EP1494506B1 - High mode microwave resonator for thermal processing - Google Patents

Abstract

The aluminum chamber has a wide floor section with elongated parallel resonance unit along each side and two microwave generators whose radiation is reflected off the other four sides of the chamber to provide a uniform pattern. A perforated door allows access.

Description

gilt, sind bei diesen Materialien die Abhängigkeiten der elektrischen Leitfähigkeit nach $${\bar{n}}^2=\sqrt{{\epsilon }_r^2+\frac{{\sigma }^2}{{\omega }^2{\epsilon }_0^2}}$$

zu berücksichtigen. Zu denken ist einerseits an polymer-gebundene Graphitplatten (Brennstoffzellmembranen), an Kohlefaserverstärkte Verbundwerkstoffe, die aus einem forminstabilen Grünzustand heraus in einen ausgehärteten, formstabilen Zustand gebracht werden sollen, durch den sie für die weitere technische Verwendung ausgezeichnete Eigenschaften aufweisen. Oder aber das Brennen/Sintern von keramischen Grünkörpern. Andrerseits aber auch weniger mit hohen Temperaturen ist an die Erwärmung von Speisen zu denken. Bei all diesen Prozessvorhaben ist gemeinsam, dass in dem Resonatorvolumen ein vorgebbares Teilvolumen bestehen muss, in dem bei einkoppelnder Mikrowelle ein vorgegeben höherer Grad an elektromagnetischer Feldhomogenität besteht, damit darin kontrolliert gleichmäßig auf die Prozesssubstanz eingewirkt werden kann.The invention relates to a modular microwave resonator and a thermal region of a process line formed therefrom. With regard to its frequency, the microwave resonator is dimensioned geometrically such that, due to the coupled-in microwave, starting from the fundamental mode, a sufficient number of modes are formed which enable an overlay in such a way that the intensity effective in the resonator volume becomes sufficiently close to the uniformity required for industrial processing , Choice of frequency, geometry of the applicator, as well as the coupling determine the nature of the overlapping wave field. In a monomode resonator, a sharp, pure geometric mode is excited, which generally has a very inhomogeneous distribution. To allow more modes, the applicator volume must be significantly increased. In the limit of very large distances, the transition to the classical geometric appearance exists. As in Feher, L., et al .: Sintering of Advanced Ceramics Using a 30GHz, 10kW, CW Industrial Gyrotron, IEEE Transactions on Plasma Science, Vol. 2, April 1999, pp.547-554 has been shown, the superposition of many modes does not necessarily lead to an even distribution or homogenization, but to focal superimpositions. The present applicator has characteristic dimensions of at least L> 2λ for each spatial dimension and remains in its maximum extent below the classical optical limit ( Feher et al .: Theoretical Aspects of Microwave Ray Tracing Calculations in Screened Structures, Proc. Latsi's Symposium 1995 on Computational Electromagnetics, ETH Zurich, Switzerland, 1995, pp. 236-241 ). In this area between pure modal excitation and classical optics is an optimized field shaping by the described technical requirements geometry, coupling design in required manner possible and to solve. In such a multi / multi-mode microwave resonator electrically very poorly conductive materials are heated in the broad sense, according to the process and use requirement. In contrast to dielectric, microwave-permeable materials (eg alumina ceramics, porcelain, glass fibers), for which the relationship $\tilde{n}=\sqrt{\epsilon }.$

applies, with these materials, the dependencies of the electrical conductivity after $${\tilde{n}}^2=\sqrt{{\mathrm{<figure-callout\; id="2"\; label="\epsilon \; r"\; filenames="imgf0001.png"\; state="\{\{state\}\}">\epsilon \; </figure-callout>}}_r^{}+\frac{{\mathrm{<figure-callout\; id="2"\; label="\sigma "\; filenames="imgf0001.png"\; state="\{\{state\}\}">\sigma </figure-callout>}}^2}{{\mathrm{<figure-callout\; id="2"\; label="\omega "\; filenames="imgf0001.png"\; state="\{\{state\}\}">\omega </figure-callout>}}^2{\epsilon }_0^2}}$$

to take into account. On the one hand, one thinks of polymer-bound graphite plates (fuel cell membranes), of carbon-fiber-reinforced composite materials which are to be brought from a dimensionally unstable green state into a hardened, dimensionally stable state, by which they have excellent properties for further technical use. Or the burning / sintering of ceramic green bodies. On the other hand, but less with high temperatures is to think of the heating of food. Common to all these process projects is that a prescribable subvolume must exist in the resonator volume in which a predetermined higher degree of electromagnetic field homogeneity exists when the microwave is coupled in so that the process substance can be controlled uniformly in a controlled manner.

