JPH07503226A - Monolithic ceramic truss structure - Google Patents

Abstract

(57) [Summary] This bulletin contains application data before electronic filing, so abstract data is not recorded.

Description

[Detailed description of the invention] Monolithic ceramic truss structure Technical field The present invention relates to a monolithic ceramic truss structure, and particularly to a monolithic ceramic truss structure useful as a mirror base material. This relates to a ceramic truss structure.

Background technology Advances in space-based imaging technology have enabled the development of dimensionally stable, rigid, lightweight, and highly rigid There is a growing need for precision mirrors with a certain frequency. Such a mirror is a laser It is also useful for ground-based studies such as research, and its dimensional stability and light weight make it It is also useful in commercial video technology, which is an important issue. used for these purposes The mirror must be a highly efficient substrate, that is, a substrate with low areal density and high structural strength. be. Areal density is the weight of the mirror per unit area of the reflective surface.

Traditionally, most precision mirrors were made from glass or glass-ceramic.

This is because these materials can be molded and polished to easily obtain optical surfaces. It is. Additionally, glass and glass ceramics have low coefficients of thermal expansion, making them difficult to use in optical applications. can have the required thermal stability. However, the structural properties of these materials is low. As a result, mirrors made from these materials provide adequate support for mirror-reflecting surfaces. Often a large amount of weight must be added to provide structure.

This means that the typical areal density of glass mirrors ranges from about 20 kg/m2 to several hundred kg/m''. It is clear from the description. Due to the heavy weight of the mirror, a large number of A problem has arisen. On the ground, the weight of these mirrors creates serious handling problems. A problem is arising. Furthermore, the gravity acting on this mirror makes it dimensionally unstable, The surface becomes distorted or deformed. Weight is not important in space, but it is The inertia caused by the large mass makes accurate and rapid alignment difficult.

Mirrors based on lightweight metals such as beryllium (Be) are an alternative to glass mirrors. be. Is such a substrate capable of supporting an attached reflective surface, or is it capable of supporting itself? May be polished to form a reflective surface. Unlike glass, beryllium (Be) has an elastic modulus It has high strength and remarkable rigidity, and has favorable structural characteristics for low-mass precision mirrors. ing. For example, a Be mirror made using current industrial methods weighs about 15 kg/a+2. may have a low areal density.

Silicon carbide (SiC) substrates are a further alternative to glass mirrors. B Like e, SiC has low mass, high elastic modulus suitable for precision mirrors, remarkable stiffness, and other features. It has the characteristics of

The combination of low coefficient of thermal expansion and high thermal conductivity makes SiC Be.

Has better thermal dimensional stability than low thermal expansion glasses and glass ceramics. Furthermore Due to the technology of manufacturing SiC, the SiC mirror base material has a structural advantage over the Be base material. . For example, the conventional SiC mirror substrate has a low areal density of about 4 kg/m2. obtain. These base materials are sandwiched between a top sheet and a back sheet that support the mirror-reflecting surface. It is a honeycomb of solid wall-reinforced cells. This reinforcing cell can be square, hexagonal or triangular. It has a polygonal cross section such as a square cross section. Various methods including chemical vapor deposition and slip casting Several different processes are effective in making SiC mirrors. One particularly useful The lip casting method is described in U.S. Patent No. 4,975,255 owned by Vivaldi et al. Disclosed. SiC mirror substrate with honeycomb sandwich structure has low areal density However, the honeycomb sandwich structure has adequate rigidity for some precision mirror applications. It uses a mirror base material that is much wider and lighter than the original.

Therefore, what is desired in industry is an ultra-low area suitable for supporting precision mirrors. It is a mirror base material with high density and dimensional stability.

Disclosure of invention The present invention relates to a dimensionally stable mirror substrate with ultra-low areal density suitable for supporting precision mirrors. .

In one aspect of the invention, the A monolithically cast open truss structure is included.

Another aspect of the invention includes a continuous ceramic facesheet and a ceramic facesheet extending from the facesheet. a ceramic mirror having a monolithic ceramic truss cast integrally therewith; A substrate is included. This integrated truss has a mirror base material with sufficient rigidity to support the precision mirror surface. have sex.

In another aspect of the invention, a plurality of internal passages are provided within a suitably shaped member made of non-porous material. By creating channels, multiple integral truss members of a monolithic ceramic truss can be A method of making a soluble core suitable for slip casting is included.

This non-porous material has a soluble core and internal passageways are liquid-containing ceramic slips. Create a filled monolithic truss member. non-porous material used to create the soluble core The material can be melted at temperatures below the freezing point of the liquid within the ceramic slip.

These and other features and advantages of the invention will be apparent from the disclosure that follows and the accompanying drawings. will be further clarified.

Brief description of the drawing FIG. 1 shows the present invention with a continuous face sheet and an integral truss with side and rear members. It is a perspective view of the ceramic mirror base material concerning this.

FIG. 2 shows a ceramic mirror substrate similar to that shown in FIG. FIG. 3 is a perspective view of the structure replaced with a continuous back sheet.

FIG. 3 is a perspective view of a planar ceramic truss structure according to the present invention.

Figure 4 shows possible ways to create the internal geometrical arrangement of the mirror base material or truss structure in items 1-3. FIG. 2 is a cross-sectional view of a tool used to make a soluble core.

Figure 5 is a cross-sectional view of the mold used to make the 1-3 mirror base material or truss structure. It is.

Optimal embodiment of the invention The monolithic, ceramic, trussed mirror substrate of the present invention can be used in many space and terrestrial applications. It can be used as a structural support for precision mirrors suitable for applications such as

The truss itself can be used as a structural member in applications that require light weight and high rigidity.

