Development of high temperature ultrasonic guided wave transducer for continuous in service monitoring of steam lines using non-stoichiometric lithium niobate piezoelectric ceramic

摘要

The dielectric, elastic, piezoelectric and electromechanical coupling coefficients of shear mode nonstoichiometric Lithium Niobate (LN) piezoelectric ceramics have been determined using the resonance method from room temperature to 600°C. An ultrasonic guided wave (UGW) transducer was manufactured by bonding a LN piezoelectric ceramic to an aluminium oxide backing block using a high temperature joining technique. Temperature dependence of the transducer from room temperature to 600 ? C has been experimentally determined. As an aid to data interpretation, a simple approximate model of the temperature dependence of the normalised received signal amplitude of a pulsed ultrasonic propagation system based on LN is developed. This model, has the advantage of avoiding the use of complex electromechanical equivalent circuits. It provides a qualitative explanation of the general form of the data from ultrasonic experiments in the range from room temperature to 600 °C. The received ultrasonic pulse power, at the highest temperature, was observed to be as good as at room temperatures. It is therefore concluded that non-stoichiometric LN is potentially suitable for use in direct contact with structures at up to 580 °C such as arise with fossil fuel plant steam lines, subject to the results of further work on long term ageing trials. However, the model indicates how the overall performance at all temperatures in the range considered might be enhanced by improved impedance matching and acoustic coupling system design. The Curie Weiss temperature for the crystals (T0) has been estimated from dielectric constant data and found to be much lower than the Curie temperature (Tc).