EP1052725A1 - Method of manufacturing a microwave reflective surface - Google Patents

Abstract

The coated and woven structure provides a sufficiently rigid structure with a very lightweight design. The rigid reflecting surface for electromagnetic waves consists of interlaced electrical conductors (2,3) which are formed superficially of a stable metallic diffusion alloy. This ensures the solid fastening together of the structure with the conductors held together, and also the rigidity of the resultant surface. The electrical conductors may either be woven together, or knitted together in order to form this mesh. The transverse dimension of the conductors in at least one direction is less than 20 microns. The mesh may be supported on an insulating reinforcing layer, which may comprise a polymerised composite fibre matrix. The thickness of the metal surface coating forming the conductors may be between 10 Angstroms and 20 microns.

Description

The present invention relates to surfaces reflecting waves electromagnetic, such as antenna reflectors, shields electromagnetic, waveguides, etc ... as well as a method for the realization of these surfaces.

We already know surfaces reflecting electromagnetic waves - hereinafter called reflective surfaces -, which are made by shaped metal sheets, for example by stamping, giving them a self-supporting structure. However, such surfaces have a large mass, so that their dimensions are generally limited. In addition, due to their mass, they cannot be on board spacecraft.

Also, to remedy these disadvantages of mass and limitation of dimensions, it has already been proposed to produce reflective surfaces by metallizing, by any known method (projection, electroplating, vacuum deposition, conductive paint, etc.) of the supports in one carbon fiber composite material - polymerized resin matrix. We can thus obtain reflective surfaces of acceptable mass and desired dimensions. However, these reflective surfaces have disadvantages. First, we see that the straight portions carbon fibers of said supports introduce a parasitic polarization undesirable in the electromagnetic radiation reflected by said surfaces. This is due to the fact that the carbon fibers reflect in part of the incident electromagnetic radiation, while the polymerized resin of the matrix, disposed between said fibers, is relatively transparent to said radiation.

In addition, parasitic electrical discharges may occur between the ends opposite broken carbon fibers - these fibers being conductive - which generates parasites in said reflected radiation.

Finally, the metallization of said composite supports present in general a surface condition so smooth that thermal radiation received by such a reflector is concentrated in the focus of the latter. Also, when the source of the reflector is at the focus, it is necessary to thermally protect said source, for example by covering the surface active reflector by diffusing paint.

The object of the present invention is to remedy the drawbacks reflective surfaces with composite support, while allowing to obtain reflective surfaces of comparable lightness.

To this end, according to the invention, the rigid surface reflecting the electromagnetic waves, especially for antenna reflector, shielding electromagnetic and waveguide, is remarkable in that it consists of an interlacing of electrically conductive wires which are superficially made of a stable metallic diffusion alloy ensuring the joining of said wires together and the rigidity of said surface.

Thus, thanks to the present invention, carbon fibers are eliminated and their disadvantages (parasitic polarization and bursts of rupture). Furthermore, since, in said interlacing, the conducting wires are cross by forming microfacets, the surface, when it occurs in the form of an antenna reflector, no longer focuses the heat energy at home alone; on the contrary, this thermal energy goes through a focal spot. As a result, the source is subjected to a thermal flux lower and that the thermal protection of the source and the reflector may be less complex. It is no longer necessary to cover the surface active diffusing paint reflector, which prevents distortion generated by it.

• réaliser un entrelacs souple de fils électriquement conducteurs dont la superficie est métallique et revêtue d'un métal d'apport, ledit métal d'apport ayant un point de fusion inférieur à celui du métal superficiel desdits fils et ledit métal d'apport et ledit métal superficiel étant aptes à diffuser réciproquement l'un dans l'autre lorsqu'ils sont portés à une température au moins égale au point de fusion dudit métal d'apport pour former un alliage métallique stable de diffusion, dont la température de fusion est supérieure au point de fusion dudit métal d'apport et croít en direction du point de fusion dudit métal superficiel avec l'intensité de ladite diffusion ;
• conformer ledit entrelacs souple à la forme souhaitée pour ladite surface rigide réfléchissant les ondes électromagnétiques ; et
• élever la température dudit entrelacs souple, ainsi conformé, au-delà du point de fusion dudit métal d'apport pour obtenir la formation dudit alliage métallique de diffusion, entraínant la solidarisation desdits fils entre eux et la rigidification dudit entrelacs, qui forme alors ladite surface rigide.

