Part IV of this study covers the additional explanations of Stirling coolers following the part III of the series. In the first section, the method for evaluating characteristics in the case of non-isothermal model is examined. The effect of imperfect heat transfer at the cylinder wall is discussed using an example calculation. In the second section, a linear driving method for the compressor piston and an expander piston are introduced. The concept of a gas spring constant is introduced. In the case of the mechanical spring supported displacer, the operating mode of the Stirling cooler and Stirling engine is discussed from the view point of the resonant frequency of the displacer.
It is known that cracks are formed when bulk superconductors are subjected to the oxygen annealing process, and that cracking is easier in larger bulk materials. Commercial bulk superconductors are subjected to oxygen annealing in a large block state, while the mechanical properties of the superconductors are often evaluated using specimens that are cut from a large pellet and then subjected to the oxygen annealing process. Therefore, it is suspected that there may be some discrepancies in the mechanical properties of these superconducting materials. We conducted bending and tensile tests on various specimen sizes after oxygen annealing. Observations of cracks formed during the oxygen annealing process were made. The cracking behavior was analyzed using a fracture toughness test.