Generation of Ultrasound

This limitation has led to the development of a number of systems that can be used to induce cavitation in homogeneous reaction mixtures. The cup-horn reactor was originally designed for use by biochemists as a cell-disruptor (Fig. 11). Its advantages are that it can deliver greater acoustic intensities and is potentially easier to thermostat. Its disadvantages are similar to those of the cleaning bath in that it is very sensitive to liquid levels and the 

Careful design of the reaction vessel allows reactions to be carried out under inert atmospheres (Fig. 13) or at moderate pressures ( 10 atmospheres) (Fig. 14). Other workers have proposed modifications to allow the reaction mixture to be simultaneously stirred. These include use of a cell with a small indentation at the bottom or a glass rosette cell (Fig. 15). Luche and coworkers have carried out extensive investigations into sonochemical preparation of 

Ultrasound is currently employed in the preparation of emulsions on an industrial scale. In this case, the reagents are pumped through a minisonic homogenizer or whistle reactor (Fig. 17). Cavitation occurs as the fluid flows across a vibrating plate and the power obtainable is limited by this factor. Most of the chemical effects observed arise from the vast increase in interfacial area rather than the ultrasonic irradiation itself. However, its advantages stem from its proven ability to process large quantities of material in this manner. 

Methods for measuring ultrasonic power have been reviewed [42] but, in short, there does not seem to be a simple method for the quantitative measurement of local ultrasonic intensity when cavitation is present. Pugin has developed a number of methods for the characterisation of sound fields in a variety of reactors [43]. These were used to develop profiles of the acoustic intensity for both cleaning probes and probe systems with a view to examining the reproducibility of reactions. 

A rough estimate of the acoustic power obtained from the source was obtained by measurement of the electrical power consumption. However, as 

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The basis for the present-day generation of ultrasound was established as far back as 1880 with the discovery of the piezoelectric effect by the Curies [1-3]. Most modern ultrasonic devices rely on transducers (energy converters) which are composed of piezoelectric material. Such materials respond to the application of an electrical potential across opposite faces with a small change in dimension. This is the inverse of the piezoelectric effect and will be dealt with in detail later (Chapter 7). If the potential is alternated at high frequencies the crystal converts the electrical energy to mechanical... 

With so many chemists turning to ultrasound as a source of energy for the acceleration or modification of chemical reactivity it becomes increasingly important that some of the electrical engineering principles which underpin the whole topic of sonochemistry are understood. The first part of this chapter is intended to provide an introduction to the principles of generation of ultrasound for the non-specialist. It is to be hoped that a chemist, armed ivith this information, will be in a much better position to decide on the type of equipment most appropriate for the intended laboratory application. 

Piezoelectric transducers are the most common devices employed for the generation of ultrasound and utilise ceramics containing piezoelectric materials such as barium titanate or lead metaniobate. The piezoceramic element commonly used in ultrasonic cleaners and for probe systems is produced in the form of a disk with a central hole. Ceramic transducers are potentially brittle and so it is normal practice to clamp them between metal blocks. This serves both to protect the delicate crystalline material and to prevent it from overheating by acting as a heat sink. Usually two elements are combined so that their overall mechanical motion is additive (Figure 10.4). Piezoelectric transducers are better than 95% electrically efficient and can operate over the whole ultrasonic range. 

Because it is a superficial structure, the penis is ideally suited to ultrasound imaging. In fact, improved spatial resolution and increased color Doppler sensitivity provided by the latest generation of ultrasound equipment allow an excellent evaluation of normal and pathological penile structures. Ultrasound evaluation of the penis, however, requires a good knowledge of penile anatomy to identify subtle changes that can be appreciated in pathological conditions. 

The Electrokinetic Sonic Amplitude (ESA) effect in this context refers to the generation of ultrasound by the application of an alternating electric field to a colloid. Previous reviews on the ESA have mainly focused on the determination of particle size and zeta potential from the ESA. While this is certainly a very important application of the ESA phenomenon, there is more information in the ESA spectmm than just particle size and zeta. It can be used, for instance, to determine the thickness of adsorbed polymer layers or the surface conductance under the shear plane. It is these other applications that will be our main interest here. To begin we will give an alternative explanation for the ESA phenomenon, one that allows a deeper understanding of the underlying physics. 

Piezoelectric transducers must be fitted with electrodes, which supply tte necessary alternating electrical field. These electrodes must not impede the irradiation of the ultrasonic waves therefore thin gold or silver films are usually depoated on the transducer surface by vacuum evaporation. A crystal prepared in this way can be used directly for the generation of ultrasound in a nonconducting liquid, e.g., oil. A typical experimental set-up of this type used for the irradiation of liquids h riiown in Fig. 1. The main disadvantage of this anangement is the difficulty in measuring... 

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