To assess the effect of the heat pipe assembly on the performance of the furnace, we compared the temperature profile inside the furnace before and after insertion of the heat pipe assembly. The temperature gradient inside the furnace was measured by maintaining the temperature of the furnace at a constant 225 °C. The results are shown in Fig. 3. The black circles and blue triangles denote the inside temperature gradient of the furnace without and with the heat pipe assembly, respectively. To block the heat loss through the furnace entrance, it was mounted with ceramic thermal insulator wool with about 5-cm depth, except for the small hole for a measuring thermometer. Without the assembly, the temperature gradient was quite steep and the temperature difference was as large as 9.5 °C at 15 cm from the furnace bottom. This difference is too large to be acceptable for the fixed-point realization, even for the usual comparison calibration of thermometers. However, by introducing the heat pipe assembly into the furnace, the temperature gradient was improved six-fold. The temperature drop decreased to as small as a 1.6 °C difference at 15 cm from the bottom with a slope of 0.11 °C/cm. This demonstrated that the assembly can dramatically improve the uniformity of the temperature gradient because of the excellent heat conduction of the individual heat pipe tubes.
The red squares and green rhombuses in Fig. 3 respectively denote the inside temperature gradient of the Sn cell installed in this furnace and in the commercial three-zone metrology furnace (ISOTECH Model ITL-M-17703). In these cases, a measuring thermometer was located in the thermometer well of the fixed-point cell. The test cell was also thermally insulated with ceramic wool by packing the wool on the top of the cell. It can be seen that incorporating the test Sn cell into the heat pipe assembly, the temperature gradient was further improved by more than four times. At 15 cm from the cell bottom, the temperature difference was very small, at only 0.38 °C (gradient of 0.025 °C/cm). This result is comparable to results obtained from the commercial three-zone furnace (green rhombus in Fig. 4). The maximum temperature variation along the Sn cell of the ISOTECH furnace was measured to be 0.12 °C from 0 to 14 cm from the bottom (gradient of 0.009 oC/cm). Commercial furnaces do perform better, but it is considered that this heat pipe assembly can be applied in a practical manner for metrological purposes.
In order to study the effects of the heat pipe assembly on the phase transition behavior of the Sn fixed point, the test Sn cell was realized before and after the installation of the heat pipe assembly at melting and freezing offset temperatures of 238 °C and 229 °C, respectively. Figure 5 shows the melting and freezing curves for the results. Without the heat pipe assembly, the melting and freezing behavior of the Sn greatly deteriorated, as shown in Fig. 5(a). At the initial stage, the melting plateau temperature is uniform and looks like a conventional melting plateau of fixed-point cells, which means that the melting transition progressed as expected. As the temperature at the bottom is highest and relatively uniform along the cell, the ingot in this area may start to melt. After this flat region, the temperature gradually increased. This increase may be caused by the nonuniform melting from the middle to the top section of the ingot inside the cell as a result of the large temperature gradient, as shown in Fig. 4. Finally, the temperature rapidly rose to the melting offset temperature. When considering the freezing curve, it was observed to be steeply declined, which means that the temperature of the inside molten liquid was severely affected by the outside temperature profile of the cell. The small supercool peak was observed at a much higher temperature than the expected melting and freezing temperatures indicated by the red broken line. All these abnormal and unnecessary melting and freezing behaviors might be caused by the worst temperature gradient along the fixed-point cell.
However, after the installation of the heat pipe assembly, the melting and freezing curves were recovered as normal, as shown in Fig. 5(b). The melting plateau lasted for about 8 h with a stable and flat plateau within ± 1.5 mK. The freezing plateau was initiated, followed by a short period of supercooling of only 0.3 °C, below the guiding red broken line. This small amount of supercooling is abnormal when compared with the commonly known deep supercooling of 20 °C ~ 30 °C [5, 7]. It was not caused by the thermal conditions when using the heat pipe assembly because it was also seen when the cell had been calibrated. Later it was found to be ascribed to the oxidation of Sn caused from the sealed cell undergoing processes [8], and it could be restored by high-temperature carbothermal reduction at 750 °C. At the freezing offset temperature of − 3 °C, the freezing plateau lasted longer than 10 h within ± 4 mK, enabling the calibration of several secondary-class thermometers, such as thermocouples and industrial PRTs, during one realization. It was therefore found that the heat pipe assembly could enhance the thermal uniformity of a single-zone electric furnace suitable for the realization of the Sn fixed point.
