DE4207009C2 - Process for producing a reflector, reflector and its use - Google Patents

Description

The invention relates to a method for producing a reflector and a reflector according to claim 1 or the preamble of claim 20 and the use of the reflector.

A large part of the optical materials is used for the orderly forwarding of rays by refraction or reflection, with both properties can be used simultaneously. Refractory materials are mainly due to the dependence of their refractive index and their transmis degrees of marked by the wavelength.

With mirrors and reflectors, the degree of reflection is of interest as a function the wavelength. If the metal layers are not too thin, they will not become reflective absorbed radiation component in the almost absorption-free adjustable dielectric mirrors is the resistance to at atmospheric influences and their increase through protective layers.

While a large number of highly transparent glasses are available in the visible spectral range, this is no longer the case in the UV and IR ranges. Here the glasses are supplemented by a small number of crystals and derived materials, the most important property of which is a high degree of transmission. Some of these materials (e.g. BaF ₂ , CaF ₂ , LiF, Al ₂ O ₃ , SiO ₂ ) are suitable for broadband and multispectral systems because their permeability ranges from UV to IR. With some of these substances, the high water solubility, the protective layers and / or use in completely dried air are problems.

Single crystals (isotropic or anisotropic, depending on the type of crystal) are made from natural finds or by crystal growth (pulling out of the melt) won. Polycrystalline material (isotropic) is produced by pressure sintering posed.  

Only isotropic materials are suitable for most applications. On Isotropic crystals are used in polarization optics.

The semiconductor silicon with an isotropic character acts as a long pass filter steep edge and thus blocks the visible and the near IR range. Butt Lycrystalline silicon has a thickness of 2 mm, without Re flex reduction, a transmittance τ ~ 0.53. After a transmission The minimum transmittance is around 16 µm in the long-wave range up to over 300 µm again at 0.4 to 0.5 and, despite its brittleness, in combination with its favorable thermal properties applications especially possible in infrared optics and as a mirror support. (Source 1: Naumann / Schröder: Components of Optics, 5th edition, page 64, Hanser Ver lay).

By means of achromatic imaging and avoidance of errors associated with the passage through refractive materials, surface mirrors offer many advantages over refractive systems. However, since the change in a beam deflection angle is twice as large as the change in the angle of incidence, the same optical requirements for shape accuracy and micro shape of the mirror surfaces make higher demands than for refractive interfaces. Another property of the mirror is the course of the spectral reflectance. The possibility of using the surface of a solid metal body directly as a mirror through good polishing is only used today in exceptional cases. In general, a mirror layer is coated on a mirror support. The metallic mirror support is polished beforehand and is responsible for the shape accuracy of the surface. The mirror layer is applied to the support by (mostly) vapor deposition in a high vacuum or chemical processes. The mirror layer adapts here completely to the shape of the support surface and determines the course of the spectral reflection function and (if appropriate together with protective layers) the temporal stability of the reflection function. Metals such as aluminum, chromium, nickel, mercury, silver, gold, platinum, rhodium but also silicon monoxide (SiO) and silicon dioxide (SiO ₂ ) are preferably used for mirror layers (source 1).

High demands are made of material for mirror supports and reflectors regarding mechanical and thermal stability. Large mirrors ver form with their own weight in different positions; Displacements by fractions of λ must be prevented or by counter forces be compensated. Temperature changes and uneven temperature ver Divisions lead to internal tensions and deformations. Essential requirements Requirements for a substrate for precision mirrors are therefore high Modulus of elasticity E and a very low coefficient of thermal expansion is α. Good polishability is also required, so that it is optimally smooth Areas with low stray light can be achieved.

In this regard, most metal surfaces are unfavorable because of their ge Joining structure due to differing properties at the grain boundaries deviations after polishing. But u. a. Pure copper, aluminum and molybdenum alloys as well as pressure sintered beryllium used as a mirror, the polishability by a layer of che mixed nickel phosphide must be improved. Metal mirror have a high thermal expansion, but can because of their favorable heat conductivity only limited z. B. can be used for high-power lasers. Glass and glass ceramics are currently of greater importance.

Certain components, especially for space travel, should be next to one high basic strength and temperature resistance also through a low one Mark density. It also has a good temperature change resistance (TWB) combined with a low coefficient of thermal expansion cient (WAK), required.

For example, future satellites should rotate with one in use the mirror structure. Such a large mirror with dimensions Solutions of, for example, 800 × 600 mm must have a front have optically reflective surface.

Because in space use with cyclical temperature changes from 0 to 700 K. in addition to the size-related component stiffness, the tempera tur- and thermal shock resistance, low density and last but not least low thermal expansion can be guaranteed. Beyond that high surface qualities for the material groups in question allow reflective optics to be achieved.  

