CHEMICAL APPLICATIONS FOR ULTRASONIC WAVES 153 , __l To radio frequency source Figure 8.--Piezoelectric transducer assembly. Metal contoiner Liquid Quartz plate sound waves is the same as that of the alternating electrical field but the frequency must be adjusted to correspond to a resonance mode of the quartz plate if intense ultrasonic waves are to be generated. With the unit in Fig. 8, the majority of the ultrasonic energy is propagated into the liquid rather than the air on the reverse side of the quartz plate. Transducers of the type described above are of limited value in processing applications because a-quartz is expensive, is generally available only in simple geometric shapes of relatively small size and requires high voltage for operation. Since World War II, the ferroelectric materials have been used exten- sively in place of quartz. Ferroelectric materials such as barium titanate can be produced in a variety of shapes since they are polycrystalline yet behave very similar to the single crystal of an ordinary piezoelectric mate- rial. In addition the ferroelectric materials cost less than a-quartz and require far lower a.-c. voltages to drive them. Several types of barium titanate transducers are shown in Fig. 9. Transducer A can be mounted in place of the quartz plate in Fig. 8. The spherically concave transducer B focuses the sound energy to substantially a point while the transducers C and D have a line focus. Through this focusing action, ultrasonic intensi- ties (e.g., 10 2 and 10 a watts/cm. •') far higher than previously available can be used for processing applications. The hollow cylindrical type can be obtained mounted within the wall of a stainless steel pipe with flanges for connection into a continuous flow processing system. Barium titanate is also available in several other forms within stainless steel jackets suitable for submergence in processing tanks.

154 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS Figure 9.--Ferroelectric transducers. A--circular plate, B--spherically concave bowl, C--hollow cylinder, D--trough type. Electrical leads to me- tallic coatings on parallel surfaces. Magnetostriction Transducers Figure 10 is a diagram of a magnetostriction transducer. In the pres- ence of a magnetic field with an alternating component, certain metals such as nickel will be periodically elongated and contracted. If one end of the bar is connected to a diaphragm in contact with a liquid, sound energy will be transmitted into the liquid. The frequency of the sound waves is the same as that of the alternating component of the magnetic field but should be adjusted to correspond to a mechanical resonance frequency of the bar if high intensity sound waves are to be generated. The metal bar is laminated to reduce eddy currents. The magnetic fields are pro- vided through the current through the coils in. Fig. 10. Since magneto- striction generators are used at sonic and relatively low ultrasonic frequen- cies, the alternating current may be obtained from a relatively inexpensive high-frequency motor-driven generator rather than an electronic generator as is usually the case with piezoelectric transducers. In general equipment is available for processing applications through the pilot plant stage and in many instances on a large industrial scale. CONCLUSIONS Cosmetic chemistry appears to be a particularly promising field for processing applications involving ultrasonic waves. In general the scale of these processing applications is expected to be relatively small compared to those in other chemical industries. Furthermore, the cost associated with ultrasonic processing should be only a small fraction of the relatively high cost of the final product. While the author is optimistic about poten-