A Schottky-Diode Model of the Nonlinear Insulation Resistance in HTSPRTs—Part II: Detailed Two- and Three-Wire Measurements

Electrical leakage is a significant factor in the uncertainty of temperature measurements employing high-temperature standard platinum-resistance thermometers, with effects as large as several millikelvin at the freezing point of silver (962◦C). The insulation resistance also exhibits complex behavior that includes non-linearities, sensitivity to the electrical environment, and the generation of spurious voltages and currents. In an earlier article, it was suggested that the behavior is consistent with the existence of metal–semiconductor diodes, also known as Schottky-barrier or pointcontact diodes, formed at the points of contact between platinum wire and fused-silica insulators supporting the platinum. In this article, we describe detailed measurements of the non-linear resistance of a fused-silica insulator supported by platinum wires. The discussion includes a detailed description of the measurement system, and the results of two experiments that show many of the features suggested by the metal– semiconductor-diode model. Observed features in the current–voltage measurements include an S-shaped feature characteristic of back-to-back diodes, temperature dependence of saturation currents consistent with thermionic emission, and a diode polarity consistent with silica being a p-type semiconductor. Some impacts of the model on thermometry practice are also noted.