ABSORPTION OF ULTRASONIC WAVES BY SINGLE CRYSTALS

Investigations of the interaction of ultrasonic vibrations with matter are of great interest, since they can yield new information concerning its structure and other physical properties. As shown by the work of S. Ya. Sokolov[^1], the absorption of ultrasonic vibrations in a metal depends on the dimensions and mutual orientation of the crystals composing it, as well as on the relation between the length of the ultrasonic wave and the dimensions of the crystals. The amplitude and phase of the resulting vibrations are the result of the statistical summation of ultrasonic waves which, after being reflected many times from the faces of the crystals, are superposed on one another with different amplitudes and phases. The picture of interaction is substantially simplified if the ultrasonic vibrations

pass through a single crystal of some substance which, moreover, contains no internal inhomogeneities. Although here, too, the theoretical analysis of the phenomena proves to be far from simple, it is nevertheless considerably more accessible than in the interaction with polycrystalline bodies.

Another work by S. Ya. Sokolov[^2] is devoted to the analysis of the interaction of ultrasonic oscillations with single crystals of quartz, as well as with table salt and Rochelle salt. The investigations were carried out as follows: the source of the ultrasonic rays was a piezoelectric quartz plate, much smaller in size than the single crystal. It was pressed tightly against the surface of the single crystal and excited ultrasonic pulses of duration one microsecond or less. The frequency of the ultrasonic oscillations was varied within the limits from \(1.6 \cdot 10^7\) to \(1 \cdot 10^9\) Hz. After passing through the single crystal, the ultrasonic pulses were amplified and recorded with a cathode oscillograph, similarly to how this is done in radar installations. Since the absorption of ultrasound in single crystals is usually small, multiple reflection from opposite faces of the crystal occurs, and on the oscillograph screen there appears a group of equally spaced peaks of decreasing height (Fig. 1). The number of them serves as a measure of the absorption of ultrasound in the crystal (in a given direction of propagation), and the distance between neighboring peaks corresponds to the path traversed by the ultrasonic pulse.

More than ten quartz single crystals were investigated. The following interesting phenomena were observed:

1. In investigating the propagation of ultrasonic oscillations in the directions of the optical, electrical, and mechanical axes of the single crystal, it was established that the absorption coefficient depends substantially on direction: its smallest value was found along the optical axis, and the largest in the direction perpendicular to the optical axis; the difference between these values in some specimens reached several tens of times. The anisotropy of absorption is evidently connected with the anisotropy of the viscosity of the crystal.

2. In some crystal specimens, transverse oscillations also arose simultaneously with longitudinal oscillations (Fig. 2). On the oscillogram two groups of peaks are clearly visible, with different distances between neighboring peaks of each group, in accordance with the different propagation velocities of longitudinal and transverse oscillations.

3. In three quartz single crystals, the simultaneous formation was observed of two groups of peaks having a considerably greater difference in the distances between neighboring peaks of each group (Fig. 3). The group of more closely spaced peaks corresponds to longitudinal oscillations. The group of peaks that are spaced more sparsely and damp more weakly corresponds to oscillations propagating 4.5 times more slowly, i.e. with a velocity of about \(1250\ \text{m/sec}\), and should be assigned to a new type of oscillation (capillary oscillations). True, the possibility is not excluded that the formation of the second group of peaks may be connected with interference phenomena.

If elastic inhomogeneities are present in single crystals, then the oscillographic pattern shown in Fig. 1 is distorted. Therefore, the method described can be used to investigate the degree of elastic inhomogeneity of single crystals.

V. Leshkovtsev

REFERENCES

1. S. Ya. Sokolov, DAN SSSR LIX, No. 5, 883 (1948).
2. S. Ya. Sokolov, DAN SSSR LXIV, No. 4, 503 (1949); ZhTF, 19, issue 2, 274 (1949).