To obtain the rigid reflecting surface in accordance with the present invention, it is possible:

• making a flexible interlacing of electrically conductive wires whose surface is metallic and coated with a filler metal, said filler metal having a melting point lower than that of the surface metal of said wires and said filler metal and said metal surface capable of diffusing into each other when brought to a temperature at least equal to the melting point of said filler metal to form a stable diffusion metallic alloy, the melting temperature of which is higher than melting point of said filler metal and increases in the direction of the melting point of said surface metal with the intensity of said diffusion;
• conforming said flexible interlacing to the shape desired for said rigid surface reflecting electromagnetic waves; and
• raise the temperature of said flexible interlacing, thus shaped, beyond the melting point of said filler metal to obtain the formation of said metallic diffusion alloy, causing the joining of said wires together and the stiffening of said interlacing, which then forms said surface rigid.

This flexible interlacing can be achieved in different ways, by example by knitting, covering, braiding, coating, weaving or even by implementing methods of manufacturing fibrous products non-woven. However, the interlacing in the form of a knitted fabric was found particularly advantageous, especially with regard to dissemination of the heat flux received by said reflecting surface.

The electrically conductive wires can consist of a metallic core covered with said filler metal. In this case, the metal superficial is therefore that of the soul. Alternatively, however, the wires are electrically conductors can consist of a plurality of layers coaxial, at least some of which are made of a material - electrically conductive or possibly insulating - different from said surface metal.

Among the metals used to make wires electrically conductors, we can cite metals that are good conductors of electricity, such as gold, silver, copper, etc ... or even low-alloys coefficient of thermal expansion, such as certain ferro-nickel, or still other metals or metal alloys.

The filler metals are chosen from metals or alloys low melting point, such as tin or indium, which can form a stable alloy by diffusion with the surface metal.

Excellent results have been obtained by choosing copper as the surface metal and indium as the filler metal.

The cross section of the electrically conductive wires can be circular, with a diameter preferably between 6 and 20 microns, or flattened, with a thickness also preferably between 6 and 20 microns and a width preferably between 0.2 and 1.5 mm. In these cases, the thickness of the coating of the filler metal can be between 10 Angström and 1 micron.

Preferably, in order to impart a thickness to said surface desired, we plan to apply uniform pressure on the interlacing flexible conformed, during the rise in temperature.

The surface according to the present invention can be uniform, without holes. In this case, a relatively tight interlacing is expected and the application of said uniform pressure makes it possible to close off any possible days of interlacing. As a variant, said surface may include holes, planned at the time of the said interlacing.

In a preferred embodiment, the surface obtained by the interlacing stiffened is reinforced by a reinforcement attached to one of the faces said interlacing and secured to it. Thus, the stiffened interlacing does not form while the active reflecting part of said surface. Such reinforcement may have a composite fiber - polymerized matrix structure. It is then advantageous that the joining of the surface and the reinforcement is obtained by bonding using the resin of said matrix, the reinforcement being formed on said surface. Of course, for this purpose, the temperature resin polymerization is lower than the melting temperature of stable diffusion metallic alloy.

It is thus seen that, thanks to the invention, a reflective surface is obtained electromagnetic waves by brazing by diffusion of electrically conductive interlacing wires.

By implementing the present invention, between others, antenna reflectors capable of operating at frequencies between 18 GHz and more than 45 GHz.

The figures in the accompanying drawing will make it clear how the invention can be realized. In these figures, identical references denote similar elements.

Figure 1 shows, in plan, an example of electrically interlaced wires conductors used in the implementation of the present invention.

Figure 2 shows, also in plan, a variant of the interlacing of figure 1.

Figures 3 and 4 are sections respectively along the lines III-III and IV-IV of Figures 1 and 2.

Figures 5 and 6 illustrate, in section, alternative embodiments conducting wires used to form the interlacing of Figures 1 and 2.

FIGS. 7A to 7F illustrate different phases of the method of realization of an antenna reflector according to the present invention.

In Figure 1, there is shown an interlacing 1 of electrically wires conductors 2 and 3 crisscrossed. In this figure 1, for the purposes of simplification of drawing, the interlacing 1 is represented in the form of a weaving with warp 2 and weft 3, although the interlacing 1 could advantageously consist of knitted stitches.

It will be noted that, in the interlacing 1 of FIG. 1, the wires electrically conductors 2 and 3 provide voids between them 4.

