Nickel–Silver Monotectic in Alumina Crucible for Use with Contact Thermometry

Previously, the authors have published work describing a pure Ni fixed point within alumina crucibles. The success of this study stimulated working with the Ni–Ag monotectic point in alumina crucibles. Similar to eutectic points, the Ni–Ag monotectic temperature is an invariant point but it differs from a eutectic reaction in such a way that the monotectic phase change takes place from Ni–Ag liquid solution to Ni–Ag solid solution and Ag rich Ni–Ag liquid solution. In the phase diagram references, the Ni–Ag monotectic phase transition temperature is assigned to be about 20 ^∘ C below the pure Ni melting/freezing point. As is the case for pure Ni, mechanical stability is one of the concerns. Therefore, proper cell design is necessary to avoid breakage of the alumina crucible. The techniques used for the fabrication and measurement of the pure Ni cell were applied to the Ni–Ag cell as well. The cells have been successfully fabricated and the temperature measurement at the fixed point was carried out for more than 20 thermal cycles in total. A Pt/Pd thermocouple was used to measure the temperature and was calibrated from the tin point to the gold point to measure the ITS-90. Freezing plateaus are realized with the technique of “recurrent offset freezing method with reserved solid”. The duration of each freezing plateau is a minimum of 30 min. The monotectic transformation temperature for the best performed cell is determined as 1428.27 ^∘ C with a combined uncertainty of ±0.06 ^∘ C ( k=1 ).