Takenori Numazawa

Performance improvement of a pulse tube cooler below 4 K by use of GdAlO3 regenerator material

Abstract 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.

Magnetic refrigerator for hydrogen liquefaction

Magnetic refrigeration which is based on the magnetocaloric effect of solids has the potential to achieve high thermal efficiency for hydrogen liquefaction. We have been developing a magnetic refrigerator for hydrogen liquefaction which cools down hydrogen gas from liquid natural gas temperature and liquefies at 20 K. The magnetic liquefaction system consists of two magnetic refrigerators: Carnot magnetic refrigerator (CMR) and active magnetic regenerator (AMR) device. CMR with Carnot cycle succeeded in liquefying hydrogen at 20K. Above liquefaction temperature, a regenerative refrigeration cycle should be necessary to precool hydrogen gas, because adiabatic temperature change of magnetic material is reduced due to a large lattice specific heat of magnetic materials. We have tested an AMR device as the precooling stage. It was confirmed for the first time that AMR cycle worked around 20 K.

The Helium Magnetic Refrigerator I: Development and Experimental Results

Development of a small helium refrigerator with high reliability and high efficiency is needed for progress in superconductive device applications.

Magneto caloric effect in (DyxGd1−x)3Ga5O12 for adiabatic demagnetization refrigeration

Abstract Gadolinium and dysprosium gallium garnet single crystals ( Dy x Gd 1−x ) 3 Ga 5 O 12 (x=0,0.5 and 1) have been investigated for adiabatic demagnetization refrigeration used as magnetic materials between 0.5 and 5 K . Specific heat measurement of (Dy 0.5 Gd 0.5 ) 3 Ga 5 O 12 showed a large and broad peak similar to that of Gd 3 Ga 5 O 12 and it suggests that the geometrical frustration in Gd 3 Ga 5 O 12 still remains in (Dy 0.5 Gd 0.5 ) 3 Ga 5 O 12 . Magneto caloric effect of (Dy 0.5 Gd 0.5 ) 3 Ga 5 O 12 was about four times larger than that of Gd 3 Ga 5 O 12 for the magnetic field of 1 T between 0.5 and 5 K . Therefore, the magnetic entropy change of Gd 3 Ga 5 O 12 by the external magnetic fields could be enhanced by substituting Dy 3+ ion for Gd 3+ ion for the magnetic fields of T between 0.5 and 5 K .

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.

The Helium Magnetic Refrigerator II: Liquefaction Process and Efficiency

Magnetic refrigeration makes use of the magneto-caloric effect in which some magnetic materials absorb or expel heat according to the entropy change in the applied magnetic field. The magnetic refrigerator is composed of two parts; the magnetic materials and the heat exchange system.

A New Ceramic Magnetic Regenerator Material for 4 K Cryocoolers

A new class of magnetic regenerator materials for 4 K cryocoolers has been developed. A hexagonal gadolinium oxisulfide Gd2O2S=GO8 has been found to provide a very high volumetric heat capacity of more than 1 J/cm3K at 5.2 K. Polycrystal ceramic GOS particles have been fabricated with diameters between 0.35 mm and 0.45 mm. The achieved smooth surface on the spherical particles and a Vickers hardness of ∼900 make the material very suitable for use in regenerators.

Properties of Czochralski‐grown RAIO3 (R: Dy, Ho, and Er) single crystals for magnetic refrigeration

Magnetization of RAlO3 (R: Dy, Ho, and Er) single crystals along the c axis have been measured by the superconducting quantum interference device magnetometer. The single crystals are nominally pure and large sized, grown by the Czochralski method. Paramagnetic properties depend on the rare‐earth elements. According to the magnetic entropy change, ErAlO3 single crystals are promising materials for the magnetic refrigerants using the Carnot cycle in the temperature range below 20 K.

Carnot magnetic refrigerator operating between 1.4 and 10 K

Abstract A magnetic refrigerator operating directly from 10 to 1.4 K has been constructed and tested. The refrigerator is a static system which operates following the Carnot cycle using a pulsed magnet and thermal switches. Typical experimental results were 550 mW at 4.5 K using a Dy 3 Al 5 O 12 refrigerant and 100 mW at 1.8 K using Gd 3 Ga 5 O 12 . Without a cooling load, the lowest temperature obtainable was 1.36 K. The Carnot efficiencies were 30% at 2.0 K and 38% at 4.4 K. A cooling test for a superconducting magnet was also performed.

Specific heat and thermal conductivity of HoN and ErN at cryogenic temperatures

The rare earth nitrides, HoN and ErN, were synthesized by the hot isostatic pressing method. Their specific heat CH(T) and the thermal conductivity κ were measured at cryogenic temperatures. In zero field, the peak values of the C0(T) of HoN and ErN are larger than those of the magnetic regenerators such as Er3Ni. The peak values of the adiabatic temperature change ΔT(T) of HoN and ErN showed similar or larger values compared with those of the candidate materials for the magnetic refrigerants such as ErAl2. The thermal conductivity of HoN and ErN are comparable to those of the magnetic regenerators such as Er3Ni. The present results indicate that HoN and ErN are promising materials as the magnetic refrigerant and regenerator for the cryogenic refrigeration system.