Non-thermal microwave effects have been posited in order to explain unusual observations in microwave chemistry

Microwave chemistry

Microwave chemistry is the science of applying microwave irradiation to chemical reactions. Microwaves act as high frequency electric fields and will generally heat any material containing mobile electric charges, such as polar molecules in a solvent or conducting ions in a solid...

. 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

Molecule

A molecule is an electrically neutral group of at least two atoms held together by covalent chemical bonds. Molecules are distinguished from ions by their electrical charge...

 or lattice

Crystal structure

In mineralogy and crystallography, crystal structure is a unique arrangement of atoms or molecules in a crystalline liquid or solid. A crystal structure is composed of a pattern, a set of atoms arranged in a particular way, and a lattice exhibiting long-range order and symmetry...

. Non-thermal effects in liquids are almost certainly non-existent, as the time for energy redistribution between molecules in a liquid is much less than the period of a microwave oscillation

Oscillation

Oscillation is the repetitive variation, typically in time, of some measure about a central value or between two or more different states. Familiar examples include a swinging pendulum and AC power. The term vibration is sometimes used more narrowly to mean a mechanical oscillation but sometimes...

. A recent review has illustrated this in application to organic chemistry, though clearly supports the existence of non-thermal effects. It has been shown that such non-thermal effects exist in the reaction of O + HCl(DCl)->OH(OD)+Cl in the gas phase and the authors suggest that some mechanisms may also be present in the condensed phase.
Non-thermal effects in solids are still part of an ongoing debate. It is likely that, through focusing of electric field

Electric field

In physics, an electric field surrounds electrically charged particles and time-varying magnetic fields. The electric field depicts the force exerted on other electrically charged objects by the electrically charged particle the field is surrounding...

s at particle interfaces, microwaves cause plasma

Plasma (physics)

In physics and chemistry, plasma is a state of matter similar to gas in which a certain portion of the particles are ionized. Heating a gas may ionize its molecules or atoms , thus turning it into a plasma, which contains charged particles: positive ions and negative electrons or ions...

 formation and enhance diffusion

Diffusion

Molecular diffusion, often called simply diffusion, is the thermal motion of all particles at temperatures above absolute zero. The rate of this movement is a function of temperature, viscosity of the fluid and the size of the particles...

 in solids via second-order effects. As a result, they may enhance solid-state sintering

Sintering

Sintering is a method used to create objects from powders. It is based on atomic diffusion. Diffusion occurs in any material above absolute zero, but it occurs much faster at higher temperatures. In most sintering processes, the powdered material is held in a mold and then heated to a temperature...

processes. Debates are still raging (January 2006) about non-thermal effects of microwaves that have been reported in solid-state phase transitions.