Piezoresistive Properties of ITO Strain Sensors Prepared with Controlled Nanoporosity

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 high-temperature 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 nanopo-rosity. 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 th