Method for the Production of a Reflector

The present invention relates to a reflector and to a method of producing a reflector or mirror.

Many optical materials are used for the orderly transmission of rays by refraction or reflection, and both properties can be employed simultaneously. Materials that function by refraction are mainly characterized by the wavelength-dependence of their refractive index and of their transmittance.

In the case of mirrors and reflectors, the feature of interest is the reflectance as a function of wavelength. A metal layer of adequate thickness will absorb the rays that are not reflected. With dielectric mirrors, which can be made nearly absorption-free, a crucial factor is their ability to resist atmospheric influences and the degree to which it can be increased by protective coatings.

Whereas a large number of highly transparent glasses are available for the visible region of the spectrum, this is not the case in the UV and IR regions. Here the glasses are supplemented by a small number of crystals and materials derived therefrom, the most important property of which is high transmittance. Some of these materials (e.g. BaF.sub.2, CaF.sub.2, LiF, Al.sub.2 O.sub.3, SiO.sub.2) are suitable for broad-band and multispectral systems, because they are transparent from the UV to the IR. A disadvantage of some of these materials is their high solubility in water, so that rotective coatings must be applied and/or the mirrors can be used only in completely dried air.

Monocrystals (isotropic or anisotropic, depending on the kind of crystal) are obtained from natural deposits or cultured artificially (pulled from a molten mass). Polycrystalline material (isotropic) is produced by pressure sintering.

For most applications only isotropic materials are appropriate. Anisotropic crystals are used in polarization optics.

The semiconductor silicon, which is isotropic, acts as a long-pass filter with sharp cutoff, thus separating the visible from the near IR range. Polycrystalline silicon as normally employed, with a thickness of 2 mm (not treated to reduce reflection), has a transmittance .tau. of ca. 0.53. As wavelength increases, transmittance goes through a minimum at about 16 .mu.m and then increases again, until it reaches 0.4-0.5 in the range above 300 .mu.m. It is this property, in combination with its favorable thermal characteristics, that enables silicon to be used primarily in infrared optics and as a mirror substrate despite its brittleness. (Source 1: Naumann/Schroder: Bauelemente der Optik, 5th edition, p. 64, Hanser Verlag)

Surface mirrors offer many advantages over refractive systems, due to their achromatic imaging and the avoidance of other errors associated with passage through refractive materials. However, because the change in the deflection angle of a ray is twice as large as the change in angle of incidence, for given optical requirements mirror surfaces must meet stricter criteria than refractive interfaces with respect to accuracy of shape and microstructure. Another characteristic of a mirror is the shape of its spectral reflectivity curve. Although it is possible to polish the surface of a massive metal body so well that it can be used as a mirror directly, this method is used today only in exceptional cases. In general a mirror layer is applied to a mirror substrate. The metallic mirror substrate is previously polished and is responsible for the accuracy of the mirror's surface configuration. The mirror layer is applied to the substrate usually by sputtering in high vacuum or by chemical methods. The mirror layer then matches exactly the shape of the substrate surface and determines the spectral reflectance function and (in some cases together with protective layers) the temporal stability of the reflectance function. The preferred materials for mirror layers are metals such as aluminum, chromium, nickel, mercury, silver, gold, platinum and rhodium, but silicon monoxide (SiO) and silicon dioxide (SiO.sub.2) are also used (Source 1).

The materials used for mirror substrates and reflectors must have a high degree of mechanical and thermal stability. Large mirrors become deformed due to their own weight when their position changes; displacements by fractions of .lambda. must be prevented or compensated by opposing forces. Temperature changes and nonuniform temperature distributions induce internal tensions and deformations. Hence crucial requirements for a substrate material for precision mirrors are a high modulus of elasticity E and a very low coefficient of thermal expansion .alpha.. Furthermore, the material must be polishable to an optimally smooth surface with a very low proportion of scattered light.

In this respect most metal surfaces are unfavorable, because their internal texture is such that inhomogeneous properties at the grain boundaries can produce surface inhomogeneities after polishing. Nevertheless several metals are used as mirrors, including pure copper, aluminum and molybdenum alloys and pressure-sintered beryllium, though it is necessary to improve their polishability with a layer of chemically deposited nickel phosphide. Metallic mirrors have high thermal expansion, but because of their favorable thermal conductivity they do find limited application, e.g. for high-performance lasers. At present glass and vitreous ceramics are of greater significance.

Certain structural components, especially for aerospace applications, are also required to have high mechanical and thermal stability combined with a low relative weight. Furthermore, good resistance to thermal shock (RTS) must be accompanied by a low thermal expansion coefficient (TEC).

For example, future satellites are to be equipped with a mirror structure that rotates when in use. These mirrors will be large, for instance measuring 800.times.600 mm, and on the front side must have an optically reflecting surface.

Apparatus used in outer space can be expected to encounter cyclic temperature changes from 0 to 700 K., so that in addition to rigidity appropriate to their size they must be guaranteed to have thermal and thermal-shock resistance, a low weight per unit volume and, not least, low thermal expansion. The groups of materials that meet these criteria must also have surface properties of the high quality required for reflecting optics.

Conventional mirror components at present are made of vitreous ceramic. The manufacturing process involves the melting of various oxide powders, such as LiO.sub.2, Al.sub.2 O.sub.3, MgO, ZnO and P.sub.2 O.sub.5, in platinum furnaces. After the melt has been homogenized, objects of the desired shapes are produced by pressing, casting and other glass-forming processes. The glass components are then suddenly cooled and removed from the molds, and subsequently tempered in a controlled manner to temperatures of ca. 700.degree. C.; in this process crystal "seeds" form in noncrystalline (amorphous) glass. If the specified temperature is maintained for a suitable time, this seed formation leads to crystal growth and completes the "ceramization" of the glass, producing a vitreous ceramic.

This crystalline vitreous ceramic possesses the advantage of low thermal expansion, only 0.+-.0.15.times.10.sup.-6 K.sup.-1 over the temperature range from 273 K. to 323 K.

