Stability and piezoresistive properties of indium-tin-oxide ceramic strain gages

Thin film strain gages were developed to assess the mechanical behavior of components used for power and propulsion systems in harsh environments. Several types of strain sensors were fabricated with indium tin oxide (ITO) active strain elements and were tested at temperatures up to 1560°C. These ceramic sensors exhibited excellent thermal stability and piezoresistive response at elevated temperature. A self-compensated ceramic strain gage (8 μm thick) with thin film platinum resistors placed in series with the ITO active element exhibited a gage factor of 20.9 and a drift rate of 0.010%/hour at 1443°C. To further increase the sensitivity and responsiveness of the ceramic sensors, uncompensated ITO strain gages prepared with an enlarged active area showed an increased piezoresistive response at 1455°C with a gage factor of 39.1. Thus, at temperatures beyond 1400°C, thickness of the ITO elements played a significant role in the high temperature stability and sensitivity of the gages. Self-compensated ITO sensors prepared with 8 μm thick survived 20 hours at 1481°C with a gage factor 2.36 and drift rate 0.0092%/hour. However, 10 μm thick ITO strain gages were operated at temperatures beyond 1500°C for tens of hours. The latter sensors exhibited a drift rate < 0.00001%/hour and a gage factor of 131 at 1530°C. SEM micrographs of thick ITO sensors revealed a partially sintered microstructure with a uniform distribution of nanopores. To reproduce this microstructure by alternative means, ceramic sensors were prepared in nitrogen rich environments and tested to verify the large piezoresistive response at temperatures above 1550°C. The results of our efforts to improve the stability and piezoresistive response of ITO strain gages at high temperature are presented here and the potential applications of these nanoporous ceramic sensors are discussed.