Ultrasonic-Pulse Propagation through Films, Foils, Fibers, and Whiskers

Ultrasonic-pulse techniques are extended to very thin samples in the forms of films, foils, thin plates, whiskers, fibers, and fine wires. Details of the experimental techniques are described. Separate sample and transducer supports are used with independent micrometer screw adjustments to bring sample and transducers into alignment and nondestructive contact. The sample behaves like an elastic waveguide that propagates in the lowest-order modes. Major emphasis is on the lowest compressional and horizontal shear modes in isotropic planar samples and lowest compressional and torsional modes in isotropic cylindrical samples. Propagation in the 10 Mc/sec region was achieved through planar samples ranging down to 1000 Å, in thickness and through cylindrical samples ranging down to a few microns in diameter. The thinnest samples require the use of buffer plates. Representative measurements of velocity and elastic constant are presented for metal, semiconducting, and insulating materials. Applications and limitations of the described techniques are discussed.