JP2003344620A - Reflective mirror and its manufacturing method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a reflective mirror whose surface can be polished at mirror surface precision, which has high strength, high rigidity and low thermal expansion, and which is lightweight and can be large-sized. <P>SOLUTION: The reflective mirror is manufactured through a process for molding a pitch-based carbon fiber dispersed in a solvent, a process for hardening the molded pitch-based carbon fiber, a process for baking the hardened pitch-based carbon fiber at a first prescribed temperature and obtaining a carbon fiber reinforced carbon molding, a process for obtaining a silicon carbide-based composite material molding impregnating the carbon fiber reinforced carbon molding with a silicon melt at a second prescribed temperature, and a process for polishing the obtained silicon carbide-based composite material molding. The volumetric content of the carbon fiber in the silicon carbide-based composite material molding is preferably 20% or more and 50% or less. In addition, the pitch-based carbon treated thermally at 2,500°C or more, preferably 2,800°C or more is preferably used in the process for molding the pitch-based carbon fiber. <P>COPYRIGHT: (C)2004,JPO

Description

Detailed Description of the Invention

[0001]

TECHNICAL FIELD The present invention relates to a reflecting mirror,
In particular, the present invention relates to a manufacturing method of a reflecting mirror that can be upsized and that is suitable for a space infrared telescope, a high-power laser device, and the like.

[0002]

2. Description of the Related Art Due to its excellent mechanical properties and low thermal expansion, carbon fiber reinforced composite materials are expected to be applied to reflecting mirrors used in space infrared telescopes and the like. Japanese Unexamined Patent Application Publication No. 10-251065 describes the characteristics of a carbon fiber reinforced composite material containing silicon carbide as a main matrix and a method for producing the same. The technical outline of this manufacturing method will be described below with reference to the process flow chart shown in FIG.

First, at least two layers of high-strength graphite fiber and prepreg (a sheet in which carbon fiber cloth is impregnated with a synthetic resin) are compressed, hardened, and carbonized. The porous carbide is impregnated with a carbonizable impregnating agent (synthetic resin or pitch), recarbonized, and the carbide is graphitized at a temperature of up to 2,400 ° C. Graphitized carbide is finely pulverized so that the graphite fibers contained in it have a predetermined length and mixed with a highly carbon-containing binder (synthetic resin, pitch, etc.), and then this mixture is molded, hardened, and carbonized. .

When molten silicon is infiltrated into the porous carbide thus obtained and the carbon matrix is converted into silicon carbide, the elongation is large, but a matrix mainly composed of silicon carbide (SiC) and containing a small amount of carbon and silicon is used as a carbon fiber. A carbon fiber reinforced silicon carbide based composite material (hereinafter, may be abbreviated as C / SiC composite material or simply composite material) reinforced by.

Here, the carbon fibers are short graphite fibers, which are bound and surrounded by two or more layers of shell made of graphitized carbon partially converted into silicon carbide. This shell prevents the silicon from penetrating into the graphite fibers during the subsequent silicidation of the carbon matrix. However, part of the shell has been converted to silicon carbide.

In the above manufacturing process, the starting material is
A so-called “roving” bundle of fibers, which is a high-strength graphite fiber existing as a fiber structure processed into a woven fabric or a woven fabric or as a short fiber.

The matrix is predominantly silicon carbide containing up to 20% by weight of elemental silicon and a small amount of carbon, the composite material having up to 5% by volume of pores (voids). The carbonization treatment before silicifying the carbon matrix is preferably performed in the range of 850 to 950 ° C, and the graphitization is preferably performed in the temperature range of 1800 to 2200 ° C. The silicidation is performed at a temperature not exceeding 1750 ° C.

[0008]

The above-mentioned composite material is
It is said to have high bending strength and excellent heat resistance, abrasion resistance, and acid resistance, but it has the following problems. That is, the starting material for making the prepreg is "crude yarn".
Because it is a "fiber bundle" called "fiber bundle", even if the graphitized carbide is subjected to a process of finely crushing the contained graphite fibers to a predetermined length, it is finished as a carbon fiber reinforced silicon carbide based composite material. In the molded product, the carbon (graphite) fibers are dispersed in a bundle. In addition, as described above, there are pores (voids) at a maximum of 5% by volume.

