Ultrasonic immersion horn

Two major sources of ultrasound are employed, namely ultrasonic baths and ultrasonic immersion horn probes [70, 71]. The former 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 in both cases the results strongly depend on the position and design of the set-up. The ultrasonic hom transducer, on the other hand, is a transducer provided with an electrically conducting tip (often Ti6A14V), which is immersed in a three-electrode thermostatted cell to a depth of 1-2 cm directly facing the electrode surface. 

Since the ultrasonic radiating surface is not in direct contact with the reaction solution, the acoustic intensities are much lower than those of the direct immersion horn, and so homogeneous sonochemistry is often quite sluggish. On the other hand, there is no possibility of contamination from erosion of the titanium horn. 

The ultrasonic cleaning bath is the most common source of ultrasound in the laboratory and was the equipment used in most of our investigations. The acoustic intensity is far less than the immersion horn but the low price, less than 200 for a 4" x 9 bath that holds flasks up to 1 liter in size, compared to nearly 2000 for a modest horn setup probably accounts for the difference in popularity. 

Disadvantages of the Ultrasonic Bath. The major disadvantage of the bath as a source of ultrasonic waves is the low acoustic intensity. This translates into less than optimum reaction rates and, for some reluctant systems, no rate enhancements at all. Companion to this is variability (often on a day-to-day basis) of the intensity and its focal point, which makes precise rate measurements a formidable challenge. Ultrasonic baths are not easily adapted to flow synthesis as are immersion horns. 

Molecular oxygen is important for the sonolysis of S(-II) at alkaline pH because it propagates a free-radical chain reaction that is initiated by OH. Furthermore, the enhancement of oxygen transfer upon sonication with a direct-immersion horn is considerable. These results may have important implications for the application of ultrasonic irradiation for the destruction of chemical contaminants in water systems. 

Concerning the laboratory devices used for sonochemistry, common cleaning baths are constructed aroimd one or more ceramics fitted to the external face of a tank (p. 304). Such devices work at a single frequency, generally between 20-50 kHz, fixed by the manufacturer with an acoustic power of ca, 1 W. Immersion horns are used when more acoustic power is required. Emitters are composed of a "pancake" of PZT ceramics compressed between a titanium rod and a steel countermass (p. 305). Usually horn devices work from 20 to 100 kHz, and the acoustic power emitted can reach several tens of W. For higher frequencies, piezoceramics are simply fixed to the reactor. The reader interested in the construction of ultrasonic devices should consult Ref. 21. 

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... 

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... 

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]. 

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.

A sonicator (Model W-370) was purchased from Heat Systems-Ultrasonic with a cup horn attachment. The horn was the resonant body, which vibrated at 20 kHz (20,000 cycles per second) and served as a second stage of acoustic amplification. The standard tapped titanium disrupter horn was immersed in circulating water at 50°C during sonication. 

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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