Production of Ultrasonics by Piezoelectric method
For generating ultrasonic waves of high frequency (about 50 MHz) a generator using the piezoelectric effect is employed. It is found that when crystals of some materials such as quartz, tourmaline, rocksalt etc. are subjected to a mechanical pressure in a certain direction, each charges of opposite sign develop as their faces, normal to the direction of the direction of the applied pressure. This phenomenon is known as the piezoelectric effect.
Referring to figure 3.3 which is a cross sectional view of a quartz crystal, if pressure is applied along the axis x2-x2 electrical charges appear on the faces ab and a’b’ conversely if two opposite faces of a crystal are subjected to a potential difference (in order to provide charge on them), a tensile pressure appears on the crystal. This pressure alters the length of the pressure alters the crystal. This pressure alters the length of crystal in that direction. If the applied potential is alternating the crystal, the crystal, the crystal begins to oscillate with a frequency which lies in the ultrasonic range.
FIGURE3.3 Cross-sectional view of a quartz crystal. If pressure is applied along the axis X2-X2, electric charge appears on the faces ab and a’b’
Figure 3.4 shows circuit arrangements that can be used to generate ultrasonic waves by using the piezoelectric effect. A thin slice of quartz crystal R is placed between two metal plates A and B to form a parallel plate capacitor, with the quartz crystal as dielectric. The plates are connected to the terminals of a coil which is inductively coupled to the oscillating circuit of a triode valve. An alternating potential difference developed across the condenser plates due to the electrical circuits. The quartz slab is thus subjected to an alternating electric field which produces alternate contraction and expansion of the slab in the perpendicular direction leading to the oscillation of the crystal.
FIGURE 3.4 Circuit arrangement used to generate ultrasonic waves using piezoelectric effect
The variable condenser C is adjusted so that the frequency of the oscillatory circuit is equal to the natural frequency of one of the modes of vibration of the crystal. This produces resonant mechanical vibrations in the crystal due to the linear expansion and contraction. If one of the faces of the crystal is placed in contact with some medium in which elastic waves can be propagated, ultrasonic waves are generated.