The Melting Point of Palladium Using Miniature Fixed Points of Different Ceramic Materials: Part I-Principles and Performances

Fifteen miniature fixed-point cells made of three different ceramic crucible materials (Al2O3, ZrO2, and Al2O3(86%) + ZrO2 (14%)) were filled with pure palladium and used for the calibration of type B thermocouples (Pt30% Rh/Pt6% Rh). The melting behavior of the palladium was investigated by using different high-temperature furnaces usable in horizontal and vertical positions. It was found that the electromotive forces measured at the melting temperature of palladium are consistent with a temperature equivalent of +/- 0.25 K when using a furnace with an adequate temperature homogeneity (+/- 1 K over a length of 12 cm), independent of the ceramic crucible materials. The emfs measured in the one-zone furnaces with larger temperature gradients along the crucibles are sensitive related to the position of the crucibles in the temperature gradient of these furnaces. This is caused by higher parasitic heat flux effects which can cause measurement errors up to about -(1-2) K, depending on the thermal conductivity of the ceramic material. It was found that the emfs measured by using crucibles with lower thermal conductivity (ZrO2) were less dependent on parasitic heat flux effects than crucibles made of material of higher thermal conductivity (Al2O3). The investigated miniature fixed points are suitable for the repeatable realization of the melting point of palladium to calibrate noble metal thermocouples without the disadvantages of the wire-bridge method or the wire-coil method.