The invention resides in a microwave resonator, a modular plant/assembly line build up by such a microwave resonator, a method and work pieces thermally processed in accordance with the method by means of microwaves which are generated in a high-mode resonator.
The heating of food by microwaves is well-known. An electromagnetic field coupled into a resonator chamber immediately assumes a stationary distribution in the chamber which depends on the frequency of the microwave, that is its wavelength, and the geometry of the resonator chamber. The household microwave has a frequency of 2.45 GHz, that is, a wavelength λ of about 12 cm in a space filled with air or in a vacuum. The geometry of microwave apparatus for households resides therefore in a simple cubic configuration in those dimensions.
Experience has shown, that food exposed to microwaves in the resonator chamber is not uniformly heated. This depends on different water contents of the food components and consequently the existence of a locally different dipole density, but most importantly, on the distribution of the electromagnetic field in the resonator and consequently the excitation of the dipoles by the electromagnetic excitation field. During heating particularly of similar substances/materials the field distribution is of atmost importance for the uniform heating in the whole substance/material volume. A rough but effective aid in avoiding large spatial field variations is a movable surface area in the resonator chamber, where impinging parts of the microwave radiation are reflected so that field strength variations are localized but are blurred. In this case, the process material is stationary in the resonator. Another or additional measure resides in moving the process material, for example, on a rotating plate or a table which is moved back and forth.
In the industrial material processing technology, it is very important that, during the heating of the goods being processed by microwaves, the electromagnetic field varies in the whole resonator volume or at least in a central part of the resonator volume as little as possible so that it is close to an ideal spatially uniform distribution in order to avoid over-heating and hot spots.
DE 196 33 245 discloses a polygonal resonator geometry into which radiation is coupled eccentrically from the front side thereof with an inclined radiation axis. Particularly with a hexagonal cross-section of the resonator geometry, in this way a field distribution is obtained, with which, at least in the central resonator area, a volume area is provided in which a usable field uniformity exists. Although an electromagnetic field is present in the whole interior of the resonator with the in-coupling of the microwave, the central process volume in which the field is homogeneous or in which the field variations are at least tolerable, is relatively small. This resonator with its type of in-coupling can only be used as individual resonator and is not usable as a modular part of a plant.
It is the object of the present invention to provide a resonator for a microwave to be coupled into the resonator, in which a mixture of multiple modes are formed in such a way that, in the resonator volume, an as large as possible process volume around the longitudinal resonator axis can be utilized for the thermal processing providing that the average field strength of the mode mixture approaches an ideal uniform distribution in this space.
Such a resonator should be usable as a module that is it should be usable in a setup of an assembly line for the thermal processing of workpieces by a serial arrangement of such resonators.
In addition, the resonators should each be operable with microwaves in such a way that, with a controllably predetermined frequency, the establishment of modes in the resonator by the in-coupling of microwaves oscillates locally or moves around the space.
With such a resonator it should be possible to process workpieces or objects on an industrial scale by the application of microwaves based on volumetric heating while saving time and energy, particularly resin-soaked compound materials, CFK and/or GFK should be uniformly cured to achieve high form stability and mechanical loading capacity, as well as to cement components and epoxy-resin coated structures.