As can be seen in FIGS. 3 and 4, each wire 2 and 3 comprises a core 5, for example of copper, coated on the surface with a coating 6 of a metal with a low melting point, such as indium. The diameter d of the wires 2 and 3 can preferably be between 6 and 20 microns, while the thickness e of the coating 6 can be between 10 Angstrom and 1 micron.

In the embodiment of Figure 2, the interlacing 7 is similar at the interlacing 1 of Figure 1 with the difference that the conductors wefts and warps 2 and 3 are woven more tightly so as to practically eliminate the voids 4.

We know that when they are heated to a temperature at least equal to the melting point of indium, indium and copper diffuse one in the other to form a stable diffusion alloy whose point of fusion is between that of indium and that of copper and is all the more higher than temperature to which copper and indium are subjected is higher.

It is therefore easy to see that, if the interlacing 1 and 7 are subject at a rise in temperature, beyond the melting point of indium, in being subjected to a uniform pressure, the conducting wires 2 and 3 in contact each other are going to be the object of the surface formation of a stable alloy of indium-copper diffusion.

Figure 3 illustrates the contact of wires 2 and 3 at one of their points crossover, while Figure 4 illustrates the contact of two wires 2 and 3 parallel.

After formation of this stable alloy, the wires 2 and 3 of the interlacing 1 and 7 are joined to each other, which stiffens said interlacing.

Of course, if during the rise in temperature, the interlacing 1 and 7 are shaped to desired shapes for the stiffened interlacing, the stiffening will fix the final shape of said interlacing.

In FIGS. 3 and 4, it has been assumed that the wires 2 and 3 have a circular section. As can be seen in a variant in the figure 5, said wires could have an oblong section. In this case, the thickness ℓ of said section can be between 6 and 20 microns and the width L can be between 0.2 and 1.5 mm, the thickness ℓ being the same as before. In addition, instead of comprising that a core 5 and a surface coating 6, the wires 2 and 3 could have a structure with several superimposed layers. In Figure 6, an alternative embodiment of said wires 2 and 3 has been shown in which an intermediate layer 8 is interposed between the core 5 and the coating surface 6. Of course, in this case, layer 8 must be of a metal capable of forming a stable diffusion alloy with the coating 6.

• sur le moule 10, on applique un entrelacs 1, 7 de fils électriquement conducteurs 2 et 3, en tendant ledit entrelacs (figure 7A) ;
• puis on fixe périphériquement ledit entrelacs 1, 7 ainsi appliqué sur le moule 10 par tout moyen 11 désiré, par exemple un cordon de mastic (voir figure 7B) ;
• sur l'entrelacs 1, 7 ainsi fixé sur le moule 10, on applique une peau de matage 12 --préalablement réalisée sur le moule 10-- que l'on fixe par tout moyen approprié 13, par exemple également un cordon de mastic (figures 7B et 7C) ;
• l'ensemble du moule 10, de l'entrelacs 1, 7 et de la peau de matage 12 est alors introduit dans un autoclave 14, dans lequel ledit ensemble est soumis à une élévation de température, au-delà du point de fusion de l'indium, tout en lui appliquant une pression uniforme P1, par exemple par l'intermédiaire d'une vessie à vide (non représentée), agissant sur la peau de matage 12 ;
• dans ces conditions, de la façon décrite ci-dessus, il se forme superficiellement un alliage métallique de diffusion cuivre-indium à la surface des fils électriquement conducteurs 2 et 3 de sorte que l'entrelacs 1, 7 se rigidifie à la forme du moule 10 ;
• après démarouflage (figure 7D), il est alors possible de napper, sur la face convexe de l'entrelacs 1, 7, un renfort 15 de matière composite fibres - matrice polymérisable (voir la figure 7E) ;
• après nappage du renfort composite 15, celui-ci est polymérisé dans un autoclave 16 avec application d'une pression P2 ;
• pendant la polymérisation du renfort 15, la résine solidarise l'entrelacs 1, 7 dudit renfort 15 et l'on obtient ainsi une surface réfléchissant les ondes électromagnétiques, constituée dudit entrelacs 1, 7 et de son renfort arrière 15.

