Piezoresistive Properties of Ceramic Strain Sensors with Controlled Nanoporosity
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Piezoresistive Properties of Ceramic Strain Sensors with Controlled Nanoporosity Otto J. Gregory and Tao You Sensors and Surface Technology Partnership Chemical Engineering Department, University of Rhode Island, Kingston, RI 02881 ABSTRACT A ceramic strain gage based on reactively sputtered indium tin oxide thin films is being developed to monitor the structural integrity of components employed in aerospace propulsion systems that operate at temperatures in excess of 1500°C. When relatively thick indium-tinoxide (ITO) strain gages were prepared by reactive sputtering in oxygen:argon atmospheres and annealed in nitrogen, an extremely stable piezoresistive response was observed at temperatures as high as 1530°C. SEM and AFM of these sensor surfaces after high temperature exposure revealed a partially sintered microstructure with interconnected nanoporosity. Specifically, the microstructure consisted of a contiguous network of uniform sized ITO particles with welldefined necks between individual particles. When these microstructures were compared to those of relatively thin ITO sensors sputtered in nitrogen:argon:oxygen atmospheres, i.e. ITO films prepared in a nitrogen rich plasma, the average pore size and particle size was estimated to be an order of magnitude smaller than those associate with thick ITO sensors. In the nitrogen sputtered films, enhanced electrical conduction along the surfaces of the contiguous ITO particles resulted in a very stable and large piezoresistive response with a gage factor of 11.4 and a drift rate of 0.0001%/hour at 1560°C. The improved performance realized when the ITO films were processed in nitrogen may be extended to other ITO based sensors including gas sensors and the advantages of films processed in this manner will be discussed. INTRODUCTION Thin film sensors are ideally suited for direct physical measurements on turbine components in harsh environments and are being used to assess the mechanical behavior of turbine components during actual engine operation, so that structural models can be validated and non-contact optical methods can be calibrated. These sensors are non-intrusive in that the gage thickness is considerably less than the gas phase boundary layer thickness and thus, the gas flow path through the engine will not be adversely affected. The active strain elements in these ceramic sensors are based on alloys of indium-tin-oxide (ITO) which are extremely oxidation resistant and do not undergo any phase changes when thermally cycled between room temperature and 1500oC [1-7]. ITO strain gages were prepared with controlled nanoporosity by subjecting relatively thick ITO films to a post deposition anneal at 800oC in nitrogen. Alternatively, very thin ITO films with these characteristics were prepared by reactive sputtering in various nitrogen/oxygen/argon partial pressures, followed by high temperature exposure. Scanning electron microscopy (SEM) and atomic force microscopy (AFM) indicated that although the microstructures of the ITO films prepared in a nitrogen
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