Conventional mirror components are now made from glass ceramics. The manufacturing process is carried out by melting various oxide powders such as LiO ₂ , Al ₂ O ₃ , MgO, ZnO and P ₂ O ₅ in platinum furnaces. After the melt has been homogenized, corresponding glass moldings are produced using the pressing, casting and other glass molding processes. After tumble cooling and demolding, the glass components are tempered to a temperature of approx. 700 ° C, so-called crystal nuclei form in the non-crystalline (amorphous) glass. With a corresponding holding time the mentioned nucleation leads to crystal growth and carries out the "ceramization" of the glass to the glass ceramic.

This crystalline glass ceramic has the advantageous property of having a low thermal expansion of only 0 ± 0.15 × 10 ^-6 K ^-1 in a temperature range from 273 K to 323 K.

As a mirror material, this glass ceramic can only be used to a limited extent, since it can only be produced using complex shaping processes and, in addition, has a relatively high density of 2.53 g / cm ³ , low tensile strength and, last but not least, brittle fracture behavior. In addition, their range of use as an optical component is limited to a constant or a maximum temperature of 423 K, since the crystalline structure of the glass ceramics in the temperature range of 200 to 300 K and 360 to 480 K are subject to a voltage hysteresis. In the temperature range above 700 K, the structure is already irreversibly damaged (source 2: SiRA; ESTEC Contract No. 5976/84 / NL / PR; October 1985).

Attempts are also known to produce lightweight mirror components from inexpensive aluminum (density 2.71 g / cm ³ ). Due to the low rigidity of aluminum, it has not been possible to manufacture precision optics from it. For use in a corrosive environment, aluminum mirrors must be provided with a thick (0.2-0.5 mm) nickel coating. Due to massive differences in thermal expansion between aluminum (23. 10 ^-6 K ^-1 ) and nickel (13. 10 ^-6 K ^-1 ), these mirrors must not be exposed to any changes in temperature, otherwise thermal cracking will occur.

Pure aluminum mirrors, such as can be used in a vacuum, show even at low temperatures - due to the very high Coefficient of thermal expansion on the optical mirror surfaces - local Deformations that are used, for example, as laser mirrors for removal measurement would lead to undefined results (source 2).

State of the art are also reflective optics based on quartz glass. Due to their extremely low coefficient of thermal expansion of almost zero in the temperature range from 0 to 273 K, quartz glass systems are predestined for so-called cryogenic applications. In the range between 273 K and 373 K, the coefficient of thermal expansion rises to 5.1. 10 ^-7 K ^-1 . After parts are the relatively high density of 2.2 g / cm ³ , the low rigidity, the low tensile strength of <50 MPa, high production costs and the limitation of the diameter to approx. 500 mm due to the complex manufacturing process (W. Englisch, R. Takke, SPIE, vol. 1113, Reflective Optics II, 1989, page 190-194).

Due to its mechanical and thermal properties and its low density of only 1.85 g / cm ³ , beryllium is particularly suitable for the production of lightweight mirror structures. For example, beryllium is five times more rigid than aluminum or glass materials. Coated beryllium plates can be polished to a roughness depth of less than 15 angstroms (R _a ) and are therefore predestined for optically reflective surfaces.

In addition to the high raw material and manufacturing costs, the generally toxic behavior of beryllium materials is particularly disadvantageous. To be able to use them as optical components under atmospheric conditions, they must first be coated with nickel. Due to different coefficients of thermal expansion of beryllium (11.2. 10 ^-6 K ^-1 ) and nickel (15. 10 ^-6 K ^-1 ), these components must under no circumstances be subjected to thermal shock and therefore only come at constant temperatures or in very narrow temperatures Temperature ranges for use.

It was also found that the vacuum hot press technology or The hot isostatic pressing made beryllium parts anisotropic Material character with different properties in different Chen have crystal directions.  

Uncoated mirrors can be used under space conditions. However, the high coefficient of thermal expansion causes the typical temperature temperature changes between 0 and 700 K local deformations on the optical Area that excludes beryllium for use in precision optics (Source 1) and also with satellite mirrors to considerable transmission pro can lead to blemen.

Such mirror structures are currently also made of monolithic ceramics based on silicon carbide using the so-called slip casting technique. In this molding process, a silicon carbide powder suspension is filled into a plaster mold designed as a negative. Depending on the dwell time of the suspension in the plaster molds, a ceramic body, i.e. the positive component green body, is formed with different wall thicknesses. After the blanks have dried, a sintering process takes place in vacuum or protective gas ovens at temperatures of up to 2200 ° C. In addition to the elaborate mold construction for the production of the green body, this manufacturing technology has the disadvantage that only certain geometries and small sizes can be realized and the production is subject to a high rejection rate. Since these silicon carbide moldings are subject to shrinkage during drying and sintering, the required dimensional accuracy can only be guaranteed by costly machining with diamond tools. Due to the heterogeneous structure, the sintered body is then additionally coated with silicon carbide via chemical vapor deposition in order to be able to reach roughness depths of less than 40 angstroms. In addition to the complex manufacturing and processing method, silicon carbide has a relatively high density of 3.2 g / cm ³ and exhibits extremely brittle fracture behavior.

