Seite - 17 - in Adaptive and Intelligent Temperature Control of Microwave Heating Systems with Multiple Sources

2.1. ElectromagneticHeating 2.1.3. Dielectricheating Dielectric heating is used to heat poorly conducting materials such as dielectrics and insulators, i.e. plastic, wood and rubber. According to the electrical frequencies used, dielectric heating is further classified into radio frequency heating and microwave heating. The frequency used in radio frequency heating is 300 kHz to 300 MHz, and in mi- crowave heating is 300 MHz to 300 GHz [Met96]. The reason why di- electricheatingisdistinguishedbytheappliedfrequencyisbecauseat radio frequency the ionic conduction mechanism dominates the loss, whereas at microwave frequency the dipole relaxation is more im- portant [Met96] [MM83]. In industry, the operating frequency of mi- crowaveheatingisdefinedbyISM(theindustrial, scientificandmedi- cal) frequenciesas915MHzor2.45GHz(inEurope). Higher frequen- cies like 24.15 GHz could also be used, but it has to be verified with respect tospecialpracticalor theoreticaladvantages [Feh09]. When a dielectric material is put into an alternating electric field, the movements of permanent dipoles and free ions or ionic species are both affected, resulting in two different loss mechanisms: the dipole relaxation and the ionic conduction. In an alternating electric field, dipoles will rotate their orientations around equilibrium status to fol- low the external electric field, which is called dielectric polarization [GC99] such as illustrated in figure 2.2. Due to the time needed for the rotation, the response of re-orientation following external electric field is not instantaneous. At low frequencies, the dipoles have suffi- cient time to follow the direction altering of the electric field. There is little heat generated due to frictions during the rotation process, and most of the energy from the external electric field is directly stored in thedielectric. As the frequency increases, the time left for the rotation gets less and less and finally it is shorter than the time needed for the rotation. It causes thedipolere-orientation lags theexternalelectricfieldandthis delay is called dipole relaxation [HS92]. In this case, the dipole align- ment within the dielectric is broken and molecules collides with each other more and more. Correspondingly, frictions get larger and more heat is generated during the re-orientation process. As the frequency further increases, a critical point is reached where the rate of direc- 17