Toshimi Satoh

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.

Development of 1.5W 4K G-M Cryocooler with Magnetic Regenerator Material

A two-stage 4K Gifford-McMahon (G-M) cycle cryocooler with magnetic regenerator material which has cooling capacity 1.5W at 4.2K has been developed. The hybrid structural second regenerator composed of lead and ErNi0.9Co0.1 was used in the cryocooler. ErNi0.9Co0.1 has a large specific heat peak at lower temperature than 10K and lead has a larger specific heat in the. higher temperature region. The intake/exhaust valve timing was optimized to improve the cooling capacity not only of the second stage but also of the first stage. A larger size second cylinder in diameter than the former one was used to get a larger pressure-volume (PV) work.

Temperature Stabilization on Cold Stage of 4 K G-M Cryocooler

A simple method of temperature stabilization on cold stage of 4 K Gifford-McMahon (GM) cryocooler has been proposed. A copper pot connected to compressor unit by a stainless steel capillary tube is mounted on the 4 K stage of a GM cryocooler. Depended on the temperature of pot, pressurized helium can naturally go into or out of the pot. The utilization of high volumetric specific heat of pressurized helium in the pot at 4 K region produces a good temperature stability on the 4 K stage. The periodic temperature fluctuation of the 4 K stage is ∼ 0.5 K (peak-to- peak) typically, but the high pressure pot described in the present paper reduces the temperature fluctuation effectively down to ∼ 0.05 K (peak-to-peak). The method is safe to deal with, and the system is easy to operate. This paper shows the experimental details, and discusses the temperature stabilization effect of the pot and other advantages of the method.

A Gifford-McMahon Cycle Cryocooler below 2K

According to theory, a Gifford-McMahon (GM) cycle cryocooler with 4He cannot cool below 2 K because of the 4He superfluid transition near this temperature. However replacing 4He by 3He removes this temperature limitation. The cooling performance of a GM cryocooler with a HoCu 2 magnetic regenerator material is investigated using 3He. The minimum temperature of 2.3 K with 4He goes down to 1.65 K when the 4He working fluid is replaced by 3He. The maximum cooling capacity at 2 K is 53.9 mW with a compressor power of about 2.5 kW, and the cooling capacity at 4.2 K is enhanced by more than 20%.

Improvement of Two-Stage GM Refrigerator Performance Using a Hybrid Regenerator

To improve the performance of two-stage GM refrigerators, a hybrid regenerator with magnetic materials of Er3Ni and ErNi0.9Co0.1 was used in the 2nd stage regenerator because of its large heat exchange capacity. The largest refrigeration capacity achieved with the hybrid regenerator was 0.95W at helium liquefied temperature of 4.2K. This capacity is 15.9% greater than the 0.82W refrigerator with only Er3Ni as the 2nd regenerator material. Use of the hybrid regenerator not only increases the refrigeration capacity at 4.2K, but also allows the 4K GM refrigerator to be used with large 1st stage refrigeration capacity, thus making it more practical.

INFLUENCE OF VALVE OPEN TIMING AND INTERVAL ON PERFORMANCE OF 4 K GIFFORD-McMAHON CYCLE CRYOCOOLER

The influence of intake/exhaust valve timing on performance of a 4 K Gifford-McMahon(GM) cycle cryocooler has been investigated. The 4 K GM cryocooler is one of the standard two-stage types with magnetic regenerator material, ErNi0.9Co0.1, in its second stage regenerator, and delivers more than 1 W cooling at 4.2 K. A reasonably early open timing of intake/exhaust valves not only brings about a cooling capacity above 1 W at 4.2 K on the second stage, but also produces a much larger cooling capacity at the first stage. Under the optimum conditions, 39.4 W cooling at 40 K on the first stage, as well as 1.18 W at 4.2 K on the second stage, was obtained with an input electric power of 7.05 kW. This paper discusses the results of cooling capacity with P-V diagrams, and indicates that the reduction in pressure drop at intake/exhaust valves is important for a 4 K GM cryocooler.

Cooling performance of ceramic magnetic regenerator material used in a GM cryocooler

A ceramic magnetic regenerator material GAP (GdAlO3) has been developed for cryocoolers below 4 K region. A new fabrication method has achieved Vickers hardness of ∼900, relative density of 99.5% to that of the single crystal, and smooth surface on the spherical particles in diameters between 133 μm and 500 μm. The cooling tests with a GM cycle cryocooler have been done for 4He and 3He as working fluids individually. The 2nd regenerator constitution was adjusted by partially replacing the HoCu2 with the GAP. The cooling capacity increases up to 30% around 3.4 K and the minimum temperature reached 2.2 K for the 4He test. 3He operation with the GM cycle was investigated to provide temperatures below 2 K without the superfluid transition appearing with 4He. The test results showed a remarkable improvement on the cooling characteristics; the cooling capacity increased 1.8 times at 2.0 K and 2.4 times at 1.8 K, compared to those without the GAP. The minimum temperature without heat load was lowered from 1.67 K...

Heat capacity of ternary compounds RNiGe(R=Gd, Dy, Er, and Y)

The heat capacity of the ternary compounds RNiGe(R=Er, Dy, Gd, and Y) was studied in the temperature range between 2 and 40 K. From the heat capacity curves, Neel temperatures are found to be 3.3, 7.4, and 10.7 K for ErNiGe, DyNiGe, and GdNiGe, respectively. The heat capacity of YNiGe was well fitted with an average Debye temperature of 272 K in the temperature range between 2 and 40 K. The changes of magnetic entropy indicate a strong influence of the crystal field on the Er+3 and Dy+3 ion in ErNiGe and DyNiGe compounds. These results suggest the possibility of using these compounds as magnetic regenerator materials near the temperature of 4.2 K.

Status of the Development of Ceramic Regenerator Materials

This paper describes the status of recent developments of ceramic regenerator materials. First, improvement of the thermal and mechanical properties of Gd2O2S (GOS) regenerator material has been achieved. Since the heat capacity of GOS is very high (> 1 J/cm3), with a γ type peak at 5.2 K, the thermal penetration depth is about 30% smaller than that of HoCu2 below 6 K. Thus, a small particle size (less than 0.3 mm) with a high homogeneity is important for optimization of cooling efficiency. The recent introduction of Hot Isostatic Pressing (HIP) for fabricating GOS particles has been shown to be very effective in increasing the quality. Higher thermal conductivity and a withstanding pressure higher than 5 MPa have been achieved.

A Simple Method of Temperature Stabilization for 4 K GM Cryocooler

Publisher Summary This chapter develops a simple method of temperature stabilization for 4 K GM cryocooler. It applies the high volumetric specific heat of pressurized helium at 4 K region to reduce the temperature fluctuation of the 4 K stage. It describes the experimental details and the results. In a proposed simple method of temperature stabilization for 4 K G M cryocooler a copper pot connected to compressor unit by a capillary tube is mounted on the 4 K stage of a GM cryocooler. The utilization of high volumetric specific heat of pressurized helium in the pot at 4 K region produces good temperature stability on the 4 K stage. The periodic temperature fluctuation of the 4 K stage is, 43.5 K (peak-to-peak) typically, but the helium pot reduces the temperature fluctuation effectively down to, 43.05 K (peak-to peak). The system can be operated continuously, and is easy to cool down, furthermore it is safe to use.