As a material for mirrors, however, this vitreous ceramic is of limited use, because it can be produced only by elaborate shaping procedures and furthermore has a relatively high weight per unit volume, 2.53 g/cm.sup.3, as well as low tensile strength and not least a brittle breaking characteristic. In addition, it can be employed for optical components only at a constant or maximal temperature of 423 K, because the crystalline structure of such vitreous ceramics is subject to tension hysteresis in the temperature ranges 200-300 K and 360-480 K. At temperatures above 700 K the internal structure of the material is irreversibly damaged (Source 2: SiRA; ESTEC-Contract No. 5976/84/NL/PR; October 1985).

Attempts have also been made to manufacture lightweight mirror components from economically favorable aluminum (relative weight 2.71 g/cm.sup.3). However, because of the low stiffness of aluminum it has been impossible so far to construct precision optics of this material. For use in corrosive surroundings, aluminum mirrors must be provided with a thick (0.2-0.5 mm) coating of nickel. Because of the massive differences in thermal expansion between aluminum (23.times.10.sup.-6 K.sup.-1) and nickel (13.times.10.sup.-6 K.sup.-1), these mirrors must not be exposed to any temperature fluctuations, which would cause thermally induced fracture.

Mirrors of pure aluminum, such as can be employed in a vacuum, exhibit local deformation under even slight thermal stress, due to the extremely high thermal expansion coefficients on the optical mirror surfaces. When they are used, e.g., as laser mirrors for distance measurement, such deformation can led to undefined results (Source 2).

Reflecting optics based on quartz glass are also state of the art. Because of their extremely low thermal expansion coefficients, nearly zero in the temperature range from 0 to 273 K., quartz-glass systems are eminently suitable for so-called cryogenic applications. In the range between 273 K. and 373 K. their thermal expansion coefficient rises to 5.1.times.10.sup.-7 K.sup.-1. Other disadvantages are the relatively high weight per unit volume, 2.2 g/cm.sup.3, the low rigidity, the low tensile strength of <50 MPa, high production costs and the restriction of the diameter to ca. 500 mm because of the complicated manufacturing process (Source: W. Englisch, R. Takke, SPIE, Vol. 1113, Reflective Optics II, 1989, pages 190-194).

Its mechanical and thermal characteristics and its low relative volume, only 1.85 g/cm.sup.3, make beryllium especially suitable for the manufacture 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 surface roughness (R.sub.a) of less than 15 Angstrom, so that they are very well suited for optically reflecting surfaces.

A particular disadvantage of beryllium structures, in addition to the high cost of the raw material and of the manufacturing process, is their general toxicity. To use them as optical components under atmospheric conditions, they must first be coated with nickel. Because of the different thermal expansion coefficients of beryllium (11.2.times.10.sup.-6 K.sup.-1) and nickel (15.times.10.sup.-6 K.sup.-1), it is essential for these components to avoid thermal shock. Therefore they can be employed only at constant temperatures or in very narrow temperature ranges.

It has also been discovered that beryllium components manufactured by the vacuum hot-pressing technique or by high-temperature isostatic pressing have an anisotropic character, such that they have different properties in different crystal directions.

Under outer-space conditions, uncoated mirrors can be used. However, because of the high thermal expansion coefficient the temperature fluctuations typically encountered there, between 0 and 700 K., produce local deformations of the optical surface that make beryllium unusable for precision optics (Source 1) and can also introduce serious transmission problems in the case of satellite mirrors.

Mirror structures of this kind are currently also constructed of monolithic ceramic based on silicon carbide, by the so-called slip casting technique. In this casting process, a suspension of silicon-carbide powder is placed in a plaster mold shaped as a negative. Depending on the time the suspension spends in the plaster mold, a ceramic body with variable wall thickness forms; this is the positive component in the green state. After the blanks have dried, a sintering process is carried out in vacuum or protective-atmosphere furnaces at temperatures as high as 2200.degree. C. This manufacturing technology not only requires the construction of elaborate molds to produce the green compacts, it also has the disadvantage that only certain geometries and small sizes are achievable, and the entire process suffers from a high percentage of rejects. Because these formed silicon carbide bodies shrink during drying and sintering, the required accuracy of their dimensions can be ensured only by expensive machining with diamond tools. Furthermore, the heterogeneous texture of the sintered compact makes it necessary for the compact to be coated subsequently with silicon carbide by chemical vapor deposition, to reduce the surface roughness to less than 40 .ANG.ngstrom. Not only does silicon carbide require these elaborate manufacturing and machining processes, it also has a relatively high weight of 3.2 g/cm.sup.3 and is extremely brittle.

From DE 32 46 755 A1 it is known that highly stable, lightweight composite materials can be constructed of various laminate layers combined with cellular or honeycomb layers. Among the raw materials used are fibrous mats soaked with artificial resin, webs of paper, plastic, foil or glass, carbon-fiber mats or polyimide. The cellular or honeycomb layer is intended to endow the formed object with better stability and increased resistance to bending.

Such bonded materials based on carbon- or glass-fiber reinforced plastics are restricted to room-temperature applications. The inhomogeneous structure of the fibrous or laminated components makes it impossible for an optical mirror surface to be created by superficial processing.

From DE 38 09 921 A1 it is known that a mirror can be produced by fastening a commercially available silicon wafer to a plate substrate. To manufacture larger mirrors, several such wafers are arranged next to one another. At the joints between them, the wafers are connected to one another by laser or electron-beam welding. The mirror surface is subsequently ground and polished. A body of lightweight metal or fiber-reinforced composite plastic is used as the substrate. Here, again, the above-mentioned disadvantages appear, caused especially by the use of different materials for substrate and reflector layer.

