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A phase-field solidification model of almost pure ITS-90 fixed points.

Large, M J*; Pearce, J V (2014) A phase-field solidification model of almost pure ITS-90 fixed points. Int. J. Thermophysics, 35 (6-7). pp. 1109-1126.

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We present a two dimensional axisymmetric phase-field model of thermo-solutal solidification in freezing-point cells used for calibrating standard platinum resistance thermometers for realisation and dissemination of the International Temperature Scale of 1990. The cell is essentially a graphite crucible containing an ingot of very pure metal (of order 99.9999%). A graphite tube is inserted along the axis of the ingot to enable immersion of the thermometer in the metal. In this study, the metal is tin (freezing temperature 231.928 °C). During the freezing of these cells a steady, reproducible temperature is realised, with a defined temperature, that can be used to calibrate thermometers with uncertainties less than 1 mK. The model is applied to understand the effect of experimental parameters, such as initiation technique and furnace homogeneity, on the measured freezing curve. Results show that freezing curves whose behaviour is consistent with the Scheil theory of solidification can be obtained with a specific furnace temperature profile, and provided that the freeze is of long duration the results are consistent with previous one dimensional models and experiments. Morphological instability is observed with the inner interface initiation technique, causing the interface to adopt a cellular structure. This elevates the measured temperature, in accordance with the Gibbs-Thomson effect. In addition, we examine the influence of initiation techniques on the solidification behaviour. The model indicates that an initially smooth inner mantle can `de-wet' from the thermometer well forming agglomerated solid droplets, following recalescence, under certain conditions. This manifests as a measured temperature depression due to the Gibbs-Thomson effect, with a magnitude of 100-200 µK in simulations. The temperature rises to that of the stable outer mantle as freezing progresses and the droplets re-melt. It is demonstrated that the effect occurs below a critical mantle thickness. A physical explanation for the origin of the effect is offered, showing that it is consistent with solid-state de-wetting phenomena. Consideration is also given to the limitations of the current model configuration.

Item Type: Article
Keywords: Phase-field, fixed-point, thermal effects, impurities, modelling
Subjects: Engineering Measurements
Engineering Measurements > Thermal
Identification number/DOI: 10.1007/s10765-014-1685-2
Last Modified: 02 Feb 2018 13:13
URI: http://eprintspublications.npl.co.uk/id/eprint/6380

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