WO1998008359A1

WO1998008359A1 - High-mode microwave resonator for the high-temperature treatment of materials

Info

Publication number: WO1998008359A1
Application number: PCT/EP1997/003328
Authority: WO
Inventors: Lambert Feher; Guido Link
Original assignee: Forschungszentrum Karlsruhe Gmbh
Priority date: 1996-08-17
Filing date: 1997-06-25
Publication date: 1998-02-26
Also published as: DE59704730D1; EP0919110A1; EP0919110B1; DE19633245C1; JP2000501880A; JP3299977B2

Abstract

The invention concerns a furnace for sintering material, said furnace being an overdimensioned microwave resonator. The design of the resonator interior permits a homogeneous field-distribution of the microwave coupled-in obliquely via a front face. Numerical calculations, the MIRA code, have established the optimum resonator geometry to be that with a hexagonal cross-section. In this geometry, the microwave is coupled in such that the beam axis of the coupling-in beam part is divided symmetrically at the closest edge of two abutting wall segments. In this way, the microwave beam is widened for the first time here and is then always widened each time it is reflected. Field magnifications in the resonator are thus prevented, and the field distribution in almost the entire resonator volume is largely homogeneous. The prerequisite for uniform heating of the sinter material introduced into the resonator is thus satisfied. A field distribution which is similar but not quite as good can be attained with a resonator having octagonal geometry. Cross-sections of an even higher order converge rapidly into cylindrical geometry which is affected by caustics.

Description

Hoctunodiger microwave resonator for the high temperature treatment of materials

The invention relates to a high-mode microwave resonator for the high-temperature treatment of materials. It should be used to sinter ceramics or to dry materials. This works better the more homogeneous the field distribution in the interior of the resonator or in the microwave oven.

DE 43 13 806 describes a device for heating materials by microwaves. The device consists of a heating chamber through which the material to be processed is transported. The heating chamber has a wall part that is concavely curved. The coupled microwave is reflected on this and focused on the material volume to be heated.

A comparable device is shown in WO 90/03714. There the heating chamber is used to heat the food in order to try to provide the food volume to be heated with a more uniform temperature field.

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

No. 5,532,462 describes a cylindrical reaction vessel, the interior of which is etched with microwave energy. For this purpose, the multi-od microwave coupled into the vessel so that it is absorbed and reflected on the inner wall, and zar derat that the absorption and reflection take place helically progressively. The inside of the boiler should be heated evenly. When sintering ceramics, inhomogeneous field distributions lead to different densities within a batch and to inhomogeneous densifications in individual samples, which ultimately cause mechanical stresses that deform or even shatter the molded parts. This problem and the knowledge gained from it that uniform volume heating, inter alia in sintering processes, are of significant advantage and great importance in thermal material processing are described in the article "Microwave Sintering of Zirconia-Toughened Alu ina Co posites" by HD Ki rey et al. dealt with (Mat. Res. Soc. Symp. Proc. Vol. 189, 1991 Material Research Sosiety, pages 243 to 255). Two high-fashion, cylindrical microwave ovens are operated, one at 2.45 GHz and the other at 28 GHz. The sintering process was successful only at the high frequency.

On the occasion of the MRS Spring Meeting in San Francisco, April 11th, 1996 (Symp. Microwave Processing of Materials V), L. Feher et al. entitled "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-fashion, cylindrical resonator with a spherical cover used by the IAP in Nizhny Novgorod. It is shown that resonators with circular cylindrical or spherical geometry have field distributions that need to be improved throughout. Due to their topology, focusing of the field inside the resonator inevitably occurs, so that compared to the resonator volume, only a relatively small working volume with a reasonably homogeneous field distribution remains. Additional technical measures such as fashion stirrers and diffuse surfaces (scattered surfaces) bring improvements, but they are too expensive for commercial or industrial use.

