Ultrasounds, high-power

In the literature we can now find several papers which establish a widely accepted scenario of the benefits and effects of an ultrasound field in an electrochemical process [13-15]. Most of this work has been focused on low frequency and high power ultrasound fields. Its propagation in a fluid such as water is quite complex, where the acoustic streaming and especially the cavitation are the two most important phenomena. In addition, other effects derived from the cavitation such as microjetting and shock waves have been related with other benefits reported for this coupling. For example, shock waves induced in the liquid cause not only an enhanced convective movement of material but also a possible surface damage. Micro jets of liquid, with speeds of up to 100 ms-1, result from the asymmetric collapse of cavitation bubbles at the solid surface [16] and contribute to the enhancement of the mass transport of material to the solid surface of the electrode. Therefore, depassivation [17], reaction mechanism modification [18], surface activation [19], adsorption phenomena decrease [20] and the mass transport enhancement [21] are effects derived from the presence of an ultrasound field on electrode processes. We have only listed the main phenomena referring to the reader to the specific reviews [22, 23] and reference therein. 

Heteropoly compounds, nanostructured adsorbents, 119 High-powered ultrasound metal oxides and supported metal catalysts, 3... 

A similar in situ HO generation can be obtained using high power ultrasound sources or by processes using subcritical or supercritical water (at very high temperatures or pressures). [9] and other references therein and in Table 3 can be consulted. 

High-power ultrasound has the potential to increase the pore volume and to reduce the crystallinity of cellulosic biomass. The effect of ultrasound pretreatment on the microstructure of alkaline-pretreated corn stover is shown in Figures 14.5a and b. The pretreated stover sample showed significant disruption of the cell wall (Figure 14.5b). Thermal and chemical pretreatments (acid) make the cellulose more accessible to enzyme by opening up cleavage sites. Such pretreatments however, also... 

The principle of sonochemistry is breaking the chemical bond with the application of high-power ultrasound waves, usually between 10 and 20 MHz. The physical... 

High-power ultrasound has been used to disrupt cells, disperse aggregates, and modify food texture and crystallization (Knorr et ah, 2004). The ultrasonic wave causes intense localized heating and this generates gas bubbles which cavitate and result in intense pressure and shear (Povey and Mason, 1998). It is the high pressure and shear which cause physical disruption of food components and materials and can change the rate of chemical reactions. Kentish et ah (2008) used a flow-through power ultrasound systems at 20-24 kHz to produce an oil-in-water emulsion with... 

Low-frequency, high-power ultrasound, with frequencies between 20 kHz and 1 MHz, can be employed directly in a wide variety of food processing applications (see Table 1). 

High power ultrasound alone is known to damage or disrupt biological cell walls for the release of contents (vide supra). This will result in the destruction of the cells, i.e. sonication has the potential for the destruction of bacteria. Unfortunately very high intensities are required if ultrasound alone is to be used for complete sterilization. It has been shown however that low-power ultrasound is capable of... 

The introduction of high power ultrasound (i.e., soimd energy with fivquencies in the range of IS kHz to lMHz) into liquid reaction mixtures is known to cause a variety of chemical transformations. In recent years, due to... 

There are many possible stimuli, both physical and chemical, which can trigger the release of the capsule content. The physical ones are, for example, high-frequency ultrasounds, " magnetic fields, microwaves, " or near-infrared radiation. " " The capsules may also respond by changing size indeed, some could both shrink and expand when heated. " However, some of these stimuli, such as low-frequency high-power ultrasounds, are not applieable under in vivo conditions. " ... 

Recently, a novel continuous process has been developed for devulcanization of rubbers as a suitable way to recycle used tires and waste rubbers [83-119]. This technology is based on the use of high-power ultrasounds. The ultrasonic waves of certain levels, in the presence of pressure and heat, can quickly break up the three-dimensional network in crosslinked rubber. The process of ultrasonic devulcanization is very fast, simple, efficient, and solvent and chemical free. Devulcanization occurs at the order of a second and may... 

Dorr, L. N., and Hynynen, K., 1992, The Effects of Tissue Heterogeneities and Large Blood Vessels on the Thermal Exposure Induced by Short High-power Ultrasound Pulses, Int. J. Hyperthermia, 8 45-59. 

X. Sun, The devulcanization of unfilled and carbon black filled isoprene rubber vulcanizates by high power ultrasound, PhD dissertation, university of Akron, 2007. 

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