From DE 32 46 755 A1 it is known that high strength Composite materials, consisting of different laminate layers in combination nation with cell nucleus or honeycomb layers, lightweight construction can. Resin-impregnated nonwovens or fabrics are used as raw materials Paper, plastic, foil, glass fabric, carbon fiber fleece or polyimide for Use where the cell nucleus or honeycomb layer has better stability and to ensure an increased bending stiffness of the molded body.  

Such composite materials based on CFRP or GFK (carbon or glass fiber reinforced plastics) are on Room temperature applications limited. Because of the inhomo Genen fiber or laminate structure can by a surface no optical mirror surface become.

From DE 38 09 921 A1 it is known that a game gel can be made by using a commercially available Silicon wafer attached to a support plate. For Making larger mirrors will be several such wafers arranged side by side. The joints between the wafers using laser or electron beam welding connected with each other. Below is the Mirror surface ground and polished. As a carrier a body made of a light metal or fiber-reinforced Composite plastic used. Here, too, the steps occur mentioned disadvantages, in particular by the different materials from carrier and Reflector layer are conditional.

From DE 36 26 780 A1 it is known that mirrors not only to reflect light but also as antennas can use. The mirrors shown there have one Carrier made of carbon fiber reinforced material. The reflect The door layer consists of glass or glass ceramic and is on the support melted directly. The disadvantages of such Mirrors are described above.

DE 30 18 785 A1 describes a method for the production known from mirrors, in which first a molded body from a high temperature resistant material, namely from Carbon is produced, which is used as a support structure for serves the mirror. On this support structure is the Bil sintered onto a glass front panel  or melted. Here is a structure again in which the support plate and the reflector layer very much have different thermal properties, so that such mirrors only a very low temperature shock have stability. The density of such mirrors is quite high.

The invention has for its object a method for Manufacture of a reflector and a reflector show, the reflector light weight and verbes Serte mechanical / thermal properties with simple Manufacturing has.

This task is procedurally carried out by the patent claim 1 mentioned features solved. A reflector that solves this problem is specified in claim 21. Before Preferred embodiments of the invention result from the Subclaims.  

The following description primarily assumes that carbon or carbon fibers as the basic building block for the carrier body be used. At this point, however, it should expressly depend on this be shown that the development of materials similar to fine structure len construction or the use of such substances from the inventive concept is included. An essential idea of the present invention lies namely in that these are "soaked" so that the silicon Re can firmly connect the reflector layer to the base body.

CFC composites, which consist of a carbon matrix and reinforcing fibers made of carbon, are manufactured industrially using the resin impregnation and carbonization process. The materials thus produced are characterized by an extremely favorable combination of material properties, such as. B. high mechanical strength in the room and high temperature range in conjunction with low density (1.0-1.7 g / cm ³ ) and low brittleness.

The excellent material properties of CFC are clouded by that this material has a low resistance to oxidation and only can be used to a very limited extent in an oxygen-containing atmosphere. The low resistance to oxygen currently limits CFC to the Va use of vacuum and protective gas, otherwise at temperatures above 400 ° C a burn begins.

In order to increase the oxidation resistance of these composite materials, developed the so-called Ceramic Matrix Composites [CMC]. With these Materials are refractory and ceramic in the porous CFC matrix components infiltrated. It is also the production of short fiber kernels possible, with short carbon-based fibers in a phenolic resin Suspension are dissolved and harden when the temperature rises. With another The resin binders of both composite qualities are subjected to temperature carbonized in the absence of oxygen.

The reflector has fiber-reinforced CFC or CMC or carbon honeycomb composites and superficially "metallic" Silicon. Under "metallic" silicon elemental silicon is too understand which by diffusion, sintering or melting processes of Silicon moldings or wafers on a CFC carrier substrate was applied. Silicon wafers are disks consisting of made of ultra-pure silicon. The silicon can be isotropic or polycrystalline Have structures.

The invention allows the production of reflectors with complex geometry, high thermal shock resistance, low density (0.5-2 g / cm ³ ) with high strength (<150 MPa), low thermal expansion coefficient (CTE) and surfaces that are suitable for reflective optics.

