EP0950341B1 - Baking oven for the high-temperature treatment of materials with a low dielectric loss factor - Google Patents

Abstract

PCT No. PCT/EP98/00003 Sec. 371 Date Aug. 12, 1999 Sec. 102(e) Date Aug. 12, 1999 PCT Filed Jan. 2, 1998 PCT Pub. No. WO98/30068 PCT Pub. Date Jul. 9, 1998In the furnace (10) for the high-temperature processing of materials with a relatively low dielectric loss factor (tan delta ) by heating the material by absorption of microwave energy in a resonant cavity (16), a uniform energy intensity of the microwave field is to be achieved for example by irradiating the microwave energy over a broad band and/or by varying in time the frequency of the irradiated microwave energy. The resonant cavity (16) and the radiation source (13) are tuned to each other such that the relation: (V/ lambda 3). B>/=20 is satisfied. V stands for the volume of the resonant cavity (16), lambda for the wavelength of the microwave radiation and B its band width. V/ lambda 3 equals at least 300 and the clear dimensions 1x, ly and lz of the resonant cavity (16) in the direction of the co-ordinates x, y and z are approximately equal to the cubic root of V. The wall (161 to 166) of the resonant cavity is made of graphite and can be heated by a heating device (28) up to the temperature of the material to be treated. The heating device is arranged outside the resonant cavity, and a heat insulting envelope (38) encloses the unit of resonant cavity (16) and heating device.

Description

The invention relates to a kiln for high temperature treatment of materials with relatively low dielectric Loss factor when the material is heated by absorption of microwave energy in a cavity and with the further generic in the preamble of claim 1 Features.

Such a kiln is known from WO95 / 05058 PCT / GB94 / 01730 known.

The well-known kiln has a design in which it is used for Sintering ceramic materials in one during sintering resting stack is suitable, a cuboid cavity, within that by a cuboid Thermal insulation device of the roughly cuboid stacking space is delimited to that area within the Corresponds to that of a sufficiently homogeneous Distribution of the electric field strength is assumed. The Uniformity of the electric field strength or the square the same is a prerequisite for the sintered material to be sufficient "evenly" can be thermally treated. To do this To counteract the effect that with increasing warming of the sintered goods the heat radiation from the areas of the Sintered stack causes a higher inside Temperature prevails than in the marginal areas mentioned Effect that is characteristic of microwave ovens a heating device is provided which allows the edge areas the sintered stack conventionally, e.g. by means of a resistance heater in addition to heat in this way balanced temperature profile within the sintered stack achieve.

The known kiln is indeed suitable, in a relative sense small treatment area about the same thermal conditions can be achieved in the entire treatment volume with the Disadvantageous is that it is exposed to microwave radiation Thermal insulation device the majority of the irradiated Absorbs microwave energy, which inevitably leads to a leads to high consumption of microwave energy, which is not for the Desired thermal treatment of the sintered goods available stands. This results from the fact that in practical cases Total volume of insulation material is significantly larger than that Volume of the sintered good. The well-known kiln is therefore as industrially usable furnace not suitable because it is efficient Utilization of microwave energy is not given, its generation but is much more expensive than the "conventional" Heating by means of an electrical resistance heater.

WO95 / 05058 also means that it is designed as a continuous furnace Kiln known as the tunnel kiln with heating zones different temperature is formed by which Sintered material is moved over transport rollers, the additional heating is arranged outside the treatment room is and the thermal insulation that the environment against the high temperature area insulated, enclosing the furnace on the outside. at However, this oven is a system with inevitable insufficient field homogeneity, i.e. an oven design, which is possible when relatively small opposites are serial be sintered, and it due to the moving through due to inhomogeneous areas not to a homogeneous field distribution arrives.

The well-known tunnel kiln is for materials with high dielectric Losses suitable, the microwave energy strong absorb, but not for the treatment of sintered goods relatively weak dielectric losses, which is practically only in significant quantities in a cavity resonator with high Field homogeneity can be treated.

The known tube furnace would be for low dielectric materials Loss factor, which is technically of great interest are not suitable.

The object of the invention is therefore a furnace of the beginning Specify the type of high temperature treatment of sintered material with a low dielectric loss factor allowed in a large volume of treatment due to its dimensions can be used as an industrial furnace and thereby nevertheless operable with a high degree of energy utilization is. Furthermore, the kiln is intended for applications within a wide temperature range up to 1800 ° C.

This object is achieved by the characterizing Features of claim 1 solved.