Therefore, the carbon fiber reinforced silicon carbide based composite material is expected to be applied to a reflecting mirror such as a space infrared telescope. However, in the conventional composite material described above, when the surface is ground into the shape of the reflecting mirror. In addition, since the machinability of the matrix, the fiber bundle, and the void portion is different, there arises a problem that the processing accuracy of mirror-finishing by polishing cannot be raised to a desired high level.

On the other hand, as a conventional material for a reflecting mirror, a specific glass or metal material can be cited as a candidate, but each has its problems. Low-strain glass has a coefficient of thermal expansion close to zero and little influence of strain due to temperature difference, but mechanical properties such as strength, elastic modulus and fracture toughness are inferior to other materials, and the manufacturing process is low. It is complicated, it is difficult to increase the size, and there is a problem that the processing cost and the processing period are long when mirror-finishing.

As the metal material, for example, aluminum has been considered. However, since aluminum has a large coefficient of thermal expansion, it is likely to be locally deformed by thermal stress, and accuracy cannot be ensured in an environment where temperature changes. Moreover, there is a problem that the specific gravity is large and becomes heavy when the size is increased.

The present invention has been made in order to solve the above-mentioned problems, and it is possible to perform surface polishing processing with mirror surface accuracy, has high strength, high rigidity, low thermal expansion property, and is lightweight. The purpose is to obtain a reflecting mirror that can be upsized.

[0013]

A method for manufacturing a reflecting mirror according to the present invention comprises a step of molding pitch-based carbon fibers dispersed in a solvent, a step of curing the molded pitch-based carbon fibers, and a cured pitch. A step of firing carbon-based carbon fiber at a first predetermined temperature to obtain a carbon fiber-reinforced carbon molded product, and impregnating the carbon fiber-reinforced carbon molded product with a silicon melt at a second predetermined temperature to mold a silicon carbide-based composite material It is provided with a step of obtaining a product and a step of polishing the obtained silicon carbide-based composite material molded product.

The volume content of carbon fibers in the silicon carbide-based composite material molded product is 20% or more and 50% or less.

In the step of molding pitch-based carbon fibers dispersed in a solvent, pitch-based carbon fibers heat-treated at 2500 ° C. or higher, preferably 2800 ° C. or higher are used.

Further, before the step of impregnating the carbon fiber reinforced carbon molded product with the silicon melt at the second predetermined temperature to obtain the silicon carbide based composite material molded product, the obtained carbon fiber reinforced carbon molded product is reflected. It is provided with a step of processing into a mirror shape.

The first predetermined temperature is 1650 ° C. or higher and 2500 ° C. or lower.

The second predetermined temperature is not lower than the first predetermined temperature and not higher than 1800 ° C.

The method further comprises the step of forming a coating film of a metal having a high reflectance on the polished silicon carbide type composite material molded article.

A reflecting mirror according to the present invention is a reflecting mirror having a silicon carbide based composite material molded product obtained by impregnating silicon into carbon fiber reinforced carbon obtained by firing pitch-based carbon fiber. The volume content of carbon fiber is 20
% Or more and 50% or less.

[0021]

BEST MODE FOR CARRYING OUT THE INVENTION The more detailed aim of the present invention is as follows. (1) The mirror surface accuracy required as a material suitable for a reflecting mirror is, for example, for an infrared telescope, a surface roughness of 50 nm (nanometer) or less. (2) In order to reduce the influence (strain, deformation) due to temperature change, the coefficient of thermal expansion should be small from the ambient temperature (about 4K) to the manufacturing temperature (room temperature). For infrared telescopes used in outer space, the desired coefficient of thermal expansion is 0.7 at 77K or less.
It is 2 ppm or less and 2.5 ppm / K or less at room temperature. (3) Lighter, stronger, and larger and lighter than conventional glass and metal. When used in space, it must be strong enough to withstand the impact of a satellite launch.

