy hydrogen from room temperature in hydrogen plants for upcoming hydrogen energy society. Higher efficient liquefaction requires magnetic materials with higher magnetocaloric effect, higher thermal conductivity and lower thermal expansion. This paper surveys thermal properties of some of promising rare earth compounds and its magnetic dependence for deep insight in its nature as well as for contribution to a database for designing and development of the practical AMRR for hydrogen liquefaction.

The single crystal of isotopically enriched silicon ( 28 Si) has been made by FZ method with 〈111〉 growth direction. The specific heat measurement at low temperature was performed at the temperature range of 1–140 K. The low-temperature specific heat of 28 Si diverges from Debye T 3 behavior from 14.5 K, and this is in contrast to natural silicon, where deviations from the T 3 behavior occur already at much lower temperature (7.2 K). In addition, the specific heat of 28 Si becomes obviously higher than that of natural silicon at T >20 K.

This paper focuses on the second stage regenerator of a 4 K Gifford-McMahon (G-M) cryocooler. A three-layer layout of lead (Pb), HoCu2 and Gd2O2S spheres in the second stage regenerator derives a good performance at 4 K. After some modifications, we confirmed that the cooling power of 1.60 W at 4.2 K was achieved by using this three-layer layout. A two-stage G-M cryocooler is RDK-408D2 (SHI) and a compressor is C300G (SUZUKISHOKAN) with a rated electric input power of 7.3 kW at 60 Hz. In order to further improve, a double pipe regenerator was applied to the second stage regenerator. As a double pipe, a stainless steel pipe with thin wall was inserted in the coaxial direction into the second stage regenerator. The helium flow in the second stage regenerator is expected to be non-uniform flow because of the distribution of helium density and the imperfect packing of regenerator material. The double pipe regenerator is considered to have an effect of restraining the non-uniform flow. From the experimental results, the second stage cooling power of 1.67 W at 4.2 K and the first stage cooling power of 64.9 W at 50 K were achieved.

The magnetocaloric effect of superparamagnetic nanocomposites was studied by evaluating their magnetic entropy changes ΔS from magnetization data sets. Influence of distribution of the magnetic moment size on ΔS was evaluated. It is pointed out that a narrow size distribution is favorable for high performance of the magnetocaloric effect.

Magnetic materials play a significant role in improvement of regenerative cryocooler performance, because they have high volumetric specific heat at magnetic transition temperatures. Gadolinium oxysulfide (Gd2O2S, GOS) that has an antiferromagnetic transition at 5 K improved the cooling performance of cryocoolers when it was used in colder side of the second stage regenerator operating below 10 K. Small magnetic susceptibility and specific heat insensitive to magnetic field is important in order to reduce influence of magnetic field on the performance of cryocooler. We measured magnetization and specific heat of ceramic GOS in magnetic field up to 5 T. The magnetization of GOS represented typical temperature dependence for antiferromagnetic materials and no metamagnetic transition was observed. As for specific heat of GOS, peak temperature decreased from 5.5 to 5.0 K with increasing magnetic field from 0 to 5 T and the transitions remained sharp in magnetic fields. Thermal conductivity of GOS was observed to have very small magnetic field dependence.

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.

The measurement of specific heat at low temperature for isotopically modified boron single crystals was performed between 0.5 and 100 K using a calorimetric technique known as the time-constant or relaxation method. The results indicate that the specific heat has obvious divergencies at 1.3 K

Magnetic regenerator materials, such as HoCu2, Er3Ni, Gd2O2S, and so on, are always needed to produce the cooling power at the temperature of 4.2 K using regenerative cryocoolers. The use of a large quantity of magnetic regenerator materials affects directly the fabrication cost of 4K cryocoolers. From this point of view, we introduce a magnetic regenerator material economizing method in this paper. An innovative regenerator structure was adopted for two-stage Gifford-McMahon (GM) cryocooler. The structure has a bakelite rod inserted in the co-axial layout of the 2nd stage regenerator in which HoCu2 spheres are filled. The cold temperature side of the 2nd stage regenerator is occupied with 50% by volume of the bakelite rod and HoCu2 spheres, and the warm side is packed with lead (Pb) spheres as a single layer. The bakelite rod is a dummy volume. Two types of conventional GM cryocoolers with power consumption of 1.3 and 7.3 kW, respectively, are tested. The experimental results show that the filling volume...

