Oxides in metal fixed points of the ITS-90

Abstract

In the range between 0 °C and 961 °C, the International Temperature Scale of 1990 (ITS-90) depends to a great extent on the freezing points of the pure metals gallium, indium, tin, zinc, aluminium and silver. An up-to-date realization of these fixed points is based on cells containing metals of ultra-high purity (6N or better) and should include a correction for the influence of relevant impurities. Still, chemical analyses of the fixed-point material can show large amounts of oxygen, which had to be neglected so far, because of the lack of detailed knowledge about it, presuming it could be removed from the cell by applying a vacuum (less than 1 Pa) for a few hours.In this paper we discuss an equilibrium of several forms of oxygen in a fixed-point cell, gaseous in the cell's atmosphere, dissolved in the fixed-point metal and as oxide in a separate (solid) phase. We will conclude that in many fixed points most of the oxygen is not dissolved in the metal, but bound in oxides of the fixed-point metal as well as oxides of some impurities. To demonstrate the impact that the precipitation of impurity oxides has on thermometry, two indium fixed-point cells were doped with magnesium and chromium, which leave the fixed-point temperature unchanged. Further evidence is drawn from earlier work. All these results support the presumed existence of (at least one) persistent separate oxide phase in the fixed points of indium, tin, zinc and aluminium, which renders them eutectic or peritectic points and is a more likely reason why the oxygen content of a cell does not influence the fixed-point temperature.To complement these studies, thermodynamic calculations show how to treat the equilibrium in the cell quantitatively. Using available chemical data, a list is provided that indicates for each fixed-point metal (including the other metal fixed points of the ITS-90: mercury, gold, copper) the impurities that probably build oxides. Due to the agreement of the calculated values with the presented experimental results, we suggest excluding those impurities from the correction of a fixed-point temperature (e.g. the SIE method), unless there is strong evidence of their dissolution.

Study on the Realization of Zinc Point and the Zinc-Point Cell Comparison

“…4, mentions the difference in impurity configuration of the zinc ingot after the cell fabrication from that before the cell fabrication. This may arise from the contamination during the cell fabrication, or the oxide layer formation as reported by Fahr and Rudtsch [9]. Such contamination may imply the risk in adopting T SIE if the calculation is based only on the impurity analysis taken before the cell fabrication.…”

“…Cr, on the other hand, disappeared from the ingot. Cr is not an element that is predicted by Fahr and Rudtsch [9] to have high affinity for oxygen, so it is unlikely to disappear from a zinc ingot due to any oxide layer formation. Comparing the vertical impurity distribution at the center, all impurity elements are more concentrated toward the upper surface.…”

“…[6] and reported values by Ancsin [7], and the uncertainties of the analysis results, u(c li ), taken as half of the concentration values for a detected element or half of the GDMS detection limit for non-detected elements as proposed by Fellmuth and Hill [8]. Based on their findings, Fahr and Rudtsch [9] suggest that the SIE calculation should consider the effect of oxidation and other possible chemical reactions that involve the impurity elements, which may result in the exclusion of individual estimates of some elements. In this work, however, the conventional calculation, including all individual estimates, is used in Eq.…”

Uncertainties in the SPRT Subranges of ITS-90: Topics for Further Research

“…1 require knowledge of the liquidus slopes of all possible impurities, and complete data are not yet available for any of the metal fixed points. The problem has been complicated by the recent observation [13] that in metal fixed points containing some oxygen, a notable number of impurities precipitate as insoluble oxide and do not significantly affect the realized temperature. This means that Eq.…”

Adiabatic Calorimetry Approach to Assess Thermal Influences on the Indium Melting Point

“…It seems that the indium melting point is unaffected by the ingot freezing history. Another study, recently published [18], has testified to the origin of the limited indium sensitivity to impurity effects.…”