Effect of oxygen on trace impurities in extremely pure metal ingots used for realising ITS-90 fixed point temperatures

Abstract

The International Temperature Scale of 1990 (ITS-90) provides a scheme for calibrating thermometers which is largely based on the freezing points of pure metals such as indium, tin, zinc, aluminium and silver. The existence of trace impurities in pure metals results in uncertainties in the realization of these freezing points. Several methods such as the sum of individual estimates (SIE) and overall maximum estimate (OME) are used to assess the uncertainties due to the presence of impurities. However, it is becoming evident that the oxidation of impurities results in their precipitation, which means that these insoluble oxides no longer play a significant role in the elevation or depression of the freezing temperature. Hence the conventional correction techniques may overestimate the magnitude of the effect. In this study, the thermodynamic modelling software MTDATA was used to evaluate the effect of oxygen on selected impurities in In, Sn, Zn, Al and Ag by calculating the binary impurity-metal phase diagram in the presence of specific amounts of oxygen. It was found that oxygen has a significant effect on many of the systems studied. The most significant effect was observed in the Ag system, and then, in decreasing order of significance, In, Sn, Zn and Al.