Supercooling behavior of BaB 2 O 4 substituted with M (M:Al or Ga) for B was investigated. Moreover, single crystals were grown by the modified Czochralski method. Since Al and Ga are glass forming elements for the borates such as BaB 2 O 4 , large supercooling was obtained by the substitution of M for B. By controlling the M-amount single crystals of the low-temperature phase were grown, preventing glass formation. The merits of the substitution are the suppression of the solid-state phase-transformation during growth and cooling, and the crystal hardening.

Recent progress in magnetic refrigeration has been reviewed. Adiabatic demagnetization refrigerators (ADR) are under development at NASA for large deployable reflectors and X-ray spectrometers. The refrigerators provide temperatures of 2K at 0.027W for 165s and 0.1K at 0.01W for 13h using gadolinium gallium garnet and paramagnetic salt crystals. The Gas-Gap heat switch is used for the isothermal portion of a Carnot cycle. Active magnetic regenerator (AMR) cycles have the advantage of the Carnot cycle for larger temperature span and refrigeration capacity, in particular, above 20K. A test apparatus of the AMR demonstrated that the refrigeration capacity of 1.5-3.5W was obtainable at the temperature spans of 13-25K with a Carnot efficiency of 42%. Practical applications of magnetic refrigeration are also described.

Single crystals of the low-temperature (β) phase of BaB2O4 have been grown by the Czochralski method using a Pt tube as a seed. A β-phase polycrystal was formed inside the Pt tube at first, but a single crystal formed subsequently. After the growth in the Pt tube, a single-crystal boule could be grown continuously by the usual Czochralski method. In addition, a single crystal could be grown along a particular crystallographic direction when a β-phase single-crystal seed was mounted inside the Pt tube and the β-α phase transformation was prevented before seeding.

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.

Examining a Bi2212 superconductor for useful applications, the rapid decline of critical current (I/sub c/) with an increase in temperature prevents the high-temperature operation for a superconducting magnet when using a Bi2212 cable. In this study, we found a phenomenon that indicates the rapid decline of I/sub c/ when using a Bi2212 single crystal. Thermoremanent magnetization shows an anomalous dispersive behavior in AC susceptibility when kept below 20 K. The analysis of decay in DC magnetization, based on weak collective pinning and vortex glass theory within the single-vortex pinning regime, describes a drastic change in the activation energy of vortex pinning. To explain these peculiar characteristics, a model that originates in variable range hopping while in the metastable state is suggested.

We have built active magnetic regenerator (AMR) test apparatuses operated with a gas displacer to transfer the heat from magnetic material unit (AMR bed). Because finding an optimum parameter by experiment is not easy, numerical simulation is necessary to confirm the experimental conditions. As the first step of the project, we developed a 1-dimensional porous media model for hydrogen magnetic refrigerator with a Brayton-likeoperation cycle. This model has been calculated separately for heat exchange fluid and magnetic material. The results using two different magnetic materials have been compared.We confirmed that the simulation results agreed with experimental data of the internal gas displacer system.

Magnetic refrigeration technology needs further development not just by the improvement of magnetocaloric properties but also the optimization of the cooling system design. One of the important problems in the cooling system design is the geometry of regenerator for the efficient heat transfer between magnetic material and fluid which is the major loss mechanism in cooling system. Two kinds of regenerators are widely used. One is flat plate regenerator which can offer the best heat transfer to pressure drop ratio [2] for common regenerator design; another is porous media regenerator which can obtain a large temperature span for the good heat transfer surface. But until now, only a few research papers actually study the regenerator geometry. This paper focuses on the influence of regenerator geometry to the performance of AMR system. The 1 dimension flat plat model and porous media model have been constructed and compared with entropy generation, cooling capacity, coefficient of performance by changing plate thickness and sphere size at frequency 0.25Hz, 0.5Hz, 1, aspect ratio 2, 7, 14. The result shows that the optimized sphere size will be around 0.2mm to 0.3mm. On the other hand, 0.1mm to 0.2mm thickness plate will be more efficient. Compared the 2 models, flat plate model can get a smaller entropy generation and achieve a higher cooling capacity.