The thermodynamic temperature of the point of inflection of the melting transition of Re-C, Pt-C and Co-C eutectics has been determined to be 2747.84 ± 0.35 K, 2011.43 ± 0.18 K and 1597.39 ± 0.13 K, respectively, and the thermodynamic temperature of the freezing transition of Cu has been determined to be 1357.80 ± 0.08 K, where the ± symbol represents 95% coverage. These results are the best consensus estimates obtained from measurements made using various spectroradiometric primary thermometry techniques by nine different national metrology institutes. The good agreement between the institutes suggests that spectroradiometric thermometry techniques are sufficiently mature (at least in those institutes) to allow the direct realization of thermodynamic temperature above 1234 K (rather than the use of a temperature scale) and that metal-carbon eutectics can be used as high-temperature fixed points for thermodynamic temperature dissemination. The results directly support the developing mise en pratique for the definition of the kelvin to include direct measurement of thermodynamic temperature.

This paper deals with the evaluation of the uncertainty in the equilibrium value of the eutectic temperature of the eutectic systems Fe-C and Co-C in their pure state. The first step towards this is deriving the equilibrium value of the liquidus temperature of the actual impure system and its associated uncertainty, the second estimating the uncertainty in the correction for the effect of impurities. The derivation of liquidus temperature and its uncertainty from the observed melting behaviour forms the core of this study. It involves the extrapolation of the measured melting plateaus, obtained at finite growth rates of the preceding freezes, towards the plateau related to zero growth rate, i.e. to the state of structural thermodynamic equilibrium. The uncertainty in the correction for the effect of impurities will be evaluated by means of 'the sum of individual estimates' and 'the overall maximum estimate', earlier shown to be applicable to eutectic systems.

WC–C, Cr3C2–C and Mn7C3–C peritectic systems were investigated for their potential of serving as high-temperature reference points in thermometry. Mixtures of high-purity graphite powder with W, Cr and Mn powder of 99.99%, 99.9% and 99.95% purity by mass, respectively, were placed in graphite blackbody crucibles and melting/freezing plateaus were observed by means of a radiation thermometer. The observed melting temperatures were 2749 °C (WC–C), 1826 °C (Cr3C2–C) and 1331 °C (Mn7C3–C), with a repeatability—in each case—of 0.02 K. The melting range for WC–C and Cr3C2–C peritectics was roughly 0.1 K. WC–C showed a flat freezing plateau that agreed with the melting plateau within the repeatability. The three fixed points are possible candidates, like the metal (carbide)–carbon eutectic fixed points, in the realization of an improved high-temperature scale above the copper point.

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The eutectic alloys rhenium-carbon, platinum-carbon and cobalt-carbon have been proposed as reference standards for thermometry, with temperature and uncertainty values specified within the mise en pratique of the definition of the kelvin. These alloys have been investigated in a collaboration of eleven national measurement institutes and laboratories. Published results reported the point-of-inflection in the melting curve with extremely low uncertainties. However, to be considered as standards it is necessary to stipulate what phenomenon a temperature value has been ascribed to; specifically, this should be a thermodynamic state. Therefore, the data have been further evaluated and the equilibrium liquidus temperatures determined based on a consideration of limits and assuming a rectangular probability distribution. The values are: for rhenium-carbon 2747.91 ± 0.44 K, for platinum-carbon 2011.50 ± 0.22 K and for cobalt-carbon 1597.48 ± 0.14 K, with uncertainties at approximately a 95% coverage probability. It is proposed that these values could be used as Metrologia

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The authors recently reported the very first radiometric plateau observation of high-temperature fixed points of metal-carbide carbon peritectics. These act in a similar way to the metal (carbide)-carbon eutectic points in the sense that they can be used at high temperature without being contaminated by their graphite crucibles. The performance seems similar in terms of repeatability and plateau shape. The peritectic transition temperatures are close to the transition temperatures of some of the metal (carbide)-carbon eutectics. In this article, results of further study to understand the melting and freezing involved in these fixed points are reported, with the focus on the Cr 3 C 2 -C peritectic point. Difficulty in producing an ingot without voids was encountered. To overcome this, a filling technique that takes advantage of the capillary effect was devised. Plateau shapes and microstructures observed with electron-probe microanalysis (EPMA) and back-scattered electron imaging (BSE) for various filling methods were compared. The observation of two fixed-point plateaux, one at the Cr 3 C 2 -C peritectic point and the other at a lower temperature of the Cr 7 C 3 -Cr 3 C 2 eutectic point, correlates to the presence of two kinds of domains in the observed microstructure. The graphite crucible is shown to play an essential role in realizing peritectic plateaux of good quality.

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