Durch die Einhaltung der Dimensionierungsrelationen gemäß Merkmal a) ergibt sich bezogen auf die äußeren Abmessungen des Resonators eine für ein großes Behandlungsvolumen, in dem bei gleichmäßiger Beladung mit dem Sintergut eine hohe Anzahl von Sinter-Objekten behandelt werden kann, geeignete Homogenität der Feldverteilung.

Durch die Verlagerung des Isoliermantels nach außen wird sichergestellt, daß der überwiegende Anteil der erzeugten Mikrowellenstrahlung auch zum jeweils gegebenen Behandlungszweck genutzt werden kann. Hierdurch ist ein wirtschaftlicher Betrieb des erfindungsgemäßen Brennofens als Industrieofen erst möglich.

Durch die Verwendung von Graphit oder Material auf Graphit basis als Wandungsmaterial für den Hohlraumresonator wird nicht nur der Temperaturbereich, innerhalb dessen eine Hochtemperaturbehandlung von Sintergut möglich ist, drastisch erhöht, sondern es wird auch, verglichen mit einem koventionell in Stahl-Bauweise erstellten Hohlraumresonator auch dessen Gewicht und damit die elektrische Heizleistung der Zusatzheizeinrichtung verringert, die für die Erzielung des erwünschten Temperaturprofils erforderlich ist. Auch dadurch wird die Wirtschaftlichkeit des Betriebs eines als Industrieofen ausgebildeten erfindungsgemäßen Brennofens erhöht.

The functional properties and advantages of the kiln according to the invention achieved thereby are at least the following:

Compliance with the dimensioning relations according to feature a) results in a homogeneity of the field distribution that is suitable for a large treatment volume, in which a large number of sintered objects can be treated with uniform loading, with respect to the external dimensions of the resonator.

Moving the insulating jacket to the outside ensures that the majority of the microwave radiation generated can also be used for the given treatment purpose. As a result, economical operation of the furnace according to the invention as an industrial furnace is only possible.

The use of graphite or graphite-based material as the wall material for the cavity resonator not only drastically increases the temperature range within which high-temperature treatment of sintered material is possible, but also the cavity resonator, which is conventionally made of steel Weight and thus the electrical heating power of the additional heating device is reduced, which is necessary for achieving the desired temperature profile. This also increases the economic viability of operating a kiln according to the invention, which is designed as an industrial furnace.

The preferred design of the furnace is as a microwave radiation source at least one magnetron is provided, around a center frequency within a bandwidth B, which is determined by the Relationship B = Δf / f is given, in which with Δf the frequency swing is designated, is tunable by about 1/100.

Such a magnetron can e.g. a center frequency of 2.45 GHz, which has a tuning range of between 2.438 GHz up to 2.462 GHz.

As a result, there are a large number of in the cavity resonator Vibration types that can be excited when tuning the magentron, e.g. periodically between the cutoff frequencies, be stimulated consecutively in time.

The beneficial consequence of this is that at different times there are different spatial distributions of the field strength, which on average a largely homogeneous field in the treatment area result.

In a practical design, the radiation source is designed so that the time for a frequency swing between the cut-off frequencies is in the tenths of a second range, e.g. between 0.05 and 1 second, i.e. within a period of time that is small against the thermal relaxation time of the sintered material.

This measure is beneficial to thermal within the sintered good To avoid tensions. Such tensions could build up if as a result of too low a rate of change the frequency is the field distribution corresponding to a certain frequency, which is necessarily inhomogeneous for too long Would be maintained over time.

In the sense of an effective broadening of the frequency band within which the cavity resonator can be excited, it can also be advantageous if a number n of magnetrons are provided as microwave radiation sources which can be operated at different center frequencies f _i (i = 1 to n) and within their respective Bandwidths Δf _{i are} tunable.

A quasi-continuous "gap-free" tuning range of the frequency results if the frequency spacings in the frequency scale of adjacent center frequencies of the magnetron satisfy the relationship (Δf _i + Δf _{i + 1} ) / 2.

In a preferred design of the furnace, its cavity resonator is cuboid, preferably such that the edge lengths l _z , l _y and l _{z of} the cavity boundary correspond to at least 10 times the wavelength λ of the microwave radiation.

Alternatively, the cavity resonator can be as claimed 7 provided, seen in the direction in which flat boundary walls of the cavity resonator along parallel Adjoin corner edges, has a polygonal shape, i.e. the shape of a prismatic hollow profile. In these designs is the resonator made of plate-shaped in a simple manner Elements can be assembled, in particular also as per Claim 8 provided, made of plate-shaped graphite material.

