Generating Ultrasound

There are numerous different types of equipment available for use as sonochemical reactors. The initial source of ultrasound comes from transducer devices which convert alternating electrical impulses to mechanical vibrations. Generally these are constructed of either piezoelectric or magnetostrictive material (p. 5). A purely mechanical low-frequency emitter is the whistle system, not frequently used by sonochemists, but of widespread usage in food processing (p. 311). Several set-ups are used to produce low-frequency ultrasoimd, from the simple cleaning baths to much more sophisticated emitters, sometimes using two 

With these emitters (e.g., Fig. 1),15 which generally deliver frequencies between 30 and 50 kHz, sonication of a reaction mixture is indirect, with a liquid, almost always water, used to transmit the energy from the emission zone to the reaction vessel. Standard baths deliver low power to avoid cavitation damage to the tank walls and low power density because of the large insonated volumes. 

In common parlance, the horn is often termed the probe by sonochemists, and the whole assembly consisting of transducer, booster, and horn is sometimes referred to as a probe system. With horn generators, the ultrasonic field is produced directly in the reaction vessel. 

The general representation of the most common type of power transducer of the piezoelectric sandwich-type is given in Fig. 3. The transducer itself forms part of a whole assembly which together constitutes the source of power ultrasound for sonochemical applications. 

This assembly can be compared with conventional engineering (Table 1). 

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Table 1. First-generation ultrasound contrast microbubble agents...

There are a number of factors which have to be considered when deciding which transducer to use for a particular application. The most important of these are the frequency, crystal diameter and acoustic matching. An ultrasonic transducer generates ultrasound over a range of frequencies which depends on its resonant frequency and the degree of damping of the crystal. The resonant frequency fr of a transducer is determined by its thickness and the... 

Part II of the book deals with lesser known aspects of US for the analytical chemists such as its use as an energy source for detection purposes. Thus, ultrasound-based detection techniques viz. US spectrometry in its various modes including ultrasound attenuation, ultrasonic velocity, resonant ultrasound, laser-generated, ultrasound reflection and acoustic wave impedance spectroscopies) are dealt with in Chapter 9. Finally, Chapter 10 is devoted to seleoted applioations of US spectrometry — mostly non-analytical applications from whioh, however, analytical chemists can derive new, interesting analytical uses for ultrasound-based deteotion techniques. 

Ultrasonic-laser instruments can use short-pulse lasers (a few nanoseconds long) to generate ultrasound and long-pulse (tens of microseconds long) or continuous lasers, coupled to an optical interferometer for the detection of US (I.e. the corresponding mechanical displacements) [22]. 

Methods of aerosol generation—ultrasound, electrohydrodynamics, hydrostatic pressure extrusion of liquid through small orifices—that, just ten years ago, might have been considered laboratory curiosities or perhaps only implemented as laboratory prototype generators, are now in late-stage development as handheld inhalers. In addition, precision dry-powder inhalers are in late-stage development. Chlorofluorocarbon (CFC)-free MDIs, often more... 

B. Audoin, and C. Bescond, Measurement by laser-generated ultrasound of four stiffness coefficients of an anisotropic material at elevated temperatures, J. Nondestructive Eval. 16(2) 91-100 (1997). 

Emulsification is performed with the help of devices that generate ultrasound, vibration, use high shear gradient (the so-called colloidal mills), collision of two thin jets of liquids, etc. 

Several variations of these are possible, particularly for large-scale operation. Figure 22.1 shows schematic sketches of different types of devices used to generate ultrasound (Berlan and Mason, 1992). By using any of these transducer designs, it is possible to introduce power ultrasound (as it is normally called) into the reacting system. A reaction conducted under the influence of ultrasound is usually represented as... 

Ultrasound, like sound and infrasound, is made up of pressure waves, i.e. mechanical as opposed to electromagnetic waves. While the latter travel in vacuo, mechanical waves require an elastic medium to propagate.To generate ultrasound, one must do mechanical work on the propagation medium. Two possibilities are exploited magnetostriction and the piezoelectric properties of some materials. 

Nestleroth et al. [15], Segal et al. [16] considered some established novel signal processing schemes to assist in adhesive bond inspection. Sinclair et al. [17] and Filimonov [18] employed acoustic resonance methods for dynamic elastic modulus measurements in adhesively bonded structures. Yost and Cantrell [19], Achenbach and Parikh [20] and Nagy et al. [21] considered a nonlinear response of bonded structures to estimate material characteristics. In Achenbach and Parikh [20], failure was preceded by nonlinear behavior of thin boundary layers at the interfaces. Billson and Hutchins [22] considered lasers and EMATS in bond investigations. It was shown that this non-contact technique was reasonable when compared to that obtained by conventional piezoelectric transducers. Ince et al. [23] also characterized bonds with laser-generated ultrasound and through-transmission measurements. 

Ince, R., Thompson, G.E. and Dewhurst, R.J., Characterisation of adhesive bonds from inspection by laser-generated ultrasound. J. Adhes., 43, 135-159 (1993). 

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