The invention is based on the object of avoiding strong inhomogeneous field increases (caustics) in a resonator which is used as a microwave oven, and distributing the pinging microwave beam through an external geometry in the volume in order to be able to expose a material to be warmed up, burned or sintered to a largely homogeneous field.

The object is achieved by a high-mode microwave resonator according to claim 1.

The resonator is a prismatic cavity with an even polygonal cross section that is symmetrical with respect to its longitudinal axis. All surface segments of the resonator are flat or equivalent, topologically flat. As a result, the coupled-in microwave beam always remains divergent in the event of reflections on the resonator wall and is not repeatedly focused as in the case of circular-cylindrical and spherical geometries.

At the first reflection, the beam is divided into two symmetrical halves, since the beam axis from the microwave coupling window initially falls on the closest, common edge of two lateral surface segments. This results in a first strong fanning out after the first reflection, which is not achieved when the beam is first reflected on only one flat wall segment.

It initially seems plausible if the injected beam is always reflected in such a way that a fanning out takes place. Then improper field elevations, as they occur in pure cylinder geometry due to the focusing effect, cannot occur. As a result, in the case of a resonator geometry with a polygonal cross section, it is not necessary to achieve a higher order than the cylinder geometry, and a much larger usable volume (including work or process volume) can be achieved.

Field calculations with a specially developed computer program confirm this plausibility check. For this purpose, resonator design studies using this computer program, the MiRa code (Microwave Raytracer), were carried out under various selected polygonal geometries as the most promising. The MiRa code is used to calculate stationary wave fields and shows good agreement with correspondingly implemented resonators.

The MiRa code was developed as a gridless analytical computer method with which complex resonator geometries can be examined. A beam formalism that represents the complete properties for electromagnetic fields in the steady state provides the theoretical basis for this code. This allows the description of a monochromatic, harmonically changing wave field with the vector potential

A (x, t) = A (x) e ^-iwt .

Including calibration transformations is the condition

(x, t) = O

always to be observed (see again MRS Spring Meeting 1996, especially "Optical field calculations with the Mira Code").

In dependent claim 2, the symmetrical hexagonal cross section of the resonator is characterized because with it the best result was achieved with regard to the homogeneous field distribution of the smallest fluctuation and thus the resonator volume can be used almost completely as the working volume. Other even polygonal resonator cross sections do not show this quality in terms of field homogeneity. Nevertheless, an octagonal resonator cross-section is still considerably cheaper for the development of a homogeneous field than the geometries mentioned in the prior art, even if they still have a mode stirrer inside the resonator. The inner walls of the resonator are metallic or covered with a metallic layer, which makes them a mirror for the microwave, which reflects better the higher the electrical conductivity of the walls. In addition, they must be stable in the process environment, ie they must be chemically inert to the contacting atmosphere, and they must be cooled in order to withstand thermal loads, which mainly result from radiation and more or less subordinate to convection. Depending on the application, a material such as silver or copper or gold or stainless steel or an otherwise suitable metallic material is used as the wall or inner wall coating for the resonator (claim 3).

The microwave is coupled into the resonator from one of the two flat end faces. The coupling opening lies outside the center of the end face (claim 4), so that there is a common edge of two abutting jacket segments which is closest to the coupling opening. The beam axis emanating from the coupling opening runs to this edge and there is initially divided into two beam axes during the first reflection, which mirror images of one another until the second reflection.

Due to the homogeneous field distribution achieved in the stationary state, the resonator is now very well suited as a microwave oven for sintering ceramic substances. However, other objects can also be heated or dried or simply tempered (claim 5).

A quasi-optical beam with a Gaussian beam profile or a microwave beam coming close to this profile is coupled into the resonator (claim 6).

