An IUT can be coated directly onto curved surfaces and op- erated at elevated temperatures [16, 17]. Figure 5a shows an IUT made of a 60 µm thick BIT-c film and deposited onto a steel tube of 25.4 mm outer diameter and 6.4 mm wall thickness and measured by a handheld EPOCH LT pulser- receiver at 400 ◦C. BIT was chosen because it can operate up to 500 ◦C [16, 23]. The diameter of the top silver paste electrode was 5 mm and was chosen to obtain the maximum
signal strength at room temperature. In Fig. 5a, 40.8 dB gain out of the available 100 dB receiver gain was used. The measured ultrasonic signal obtained in pulse-echo mode at 400 ◦C is presented in Fig. 5b, where L n is the n th travel of the ultrasonic echo through the tube wall thickness. At 400 ◦C the center frequency and the 6 dB bandwidth of the L 2 are 12.7 MHz and 5.8 MHz respectively. The low repetition rate of 10 Hz was chosen to per-form all the experiments presented below. Figures 6a and 6b present the IUT received ultrasonic signals averaged over 10 laser pulses of 2 mJ energy and 5 ns duration with sample at room temperature and at 400 ◦C, respectively. The averaging over 10 laser pulses is used to reduce the electrical noises. The laser beam spot size was 0.5 mm and impinged at the center of the top silver electrode of the IUT. Since the laser generation is in the ablation regime, the silver top electrode impinged by the laser beam eventually disappeared. How-ever, there is little change in the measured ultrasonic signal. If the center of the IUT does not have a BIT-c film, the 0.5 mm laser spot directly impinge onto the steel tube and the signal received signal by the IUT surrounding the 0.5 mm spot is expected to be somewhat different but should be suffi-cient strong for thickness measurements. At 400 ◦C the cen-ter frequency and the 6 dB bandwidth of L 2 echo generated by the pulsed laser and received by the IUT are 13.7 MHz and 7.4 MHz respectively. Figures 6a and 6b show well sep-arated echoes which indicate that the frequency bandwidth achieved by the IUT in combination with pulsed laser gen-eration is sufficient to inspect a 6.4 mm wall thickness steel pipe. This confirms that this IUT made of a BIT-c film is a sensitive receiver on curved surface for laser generated ul-trasound up to at least 400 ◦C.