320 EARLE C. GREGG, JR. 



along the x axis. For very low frequencies, some rc-cut quartzes are 

 cut in the form of rods; however, magnetostriction is superior at 

 those frequencies. 



The efficiency of generation of ultrasound depends not only on 

 the purity of the quartz but also on the manner in which it is clamped 

 and the type of electrodes on the faces. As a general rule, the faces 

 are metallized by evaporation of copper or silver and then thickened 

 by electroplating. Some investigators have merely laid thin metal 

 sheets or gauze on the faces but this is not as satisfactory as a metallic 

 plating. Care must be taken not to have the electrodes too near the 

 edge of the crystal since the dielectric breakdown strength diminishes 

 near the edge. Once a breakdown occurs, a conduction path results 

 and the crystal usually cracks from the heat generated. 



At a given frequency the amplitude of vibration of a piezo crystal 

 is directly proportional to the voltage applied to its faces. For large 

 \dbrations then, rather large alternating voltages must be applied to 

 the crystal. Care must be taken not to puncture the crystal nor to 

 mechanically shatter it by forcing it at too large an amplitude. 



Some investigators have used potentials as high as 34,000 v. but 

 this has been for short time service with painstaking insulation pre- 

 cautions. The potentials usually range from 1000 to 10,000 v. de- 

 pending on the thickness of the crystal involved. About 2000 v. 

 per millimeter is a satisfactory field strength. A special crystal cut — 

 Straubel contour {2, p. 24) — has been developed which produces a 

 more uniform motion of the crystal faces. This in turn allows higher 

 voltages and hence higher amplitudes of vibration with less danger 

 of fracture. 



The type of mounting and acoustic loading of the crystal play an 

 important part in the design and use of a crystal generator. Several 

 different arrangements are shown in Figure 7. In a, the crystal and 

 the upper thin metallic electrode are merely clamped by a ring to a 

 metallic block. This block should be of lead since lead is less hkely 

 to shatter the crystal than a harder metal. The most favorable size 

 of the upper clamping electrode and clamping electrode pressure 

 must be found by experience. Electrodes with too small a hole 

 produce small ultrasonic outputs while large holes lead to irregular 

 excitation of the quartz. If the crystal is plated, the latter factor is 

 of little consequence and small clamping pressure need be used. In 

 h, the crystal is backed by an air space rather than a metallic block. 

 This means that, since little sound energy is radiated into air, most 



