Melting Plateau Research Articles

Argon Triple-Point Device for Calibration of SPRTs

This paper presents an apparatus for the calibration of long-stem platinum resistance thermometers at the argon triple point $$(T_{90} = 83.8058\,\hbox {K})$$ , designed at the Institute of Low Temperature and Structural Research, Poland (INTiBS). A hermetically sealed cell filled at the Istituto Nazionale di Ricerca Metrologica, Italy with high purity gas (6N) is the main element of this apparatus. The cell is placed in a cryostat fully immersed in liquid nitrogen. A temperature-controlled shield ensures the quasi-adiabatic condition needed for proper realization of the phase transition. A system for correcting the temperature distribution along the thermometer well is also implemented. The cell cooling and argon solidification is carried out by filling the thermometer well with liquid nitrogen. A LabVIEW computer program written at INTiBS automatically controls the triple-point realization process. The duration of a melting plateau in the apparatus lasts for about 24 h. The melting width for $$F$$ between 20 % and 80 % was $${<}0.3$$ mK. The reproducibility of the plateau temperature is better than $${\pm }20\,\upmu \hbox {K}$$ .

A Method to Improve the Temperature Distribution of Holder Around the Fixed-Point Cell Position

The temperature profile along the furnaces used in heating high-temperature fixed points has a crucial impact on the quality and duration of melting plateaux, accordingly the accuracy of thermodynamic temperature determination of such fixed points. This paper describes a simple, yet efficient, approach for improving the temperature uniformity along a cell holder in high-temperature blackbody (HTBB) furnaces that use pyrolytic graphite rings as heating elements. The method has been applied on the KRISS’ HTBB furnace. In this work, an ideal solution for arranging the heating elements inside the furnace is presented by which the temperature gradient across the cell holder can be kept as low as possible. Numerical calculations, based on a finite element method, have been carried out to find the best possible arrangement of the rings. This has been followed by measuring the temperature gradient along an empty cell holder to validate our calculations. A temperature gradient of 100 mK has been achieved at $$1500\,^{\circ }\mathrm{C}$$ over a length of 50 mm within a cell holder of 10 cm in length. It has also been shown that for a 20 cm long holder surrounded by rings with an arbitrary resistance profile, the temperature uniformity can be improved by adding a few “hot” rings around the cell holder.

The Effect of the Rear Cavity Wall Ingot Shape on the Evolution of the Liquid–Solid Interface During Melting for the Eutectic Pt-C

When characterizing high-temperature fixed points, the fraction of the melting time of the regular part of the plateau with respect to the total melting time, is critical. Maximizing the melting duration minimizes the uncertainty associated with the determination of the fixed-point temperature. One factor that affects this quality is the effect of the thermal bridging between the external and internal surfaces of the ingot enclosed by the cell. This paper presents the results of simulations for the eutectic Pt-C, investigating the effects of different ingot shapes on the duration of the melt plateau. It was found that the formation of a thermal bridge from the rear of the blackbody cavity toward the outer surface of the ingot was critical and that its formation could be delayed or suppressed through a proper choice of the ingot shape. The shapes considered included, firstly, the shape of the rear of the cavity, in contact with the ingot, either cone-shaped or dome-shaped, and secondly, the inside rear surface of the cell, in contact with the ingot, being a cone, a convex dome, or flat. The presence of impurities in the alloy was taken into consideration, and its influence in the evolution of the liquid–solid interface compared with that for the pure alloy. The effect of changing the thermal isolation of the cell, at its front side, was also considered. A dome-shaped surface for the rear of the cavity was found to be more favorable for the development of a regular melting front, in conjunction with the segregation of impurities during melting. At the rear of the cell, a flat surface ensures the back wall is the last to experience thermal bridging, resulting in more extended melting plateaus.

