SPUTTERED HIGH-TEMPERATURE THIN-FILM THERMOCOUPLES

Thin film thermocouples have the advantage of extremely fast response (< 1 ms), low cost, and very fine spatial resolution. These capabilities have permitted measurements never before possible, such as the instantaneous temperature measurements of the inside surfaces of a diesel engine cylinder during operation. Although these type S platinum plus platinum 10% rhodium thermocouples are stable at the 900-1100-degrees-C range, oxidation of rhodium can take place in the 600-800-degrees-C range in air which lessens the thermoelectric potential of the positive element. We evaluated the effect of air exposures to high temperature thin film thermocouples and attempted to find solutions to the problem. We found that after 50 h at 750-degrees-C the Seebeck coefficient of a platinum 10% rhodium element was reduced 50% compared to the as-sputtered condition. The reduction of Seebeck coefficient at 700 and 800-degrees-C was also substantial. This reduction in Seebeck coefficient could be recovered by annealing the thin film at 900-degrees-C. The degradation after 750-degrees-C could be suppressed using a 1 mum sputtered aluminum oxide coating. The reversible behavior suggests the formation of Rh2O3 at 700-800-degrees-C and its subsequent reduction at temperatures at which the free energy of formation is zero or positive. Microstructural evidence confirm these conclusions.