Metal-carbon Eutectics Research Articles

Freezing and melting in the peritectic W–C–X

The tungsten–carbon (WC) peritectic, with a liquidus temperature near to 3020 K, shows promise as a high-temperature reference fixed point. It appears that the WC phase is a discrete phase with full stoichiometry of the components W and C, in which impurities cannot be dissolved. In this paper we describe the use of Thermo-Calc software to derive the liquidus and solidus temperatures versus the impurity concentration for the impurities V and Si. It is found that the liquidus is univariant, whereas the solidus is invariant. Additionally, the transition curves between the solidus and the liquidus are modeled. The liquid fraction at the liquidus point is found to be less than 1; in contrast, it is unity for metal–carbon eutectics. Measurements are presented for a series of melting and freezing curves for two WC fixed-point cells. The Gibbs phase rule requires segregation of a fourth compound phase, involving the impurity in question, at the solidus point. The observations confirm that during supercooling, impurities are trapped in compound phases consisting of the impurity together with W and/or C, encapsulating the impurity, and resulting in a flat freezing plateau characteristic of the pure system. Finally, an expression for the melting temperature, as a function of the liquid fraction, is derived, spanning the temperature range from an initial melting temperature to the liquidus temperature. Within this range just free impurities are involved.

Thermodynamic temperature assignment to the point of inflection of the melting curve of high-temperature fixed points

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.

Evaluation and Selection of High-Temperature Fixed-Point Cells for Thermodynamic Temperature Assignment

A multi-partner project to determine the thermodynamic temperatures of a selected set of high-temperature fixed points based on metal-carbon eutectics is underway as a working group activity within the Comite International des Poids et Mesures. The investigation focuses on four fixed-point types, namely, the three metal-carbon eutectic points of Re-C \((2474\,^{\circ }\hbox {C})\), Pt-C \((1738\,^{\circ }\hbox {C})\), and Co-C \((1324\,^{\circ }\hbox {C})\), and the Cu point \((1084.62\,^{\circ }\hbox {C})\). This paper describes the construction, pre-evaluation, and screening stage of the cells prior to their thermodynamic temperature determinations. The construction of the HTFP cells was undertaken by nine national metrology institutes (NMIs) according to instructions laid out in a pre-agreed protocol that ensures production of best quality cells. Four NMIs conducted the evaluation, each for a certain fixed-point type, and screened out cells that did not meet pre-determined selection criteria while assuring sufficient variety in the sources of the cells in the final selected sets. In autumn 2012, the selected cells were successfully passed on to the final stage of the project, the thermodynamic temperature measurement, and assignment.

Measurement of the thermodynamic temperature of high-temperature fixed points

The thermodynamic temperature of high-temperature fixed points is measured using cobalt–carbon, platinum–carbon and rhenium–carbon metal–carbon eutectics employing a radiation thermometer in the luminance mode. This thermometer is calibrated using a high-temperature model of a black body and a filter radiometer with a known spectral sensitivity in the illuminance mode.

A new approach to the determination of the liquidus and solidus points associated with the melting curve of the eutectic Co–C, taking into account the thermal inertia of the furnace

In the case of metal–carbon eutectics, with transition temperatures above 1000 °C, the liquidus-point temperature, associated with melting, serves as a prime reference temperature. Factors influencing the shape of the melting curve and how to deal with them when specifying the liquidus temperature are the main subject of this paper. Influence factors investigated are the impurity factor, the furnace-temperature gradient, and a factor not considered before: the thermal inertia of the furnace and the associated transient in furnace temperature during melting. These investigations resulted in an improved method for determining the liquidus point, demonstrated for Co–C, which implicitly corrects for the post-melting effect, if any. In addition, a new method, the step-shift method, is proposed for the determination of the solidus point by direct observation. As shown, there is a clear justification for establishing reference methods to determining the solidus and liquidus points of M–C eutectics along the lines set out in this paper.