Construction and in-situ characterisation of high-temperature fixed point cells devoted to industrial applications

Among the activities of the European Metrology Research Programme (EMRP) project HiTeMS one work package is devoted to the development and testing of industrial solutions for long-standing temperature measurement problems at the highest temperatures. LNE-Cnam, NPL, TUBITAK-UME have worked on the design of high temperature fixed points (HTFP) suitable for in-situ temperature monitoring to be implemented in the facilities of CEA (Commissariat a l’energie atomique et aux energies alternatives). Several high temperature fixed point cells were constructed in these three national metrology institutes (NMIs) using a rugged version of cells based on the hybrid design of the laboratory HTFP developed and continuously improved at LNE-Cnam during the last years. The fixed points of interest were Co-C, Ru-C and Re-C corresponding to melting temperatures of 1324 °C, 1953 °C and 2474 °C respectively. The cells were characterised at the NMIs after their construction. Having proved robust enough, they were transported to CEA and tested in an induction furnace and cycled from room temperature to temperatures much above the melting temperatures (> +400 °C) with extremely high heating and cooling rates (up to 10 000 K/h). All the cells withstood the tests and the melting plateaus could be observed in all cases.

Water phase transition under pressure and its application in high pressure thawing of agar gel and fish

Experiments were carried out with a high-pressure (HP) differential scanning calorimeter (DSC) and a HP unit using frozen agar gel (3%, w/w) and Atlantic salmon (Salmo salar). Small samples (0.54–0.7g) were prepared for HP DSC tests. Frozen samples of agar gel and salmon muscle in cylinders (47.5mm diameter, 135mm length) were subjected to water immersion thawing (WIT) (20°C) and HP thawing at 100, 150 and 200MPa with a water temperature of 20°C. Phase transition temperature of agar gel was close to the phase diagram of pure water. Melting temperature of salmon was significantly lower than phase diagram of pure water probably due to the presence of solutes and cellular structures in fish. HP DSC tests demonstrated a good correlation between temperature (T) and average pressure (P): T=−1.22–0.0946P−0.000115P2 (R2=0.99, n=10). High pressure caused a depression of the ice-melting temperature resulting in an accelerated thawing process. The reduction of melting plateau time can be predicated by using Plank’s model. For frozen agar gel, the total thawing time was 50.3±2.7, 36.4±2.2 and 30.8±1.8min, or 73, 53 and 45% of WIT time (68.7±4.3min) at 100, 150 and 200MPa, respectively. For frozen fish, the total thawing time was 58.9±2.8, 41.8±4.7, 37.2±2.6 and 33.8±1.9min for WIT, HPT at above pressures, respectively.

The correlation between the lower temperature melting plateau endotherm and the stretching-induced pore formation in annealed polypropylene films

The room temperature stretching process of annealed polypropylene hard elastomer films was analyzed and the relationship between the low-temperature endotherm plateau in the differential scanning calorimetry curves generated during annealing and the stretching-induced pore formation was studied. It was found that compared with that of precursor film, an endotherm plateau and some diffraction phenomena around 2θ angles of 13.0 degrees and 18.4 degrees were observed after annealing. During stretching, the plateau disappeared progressively and these diffraction phenomena also vanished. At the same time, the pores and bridges connecting lamellae were formed. The endotherm plateau came from the crystallization of tie chains around initial row-nucleated lamellar structures. During pore creation stage, the grown crystals were first drawn and converted to bridges connecting lamellae. Further stretching into the strain-hardening stage led to the deformation of the row-nucleated lamellar structures. The experimental results indicated that there was a direct relationship between the endotherm plateau and pore formation during the fabrication of microporous membrane based on melt stretching mechanism.

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.

Palladium–Carbon Eutectic Fixed Point for Thermocouple Calibration

A Pd–C eutectic fixed point has been produced using a molybdenum disilicide element, electrically heated furnace that was built in-house. The eutectic fixed point was measured with two Pt/Pd thermocouples calibrated at the fixed points of Sn, Zn, Al, Ag, and Au. An ITS-90 temperature of (1490.69 ± 0.88) °C (k = 2) was obtained for the inflection point of the melting plateau. Diffusion of Pd into the thermowell and onto the thermocouple protection tube was observed.

