Melting and freezing behavior of pure Ni cells in alumina crucibles with different internal structures

Abstract

Two pure Ni cells for thermocouple calibration were fabricated in alumina crucibles with different internal structures to investigate their melting and freezing behaviors. One of the produced crucibles had a smooth surface, and the other was made with internal circular grooves. The melting and freezing processes of each cell were investigated in 10 melting/freezing cycles using a type B thermocouple calibrated with a standard radiation thermometer. The reproducibility of the melting point was better than that of the freezing point by a factor of more than 2, regardless of the internal shape of the crucible. The freezing temperature gradually decreased with the degree of supercooling, and the slope was observed to be about −0.08 • C. It was found that in terms of reproducibility and easiness of realization the melting point was better than the freezing point for use as a fixed temperature point. Therefore, it is proposed to use the melting point as a fixed point for thermocouple calibration. Regarding the difference in internal structure of the crucible, it was observed that a cell with circular grooves was slightly better in terms of reproducibility of melting and freezing points, but it was still unclear whether this was due to the grooves formed inside the cells, and, thus, additional research is needed. The average melting temperature and uncertainty of the Ni cells fabricated in this study was evaluated as (1454.1 ± 0.9) • C.

Phase transition behavior of pure Fe cells for thermocouple calibration

“…The first reason was the practical and easy operation process for melting, compared with freezing. Next was the negligible contribution of the uncertainty from the melting point determination (≈0.06 • C if a rectangular distribution is assumed) to the combined uncertainty when considering the nine times larger calibration uncertainty of the reference thermocouple (0.53 • C at 1534 • C, k = 1) [13]. Finally, selecting the melting process as a fixed point made it easy to automate the realization cycles.…”

“…Because there were only five measurements, the reproducibility of the melting temperature was determined by applying a t-distribution. Other components were analyzed with the same procedures as described in a previous report [13]. The final expanded uncertainty of the Fe-20 cell was calculated to be 1.16 • C with k = 2.…”

“…Sintered alumina crucibles were prepared together with a thermometer guide well. The design and size of the crucible in figure 1(a) were the same as those of previously reported pure Ni cells [13]. The inner surface of this cell was machined to be smooth and plain.…”

“…The small difference in cell volume between the cells was due to the machining tolerance variation of the ceramic materials. An electric high-temperature furnace with six vertical U-shaped Kanthal super heaters was used [4,9,12,13]. To protect the Fe sample from oxidation at high temperature, a mixture of Ar (99.999% purity) and 0.5 vol% hydrogen continuously flowed into the cell during all experiments.…”

“…The HBTC-1701 was not calibrated but just used for monitoring of melting and freezing behaviors of the Fe cells. The calibrated thermocouple (S/N: HBTC-1702), which was the same as was used in previous reports [13], was used to measure the exact transition temperature. The thermocouple emf was measured using a Keithley nanovoltmeter 2182 A with a calibration uncertainty of 10 μV V −1 (k = 2) and long-term stability of about ±20 μV V −1 .…”