Thermophysical Data on Platinum
Thermophysical Data on Platinum
Resistivity and Conductivity Values Recommended
Platinum is one of several key materials whose thermophysical properties such as electrical and thermal conductivities, heat capacity, coefficient of thermal expansion and diffusivity are used internationally as a basis for calibration and reference purposes. The choice of platinum among this key group of materials (which includes iron, copper, tungsten, silicon and sapphire) is based on its stability and inertness, its ready availability in high purity, its well-established electrical resistance-temperature relationship between 13.8 and 1200K, measured for the International Practical Temperature Scale (IPTS), and the consistency of measurements of thermal conductivity made at several national laboratories over the temperature range 100 to 1200K.
“Recommended” values of these properties for this group of materials are currently being prepared through the codata Task Group on Thermophysical Properties of Solids, which consists of an international co-operation between national laboratories. The recommended values of the electrical resistivity and thermal conductivity of platinum were reported by Guy K. White of the CSIRO Division of Applied Physics, Sydney, Australia, during the conference, “Thermal Conductivity 17”, held at Gaithersburg, Maryland, U.S.A., in June 1983.
The task of selecting data for platinum has been made easier because of its role in the realisation of the temperature scale (IPTS) which is defined from 13.8 to 904K. Further efforts to extend the range of platinum resistance thermometers to the gold point (1337.6K) has contributed further data. The data have been corrected for factors such as thermal expansion and impurity scattering and fitted to polynomial equations. The recommended values are summarised in the Table as a function of temperature between 20 and 2000K. Resistivity at temperatures below 13.8K is not considered to be sufficiently accurate for recommended values to be assigned. Above this temperature accuracy is considered to be within 0.1 per cent, although only 0.3 per cent above 1300K.
The 1972 survey of the data by cindas has been taken as the basis for this study with more recent data at temperatures above 1000K being included. The influence of the purity of the platinum, defined by the average resistance ratio, on the low temperature values has also been examined. This leads to the conclusion that the accuracy of the recommended values lies within 10 per cent below 100K, but improves to less than 3 per cent between 100 and 300K and less than 2 per cent in the range 300 to 1200K. Above 1200K accuracy decreases to 10 per cent uncertainty. The recommended values are again summarised in the Table; the values above 100K have been corrected for expansion. Those given for temperatures above 1600K are based on data that leads to values of the Lorenz ratio that are considered to be too high. More plausible values of the conductivity are given in the Table in brackets, but the author cautions that more measurements in this temperature range are needed.
The conference proceedings of “Thermal Conductivity 17” have been published by Plenum Press, New York, 1983 and readers of Platinum Metals Review are advised to refer to the original paper for a fuller description of the recommended values for both resistivity and conductivity, and for the basis on which they have been derived.