At this point, it was shown that a heat pipe assembly made of several water heat pipes sealed in a Cu tube is economical and useful for the calibration of thermometers, and can even be applied for the realization of an Sn fixed point. To improve this approach, it was suggested that the weight of the heat pipe assembly be reduced for easy and convenient handling. The heat pipe assembly used was made of Cu or Cu alloy. Therefore, it was somewhat heavy, and it also took a long time to reach the target temperature of the furnace. To avoid this, it was thought that Al alloy could be an alternative, sacrificing 10% of thermal diffusivity (αAl = 102.0 mm2/s, αCu = 115.4 mm2/s at 25 °C) [9]. More specifically, the inner tube, base plate, and tube holder could be replaced by an Al alloy such as Al 6061. Another possible step for improvement would be eliminating the Cu outer cover pipe and tube holder, which was designed to protect and fix the heat pipes. However, after the heat pipe assembly was installed into the furnace, it was found that there was no opportunity for it to be damaged mechanically. The outer cover and tube holder are therefore not considered to be necessary components of the assembly. Eliminating these parts would also be helpful to increase the thermal reaction kinetics. From this consideration, an attempt was made to make an Al-base heat pipe container.
Figure 6 shows the three-dimensional schematic diagram of the container and the constructed heat pipe assembly, respectively. As shown in the figure, the container is composed of two parts, a commercial Al 6061 pillar tube well (outer diameter 58 mm, inner diameter 52 mm, and length 370 mm) and a base plate (outer diameter 80 mm, inner diameter 51.9 mm, and step 10 mm). These two parts were joined using four screw bolts. Finally, 20 heat pipes were attached to the surrounding outer surface of the pillar based on the grooves formed on the base plate, and fixed using an O-shaped clamp. A high-temperature heat-conductive ceramic epoxy from Cotronics (Resbond 920 powder), which can be activated by mixing 14 parts of water into 100 parts of powder by weight, was used to increase the thermal contact between the heat pipes and the pillar well. As a result, this Al-base container was considerably simpler and substantially lighter than the Cu-base container. The number of main parts was reduced to only two and the weight was decreased from 4.0 kg to 1.4 kg, including the 20 heat pipe tubes. In addition, the manufacturing cost was less than a third of that for the Cu-base container.
To examine the heat-conducting performance of the Al-base heat pipe assembly, it was loaded into the furnace, and then the Sn test cell was installed into the heat pipe assembly. Figure 7 shows the result of the temperature gradient profile at 225 oC, represented by red reverse triangles. For comparison, data from the Cu-base holder (black circles) is also presented. Up to 15 cm from the bottom, the gradient was measured as small as 0.015 oC/cm, which was a little less than that of the Cu-base. This performance may come from the smaller thermal mass of Al-base holder, leading to the higher heat-pumping capability. Above 15 cm from the bottom, however, the gradient became much steeper than for the Cu-base system, owing to the increased heat loss because the thermometer is moved closer to the ambient temperature as the distance from the bottom is increased. From the gradient tests, it was found that the heat pipe assembly with Al-base holder showed good thermal uniformity, which was more comparable to the commercial three-zone furnace.
Figure 8 shows the three repetitive melting and freezing transitions of the Sn fixed point in the Al-base heat pipe assembly. The melting and freezing offset temperatures were the same as for the case of Fig. 5, i.e., 238 oC for melting and 229 oC for freezing, respectively. As shown in the figure, it was reproducible and showed typical phase transition behaviors of the metal fixed points in thermometry. It also showed much flatter freezing plateaus when compared with the Cu-base heat pipe assembly shown in Fig. 5(b). In order to check the reproducibility of the melting temperature, the melting temperature of each realization was considered as the average value of 10 hours in the middle of the freezing plateau as indicated by the arrow in the Fig. 8. The resultant reproducibility (one standard deviation) was calculated to 4 mK. Its variation may come mainly from the instability of the Pt100 thermometer and the measuring device. Because the purpose of this work is not the exact determination of the Sn fixed-point temperature, but rather investigating the suitability of the assembly as a good uniform thermal enclosure, this level of standard deviation was considered as a successful result.
From the studies, it was found that using Al alloy as a heat pipe container was practical and useful. It is possible to also make a Cu-base container similar to the Al-base, but it is recommended that Al alloy be used because of the raw material price and weight. Conclusively, the heat pipe assembly manufactured in this study, regardless of raw materials, was demonstrated as a successful alternative for the expensive three-zone furnace or uncomfortable liquid bath as a uniform thermal enclosure for the calibration of thermometers. In addition, further studies can also investigate the application of the heat pipe assembly to an In fixed-point cell for which a dry system would be efficient and more convenient than a liquid bath.