This design of the cavity resonator has the advantage that the Kiln can be operated at very high temperatures, so that Sintering processes in temperature ranges up to 1800 ° C possible become.

With a correspondingly numerous polygonality and possibly regular polygonal The design of the cavity resonator is also the Limit case of the cylindrical-tubular resonator in good approximation reachable.

This design has the constructive point of view Advantage that the design of the resonator better than usual cylindrical outer vessel can be approximated, which can be evacuated is and / or can be flushed with protective gas.

To e.g. required for sintering the material to be treated high microwave power in an even spatial distribution To be able to couple into the cavity resonator, it is advantageous to choose an antenna arrangement according to claim 9 has an omnidirectional characteristic, i.e. a directivity largely avoided. Such an antenna is according to the Features of claim 10 as a multiple single radiator comprising Group emitter trained, the individual emitters in a statistically distributed phase position can be fed.

Such a group radiator is designed in a preferred embodiment of the oven according to claim 11 as a slot radiator, which comprises a plurality of radiation slots with a slot length between λ / 4 and λ / 2 and a compared to this small slot width w, which, in the direction of propagation of the microwave field in the feeding waveguide seen, distributed over its length such that the same or approximately the same amounts of microwave energy can be coupled into the cavity resonator per slot, the extent of the individual slots being between w and λ / 2, viewed in the direction of propagation of the microwave field in the waveguide, furthermore, the distance of successive slots of the slot antenna measured in the direction of propagation of the microwave field in the waveguide is between λ / 2 and 3λ / 4 and, based on the longitudinal center plane of the waveguide running in the direction of propagation, the lateral distance of the slots from this center live, increases gradually over the length of the waveguide, and that a statistical distribution of the longitudinal slots, which form the individual radiation elements, is provided with respect to the longitudinal median plane of the waveguide

This design of the slot antenna is a very good one Omnidirectional characteristics already achieved when at least 20 Single slots are provided, with increasing numbers of the slots an increasingly effective approximation of the antenna characteristics to the omnidirectional characteristic.

In the special design provided according to claim 13 the slot radiator can at least some of its slots also obliquely to the spread of the microwave field in the waveguide run.

From the point of view of an even energy input in the cavity resonator, it can also be advantageous if several Group spotlights of the aforementioned type are provided, wherein statistically a more even distribution of the phase positions of those coupled in via the individual antenna elements Microwave energy can be achieved and secondly a correspondingly increased energy input is possible, which for Heating a large-volume sintered stack is suitable.

Both for constructional reasons and for reasons of radiation characteristics ("Horn" effect of the resonator walls) can it may be particularly useful if the antenna (s) are strip-shaped Edge areas of flat parts of the resonator walls arranged is / are in the immediate vicinity of edges of the resonator wall run along the flat inner surfaces of the resonator bump into each other.

The resonator and the waveguide (s) via which the Antenna (s) is / are fed, surrounding auxiliary heating is designed as an electrically controllable resistance heater, which, according to a temperature curve specified by a program is controlled that the temperature curve in Sintered material, which in turn by means of a temperature sensor, preferably a pyrometer is monitored and for the target-actual value comparison for heating the resonator wall whose temperature is used in the sense of a run-on control adjusted to the temperature of the sintered material is essentially due to the radiated microwave power is determined.

It is expedient that temperature sensors for different Wall areas of the resonator are provided, by means of which the possibly different resonator wall temperatures can be detected and that the heating is monitored individually Wall areas associated heating elements, which in turn are individually controllable, it being useful in the case of the cuboid resonator, each of the resonator walls has its own Assign heating element and its own temperature sensor.

In the arrangement of the thermal insulation provided according to the invention Isolation outside the resonator cavity and also outside the heating element is the insulation itself made of graphite or of a Graphite based material formed, e.g. Graphite felt and then mediates an arrangement on the inside of the resonator surrounding housing provided, due to the conductivity effective suppression of any of the graphite material Microwave leakage radiation to the outside.