Another advantage of the method according to the invention is that reasonably priced, commercially available materials can be shipped Have it processed on every machine tool.  

In addition, the density and strength of the component can be targeted the selection of the carrier body and the quantity or quality of the ge selected infiltration processes. The coefficient of thermal expansion efficiency of the material groups used are below very similar to each other; this results in very precise, dimensionally stable parts, even with large dimensions.

Any CFC raw materials can advantageously be used, in particular Long or short fiber base as well as with an oriented or unoriented company can be used. In addition, the Ver also drive on known honeycomb structures on coal fiber base can be applied.

The CFC carrier body used has a density of at most 1.4 g / cm ³ , ie it has a high porosity. Apart from the pores, the carrier body has no cavities, that is, it is a massi ver body in shape, z. B. a plate, a block or a solid cylinder. With multi-dimensionally oriented CFC grades with long fiber construction, one generally starts from resin-soaked carbon fiber fabrics, so-called prepregs, which are pressed into CFRP sheets in heatable axial presses.

For the production of CFC full bodies based on short fibers are known in Way carbon or graphite fibers in a thermosetting resin binder suspended. The suspension is placed in a mold. Then the Solvent e.g. B. removed by heating and the resin binder and there cured with the CFRP full body. For all CFC qualities, the company server strengthening to prevent embrittlement of the ceramic CFC materials act and maintain quasi-ductile fracture behavior. The from the known molding process honeycomb structures for example Hard paper or carbon fiber are used to increase carbon Yield impregnated with a resin binder, preferably phenolic resin and cured in a subsequent heat treatment.

Common to all CFC or honeycomb structures is the subsequent carbonization tion of the binder resin in a vacuum or protective gas at temperatures of for example 900 to 1300 ° C. The CFC solid or honeycomb obtained  structures are then preferably in a vacuum or protective gas atmosphere Temperatures of more than 2000 ° C heated to at least partially Gra phitation of the C-matrix and fibers.

The material is then removed from the solid by machining talking CFC blank made of the dimensions of the construction to be made partly, for example the basic mirror structure of a satellite or other optically reflective systems. The material-removing Be work can be done, for example, by turning, milling or grinding, the be for machining metal materials known machines can be used. For further weight savings can advantageously when processing the CFC raw materials on the Back of the mirror structures pockets of any geometry milled, be eroded or drilled. The carbon honeycomb structures can after carbonization on the end faces with carbon fiber fabrics of any kind or tissue prepregs using a resin binder to form a composite lami be kidneyed. It is also possible to insert the honeycomb structures into the hochpo Press in red short-fiber CFC or honeycomb coated with CFC fabrics structures to form a sandwich system consisting of several honeycomb structures to be pressed, so that after carbonization a high temperature permanent lightweight construction material with high rigidity and quasi to obtain ductile fracture behavior.

The CFC or honeycomb blank obtained after processing, which like the solid body has a low density of 0.1 to 1.3 g / cm ³ and thus a high porosity of up to 90% by volume, can then be re-impregnated with resin binders and infiltrate and solidify their carbonization.

Infiltration via chemical vapor deposition [CVD] with pyrolytic carbon up to a maximum density of 1.4 g / cm ³ , preferably 0.3 to 1.0 g / cm ³ , can also be necessary to solidify the CFC blanks and thus stiffen the mirror base structure. While phenolic resins are preferably used in resin impregnation, in the chemical vapor deposition of carbon, a mixture of hydrocarbons, such as methane or propane, and an inert gas, such as argon or nitrogen, is preferably used at a temperature between 700 and 1100 ° C. and one Pressure from 1 to 100 mbar carried out. The gas diffuses into the open-pore structure, decomposes into carbon and hydrogen, the carbon preferably depositing as pyrolytic carbon on the fiber surfaces or at the fiber crossing points, thus increasing the strength.

The CFC blanks obtained from both infiltration processes are on the for the side provided for the mirror surface, ground and in a vacuum or inert gas oven installed. On the sanded side who the one or more molded silicon body and the Pro be at temperatures from 1300 to 1600 ° C, preferably 1350 to 1450 ° C heated. Through a chemical reaction of the carbon with the silicon silicon carbide forms at the interfaces, which leads to a Verfe Fixing or joining and thus for the application of the silicon wafer on the CFC carrier substrate leads. In addition to the chemical reaction, An melting processes of silicon or diffusion for application the wafer on the CFC blank and so optically reflective Train structures.

On so-called ceramic matrix composites [CMC], which are described, for example, in the matrix have silicon carbide and silicon, Sili cium wafers can be firmly applied at temperatures from 1300 to 1600 ° C.