Use of Different Furnaces to Study Repeatability and Reproducibility of Three Pd-C Cells

Three different Pd-C eutectic fixed-point cells were prepared and investigated at INRIM. Several tens of phase transition runs were carried out and recorded with both a Si-based radiation thermometer at 950 nm and a precision InGaAs-based thermometer at 1.6 μm. Two of the cells were of the same design with an inner volume of 12 cm3. The third one was smaller with a useful inner volume of 3.6 cm3. The three cells were filled with palladium powder 4N5 or 4N8 pure and graphite powder 6N pure. The repeatability and stability of the inflection point were investigated over a period of 1 year. The noticeably different external dimensions of the two cells, namely, 110 mm and 40 mm in length, allowed the influence of the longitudinal temperature distribution to be investigated. For this purpose, two different furnaces, a single-zone with SiC heaters and a three-zone with MoSi2 heaters, were used. Different operative conditions, namely, temperature steps, melting rate, longitudinal temperature distributions, and position of cells within the furnace, were tested to investigate the reproducibility of the cells. Effects on the duration and shape of the plateaux were also studied. This article gives details of the measurement setup and analyses of the melting plateaux obtained with the different conditions.

Current Work on Furnaces and Data Analysis to Improve the Uniformity and Noise Levels for Metal Fixed Points

Ongoing work to improve the uniformity of vertically mounted furnaces, manufactured by Carbolite (e.g., Type TZF12/75—three-zone furnace capable of 1200 °C, with 75 mm inner bore) along the axis and across the working tube and/or equalizing block is reported. This involves adjusting the size of the end zones, the position of the control thermometers, and the use of cascade-control methods. Means regularly used at NPL to reduce electrical noise in some commercially available ac furnaces through a reduction in the voltage used to “fire” the heaters, and better use of thyristor controllers (by extending their cycle time) are described. The need to shield the controllers from local magnetic fields is described. With these measures, the electrical noise from ac furnaces can approach that of dc furnaces, without the large cost of a dc power supply. The application of new data analysis techniques (Allan deviation) will be shown to improve the representation of uninterrupted fixed-point traces (as used in ingot verification rather than PRT calibration). Reduction of statistical noise on the temperature measurements has been achieved for data on the freezing plateau by determining the statistically optimum averaging time. This shows that the statistical uncertainty in the determination of the temperature of a particular freezing plateau is less than 25 μK and that noise (drift) from other sources, possibly due to variations in room temperature, starts to become appreciable over periods longer than a few tens of minutes. The measurement of freezing and melting plateaux at this level is aided by the introduction of new ASL-F900 bridge(s), and quieter/larger standard resistor baths.

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.

Design and Evaluation of Large-Aperture Gallium Fixed-Point Blackbody

To complement existing water bath blackbodies that now serve as NIST primary standard sources in the temperature range from 15 °C to 75 °C, a gallium fixed-point blackbody has been recently built. The main objectives of the project included creating an extended-area radiation source with a target emissivity of 0.9999 capable of operating either inside a cryo-vacuum chamber or in a standard laboratory environment. A minimum aperture diameter of 45 mm is necessary for the calibration of radiometers with a collimated input geometry or large spot size. This article describes the design and performance evaluation of the gallium fixed-point blackbody, including the calculation and measurements of directional effective emissivity, estimates of uncertainty due to the temperature drop across the interface between the pure metal and radiating surfaces, as well as the radiometrically obtained spatial uniformity of the radiance temperature and the melting plateau stability. Another important test is the measurement of the cavity reflectance, which was achieved by using total integrated scatter measurements at a laser wavelength of 10.6 μm. The result allows one to predict the performance under the low-background conditions of a cryo-chamber. Finally, results of the spectral radiance comparison with the NIST water-bath blackbody are provided. The experimental results are in good agreement with predicted values and demonstrate the potential of our approach. It is anticipated that, after completion of the characterization, a similar source operating at the water triple point will be constructed.

