Part III of this study covers the basic characteristics of a Stirling cooler based on a simplified Stirling cycle. Two different types of Stirling coolers—one with a piston and displacer, and the other with two-pistons—are described by a simplified isothermal model using Schmidt equations. Examples of calculation results are given to explain the optimum phase difference between the piston movement and the pressure wave of the working gas. The regenerator, as one of the critical components of cryocoolers, is analyzed in detail. Numerical investigations have been conducted to study the regenerator efficiency in terms of the geometry, materials selection, pressure ratio, and the phase between mass flow and pressure in the regenerator. Improvement in regenerator performance is discussed as related to minimization of the enthalpy flow within the regenerator.
We performed tensile tests at low temperatures from room temperature to 4 K for SUS 304L and SUS 316L in order to obtain basic data on the mechanical properties of materials to be used in a liquid hydrogen tank service. First we studied tensile curves and specimen heating during tensile testing from 70 K to 4 K, and then evaluated the ferrite contents of those alloys during tensile deformation from room temperature to 4 K. For both SUS 304L and SUS 316L, tensile strength showed a small peak around 10 K, and specimen heating decreased above 30 K as the environment temperature increased. Below 77 K, there was almost no clear influence of testing temperature on strain-induced martensitic transformation. However, for SUS 304L, the amount of ferrite content increased as the temperature decreased down to 233 K, and increased only slightly below 233 K. For SUS 316L, it increased as the temperature decreased down to 4 K, and increased steeply after 60% plastic strain above 243 K.
In the production of melt-textured RE-Ba-Cu-O bulk superconductors, the precursors are generally prepared by uniaxial and/or cold-isostatic pressing of raw powders. The employment of fine starting powders is effective in fabricating high-performance RE-Ba-Cu-O bulk superconductors. However, cracks often form in the green bodies when fine powders are unaxially pressed. In this work, we have investigated the defects in green bodies during the powder compression process for bulk superconductor fabrication. Fine and large YBCO powders and granulates of binder-added fine powder were uniaxially pressed into tablets (20 mm in diameter) using different pressures from 31 to 125 MPa. When the fine powder was pressed at pressures above 62 MPa, several defects such as lamination, capping and cracks on the parallel in the direction of pressing were found in green bodies. These defects in green bodies partially remained after melt-processing and affected the field-trapping abilities of melt-textured bulk samples. The addition of an organic binder is very effective method for reducing defects in green bodies.