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Microwave chemistry is the science of applying microwave irradiation to chemical reactions    . Microwaves act as high frequency electric fields and will generally heat anything with a mobile electric charge. Polar solvents are heated as their component molecules are forced to rotate with the field and lose energy in collisions. Semiconducting and conducting samples heat when ions or electrons within them form an electric current and energy is lost due to the electrical resistance of the material. The concept was introduced in 1986 .
MORE synthesis stands for Microwave-organic Reaction Enhancement.
Additional recommended knowledge
Conventional heating usually involves the use of a furnace or oil bath, which heats the walls of the reactor by convection or conduction. The core of the sample takes much longer to achieve the target temperature, e.g. when heating a large sample of ceramic bricks.
Microwave heating is able to heat the target compounds without heating the entire furnace or oil bath, which saves time and energy. It is also able to heat an object throughout the volume (instead of through its outer surface), in theory producing more uniform heating. However, due to the design of most microwave ovens and to absorption by the object being heated, the microwave field is usually non-uniform and localized superheating occurs.
Some compounds absorb microwave radiation differently than others. This selectivity allows some parts of the object being heated to heat more quickly or more slowly than surrounding parts.
Microwave heating can have certain benefits over conventional ovens:
A heterogeneous system (composed by different substances or different phases) is anisotropic if regarded to the loss tangent. As a result a different dissipation of the electric field into heat in different system domains can be expected. A different dissipation means a Selective heating of different parts of the material, leading theoretically to temperature gradients between them. Nevertheless, the presence of zones with a higher temperature than others (called hot spots) must be subjected to the heat transfer processes between domains. Under conditions where a high amount of heat could be transferred between system domains a possible hot spot would be canceled by the exchange of heat from the hot zones to the cold zones until reaching the thermal equilibrium. Only in a system where the heat transfer would be hindered, it would be possible to have the presence of a steady state hot spot able to enhance the rate of the chemical reaction happening in its surrounding.
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|This article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia article "Microwave_chemistry". A list of authors is available in Wikipedia.|