FR2669557A1 - Device for homogeneous treatment, using microwaves, of materials under mechanical pressure stress - Google Patents

Abstract

Device for treating dielectric materials with the aid of microwaves, including a mould intended to contain the material to be treated, means (23) for pressurising the material (3) contained in the mould, and a microwave generator (25) intended to apply to the mould the ultrahigh frequency energy necessary for treating the material, characterised in that the mould includes at least one dielectric material (4, 5, 6, 7, 8) transparent to microwaves, surrounding the material (3) to be treated, the dielectric material resisting the application of a mechanical stress and having a dielectric permittivity decreasing from the material (3) to be treated as far as the outer wall of the mould, the value of the mean dielectric permittivity of the dielectric material being lower than the value corresponding to the situation of cutting of the first mode of an order higher than the fundamental mode of propagation of the electromagnetic wave produced by the generator (25).

Description

 The present invention relates to the microwave treatment of dielectric materials and relates more particularly to a treatment of this type produced under mechanical pressure stress.

 The microwave treatments usually used for the preparation or implementation of dielectric materials, especially organic matrix composite materials, whose dimensions are greater than the half-wavelength of the electromagnetic radiation involve either a system ensuring the relative displacement of the material with respect to the spatial structure of the electromagnetic field, ie a multimode cavity type system with a propagation mode stirrer.

 In the first solution, the application of a pressure constraint is generally impossible because of the movement of the material.

 In the second solution, the material to be treated and the dielectric elements which carry out the pressure holding of this material (matrix plus punch) constitute by their average permittivity, their dimensions and the configuration of their interfaces, a microwave electrical circuit which select the propagation modes. Indeed, the experiments show that, with or without a mode stirrer, homogeneity defects or "heterogeneous markings" appear within the material to be treated, if the treatments are not sufficiently soft and their sufficiently prolonged durations.

 The defects of homogeneity or markings observed in these experiments result from the establishment of stationary wave regimes which are generated in the material by the reflections internal to the interfaces, as a function of the propagation modes excited by these interfaces and as a function of the distances which separate them. These effects are all the more important because the attenuation of the waves in these types of materials is relatively low because of the low dielectric constant of losses and because the diffusion of heat favors little homogenization between the marked zones, because thermal conductivity also low and the duration of treatments voluntarily reduced.

Such an observation of markings due to heterogeneities and the incidence of heat diffusion conditions are well specified in the patent.
In the device described in this patent, the homogenization only results from the conditions of thermal conductivity, the applicator remaining of the multimode type.

 For the device described in patent JP60-135230, the pressure stress is reduced to a vacuum of the materials to be treated or assembled by gluing. As a result, the filling of the applicator remains low and does not require particular dielectric conditions for the permittivity constant values.

 Finally, patent FR 88 08 081 does not specify any element relating to the application of microwave energy or wave propagation conditions.

 The invention aims to overcome the drawbacks of the prior art by providing a "mold" where the electric field of the fundamental mode is guided so as to avoid the excitation of hybrid modes that hinder the homogeneity of the treatment.

 The great difficulty encountered in microwave material processing processes is the control of the spatial distribution of electrical energy, that is to say the obtaining of a homogeneous treatment.

 The aim of the invention is to create a new device which allows homogeneous treatment by establishing an appropriate configuration of the electric field in a mold under mechanical stress.

 It therefore relates to a device for processing dielectric materials using microwaves comprising a mold for containing the material to be treated, means for pressurizing the material contained in the mold, and a microwave generator for to apply to the mold the hyperfrequency microwave necessary for the treatment of the material, characterized in that the mold comprises at least one microwave-transparent dielectric material, surrounding the material to be treated, the dielectric material resistant to the application of a mechanical stress and having a decreasing dielectric permittivity from the material to be treated to the outer wall of the mold, the value of the dielectric mean permittivity of the dielectric material being lower than the value corresponding to the breaking position of the first higher order mode to the fundamental mode of the electromagnetic wave generated by the generator.

 According to a particular characteristic of the invention, at least the input interface of the electromagnetic wave in the mold is constituted by a surface inclined with respect to the direction of propagation of the electromagnetic wave of an angle propagation of the only fundamental mode TEOl in the mold.