Fig. 1

ein Ausführungsbeispiel eines erfindungsgemäßen Brennofens für eine Hochtemperaturbehandlung von keramischem Sintergut mit niedrigem dielektrischem Verlustfaktor, das innerhalb eines quaderförmigen Hohlraumresonators des Brennofens durch Absorption von Mikrowellenenergie aufheizbar ist, in schematisch vereinfachter Blockschaltbild-Darstellung,

Fig. 1a

eine schematisch vereinfachte, perspektivische Ansicht des Hohlraumresonators und der Anordnung der Behandlungs-Toleranzen;

Fig. 2

Einzelheiten einer zur Einkopplung von Mikrowellenenergie in den Hohlraumresonator des Brennofens gemäß Fig. 1 vorgesehenen Schlitzantennenanordnung, in schematisch vereinfachter, teilweise abgebrochener perspektivischer Ansichtsdarstellung und

Fig. 2a

die Schlitzantenne gemäß Fig. 2 in vereinfachter Draufsicht.

Further details of the kiln according to the invention will become apparent from the following description of a specific exemplary embodiment and possible modifications thereof with the aid of the drawing. Show it:

Fig. 1

1 shows an exemplary embodiment of a kiln according to the invention for high-temperature treatment of ceramic sintered material with a low dielectric loss factor, which can be heated within a cuboid cavity of the kiln by absorption of microwave energy, in a schematically simplified block diagram representation,

Fig. 1a

a schematically simplified, perspective view of the cavity resonator and the arrangement of the treatment tolerances;

Fig. 2

Details of a slot antenna arrangement provided for coupling microwave energy into the cavity resonator of the furnace according to FIG. 1, in a schematically simplified, partially broken away perspective view and

Fig. 2a

2 in a simplified top view.

The kiln designated 10 in FIG. 1 as a whole for tempera treatment, especially for sintering, only schematically indicated workpieces 11 thought by this thermal treatment is only for an intended use Use of the finished workpieces required material properties and / or achieve spatial dimensions.

Typical workpieces 11 which are produced on the basis of nitride-ceramic material, in particular Si ₃ N ₄ , for example ball bearings, valve body and housing, nozzles, or can be produced on the basis of oxide-ceramic material, for example sealing disks and rings, and require a sintering treatment, should be exposed to this thermal treatment in the furnace 10.

These are relatively low materials dielectric loss factor (tan δ <0.01), which in one a total of 12 stacks are arranged.

liegen, wobei mit V_res das Volumen des Hohlraumresonators 16 bezeichnet ist (V_res = l_x . l_y . l_z). Der Behandlungsraum, innerhalb dessen das Sintergut stapelförmig als dielektrische Beladung des Hohlraumresonators 16 auf nicht eigens dargestellte Weise gehalten wird, ist in der Fig. 1a schematisch als mit dem Inneraum des Hohlraumresonators 16 geometrisch ähnlicher, zentraler Teilraum 17 repräsentiert, dessen zur thermischen Behandlung des Sintergutes 11 nutzbares Volumen ca. 1/3 des Resonatorvolumens V_res betragen kann.The sintered material formed by the workpieces 11 as a whole is heated by absorption of microwave energy which is generated by a microwave source 13 and via an antenna arrangement, generally designated 14, with omnidirectional characteristic into a cavity resonator, generally designated 16, with electrically conductive walls 16 ₁ to 16 _{6 is} coupled in, which in the special embodiment shown has the shape of a cuboid whose dimensions l _x , l _y and l _{z are} significant, for example approximately 10 times larger than the wavelength λ of the microwaves that can be generated by means of the microwave source 13, and each in the order of magnitude

lie, with V _res the volume of the cavity resonator 16 is designated (V _res = l _x . l _y . l _z ). The treatment room, within which the sintered material is held in a stack as a dielectric loading of the cavity resonator 16 in a manner not specifically shown, is schematically represented in FIG. 1a as a central subspace 17 geometrically similar to the interior of the cavity resonator 16, the one for the thermal treatment of the sintered material 11 usable volume can be approximately 1/3 of the resonator volume V _res .

In such a resonator 16, the resonance condition is for the wavelength of the microwave radiation which is resonant in the resonator 16 λ r = 2 m L x 2 + n L y 2 + O L z 2 where m, n and o denote the integer quantum numbers with which the relationship (1) can be fulfilled.

The types of vibrations that can be resonantly excited in such a cavity result in a periodically varying field course in the three coordinate directions x, y and z within the cavity, the square (E ² ) of the electric field strength (E) of the electric field generated in the cavity between 0 and one Maximum amount varies, ie a field distribution that is extremely inhomogeneous in space.