From DE 36 26 780 A1 it is known that mirrors can be used not only to reflect light but also as antennae. The mirrors disclosed there comprise a carrier made of carbon-fiber reinforced material. The reflector layer consists of glass or vitreous ceramic and is melted directly onto the substrate. The disadvantages of such mirrors have been described above.

From DE 30 18 785 A1 a method of producing mirrors is known in which the first step is to produce a preform made of a highly thermally stable material, namely carbon, which serves as substrate structure for the mirror. To form the mirror, a front plate of glass is sintered or melted onto this substrate structure. The result, again, is a structure in which the substrate plate and the reflector layer have very different thermal properties, so that such mirrors are only very slightly resistant to thermal shock. The weight per unit volume of such mirrors is relatively high.

The object of the present invention is to provide a reflector and a method of producing same whereby the reflector has improved mechanical/thermal properties with simultaneously improved optical properties with respect to the prior art and is also simple to manufacture.

According to a first aspect of the present invention there is provided a reflector for the reflection of electromagnetic radiation comprising a substrate element of porous and at least partly carbidized carbon, a first coating of at least one of silicon and silicon carbide attached to the substrate element with its surface facing away from the substrate element having been smoothed and subsequently provided with a reflecting layer of at least one of silicon, silicon carbide, silicon oxide, silicon nitride, gold, silver, nickel, copper, and alloys of the afore-mentioned metals.

According to a second aspect of the present invention there is provided a method of producing a reflector for the reflection of electromagnetic radiation, comprising the steps of:

a. producing a porous substrate element from one of carbon and a carbon-containing material that has been carbonized or graphitized;

b. coating the substrate element with a first silicon coating in the form of one of powder, silicon preforms, and silicon disks;

c. infiltrating the substrate element with silicon at a temperature in the range 1300.degree. C. to 1600.degree. C. in an oxidation-preventing atmosphere or in vacuum, so that the porous substrate element is impregnated with liquid silicon and, at the interfaces between the porous material of the substrate and the liquid silicon, a silicon carbide layer is formed, and so that the first silicon coating is joined to the infiltrated substrate element by melting-on or sintering in a high-temperature treatment;

d. cooling the substrate element with its first silicon coating thereon; and

e. applying a reflecting layer of at least one of silicon, silicon carbide, silicon oxide, silicon nitride, gold, silver, nickel, copper, and alloys of the aforementioned metals.

It is an important feature of the invention that the substrate element is initially made of an easily processed material, namely substantially carbon, which is not endowed with its special characteristics until it is impregnated with silicon, after which it can be processed only with greater effort. An additional feature is the fact that the actual reflecting layer is applied to a silicon layer that is deposited on the (silicon-impregnated) substrate element and firmly joined to the latter. It is also possible, instead of depositing the silicon layer separately, to impregnate the substrate element with molten silicon in such a way that its outer surfaces substantially comprise a silicon layer free of openings or pores. This can then be smoothed and coated with the actual reflecting layer.

In the following description, the use of carbon or carbon fibers is described as the basic material for construction of the preforms. At this juncture, however, it should be stated expressly that the development of materials with similar fine-structural characteristics or the use of such substances is included within the ambit of this invention. Any suitable material that can be "impregnated" in order to permit firm fixation of the silicon reflector layer onto the core structure can be used.

Furthermore, it should be pointed out that the following description refers in general to the use of preforms or wafers made of silicon for construction of the outer layer. It is also possible to use metallic silicon in granular or powder form, in which case subsequent mechanical processing such as grinding and/or polishing will be required.

CFC composites comprising a carbon matrix with carbon-fiber reinforcement are produced industrially by the resin-impregnation and carbonization method. The resulting materials are distinguished by an extraordinarily favorable combination of characteristics, such as high mechanical stability in space and at high temperatures, combined with low weight per unit volume (1.0-1.7 g/cm.sup.3) and low brittleness.

Set against the excellent material characteristics of CFC is its low resistance to oxidation, which greatly limits the possibilities for employing the material in an oxygen-containing atmosphere. At present its low resistance to oxidation restricts CFC to use in vacuum and protective atmospheres, because otherwise at temperatures above 400.degree. C. it begins to burn away.

To increase the oxidation resistance of this material, the so-called ceramic matrix composites (CMC) were developed. Here refractory and ceramic components are infiltrated into the porous CFC matrix. It is also possible to manufacture short-fiber preforms, in which short carbon-based fibers are dissolved in a phenolic-resin suspension and harden when the temperature is increased. When the temperature is further increased in the absence of oxygen, the resin binders of the two composite qualities are carbonized.

The reflector in accordance with the present invention has as its basic structural element fiber-reinforced CFC or CMC or carbon-honeycomb composites plus superficial metallic silicon. Here metallic silicon is understood to mean elementary silicon that as been applied to CFC carrier substrates as silicon preforms or wafers or silicon powder, by processes of diffusion, sintering or melting-on. Silicon wafers are thin metallic pieces comprising highest-purity silicon. The silicon can have an isotropic or polycrystalline structure.

On the whole, carbon-fiber reinforced carbon bodies differ from other known carbon materials in that they have a number of improved mechanical and physical properties and can be employed at temperatures up to 2200.degree. C. in an inert atmosphere. These characteristics can be varied over wide ranges because of the many ways to modify the manufacturing process, i.e. by altering the type of carbon fiber and fiber pretreatment, the fiber orientation or arrangement, the fiber content, the forming method (pressing, lamination or rolling), the number of subsequent densification cycles and in some cases the annealing and graphitization temperatures. By these means the constitution of the material, in particular the structure, the matrix and the porosity, can be influenced in specific ways and matched to the requirements of the intended application.

In principle it is likewise possible to use silicon carbide reinforced with silicon carbide fibers rather than carbon fibers in the present invention.

It is also in accordance with the invention to use a vapor-silication process. Here the porous substrate elements are infiltrated by exposing them to a silicon-containing atmosphere (silicon vapor) at temperatures between 1600.degree. C. and 2300.degree. C.

