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Abstract
Piezoelectric transducers are used extensively as acoustic sensors and actuators in the non-destructive testing (NDT) of engineering structures and materials at room temperature. There is also a need for acoustic sensors that can operate at high temperatures, for example the monitoring of acoustic disturbances in gaseous combustion processes that cause combustion instability in the gas phase and structural vibrations in combustion apparatus. The sensors could be integrated into a smart control system to stabilize the combustion process and minimize structural vibrations. Another application is in an embedded sensor network for NDT of high temperature composite vessels.
Tantalum pentoxide (Ta2O5) was investigated as a possible high temperature resistant piezoelectric material. Thin films of Ta2O5 were deposited by reactive sputtering with and without substrate heating. Crystalline films were obtained by film deposition at 450°C and by annealing films deposited without substrate heating. Crystalline β-Ta 2O5 is formed at these conditions with either monoclinic or orthorhombic crystal structure, which is stable up to 1360°C and piezoelectric. The piezoelectric response of thermally crystallized Ta2O 5 film was investigated qualitatively for different annealing temperatures. The piezoelectric response for Ta2O5 films deposited in the crystallized form at 450°C were found to have a stronger piezoelectric response than thermally crystallized Ta2O5 films and the piezoelectric constant, dz1, of the film determined, based on a freely vibrating cantilevered beam technique. The technique could possibly be extended to measurement of piezoelectric properties of films at elevated temperatures. The constant dz1 was determined for Ta2O 5 film deposited at 450°C, and for film subsequently exposed to 700°C in air.
A spring-node model with simulation results is also presented for stress wave propagation in isotropic and transversely isotropic materials with or without piezoelectric properties for intact and flawed material. The model was applied to the impact-echo NDT technique for thick fiber composite materials where a piezoelectric film sensor is used. Experimental and theoretical sensor responses are compared.
