Thermometry's dependence on chemical metrology: a needs-based assessment

NRC publication: Ye

Further findings of impurity precipitation in metal fixed points

Impurities are believed to be one of the major issues in realizing the metal fixed-point temperatures of the ITS-90 with a low degree of uncertainty. This has raised interest in the individual effects of impurities on the phase-transition temperature of fixed-point metals. Surprisingly, impurities that do not affect a fixed-point temperature have been found experimentally. A possible explanation for this behavior is the formation of insoluble oxides of the added impurities consuming oxygen already present in the fixed-point cell (mostly as an oxide of the fixed-point metal). This is supported by several recent publications. However, all the results could be coincidental. This article presents more convincing proof for the formation of insoluble compounds born from impurities dissolved in the fixed-point metal. Based on refined doping experiments and using impurities that have not been investigated before, both the impurities' dissolution and the precipitation could be observed as an initial decrease (or increase) of the fixed-point temperature followed by a gradual return to its original value. The selected impurities (gallium and zinc in indium) were found to dissolve within a few days and precipitate out within no more than two weeks. The behavior of iron in indium was investigated as well, but the results are not conclusive. Finally, another series of doping experiments indicates that sulfur does not dissolve in indium in significant amounts, but forms insoluble compounds (probably sulfides) when added to the metal. This supports the general assumption that metal-non-metal compounds might be present in the cell without being noticed. \ua9 2011 Her Majesty the Queen in Rights of Canada.Peer reviewed: YesNRC publication: Ye

Fixed-point comparison uncertainties for two cell geometries

To realize the ITS-90 according to its definition, among others, the melting and freezing temperatures of ideally pure metals are needed. Therefore, many national metrology institutes (NMIs) utilize a group of cells instead of one single cell as the national reference for each temperature. With direct fixed-point cell comparisons on a regular basis, it is feasible to account for the small differences between the individual fixed-point temperatures and to detect possible temperature drifts of the cells. At PTB (the German NMI), in recent years, these groups of national standard cells and the so-called transfer cells for calibrations have been complemented by newly developed slim fixed points. These cells typically contain 75% to 80% less fixed-point material compared with standard cells. Slim cells are used for homogeneity investigations of large batches of fixed-point material, for doping experiments to determine the influence of very small amounts of impurities on the fixed-point temperature with very small uncertainties, and for the investigation of contamination or purification effects after the manufacture of a fixed-point cell. These investigations have shown that the main limitation of slim cells is the quality of the phase boundary. The small dimensions of the cell do not allow the formation of a closed phase boundary (or even two of them). However, this can be compensated using a quasi-adiabatic realization procedure, and in this way, uncertainties comparable to those of standard fixed-point cells can be achieved. In this article, the design of the cells as well as typical measurement results and uncertainties for the direct comparison of fixed-point cells of both types, the standard size and slim design, are presented. \ua9 2011 Springer Science+Business Media, LLC.Peer reviewed: YesNRC publication: Ye

Thermal diffusivity of the candidate standard reference material cordierite

The results of an international programme carried out to qualify transfer or reference standards for thermal transport properties, thermal conductivity and thermal diffusivity, in the range between 1 W m(-1) K-1 and 5 W m(-1) K-1, are reported. It was necessary to cover the gap in the availability of reference standards among ceramic high-temperature materials appropriate for use above 800 degrees C. A well-suited candidate material has been found in commercial cordierite manufactured by Ceramtec AG, large batches of which are regularly produced for various articles and components under a standard procedure. The programme carried out under the auspices of the German task group Thermophysik involved thermal diffusivity measurements at the Institute for Nuclear Technology and Energy Systems (IKE) of the University of Stuttgart, Germany, the Institute of Nuclear Sciences Vinca (Vinca), Yugoslavia, the Austrian Research Centre Seibersdorf (FZS), Austria, and the Brandenburg Technical University (BTU) at Cottbus, Germany. Measurements of thermal diffusivity carried out by different experimental techniques and by varying various relevant parameters gave results which are in very good agreement. Experimental results are presented and discussed