The invention enables the manufacture of components with complex geometry, high temperature-shock resistance, low relative weight (0.5-2 g/cm.sup.3) and simultaneously high tensile strength (>150 MPa), low thermal expansion coefficient (TEC) and surfaces suitable for reflecting optics.

Another advantage of the method in accordance with the invention is that economically priced, commercially available materials can be used, which can be processed with machine tools before they are "treated" with silicon.

Furthermore, the density and strength of the component can be adjusted as desired, by suitable choice of substrate structures and of the quantity or quality of the infiltration processes. The thermal expansion coefficients of the groups of materials used in accordance with the invention closely resemble one another, resulting in very precise elements of stable form even when the dimensions are large.

Any CFC raw materials can be used advantageously, in particular those based on long or short fibers and with oriented or unoriented fiber structure. Moreover, the method in accordance with the invention can also be applied to known honeycomb structures based on paper, cellulose or carbon fiber.

The density of a CFC block is at most 1.4 g/cm.sup.3 ; that is, it has high porosity. Apart from the pores, the block contains no cavities, so that with respect to its shape it is a massive body such as a plate, a brick or a solid cylinder. For multidimensionally oriented CFC qualities with long-fiber construction, as a rule, one begins with resin-impregnated carbon-fiber webs, so-called prepregs, which are compressed into CFC plates in heatable axial presses

To produce CFC blocks on a short-fiber basis, carbon or graphite fibers are suspended in a thermosetting resin binder, as known in the art. The suspension is poured into a mold and then the solvent is removed, e.g. by heating, and the resin binder and hence the CFC block is hardened. In all CFC qualities the fiber reinforcement is intended to counteract embrittlement of the ceramized CFC material and maintain quasiductile fracture behavior. The honeycomb structures manufactured by the known molding methods, from hard paper or carbon fibers, are treated to increase the carbon yield by impregnation with a resin binder, preferably phenolic resin, and hardened by subsequent heat treatment.

The next step is common to all CFC or honeycomb structures: carbonization of the binder resin in vacuum or protective atmosphere at temperatures of, e.g., 900.degree.-1300.degree. C. The CFC blocks or honeycomb structures so obtained are then preferably heated in vacuum or protective atmosphere to temperatures of more than 2000.degree. C., to achieve at least a partial graphitizing of the carbon matrix and fibers.

The block is then machined down to produce the CFC blank, which has the dimensions of the component to be manufactured, such as the basic mirror structure of a satellite or other optically reflecting system. The removal of material in this process can be done, for example, by turning, milling or grinding, with the same machines as are ordinarily used for the machining of metallic materials. To achieve further weight savings, during the machining of the CFC blanks pockets of any desired geometry can advantageously be milled, eroded or drilled into the back surface of the mirror structures. After carbonization the carbon honeycomb structures can be laminated to form a composite by attaching to the front surface carbon-fiber webs of any kind or web prepregs, by means of a resin binder. Furthermore, it is possible to press the honeycomb structures into the highly porous short-fiber CFC or to construct a sandwich system by pressing together several honeycomb structures, each encased in a CFC web. In this way, after carbonization a highly thermostable, lightweight construction material is obtained, with great rigidity and quasiductile fracture behavior.

The CFC or honeycomb blank obtained after machining, which like the block has a low density of 0.1-1.3 g/cm.sup.3 and hence a high porosity, up to 90 vol. %, can subsequently be further infiltrated and stabilized by impregnating it with resin binders and carbonizing them.

Another way to achieve the necessary strengthening of the CFC blanks and thereby increase the rigidity of the basic mirror structure is by infiltration with pyrolyric carbon by chemical vapor deposition (CVD), until the density reaches a maximum of 1.4 g/cm.sup.3 but preferably 0.3-1.0 g/cm.sup.3. Whereas in resin impregnation phenolic resins are preferred, in the chemical vapor deposition of carbon it is preferable to use a mixture of hydrocarbons, such as methane or propane, and an inert gas, such as argon or nitrogen, at a temperature between 700.degree. and 1100.degree. C. and a pressure of 1-100 millibar. The gas mixture diffuses into the porous structure and breaks down to form carbon and hydrogen, the carbon preferably being deposited as pyrolyric carbon on the surfaces or at the intersection points of the fibers and thereby strengthening the structure.

The CFC blanks obtained from either infiltration process are polished on the surfaces intended for reflection and fitted into a vacuum or protective-atmosphere furnace. One or several metallic silicon preforms are laid onto the polished side and the sample is heated to temperatures of 1300.degree.-1600.degree. C., preferably 1350.degree.-1450.degree. C. As the result of a chemical reaction between the carbon and the silicon, silicon carbide forms at the interfaces, creating a consolidation or joint and thereby attaching the silicon wafer to the the CFC substrate. In addition to the chemical reaction, melting-on of the metallic silicon and diffusion can also cause the wafer to become attached to the CFC blank, forming optically reflecting structures.

Metallic silicon wafers can be fixed to so-called ceramic-matrix composites (CMC), which for example contain silicon carbide and silicon in the matrix, at temperatures of 1300.degree.-1600.degree. C.

An especially advantageous version of the method in accordance with the invention also provides for the surfaces that are to be made reflective to be covered with one or more silicon preforms or silicon wafers and for the substrate thus prepared to be positioned with its lower end in a pool of molten doped silicon. Due to the capillary forces in the substrate structure, the molten silicon ascends in the blank until it reaches the high-purity silicon preforms or powder resting on its upper surface. Firstly, the blank is thereby upgraded to a CMC and the silicon preforms or wafers or the powder mass resting on it are joined to one another and fixed to the substrate. In addition, substrate structures comprising several discrete elements assembled by mechanical means can be consolidated into a single unit by the ascending silicon.

