Stabilization of Indium Tin Oxide Films to Very High Temperatures
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Stabilization of Indium Tin Oxide Films to Very High Temperatures Otto J. Gregory, Tao You, Michael Platek and Everett Crisman Chemical Engineering Department, University of Rhode Island, RI, 02881 Summary: Thin film strain gages based on indium-tin-oxide (ITO) are being developed to measure to static and dynamic strain at temperatures approaching 1500oC. These ceramic strain gages exhibit excellent oxidation resistance and high temperature stability, surviving more than 25 hours of testing in air at 1470oC. Electron spectroscopy for chemical analysis (ESCA) studies indicated that interfacial reactions between ITO and alumina can increase the stability of ITO at elevated temperature. Solid state diffusion of aluminum into the ITO at these temperatures can produce a very stable ITO/Al2O3 solid solution [1, 2]. To determine the nature of the interfacial reaction product, ITO films were deposited onto both Al2O3 and AlN surfaces and thermally cycled to 1500oC. AlN films were used to reduce/eliminate oxygen transport to the interface, so that aluminum-indium interactions alone could be studied. ITO films were deposited onto Al2O3 and AlN films, which were rf sputtered on platinum-coated alumina substrates. The resulting ESCA depth files showed that an interfacial reaction had occurred between the ITO and the Al2O3 and AlN. The presence of two new indium-indium peaks at 448.85 and 456.40eV, corresponding to the indium 3d5 and 3d3 binding energies were observed in both cases; i.e. the AlN and the Al2O3. These binding energies are significantly higher than those associated with stoichiometric indium oxide. In addition, aluminum doped ITO films were formed by co-sputtering from multiple targets and electrical stability of these films was compared to undoped ITO films over the same temperature range (25-1500oC) [1-4]. Introduction: As the temperature capability of engine components is increased and new materials are developed to meet these new challenges, there is a growing need to assess material behavior in these environments so that structural models can be validated and engine materials can be further characterized. Towards this end, a ceramic strain gage based on alloys of indium-tin oxide (ITO) was developed to monitor both static and dynamic strain of components employed in propulsion systems operating at temperatures in excess of 1500°C. The sensor elements are oxidation resistant and do not undergo any phase change when thermally cycled between room temperature and beyond1500oC. To further improve the stability of ITO film, aluminum was deposited onto the ITO surface as a protection. Electron spectroscopy for chemical analysis (ESCA) was used to verify the existence of an interfacial reaction between ITO and aluminum and its stabilizing effect on the ITO films. Experiments were performed on ITO films rf sputtered onto alumina and compared with films deposited onto AlN film. Experimental: High temperature test coupons were fabricated by sequentially depositing almuina or aluminumnitride films, platinum films
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