Abstract
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.
Highlights
When characterizing high-temperature fixed points (HTFPs), the quality of the melting plateaus is a critical aspect to be considered
The contours delimiting the liquid phase for the impure system during melting at tbridge, at which thermal bridging is initiated, are given in Fig. 4 for all the configurations 1) to 6)
The prime parameter to judge the effect of varying the backside of the shape of the ingot is the liquid fraction F(tbridge) =/(tliq − tsol), where tbridge is the abrupt upturn in the melting curve, where thermal bridging is initiated, followed by the post-melting part of the curves, see Figs. 2 and 3
Summary
When characterizing high-temperature fixed points (HTFPs), the quality of the melting plateaus is a critical aspect to be considered. Where T and t represent the temperature and time, respectively, F is the liquid fraction, TE = 2012.229 K is the eutectic temperature of the pure system, c = 94×10-6 mol-1 is the impurity concentration, m = -0.915×10-6 mK·mol is the associated liquidus slope, and k = 0.316 is the distribution coefficient. These parameters were chosen for modeling the effect of impurities and were taken from real data [5]
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