Thermoacoustic generation of airborne ultrasound using carbon materials at the micro- and nanoscale
(2012)
The generation of airborne ultrasound is presented using the thermoacoustic principle applied to carbon materials at the micro- and nanoscale. Such materials are shown to be capable of emitting ultrasound when being fed by an alternating current. We tested the acoustic performance of carbon fibers, bucky papers and electrospun polyacrylonitrile-derived carbon nanofibers and determined the sound pressure for frequencies up to 350 kHz. A comparison between the experimental results and the theoretical prediction showed remarkable agreement for frequencies up to 150 kHz. Beyond 150 kHz, we found slight deviations from the expected sound pressure dependence on the square root of the frequency.
A method has been developed for the generation of airborne ultrasound using the thermoacoustic principle applied to carbon materials at the micro- and nanoscale. Such materials are shown to be capable to emitting the ultrasound. We tested the acoustic performance of electrospun polyacrylonitrile-derived carbon nanofibers tissues and determined the sound pressure for frequencies up to 350 kHz. The experimental results are compared to analytic calculations.
Novel air-coupled ultrasonic transducer combining the thermoacoustic with the piezoelectric effect
(2016)
In recent years, there has been an increasing industrial demand for one-sided inspection of various structures by means of air-coupled ultrasonic technique. Lightweight structures based on carbon-fibre-reinforced polymers may have very complex shapes, making air-coupled transmission difficult or even impossible. The inspection of concrete structures is another example where one-sided inspection is required.
To address these challenges a new type of transducer for air-coupled pulse-echo inspection was developed, which unites two principles: thermoacoustic emission and piezoelectric reception. The thermoacoustic emitter is a titanium electrode with a thickness of several tens of nanometer. This electrode was deposited onto charged cellular polypropylene, which serves as a piezoelectric receiver. The thermoacoustic transmission is based on a transformation of the thermal energy of an electrically heated electrode into the acoustic energy of an ultrasonic wave. Thermoacoustic emitters provide resonance-free behaviour and thus extremely broadband pulses. Charged cellular polypropylene is piezoelectric due to the polarization of its cells and it is well matched to air, with a Young modulus in the order of magnitude of MPa. In this contribution we present some pulse-echo measurements with the first prototypes of the combined thermoacoustic-piezoelectric transducer.