Ultrasonic transduction for high temperatures
Beating the heat with pulsed electromagnet EMATs
In general, ultrasonic methods are well established for material characterization and Non-destructive Testing (NDT) in research and industry. However, most transducers are restricted to temperatures below 200-300 °C. Currently, there are a number of methods used for inspection of components at high temperature, including eddy current based methods and laser based interferometric methods, amongst others. They are usually expensive, highly dependent on surface condition and require fairly large diameter optics, even when fibre-coupled. Additionally, high temperature applications are receiving increased attention in the piezoelectric materials community, but their long term performance is yet to be demonstrated.
Therefore, the design of novel ultrasonic generation and detection devices and their implementation with combined measurement systems to deal with high temperature inspection problems are of great interest to both the academia and the industry.
Previous work has shown that EMATs (Electromagnetic Acoustic Transducers) using permanent magnets can be employed to find and size defects at elevated temperatures. However, the temperature of the magnet must be kept below its Curie point, and thus cooling is required. An alternative approach is to employ an electromagnet, as was typical in the earlier generations of EMATs and is still used in various designs today. Electromagnets can be used at elevated temperatures without cooling, beyond the maximum operation temperature of permanent magnets such as NdFeB (Neodymium Iron Boron) or SmCo (Samarium Cobalt).
Preliminary results in the design of an EMAT with a pulsed electromagnet; intended for high temperature operation with the ability to generate and/or detect the ultrasonic modes across a range of frequencies for different application, can be seen in the following conference papers [QNDE 08 , QNDE 09 ].
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