A magnetic refrigeration technology has been developed in all parts of the world. Altering the magnetic field around certain types of magnetic materials changes their temperature (magneto-caloric effect). We have conducted R&D to convert magnetic refrigeration technology, which uses this effect and is a highly-efficient, energy-conserving /environment-friendly technology, into practical application. In this special issue on magnetic cooling technology, the current state from system development concerning magnetic cooling technology to the magnetocaloric material development in Japan is introduced. In addition, the current state of the development of the Astronautics company who is the pioneer of magnetic refrigeration technology development is introduced.

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Magnetic refrigeration has been paid much attention with the view of alternating conventional gas-expansion cooling techniques which use some sort of harmful gases such as ozone-depleting (CFC;chlorofluocarbon), green-house (HCFC;hydrochlorofluocarbon, HFC; hydrofluocarbon) toxic (NH₃:ammonia) or flammable (CH₃CH₂CH₃) gases as refrigerant. A concept of the Active Magnetic Regenerator (AMR) cycle and use of magnetocaloric effect at the ferromagnetic transition bring magnetic refrigeration to a successful in room temperature range. We have developed a room temperature magnetic refrigeration apparatus with Halbach-type permanent magnets. The magnitude of magnetic field is 1.1 T at the center of the bore. We have obtained the lowest temperature of 10 degrees below zero, and the maximum temperature span DT_span of 44.2 K by operating AMR cycle using GdY spheres as the magnetic refrigerant. It needs breakthrough of magnetic field changing mechanism or advanced magnetocaloric materials or feasible high-frequency operation toward the practical use of magnetic refrigeration.

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Tokyo Institute of Technology and Chubu Electric Power Company have been developing a room-temperature magnetic refrigerator. In order to improve the performance of the magnetic refrigerator, mainly the following components have been modified; (1) magnets, (2) AMR (active magnetic regenerator) ducts, (3) a pump, (4) piping. Coefficient of performance (cooling power at 5 °C temperature span / power consumption of the system) of 2.5 was achieved after the modifications using gadolinium as a cooling source and 1.1 T permanent magnets.

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The purpose of this study is to understand the fundamental cooling characteristics of room temperature magnetic refrigerator using an active magnetic regenerator (AMR). The AMR is one of the regeneration cycles and has four sequential processes: adiabatic magnetization, heat exchange fluid flow, adiabatic demagnetization, and fluid flow. Gadolinium particles of 0.6mm diameter are adopted as a magnetic refrigerant and fully packed into a test section. The temperature change based on the magnetocaloric effect is distributed to the hot end and the cold end with heat transport fluid. The results reveal that the AMR cycle shows the improvement of the cooling performance of the room temperature magnetic refrigerator. In addition, it is found that the temperature difference between the hot and cold ends is strongly affected by the fluid transfer volume.

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Astronautics Corporation has designed, constructed, and extensively tested two generations of magnetic refrigerators employing a rotary magnet-fixed bed architecture. This paper reviews and summarizes the performance of these prototypes. In particular, the testing on the 1st-generation prototype demonstrates the significant performance advantage associated with the use of layered beds of LaFeSiH, a magnetocaloric material with a sharp, first-order transition and a readily adjustable Curie temperature. The 2nd-generation prototype exhibits peak performance of 2.4 kW of cooling power over a temperature span of 11 °C, corresponding to 168 W of specific exergetic cooling power. We believe that this is the highest performance yet reported for a magnetic refrigeration system. Moreover, the system performance is in good agreement with theoretical prediction.

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This study aims at practical use of a cooling technology using a magnetocaloric effect for the purpose of energy saving of air conditioners of railway. A method to cascade magnetocaloric materials having the different temperatures is suggested to obtain an operating temperature required by air conditioner because a work point of the magnetocaloric material is limited to temperature around the Curie point of the material. La(Fe_0.84Co_0.06Si_0.10)₁₃ has an operating temperature that is lower than Gd. Accordingly, a magnetocaloric refrigerator system which has a magnetocaloric material of La(Fe_0.84Co_0.06Si_0.10)₁₃ is reported in this paper.

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This report explains recent magnetic refrigeration technology in low temperatures. As the case study, continuous ADR (Adiabatic Demagnetization Refrigerator) and hydrogen magnetic refrigeration are shown here. Continuous ADR system has been developed to provide the temperatures lower than 0.1K continuously. This system operates the cascade Carnot cycle with very high efficiency of 90% Carnot. The hydrogen magnetic refrigeration consists of Carnot stage for liquefaction and AMR stages for pre-cooling the hydrogen gas. A model study covers from room temperature to liquid hydrogen temperature (20K) is described.

