Microwave heating mechanisms

The second microwave heating mechanism arises from the migration of ions in the electric field. The resulting current from the oscillating ions gives rise to heat in the familiar way, following the i2r law, where / is the current and r reflects the resistance or impedence to ionic movement through collisions with other ions and molecules present in the medium. Ionic conduction is important in situations where the ions are free to move to some extent. 

In addition to the above mentioned thermal/kinetic effects, microwave effects that are caused by the unique nature of the microwave dielectric heating mechanisms (see Section 2.2) must also be considered. These effects should be termed specific... 

Although microwave activation of catalytic reactions has been the subject of many studies (Sects. 10.3.1 and 10.3.2), the mechanism of these reactions is not yet fully understood. In heterogeneous catalytic liquid/solid and gas/solid systems many results have revealed significant differences between the rates of conventionally and microwave heated reactions. As a rule, at the same temperature microwave heated reactions were faster than conventional and their rate enhancement was over one or-... 

Microwave heating has already been used in combination with some other (unconventional activation processes. Such a combination might have a synergic effect on reaction efficiencies or, at least, enhance them by combining their individual effects. Application of MW radiation to ultrasound-assisted chemical processes has been recently described by some authors [18, 19]. Mechanical activation has also been successfully combined with MW heating to increase chemical yields of several reactions [1]. 

Microwave-assisted organic synthesis offers a very quick and direct route to intermediate quantities of material. When working on a large scale it is important to understand the mechanisms of microwave heating. When synthesis is designed correctly the microwave-assisted approach to scale is a safe and efficient tool for any chemist. 

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