The amount of molten metallic silicon used to infiltrate the CFC blank should be such that the density of the latter is less than 2.0 g/cm.sup.3, preferably 1.5-1.8 g/cm.sup.3. It is advantageous for the commercially available metallic silicon-monocrystal wafers to have been pre-ground and polished in such a way that after application to the substrate they immediately form optically reflecting surfaces, so that machining cycles can be reduced to a minimum or eliminated altogether. To prevent massive melting-on, deformation and volatilization of the applied silicon elements, a maximal process temperature of 1550.degree. C. must not be exceeded; the process temperature is preferably between 1350.degree. and 1500.degree. C. In a particularly advantageous version of the method in accordance with the invention, the silicon preforms are attached to the CFC or CMC or honeycomb substrate prior to fixation by means of an adhesive or resin binder, which during the subsequent heat treatment facilitates the processes of diffusion, sintering or melting-on at the interface between the substrate and the silicon preforms.

As adhesives or resin binders it is advantageous to use precursors based on polysilane or silicon carbonitride and/or adhesives based on silicon, silicon carbide or carbon or silicones. Before the reactive fixation process the adhesives must be dried or hardened at temperatures between 100.degree. and 200.degree. C. Pyrolysis of the resin binders is carried out at 1000.degree. C. in vacuum or in a protective atmosphere.

Polysilanes (polymethylphenylsilanes), for example, after pyrolysis under inert gas at 1200.degree. C. give a ceramic solid yield of 30% to maximally 70% by weight, depending on the solvent.

For mirror and reflector applications in the temperature range of, for example, 250 and 330 K. with no major temperature fluctuations, a sufficiently firm attachment between the substrate and the mirror layer can be achieved by attachment of the silicon parts to the substrate structures with silicones.

Glass frits can also be used, with suitable tempering above 600.degree. C., to fix the silicon preforms to the CFC or CMC substrates by melting-on processes.

The method in accordance with the invention further provides for several silicon preforms or Si wafers or also powder differing in doping or melting points to be applied as so-called multilayers to the CFC or CMC or honeycomb substrate structures, the silicon preforms preferably being arranged in such a way that the one with the lowest melting point is placed directly on the substrate and all the silicon preforms arranged above it have a higher melting point. Undoped highest-purity silicon, for example, has a melting point at 1412.degree. C. Depending on the amount of impurities in silicon (doping), the melting point can be lowered by various amounts due to the formation of eutectics.

It is in accordance with the invention for the reflecting layers, comprising carbon or silicon carbide or silicon or silicon dioxide or silicon nitride or gold or silver or nickel or copper or alloys thereof, to be applied to the unprocessed or ground reflector surface of the substrate made of ceramic matrix composites (CMC) by physical vapor deposition (PVD), preferably in the temperature range from 20.degree. C. to 900.degree. C., in vacuum or under a protective atmosphere.

In contrast to layers obtained by thermal spraying, which are applied by transferring droplets or particles to the substrate surface, these PVD coatings are produced by the deposition of atoms or molecules from the gas phase to form a layer the thickness of which can range from a few .ANG.ngstroms to several millimeters. The most widely used forms of PVD layers include vacuum metallized layers, sputter layers and ion-plating layers. By this means various materials, for example, metals, metal alloys, glasses, ceramics and non-metals as well as plastics, can be deposited on surfaces.

In vacuum metallization methods, metals or compounds are vaporized in a vacuum and deposited on mirror-substrate surfaces that are at a considerably lower temperature than the vapor. The energy with which the atoms or molecules strike the substrate surface is low and insufficient to produce good adhesion of the layer. Therefore the surface of the side of the mirror to be coated is specially prepared. This can be done, for example in the vaporization apparatus, by bombarding the surface with ions of inert gases in a glow-discharge plasma and/or by heating the parts to several 100.degree. C. in order to volatilize impurities and adsorbed films (moisture). The coating material is vaporized thermally by resistance heating or high-frequency heating or by means of an electron or laser beam. In the vacuum method the free path length of the atoms/molecules is some meters, so that the particles can pass directly, i.e., without colliding with the molecules of the residual-gas atmosphere, to the substrate surface and condense there to form the deposited film.

Reflecting layers of the reflectors in accordance with the invention can also be produced by so-called sputter methods. Here a cloud of particles is produced in a glow-discharge plasma. By applying a voltage of ca. 1-10 kV across a noble gas (e.g. Ar) between the cathode, which consists of the coating material, and the anode, noble-gas ions (Ar.sup.+) are produced, with which the cathode is bombarded. Atoms of cathode material thereby removed are deposited on the mirror substrate, which is positioned near the cathode. Component (sample holder) and recipient are grounded. In a modification of the method, the sputtering can also be done in a reactive-gas atmosphere, with which the metal atoms leaving the cathode react to form compounds such as carbides, nitrides or oxides, which then deposit to form a coating.

In ion plating the coating material is vaporized into a plasma, which is maintained between the vaporization source and the mirror component to be coated, and deposited from the plasma onto the substrate. In an evacuatable apparatus the coating material is vaporized by means of thermal vaporizers, induction heating or electron gun. Between the substrate, which serves as the cathode, and the anode (vaporization source) a plasma is produced. The mirror substrate is bombarded with anions produced in the plasma by impact ionization of the noble gas. As a result, layers of foreign material are removed from the substrate and its surface is cleaned, slightly etched and activated. About 1% of the particles of coating material that enter the plasma from the vaporization source are ionized by impact ionization and accelerated in the electrical field toward the mirror substrate, which if it consists of nonconducting material is covered with electrically conducting wire mesh to serve as the cathode. The accelerated ionized particles lose their charge by charge exchange, but even in this state they retain the velocity they had acquired as ions, so that they strike the substrate surface with high energy. Because the sputtering away from the surface and deposition of the coating material occur at the same time, the clean surface required for the optimal build-up of a layer is always available. Penetration of the substrate surface by the accelerated particles produces a kind of diffusion layer, which is responsible for the outstandingly firm adhesion of the ion-plated coating. In ion plating, again, the variant of reactive ion plating exists. Substances introduced into the plasma react either here or on the substrate with the partner, to form new layer-producing substances.

