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A ceramic thermocouple based on indium-tin-oxide (ITO) thin films is being developed to measure the surface temperature of gas turbine engine components employed in power and propulsion systems that operate at temperatures in excess of 1500 C. By fabricating ITO elements with substantially different charge carrier concentrations, it was possible to construct a robust ceramic thermocouple. A thermoelectric power of 6.0 V C, over the temperature range 25-1250 C, was realized for an unoptimized ITO ceramic thermocouple. The charge carrier concentration difference in the legs of the ITO thermocouple was established by r.f. sputtering in oxygen-rich and nitrogen-rich plasmas. SEM micrographs revealed that after high-temperature exposure, the surfaces of the nitrogen prepared ITO films exhibited a partially sintered microstructure with a contiguous network of ITO nanoparticles. Thermal cycling of ITO films in various oxygen partial pressures showed that the temperature coefficient of resistance was nearly independent of oxygen partial pressure at temperatures above 800 C and eventually became independent of oxygen partial pressure after repeated thermal cycling below 800 C. Based on these results, a versatile ceramic sensor system has been envisioned where a ceramic thermocouple and strain sensor can be combined to yield a multifunctional ceramic sensor array.Index Terms-Ceramic thermocouple, indium-tin-oxide (ITO).

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A ceramic strain gage based on reactively sputtered indium-tinoxide (ITO) thin films is being developed to monitor the structural integrity of components employed in aerospace propulsion systems operating at temperatures in excess of 1500°C. The hightemperature stability and piezoresistive properties depend to a large extent on the thickness of the active ITO strain elements comprising these ceramic strain gages. Scanning electron microscopy of the thick ITO sensors revealed a partially sintered microstructure consisting of a contiguous network of submicrometer ITO particles with well-defined necks and isolated nanoporosity. It appeared that densification of the ITO particles was retarded during high-temperature exposure with nitrogen playing a key role in stabilizing the nanoporosity. Based on these preliminary results, ITO strain sensors were also prepared by reactive sputtering in various nitrogen/oxygen/argon partial pressures to incorporate more nitrogen into the films. Under these conditions, sintering and densification of the ITO particles containing these nitrogen-rich grain boundaries was retarded and a contiguous network of nanosized ITO particles was established. The influence of nitrogen in the sputtered and annealed ITO films on the microstructure and the high-temperature piezoresistive properties was investigated, and the results are presented in this paper. © 2004 The Electrochemical Society. [DOI: 10.1149/1.1767839] All rights reserved

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