A SURFACE LAYER MODEL FOR THE TOTAL RESISTIVITY OF THERMOMETRIC-GRADE PLATINUM WIRE

Abstract

A model for the temperature dependence of the total resistivity of thermometric-grade platinum wire is developed. The evidence supports as a basic model the two-electronic conduction-band theory of Sondheimer and Wilson. However, in the small-diameter (~80 μm) wire, here considered, surface contamination would lead to deviations from the simple model. A theory of multiple conduction bands, due to surface contamination, has been developed. The theory enables us to infer, from known calibration data on platinum resistance thermometers, that there are two regions of contamination. One is a region of heavily concentrated impurity over a few atomic layers at the surface, which accounts for variations in resistivity near 700 °K. The other is a subsurface layer about 0.1 μm thick with about 0.1% impurity level, which accounts for variations in resistivity in the region of 90 °K.