Hideki Yayama

New application of complex magnetic materials to the magnetic refrigerant in an Ericsson magnetic refrigerator

A complex new magnetic refrigerant, suitable for the ideal Ericsson cycle, has been investigated. Above ∼15 K it is necessary to use ferromagnets as a magnetic refrigerant. However, temperature variation for the magnetic entropy change in a homogeneous ferromagnet is not suitable for the Ericsson cycle. The present paper verifies, from theoretical analysis, that a complex ferromagnetic material, for instance, (ErAl2)0.312(HoAl2)0.198 (Ho0.5Dy0.5Al2)0.490, has the most suitable characteristics for the ideal Ericsson cycle, including two kinds of isomagnetic field processes. On the basis of the above consideration, a sintered layer structural complex has been prepared, composed of three kinds of RAl2.15 layers, where R’s are rare‐earth atoms. From specific heat measurements made on this complex, its entropy and entropy change have been determined. It has been concluded that the complex magnetic material is the most hopeful refrigerant for the Ericsson cycle.

Pressure-Induced Valence Transition in Antiferromagnet EuRh2Si2

Considering the unique properties of EuRh 2 Si 2 from the viewpoint of the Eu valence, we have examined its physical properties under external pressure. At ambient pressure, EuRh 2 Si 2 is an antiferromagnet with a Neel temperature T N of 25 K, and the Eu ion is in the divalent state. The application of pressure up to 0.84 GPa slightly shifts T N toward higher values. Under pressures higher than 1.00 GPa, an abrupt first-order valence transition emerges simultaneously with the disappearance of antiferromagnetism. For P =1.17 GPa, the valence change associated with valence transition is roughly estimated to be ∼0.19 from the thermal expansion anomaly. The valence transition temperature T v increases rapidly with increasing pressure. The temperature–pressure phase diagram of EuRh 2 Si 2 is very similar to those of the other systems showing pressure-induced valence transition.

Electrode Resistance of Metal Hydride in Alkaline Aqueous Solution

The electrode resistance of metal hydride in constant-current electrolysis is investigated. It is found that the diffusion of hydrogen in the electrode is the rate-determining step in the reaction and is the dominant cause of the electrode resistance. The activity of the electrode, corresponding to the reaction rate, is determined by the effective surface area and the diffusion coefficient of hydrogen in the bulk of the electrode. Some parameters concerning the electrode kinetics are estimated. TiMn1.5 is an excellent electrode material for a secondary battery with high current and energy densities.

INVESTIGATIONS ON THE POSSIBILITY OF THE RAl//2 SYSTEM AS A REFRIGERANT IN AN ERICSSON TYPE MAGNETIC REFRIGERATOR.

We investigated the Ericsson type magnetic refrigerators in the range below 77 K. This is the first report of experimental results of the refrigeration character, especially the magnetocaloric character of RAl2, where R is a rare earth atom.

Specific heat and entropy of dysprosium gallium garnet in magnetic fields

Abstract The specific heat of dysprosium gallium garnet (DyGaG) single crystal has been measured between 20 and 2 K in magnetic fields up to 5 T. From the specific heat and adiabatic demagnetization measurements, the temperature dependence of the entropy in various magnetic fields is determined. It is shown that DyGaG is a good material for magnetic refrigeration between 12 and 2 K.

Equilibrium Potential and Exchange Current Density of Metal Hydride Electrode

The equilibrium potential and exchange current density were measured as a function of the hydrogen content in a TiMn1.5Hx (x<0.31) electrode. The reaction order of hydrogen in the electrode is estimated as 0.67 and the activity coefficient of hydrogen is found to be unity. The influence of the absorbed hydrogen on the equilibrium potential is discussed. It is shown that TiMn1.5Hx has low power drop during discharge and is a promising electrode material for use in hydrogen cells and other devices.

Charge density wave and superconductivity of RPt2Si2 (R = Y, La, Nd, and Lu)

The temperature dependence of electrical resistivity and magnetic susceptibility was studied for RPt2Si2 compounds with R = Y, La, Nd, and Lu. LaPt2Si2 and NdPt2Si2 show steps in the resistivity vs temperature curve at \(T^{*} = 112\) and 77 K, respectively. We performed X-ray diffraction and selected-area electron diffraction (SAED) measurements for LaPt2Si2 at low temperatures. It was found that LaPt2Si2 undergoes a structural transition from tetragonal to orthorhombic at \(T^{*}\). Moreover, \((n/3, 0, 0)\) superlattice reflections with \(n = 1\) and 2 were observed at 10 K in the SAED patterns. These results strongly suggest the formation of a charge density wave (CDW) in LaPt2Si2 below \(T^{*}\). On the other hand, the compounds R = Y, La, and Lu were found to show superconductivity below 1.5–1.9 K. The origin of a CDW and its coexistence with superconductivity in RPt2Si2 are discussed.

Recent Progress in Magnetic Refrigeration Studies

After the 1985 Cryogenic Engineering Conference, two directions for the fundamental investigations on the magnetic refrigeration to expand the refrigeration range above ~15 K have been developed by our group. One is the improvement of the refrigeration characteristics able to refrigerate from ~20 K for the Carnot magnetic refrigerator and the other is the fundamental study of the Ericsson magnetic refrigerator. As for the former purpose, we used a new magnetic material, Dy3A15012, as the refrigerant in a reciprocating Carnot magnetic refrigerator instead of Gd3Ga5012. Consequentially, we succeeded in expanding the refrigeration range. As for the latter, we have established the method to make the refrigerant suitable for the ideal Ericsson cycle including two kinds of iso-magnetic field processes. Now, the investigation of the Ericsson magnetic refrigeration cycle and refrigerator is starting.

Hybrid Cryogenic Refrigerator: Combination of Brayton Magnetic-Cooling and Gifford-McMahon Gas-Cooling System

We propose a new cryogenic refrigerator which is a hybrid of the Brayton magnetic-cooling cycle and the Gifford-McMahon (GM) gas-cooling cycle. We evaluate the refrigeration power of the refrigerator with ErNi regenerator material by a numerical simulation. The results show a remarkably high refrigeration power in contrast to the conventional simple GM gas-refrigerator.

A new method of producing the magnetic refrigerant suitable for the ericsson magnetic refrigeration

A layer structural complex magnetic refrigerant, which is suitable for the Ericsson cycle, satisfying the Carnot principle, has been investigated. Above ∼ 15 K, since the thermal agitation energy and the lattice entropy increase considerably, the ferromagnetic material has to be used for the refrigerant in the magnetic refrigeration. However, the variation of the magnetic entropy of the ferromagnets is not appropriate to the Ericsson cycle in which the entropy difference ΔS J between two kinds of the isomagnetic field processes should be constant in the refrigeration range. In order to make the refrigerant having constant ΔS J mentioned above, we made the layer structural sintered material composed of four kinds of ferromagnets, ErAl 2.2 , HoAl 2.2 , (Dy 0.5 Ho 0.5 )Al 2.2 and DyAl 2.2 , and investigated its entropy experimentally. As the result, it is verified that the layer structural magnetic material is one of the most promising refrigerant for the Ericsson cycle.