The advantages of the prismatic resonator with an even, symmetrical polygonal cross-section and the beam coupling inclined against the longitudinal axis with subsequent symmetrical beam splitting after the first reflection has been found after predictions based on the MiRa code were found to be optimal and advantageous. The theoretical findings have been confirmed experimentally. Above all, other known technical aids such as the mode stirrer and spreading disks (diffusers) can be omitted. They no longer bring any additional improvement. This creates the prerequisite for uniform processing of several bodies to be glowed or burned, so-called green bodies, and suggests industrial use.

The exemplary embodiment of the invention described in more detail below with reference to the drawing is intended as an oven for ceramic sintering.

It shows:

Figure la projection of the injected beam perpendicular to

Resonator axis,

FIG. 1b projection of the injected beam parallel to the resonator axis,

FIG. 2 shows the microwave oven with a hexagonal applicator insert and edge loading by the microwave,

FIG. 3 dependence of the field homogeneity and energy density in the working volume on the order of the polygonal cross-section and the type of wall application,

Figure 4 Block diagram of the field calculation with the MiRa code.

The quasi-optical microwave beam 2, which is coupled into the resonator 1 with a hexagonal cross section, is shown in a simplified manner in the two FIGS. 1 a and 1 b with the two first reflections. The microwave beam 2 enters the resonator 1 through the coupling opening 3 in the lower end face 4 in FIG. The beam axis 5 of the in The first beam part entering the resonator 1 is inclined at an angle a to the end face 4 with the coupling opening 3. It is aligned so that it meets the closest edge of the two abutting, flat polygon surfaces. The beam 2 is reflected for the first time on these two abutting polygon surfaces and simultaneously divided into two parts that are symmetrical to one another. The interior of the resonator 1 is filled more and more uniformly by the always divergent beam path with increasing reflections.

In Figures la and lb, this process is only shown for the first two reflections to indicate how the field filling of the room and thus of the microwave oven progresses (in reality, the stationary field filling in the resonator is almost immediately present after coupling). Stronger local field elevations (caustics) are avoided, as a result of which no so-called hot spots can arise in the ceramic molds heated in the resonator 1. The ceramic molds to be processed are exposed to the microwave field in the working volume (process volume) of the furnace (resonator).

In Figure 2, the microwave oven consists of a cylindrical structure 6 with two connecting pieces 7 and 8, of which one 8 attaches to the lateral surface and serves for temperature measurement and pumping out or flooding the interior of the resonator and the second 7 obliquely attaches to one of the two end faces 4 . The microwave 2 is coupled into the interior of the resonator via the latter. Therefore, it is also completed with the coupling window 9 at the joint to the beam-guiding waveguide.

The inside of the original cylinder 6 is designed from end face 4 to end face 4 with the applicator insert 10 having a hexagonal cross section. In Figure 2, the applicator 10 is rotated so far about the cylinder axis that the incident beam axis 5 falls on the closest edge of two abutting polygon walls of the applicator insert 10. So that he- the first symmetrical division of the incident microwave beam 2 then follows.

The MiRa code as an instrument for determining and designing the optimal resonator geometry is a crucial tool. Its essential features and its basic use are explained in FIG. 4. The more detailed connections of this code are in the above. Literature comprehensible by the authors H. Feher et al. described. Essentially, a resonator model with a polygonal cross section is initially assumed, modeled and used to calculate the field distribution occurring in this resonator geometry. The numerical calculation is carried out using the MiRa field calculation, in which the microwave 2 entering the resonator 1 is followed optically. The successive field filling in the resonator 1 can finally u. a. represent video suitable so that z. B. as a result, the longitudinal and cross-sectional development of the field distribution inside the resonator can be demonstrated.

When designing the furnace, the aim is to keep the energy density in the defined working volume as large as possible, while at the same time little variation in the field strength values around the mean value (homogeneous distribution). The working volume, for comparison of the conditions, is defined as the continuous volume, which has the best field quality with the original cylindrical geometry. The study with the MiRa code for examining the field homogeneity of various prismatic applicator designs showed an optimum for the hexagonal structure with edge loading according to FIGS. La, b and 2b. Here is the relationship

(Scattering of the energy density in the work volume): (Average energy density available in the work volume)

minimal. In FIG. 3, the quotient normalized to the maximum (worst case) for the application of edges or walls shown. The edge loading shows a better homogeneous energy yield except for the pentagonal cross section.

