Superconductive Transition Point of Tantalum and Niobium as a Reference Temperature

The applicability of the superconductive transition point of Ta and Nb for use as a temperature reference point was examined. An unusual impurity effect was found in both metals; In a plot of the transition temperature (Tc) vs. the inverse resistance ratio (1/Γ), a substitutional impurity had a larger reducing effect onTcthan an interstitial impurity. The purest specimens we obtained, with Γ=5000 for Ta and Γ=4500 for Nb, were prepared by zone-melting and subsequent annealing just below the melting points under ultra-high vacuum. From theirTcvalues and analyses of the impurity effect, theTcvalues of impurity-free Ta and Nb were determined as 4.4924 K and 9.2877 K, respectively, based on the 1976 Provisional 0.5 K to 30 K Temperature Scale, the currently-accepted temperature standard.