We have designed and manufactured a reciprocating magnetic refrigerator (Carnot cycle) operating between 1.8K and 4.2K. The magnetic refrigerant was gadolinium gallium garnet (GGG). Operating tests were carried out varying maximum applied magnetic field Bm and operating frequency f. The experimental Bm and f dependence of useful cooling power showed good agreement with the calculated results. The maximum useful cooling power was 0.50W at 1.80K for Bm=3.0T and f=0.45Hz.
A review is given on the current research subjects on the upper critical field Hc2 in A15 superconductors such as Nb3Sn and V3Ga. After the brief introduction of the physical mechanisms determining Hc2, the following topics are dealt with; (1) the correction to the Pauli spin paramagnetic effect due to many-body interactions, (2) the effects of the martensitic transformation and disorder onHc2 in both single crystals and practical conductors, (3) the tertiary element effects and (4) problems remained in future.
The relationship between superconducting properties and microstructures of the bronze-processed Nb3Sn composite wires is reviewed in connection with the stress effect, the martensitic transformation, the pinning mechanism and the nature of the Nb3Sn layer (i.e. grain size, and grain boundary segregation and structure etc.). The Nb3Sn layer in the composite wire is compressively prestrained at cryogenic temperatures because of the difference of the thermal contraction between the Nb3Sn layer and the bronze matrix. This resultant compressive prestrain affects superconducting properties and the martensitic transformation of the bronze-processed Nb3Sn compounds. Martensitic transformation is inevitable for the pure Nb3Sn compound at and near the stoichiometric composition, and this transformation brings about a derease in Hc2 of Nb3Sn. The addition of Ti and Ta to Nb3Sn not only prevents the martensitic transformation but increases its electrical resistivity and Hc2. Therefore, the high field performance of the multi-filamentary Nb3Sn composites is improved by the addition of Ti and Ta. The possibility of an electron scattering flux pinning mechanism has been recently proposed for Nb3Sn compounds. Although Jc of the bronze-processed Nb3Sn wires depends much on the nature of Nb3Sn layers, there are few studies about it. More detailed studies are desired for understanding and improving superconducting properties of the bronze-processed Nb3Sn wires.