The invention will be better understood from the following description, given solely by way of example and with reference to the accompanying drawings, in which:
- Fig.1 is a schematic representation in perspective of the input of the mold highlighting the stack of different dielectric lining the mold and the position of the material to be treated according to the invention;
- Fig.2 is a profile view of the mold of Fig.1 containing the material to be treated;
FIGS. 3A and 3B show sections AA and BB of FIGS. 2, 5 and 6;
Fig.4 is a cross-sectional representation of the mechanical pressure application fitting;
FIGS. 5 and 6 are views similar to that of FIG. 2 and represent two other possible mold shapes; and
- Fig.7 represents the treatment device in its entirety.

 The approach followed in the present invention is to allow the establishment of only one wave propagation mode in the material to be treated, in the mold that contains it and in the applicator associated with it.

 For the mode chosen to be unique, it must be the fundamental mode TEO1 and the configuration of the dielectric assembly formed by the mold and the material to be treated must be designed to prohibit the propagation of higher order modes and, corollary way, the hybrid modes that could result from the possibility of existence of these modes of higher order and their excitation by the interfaces at the entrance and exit of the mold. The fundamental mode being filtered, its spatial distribution is known.

 The material to be treated is thus available in a zone with a maximum electric field and homogeneous.

In FIG. 1, the input of the mold of the device according to the invention is schematically represented.
The actual mold is materialized by a waveguide 1 in which is placed a stack of filling pieces made of dielectric material which delimit a cavity 2 in which the material to be treated 3 is arranged.

 This stack consists in the present example of five layers of dielectric material 4,5, 6, 7 and 8 arranged symmetrically with respect to the median horizontal plane of the guide.

 The layers 4 and 8, and 5 and 7, are made in pairs of dielectric materials of the same kind.

 The permittivity of material II of layers 5 and 7 is greater than that of materials I of layers 4 and 8.

 The layer 6 is made of a material III of permittivity greater than that of the layers 5 and 7 which surround it.

 This layer is interrupted to define with the layers 5 and 7 adjacent four walls of the cavity 2 for receiving the material to be treated.

 This cavity is closed by two side walls which are formed by layers 9 and 10 of a fourth dielectric material IV.

 The mold is placed in a pressure application device shown in phantom and will be described later.

 As can be seen in FIG. 1, the mold has an input interface 11 for the electromagnetic wave in the mold, constituted by a surface inclined with respect to the longitudinal direction of the waveguide of a angle o such that the propagation of the only basic mode TEO1 in the mold is favored.

Experiments carried out in applicators containing a material to be treated and filled with dielectric materials of different characteristics have shown that
- the input angle o determines on the one hand the values of the transmission and the reflection of the wave for the fundamental mode and, on the other hand, is responsible for the excitation of the modes of higher order and in particular stable and reproducible hybrid modes,
the low values of the inclination or bevel angles reduce the reflection of the fundamental mode but also reduce the transmission thereof and increase the transmission of the other undesirable modes.

 Furthermore, the experiments also showed that the propagation mode length (guided wavelength) is unique and is a function of the average value of the permittivities of the constituents; this "average" also depends on the respective position of these constituents with respect to the structure of the propagation mode.

où Em est la permittivité relative moyenne du matériau formant le moule.The input to the mold interface inevitably excites higher order modes. In order to prevent their propagation in the mold in order to filter only the fundamental mode, it is necessary, according to the invention, for the dielectric materials of constitution of the mold to have an "average" permittivity lower than the value corresponding to the breaking position of the first higher order mode. The threshold value of the average permittivity of the dielectrics constituting the mold is given by the following expression

where Em is the average relative permittivity of the material forming the mold.

c, the speed of light,
f, the frequency of the wave,
a, the big side of the applicator.

 For example, for a standard waveguide which, for Europe, corresponds to a prismatic shape of rectangular section where a = 86.36 mm, the threshold permittivity is 2.0.

 This permittivity must take into account the amplitude of the variations in the permittivity of the material to be treated during the entire transformation cycle.

 This condition being fulfilled, the undesirable modes excited at the interface are attenuated exponentially (evanescent modes) and the fundamental mode happens to be the only mode to propagate in the mold.

 As can be seen in FIG. 2, the output interface 12 of the mold is also a bevel parallel to the input interface 11 in order to avoid the reflection of the waves on said interface and their return towards the mold.

 Fig.3A and 3B are sectional views along the lines AA and BB of Fig.2, 5 and 6 and respectively show the positions of the various layers of dielectric material in the mold outside the cavity containing the material to be treated and facing this cavity.