A particularly advantageous embodiment of the method according to the invention also provides that the surfaces to be mirrored with one or more Ren molded silicon bodies or Si wafers and the so prepared Mirror support with its lower end in a doped silicon melt poses. Due to the capillary forces in the support structure, the melted content increases zene silicon in the blank up to the high-purity silicon moldings above, whereby on the one hand the blank is refined to the CMC and the Silicon molded body or wafer connected to each other and back be firmly attached to the component. Support structures made up of several Sub-elements are inserted or joined, can also by the rising silicon can be solidified into an overall structure.

The CFC carrier body is infiltrated with such an amount of molten silicon that its density is less than 2.0 g / cm ³ , preferably 1.5 to 1.8 g / cm ³ . Advantageously, the commercially available metallic silicon single crystal wafers are pre-sanded and polished in such a way that they form optically reflective surfaces after application and so machining processes can be reduced to a minimum or excluded. In order to prevent massive melting, deformation and evaporation of the silicon parts, a maximum process temperature of 1550 ° C must not be exceeded; the process temperature is preferably between 1350 and 1500 ° C. A particularly advantageous embodiment of the method according to the invention also provides that the silicon moldings are stuck onto the CFC or CMC or honeycomb base structures before application with an adhesive or resin binder, these diffusion, sintering or melting processes being applied to the temperature treatment promote the interface between the carrier substrate and the silicon moldings.

Precursors can advantageously be used as adhesives or resin binders Polysilane or silicon carbonitride-based and / or adhesive based on silicon, Silicon carbide or carbon base or silicones are used. In front the reaction application, the adhesives must be at temperatures between 100 and 200 ° C dried or hardened. Pyrolysis of the resin binder tel takes place at 1000 ° C in a vacuum or protective gas atmosphere.

Depending on the solder, polysilanes (polymethylphenylsilanes) have, for example proportion of the solvent after pyrolysis under inert gas at 1200 ° C a kera mix solids yield of 30 to a maximum of 70 wt .-%.

For mirror and reflector applications in the temperature range of, for example wise 250 and 330 K without major temperature fluctuations leads to an adhesive of the silicon parts by means of silicones onto the carrier structures sufficient consolidation between the supports and the mirror layer.

The method further provides that several sili cium shaped body or Si wafer with different doping or melting point te as a so-called multilayer on the CFC or CMC or honeycomb support structure introduces doors, the silicon moldings preferably being arranged in this way net are that the Si molding with the lowest melting point directly on is applied to the carrier substrate and all the silicon Bodies have a higher melting point. Undoped high-purity silicon be for example, its melting point is at 1412 ° C. Depending on the quantity the melting point can result from the doping of eutectic formations are reduced accordingly.  

The components produced in this way can then be reworked be drawn, for example around optically reflective surfaces at one Generate satellite mirrors. For post-processing, the data from Me tallverarbeitung her known grinding, lapping or polishing machines and Tools are used, especially diamond tools.

If the reflecting / reflecting surface is generated from wafers, then the Temperature for the melting process chosen so that the smooth outer surface surface of the wafer does not melt. For silicon moldings with non-smooth surfaces are created surface from the solidified melt with subsequent grinding and buttocks process.

Another embodiment provides that the silicon moldings applied to the CFC or CMC carrier bodies can be converted, at least on the surface, to hard silicon carbide (SiC). Particularly in the application areas in which chemical attack or abrasion is to be expected, the mirrors or reflectors with their silicon mirror layer can be exposed to a carbon-containing atmosphere above 700 ° C, the silicon reacting with the carbon and forming a wear-resistant surface layer made of silicon carbide . For example, the silicon reacts with a gas mixture consisting of methane and argon or hydrogen at temperatures of 1200 to 1300 ° C to silicon carbide. In the temperature range of 900 to 1200 ° C, a gas mixture consisting of C ₃ H ₈ and H ₂ also leads to a corresponding silicon carbide formation.

Another advantageous embodiment of the method provides that the silicon mirror layers on the CFC or CMC mirrors can be superficially converted into silicon dioxide (SiO ₂ ) or quartz for applications such as in broadband or multispectral systems. For this purpose, the entire mirror or reflector structure or other optical components with their reflecting silicon surfaces in an oxygen-containing atmosphere, preferably air, are exposed to temperatures above 500 ° C, preferably 800 to 1000 ° C, and so the metallic silicon superficially to silicon dioxide ( SiO ₂ ) oxidized.