An International Study of the Long-Term Stability of Metal–Carbon Eutectic Cells

For high-temperature fixed points to be accepted as temperature references, it is of prime importance that their long-term stability is demonstrated. This evaluation is part of the CCT-WG5 high-temperature research project (Machin et al. in Int. J. Thermophys. 28, 1976 2007) devoted to a comprehensive evaluation of three high-temperature fixed points: Co–C (1324 °C), Pt–C (1738 °C), and Re–C (2474 °C). The assessment of the long-term stability, as well as the robustness of the cells, is examined in the first workpackage of this project. Four cells for each of the eutectic points have been constructed by NMIJ (4 Co–C, 2 Pt–C, and 4 Re–C) and NPL (2 Pt–C), and stability tests have subsequently been performed by NMIJ (Co–C), NIM (Pt–C), and VNIIOFI (Re–C). These tests consisted of ageing one cell among the set of four, for a period of 50 h around the melting temperature. For each of the three eutectic points, before and after ageing, the aged cell was compared to one of the three cells so that any drift due to ageing could be determined. The aged Co–C, Pt–C, and Re–C cells showed no significant damage and demonstrated highly repeatable melting plateaus. In this paper, after a short description of the cells and ageing process (described more completely elsewhere (Sadli et al. in Acta Metrol. Sinica 29, 59, 2008)), the results for the three fixed points are presented and discussed.

New Method of Filling of High-Temperature Fixed-Point Cells Based on Metal-Carbon Eutectics/Peritectics

A new method of filling of high-temperature fixed-point cells based on metal-carbon eutectics and peritectics is suggested and tested. In this method a metal and carbon powder mixture is introduced not directly into the crucible, but into an additional container located just above the crucible. The mixture melts inside the container, and the already molten eutectic drops through a small hole in the bottom of the container and fills the crucible drop by drop. The method can be used to obtain a uniform ingot without porous or foundry cavities, to minimize the risk of contamination, and to avoid some other disadvantages. The method was applied to fabricate Re-C and WC-C cells using 5N purity materials. The cells demonstrated a good plateau shape with melting ranges of 0.2 K and 80 mK for Re-C and WC-C, respectively. The Re-C cell was compared with a cell built at NMIJ and showed good agreement with a difference of melting temperatures of only 45 mK.

Thermal Effects in the HTBB-3200pg Furnace on Metal-Carbon Eutectic Point Implementation

The general statement that a temperature fixed-point cell will show better melting and freezing plateaux with better temperature uniformity along the dimensions of the fixed point is understood to be valid for metal-carbon (M-C) eutectics as well as for pure metal fixed points. In this article, it is shown that improved temperature uniformity in the central part of the high-temperature blackbody BB3200pg (HTBB), where the M-C fixed point is implemented, results in flatter and longer plateaux. Pyrolitic graphite rings, clamped together by a spring, form the heated cavity of the HTBB. As a first step, the relative electrical resistivity of each pyrolitic graphite ring was measured using a method advised by the furnace manufacturer. Next, the ring positions were optimized, taking into account their relative resistivities, in order to obtain a more homogeneous temperature distribution. Subsequent measurement of the temperature uniformity at the furnace walls confirmed the improvement. Measuring the melting plateaux of the Pt-C eutectic with different arrangements of the rings, and thereby operating the fixed-point cell in different temperature distributions, confirmed the influence of the temperature distribution on the plateau shape, with the best plateau shape corresponding to the most homogeneous temperature distribution.

Experience of Construction and Study of Pt–C Eutectic in VNIIM and Cooperation with LNE-INM

A new high-temperature furnace was developed in VNIIM and manufactured by the Omsk plant “Etalon.” This furnace is intended for the realization of various fixed points of pure metals and high-temperature metal–carbon eutectics up to a temperature of 3000°C. The construction of the furnace allows measurements to be carried out either with or without the protective window, in a protective atmosphere of argon. The furnace will be described in this article. Cooperation with LNE-INM was initiated with the aim of sharing experience in metal–carbon eutectic cell construction and comparing two Pt–C cells (one cell was made by each partner) as a means of comparing scale realizations around 1740°C. The filling technique used in each laboratory will be described. The average melting temperature of the VNIIM Pt–C eutectic cell, characterized and studied at VNIIM, was 1738.4°C with a standard uncertainty of about 0.13 K. The repeatability of the melting temperature with various measurements was estimated to be within the limits of (0.02–0.2) K. The cell supplied by LNE-INM is the one that was constructed and studied in the framework of the HIMERT European project and described elsewhere. The difference in the melting temperatures of the fixed points (VNIIM—LNE-INM) is about 0.4 K with a standard uncertainty not exceeding 0.18 K. The cells constructed and characterized by each partner were exchanged and measured by the other partner. The results of the study of the melting and freezing of the Pt–C cells carried out by VNIIM and LNE-INM will be presented. The reasons for the difference in the melting temperatures of the two cells are analyzed, and the steps preceding the comparison of the scale realizations of the two laboratories are presented.