 In FIG. 3A, the layers 4,5 and 7,8 of dielectric filler material I and II are disposed on either side of the layer 6 of dielectric filler material III and constitute a decreasing permittivity configuration of the center towards the edges of the mold.

 In Fig.3B, the material to be treated 3 occupies the center of the cavity. It is confined on its horizontal lateral faces by the layers 5, 7 of dielectric material 11 which in turn are taken between the two layers 4.8 of dielectric material I.

 The material 3 to be treated is further confined on its vertical faces by the layers 9, 10 of filling dielectric material IV.

 In FIG. 4, the device for applying mechanical pressure to the material to be treated is shown in section.

 This arrangement, which is advantageously incorporated in the waveguide 1 of FIG. 1 opposite the stack of layers of dielectric material delimiting the cavity 2 containing the material to be treated, comprises a metal matrix 14 provided with a base 15 and in which is provided a chamber 16 open at its upper part having the inner dimensions of the waveguide 1.

 A punch 17 is placed on the open upper part of the die in order to apply a pressure to the stack of layers of dielectric material described with reference to FIGS. 1 to 3B and consequently to the material to be treated 3 contained in FIG. cavity 2.

 The stack shown in FIG. 5 consists of the same layers 4 to 8 of dielectric materials of decreasing permittivity from the center towards the walls of the waveguide which contains it, as those of the arrangement of FIG.

 It differs from it by its input and output interfaces 18 and 19 which each consist of a hollow wall formed of two intersecting planes 18a, 18b, 19a, 19b inclined at an angle o with respect to the direction longitudinal guide.

 The stack shown in FIG. 6 differs from that of FIG. 5 in that its input and output interfaces 20 and 21 are constituted by projecting walls 20, 21 each formed of two secant planes 20a, 20b , 21a, 21b inclined at an angle e with respect to the longitudinal direction of the guide.

 In Fig. 7, there is shown schematically the processing device as a whole.

This device comprises an applicator 22 whose configuration is that described with reference to
FIGS. 4 and 5, the pressurization assembly of which is diagrammatically represented by a punch 23.

 The applicator is connected to a waveguide 24 to which is connected at one end a microwave generator 25, an insulator 26 being interposed between the generator 25 and the applicator 22.

 The magnetron type generator 25 generates electromagnetic waves at a frequency of 2.45 GHz and the insulator 26 protects the generator against any return waves.

 The insulator 26 is of the water circulation type.

 At its end opposite the generator 25, the waveguide 24 is connected to an adaptable load 27 which, too, is of the water circulation type, and which is intended to prevent any wave return to the mold.

 The electromagnetic waves delivered by the microwave generator are guided towards the mold by the waveguide 24 in which only the fundamental mode is propagated.

 In the device represented here by way of example, the electric field is maximum and homogeneous in the median plane of the applicator 22 and the substance 3 to be treated is thus disposed in the center of the mold as indicated in the figures.

The homogeneous distribution of the intensity of the electric field in the substance to be treated is controlled thanks to the following properties of the dielectrics I,
II, III and IV which constitute the mold.

 The dielectric permittivity III is very close to that of the material to be treated to avoid dielectric discontinuity so that the electromagnetic wave easily enters the substance.

The dielectric permittivity of the material to be treated being the highest, those of the surrounding dielectrics I and II are in the order E '(I) <E'(II);
E '(II) is slightly less than the permittivity of the material.

 The dielectric It has the qualities of a mold material. Indeed, this material being in direct contact with the material to be treated, it is therefore its surface state that depends on the appearance of the part to be developed. In addition, these performances must not change with temperature.

 The thermal conductivity of the dielectric It has anisotropy. It is preferable that it is a good thermal conductor in the direction of propagation of the waves in order to facilitate the uniformization of the heating in the longitudinal direction. On the other hand, the heat losses towards the dielectric II are reduced to the minimum possible thanks to a very low thermal conductivity of the dielectric Il in the direction perpendicular to the direction of propagation of the electromagnetic wave and the direction of the electric field. This property can be acquired by using for the dielectric II, for example, a fiber material.

 The dielectric IV constituting the layers 9 and 10 (FIG. 3B) has thermal insulation properties.

 In order to limit the lateral losses of heat, it is inserted between the substance to be treated and the metal wall of the applicator.

 Finally, all these dielectrics have the common property of being all resistant to mechanical pressure.

