Ultrasonic horns

Two major sources of ultrasound are employed, namely ultrasonic baths and ultrasonic immersion hom probes [79, 71]- The fonuer consists of fixed-frequency transducers beneath the exterior of the bath unit filled with water in which the electrochemical cell is then fixed. Alternatively, the metal bath is coated and directly employed as electrochemical cell, but m both cases the results strongly depend on the position and design of the set-up. The ultrasonic horn transducer, on the other hand, is a transducer provided with an electrically conducting tip (often Ti6A14V), which is inuuersed in a three-electrode thenuostatted cell to a depth of 1-2 cm directly facing the electrode surface. 

Eig. 9. A typical sonochemical apparatus with dkect immersion ultrasonic horn. Ultrasound can be easily introduced into a chemical reaction with good control of temperature and ambient atmosphere. The usual pie2oelectric ceramic is PZT, a lead 2kconate titanate ceramic. Similar designs for sealed... 

FIG. 20 22 Schematic of supercritical antisolvent with enhanced mass-transfer process to produce nanoparticles of controllable size. R, precipitation chamber SCF pump, supply of supercritical COg I, inline filter H, ultrasonic horn P, pump for drug solution G, pressure gauge. 

Figure 3.5 (a) Indirect sonication using an ultrasonic bath, (b) Direct sonication using an ultrasonic horn, (c) Direct sonication... 

There are two types in sonochemical reactors. One is a bath-type reactor, and the other is an ultrasonic horn. 

In order to generate a high intensity ultrasound, an ultrasonic horn has been used (Fig. 1.14). The electric system is the same as that in a bath-type reactor. 

An ultrasonic horn has a small tip from which high intensity ultrasound is radiated. The acoustic intensity is defined as the energy passing through a unit area normal to the direction of sound propagation per unit time. Its units are watts per square meter (W/m2). It is related to the acoustic pressure amplitude (P) as follows for a plane traveling wave [1]. 

Fig. 1.15 Calculated acoustic amplitude under an ultrasonic horn as a function of the distance from the horn tip on the symmetry axis. The dotted curve is the calculated result by (1.21) when v0 = 0.77 m/s, X = 51.7 mm (29 kHz), and a = 5 mm. The solid curve is the estimated one in a bubbly liquid. Reprinted figure with permission from Yasui K, Iida Y, Tuziuti T, Kozuka T, Towata A (2008) Strongly interacting bubbles under an ultrasonic hom. Phys Rev E 77 016609 [http //link.aps.org/abstract/PRE/v77/e016609]. Copyright (2008) by the American Physical Society...

Yasui K, Iida Y, Tuziuti T, Kozuka T, Towata A (2008) Strongly interacting bubbles under an ultrasonic horn. Phys Rev E 77 016609... 

Probe systems, also called as the ultrasonic horn are being most frequently used for the sonochemical research at laboratory scale of operation. A typical schematic representation of the setup of probe systems has been given in Fig. 2.5. These are typically immersion type of transducers and the most important advantage of using... 

Fig. 2.7 Schematic representation of flow systems based on ultrasonic horn...

The extent of immersion of the transducer in an ultrasonic horn or the extent of liquid height, which affects the extent of reflection of the incident sound waves from the liquid surface as well as the reactor bottom, also shows an optimum value [53]. 

Kumar A, Kumaresan T, Joshi JB, Pandit AB (2006) Characterization of flow phenomena induced by ultrasonic horn. Chem Eng Sci 61 7410-7420... 

Fig. 9. Direct immersion ultrasonic horn equipped for inert atmosphere work. [Design of K. S. Susiick (183). ...

Figure 2. Direct Immersion Ultrasonic Horn Equipped for Inert Atmosphere Work.

Soil extraction using all types of aqueous solutions and ultrasonic agitation has been carried out. Simple apparatus such as extractant and aqueous solvent in an Erlenmeyer flask or test tube are used. An ultrasonic bath with Erlen-meyer flask is shown in Figure 11.7 (the use of an ultrasonic horn is shown in Figure 12.9). Ultrasonic extraction is typically carried out when the soil particles are not well separated, such as high clay soils, and thus the surfaces are not exposed to the extracting solutions. 

Figure 12.9. Ultrasonic horn for disruption and extraction of soil samples. The horn tip is small enough in diameter and long enough to reach the bottom of a test tube. 

Let us now turn our attention to the application of the sound wave to a liquid since this is the medium of importance to the practising chemist. The sound wave is usually introduced to the medium by either an ultrasonic bath or an ultrasonic horn (see Chapter 7). In either case, an alternating electrical field (generally in the range 20-50 kHz) produces a mechanical vibration in a transducer, which in turn causes vibration of the probe (or bottom of the bath) at the applied electric field frequency. The horn (or bath bottom) then acts in a similar manner to one prong of a tuning fork. As in the case of air, the molecules of the liquid, under the action of the applied acoustic field, will vibrate about their mean position and an acoustic pressure (P = P sin 2k ft) will be superimposed upon the already ambient pressure (usually hydrostatic, Pjj) present in the liquid. The total pressure, P, in the liquid at any time, t, is given by Eq. 2.4. 

A novel method of generating finely divided zinc metal is by the use of pulsed sonoelectrochemistry using an ultrasonic horn as the cathode [85], Normal electrolysis of ZnCl2 in aqueous NH4CI affords a zinc deposit on the cathode. When the electrolysis is pulsed at 300 ms on/off and the cathode is pulsed ultrasonically at a 100 ms 200 ms on off ratio the zinc is produced as a fine powder. This powder is considerably more active than commercial zinc powder e. g. in the addition of allyl bromide to benzaldehyde (Eq. 3.9). 

Ultrasonic irradiation of aqueous solutions of the chlorophenols was carried out with a Vibra Cell Model VC-250 direct immersion ultrasonic horn (Sonics Materials Newtown, CT) operated at a frequency of 20 kHz with a constant power output of 50 W (the actual insonation power at the solution was 49.5 W, and the power density was 52.1 W/cm2). Reactions were done in a glass sonication cell (4.4 cm i.d. by 10 cm), similar to the one described by Suslick (1988). The temporal course of the sonochemical processes was monitored by HPLC. 

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