Thin-Film Thermocouples Based on the System In2O3-SnO2

Ceramic thermocouples are being developed to replace noble-metal thermocouples that are unable to withstand the harsh environments inside the hot sections of turbine engines used for power generation and propulsion. A number of alloys in the system indium oxide (In2O3):tin oxide (SnO2) were systematically investigated as thermocouples. Specifically, solid solutions containing up to 10 wt% SnO2 were initially tested relative to a platinum reference electrode and the resulting thermoelectric properties were measured. The results indicated that the thermoelectric response was dependent on the SnO2 content in the alloy. Seebeck coefficients ranged from 53 to 224 mu V/degrees C at temperatures up to 1300 degrees C, which are considerably larger than those generated from metal thermocouples. Bi-ceramic thermocouples based on selected solid solutions of indium tin oxide (ITO) exhibited high temperature stability and Seebeck coefficient on the order of 160 mu V/degrees C. Postdeposition treatments had a significant effect on the stability of the ceramic thermocouples. High-temperature annealing improved the film uniformity, stability, and reproducibility of the ITO thin-film thermocouples. A bi-ceramic thermocouple consisting of In2O3 and In2O3:SnO2 (95:5 wt%) was the best-performing thermocouple of all compositions studied.