With reference to FIG. 1, a manufacturing process of a reflecting mirror made of a carbon fiber reinforced silicon carbide based composite material according to the present invention will be described. In the flow diagram of FIG.
C / C (Carbon Fiber Reinforced Carbon; Carbon / Carb) by graphitizing and firing from the starting carbon fiber base material so that the morphology and dispersion state of the fibers and matrix in the material can be easily understood.
on) As a two-dimensional schematic diagram of the cross-sectional structure up to the molded product, and as a three-dimensional solid view from the machined C / C reflecting mirror to the mirror-polished C / SiC reflecting mirror, each process element It is drawn.

First, short fibrous pitch-based carbon fibers are stirred and homogeneously dispersed in a container containing an aqueous solution (solvent) to which an appropriate amount of binder (a sizing agent for joining carbon fibers) is added. For example, cellulosic materials (insoluble) used in food additives are thin, short fibrous,
It is a preferred binder because it is easily entangled with carbon fibers.

Thereafter, the dispersion liquid is transferred to a jig having a predetermined size, filtered, and further pressurized to obtain a carbon fiber base material (a molded body in which carbon fibers are homogeneously dispersed) molded into a constant size. Next, the carbon fiber base material is impregnated with pitch or resin and cured. At this time, the pitch is impregnated when heated and melted, and cured when returned to room temperature, but after impregnated with the resin,
For example, it is heated in the atmosphere at 180 ° C. for about 1 hour to be cured. Then, it is heated in an inert atmosphere to carbonize the impregnated pitch or resin to form a C / C molded product.

Further, the C / C molded product is impregnated with pitch or resin, heated and recarbonized, and then heat treated at 2500 ° C. or less in a vacuum or an inert atmosphere to graphitize. This graphitized C / C molded product is ground into a reflector shape, and then metal silicon is melted and impregnated in vacuum at a temperature of 1800 ° C. or lower to convert the matrix carbon into silicon carbide. And finally, with this carbon fiber
A C / SiC reflecting mirror is completed by mirror-polishing a C / SiC composite material molded product mainly composed of a matrix containing SiC and a small amount of carbon and silicon. After that, a metal such as gold or aluminum may be coated in order to improve the reflectance of the reflecting mirror.

Here, the pitch is a kind of polymer compound. There are petroleum-based and coal-based pitches, which are solid at room temperature, but melt when heated. Pitch-based carbon fibers are carbon fibers obtained by melt-spinning the pitch, and then subjecting it to an inert treatment and a carbonization treatment.

The pitch-based carbon fiber is previously subjected to the following dispersing treatment in order to enhance the dispersibility. First, the sizing agent (resin that prevents the fibers from falling apart and is easy to handle) attached to the fibers is removed (washed with a solvent or burned off at a high temperature). Next, the carbon fibers are put into an aqueous solution containing a surfactant (soap or the like), stirred and uniformly dispersed, and then water is filtered to obtain a carbon fiber base material having improved dispersibility. When carbon fiber is put into the resin and stirred, it becomes pills that grow steadily and do not disperse, so in the gap between the carbon fibers,
Impregnate with resin or pitch.

The reason why the pitch-based carbon fiber is used as a starting material is that when the graphitized C / C molded product is impregnated with molten metal silicon to siliconize the matrix carbon, the pitch-based carbon fiber is hard to react with silicon. Of nature and of fiber
This is because strength reduction and embrittlement due to the use of SiC are prevented.
JP-A-7-198917 suggests that when rayon-based or PAN-based carbon fiber is used as a starting material, the carbon fiber also reacts with silicon during the impregnation with silicon to form SiC, resulting in strength reduction and embrittlement. , It was also confirmed from our experimental results. The PAN-based carbon fiber is a carbon fiber obtained by carbonizing a polymer called polyacrylonitrile.

It is desirable that the pitch-based carbon fiber be heat-treated before being impregnated with pitch (or resin). When pitch (or resin) impregnation, firing, and silicon impregnation treatment are performed without heat treatment, the fiber undergoes a SiC reaction and the mechanical properties deteriorate. On the other hand, when the fibers are heat-treated at 2500 ° C. or higher before being impregnated with pitch (or resin), and then impregnated with pitch (or resin), fired, or impregnated with silicon, the reaction of SiC formation of the fibers is suppressed, and the reaction Deterioration is reduced and good properties are exhibited.