Preferred embodiments of the invention are described below with reference to Illustrations described in more detail. Show here

Fig. 1 shows a cross section of a silicon wafer Applizierung on a CMC substrate at 20 × magnification,

Fig. 2 shows the polished section of FIG. 1 × 40 under magnification,

Fig. 3 shows the polished section of FIG. 1 at 100 times magnification,

Fig. 4 shows the polished section of FIG. 1 at 200 times magnification,

Fig. 5A is a large mirror which is composed of facets,

FIG. 5B is a partial section of the mirror of FIG. 5A,

5C shows another embodiment of a large mirror in a presen- tation similar to FIG. 5B,

Fig. 6A is a longitudinal section through a preferred embodiment of a support structure,

Fig. 6B is a section along the line VI-VI of Fig. 6A, and

Fig. 7 is a diagram of a silicon wafer coated on a Applizierung with Kohlefa ser tissues honeycomb structure,

Fig. 8A, an embodiment of infrared telescope mirrors and

Fig. 8B, an embodiment of infrared telescope mirrors.

Examples 1 and 2 are used to illustrate processes explained to understand the basis of the further examples 3 and 4 explained the inventive method not are agile.

Example 1:

40 carbon fiber prepregs with a satin bond, a phenolic resin content of 35% by weight and a diameter of 150 mm are made into a CFRP molded body in a heatable axial press at a temperature of 200 ° C and a pressing time of 8 minutes a diameter of 150 mm and a wall thickness of 12 mm. The body obtained after demolding, or this plate, is now carbonized in a reactor with the exclusion of oxygen, ie in a vacuum or in a protective gas atmosphere, at about 1000.degree. In order to minimize the reactivity of the carbon fibers or to influence the modulus of elasticity, the solid body is exposed to a temperature of more than 2000 ° C. in the absence of oxygen, ie in a vacuum or in a protective gas atmosphere, whereby the matrix carbon formed by carbonizing the phenolic resin at least is partially graphitized. This graphitization takes place, for example, at a heating rate of 30 K / min and a holding time of 2 hours at 2100 ° C. The CFC solid obtained has a density of 1.0 g / cm ³ .

From the solid body is then machined by turning, milling and / or grinding the blank shown in Fig. 6, which serves as a support body for optically reflective mirror structures.

The blank is again in a pressure autoclave at 500 bar with phenol resin impregnated. After the pressure impregnation, the component is covered with a Silicon molded body (diameter 123 mm, wall thickness 0.8 mm) pasted. As an adhesive z. B. a commercially available silicon carbide adhesive from Type RTS 7700 from Kager used, which at 100 ° C in air without Shrinkage dries up. The impregnated component is placed in a reactor 1000 ° C and carbonized again at a pressure of 10 mbar. The heater Speed is 2 Kelvin per minute, the holding time is 12 hours.

The blank with glued silicon molded body and a density of 1.18 g / cm ³ is now heated in a vacuum oven at a heating rate of 20 K per minute to a temperature of 1390 ° C. and held there for 30 minutes. After cooling to room temperature, the silicon wafer is firmly interlocked with the CFC without deformation.

The microscopic examination of the cut surface by means of a reference component confirmed that the silicon molded body is firmly toothed and joined to the CFC base substrate without recognizable cracks and pores. Experiments in polishing with submicron diamond suspensions showed that the silicon surfaces can easily be ground to roughness depths R _a of less than 15 angstroms and are therefore outstandingly suitable as optically reflective structures.

Example 2:

Short carbon fibers with a length of 10 to 30 mm are in one Slurry of phenolic resin suspension. The fiber content in the suspension be carries 40% by weight. The suspension comes in a cylindrical shape with a Filled with a diameter of 150 mm and a height of 100 mm. At 60 to 70 ° C the solvents contained in the phenolic resin are removed under vacuum away. When the temperature rises to 180 ° C, curing takes place of the phenolic resin. After removal from the mold, the cylindrical CFRP solid body carbonated in the absence of oxygen as in Example 1.

The CFC solid body obtained with a quasi-isotropic structure has a density of 0.55 g / cm ³ and a porosity of approximately 70% by volume. In order to minimize the reactivity of the carbon fibers used or at least partially convert matrix carbon formed from the phenolic resin to graphite, a graphitization is carried out at temperatures of greater than 2000 ° C., as described in Example 1.

The components shown in FIG. 6 are then machined from the cylindrical solid body by turning, milling and / or grinding, which can serve as a carrier substrate for the satellite mirror structures.

The components are now in a high temperature vacuum chamber in a Gra placed phitgefäß, its bottom with molten silicon is covered. The molten silicon rises due to the capillary forces in the blank upwards, whereby the pores are almost completely filled with silicon become. When the temperature increases further to about 1750 ° C to 1800 ° C part of the silicon with pyrolytic carbon to sili cium carbide implemented.

After cooling to room temperature, the component has a density of 1.75 g / cm ³ , 20% of unreacted, free metallic silicon being present in the matrix.