The uniform distribution of the electrical field energy required for a qualitatively equivalent treatment of a sintered material distributed over the treatment subspace 17 can be achieved to a good approximation if the cavity resonator can be excited in a large number of resonant vibration types and these vibration types are at least capable of being superimposed on average over time, whereby the Number ΔN of the types of vibrations that can be excited by the relationship ΔN = 8.π.v res λ 3 · 1 Q total is given in which V _{res is} the volume of the cavity resonator, λ is the vacuum wavelength of the microwave radiation and Q is the _total quality of the arrangement 10, 11, 12, 13, 14 explained so far, which in turn is represented by the relationship 1 Q total = 1 Q res + 1 Q ant + 1 Q diel + 1 Q source given is. In this regard, Q _res denotes the quality of the resonator wall, which is given by the relationship Q res = 3 · V res 2A res · E is given with Q _ant the quality of the antenna arrangement for which the relationship Q ant = 8th . π. V res A ant , λ with Q _diel the quality of the dielectric sintered good for which the relationship Q diel = V res V diel · 1 ε r · tans applies and with Q _source the quality of the microwave _source (13) is designated by the relationship Q source = 1 / B given is.

A_res

die Fläche der Resonatorwand insgesamt,

e

die Eindringtiefe in die Resonatorwand

A_ant

die abstrahlenden Flächen der Antennenanordnung 14, mit

V_diel

das Volumen des dielektrischen Behandlungsgutes 11, mit

ε_r

die Dielektrizitätszahl des Sintergutes 11, mit

tan δ

der dielektrische Verlustfaktor des Sintergutes und mit

B

die Bandbreite der Mikrowellenquelle 13

bezeichnet. In relationships (4), (5), (6) and (7) are with

A _res

the total area of the resonator wall,

e

the depth of penetration into the resonator wall

A _ant

the radiating surfaces of the antenna arrangement 14, with

V _diel

the volume of the dielectric material to be treated 11, with

ε _r

the dielectric constant of the sintered material 11, with

tan δ

the dielectric loss factor of the sintered material and with

B

the bandwidth of the microwave source 13

designated.

In the kiln 10 selected for explanation, a magnetron with a fundamental frequency of 2.45 GHz is provided as the microwave radiation source 13. The resonator volume V _res is 1.4 m ³ , so that the ratio V _res / λ ^{3 has} a value of about 770. A value of 7.6 m ^{3 is} assumed for the value A _{res of} the total area of the resonator walls 16 ₁ to 16 ₆ . The resonator walls 16 ₁ to 16 ₆ consist of plate-shaped graphite material, so that at the specified frequency of the microwave source there is a penetration depth e of 32 μm, which corresponds to a quality of the resonator wall of approximately 8600.

A value A _ant of 60 cm ^{2 is} assumed for the "radiating" antenna area, which corresponds to a quality factor Q _{ant of} the antenna arrangement of approximately 48,000. For the volume of approx. 0.03m ³ occupied by the sintered good 11, the value of the quality Q _{diel of} the sintered _{good is} 2100 if a value of 8 and a loss factor of 0.008 are used for the dielectric constant. When the magnetron 13 is operated at a fixed frequency, the bandwidth B of the microwave radiation generated by the magnetron is less than 10 ^-6 , which corresponds to a source quality Q _source of more than 10 ⁶ . With dielectric loading of the cavity resonator to the specified extent, the total quality Q _ges approximately corresponds to the quality Q _{diel of} the dielectric material and the number of excitable vibration types ΔN approximately a value of 9. This results in a sufficient number of vibration types that are sufficient for a sufficiently uniform Distribution of the electrical field in the cavity are necessary, can only be achieved by a broadband microwave source.

Accordingly, the furnace 10 is designed to have the following relationship: V res · B / λ 3 ≥ 20

The antenna arrangement 14, by means of which microwave energy generated by the magnetron 13 can be coupled into the cavity resonator 16, is designed as a slot radiator which comprises a plurality of radiation slots 18, each of which forms an antenna element, the radiating antenna area of which corresponds to the clear slot area. These radiation slots 18 are arranged in a longitudinal wall 19 of a rectangular waveguide 21 (FIG. 2), which at the same time also forms an inner wall region of the cavity, in which the microwave energy generated by the magnetron 13 and fed into it at one end of the waveguide 21 only in the TE ₁₀ mode (basic vibration type) in the arrangement example shown is capable of spreading in the z direction, such that the electric field vector is perpendicular to the longitudinal waveguide wall 19 provided with the slots 18 and the field distribution of the electric field in the interior of the rectangular waveguide runs essentially symmetrically to its longitudinal center plane 23, which in turn extends in the direction of propagation of the microwave field in the waveguide 21. These radiation slots 18 are distributed over the length l _{z of} the rectangular waveguide 21 such that the same or approximately the same amounts of microwave energy can be coupled into the cavity resonator 16 per radiation slot 18, and that the phase positions of the radiation slots through the radiation slots into the cavity resonator 16 coupled electromagnetic fields are different in a statistical sequence.

