Dielectric Heating - Use of RF Electric Fields in Dielectric Heating

Use of RF Electric Fields in Dielectric Heating

The use of high-frequency electric fields for heating dielectric materials had been proposed in the 1930s. For example, US patent 2,147,689 (application by Bell Telephone Laboratories, dated 1937) states "This invention relates to heating systems for dielectric materials and the object of the invention is to heat such materials uniformly and substantially simultaneously throughout their mass. ... It has been proposed therefore to heat such materials simultaneously throughout their mass by means of the dielectric loss produced in them when they are subjected to a high voltage, high frequency field." This patent proposed radio frequency (RF) heating at 10 to 20 megahertz (wavelength 15 to 30 meters). Such wavelengths were far longer than the cavity used, and thus made use of near-field effects and not electromagnetic waves. (Commercial microwave ovens use wavelengths only 1% as long).

In agriculture, RF dielectric heating has been widely tested and is increasingly used as a way to kill pests in certain food crops after harvest, such as walnuts still in the shell. Because RF heating can heat foods more uniformly than is the case with microwave heating, RF heating holds promise as a way to process foods quickly.

In the natural sciences, the term "diathermy" is understood to mean "electrically induced heat" and is commonly used to cause muscle relaxation. It is also a method of heating tissue electromagnetically or ultrasonically for therapeutic purposes in medicine. In surgery, it is used as a method of electrocauterization, for example to control bleeding or cut through tissues.

"Electric diathermy" (not to be confused with therapeutic ultrasound) may be loosely called "diathermy," but it does not use dielectric heating but rather high frequency alternating electric or magnetic fields, sometimes with no electrode or device contact to the skin, to induce gentle deep tissue heating by induction or dipole rotation. If precautions are followed no tissue is ordinarily damaged. It is generally used in physical therapy biomedical applications.