Recent advancement in materials processing enabled us to grow large single-grain bulk RE-Ba-Cu-O superconductors (RE: rare earth elements) with high critical current densities. These superconductors can exhibit a large electromagnetic force with the interaction of external magnetic fields. Various devices have been developed by utilizing such a force: magnetic bearings, flywheels for energy storage, load transport, hysteresis motors, and several levitation devices. A large magnetic field can also be trapped by bulk superconductors, which can function as a quasipermanent magnet. Trapped field values have already reached 10T, thus leading to many novel applications of high trapped field magnets. The final target will be a second-generation Maglev train.
Resin cracking, a cause of coil quench in superconducting magnets, occurs when a resin contains small flaws and sustains high thermal stress caused by coil restraint during cooling. Seven epoxy resins were chosen to evaluate thermomechanical properties, small flaw strength, and creep deformation at low temperatures. Although the plain specimen strengths consistently increased as the temperature decreased, the fracture toughness values resulting from large cracks reached maximum values at about 80K, then decreased to the lower values at 4K. The loss factor during cyclic loading behaved similarly because of low-temperature relaxation of the resin, having a maximum value at about 150K. The strengths corresponding to small flaws leveled off to those of plain specimens; i.e., they deviated from linear fracture mechanics. Thermal stress in the epoxy resin caused by coil restraint was experimentally measured by simulating the coil-molding process. These stresses were close to the calculated ones obtained by using the elastic moduli and the coefficients of linear thermal expansion of the resin. However, they were slightly lower because of stress relaxation at about the glass transition temperature. By using the thermal stress values and the small flaw strengths at 4K, we could calculate the allowable flaw sizes and the critical ratios of resin volume to conductor volume that produce coil quench. These values varied greatly among the resins. Even at as low as 77K, creep deformation was significant, which may possibly cause a cryogenic delayed fracture of the resin and result in an unexpected coil quench during a steady-state persistent current operation. The ratios of the creep strain to the initial strain decreased uniformly for all resins as the difference between glass transition temperature and the test temperature increased.
The thermoacoustic theory is applied to discussions on cooling power of a regenerator with narrow channels (ωτa<<1). Heat flux in the regenerator is shown to be a quadratic equation of the operating frequency ω, and there is a characteristic frequency ωQ. If ωQ is independent of ω, the cooling power is also a quadratic equation of ω, and it shows maximum at ω=ωQ. The cooling power depends on the temperature of the cold end partly through ωQ, which depends on material constants of the working fluid, the oscillating amplitudes, and the phase difference between pressure and displacement of the fluid. Discussions on ωQ and the maximum cooling power give a qualitative explanation of recent experimental results on 4K G-M cryocoolers only if ωQ is identified with experimentally obtained operating frequency corresponding to a maximum cooling power of 4K G-M cryocoolers.