As described above, the prior heat treatment of the carbon fiber is
It has the effects of promoting the graphitization of carbon to increase the Young's modulus of the fiber and suppressing the SiC reaction of the fiber to improve the mechanical and thermal characteristics of the C / SiC reflecting mirror. If the heat treatment temperature of the carbon fiber in advance is higher than the firing temperature, the above effect is easily obtained, and 2800 ° C. or higher is more preferable.

If the carbon fibers are homogeneously dispersed in the matrix before firing, the mirror surface accuracy is improved in the polishing process of the reflecting mirror. For reference, JP-A-10-25106
The shape and shape of the C / SiC composite material molded product using bundled fibers as in Japanese Patent No. 5 and the C / SiC composite material molded product according to the present invention (using the fiber subjected to the dispersion treatment) FIG. 2 shows the result (photograph) of comparing the distribution state by observing the structure. In the photograph (b) using the bundled fibers, the carbon fibers are in a bundled state and are in a non-homogeneous dispersed state, but in the photograph (a) according to the present invention, the fibers are not in the bundled shape and are homogeneous. It is clearly shown to be dispersed.

The impregnation temperature of silicon can be selected in the range of 1800 ° C. from the melting temperature (melting point) of silicon, but the range of 1650 ° C. or more and 1800 ° C. or less is preferable. When the impregnation treatment is performed at a temperature below 1650 ° C, C /
C The voids (holes) that are internal to the molded product are not sufficiently impregnated with silicon, leaving voids and decreasing the mechanical properties.
It also affects the mirror finish. On the other hand, when the silicon is impregnated at a temperature exceeding 1800 ° C., the carbon fibers react with each other and the strength characteristics deteriorate.

In the composite material reflecting mirror, in order to obtain various characteristics suitable for practical use as a reflecting mirror, the volume content of carbon fiber is within the range of 20% to 50%, preferably 30%.
To be within the range of 40% to 40%. If the volume content of carbon fibers is less than 20%, the mechanical strength is insufficient and the thermal expansion coefficient is large, which is a problem. Further, when the volume content exceeds 50%, it becomes difficult to uniformly disperse the fibers, and the voids contained in the C / C molded product are reduced, and the amount of silicon to be impregnated is reduced, which results in a matrix. It becomes difficult for carbon to be converted into silicon carbide.

The heat treatment (calcination) temperature for obtaining the carbon fiber reinforced carbon (C / C) is preferably the same as or higher than the temperature at which the metallic silicon is impregnated, and 2500 ° C. or less. This setting of the temperature range for C / C conversion is important for promoting C / C conversion to an appropriate degree and for sufficiently promoting SiC conversion of the matrix in the subsequent process. When the C / C conversion temperature is lower than the temperature at which the metallic silicon is impregnated, the impregnation property is poor during the silicon impregnation, and voids are likely to remain. Also 2500
C / C conversion at a temperature exceeding ℃ is not suitable because the expansion (dimension change) of the molded product due to the SiC conversion reaction becomes severe during silicon impregnation, and the molded product is cracked or split.

In the C / SiC reflecting mirror product according to the present invention, it is possible to obtain excellent characteristics such as a high degree of mirror surface precision, low thermal expansion and high rigidity, which are suitable for practical use.
High-performance reflectors can be manufactured.

Example Chopped fibers of pitch-based carbon fiber (manufactured by Mitsubishi Kagaku Sanshi Co., Ltd.) subjected to dispersion treatment (average fiber length: about 3 m)
m) was heat-treated at 3000 ° C. for graphitization, and then uniformly dispersed in a container containing an aqueous solution containing a binder. This dispersion was transferred to a jig having a predetermined size and further heated to obtain a molded product of a carbon fiber base material. At this time, the molding pressure was changed to prepare four types of samples having different carbon fiber filling rates.