In the DE 43 13 806 An apparatus for heating materials by microwaves is described. The device consists of a heating chamber, through which the material to be processed is transported. The heating chamber has a wall portion which is concavely curved. At this is the coupled microwave reflected and focused on the volume of material to be heated.
A comparable device shows the WO 90/03714 , There, the heating chamber is used for food heating in order to try to surround the food volume to be heated with a volume in which an electromagnetic field with still tolerable deviation from homogeneity exists, so that a more uniform temperature field is established.

In the JP 4-137391 the heating chamber is widened by a second reflection wall opposite the first reflection wall, with the aim of fulfilling the process volume with a reinforced, uniform field in order to achieve a uniform heating of the object.

In the US 5,532,462 a cylindrical reaction vessel is described, the interior of which is heated by microwave energy. For this purpose, the multimode microwave is coupled into the vessel in such a way that it is absorbed and reflected on the inner wall, in such a way that the absorption and reflection occur helically progressing. The interior of the boiler should be heated so evenly.

Inhomogeneous field distributions lead to different densities in the sintering of ceramics within a batch and to inhomogeneous densities in individual samples, which ultimately cause mechanical stresses that deform or even shatter the shaped parts. This problem and the resulting finding that a uniform volume heating, inter alia, in sintering processes of significant advantage and great importance in the thermal material processing, are in the article " Microwave Sintering of Zirconia-Toughened Alumina Composites "by HD Kimrey et al. (Mat. Res. Soc. Symp. Proc. Vol. 189, 1991 Material Research Society, Pages 243 to 255 ). Two high-speed cylindrical microwaves are described, one at 2.45 GHz and the other at 28 GHz, and the sintering process was only successful at the high frequency.

At the MRS Spring Meeting in San Francisco, April 11th, 1996 (Symp. Microwave Processing of Materials V), L. Feher et al. titled "The MiRa / THESIS 3D Code Package for Resonator Design and Modeling of Millimeter-Wave Material Processing" On the simulation of the field distribution in a design of a high-capacity, cylindrical spherical resonator with a spherical cover used by the IAP in Nizhny Novgorod. It is shown therein that resonators with a circular cylindrical or spherical geometry have a field distribution which is in need of improvement. Due to their topology, focusing of the field inevitably occurs in the interior of the resonator, so that only a comparatively small working volume with reasonably homogeneous field distribution remains compared to the resonator volume. Additional technical measures such as fashion stirrers and diffuse surfaces (scattered surfaces) bring about improvement, but they are too costly for commercial or industrial use.

In the DE 196 33 245 a prismatic, with respect to its longitudinal axis symmetrical cavity with even polygonal cross section is described as a resonator. All surface segments of the resonator are flat. As a result, the coupled-in microwave beam always remains divergent in the case of reflections on the resonator wall and is not repeatedly focused, as in the case of circular-cylindrical and spherical geometries. The microwave beam is coupled through a coupling opening in one of the two end walls, its beam axis is inclined to the longitudinal axis, in such a way that at the first reflection, a symmetrical beam splitting takes place. The theoretical Findings for the field division were confirmed mathematically as well as experimentally in good measure. Uniform processing of several bodies that are too glowing or burning can be carried out with reduced rejects.

The previously presented, existing technical devices solve the problem by monomode or optical approaches, limited in a finite geometry, and are in terms of technical use under the requirements of large-scale, membrane-like structures and loads in their training, especially for the realization of linear process lines not useful or usable ,

The invention has for its object to enable heating, temperature control and processing of extended sheet materials in the mold for industrial application that, due to extraordinary field homogeneity, by the structural geometry, the type of source and waveguide coupling and tuning frequency and Size of the applicator even sensitive polymer structures can be thermally processed to high quality products with previously unattainable material properties and thus cured. In this case, the charge should be possible in a stack-like manner, ie by full packing of the applicator, or in the other embodiment in the flow-through method.