The resulting Ceramic Matrix Composite (CMC) component ( 10 ) with recesses ( 16 ) and bores ( 17 ) is now ground on the end face provided for the mirror surface using a grinding machine. A silicon molded part ( 11 ) with a diameter of 123 mm and a wall thickness of 1.0 mm is placed on the ground surface without the use of adhesives or resin binders and installed in a protective gas oven. In an argon atmosphere, the component is heated to a temperature of 1405 ° C at a heating rate of 30 K / min. After a holding time of 20 minutes, the structures are cooled to room temperature.

In order to check the application or joining of the silicon parts, a component was sawn and a bevel made. The crack, pore and joint-free sintering of the silicon is shown in the microscope images shown in FIGS . 1 to 4. The toothing is obviously based on diffusion and sintering processes between the silicon from the CMC carrier component and the silicon molding.

After processing, the mirror structures were placed on their thermal shock constantly examined. In a hundred attempts, the structures are zy cical temperature changes in the range of 0 to 700 K have been exposed. Micrographs showed nothing after the temperature change examination Cracks in the structure and at the interface between the support body and the silicon.  

A rod measuring 50 × 4 × 4 mm was sawn out of the surface area of a mirror structure and a dilatometer measurement was carried out. In the temperature range from 0 to 700 K, the mirror material shows a coefficient of thermal expansion of only 2.0. 10 ^-6 K ^-1 .

Example 3:

A CFC solid body is produced according to example 1. After carbonization, graphitization and infiltration via the chemical gas phase separation, the CFC cylinder, as indicated in FIG. 6, is mechanically processed. A silicon wafer with a wall thickness of 0.8 mm is glued to the highly porous CFC component using polysilane precursors from Wacker. After the resin binder has dried and hardened in an argon atmosphere at 180 ° C., the component is heated further to 1200 ° C. at a rate of 3 Kelvin per minute, thus pyrolyzing the polysilane precursors.

The CFC structure is then placed in a vacuum furnace in a graphite crucible filled with powdered silicon. The system is heated to 1400 ° C at a heating rate of 20 K per minute and then cooled to room temperature after a 30-minute hold time. The silicon, which melts at around 1350 ° C due to a doping, diffuses into the porous CFC matrix as far as the carrier substrate-wafer interface and causes a so-called reaction application of the silicon wafers to the composite. A micrograph shows that the infiltrated silicon has partially reacted with pyrolytic carbon to form silicon carbide and the content of unbound silicon in the component is 21%. The silicon wafer has no pores, cracks or joints and is firmly toothed on the CMC composite. This optically reflective structure with a density of 1.7 g / cm ³ is briefly polished to the required roughness depth using a lapping machine.

This method is particularly advantageous in that situ both the CMC production as well as the reaction application or Fü silicon wafers on the CFC blanks and thus high-gloss Mirror structures are formed.  

Example 4:

A commercially available honeycomb material based on hard paper with a density of 0.2 g / cm ³ and a wrench size of 6 mm is impregnated with a phenolic resin binder and dried at 70 ° C.

On the honeycomb body with a diameter of 400 mm and a wall thickness of 10 mm are now in each case in a heatable press at 200 ° C three layers of carbon fiber prepregs pressed or laminated.

The resulting CFRP structure is heated to 1000 ° C. with the exclusion of oxygen at a heating rate of 2 Kelvin per minute and carbonized. After a holding time of 6 hours, the mixture is cooled to room temperature and a carbon-based honeycomb structure is obtained which, apart from a linear shrinkage of about 17%, corresponds to the original CFRP component. For further solidification, the still porous structure is infiltrated with pyrolytic carbon via chemical vapor deposition, as described in Examples 2 and 3. The component thus obtained with a density of 0.22 g / cm ³ shows 4-point flexural strength of greater than 150 N / mm ² .

One of the end faces laminated with carbon fiber fabric becomes superficial ground and covered with two silicon wafers. As a resin binder a phenolic resin-silicon powder mixture with a weight ratio of 2: 1 used.

This is followed by silicon infiltration according to Example 3, the wafers being applied to the honeycomb structure. The honeycomb-based mirror structure obtained after cooling with a real density of 0.42 g / cm ³ is shown in FIG. 7.

The resulting extremely light construction material of the carrier body also stands out its high rigidity and pressure resistance due to low thermal conductivity ability from. It also proves to be particularly advantageous that the inner flexible honeycomb structures due to their thin-walled thermal shock bean Compensate for the corresponding thermal expansion and thus expected thermally induced cracks can be reduced to a minimum.  

In Fig. 5 it is shown how one can assemble a large area mirror of the portions. Particularly advantageous here is the fact that with basic modules consisting of silicon wafers ( 11 ) and size-matched support structures ( 10 ) any mirror size can be put together. The support structures ( 10 ) are held together by webs ( 14 ) (preferably made of CFC material). If the gaps between the individual components do not interfere, the construction can be made from finished individual reflectors. Of course, it is also possible to connect the "raw parts" with one another and to jointly subject them to the above process, so that all the parts are firmly connected to one another.

