Abstract
Continuous ultrasonic in-situ monitoring for industrial applications is difficult owing to the high operating temperatures in industrial fields. It is expected that ultrasonic transducers consisting of a CaBi4Ti4O15(CBT)/Pb(Zr,Ti)O3(PZT) sol-gel composite could be one solution for ultrasonic nondestructive testing (NDT) above 500 °C because no couplant is required and CBT has a high Curie temperature. To verify the high temperature durability, CBT/PZT sol-gel composite films were fabricated on titanium substrates by spray coating, and the CBT/PZT samples were tested in a furnace at various temperatures. Reflected echoes with a high signal-to-noise ratio were observed up to 600 °C. A thermal cycle test was conducted from room temperature to 600 °C, and no significant deterioration was found after the second thermal cycle. To investigate the long-term high-temperature durability, a CBT/PZT ultrasonic transducer was tested in the furnace at 600 °C for 36 h. Ultrasonic responses were recorded every 3 h, and the sensitivity and signal-to-noise ratio were stable throughout the experiment.
Highlights
Ultrasonic monitoring at elevated temperatures is desired in industrial fields because it can detect failure phenomena such as thickness reduction, corrosion cracks, micro-void generation, and processed material deterioration in an early stage [1,2,3]
Air-coupled ultrasonic transducers have good acoustic impedance matching with air; it is suitable for the nondestructive testing (NDT) of light materials such as graphite composites but not for the NDT of metals, which are the main targets of high-temperature ultrasonic NDT
CaBi4 Ti4 O15 (CBT)/PZT films were fabricated on titanium substrates with a 3 ± 0.2 mm thickness, a 30 ± 1 mm length, and a 30 ± 1 mm width
Summary
Ultrasonic monitoring at elevated temperatures is desired in industrial fields because it can detect failure phenomena such as thickness reduction, corrosion cracks, micro-void generation, and processed material deterioration in an early stage [1,2,3]. Even though ultrasonic nondestructive testing (NDT) is one of the common NDT techniques [4], long-term high-temperature application of ultrasonic NDT is difficult. For the general ultrasonic transducer using the contact method, an acoustic couplant to transmit ultrasound into test objects and a backing material to achieve a broad frequency bandwidth are used [5]. These materials cannot withstand elevated temperatures in the long term. Air-coupled ultrasonic transducers have good acoustic impedance matching with air; it is suitable for the NDT of light materials such as graphite composites but not for the NDT of metals, which are the main targets of high-temperature ultrasonic NDT. The use of a delay line is another method to avoid the direct contact of the ultrasonic measurement system with high temperatures [8], though the measurement system becomes bulky
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