Impurity Correction Techniques Applied to Existing Doping Measurements of Impurities in Zinc

Abstract

Impurities represent the most significant source of uncertainty in most metal fixed points used for the realization of the International Temperature Scale of 1990 (ITS-90). There are a number of different methods for quantifying the effect of impurities on the freezing temperature of ITS-90 fixed points, many of which rely on an accurate knowledge of the liquidus slope in the limit of low concentration. A key method of determining the liquidus slope is to measure the freezing temperature of a fixed-point material as it is progressively doped with a known amount of impurity. Recently, a series of measurements of the freezing and melting temperature of ‘slim’ Zn fixed-point cells doped with Ag, Fe, Ni, and Pb were presented. Here, additional measurements of the Zn–X system are presented using Ga as a dopant, and the data (Zn–Ag, Zn–Fe, Zn–Ni, Zn–Pb, and Zn–Ga) have been re-analyzed to demonstrate the use of a fitting method based on Scheil solidification which is applied to both melting and freezing curves. In addition, the utility of the Sum of Individual Estimates method is explored with these systems in the context of a recently enhanced database of liquidus slopes of impurities in Zn in the limit of low concentration.