In vacuum metallization and in ion plating, the mirror components are exposed to temperatures up to 800.degree. C. or more, so that there may advantageously be an influence on the substrate material (an interlocking or adhesion-mediating layer). When layers are deposited by sputtering, the mirror substrate is heated only to ca. 300.degree.-500.degree. C. In the case of reflector components with complex shapes, sputtering is the method of choice because of the high degree of scattering achieved. Excellent adhesion of the layer to the mirror substrate is produced by sputtering and ion-plating.

Alternatively, it is in accordance with the invention to apply to the unprocessed substrate structure, or to the ground reflector surface of the substrate structure made of ceramic matrix composites (CMC), surface-refining and/or reflecting layers consisting of carbon or silicon carbide or silicon or silicon dioxide or silicon nitride or nickel or alloys thereof by chemical vapor deposition (CVD), preferably in the temperature range from 600.degree. C. to 2000.degree. C., in vacuum or under a protective atmosphere.

The term chemical vapor deposition basically denotes that various reactive and carrier gases are thermally decomposed in particular stoichiometric proportions and thus react to form the deposited solids plus gaseous side products. As solid products, metals, metallic and non-metallic compounds and organic structures in the gas atmosphere surrounding the object to be coated can be formed on the surface of the object and deposited there. The substrate material can be any metal or non-metal that is not damaged by the high temperatures necessary for the chemical reaction. The CVD method permits, for example, the manufacture of ceramic layers and structures of extremely high purity, with unique mechanical, chemical, thermal, electrical and/or optical properties.

In a CVD process (superficial deposition) the flowing gas or gas mixture is thermally decomposed, in the gas phase and/or at the hot component to be coated. The non-volatile products of this decomposition, such as carbon or silicon or silicon carbide, form nuclei that grow and produce a layer on the mirror component. The most important physical process parameters for deposition in the CVD reactor are the temperature, the temperature distribution, the partial pressure, the amount and composition of the gas and the gas velocity, together with the associated flow conditions at the component and in the reactor. Aspects specific to the apparatus and component, such as the design and dimensions of the furnace, the geometry of the object being treated and its position in the furnace, also play a role, and the material of which the substrate is made is not least of the factors with a decisive influence on the operation of the process.

Another embodiment in accordance with the invention provides that the optically reflecting layer, which comprises nickel or silver or gold or copper or alloys thereof, is applied to the ground reflector surface or to the substrate structure made of CMC by electrolysis, preferably in the temperature range from 20.degree. C. to 200.degree. C.

Thermal spray-coating methods such as flame spraying or plasma spraying can also be employed to apply reflecting layers consisting of silicon or silicon carbide or silicon oxide or silicon nitride or nickel or alloys thereof. The reflector layer is produced by transferring to the mirror-substrate surface droplets or particles of the material to be applied.

Fixation of the silicon preforms to the substrate structures is made possible at considerably lower process temperatures (so that in the case of certain applications, it is possible to work in a normal atmosphere, without protective gas or vacuum) by coating the silicon preforms with gold before they are attached to the substrate structure. In the temperature range 350.degree.-400.degree. C. silicon forms a plastic deformable melt eutectic with gold. As a result, it is possible to attach the silicon preform, e.g. wafer, to the substrate structure by melting-on at temperatures of 300.degree.-600.degree. C.

Gold has the advantage that its thermal expansion coefficient is similar to that of silicon, so that there is no danger of thermal damage as a result of temperature fluctuations. Another advantage of gold is that, because of its chemical inertness, it reacts only with silicon. As a result, in the limiting case gold-coated silicon preforms can be melted onto the substrate structures in a normal atmosphere in the low temperature range of ca. 300.degree.-450.degree. C., which considerably reduces the cost in comparison to melting-on in a protective atmosphere or in vacuum. Where the contact between the silicon preform and the substrate structure must be of high quality, however, sintering in vacuum is preferable.

The silicon preforms can be coated with gold, e.g., by applying gold foil or by physical vapor deposition (sputtering), and the thickness of the coating should be approximately in the range 0.5-50 .mu.m. In principle the gold coating can be applied to the surface of the substrate that is to be made reflective, rather than to the silicon preform.

Instead of gold it is also possible, in principle, to use other nonferrous metals or materials that form a plastic deformable melt eutectic with silicon. An example is aluminum, which forms a melt eutectic with silicon in the temperature range ca. 500.degree.-650.degree. C. However, because aluminum can oxidize to form an Al.sub.2 O.sub.3 layer which would inhibit sintering, it must be applied under vacuum or in a protective atmosphere. Furthermore, aluminum has a higher thermal expansion coefficient than silicon, which limits its use for applications involving marked temperature fluctuations.

The choice of a coating material in any individual case will depend on the conditions of use, quality considerations and the costs of materials and manufacturing processes.

The present invention will now be described by way of four specific examples of preferred embodiments, with reference to the accompanying drawings.

FIG. 1 shows a polished cut section of a silicon wafer fixed to a CMC substrate at 20-fold magnification;

FIG. 2 shows the section of FIG. 1 at 40-fold magnification;

FIG. 3 shows the section of FIG. 1 at 100-fold magnification;

FIG. 4 shows the section of FIG. 1 at 200-fold magnification;

FIG. 5A is a perspective view of a large mirror composed of facets;

FIG. 5B is a perspective view to an enlarged scale of part of the mirror shown in FIG. 5A;

FIG. 5C is a view similar to that of FIG. 5B but of another embodiment of mirror;

FIG. 6A is a longitudinal section through a preferred embodiment of a substrate preform,

FIG. 6B is a section along the line VI--VI in FIG. 6A;

FIG. 7 shows a silicon wafer fixed to a honeycomb structure enclosed in carbon-fiber web;

FIGS. 8A to 8D show different aspects of a preferred embodiment of an infrared telescope mirror;

FIGS. 9A to 9C show steps in the production of another preferred embodiment; and

FIGS. 10A and 10B show steps in the production of a further embodiment similar to that shown in FIGS. 9A to 9C.