Subsequently, these four types of samples were placed in a constant temperature bath for 9 minutes.
It was dried at 0 ° C. for 24 hours and then impregnated with pitch. Furthermore, this molded impregnated material is heated to about 1000 ° C in an inert atmosphere.
Then, it was calcined to C / C, and then heat-treated at 2000 ° C. in an inert atmosphere to graphitize. The metallic silicon is then melted and this graphitized C /
The C molding was impregnated in vacuum at 1750 ° C.

The volume fraction of carbon fibers contained in the C / SiC composite material molded product thus obtained was 15%, 30%,
40% and 60%. Further, structural analysis of this molded product confirmed that the carbon in the matrix reacted with the impregnated silicon and changed to SiC, but the carbon fiber did not react. Furthermore, the voids (voids) in the C / SiC composite material molded product are almost completely filled with silicon impregnation, and the proportion of the voids is 1% by volume in the samples with carbon fiber volume contents of 30% and 40%. It was below.

As a result of measuring various characteristics of these C / SiC composite material molded products, values shown in FIG. 3 were obtained. A C / SiC composite material molded product with a carbon fiber volume content of 40% has a bending strength (210 MPa) and Young's modulus (260 G
Pa), and the fracture toughness value was about 7 times.
It has higher strength and Young's modulus than aluminum
A small linear expansion coefficient (room temperature) of 10 or less was obtained. This C /
When the surface of the SiC composite material molded product was polished, the surface (mirror surface) accuracy was at a high level (50 nm
It was confirmed that

Even with a C / SiC composite material molded product having a carbon fiber volume content of 30%, Young's modulus (240 GPa), flexural strength (18
0 MPa), linear expansion coefficient (room temperature), and fracture toughness values were superior to those of low strain glass. JP 10-2
Even when compared with the C / SiC composite material molded product (bending strength: 57.3 Mpa, Young's modulus: 42.6 GPa) described in Japanese Patent No. 51065, C / SiC composite materials having a carbon fiber volume content of 30% and 40%. The molded product was excellent in both bending strength and Young's modulus.

On the other hand, when the volume content of carbon fiber is 15%, the fracture toughness value and the bending strength are extremely lowered, which is an unfavorable result. The volume content of carbon fiber is about 6
In the case of 0%, the fibers are partially oriented or aggregated and are not uniformly dispersed, so that the surface does not become a mirror surface and the matrix carbonization of silicon carbide is insufficient. Fell.

Although not shown in Table 3, when the volume content of carbon fiber is 30% and 40%, the linear expansion coefficient at 77K is 0.2 ppm or less, which is used at cryogenic temperature. It turns out that it can handle cases.

[0043]

The method of manufacturing a reflecting mirror according to the present invention comprises:
A step of molding a pitch-based carbon fiber dispersed in a solvent,
A step of curing the molded pitch-based carbon fiber, a step of firing the cured pitch-based carbon fiber at a first predetermined temperature to obtain a carbon fiber-reinforced carbon molded product, and a melt of silicon in the carbon fiber-reinforced carbon molded product. Of a silicon carbide based composite material and a step of polishing the obtained silicon carbide based composite material molding, thereby providing high strength, high rigidity and low thermal expansion. It is possible to obtain a reflecting mirror having characteristics.

The volume content of carbon fibers in the silicon carbide-based composite material molded product is 20% or more and 50% or less, so that a reflecting mirror having high strength, high rigidity, and low thermal expansion can be obtained. It is a thing.

Further, in the step of molding the pitch-based carbon fiber dispersed in the solvent, by using the pitch-based carbon fiber heat-treated at 2500 ° C. or more, preferably 2800 ° C. or more, the silicon carbide conversion reaction of the fiber is performed. It can be suppressed.

Before the step of impregnating the carbon fiber reinforced carbon molded product with the silicon melt at the second predetermined temperature to obtain the silicon carbide based composite material molded product, the obtained carbon fiber reinforced carbon molded product is reflected. By including the step of processing into the shape of a mirror, grinding of the reflecting mirror can be easily performed.

