Estimation of impurity effect in aluminium fixed-point cells based on thermal analysis

Abstract

The impurity effect on fixed-point temperature realization by thermal analysis has been assessed. For such an assessment, the following actions were conducted: (1) the fabrication of aluminium point cells using 6N or higher-grade aluminium samples from different sources (manufacturers), (2) temperature measurements during solidification and thermal analyses based on freezing curves obtained from the measurements, (3) direct cell comparison among cells of different nominal purities and (4) calculation of the departure of the freezing point from the ideally defined freezing point by applying the sum of individual estimates (SIE). Two aluminium point cells were prepared in action (1) using 6N-grade and one cell using 6N5-grade aluminium samples. To realize a fixed point using the cells, a fixed-point furnace was developed and evaluated. Temperature measurements in action (2) were conducted at different rates of solidification and in accordance with the one using the liquid–solid interface technique. Gradients of freezing curves were derived in the thermal analysis, and from their dependence on the rate of solidification, the impurity effect was evaluated. Indirect cell comparison was also derived using the difference in the gradients. It was found that the indirect cell comparison was in satisfactory agreement with the direct cell comparison, which was obtained from action (3). It was also found that the departure of the thermal analysis from the SIE obtained from action (4) was within the uncertainty. This fact may imply a possible application of thermal analysis for estimating the effect of impurities in the realization of the aluminium point, especially for 6N-grade aluminium fixed-point cell as used in the present study.