Development of Gallium and Gallium-Based Small-Size Eutectic Melting Fixed Points for Calibration Procedures on Autonomous Platforms

Melting/freezing temperature curves are studied for the single-component Ga and bimetallic eutectic alloys Ga–In, Ga–Sn, Ga–Zn, and Ga–Al in small-size cells. These phase-transition studies were conducted at VNIIOFI in order to design small-size fixed-point devices for metrological monitoring of temperature sensors on autonomous (e.g., space borne) platforms. The results show that Ga and some Ga-based eutectic alloys in small cells can be used as melting fixed points. The repeatability of melting temperatures of Ga, Ga–In, Ga–Sn, and Ga–Zn fixed points is studied. The effects of the concentration of the second element of Ga-based eutectic alloys and the thermal history on the melting plateau’s shape and the melting temperature are studied.

Experimental Investigation of the Cr3C2–C Peritectic Fixed Point

The Cr3C2–C (1,826°C) peritectic point was investigated for its performance as a high-temperature fixed point. Dependence on the impurity content was observed, although it was less severe for the higher of the two equilibrium temperatures obtained with the same cell, the Cr3C2–C peritectic point, than for the lower, the Cr7C3–Cr3C2 eutectic point. Thermal history had an effect on the melting plateau duration, but not on the point-of-inflection temperature nor on the melting range. The melting rate had no apparent effect. The repeatability evaluated as the standard deviation of the repeated melting plateaux within a day was 20 mK for the Cr3C2–C peritectic point, while for the Cr7C3–Cr3C2 eutectic point, this was 210 mK. For both the Cr3C2–C peritectic and the Cr7C3–Cr3C2 eutectic, the freezing plateaux often showed deep supercools, which made them unsuitable for use. The observed good repeatability shows the peritectic-point performance to be comparable to the best MC-eutectic high-temperature fixed points investigated so far. The insensitivity to thermal history constitutes an important and practical advantage. The low price of chromium is a clear benefit as compared to Pt–C (1,738°C) or Ru–C (1,953°C) eutectic points, the M–C eutectic points in this temperature range.

Investigation of Furnace Uniformity and its Effect on High-Temperature Fixed-Point Performance

A large-area furnace BB3500YY was designed and built at the VNIIOFI as a furnace for high-temperature metal (carbide)–carbon (M(C)–C) eutectic fixed points and was then investigated at the NMIJ. The dependence of the temperature uniformity of the furnace on various heater and cell holder arrangements was investigated. After making some improvements, the temperature of the central part of the furnace was uniform to within 2K over a length of 40 mm—the length of the fixed-point cell—at a temperature of 2,500°C. With this furnace, the melting plateaux of Re–C and TiC–C were shown to be better than those observed in other furnaces. For instance, a Re–C cell showed melting plateaux with a 0.1K melting range and a duration of about 40 min. Furthermore, to verify the capability of the furnace to fill cells, one Re–C and one TiC–C cell were made using the BB3500YY. The cells were then compared to a Re–C cell made in a Nagano furnace and a TiC–C cell filled in a BB3200pg furnace. The agreement in plateau shapes demonstrates the capability of the BB3500YY furnace to also function as a filling furnace.

The Influence of Antimony on the Tin Point

In 2005, an agreement was reached on how to estimate uncertainties and how to correct fixed-point temperatures for the influence of chemical impurities. Although the general procedure is now specified, some problems remain. The slope of the liquidus line at very low-impurity concentrations must be extracted either from binary-phase diagrams or from doping experiments. Apart from this, there is little experimental evidence to prove that the models used to characterize the freezing and the melting plateaux are adequate, especially for impurities that increase the fixed-point temperature. Therefore, a series of measurements were carried out using a tin fixed-point cell doped with antimony. By varying the freezing and the melting conditions, some useful experimental data were collected.