In the direction of propagation of the microwave field in the waveguide 21 seen, the distance d (Fig. 2a) is consecutive Slots of slot antenna 14 between λ / 2 and 3λ / 4, where deviating from the illustration chosen for explanation, in the longer slot edges parallel to the longitudinal median plane 23 of the waveguide 21, also slot configurations with diagonally to this or even perpendicular to this Longitudinal edges are possible. In the configuration shown the slot antenna 14, in which the radiation slots parallel to of this longitudinal center plane 23, the length is 1 individual slots 18 between λ / 4 and λ / 2 and is significant larger than that perpendicular to the longitudinal median plane 23 or the direction of propagation of the microwave energy in the rectangular waveguide measured width w of the slots. Over the length of the Rectangular waveguide 21 seen away at one end the microwave energy generated by the magnetron 13 is fed in is the lateral distance a of the radiation slots from the longitudinal median plane 23 of the rectangular waveguide 21 gradually.

The order of the arrangement on each side of the median longitudinal plane arranged radiation slots 18 'and 18' '(Fig. 2a) corresponds to the slot spacing d, seen in Direction of propagation of the microwave field in the rectangular waveguide 21, a "binary" random sequence of slot pairings (1,0) and (0,1), where (1,0) means that a slot 18 'on one, "left" side of the longitudinal median plane 23 of the rectangular waveguide 21 is present, but not a symmetrical to this arranged slot 18 '' and the combination (0.1), that on the other "right" side of the median longitudinal plane 23 Beam slot 18 '' is available, but not on the opposite, "left" side. The combination (1,1) that a phase difference of exactly opposite one another arranged radiation slots 18 'and 18' 'emitted Field of π / 2, and the combination (0.0) are in the exemplary embodiment chosen for explanation, without restriction of generality, excluded. The so far slot antenna explained in its basic structure acts as a group radiator, whose through the slots 18 or 18 'and 18 "formed single radiators with statistically distributed Phases can be fed, which reduces the radiation characteristics the antenna arrangement 14 in a very good approximation Omnidirectional characteristic is.

The rectangular waveguide 21 provided for feeding the radiation slots 18 of the antenna arrangement 14 is integrated into a prismatic graphite body 24, corresponding to the schematic illustration in FIG 1 represents a resonator cavity boundary surface which mediates in a corner region of the cavity resonator 16 between the resonator walls 16 ₂ and 16 ₄ which adjoin one another at right angles in the region of the antenna arrangement 14, the waveguide surfaces delimiting the waveguide interior 22 in pairs in parallel or run perpendicular to the inclined inner longitudinal boundary surface 26 of the cavity resonator 16, which is formed by the "hypotenuses" surface of the graphite body 24.

In order to reduce the "effective" quality Q _{source of} the magnetron provided as the energy source, in order to increase the number of oscillation types that can be excited in the cavity resonator, which benefits the uniformity of the field distribution in the cavity resonator, a design of the magnetron 13 is provided in which its oscillation frequency within one Bandwidth of 1/100 of the fundamental frequency f of 2.45 GHz is variable. The cycle times of the frequency variation, which can be controlled by means of an electronic control unit 27, are matched to the thermal relaxation behavior of the sintered good 11 in that they are small compared to the thermal relaxation time of the sintered good to be treated. Accordingly, the electronic control unit 27 is designed so that the cycle times can be between 0.05 and 1 second.

The purpose of a - on average over time - reduction of the source quality Q _source , which is not specifically shown, can also serve the measure that several magnetrons are provided as microwave radiation sources, which at different fundamental frequencies fi (i = 1 ... n) are operable and each have corresponding characteristic bandwidths B _i , it then being expedient that the frequency spacings Δf _{l of} the magnetron oscillation frequencies adjacent to one another in the frequency scale, at least approximately the value .delta.f i + Δf i + l 2 correspond.

When to radiate microwave energy into the cavity 16 two or more antenna arrangements 14 are provided are, it is expedient if they are approximately equidistant azimuthally around a parallel to the polygon edges of the resonator cavity running "central" axis are grouped around a uniform radiation of microwave energy into the treatment room 17 to achieve the cavity resonator.

