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

Abstract

As a continuation to earlier work on the silver-point realization, already reported at TEMPMEKO 2007, a new silver-point cell has been fabricated using 6N-nominal grade material that was analyzed by means of glow discharge mass spectrometry (GDMS) by the manufacturer. This new cell is evaluated by a direct cell comparison with one of the existing cells, which was also already reported at TEMPMEKO 2007. One of those existing cells was drawn out from its crucible, and its ingot was analyzed by GDMS at four positions, namely, at around the center of the top, of the middle, of the bottom, and around the outer part of middle areas for the purpose of confirming whether or not there has been differences in the content of impurities before and after the cell fabrication, as well as differences in impurity homogeneity within the ingot. As results of the aforementioned measurements, it was found that the homogeneity of impurities in the silver ingot was on average within 50 %. It was also found that cell-to-cell temperature differences change along with the progressing solidification process. As a consequence, it was concluded that, for an accurate cell-to-cell comparison, the location in the freezing plateau, where the comparison is done, should be determined. Also obtained here is that the slope analysis was consistent with both the cell-to-cell comparison and the impurity analysis.