Evaluation of Cobalt–Carbon and Palladium–Carbon Eutectic Point Cells for Thermocouple Calibration

Two cobalt–carbon (Co–C) eutectic point (1,324 °C) cells and one palladium–carbon (Pd–C) eutectic point (1,492 °C) cell were constructed for thermocouple calibration. The lengths of the Co–C and Pd–C cells were 297 mm, 140 mm, and 140 mm, respectively. The melting and freezing plateaux at the Co–C and Pd–C eutectic points were observed using Pt/Pd thermocouples. The repeatability of the plateau, the effect of the surrounding temperature, and the temperature profile in the cell were measured, and the heat flux effect along the thermometer well was evaluated. When the plateaux of Co–C (297 mm height), Co–C (140 mm height), and Pd–C cells, were measured three times, seven times, and six times, respectively, the standard deviations of the melting points were 0.1 μV, 0.1 μV, and 0.4 μV, respectively. According to the temperature profiles along the thermometer well during the melting plateaux, it was found that the Pt/Pd thermocouple should be inserted at least 9.5 cm, 5 cm, and 6 cm below the surface of the eutectic alloys in the Co–C (297 mm height), Co–C (140 mm height), and Pd–C cells with the furnace set-point 16 °C above the melting point.

Optimizing Contact Thermometry High-Temperature Fixed-Point Cells ( &gt; 1,100 °C) Using Finite-Element Analysis

The advent of high-temperature (i.e., metal–carbon eutectic) fixed points (HTFP) has placed high demands on the equipment needed to implement them. In particular, the HTFP performance is sensitive to the thermal environment. The temperature gradient in the crucible volume determines the duration and form of the melting plateau, and a gradient in the wrong direction along the crucible can result in mechanical damage. With an emphasis on crucibles for contact thermometry, a transient thermal model, which employs the finite element method is described. The aim is to optimize the HTFP environment, and to evaluate the relationship between the temperature of the liquid–solid interface (the actual fixed-point temperature) and the temperature measured by the contact thermometer (the measured fixed-point temperature). A simple mechanism to minimize temperature gradients along the HTFP cell axis is also presented. Importantly, the model shows that the actual temperature of the liquid–solid interface during melting is given by the indicated temperature at the end of the plateau, i.e., the liquidus point, not the point of inflection of the plateau, as is currently the convention. This does not significantly affect the conventional pure metal fixed points of the ITS-90, but could have ramifications for the new generation of high-temperature fixed points where the melt takes place over a temperature range and the liquidus temperature has yet to be identified.

Thermal Analysis of the Heater-Induced Realization of the Tin Fixed Point

In this article, work concerning the thermal analysis of the tin fixed-point is reported. First, the development of a new fixed-point furnace is described. Improvements in the design of the furnace and in the control system enable measurement of the heater power during the phase change. The furnace is sufficiently thermally insulated to produce excellent uniformity and stability, leading to high quality freeze-initiation and minimal thermal influences on the freezing point. By employing the improved furnace and newly-fabricated tin fixed-point cells, the start and end of the melting plateau and the end of the freezing plateau were accurately determined, enabling reliable evaluation of the liquid fraction during the realization of the tin fixed-point compared to conventional methods. Two open-type tin fixed-point cells were fabricated using high-purity tin that was chemically analyzed for impurity content. Thermal analysis results of freezing-point depression are compared to those based on the chemical analysis.

The dependence of the iron–carbon eutectic transition temperature on thermal history and its implications for thermometry

The melting behavior of the eutectic Fe–C fixed point was investigated as a function of the thermal history prior to melting using degassed and non-degassed Fe–C cells. The liquidus temperature and the melting range depend on the preceding growth rate: slower freezing entails a higher liquidus temperature and a smaller melting range. Annealing at temperatures just below the eutectic temperature has similar effects. The inflection point of the melting plateau and the maximum temperature of the freezing plateau showed a linear variation with the square root of the growth rate during the (preceding) freeze and this variation is approximately five times smaller for the melt than for the freeze. No clear evidence of the effect of gaseous impurities could be detected on the shape of the melting plateau obtained in conjunction with the lowest growth rate. However, both the inflection point of the melting plateau and the maximum temperature of the freezing plateau showed a larger dependence on the growth rate for the degassed cell.