The object is achieved by a high-mode microwave resonator according to the characterizing features of claim 1, in which, in addition to the fundamental mode sufficiently many higher modes can form, in particular.

The resonator has a prismatic columnar shape with a pentagonal, outwardly curved (convex) cross section. The microwave is via Einkoppelöffnungen in one of the five Jacket sides coupled into the resonator. These coupling openings are line radiators and lie parallel to the edge of the jacket wall. As a result, a divergent microwave beam with a beam plane instead of the beam axis, a line beam, emerges from each coupling-in opening. The beam planes are directed so that the coupled-in microwave line beam bundles fan out in the resonator and superimpose themselves in a predetermined central volume around and along the longitudinal axis of the resonator to an at least largely homogeneous distribution of the electromagnetic field therein.

• Eine spezielle, symmetrische Querschnittsform des Resonators ist die zur Seitenhalbierenden der Grundseite symmetrische Querschnitt (Anspruch 2), insbesondere wenn die beiden Seitenwände auch noch senkrecht auf der Grundplatte/Rückwand stehen (Anspruch 3). Der letztere Fall insbesondere ergibt sich aus rechnerischen Feldbetrachtungen, Felduntersuchungen und Symmetriebetrachtungen an einem Resonator mit hexagonalem Querschnitt (siehe DE 196 33.245 ). Aus diesen Untersuchungen und den Ableitungen aus Symmetriegründen darin wird dieser spezielle Querschnitt des Resonators gemäss Anspruch 4 als semihexagonal bezeichnet, weil er sich durch den mittigen Schnitt durch die jeweilige Längsmitte zweier paralleler und einander senkrecht gegenüberliegender Wände des Mantels eines Resonators mit hexagonalem Querschnitts ergibt.

In the dependent claims 2 to 10 advantageous and suitable for the operation of the resonator embodiments are described:

• A special, symmetrical cross-sectional shape of the resonator is symmetrical to the Seitenhalbierenden the base side cross-section (claim 2), in particular when the two side walls are still perpendicular to the base plate / rear wall (claim 3). The latter case in particular results from computational field observations, field investigations and symmetry considerations on a resonator with hexagonal cross-section (see DE 196 33,245 ). From these investigations and the derivations for reasons of symmetry therein, this particular cross-section of the resonator according to claim 4 is referred to as semi-hexagonal, because it results through the central section through the respective longitudinal center of two parallel and mutually perpendicular walls of the shell of a resonator with hexagonal cross-section.

With regard to the required quality of the electromagnetic field distribution in the interior of the resonator, it has been shown experimentally that the mounting of the microwave belonging to the coupling device is similar on one or the other end side, and thus in the case of both coupling devices or on a different face - in one case or another measurable, but in general not very pronounced improvements brings (claim 5).

Deposits in the coupling openings would adversely affect the decoupling of the microwave and thus the field distribution in the resonator. It is therefore useful and useful to close the coupling openings microwave transparent, ambient and prozessinert with a dielectric / cover (claim 6). This may for example be a Teflon film but also otherwise, possibly additionally mechanically loadable cover / layer.

In claim 7, the frontal access to the resonator is highlighted. This can be from one end face, it is then loaded from there with process material and the same taken from it. However, the resonator can also be used in the pass if access is available via both end faces. A resonator used in this way will then generally stand on one of the five jacket walls, wherein the jacket wall with the coupling openings can be exposed as required. For example, if the resonator is seated on a rack, this casing wall could be the bottom wall at the same time. If required easy accessibility to the microwave equipment, this shell wall may also be exposed to the side or upwards. But this is ultimately determined by process conditions.

In claim 8 it is described that the entrance into the interior of the resonator is arranged via at least one of the remaining four shell walls except the jacket wall with the coupling openings, preferably via one of these shell wall with coupling openings opposite or both. In this case, the resonator could then be placed on an end wall and be cabinet accessible. If he is in this way on wheels or a caster frame, he is also still mobile. In which direction, viewed from the resonator feed and removal, the lateral surface with the coupling openings is exposed, as noted above, depends on the other process conditions. An example is the access to a cabinet with hinged door or folding doors with the microwave equipment on the rear wall.

In addition to the two along the mantle wall edge fitting Einkoppelvorrichtungen exists according to claim 9 another, similar intermediate, can be coupled via the additional electromagnetic to finely manipulate the field homogeneity in the useful volume within the resonator with respect to the distribution characteristic. The main field is set via the two outer couplings.