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The power consumption of an air conditioner in an electric vehicle has a considerable impact on the cruising range. A high-performance magnetic refrigerator is therefore one of the important technologies needed for future electric vehicles. The requisite conditions for an in-vehicle magnetic refrigerator are higher with respect to cooling power density, temperature difference between the hot and cold sides, transient properties, and coefficient of performance than those of household refrigerators or home air-conditioners. We have devised a new model without a refrigerant in which heat is transported from the cold to the hot side through well-controlled thermal switches. A test rig to verify a heat transfer mechanism of the model has been created. The heat transfer mechanism has been experimentally verified in addition to an investigation of the heat transport characteristics. Furthermore, the refrigerating capacities also have been clarified by numerical simulation under the conditions for an in-vehicle refrigerator.

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The Mn-based compounds with the Fe₂P-type structure undergo a first-order magnetic transition near room temperature. Since these compounds exhibit giant magnetocaloric effects, they are expected to be promising candidates for magnetic refrigerant materials. In this paper, we show our recent experimental data on the magnetocaloric properties of MnFeP_1-xSi_x. It is found that the refrigeration capacity, q, of MnFeP_1-xSi_x with the temperature span of 30 K is 1.5-1.9 times as large as that of the conventional magnetic refrigerant, Gd. In order to obtain higher cooling capacity, a composite material is proposed.

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Magnetic entropy is basically connected to freedom of magnetic moments or spins, and the discontinuity between the magnetically ordered and disordered states due to the first-order transition brings about large caloric changes. Meanwhile, the first-order transition in a real condition occurs within finite time, therefore, the transition rate is characterized by its kinetics. Especially, kinetics of the magnetic transition is affected by the magnetic dipole interaction. We discuss these issues with referring the data of La(Fe_xSi_1-x)₁₃ itinerant metamagnetic compounds.

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This paper presents a fully implantable electromyogram (EMG) sensing system for application with myoelectric prosthesis. The internal unit of the system, an EMG sensor consisting of recording electrodes, a bio-amplifier, a FM transmitter and a photodiode panel of six photovoltaic cells connected in series, is implanted subcutaneously and powered by transcutaneous near-infrared light irradiation. The external unit placed on the skin facing the internal unit has a near-infrared LED to illuminate the photodiode panel through the skin and a FM receiver to receive the EMG signals. The micro power circuit was designed for the implantable device. Internal units were implanted into rat thigh flexor muscle and examined. The system successfully recorded the EMG signals from the specific muscle of freely moving rats, and has been functioning normally in long-term recordings for more than twelve weeks. This system may offer a method to secure stable EMG signal sources for myoelectric prostheses by minimally invasive surgery.

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In this study, multiple cracks on a structural member were detected using the magnetic flux leakage method. The distribution of magnetic flux density was measured using a flux gate sensor. A function that relates the magnetic dipoles and the magnetic flux density measured using the flux gate sensor was improved to capture the average value of the sensor length. The effectiveness of this improvement was verified through a numerical model with two adjacent semi-elliptical surface cracks. Cracks were detected separately using inverse analysis. In contrast, cracks were detected as being connected using the unimproved function.

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Wireless power transfer is expected in the use of an electric vehicle. It requires a high efficiency and takes a long distance. In this paper, we propose the use of a litz magnetoplated wire (LMW), which is a copper wire (COW) whose circumference is plated with a magnetic thin film, to improve transmission efficiency. The LMW can reduce resistances due to the proximity effect comparison with the COW and Litz wire (LCW) using COW. The inner diameter of coils is d_i = 36 mm and their number of turns is n = 8. As a result, the resistances of the COW, LCW and LMW at the frequency f = 6.78 MHz are 1.36 Ω, 1.91 Ω and 0.77 Ω, respectively, which show a reduction of resistance of LMW. The efficiencies of the COW, LCW and LMW at a transmission distance of 18 mm are 85.2%, 79.8% and 89.9%, respectively, which show an increase of efficiency of LMW. The temperature rise values of COW, LCW and LMW at received power 5 W are 14 deg., 17 deg.and 10 deg.respectively, which show a decrease of temperature rise of LMW.

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