The furnace 10 is provided with a heating device, designated overall by 28, which in turn comprises six electrical resistance heating elements 28 ₁ to 28 ₆ according to the number of large wall elements 16 ₁ to 16 _{6 of} the cavity resonator 16 (the resistance heating elements 28 ₅ and 28 ₆ are not shown because Figure 1 shows a cross section), the heating powers of which can be controlled individually, so that the temperature of the wall elements 16 ₁ to 16 _{6 can be} influenced individually. The wall elements 16 ₁ to 16 ₆ are each equipped with at least one temperature sensor 29 ₁ to 29 ₆ (the temperature sensors 29 ₅ and 29 ₆ are not shown because FIG. 1 shows a cross section), which generate characteristic electrical output signals for the actual values of the wall temperatures.

Furthermore, a pyrometer, designated as a whole by 32, is provided, by means of which the temperature of the sintered material 11 can be detected. This pyrometer 32 comprises a specimen 33 arranged at a suitable location in the stack 12 and an electronic-optical sensor 34, by means of which the radiation temperature of the specimen 33 can be detected, so that a characteristic electrical output signal of the sensor 34 is a precise measure of the temperature of the sintered material 11 is. The electronic control unit 31 of the heating device 28 mediates comparative processing of the actual value output signals of the pyrometer arrangement 32 and the temperature sensors 29 ₁ to 29 ₆ and also mediates a control of the heating elements 28 ₁ to 28 ₆ and the power control of the microwave source 13 in the Meaning that the wall temperature of the cavity resonator 16 corresponds as exactly as possible to the temperature of the sintered material 11. The course of the furnace temperature over time, ie both the temperature of the sintered material and the resonator wall temperature (s) is controlled according to a program which, taking into account the material properties and the geometric dimensions of the workpieces 11, gives a qualitatively good treatment result.

The cavity resonator 16 and the heating elements 28 ₁ to 28 _{6 of} the heating device 28 provided for heating its walls 16 ₁ to 16 ₆ are arranged within a stable steel housing 36 which, for the purpose of the possibility of a protective gas purging of its interior 17 including the resonator cavity or evacuation of the same is carried out gastight. The steel housing 36 is lined on the inside with a heat insulation layer 38, which consists of a high-temperature-resistant insulation material, for example graphite felt, for the purpose of heat insulation of its interior from the surrounding space of the furnace 10.

Claims (19)

1. Baking oven (10) for the high-temperature treatment of materials with relatively low dielectric loss factor (tan δ) by heating the material by absorption of microwave energy in a resonant chamber, in which the material to be treated is arranged within a central area of the resonator, wherein a uniform energy density of the microwave field is achieved, for example, by irradiating with broadband microwave energy and/or by varying the frequency of the irradiated microwave energy over time, so that in each volume element of the treatment area the square of the electric field strength of the microwave field has the same value, at least over time, within a minor tolerance, wherein an electric heating device is provided, with which the resonator wall can be heated to the same temperature as within the material to be treated, for example, in the sense of a servo control the temperature of the material to be treated can be followed, and wherein a heat insulating envelope (38) is provided, which insulates the baking oven against heat loss into the environment, characterized by the following features:

   a) the resonator chamber (16) and a the microwave energy generating radiation source (13) are sufficiently attuned to each other, so that the relation V λ3 · B ≥ 20 is satisfied, wherein V is the volume of the resonator chamber (16), λ is the wavelength of the microwave radiation and B is their band width, further the amount V/λ³ has a value of at least 300 and the transparent dimensions l_x, l_y and l_z of the resonator chamber (16) in the coordinate directions x, y and z have a value of

   

   each;

   b) the heating device (28) is arranged outside of the resonator chamber (16) in the immediate vicinity of the resonator wall, and the heat insulating envelope (38) is arranged so that it encompasses the resonator chamber (16) and the heating device (28) from the outside,