In the embodiment shown in Fig. 5C, webs ( 14 ) are no longer necessary, since the support structures ( 10 ) each have tongue and groove connections ( 15 ).

Claims (25)

1. A method for producing a reflector for reflecting electromagnetic waves with the following method steps:

• a) producing a porous support body made of carbon or of carbon-containing material which is carbonized or graphitized,
• b) covering the carrier body with silicon in the form of shaped silicon bodies, in particular in the form of silicon wafers,
• c) Introducing the porous support body with the silicon on it into a vessel with silicon and heating the arrangement to a temperature between 1300 ° C. and 1600 ° C. in an atmosphere preventing an oxidation or in a vacuum, so that the silicon melt in the vessel rises in the porous material and thereby creates a silicon carbide layer at the interface between the porous material and the melt and the coating of silicon combines by melting or sintering with a high-temperature treatment with the carrier body, and
• d) cooling of the carrier body with a silicon coating thereon and, if desired, reworking of the coating to form a reflecting surface.

2. The method according to claim 1, characterized, that the molded body of highly reflective silicon slices is formed, the highly reflective Areas are facing away from the support body. 3. The method according to any one of the preceding claims, characterized, that the carrier body made of CFC composite or CMC material is made. 4. The method according to any one of the preceding claims, characterized, that at a temperature between 1300 ° C to 1450 ° C in Step c is heated. 5. The method according to any one of the preceding claims, characterized, that the silicon moldings before connecting with sticks the carrier body onto the carrier surface. 6. The method according to claim 5, characterized, that the gluing takes place by means of an adhesive which Diffusion, sintering or melting processes promotes. 7. The method according to any one of claims 5 or 6, characterized, that the glue using a resin binder or an adhesive on carbon, silicon, or sili cium carbide-based or precursors based on polysilane or Silicon carbonitrite base or silicones.   8. The method according to claim 1, characterized, that for the production of larger reflector surfaces with Si moldings are such that the Carrier surface covered essentially without gaps is. 9. The method according to any one of the preceding claims, characterized, that the silicon moldings in several layers applies, the layers of silicon molding pern preferably be of different levels of doping can stand. 10. The method according to claim 9, characterized, that when building up the layers of silicon moldings different doping levels of silicon Moldings with the highest doping next Apply layer to the carrier surface. 11. The method according to any one of claims 9 or 10, characterized, that as the top layer of a layer structure with Silicon moldings of different levels of doping a layer of silicon moldings particularly high Brings purity. 12. The method according to any one of the preceding claims, characterized, that the silicon in the vessel is doped and therefore low is melting silicon. 13. The method according to claim 12, characterized,  that a support structure is made of meh reren carrier bodies is inserted or joined and by the rising silicon ver to an overall structure is consolidated. 14. The method according to any one of the preceding claims, characterized, that the support surface before applying the sili cium molded body smoothes, especially grinds. 15. The method according to any one of the preceding claims, characterized, that recesses are provided in the carrier body. 16. The method according to any one of the preceding claims, characterized, that one in the carrier pyrolytic carbon, before preferably by chemical vapor deposition brings. 17. The method according to any one of the preceding claims, characterized, that the reflective surface of the silicon Be carbides. 18. The method according to claim 17, characterized, that the carbidization in a kohlstoffhalti atmosphere, especially in a hydrocarbon substances such as an atmosphere containing propane or methane at elevated temperature, preferably above 700 ° C executes. 19. The method according to any one of the preceding claims, characterized in that the surface of the coating is oxidized to silicon dioxide (SiO ₂ ) by being exposed to temperatures above 500 ° C in an oxygen-containing atmosphere, preferably in air. 20. A reflector for reflecting electromagnetic waves, comprising a support body and a coating of silicon firmly connected thereto, characterized in that the support body ( 10 ) is porous and consists of carbon, wherein the carbon is at least superficially converted to silicon carbide and in the porous structure of silicon is embedded. 21. A reflector according to claim 20, characterized in that the carrier body ( 10 ) and / or the covering ( 11 ) is composed of several pieces and the pieces are preferably firmly connected to one another to form one-piece bodies. 22. Reflector according to one of claims 20 or 21, characterized in that the carrier body ( 10 ) has recesses ( 13 ). 23. Use of a reflector according to one of the claims 20 to 22 in space. 24. Use of a reflector according to one of the claims 20 to 22 in moving reflection or mirror systems men. 25. Use of a reflector according to one of the claims 20 to 22 as a reflector for solar energy generation.