The normalized scatter is shown in FIG. With the hexagonal applicator, the prediction results that the least scatter with the highest possible energy density is to be expected. This finding has been confirmed experimentally, namely that there is a large-scale, completely homogeneous blackening of the thermal papers brought into the resonator in all measured levels up to the applicator wall. The preliminary calculations are thus confirmed by the experiment, so that the MiRa code is characterized by a high degree of reliability. Calculations for higher-order polygonal cross sections converge rapidly in the scattering behavior of the resonator field against the cylinder geometry.

As a bar for edge loading by the coupling microwave, the ratio for the mean energy and scatter of the original (cylinder) geometry can be seen when the mode stirrer is at a standstill. The second bar shows the profit from a running mode stirrer, which rotates so quickly that the fluctuation due to the individual positions of the mode stirrer can no longer be proven. The original configuration can be viewed in terms of scatter and available energy density comparable to a cubic (square resonator cross-section) applicator geometry, but here the homogeneity is achieved without a technical aid such as a mode stirrer or a diffuser.

In the study of the field distribution with the MiRa code with regard to the experimental verification, the polygons, starting with a square cross section, were inscribed in the cylindrical cross section of the original resonator. The volume increases with an increasing number of edges and consequently the energy density available in the volume decreases with the same coupled power. This is particularly evident in the Pentagon. For even-numbered polygonal cross-sections, there is a clear decrease in the scatter from the original geometry without a running mode stirrer, through the square cross-section up to the hexagonal cross-section. Only then does the scatter increase again, but is consistently better for the even-numbered polygons than for wall application. The standardized field scatter for odd-numbered polygons is significantly stronger. The standardized scatter for polygonal cross-sections of higher order quickly converges to the original geometry without a running mode stirrer.

Reference character list

1 resonator

2 microwave beam, microwave

3 coupling opening

4 end face

5 beam axis

6 cylinders, structures

7 connecting pieces

8 connecting pieces

9 coupling window

10 prismatic cavity, applicator insert

Claims

Claims:

1. High-fashion microwave resonator for the high-temperature treatment of materials, characterized in that the resonator (1) is a prismatic, with respect to its longitudinal axis symmetrical cavity with an even polygonal cross section, the lateral surface segments and the two end faces (4) of the resonator (1) are flat , the beam axis (5) of the microwave beam (2) to be coupled in from one of the two end faces (4) falls obliquely onto the closest edge of two abutting shell surface segments, as a result of which the divergent microwave beam (2) coupled into the resonator (1) is close at the first reflection the coupling (3) is fanned out into two symmetrical reflection and diffraction components, and is always fanned out during the further reflections on the resonator inner walls, so that there is a largely homogeneous field distribution in the entire resonator volume.

2. High-fashion microwave resonator according to claim 1, characterized in that in order to achieve a homogeneity of the field with minimal fluctuation, the cross section of the resonator (1) is hexagonal or octagonal.

3. High-fashion microwave resonator according to claim 2, characterized in that the inner walls of the resonator (1) are coated with a suitable metal material for the intended process high electrical conductivity such as silver or copper or gold or aluminum or stainless steel, whereby the walls mirror for represent coupled microwave (2).

4. High-mode microwave resonator according to claim 3, characterized in that the resonator (1) is an oven for high-temperature treatment of materials such as heating or drying or sintering and / or welding of ceramics or for annealing semiconductors.

5. High-fashion microwave resonator according to claim 4, characterized in that the coupling window (3) is offset from the center of one end face (4).

6. High-mode microwave resonator according to claim 5, characterized in that the coupled microwave beam (2) is a quasi-optical beam with a Gaussian beam profile or a profile close to this.