   c) the resonator wall (16₁ through 16₆) consists of graphite or of temperature maintaining and electrically conductive material based on graphite.
2. Baking oven according to claim 1, thereby characterized, that, as microwave radiation source (13) a magnetron is provided, which is tunable about a basic frequency f within a band width B = Δf/f of preferably 1/100.
3. Baking oven according to claim 1 or claim 2, thereby characterized, that time intervals in which within a continuous or stepwise variation of an oscillation frequency of the microwave radiation source (13) occurs, lies between 0.05 and 1s, preferably around 100ms.
4. Baking oven according to one of claims 1 through 3, thereby characterized, that an amount n of magnetrons are provided, which are operable at various central frequencies f_i (i = 1 through n) and each have characteristic band widths B_i.
5. Baking oven according to claim 4, thereby characterized, that frequency separations of the center frequencies of the magnetrons which are next to each other in a frequency scale satisfy the equation (Δf_i + Δf_i+i)/2.
6. Baking oven according to one of claims 1 through 5, thereby characterized, that the resonator chamber (16) has a cuboidal design, such that the dimensions or edge lengths 1_x, 1_y and 1_z of the resonant chamber boundary correspond at least to the 10-fold of the wavelength of the microwave radiation.
7. Baking oven according to one of claims 7. through 5, thereby characterized, that the resonator chamber (16) has a polygonal cross-section.
8. Baking oven according to one of claims 1 through 7, thereby characterized, that the resonator chamber (16) is assembled of preferably plate-shaped graphite material (16₁ through 16₆).
9. Baking oven according to one of claims 1 through 8, thereby characterized, that for introduction of the microwave energy into the resonator chamber (16) an antenna-arrangement (14) is provided, which has an omnidirectional characteristic.
10. Baking oven according to claim 9, thereby characterized, that the antenna-arrangement (14) is formed as a group emitter comprising multiple individual emitters, which the individual emitters can be supplied by a statistically distributed phase position.
11. Baking oven according to claim 10, thereby characterized, that the group emitter is designed as a slit emitter, which includes a plurality of radiation slits with a slit length of between λ/4 and λ/2 and, in comparison thereto, a small slit width w, which viewed in the direction of radiation of the microwave field in the feeding wave guide, are distributed in such a manner over the length thereof, that per slit the same or approximately similar amount of microwave energy can be introduced into the resonant chamber, wherein, viewed in the direction of propagation of the microwave field in the wave guide, the extension of the individual slits corresponds to between w and λ/2, of which further in the distance measured in the direction of radiation of the microwave field in the wave guide sequential slits of the slit antenna have a value of between λ/2 and 3λ/4, and, with reference to the center plane of the wave guide running in the direction of propagation, the sideways separation of the slits from this center plane, over the length of the wave guide, increases stepwise, and wherein a statistic distribution of the longitudinal slits, which form the individual radiation elements, is provided with respect to the longitudinal center plane of the wave guide.
12. Baking oven according to claim 11, thereby characterized, that over the length of the wave guide (21) provided to feed the antenna slits (18) at least 20 individual slits are provided.
13. Baking oven according to claim 12, thereby characterized, that at least some of its slits run perpendicular to the direction of propagation of the microwave field in the wave guide.
14. Baking oven according to one of claims 9 through 13, thereby characterized, that for introduction of the microwave energy into the resonator chamber (16) at least two group emitters are provided, preferably with slit-antenna arrangement (14, 18).
15. Baking oven according to claim 14, thereby characterized, that the group emitters (14) are arranged symmetrically with regard to a significant or distinct axis of the resonator chamber.
16. Baking oven according to one of claims 9 through 15, thereby characterized, that the corresponding antenna-arrangement (14) is arranged in a strip-shaped edge area of the resonator wall, which runs very close to the inner edge of the resonator wall.
17. Baking oven according to one of claims 1 through 16, thereby characterized, that for the adjustment of a controllable heating device (28) for achievement of equalization of the temperature profile within the resonator chamber, preferably constructed as an electric resistance heater, which maintains the temperature of the resonator walls (16₁ through 16₆) at a value which corresponds to the value of the temperature-value in a central area of pile of material being sintered (12), which is sensed as actual value, preferably via a pyrometer (32), and which for its part in accordance with a control program follows a specific temperature profile over time.
18. Baking oven according to claim 17, thereby characterized, that various wall areas (16₁-16₆) of the resonator chamber (16) are provided with associated temperarture sensors (29₁ through 29₆), by means of which the possibly varying resonator wall temperatures may be sensed, and that the heating device (28) includes various heater elements (28₁ through 28₆) for heating the various walls being monitored, which are individually controllable.
19. Baking oven according to one of claims 1 through 18, thereby characterized, that the heat insulating envelope intended for heat insulation of the resonator chamber (16) against the outer surroundings of the baking oven (10) is formed internal to oven housing (36) for receiving the resonator chamber (16) and to the heating device (28), and for its part is made of graphite material, in particular graphite felt, with a minimally conductive outer layer.

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