Metal–Carbon Eutectics to Extend the Use of the Fixed-Point Technique in Precision IR Thermometry

The high-temperature extension of the fixed-point technique for primary calibration of precision infrared (IR) thermometers was investigated both through mathematical simulations and laboratory investigations. Simulations were performed with Co–C (1,324°C) and Pd–C (1, 492°C) eutectic fixed points, and a precision IR thermometer was calibrated from the In point (156.5985°C) up to the Co–C point. Mathematical simulations suggested the possibility of directly deriving the transition temperature of the Co–C and Pd–C points by extrapolating the calibration derived from fixed-point measurements from In to the Cu point. Both temperatures, as a result of the low uncertainty associated with the In–Cu calibration and the high number of fixed points involved in the calibration process, can be derived with an uncertainty of 0.11°C for Co–C and 0.18°C for Pd–C. A transition temperature of 1,324.3°C for Co–C was determined from the experimental verification, a value higher than, but compatible with, the one proposed by the thermometry community for inclusion as a secondary reference point for ITS-90 dissemination, i.e., 1,324.0°C.

The use of fixed points based on metal-carbon eutectic alloys to increase the accuracy of measurements in the temperature range above 1357.77 K

It is shown that it is possible in practice to use high-temperature models of black bodies based on phase transitions of metal-carbon eutectics for radiometry, radiation thermometry, and photometry.

Measurement of Rh-C, Pt-C and Ru-C eutectic points by four national metrology institutes

The melting temperatures of three metal-carbon eutectics, rhodium-carbon, platinum-carbon and ruthenium-carbon, were measured by four national metrology institutes: the National Research Laboratory of Metrology (NRLM, Japan, now the National Metrology Institute of Japan, NMIJ), the National Institute of Metrology (NIM, China), the National Measurement Laboratory (NML, Australia), and the Korea Research Institute of Standards and Science (KRISS, Rep. of Korea). The same cells were measured using different radiation thermometers and different high-temperature carbon-tube furnaces by the four institutes, on their independently realized temperature scales. The results were compared to examine the performance of these high-temperature fixed points and to determine their melting and freezing temperatures on the International Temperature Scale of 1990 (ITS-90). The measured fixed-point temperatures were all within 0.4 °C of the calculated mean for each cell, and were within each laboratory's estimated uncertainties. The mean values were (1656.8 ± 0.6) °C, (1737.4 ± 0.3) °C and (1953.0 ± 0.4) °C, respectively, for the melting plateaus of the Rh-C, Pt-C and Ru-C eutectic points, with the uncertainties given for k = 2.

Thermocouple observations of melting and freezing plateaus for metal-carbon eutectics between the copper and palladium points

Melting and freezing plateaus were observed with type-R thermocouples for the metal-carbon eutectics Pd-C, Ni-C, and Fe-C. For Pd-C, no apparent difference between the melting and freezing temperatures was observed at a heating/cooling rate of 3 °C/min. For Ni-C, the difference was 0.4 °C, and for Fe-C, 1.6 °C. The freezing temperature for Fe-C showed dependence on cooling rate, and the difference between the melt and the freeze decreased to 0.5 °C, though still significant, at a heating/cooling rate of 0.1 °C/min. The melting temperatures, measured with type-R thermocouples, were 1491 °C, 1329 °C, and 1153 °C, respectively, for Pd-C, Ni-C, and Fe-C eutectics, which agreed to within 1 °C with the values previously measured by radiation thermometers or the literature values. The crucibles, being made from graphite, were able to withstand heating cycles without breaking. The three metal-carbon eutectics, with melting temperatures between the copper point and the palladium point, are potentially useful practical reference points for calibration and evaluation of high-temperature thermocouples.

Radiometric observation of melting and freezing plateaus for a series of metal-carbon eutectic points in the range 1330 °C to 1950 °C

Melting and freezing plateaus were radiometrically observed for the metal-carbon eutectics Ni-C, Pd-C, Pt-C, and Ru-C, using graphite crucibles and black-body cavities. The graphite crucibles were able to withstand heat cycles without breaking. Since the crucible material is a component of the eutectic fixed-point material, the latter is inherently free of contamination from the crucible. The temperature differences between the melting and freezing plateaus were generally less than 0.1 °C. The method seems to be a promising way of realizing a series of practical reference points above the copper point.