BACKWARD LAMB WAVES IN CdS CRYSTAL PLATE

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BACKWARD LAMB WAVES IN CdS CRYSTAL PLATE

Journal Title: Machinery and Energetics - Year 2018, Vol 0, Issue 283

Abstract

Backward elastic waves have oppositely directed phase and group velocity. They are interesting to research due to some of their unusual properties. Backward elastic waves have promise for use in acoustoelectronics and non-destructive testing because they are able to have a wide range of the phase velocity values with insignificant dispersion of group velocity. Recently, a lot of attention has been given to elastic waves those have zero group velocity and the finite phase velocity. Such waves are intermediate between the backward wave and forward wave (in which direction of the phase and group velocities coincide). A necessary and sufficient condition for the existence of waves with zero group velocity is the existence of backward waves. At present, the existence of reciprocal waves in piezoelectric materials has been experimentally confirmed only for dielectric materials of lithium niobate and bismuth germanium oxide. In piezoelectric semiconductors, the interaction of elastic oscillations with electric charge carriers can be used. But in such materials, backward waves have not yet been observed. The aim of the research was to study the existence of backward Lamb waves and the frequency dependence of their phase velocity on the plates of a piezoelectric semiconductor cadmium sulphide CdS. The Cds plate had a thickness of 2h = 0,66 mm. Under wave propagation along axis of sixth order the symmetrical Lamb mode of first order s1 was investigated. The one length of shear wave fit the plate thickness at cutoff frequency of this mode. For mode s1 the frequency dependence of phase velocity of backward wave, and also of forward wave (its directions of phase and group velocities coincide), were measured. The theoretical calculations, given in the literature, point to the possibility of the existence of backward wave of mode s1 in CdS. To excite the direct Lamb waves the high-frequency voltage was applied to the metal electrodes deposited symmetrically on the opposite surfaces of the plate. A piezoelectric ceramic transducer with a resonant frequency of 2.75 MHz was used to excite the backward wave; when applying an alternating voltage, shear oscillations arose in the transducer. The studies were carried out in the pulsed mode in the frequency range The studies were carried out using pulse of waves in the frequency range MHz with the duration of the radio pulses ~ . The phase velocity of a wave is . Then for determination it is necessary to measure the frequency and wavelength . Wave propagation in piezoelectric materials was accompanied by a high-frequency electric field. Thus the registration of the waves was carried out with movable electrode that could move along the plate with a micrometer screw. For the measurements of phase velocity the phase method of variable base was used. It is based on the fact that the change in the distance between the source of waves and the receiver per wavelength causes the change of the phase of the oscillation of the receiver to . The increasing of the distance resulted in decreasing of phase difference between high-frequency voltage on wave source and wave receiver in case of backward wave, and resulted in increasing of phase difference in case of forward wave. The relative measurement error of wavelength and phase velocity was less than 1%. The existence of backward elastic waves in a CdS crystal plate has been experimentally confirmed. From this follows the existence of waves with zero group velocity in such plates. Since CdS is a piezoelectric semiconductor, it possible to realize the interaction of these waves with electric charge carriers and to use this interaction in acoustoelectronic devices. The measurement results of the dependence of the dimensionless phase velocity on dimensionless frequency of backward and forward waves of symmetrical Lamb mode of the first order s1 are presented. m/s is the velocity of shear elastic waves in directions perpendicular to the axis of the sixth order. For backward wave , since its phase velocity is directed to a wave source. For a direct wave , since its phase velocity is directed from the source of waves. The cutoff frequency = 2.68 MHz corresponds to the value [7]. The magnitude of the phase velocity of the backward waves varied within m/s due to the wave dispersion. The existence of backward elastic waves in a CdS crystal plate has been experimentally confirmed. From this follows the existence of waves with zero group velocity in such plates. Since CdS is a piezoelectric semiconductor, it makes possible interaction of these waves with electric charge carriers and to use this interaction in acoustoelectronic devices.

Authors and Affiliations

П. П. Ільїн

Keywords

Lembe wave reverse waves

EP ID EP391086
DOI -
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