40 carbon-fiber web prepregs with sateen weave, a phenolic-resin component amounting to 35% by weight and a diameter of 150 mm are compressed for 8 minutes in a heatable axial press at a temperature of 200.degree. C., producing a CFC preform with a diameter of 150 mm and a wall thickness of 12 mm. The object produced by the forming process, for example this disk, is then carbonized in a reactor with oxygen excluded, i.e. in vacuum or a protective atmosphere, at ca. 1000.degree. C. To minimize the reactivity of the carbon fibers and/or to influence the modulus of elasticity, the block is exposed to a temperature of more than 2000.degree. C. in the absence of oxygen, i.e. in vacuum or a protective atmosphere, as a result of which the matrix carbon formed by carbonization of the phenolic resin is at least partially graphitized. This graphitization is produced, for example, by heating at a rate of 30 K./min and holding for 2 hours at 2100.degree. C. The CFC block thus obtained has a density of 1.0 g/cm.sup.3.

The block is then machined by turning, milling or grinding to produce the blank shown in FIG. 6 which serves as the substrate for optically reflecting mirror structures.

The blank is again impregnated with phenolic resin, in a pressure autoclave at 500 bar. After the pressure impregnation, a preform made of metallic silicon (diameter 123 mm, wall thickness 0.8 mm) is glued to the component. As the glue, for example, a commercially available silicon carbide adhesive of type RTS 7700 made by the firm of Kager is used, which dries at 100.degree. C. in air without shrinking. The impregnated component is carbonized anew in a reactor at 1000.degree. C. and a pressure of 10 mbar. The rate of heating is 2 kelvin per minute, and the holding time is 12 hours.

The blank with attached silicon preform and a density of 1.18 g/cm.sup.3 is now heated in a vacuum furnace at a rate of 20 K. per minute to a temperature of 1390.degree. C. and held there for 30 minutes. After it has cooled to room temperature, the silicon disk is firmly interlocked with the CFC, without deformation.

Microscopic examination of the surface of a section through a reference component confirmed that the silicon preform was joined to the CFC substrate by firm interlocking with no discernible fissures and pores. Polishing trials employing submicron diamond suspensions showed that the silicon surface can be polished with no difficulty so as to reduce the surface roughness R.sub.a to less than 15 .ANG.ngstrom, so that it is eminently suitable for optically reflecting structures. Finally, gold is applied to the mirror surface by PVD.

Short carbon fibers, 10-30 mm in length, are reduced to a slurry in a phenolic-resin suspension. The fiber content of the suspension is 40% by weight. The suspension is poured into a cylindrical mold with a diameter of 150 mm and a height of 100 mm. The solvents contained in the phenolic resin are removed at 60.degree.-70.degree. C. in vacuum. When the temperature is raised to 180.degree. C., the phenolic resin hardens. After removal from the mold, the cylindrical CFC block is carbonized in the absence of oxygen, as in Example 1.

The CFC block so obtained, with a quasi-isotropic internal structure, has a density of 0.55 g/cm.sup.3 and a porosity of about 70 vol. %. To minimize the reactivity of the carbon fibers and/or to convert the matrix carbon formed from the phenolic resin at least partially to graphite, graphitization is carried out at temperatures of more than 2000.degree. C., as described in Example 1.

From the cylindrical block the components illustrated in FIG. 6, which can serve as substrates for the satellite mirror structures, are produced by machining in a lathe, milling machine and/or polisher.

The components are infiltrated with pyrolytic carbon by chemical vapor deposition. This process is carried out in a vacuum furnace for 50 hours at 750.degree. C., with a partial pressure of 2 mbar, the gas phase comprising propane and argon in the ratio 1:5. When it has been completed, the density of the components is 0.90 g/cm.sup.3 and their open porosity has been reduced to ca.

Now the components are placed in a high-temperature vacuum chamber in a graphite vessel, the floor of which is covered with molten metallic silicon. The molten silicon rises within the blank by capillary action, filling its pores almost entirely with silicon. When the temperature is raised further, to about 1750.degree.-1800.degree. C., some of the metallic silicon is converted to silicon carbide by combination with pyrolytic carbon.

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

The resulting ceramic-matrix-composite (CMC) component 10, with recesses 16 and drilled holes 17, is now polished with a polishing machine on its front side, the side intended for the mirror surface. A silicon preform 11 with a diameter of 123 mm and a wall thickness of 1.0 mm is laid onto the polished surface, with no application of adhesives or resin binders, and the structure is fitted into a protective-atmosphere furnace. In an argon atmosphere the component is heated to 1405.degree. C. at a rate of 30 K./min. After a holding time of 20 minutes the structures are cooled to room temperature.

A reflecting layer is applied.

To check the fixation or joining of the silicon parts, a component was sawn apart and the cut surfaces polished. That the silicon had been sinter-fused without fissures, pores or other gaps is illustrated by the photomicrographs in FIGS. 1 to 4. The interlocking has evidently been brought about by processes of diffusion and sintering between the silicon in the CMC substrate and the silicon preform.

After processing, the mirror structures are examined with respect to their resistance to thermal shock. In a hundred trials the structures were exposed to cyclic temperature fluctuations in the range 0-700 K., after which they were polished to reveal the internal structure. No fissures had formed in substrate or silicon, or at the interface between them.

A rod with the dimensions 50.times.4.times.4 mm was sawn out of the superficial part of a mirror structure and subjected to dilatometer measurement. In the temperature range 0-700 K. the mirror material exhibited a thermal expansion coefficient of only 2.0.times.10.sup.-6 K.sup.-1.

