We investigated the effects of adding indium, lead or tin to SmFeAsO1-xFx. It is found that these metal additions to polycrystalline SmFeAsO1-xFx remove an amorphous FeAs phase, which is formed between superconducting grains, and induces clustering of the superconducting grains. Interestingly, addition of each metal showed different effects on the superconductivity. Indium addition increases the magnetic critical current density. Lead addition effectively introduces a large amount of fluorine to SmFeAsO1-xFx. Tin addition enhances coupling between the superconducting grains.
High-temperature superconducting (HTS) power devices such as power cables, transformers, current limiters, and superconducting magnetic energy storage (SMES) are approaching the stage where they can be adopted for practical use. A cooling system requires cooling capacity of 2 to 10 kW at 65 K and high COP, and is also required to be maintenance-free and compact. However there are no adequate cooling systems among commercial systems. Therefore, we are developing a turbo-Brayton cycle refrigerator using neon gas as the working fluid with cooling capacity of 2 kW at 65 K. Details of the turbo-Brayton cycle refrigerator are described in this paper.
A novel regenerator structure for a two-stage Gifford-McMahon (GM) cryocooler is introduced in this paper. The structure has a bakelite rod inserted in the co-axial layout of the 2nd stage regenerator in which lead (Pb) spheres are filled as the regenerator material. Inserting the bakelite rod reduces the Pb weight, leading to a decrease in the heat capacity of the regenerator. To clarify the effect of the bakelite rod, two types of conventional GM cryocoolers with power consumption of 1.3 and 7.3 kW, respectively, are tested at the operating frequency of 1.2 Hz. From the experimental results, the lowest attainable temperature of the 2nd stage rises slightly as a result of reducing the Pb weight which is adjusted by the volume of bakelite rod. However, a reduction of Pb weight by approximately 40% does not affect the deterioration of the cooling power at above 9 K. These two GM cryocooler models show almost the same characteristics. Thus, this novel regenerator structure may be applied to most two-stage GM cryocoolers.
For the purpose of cooling high-temperature superconductor (HTS) devices, superconducting motors, SMES and fault current limiters, a large cooling capacity Stirling pulse tube cryocooler (SPTC) has been developed by Sumitomo Heavy Industries, Ltd. (SHI). The design concepts of the cryocooler are high efficiency, compact size, light weight and high reliability. According to the experiment results, with an in-line expander, a cooling capacity of 210 W at 77 K, and a minimum temperature of 37 K have been achieved. The compressor is operated at 49 Hz and the input power is approximately 3.8 kW when a heat load of 210 W is added to the cold-end. The compressor efficiency is approximately 74% and COP is approximately 0.055. However, a cryocooler with a COP of 0.1 is very important to realize the widespread use of HTS devices worldwide. In order to further improve the cooling efficiency, we study the energy flow and losses in the SPTC. The experiment results and the theoretical analysis are reported in this paper.
Sumitomo Heavy Industries, Ltd. (SHI) has been continuously improving the efficiency and reducing the vibration of a 4K pulse tube cryocooler. Recently, the efficiency of a 4K pulse tube cryocooler has been competitive with a 4K GiffordMcMahon (GM) cryocooler. Owing to its low vibration and high reliability, pulse tube cryocooler has been widely used for cooling superconducting magnets and pre-cooling ultra-low temperature cryocoolers. In many cases, pulse tube cryocoolers are used to directly recondense helium. In a helium atmosphere, the performance degradation of a pulse tube cryocooler is larger than that of a GM cryocooler because of extra convection losses, which are caused by the temperature difference between the pulse tube and regenerator tube. In order to reduce this degradation, the temperature profiles are shifted with several novel concepts. Firstly, the configuration of the holes, which are drilled on the second stage heat station to increase the recondensing surface area, is investigated. The performance degradation is reduced by 0.04 W by replacing the holes from go-through holes to one-end-close holes. Then, a spacer is inserted at the cold end of the second stage regenerator to shift the temperature profile. Accordingly, after these improvements, the temperature difference between the second stage pulse tube and the regenerator tube is reduced and the performance degradation is reduced from 0.39 to 0.24 W, which is almost the same as that of a GM cryocooler.
It is thought that the temperature range for operation of high-temperature superconducting (HTS) magnets is 20 to 50 K if the HTS magnets are made from rare-earth materials. An HTS wire made from rare-earth materials has several advantages in the high-temperature region close to 50 K. In this temperature region, single-stage cryocoolers are available. Therefore, we examined via tests the practicability of applying a pulse-tube cryocooler, which has advantages of simple structure, high reliability and simple maintenance, as cooling systems of on-board HTS magnets for the magnetic levitation train (Maglev). We have developed a high-efficiency cooling system that consists of multiple pulse-tubes and one compressor. When the two pulsetubes are operated in “reverse phase mode,” the cooling system has a large COP value. In this paper, we improved the performance of the multiple pulse-tubes system, and obtained a cooling capacity of 170 W at a temperature of 50 K, and a COP of 0.023.