An apparent n to p transition in reactively sputtered indium-tin-oxide high temperature strain gages

A robust strain gage based on alloys of indium-tin-oxide (ITO) has been developed, which is capable of measuring strain at temperatures up to 1450 degreesC. These thin film sensors are ideally suited for in-situ strain measurement in harsh environments since they are non-intrusive, have minimal impact on vibration patterns due to their negligible, mass and are robust enough to withstand the high 'g' loading associated with rotating components. Thus, this ITO, strain gage is well suited to meet instrumentation requirements in advanced propulsion systems. Static strain tests performed at temperatures as high as 1,400 degreesC have resulted in a relatively large and repeatable piezoresistive response. However, in the vicinity of 950 degreesC, a change in sign of the piezoresistive response from -G to +G was observed, suggesting that the active ITO strain element had been converted from a net 'n-carrier' to a net 'p-carrier' semiconductor. The 'n' to 'p' transition has been shown to be reversible over many temperature cycles from room temperature to 1400 degreesC. This repeatability implies that the carrier species is the predominate factor controlling the observed changes in the resistance and gage factor. Consistent with this change in sign of the gage factor was a change in the sign of the slope of emf vs. temperature (dV/dT) for hot probe measurements made on the same ITO films that were thermally cycled over the temperature range (600 degreesC to 1300 degreesC). This finding, supports the premise that change in sign of the gage factor from -G to +G occurred within the same temperature range (similar to950 degreesC) and that, a change in the charge carrier type was responsible for observed transition in both cases. (C) 2002 Elsevier Science B.V. All rights reserved.