Further, the first predetermined temperature is not lower than 1650 ° C. and not higher than 2500 ° C., so that the carbon fiber reinforced carbon molded product can be prevented from being cracked.

Further, the second predetermined temperature is not lower than the first predetermined temperature and not higher than 1800 ° C., so that the impregnation of silicon proceeds well.

Further, the reflectance can be improved by providing a step of forming a metal coating film having a high reflectance on the polished silicon carbide type composite material molded article.

A reflecting mirror according to the present invention is a reflecting mirror having a silicon carbide based composite material molded product obtained by impregnating silicon into carbon fiber reinforced carbon obtained by firing pitch-based carbon fiber. The volume content of carbon fiber is 20
When it is at least 50% and at least 50%, a reflecting mirror having high strength, high rigidity and low thermal expansion can be obtained.

[Brief description of drawings]

FIG. 1 is a flow chart for explaining a method for manufacturing a reflecting mirror according to the present invention.

FIG. 2 is a diagram for explaining a difference in surface morphology of a molded product due to a difference in carbon fiber used.

FIG. 3 shows various characteristics of the composite material molded article according to the present invention.
It is a table | surface which compared with the molded object comprised by the other material.

FIG. 4 is a flow chart for explaining a method for producing a carbon fiber reinforced composite material described in Japanese Patent Laid-Open No. 10-251065.

[Explanation of symbols]

C / C carbon fiber reinforced carbon, C / SiC carbon fiber reinforced silicon carbide.

   ─────────────────────────────────────────────────── ─── Continued front page    (72) Inventor Shigenori Kabashima             2-3 2-3 Marunouchi, Chiyoda-ku, Tokyo             Inside Ryo Electric Co., Ltd. (72) Inventor Gen Takeya             2-3 2-3 Marunouchi, Chiyoda-ku, Tokyo             Inside Ryo Electric Co., Ltd. F-term (reference) 2H042 DA01 DA12 DA15 DC01 DC05                       DC08 DE09

Claims (8)

[Claims] 1. A step of molding a pitch-based carbon fiber dispersed in a solvent, a step of curing the molded pitch-based carbon fiber, and a step of firing the cured pitch-based carbon fiber at a first predetermined temperature. A step of obtaining a reinforced carbon molded article, a step of impregnating a carbon fiber reinforced carbon molded article with a silicon melt at a second predetermined temperature to obtain a silicon carbide-based composite material molded article, and the obtained silicon carbide-based composite material molding A method of manufacturing a reflecting mirror, comprising a step of polishing an object. 2. The method for producing a reflecting mirror according to claim 1, wherein the volume content of carbon fibers in the silicon carbide-based composite material molded product is 20% or more and 50% or less. 3. In the step of molding pitch-based carbon fibers dispersed in a solvent, the temperature is 2500 ° C. or higher, preferably 28 ° C.
The method for manufacturing a reflecting mirror according to claim 1, wherein pitch-based carbon fibers heat-treated at a temperature of 00 ° C. or higher are used. 4. The obtained carbon fiber reinforced carbon molded product is reflected before the step of impregnating the carbon fiber reinforced carbon molded product with a melt of silicon at a second predetermined temperature to obtain a silicon carbide based composite material molded product. The method of manufacturing a reflecting mirror according to claim 1, further comprising a step of processing into a mirror shape. 5. The first predetermined temperature is 1650 ° C. or higher, 2
The method for producing a reflecting mirror according to claim 1, wherein the temperature is 500 ° C. or lower. 6. The method of manufacturing a reflecting mirror according to claim 1, wherein the second predetermined temperature is not lower than the first predetermined temperature and not higher than 1800 ° C. 7. The method of manufacturing a reflecting mirror according to claim 1, further comprising the step of forming a coating film of a metal having a high reflectance on the polished silicon carbide-based composite material molded product. 8. A reflecting mirror having a silicon carbide-based composite material molded product obtained by impregnating silicon into carbon fiber reinforced carbon obtained by firing pitch-based carbon fiber, the volumetric content of carbon fiber of the reflecting mirror. The reflecting mirror is characterized by having a rate of 20% or more and 50% or less.