Producing Ultrasound

By directly immersing a vibrating metal plate or a horn into the reaction medium (direct sonication) 

By placing the reaction vessel in a tank containing a coupling fluid (usually water) which is sonicated either by a metal plate or a horn (indirect sonication) ultrasonic waves travel through this fluid before contacting the reaction vessel 

By in situ generation of ultrasonic power in the reacting liquid by forcing it across a vibrating blade (the whistle referred to earlier). 

Sonochemistry has developed essentially around reactions that can be carried out in a liquid medium. This medium can be a homogeneous liquid phase or a heterogeneous medium in which at least one phase is liquid—to serve as the vehicle for transmitting ultrasonic power. We shall consider the effects of ultra sound in both systems. 

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Special small ultrasound transducers, often referred to as endoscopic transducers, have been designed which can be inserted into blood vessels to examine blockages in arteries (43). These transducers operate at approximately 20 MHz and have a viewing distance of less than a centimeter. Such devices are capable of producing ultrasound images of the inside of arteries and veins. The quaUty of the ultrasound image is sufficient to determine the type of blockage. 

Figure 3.54 Conceptual view of integrated microflow system employing FPW pumps, mixer, process sensor and insonicator to produce ultrasound-assisted chemical reactions. Heater would be deposited metal or polysilicon meanderline formed on a surface of the chamber.

In co-operation with LM Glasfiber, a complete section of a rotor blade was produced with a number of well defined defects in order to perform an initial sensitivity test by means of ultrasound, vibrations techniques and real-time radiography. Based on the results of this initial test it was found that automated ultrasonic inspection was the best suited teclmique. In co-... 

For example, at a frequency of 1 MHz, the effect of ultrasound in water of surface tension 73 dyn-cm and density 1 g-cm- is to produce longitudinal waves of about 12 pm. The resulting mean droplet diameter (D) is given by Equation 19.2. 

Homogeneous sonochemistry typically is not a very energy efficient process (although it can be mote efficient than photochemistry), whereas heterogeneous sonochemistry is several orders of magnitude better. Unlike photochemistry, whose energy inefficiency is inherent in the production of photons, ultrasound can be produced with neatly perfect efficiency from electric power. A primary limitation of sonochemistry remains the small fraction... 

Sonochemistry is also proving to have important applications with polymeric materials. Substantial work has been accomplished in the sonochemical initiation of polymerisation and in the modification of polymers after synthesis (3,5). The use of sonolysis to create radicals which function as radical initiators has been well explored. Similarly the use of sonochemicaHy prepared radicals and other reactive species to modify the surface properties of polymers is being developed, particularly by G. Price. Other effects of ultrasound on long chain polymers tend to be mechanical cleavage, which produces relatively uniform size distributions of shorter chain lengths. 

The phenomenon of acoustic cavitation results in an enormous concentration of energy. If one considers the energy density in an acoustic field that produces cavitation and that in the coUapsed cavitation bubble, there is an amplification factor of over eleven orders of magnitude. The enormous local temperatures and pressures so created result in phenomena such as sonochemistry and sonoluminescence and provide a unique means for fundamental studies of chemistry and physics under extreme conditions. A diverse set of apphcations of ultrasound to enhancing chemical reactivity has been explored, with important apphcations in mixed-phase synthesis, materials chemistry, and biomedical uses. 

Biomedical and Biotechnology. The use of mictocapsules for a variety of biomedical and biological appHcations has been promoted for many years (50,51). Several biomedical mictocapsule appHcations ate in clinical use or have approached clinical use. One appHcation is the use of air-fiUed human albumin mictocapsules as ultrasound contrast agents. Such mictocapsules, caUed mictobubbles, ate formed by ultrasonicating 5% albumin solutions to produce 4—10-)J.m diameter air-fiUed capsules (52). When injected the capsules act as a useful transpulmonary echo contrast agent (53) that has been approved for use in humans by the U.S. FDA. 

Ultrasound frequencies can be introduced into the walls of the vacuum system. If a source of ultrasound is placed on the wall of an ultrahigh vacuum system, a large hydrogen peak is observed. Related phenomena, presumably from frictional effects, are observed if the side of a vacuum system is tapped with a hammer a desorption peak can be seen. Mechanical scraping of one part on another also produces desorption. 

Unlike vibration monitoring, ultrasonics monitors the higher frequencies, i.e. ultrasound, produced by unique dynamics in process systems or machines. The normal monitoring range for vibration analysis is from less than 1 Hertz to 20,000 Hertz. Ultrasonics techniques monitor the frequency range between 20,000 and 100 kHz. 

Many reactions have been shown to benefit from irradiation with ultrasound (ref. 19). We therefore decided to investigate the effect of ultrasound, different catalysts and the presence of solids on Ullmann diaryl ether synthesis. Indeed, sonication of mixtures of a phenol and a bromoaromatic compound, in the absence of solvent and presence of copper (I) iodide as catalyst and potassium carbonate as base, produces good yields of diaryl ethers at relatively low temperatures (Fig. 10) (ref 20). 

Kleinjan WE, de Keizer A, Janssen AJH (2003) Biologically Produced Sulfur. 230 167-188 Klibanov AL (2002) Ultrasound Contrast Agents Development of the Field and Current Status. 222 73-106... 

Sonochemistry started in 1927 when Richards and Loomis [173] first described chemical reactions brought about by ultrasonic waves, but rapid development of ultrasound in chemistry really only began in the 1980s. Over the past decades there has been a remarkable expansion in the use of ultrasound as an energy source to produce bond scission and to promote or modify chemical reactivity. Although acoustic cavitation plays... 

Piezoelectric ceramics, which depend on lead compounds, are used to produce transducers and sensors which make possible ultrasound technologies used in wide-ranging medical and commercial applications, guidance and sensing systems used in defense and commerce, and in addition, new "smart materials" research projects. 

Abstract Acoustic cavitation is the formation and collapse of bubbles in liquid irradiated by intense ultrasound. The speed of the bubble collapse sometimes reaches the sound velocity in the liquid. Accordingly, the bubble collapse becomes a quasi-adiabatic process. The temperature and pressure inside a bubble increase to thousands of Kelvin and thousands of bars, respectively. As a result, water vapor and oxygen, if present, are dissociated inside a bubble and oxidants such as OH, O, and H2O2 are produced, which is called sonochemical reactions. The pulsation of active bubbles is intrinsically nonlinear. In the present review, fundamentals of acoustic cavitation, sonochemistry, and acoustic fields in sonochemical reactors have been discussed. 

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