By way of the number of coupling devices, a different electromagnetic field distribution than that formed here about the central longitudinal axis of the resonator can in principle also be set. The decoupled microwave / n reflect on the inner walls of the resonator expanding and not focusing. This is a fundamental prerequisite for a homogeneous field distribution, because focal field peaks, caustics, as in the case of a circular mantle wall, can not occur.

bewegt (Anspruch 10).Finally, it results from microwave theoretical considerations that in simple structures a ripple with frequency-taking dimension is advantageous for the degree of uniformity of the electromagnetic field in the partial volume of the resonator. Experimentally, this was confirmed for the resonator when the ripple w is in the band. $$\mathrm{\lambda }\mathrm{/}\mathrm{16}\mathrm{<}\mathrm{w}\mathrm{<}\mathrm{\lambda }\mathrm{/}\mathrm{2}$$

moved (claim 10).

Based on the process being driven and the material to be processed, as well as the monetary outlay that must be expended for such a microwave equipment, one will access microwave components / sources ranging from 100 MHz up to the area of construction of 25 GHz standard. For the food warming example, the household microwave is a known device. It works with a magnetron as a microwave source and generates a high frequency of 2.45 GHz. In ceramic sintering, the thermal processing at this frequency but also at about 24.5 GHz makes sense. Here, the coupling property of the process substance plays an important role, which is also still temperature-dependent. From the homogeneity requirement to the electromagnetic field in at least a partial volume of the resonator interior and the process body dimensions results in the frequency selection and geometry of the resonator, the diameter of the Resonatorquerschnitts and the length of the resonator from the field calculations and considerations out to set the required degree of field homogeneity in necessary sub-volume is always greater than the wavelength λ of the applied microwave, preferably 2λ.

• Figur 1 die Erzeugung des Resonatorquerschnitts,
• Figur 2 der Resonator mit semihexagonalem Querschnitt perspektivisch,
• Figur 3 die Energiedichteverteilung über dem Querschnitt,Figur 4 die mittige Energiedichteverteilung über der Resonatorlänge.

The implementation example is a resonator in the geometric shape according to claim 4 with pentagonal cross-section, especially, as from a regular hexagonal cross section by halving out, semi-hexagonal cross-section. This special, exemplary geometry will be explained in more detail below. The drawing to it consists of the FIGS. 1 to 4 :

• FIG. 1 the generation of the resonator cross-section,
• FIG. 2 the resonator with semi-hexagonal cross section in perspective,
• FIG. 3 the energy density distribution over the cross section, FIG. 4 the central energy density distribution over the resonator length.

The resonator with semi-hexagonal cross section is constructed in frame construction of aluminum profiles, such as FIG. 2 shows. He is a laboratory setup. The shell walls are made of aluminum sheet, which is attached to the frame from the inside. The two end faces are here perforated sheets, which are pivotable on the bottom frame via a hinge. Along the two lateral edges of the bottom wall are the two longitudinal couplings, see FIG. 1 , The right coupling device has on its rear forehead, on the rear resonator end wall, the microwave source, a magnetron, sitting with tuning unit (slide sit) for adjustment. In FIG. 2 This is indicated by the hose feeders and visible rectangular parts. The microwave source of the parallel opposite coupling device sits in front of the bottom left corner in the figure accordingly. The structure is necessarily high frequency-tight, as seen at the front edges by the fitting, fabric-like metal band. In the bottom wall lies inside a Teflon plate, which covers the entire floor and covers the two band / linear coupling openings along the respective outer wall edge. FIG. 1 shows its position in the resonator cross section. The resonator cross section is convex pentagonal and can be completed by mirroring at the base edge to a regular hexagon, as in FIG. 1 shown.

In FIG. 1 is on the left in the figure source of the beam path of the coupling-out microwave beam with its point-dashed beam axis, correct beam plane, indicated. The left ray representation in the picture reflects twice, on the side wall and left roof wall, the right only at this Roof wall. More leads to confusion and is therefore omitted.