 The waves leaving the mold are trapped in the load constituted by the adaptable load 27. No return waves are observed, which gives the system a traveling wave structure.

 Thus, the homogeneity of the spatial distribution of the electric field in the material to be treated is controlled.

 The fact that the substance to be transformed has the highest dielectric permittivity allows a good concentration of the field in the susbtance and therefore a good homogeneity of its treatment.

 The treatment device according to the invention is particularly well suited to ensure the heat treatment of composite materials consisting of a mass of organic material and filler materials such as glass fibers, carbon fibers or the like.

 It is also suitable for the treatment of homogeneous organic matter.

 In the embodiments that have just been described, the inclination angles of the input and output interfaces are equal.

 However, these angles can be chosen different given the transmission and reflection of the fundamental mode of the electromagnetic wave.

Claims (12)

 1. Device for treating dielectric materials using microwaves comprising a mold intended to contain the material to be treated, means (14, 17; 23) for pressurizing the material (3) contained in the mold, and a microwave generator (25) for applying to the mold the microwave energy necessary for processing the material, characterized in that the mold comprises at least one dielectric material (4,5,6,7,8,9, 10) transparent to the microwaves, surrounding the material (3) to be treated, the dielectric material resistant to the application of mechanical stress and having a decreasing dielectric permittivity from the material (3) to be treated to the outer wall of the mold, the value of the dielectric material mean permittivity of the dielectric material being less than the value corresponding to the breaking position of the first higher order mode to the fundamental mode of the electromagnetic wave generated by the generator (25).  2. Device for processing dielectric materials according to claim 1, characterized in that at least the input interface (11; 18; 20) of the electromagnetic wave in the mold is constituted by a surface inclined with respect to the direction of propagation of the electromagnetic wave of an angle (o) favoring the propagation of the only fundamental mode TEO1 in the mold.  3. Treatment device according to one of claims 1 and 2, characterized in that said mold contains several dielectric materials (4,5,6,7,8,9, 10) having different dielectric permittivities and being arranged in the mold so that the dielectric permittivity of the assembly varies in decreasing from the material (3) to be treated to the outer wall of the mold.  4. Device for treating dielectric materials according to one of claims 1 to 3, characterized in that the dielectric material (5, 7, 9, 10) in contact with the material (3) to be treated has an anisotropy of thermal conductivity , the thermal conductivity of said material being strong in the direction of propagation of the electromagnetic wave and low in the direction perpendicular to the direction of propagation and the direction of the electric field.  5. Device for processing dielectric materials according to one of claims 2 to 4, characterized in that in the direction of the electric field a dielectric (9,10) thermally isolates the material to be treated metal walls means (14,17 23) pressurizing the material (3) to be treated.  6. Device for processing dielectric materials according to one of claims 2 to 5, characterized in that the output interface (12; 19; 21) of the electromagnetic wave is also constituted by a surface inclined relative to the direction of propagation of the electromagnetic wave.  A dielectric material processing device according to claim 6, characterized in that the inclination angles of the inclined surfaces forming the inlet and outlet interfaces (11, 12, 18, 19, 20, 21) of the mold are equal.  8. Device for processing dielectric materials according to claim 6, characterized in that the inclination angles of the inclined surfaces forming the input and output interfaces are chosen to be different, in order to optimize the transfer associated with the fundamental mode and reduce the possible excitation of the other modes.  9. Apparatus for processing dielectric materials according to one of claims 2 and 6 to 8, characterized in that the interface or the interfaces are each constituted by a flat surface (11,12) inclined relative to the direction of propagation of the electromagnetic wave.  10. Apparatus for treating dielectric materials according to one of claims 2 and 6 to 8, characterized in that the interface or the interfaces are each constituted by a recessed wall (18,19) formed of two intersecting planes (18a , 18b, 19a, 19b) inclined at an angle (o) with respect to the direction of propagation of the electromagnetic wave.  11. Apparatus for treating dielectric materials according to one of claims 2 and 6 to 8, characterized in that the interface or the interfaces are each constituted by a projecting wall (20,21) formed of two intersecting planes (20a). , 20b, 21a, 21b) inclined at an angle (o) with respect to the direction of propagation of the electromagnetic wave.  12. Apparatus for treating dielectric materials according to one of claims 1 to 11, characterized in that it further comprises a tunable microwave trap (27) connected to the waveguide (24) downstream of the mold and pressurizing means (22) surrounding it and intended to prevent any wave return to the mold.