Effect of Ultra-Trace Impurities on the Freezing Point of Zinc

Abstract

Impurities are the most significant source of uncertainty in most metal fixed points for the realization of the International Temperature Scale of 1990 (ITS-90). The methods for the estimation of uncertainties and corrections of fixed-point temperatures attributable to the influence of chemical impurities were summarized in 2005, and the sum of individual estimates (SIE) method was recommended to be used with the known concentration and liquidus slope of each impurity. This method requires the concentrations and the liquidus slopes of all impurities. For applying the SIE method, efforts still need to be made to solve a series of problems including the unsatisfactory chemical analysis, inadequate data of the liquidus slopes, and information about the dissolution and precipitation of impurities during the filling and the operation of a fixed-point cell. In the present work at the National Institute of Metrology (NIM), great attention is paid to the effect of ultra-trace impurities on the freezing point of zinc. Five slim graphite crucibles were filled with the same batch of zinc with a nominal purity of 6 N for this research. One of them was used to investigate the concentration and distribution of the impurities in the freezing point of zinc by chemical analysis. The remaining crucibles were used to carry out the ultra-trace impurity doping experiments. The liquidus slopes of Ag–Zn, Pb–Zn, Fe–Zn, and Ni–Zn were measured. All results are reported and discussed.