A CFC block is produced according to Example 1. After carbonization, graphitization and infiltration by chemical vapor deposition, the CFC cylinder is machined to the configuration shown in FIG. 6. A silicon wafer with a wall thickness of 0.8 mm is glued to the highly porous CFC component by means of polysilane precursors supplied by the firm of Wacker. After the resin has dried and hardened in an argon atmosphere at 180.degree. C., the component is heated further at a rate of 3 kelvin per minute until it reaches 1200.degree. C., so that the polysilane precursors are pyrolyzed.

The CFC structure is then put into a vacuum furnace, within a graphite crucible filled with pulverized metallic silicon. At a heating rate of 20 K. per minute the system is brought to 1400.degree. C., and after 30 minutes at that temperature it is cooled to room temperature. Due to doping, the silicon melts at only about 1350.degree. C. and diffuses into the porous CFC matrix as far as the substrate-wafer interface, bringing about a so-called reactive fixation of the silicon wafer to the composite. Inspection of the structure in a polished section shows that some of the infiltrated silicon has combined with pyrolytic carbon to form silicon carbide and that the content of unbound silicon in the component is 21%. The silicon wafer has no pores, fissures or other gaps and is joined to the CMC composite in a firmly interlocking manner. This optically reflecting structure, with a density of 1.7 g/cm.sup.3, is polished briefly with a lapping machine to reduce the surface roughness to the required level.

In this method in accordance with the invention it is especially advantageous that the production of CMC and the reactive fixation or joining of the silicon wafer to the CFC blanks both occur in situ, thus forming extremely smooth structures that are subsequently coated.

A commercially available honeycomb material made of hard paper, with a relative weight of 0.2 g/cm.sup.3 and a cell (inscribed circle) diameter of 6 mm is impregnated with a phenolic-resin binder and dried at 70.degree. C.

To each face of a disk of the resulting honeycomb structure, measuring 400 mm in diameter and 10 mm in thickness, three layers of carbon-fiber web prepregs are now pressed on or laminated in a heatable press at 200.degree. C.

The resulting CFC structure is carbonized by heating to 1000.degree. C. in the absence of oxygen, with a heating rate of 2 kelvin per minute. After a holding time of 6 hours the structure is cooled to room temperature, producing a carbon-based honeycomb structure which, apart from a linear shrinkage of ca. 17%, corresponds to the original CFC component. The still-porous honeycomb structure is further strengthened by infiltration with pyrolyric carbon, performed by chemical vapor deposition as described in Examples 1 and 3. The component so obtained, with a relative weight of 0.22 g/cm.sup.3, has 4-point bending strengths of more than 150N/mm.sup.2.

One of the faces laminated with carbon-fiber web is superficially polished and two silicon wafers are glued to it. The resin binder used is a mixture of phenolic resin and silicon powder in the proportions 2:1 by weight.

This procedure is followed by silicon infiltration as described in Example 3, whereby the wafer is fixed to the honeycomb structure. The honeycomb-based mirror structure obtained after cooling, with a real weight per unit volume of 0.42 g/cm.sup.3, is illustrated in FIG. 7.

The resulting extremely light construction material is distinguished not only by its great rigidity and compression strength but also by low thermal conductivity. It is also particularly advantageous that the inner flexible honeycomb structures compensate for the expansion caused by thermal shock because of their thin walls, so that thermally induced fissures are kept to a minimum.

FIG. 5 shows how a large-area mirror can be put together from subunits. Here it is especially advantageous that basic elements consisting of silicon wafers 11 and substrate structures 10 of matching dimensions can be combined to produce mirrors of any desired size. The substrate structures 10 are held together by bridging units 14 (preferably of CFC material). If the presence of gaps between the individual elements is permissible, the mirror can be constructed of ready-made single reflectors. It is also possible, of course, to join the "blanks" to one another and subject them all together to the procedure described above, so that all the parts are fixed together.

In the embodiment shown in FIG. 5C, no bridging units 14 are necessary because the substrate structures 10 are so constructed as to form tongue-and-groove joints 15.

In the embodiment of the invention shown in the FIGS. 8, the CMC component forming the substrate structure 10 is provided with honeycomb-like recesses. Furthermore, in this sample embodiment three mounting recesses 117 are also included, into which connection pins or the like can be fixed to mount the finished mirror. The other reference numbers for this embodiment of the invention apply to the same parts as previously or to parts with the same function. Manufacture can be carried out as described above.

Another version of the method of the invention will now be described with reference to FIGS. 9 and 10.

As shown in FIG. 9A, the first step here is to coat a preform 11 of metallic silicon (a wafer) on the side opposite the reflecting surface 12 with a nonferrous metal, in particular with gold. This coating can be achieved by applying gold foil or by sputtering. The element so produced is shown in FIG. 9A.

This element is now heat-treated so that the gold layer 2 forms a melt eutectic with the silicon 11. This eutectic is indicated by the number 3 in FIG. 9B.

The element thus produced, shown in FIG. 9B, is now attached to the substrate structure 10, and then the whole apparatus is subjected to further heat treatment in a temperature range between 300.degree. and 600.degree. C. As a result, the upper (in FIG. 9C) silicon preform 11 becomes intimately joined to the lower substrate structure 10 over the whole region of the melt eutectic 3.

It is also possible to avoid a separate processing step for forming the melt eutectic on one surface of the wafer 11 (as in FIG. 9), by applying a wafer 11 directly to the substrate structure 10 with an intervening gold layer 2 (FIG. 10A). Then, in a single step, the two elements 10 and 11 are joined to one another by formation of a zone 3 of melt eutectic, as shown in FIG. 10B.

In this version of the method, the connection between the silicon preform (the wafer) and the substrate structure can be made at temperatures between about 300.degree. and 600.degree. C., whereas in the other versions of the method in accordance with the invention described above, the process temperatures are between ca. 900.degree. and 1500.degree. C. Furthermore, especially when gold is used as the nonferrous metal, it is possible to operate in a normal atmosphere and/or at normal pressure.

It will be appreciated from the above description that various steps of the method can readily be combined with one another.