Investigation of the Superconducting Transition Point of Niobium as a Reference Temperature

The superconducting transition of niobium was investigated to test and to improve its suitability as a thermometric fixed point of the temperature scale. The superconducting transition temperatures (ST-temperatures) of three high-purity niobium samples (two single crystals and a polycrystalline wire, having residual resistance ratios larger than 10,000 and 6000, respectively) were found to differ by no more than the resolution of the measuring apparatus, 0.2 mK. The transition widths were 0.3 mK. The ST-temperature was T c 76 = (9.288 5 ± 0.002) K. When using the sample parameters as a basis, theoretical estimations indicate that the ST-temperature of the three samples may differ by less than 1 mK from that of ideal niobium which is in agreement with the experimentally obtained results. The value of the ST-temperature and the transition width proved to be nearly insensitive to mechanical stresses and no drift could be found after storage of the samples in air for long periods. Therefore niobium appears to be the most suitable material to establish a superconducting thermometric fixed point with a precision of 0.1 mK.