TEION KOGAKU (Journal of Cryogenics and Superconductivity Society of Japan)

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In this article, the fabrication process, microstructure and superconducting performance of B1(NaCl), C15(Laves) and Chevrel crystal-type compounds are surveyed. B1-type compounds, such as NbN, show a B_c2 (4.2 K) exceeding 30 T and a high J_c in a film configuration. NbN and NbCN tapes were fabricated using high-rate sputtering and CVD processes. The performance of NbN films is fairly structure sensitive. C15-type compounds, such as V₂(Hf, Zr) are attractive due to the combination of high B_c2 (4.2 K) of ~24 T and better mechanical properties than other compound superconductors. Multifilamentary V₂(Hf, Zr) wires were fabricated from a composite of V matrix and Hf-Zr cores. The superconducting properties of B1 and C15-type compounds are much less sensitive to mechanical strain and neutron irradiation than other compounds. Chevrel-type compounds, such as PbMo₆S₈, have attracted interest due to their extremely high B_c2 (4.2 K) of ~50 T. The Chevrel-type compound wires were fabricated using a powder-in-tube process, and their J_c is effectively improved by hot isostatic pressing. A substantial stress on superconducting wires arises from the magnet winding, the thermal contraction as a result of cooling to operation temperature, and the magnetic Lorentz force during operation. The superconducting performance of compound wires is quite sensitive to mechanical strain. Then finally, strain effects in different compound wires are briefly described.

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Cryogenic slush fluids, such as slush hydrogen and slush nitrogen, are two-phase single-component fluids containing solid particles in a liquid. Their density and refrigerant capacity are greater than those of liquid-state fluid alone. Owing to these advantages, there are high expectations for use of slush fluids in various applications such as a clean-energy fuel, fuel for space-planes to improve the efficiency of transportation and storage, and as a refrigerant for high-temperature superconducting power machines. Experimental tests were performed with slush nitrogen to obtain the frictional pressure drop flowing in a horizontal pipe with an internal diameter of 15 mm and a length of 400 mm. The primary objective of the study was to investigate the pressure drop reduction phenomenon according to changes in velocity and the solid fraction. The pressure drop correlation between the friction factor and the Reynolds number was obtained and an empirical correlation between them was derived. The flow pattern for slush nitrogen inside a pipe and the behavior of solid particles were observed using a high-speed video camera and the PIV method. From the experimental results, the pressure drop reduction phenomenon emerged clearly when the flow velocity was higher than 3.6 m/s and the flow pattern of solid particles inside the pipe was pseudo-homogeneous.

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