ITO-Nanocomposite Thin Film Strain Gages with Low TCR
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1024-A05-06
ITO-Nanocomposite Thin Film Strain Gages with Low TCR Otto J. Gregory, and Ximing Chen Chemical Engineering, University of Rhode Island, Kingston, RI, 02881 ABSTRACT Dynamic strain gages based on alloys of indium tin oxide (ITO) have been developed to monitor the structural integrity of gas turbine engine components operating at temperatures in excess of 1200 °C. Static strain measurements in these harsh environments have been hampered by apparent strain effects that can be mitigated by employing sensors with a low temperature coefficient of resistance (TCR). Since refractory metals typically exhibit a positive TCR and semiconductors such as ITO exhibit a negative TCR, an optimal combination of materials could produce a sensor with little or no apparent strain. These materials were combined in a single strain gage element, in which phase separation occurred at very small length scales. Specifically, nanocomposite strain elements comprised of ITO/(Pt, Pd, W, NiCoCrAlY, Ni and Ir) combinatorial libraries were prepared by co-sputtering from ITO and refractory metal targets onto alumina constant strain beams. TCR was measured from room temperature to 1100 °C and the piezoresistive response of the more promising combinations was measured at strain levels approaching 1000 µm/m and the chemical composition of the most promising combinatorial libraries was analyzed by SEM/EDS. Preliminary results indicated that a strain gage based on an ITO/platinum (12% ITO: 88% Pt) nanocomposite was fabricated with a near zero TCR over the temperature range 500-1100 °C. The gage factor of this nanocomposite remained unchanged (~-26.0) compared to that of ITO and thus, it appears that a simple additive mixture rule does not apply here and that other physics are controlling the piezoresistive response in these materials. INTRODUCTION The development of advanced propulsion systems requires the assessment of the mechanical behavior of blades and disks in the hot section of gas turbine engines. Thin film strain gages are ideally suited for in-situ strain measurement in these turbine engines since they are non-intrusive, have negligible mass and do not affect the gas flow path through the engine. Ceramic strain gages based on reactively sputtered indium tin oxide (ITO) have been developed for dynamic strain measurement at elevated temperature [1-3]. These ceramic strain gages exhibit excellent oxidation resistance and high temperature electrical stability and are capable of surviving tens of hours of testing at temperatures as high as 1550°C in air [4-6]. However, static strain measurement under typical engine conditions using resistance strain gages is difficult because resistance changes are functions of temperature and time as well, i.e., R=R(ε, T, t). To a first approximation, the relative change in resistance of a strain gage is: ∆R 1 ∂R 1 ∂R 1 ∂R = ⋅ ⋅ ∆ε + ⋅ ⋅ ∆T + ⋅ ⋅ ∆t R R ∂ε T , t R ∂T ε , t R ∂t ε , T
(1)
where the first term, gage factor (G), is a measure of the piezoresistive response or sensitivity of the gage. The
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