Microwave effect

The phrase microwave effect is a term that is applied to a range of observations in microwave chemistry. There are two general classes of microwave effects:

• Specific microwave effects.
• Non-thermal microwave effects.

A recent review has proposed this definition^[1] and examples of microwave effects in organic chemistry have been summarized.^[2]

Specific microwave effects are those effects that cannot be (easily) emulated through conventional heating methods. Examples include: (i) selective heating of specific reaction components, (ii) rapid heating rates and temperature gradients, (iii) the elimination of wall effects, and (iv) the superheating of solvents. Microwave-specific effects tend not to be controversial and invoke "conventional" explanations (i.e. kinetic effects) for the observed effects.

Non-thermal microwave effects have been proposed in order to explain unusual observations in microwave chemistry. As the name suggests, the effects are supposed not to require the transfer of microwave energy into thermal energy. Instead, the microwave energy itself directly couples to energy modes within the molecule or lattice. Non-thermal effects in liquids are almost certainly non-existent,^[3][4] as the time for energy redistribution between molecules in a liquid is much less than the period of a microwave oscillation. Non-thermal effects in solids are still part of an ongoing debate. It is likely that, through focusing of electric fields at particle interfaces, microwaves cause plasma formation and enhance diffusion in solids via second-order effects.^[5][6][7] As a result, they may enhance solid-state sintering and diffusion processes.^[8] Debates are still raging (January 2006) about non-thermal effects of microwaves that have been reported in solid-state phase transitions.^[9]

References

1. ^ Stuerga, D.; Gaillard, P. J. Microwave Power Electromangn. Energy, 1996, 31, 101-113.
2. ^ Stuerga, D.; Gaillard, P. J. Microwave Power Electromangn. Energy, 1996, 31, 87-99.
3. ^ Kappe, C. O. Angew. Chem. Int. Ed. 2004, 43, 6250-6285.
4. ^ De la Hoz, A.; Diaz-Ortiz, A.; Moreno, A. Chem. Soc. Rev. 2005, 164-178.
5. ^ Booske, J. H.; Cooper, R. F.; Dobson, I. J. Mater. Res. 1992, 7, 495-501.
6. ^ Booske, J. H.; Cooper, R. F.; Freeman, S. A. Mater. Res. Innovations 1997, 1, 77-84.
7. ^ Freeman, S. A.; Booske, J. H.; Cooper, R. F. J. Appl. Phys. 1998, 83, 5761.
8. ^ Whittaker, A.G. Chem. Mater. 2005 17 (13), 3426 -3432.
9. ^ Robb, G.; Harrison, A.; Whittaker, A. G. Phys. Chem. Comm. 2002, 135-137