Study on the Realization of Zinc Point and the Zinc-Point Cell Comparison

Abstract

Continuing our study on aluminum, tin, and silver points, a study on the realization of the zinc point was conducted. Zinc-point cells were newly fabricated using 6N-nominal grade zinc samples, impurity elements of which were analyzed extensively based on glow-discharge mass spectrometry (GDMS). The present paper reports the temperature measurements done using the newly fabricated cells during the zinc freezing process, under which the zinc fixed point is defined, and the analysis of the freezing curve obtained. Comparisons of zinc-point temperatures realized by the newly fabricated cells (cell-to-cell comparisons) were also conducted. Zinc-point depression due to impurity elements was calculated based on the sum of individual estimates and the impurity element analysis. One of the cells evaluated was drawn out from its crucible and analyzed by GDMS at four points, namely, at around the center of the top, of the middle, of the bottom, and around the outer part of the middle area. The purpose of this cell disassembly is to see whether or not there has been some difference before and after cell fabrication, as well as difference in impurity element distribution within the ingot. From the aforementioned studies, some findings were obtained. First finding is that the homogeneity of the zinc ingot was within 30 %, except for Pb, which was more concentrated in the center part. Second finding is that the cell-to-cell temperature difference changes along with the progressing solidification process. As a consequence, for an accurate cell-to-cell comparison, the locus in the freezing plateau where the comparison is done should be determined. Third finding is that the slope analysis estimates accurately the cell-to-cell comparison, and is consistent with the impurity analysis. This shows that the slope analysis gives extensive information about the effect of impurity to the zinc-point realization, especially after the cell fabrication.

Confirming Impurity Effect in Silver-Point Realization from Cell-to-Cell Comparisons

“…In our comparison that involves various combinations of zinc-point cells and identical zinc-point furnaces [8], we found that temperatures measured by cells under comparison show trends to arrive at one common point on an imaginary axis 1/F = 0. Temperatures shown in Fig.…”

Measurement of the in freezing-point temperature: Effect of the liquid-solid interface structure

“…Since the main uncertainty of the fixed point temperature is caused by the influence of the impurities, its estimation based on impurities diffusion processes in a metal seems to be doubtful. The process of irregular growth of dendrites in liquid metal can result in a nonuniform impurities distribution after crystallization [4,5].…”

“…It is the practice to consider that in case all recommended conditions of the fixed point realization are fulfilled, the outer interface moves from the walls of the crucible to its center, the inner interface moves from the thermometer well to the crucible walls, and diffusion process results in formation of heterogeneous distribution over the crucible radius. Using the model accepted for the process of crystallization in a cell, it is difficult to explain a significant nonuniformity of impurities distribution over a metal height, which has been revealed in [4,5]. The understanding of the processes which cause a deviation of the real crystallization process from the model with a planar interface is important for evaluating the uncertainties of a measurement result due to the influence of impurities.…”

Thermal analysis on the realization of the tin fixed point

“…The metal fixed points are operated at higher temperature than ambient conditions such that it's difficult to reduce the heat fluxes below a few watts as the thermal resistance between the fixed-point cells and furnaces are low. The usual solution is to establish two solid-liquid interfaces in the fixed-point cell; however some publications manifested nice realizations with single solid-liquid interfaces by improving in the design of the furnace and in the control system [1][2][3][4]. It allows the heater power measurements during the phase change of fixed-point cells such that F can be accurately determined.…”

“…With a view to evaluating the influence of impurity on the temperatures of the solid-liquid phase transformation of high-purity substances in metal fixed-point cells, two methods have been recommended [4]. The sum of individual estimates (SIE) method leads to a temperature correction when detailed information about impurities is available, while the overall maximum estimate (OME) method is to estimate the overall impurity using the first cryoscopic constant and expressed as a mole fraction.…”