Thick film piezoelectric ceramic sensors have been successfully deposited on different metallic substrates with different shapes by a sol–gel spray technique. The ball-milled bismuth titanate fine powders were dispersed into PZT solution to achieve the gel. The films with desired thickness up to 200 µm have been obtained through the multilayer coating approach. These thick films were also effectively coated onto thin sheet metals of thickness down to 25 µm. Self-support films with flat and shell geometries were made. Piezoelectricity was achieved using the corona discharge poling method. The area of the top silver paste electrode was also optimized. The center frequencies of ultrasonic signals generated by these films ranged from 3.6 to 30 MHz and their bandwidth was broad as well. The ultrasonic signals generated and received by these ultrasonic transducers (UTs) operated in the pulse/echo mode had a signal to noise ratio more than 30 dB. The main advantages of such sensors are that they (1) do not need couplant, (2) can serve as piezoelectric and UT, (3) can be coated onto curved surfaces and (4) can operate up to 440 °C. The capability of these thick film UTs for non-destructive evaluation of materials at 440 °C has been demonstrated.

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Hydrogen atoms have been stabilized in nonequivalent lattice sites in matrices of the rare gases at liquid helium temperature. Electron spin resonance spectra of H atoms in argon, krypton, and xenon show that at least two trapping sites are involved in each case. In a neon matrix, H atoms have been stabilized in only one site. Attainability of the various trapping sites apparently depends on the initial energy of the H atom, a simple doublet spectrum being obtained when the atoms are deposited from the gas phase, while multiple trapping spectra are obtained when the atoms are produced by photolysis in the solid. The hyperfine coupling contants and the electronic g factors for H atoms trapped in the various matrix sites have been determined. The deviation of the hyperfine coupling constant from the free-state value is positive in some cases and negative in others. The experimental results are in good agreement with theoretical predictions. A complex multicomponent H atom spectrum was obtained by photolysis of HI in xenon. The subsplitting in the spectrum is attributed to magnetic hyperfine interactions with matrix nuclei (Xe129 and Xe131). The nature of the trapping sites in rare gas matrices is discussed. Evidence for trapping in substitutional sites and octahedral sites is presented.

Flexible ultrasonic transducers

Ultrasonic imaging, particle detection, and V(z) measurements have been performed using focused clad buffer rods in molten zinc at temperature more than 600 °C. The focused ultrasonic waves are generated by a spherical or cylindrical acoustic lens which is fabricated at the end of the clad buffer rod. In order to evaluate its focusing ability, several experiments are carried out at 10 MHz in a pulse-echo mode. The lateral resolution at the focus of the spherical acoustic lens in molten zinc is quantitatively examined and compared with that in water using a thin stainless wire with a diameter of 380 μm. High resolution ultrasonic imaging is carried out by the common C-scan technique. The signal-to-noise ratio of the reflected signals from the flat sample surface at the focus is better than 35 dB. Ultrasonic images are obtained from the amplitude and time delay variations of the reflected signals. An attempt has also been made to detect particles suspended in molten zinc. Backscattered signals from particles are clearly visible at the focal region of the lens. For quantitative materials evaluation, V(z) curve measurements are performed using both spherical and cylindrical surface lenses and the leaky surface acoustic wave velocity of a ceramic (SiC) plate immersed in molten zinc is successfully determined.

Acoustic characterization of doped silica glasses with a GeO,, Pz05, F, TiO,, Al,Oj, or B,O, dopant having different concentrations is presented. Quantitative measurements were performed with a 225-MHz line-focus-beam scanning acoustic microscope. The acoustic velocity variation due to different dopant concentrations is given. It has been found that the AI,O, dopant increases, but the other dopants decrease, the acoustic velocity as compared with that of the pure fused silica. We have also found that the fractional change in acoustic velocity is greater than that in refractive index for a given dopant concentration.