Comparison between the liquidus temperatures of tin samples having different impurity compositions and correction of the impurity effect

Abstract

The liquidus temperatures of two tin fixed-point ingots having different impurity compositions have been determined using the adiabatic heat pulse-based melting technique; the employed samples were analyzed at three different chemical assay laboratories for characterization of the impurities present. The shape of the melting curves, which showed a rapid increase in the early part of the melt and subsequent gradual and nearly linear increase in the later part, were found to be qualitatively well described by the Scheil-Gulliver solidification model corresponding to the detected impurities of the two samples; it was understood that the impurities having distribution coefficients (k i ) less than 1 resulted in a rapid melting temperature change in the early part of the melt, and impurities having k i > 1 manifested themselves in the later part as a gradual increase in the melting temperature. The liquidus temperatures of the different tin samples were then determined by extrapolating the melting temperatures in the later parts of the melting range, where the variations of the melting temperatures were satisfactorily described by the corresponding parts of the Scheil-Gulliver model, to the melted fraction of unity. The determined liquidus temperatures of the two samples were corrected for the effect of the detected impurities, resulting in an agreement between the corrected temperatures within the claimed uncertainties. It was concluded that with the aid of the consistent chemical assays, the liquidus temperatures determined by the heat pulse-based melting and corrected for the impurity effect were able to produce the closest approximation to the ideal definition of the fixed-point temperature of the ITS-90, which is the melting temperature of the pure fixed-point substance.