Recently, much attention has been given to magnetic refrigeration, because of its energy efficiency and environmental safety. The concept of magnetic refrigeration is based on the magnetocaloric effect (MCE); therefore the development of magnetic materials with a large MCE is strongly desired. In this article we first review the recent development of magnetic refrigerator at temperatures of 20-300K. We then report on the recent progress of magnetic materials with a large MCE. The giant MCE was observed in Er (Co1-xNix)2, MnAs1-xSbx and Mn3GaC in temperature ranges of below 30K, above 220K and at around 160K, respectively. The origin of the giant MCE in these compounds is the first-order magnetic transition (FOMT). The advantage of utilizing the FOMT system as a magnetic refrigerant material is also described.
A new generation of high gradient magnetic separation (HGMS) has recently received attention again, especially for its applications in the fields of water and wastewater treatment. The reason for this attention is that a newly developed superconducting magnet can be used to easily generate a high magnetic field, under which even weakly paramagnetic materials can be separated at high efficiency. Furthermore, new magnetic seeding processes without the addition of magnetite have been developed for the separation of nonmagnetic particles from the viewpoint of colloid chemistry and material science. We also have developed a new wastewater treatment process that uses magnetic gel particles containing immobilized microorganisms and magnetic particles. The magnetic gel particles are magnetically separated and recovered from the effluent in wastewater treatment processes, and are then recycled to a bioreactor directly or reused after storing. In this paper, the applications of magnetic separation to the recovery of microorganisms for wastewater treatment use were reviewed. Further, a novel type of magnetic separator without a filter matrix was introduced to the continuous separation and recovery of magnetic gel particles with different magnetic characteristics.
Current distribution in the superconducting film for a resistive fault current limiter is important because it influences AC loss and a uniformity of S/N transition. The lateral current distribution of the film was reconstructed from the magnetic field distribution, which is measured by multiple Hall probes. The following results were obtained. (1) Non-uniform current distribution in the superconducting film was observed when the current was less than 1.3times critical current (Ic). (2) The current in a superconducting film was uniform when the current was much higher than Ic. The current can be considered uniform when the film works as a fault current limiter because the S/N transition starts at about twice Ic. (3) The validity of the measurement was verified by the comparison with the electric circuit simulation.
Several samples of TbAlO3 and related magnetic materials were developed. These materials have large magnetic moment, and most of them exhibit the magnetic phase transition below 4K. The heat capacity of all samples has been measured with the adiabatic heat-pulse method. The dependence of the heat capacity on the preparation temperature has been studied. We discussed possibility of using these materials as regenerator materials in pulse tube refrigerator.
Pinning properties were investigated for bulk Y-123 superconductors in an underdoped condition by heat treatment at a high temperature. The critical current density was drastically decreased. It is ascribed to the weakened pinning strength of 211 phase particles resulting from a degradation of the superconductivity in the block layer. A decrease of g2, the number of flux lines in the flux bundle, was found to be stronger than that of the pinning strength. This indicates that an additional disorder because of the thermal activation of flux lines also makes g2 smaller. The present result shows that the disorder transition that causes the peak effect is more strongly affected by the elastic properties than by the pinning strength.
An AC current source of the 1, 000A class for AC transport loss measurements was designed and fabricated with the use of an oxide superconducting current transformer. Two cryocoolers were installed for cooling the transformer and a sample holder separately. A parallel conductor of 6 tapes was wound for a secondary winding, and a transposition was performed to make the current distribution uniform in the parallel conductor. Temperatures of the transformer and the sample holder were controlled in the range of 35-50K and 25-77K, respectively. The transformer and the sample holder were thermally separated and independently controlled. The peak current in the secondary winding exceeded 1, 000A in the frequency range of 1-75Hz when the primary peak current was 14A.