In FIGS. 7A to 7F, a mold 10 has been shown corresponding to the convex shape of an antenna reflector. To obtain said antenna reflector, the following operations are carried out:

• on the mold 10, an interlacing 1, 7 of electrically conductive wires 2 and 3 is applied, by stretching said interlacing (FIG. 7A);
• then said interlacing 1, 7 is thus peripherally applied to the mold 10 by any desired means 11, for example a bead of mastic (see FIG. 7B);
• on the interlacing 1, 7 thus fixed on the mold 10, a matting skin 12 is applied - previously made on the mold 10 - which is fixed by any suitable means 13, for example also a bead of mastic ( Figures 7B and 7C);
• the whole of the mold 10, of the interlacing 1, 7 and of the matting skin 12 is then introduced into an autoclave 14, in which the said assembly is subjected to a rise in temperature, beyond the melting point of the indium, while applying a uniform pressure P1, for example by means of a vacuum bladder (not shown), acting on the matting skin 12;
• under these conditions, as described above, a metallic alloy of copper-indium diffusion is formed on the surface of the electrically conducting wires 2 and 3 so that the interlacing 1, 7 stiffens to the shape of the mold 10;
• after demouflaging (FIG. 7D), it is then possible to coat, on the convex face of the interlacing 1, 7, a reinforcement 15 of composite material fibers - polymerizable matrix (see FIG. 7E);
• after coating of the composite reinforcement 15, the latter is polymerized in an autoclave 16 with the application of a pressure P2;
• during the polymerization of the reinforcement 15, the resin secures the interlacing 1, 7 of said reinforcement 15 and one thus obtains a surface reflecting electromagnetic waves, consisting of said interlacing 1, 7 and its rear reinforcement 15.

The rise in temperature in the oven 14 leading to soldering by diffusion of the interlacing 1, 7 can be 0.1 ° C per minute, from the room temperature up to the desired temperature for diffusion, compatible with the temperature of the subsequent polymerization of the resin reinforcement 15.

The interlacing 1, 7 is maintained at this desired diffusion temperature for a period suitable for diffusion brazing, after which the cooling can be natural.

Claims (11)

Rigid surface reflecting electromagnetic waves, especially for antenna reflector, electromagnetic shielding and waveguide,
characterized in that it consists of an interlacing (1, 7) of electrically conductive wires (2, 3) which are made up superficially of a stable metallic diffusion alloy ensuring the joining of said wires together and the rigidity of said area. Surface according to claim 1,
characterized in that said interlacing is a knitting of said electrically conductive son. Surface according to either of Claims 1 or 2,
characterized in that at least one of the transverse dimensions ( d, ℓ) of the section of said wires is less than 20 microns. Surface according to one of claims 1 to 3,
characterized in that it is uniform. Surface according to one of claims 1 to 3,
characterized in that it has holes. Surface according to one of claims 1 to 5,
characterized in that it comprises a reinforcement (15) placed against one of its faces and secured to the latter. Surface according to claim 6,
characterized in that said reinforcement (15) has a composite fiber - polymerized matrix structure. Method for producing a rigid surface reflecting electromagnetic waves,
characterized: in that a flexible interlacing (1, 7) of electrically conductive wires (2, 3) is made, the surface of which is metallic and coated with a filler metal (6), said filler metal (6) having a melting point lower than that of the surface metal of said wires and said filler metal (6) and said surface metal (5) being capable of diffusing into each other when brought to a temperature at least equal to the melting point of said filler metal (6) to form a stable diffusion metal alloy, the melting temperature of which is higher than the melting point of said filler metal (6) and increases in the direction of the melting point of said surface metal with the intensity of said diffusion; in that said flexible interlacing (1, 7) conforms to the shape desired for said rigid surface reflecting the electromagnetic waves; and in that one raises the temperature of said flexible interlacing (1, 7), thus shaped, beyond the melting point of said filler metal (6) to obtain the formation of said metallic diffusion alloy, causing the joining of said wires (2, 3) between them and the stiffening of said interlacing (1, 7), which then forms said rigid surface. Method according to claim 8,
characterized in that the thickness ( e ) of the filler metal coating (6) is between 10 Angstrom and 1 micron, when the section of said wires has a dimension ( d, ℓ) at most equal to 20 microns. Method according to one of claims 8 or 9,
characterized in that, during the rise in temperature, the flexible flexible interlacing (1, 7) is subjected to pressure. Method according to one of claims 8 to 10 for obtaining the surface according to claim 7,
characterized in that said reinforcing composite structure is produced directly on said interlacing (1, 7) stiffened and is joined to the latter by the resin of said matrix.

Images