Sono-Electrochemistry

Sonoelectrochemistry - The Compton Group

Waves of ultra-sound (20-100 kHz) alternately compress and stretch the structure of the liquid. If a critical distance is exceeded then the liquid breaks down and voids are formed so creating bubbles. During the compression stage the cavities can collapse leading to huge local temperatures (~ 1000s of K) and pressures (~ 100s of atmospheres). Needless to say, this may have interesting chemical consequences! We use voltammetry to monitor ultrasonically induced radicals and other high energy species and are engaged in "dual activation" experiments in which electrolytically formed species are further transformed by ultrasound directed at the electrode surface with the aim of generating new chemistry and novel intermediates. In other experiments ultrasound is used to continuously activate an electrode ~ literally by stripping off surface atoms ~ so as to open up the electrochemistry of either otherwise passivating species.

The sono-activation of electrodes finds particular relevance in facilitating the use of electroanalytical methods (e.g. for trace metals) in real, "dirty" media where conventional voltammetry fails due to the adsorption of surface active species which inhibit or interfere with the intended analytical determination.

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dielectric material

A dielectric material is a substance that is a poor conductor of electricity, but an efficient supporter of electrostatic field s. If the flow of current between opposite electric charge poles is kept to a minimum while the electrostatic lines of flux are not impeded or interrupted, an electrostatic field can store energy. This property is useful in capacitor s, especially at radio frequencies. Dielectric materials are also used in the construction of radio-frequency transmission lines.

In practice, most dielectric materials are solid. Examples include porcelain (ceramic), mica, glass, plastics, and the oxides of various metals. Some liquids and gases can serve as good dielectric materials. Dry air is an excellent dielectric, and is used in variable capacitors and some types of transmission lines. Distilled water is a fair dielectric. A vacuum is an exceptionally efficient dielectric.

An important property of a dielectric is its ability to support an electrostatic field while dissipating minimal energy in the form of heat. The lower the dielectric loss (the proportion of energy lost as heat), the more effective is a dielectric material. Another consideration is the dielectric constant , the extent to which a substance concentrates the electrostatic lines of flux. Substances with a low dielectric constant include a perfect vacuum, dry air, and most pure, dry gases such as helium and nitrogen. Materials with moderate dielectric constants include ceramics, distilled water, paper, mica, polyethylene, and glass. Metal oxides, in general, have high dielectric constants.

The prime asset of high-dielectric-constant substances, such as aluminum oxide, is the fact that they make possible the manufacture of high-value capacitors with small physical volume. But these materials are generally not able to withstand electrostatic fields as intense as low-dielectric-constant substances such as air. If the voltage across a dielectric material becomes too great -- that is, if the electrostatic field becomes too intense -- the material will suddenly begin to conduct current. This phenomenon is called dielectric breakdown . In components that use gases or liquids as the dielectric medium, this condition reverses itself if the voltage decreases below the critical point. But in components containing solid dielectrics, dielectric breakdown usually results in permanent damage.

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