The present mass standard based on the prototype of the kilogram has some serious problems such as surface contamination and unrecoverablity in the case of damage or loss. Research on monitoring and finally replacing the present mass standard are therefore encouraged and some approaches have been proposed. In this article, the superconducting magnetic levitation method, which is one of the approaches that also makes use of the unique properties of superconductivity, is reviewed together with a brief description about mass and electric standards.
On basis of the Meissner effect, which assures the reversibility of the normal-superconducting state transition, thermodynamical relations describing the difference in superconducting and normal states were derived in the preceding chapter. In this chapter, a two-fluid theory is described where the free electrons in the superconducting state are assumed to be a mixture of superelectrons that flow freely without resistance and normal electrons that have Ohm resistance. The theory is phenomological in that the property of the superelectron is based on experiment and the theory does not attempt to describe the nature and origin of the superelectrons.
Nb3Al multifilamentary wire made by a rapid heating, quenching and transforming (RHQT) process shows not only 3-5 times higher Jc (4.2K), but also much lower strain sensitivity of superconductivity than that of commercialized (Nb, Ti)3Sn multifilamentary wire, and is therefore one of the most promising candidates as a next-generation superconductor. However, the Cu stabilization process is an unsolved key technology required to commercialize the RHQT-processed Nb3Al conductor. Because the RHQT process includes heat treatment at about 2, 000°C, which is much higher than the melting point of pure Cu, we must cover the Nb3Al wire with a Cu stabilizer after quenching. A stable Nb oxide layer is formed on the surface of the Nb3Al wire and prevents electrical and thermal conductivities between the Cu stabilizer and Nb3Al wire. We found that a 1μm thick Cu-ion plating on the Nb3Al wire removes the Nb oxide layer effectively. The Nb3Al wire, Cu-plated electrically with several tens of micrometers in thickness after being Cu-ion plated, shows improved Jc in lower fields and an increase in recovery current, which indicates the improved stability of the Nb3Al wire.