• Die Betriebsfrequenz ist 2,45 GHz und damit eine Wellenlänge λ im Vakuum von etwa 12 cm. Als Mikrowellenquelle wird pro Einkoppelvorrichtung ein Magnetron verwendet. Die beiden Magnetrone sind pulsbar mit steuerbarem Puls-Breiten-Verhältnis, so dass kontinuierlich eine Mikrowellenleistung von null bis zum Nennmaximum eingestellt werden kann. Das Resonator hat die zehnfache Vakuumwellenlänge, also etwa 1,2 m, die Seitenwand hat eine Innenhöhe von etwa 30 cm und die beiden Dachmantelwände sind jeweils 60 cm breit.

The operating data and geometry of the semi-hexagonal section resonator are:

• The operating frequency is 2.45 GHz and thus a wavelength λ in a vacuum of about 12 cm. The microwave source used per coupling device is a magnetron. The two magnetrons are pulsable with controllable pulse width ratio, so that a microwave power can be continuously set from zero to the nominal maximum. The resonator has ten times the vacuum wavelength, ie about 1.2 m, the side wall has an internal height of about 30 cm and the two roof walls are each 60 cm wide.

The technical data are exemplary. In this resonator plates and band-shaped green bodies were cured in homogeneous or composite form over the action of the coupled microwave depending on the extent in a short time for dimensional stability and optionally for mechanical stability. A green sheet of CFRP material, 3 mm thick and 20 cm ^2, can thus over cross section and area in less than 20 minutes. evenly cured, fuel cell membranes in less than 5 minutes. This is only possible in several hours in a classic autoclave with purely thermal action over the surface of the article.

The excellent process times are based on the Figures 3 and 4 explained with the illustrated field distributions. FIG. 3 shows the energy density distribution over the central cross section. In the upper half, in the central area, there is a relatively uniformly uniform distribution, which in this central area is characterized by only slight fluctuations. Strong also occur the two immediate Einkoppelbereiche with the respectively connected rectangular waveguide cross section.
In FIG. 4 the distribution is shown perpendicular thereto in the center plane to the base plate along the longitudinal center. Process articles which are exposed in the partial volume of the resonator in which these useful, little fluctuating field conditions exist and are exposed to the microwave are uniformly shaped-solidified in the comparatively short process times.
In both planes considered, the largest variation is less than 5%.

Claims (10)

1. High mode microwave resonator for thermally processing materials, wherein the resonator has a prismatic columnar shape with a pentagonal cross section curved outwards, characterized in that

   a) at least two identically linear coupling-in devices for a microwave are situated parallel to the two outside edges of one of the five outside walls in the same, via which coupling devices the microwave is coupled-in into the resonator in each case in the form of a line beam bundle,

   b) one microwave source each is used per coupling device and

   c) the two microwave sources are pulsable with controllable pulse rate/width ratio such that a microwave output is adjustable in a continuous manner from zero up to the rated maximum and

   d) the coupled-in microwaves overlap in the resonator.
2. Microwave resonator according to Claim 1, characterized in that the cross section of the resonator is mirror-symmetrical in relation to the median of the baseline of the cross section.
3. Microwave resonator according to Claim 2, characterized in that the two side walls to be placed on the bottom surface of the resonator stand vertically on said surface.
4. Microwave resonator according to Claim 3, characterized in that the cross section of the resonator is the symmetrical half of a hexagonal cross section - semi-hexagonal.
5. Microwave resonator according to Claim 4, characterized in that the respective microwave source of the two outer coupling-in devices is attached to the identical or to the oppositely situated end face of the resonator.
6. Microwave resonator according to Claim 5, characterized in that the coupling-in openings for the microwave are closed/covered by a microwave-transparent, environment-inert and process-inert dielectric.
7. Microwave resonator according to Claim 6, characterized in that said microwave is accessible via at least one of its two end walls.
8. Microwave resonator according to Claim 6, characterized in that said microwave resonator is accessibly via at least one of the two outside walls, which are situated opposite the outside wall with the coupling-in openings.
9. Microwave resonator according to Claims 7 and 8, characterized in that between the two outer coupling-in openings there is another parallel one, via which a microwave of adjustable power is also coupled for adapting to the field homogeneity.
10. Microwave resonator according to Claim 9, characterized in that the inside walls of the resonator have an undulation w within the range $$\mathrm{\lambda }\mathrm{/}\mathrm{16}\mathrm{<}\mathrm{w}\mathrm{<}\mathrm{\lambda }\mathrm{/}\mathrm{2}$$

    

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