As shown in FIG. 1, the mirror base material 2 of the present invention is integrated with a three-dimensional ceramic truss 8. It has a thin, continuous ceramic face sheet 4 connected to it.

This top sheet 4 has a mirror surface 6 on one side. This truss 8 is a distortion of the mirror surface. The mirror base material 2 should be made sufficiently strong so as to limit it to the smallest unit of micrometer. Ru. Distortion of mirror surfaces can occur due to mechanical and thermal loads. Truss 8 has four sides It can have several conventional three-dimensional shapes, such as body shapes. In this shape, The integrated horizontal member 10 has a surface sheet reinforcing rib 14, a plurality of tetrahedrons, and the base of the truss 8. It extends to form a structural element. The top sheet reinforcing ribs 14 and the top sheet 4 It is an integrated part. The integral rear member 12 is attached to the front to complete the three-dimensional truss. The individual tetrahedra described above are connected to form an inverted tetrahedron.

Many different shapes are possible based on the design of FIG. Improved rigidity of mirror base material 2 Multi-layer monolithic trusses may be used to increase the height. For example, the small tetrahedron A truss layer is placed next to the face plate to provide local stiffness to the face plate. It may be arranged so that the This layer consists of different tetrahedrons of the same size or larger. It may be supported by a monolithic layer to strengthen the whole. necessary to obtain the desired stiffness. Integral truss layers of any size required can be used.

Another way to further strengthen the mirror base material 2 is to Replace the component with a thin, monolithic, continuous ceramic backsheet 16 There is a way to do this, or add it. Other configurations include surface sheeting. The front sheet 4 and the back sheet 16 are arranged so that they constitute an open truss structure 18 as shown in FIG. May be omitted. During this time, the truss structure 18 has sufficient capacity to be used for structural purposes. It has multiple rear members 12 and front members 20 that provide strength and rigidity. Depends on usage Therefore, the truss structure 18 may be a planar sheet as shown in FIG. may be any other useful shape, such as a cone or an ellipse.

The area density of the open truss structure and the mirror base material according to the present invention is the dimension of the finished product based on these. and changes depending on the structure. However, in general, these things can be compared It has a lower areal density than prior art honeycomb sandwich products. For example, open The truss structure may have an areal density of less than about 1 kg/12. Approximately 0.0 in diameter. For mirrors with a reflective surface of 25 m (10 inches) or less, a surface of approximately 2 kg/m2 or less Stacking density is possible. For example, a mirror with a reflective surface of about 0.25m in diameter is 0°5m. m (0,020 inches) thick surface sheet and a diameter of 0.75 mm (0°030 inches). ) and a 25 mm (1 inch) thick truss.

Such a substrate has a front sheet with an areal density of 1.4 kg/m" and an areal density Q of 4 kg/m". m2 truss, the overall areal density is 1. 8kg/m”.Current production A comparative mirror with a honeycomb sandwich substrate designed at the limit is 0.5 m. l with m-thick front and back sheets and 0.5 mm walls 2.7 mm (0.5 inch) Consists of a 11.7 mm (0,461 inch) thick core made of sphere cells be able to. Such a base material has front and back sheets with an areal density of 1,4 kg/m". and a core with an areal density of 1.4 kg/+2, the total weight is 4゜2 kg/m' has an areal density of

The mirror base material 2 and the truss structure 18 are made of ceramic materials that provide desired weight and rigidity characteristics. made from Suitable ceramics include SIC, silicon nitride, silicon carbide and the like. Contains materials. A preferred ceramic is silicon carbide. Mirror base material 2 and truss structure 18 may be manufactured using conventional methods used to make similar ceramic products. . For example, structures of the invention may be manufactured by chemical vapor deposition or slip casting. . U.S. Patent No. 4,975.255, commonly owned by Vivaldi, Discloses an advanced slip casting method. If you use the slip casting method, mirror The base material 2 and the truss structure 18 are made from metal powder as is known from slip casting techniques. It's okay to be made.

Slip casting involves adding ceramic or metal powder to a liquid, usually water, to create the desired product. Use a slip with dispersed This slip is known from the prior art for nucleation. It may contain additional components such as additives. For example, a suitable SiC slip is F- 320 mesh about 40% to about 60% by weight SiC powder, about 30% to about 45% by weight of 1.0mm 5jC powder, about 7% to about 15% by weight of water, about 0.0 5% to about 0.55% by weight sodium silicate bonding material, and about 0.3% by weight % to about 2.5% by weight of a nucleating agent such as urea or dimethyl sulfoxide.

These materials are available from a variety of commercial sources. to obtain the desired product The slip is poured into a mold 36 containing a soluble core 22 as shown in FIG. soluble The decomposable core 22 includes a plurality of truss members 10, 12, 20 of the three-dimensional truss 8. It has an internal passageway 23 and a suitable external geometry. If there are front and back sheets, Mold 36 also defines its external geometry.

The fusible core 22 defines an internal passageway in a suitably shaped member made of non-porous material. You can also make it depending on the situation. This non-porous material is suitable for slip casting and Any non-porous material that can be easily melted at temperatures below the freezing point of the liquid in the liquid may be used. stomach. A suitable non-porous material is polystyrene. To make the soluble core 22 In this case, a jig having a top plate 26, a bottom plate 28, and a spacer 30 as shown in FIG. 24 may also be used. The spacer 30 separates the two plates 26 and 28 so that the core 22 Create a closed space where possible. In order to form the passage 23 in the desired tetrahedral shape, the plate 2 is A pin 34 is inserted into the jig 24 through the hole 32 at 6.28. board 26.28 also has ribs 35 forming back member 12 and front sheet reinforcing ribs 14. ing. To form the core 22, the core 22 is placed in a tool 24 and steam is injected into the tool. Heat the tool by soaking it in water or in a hot water bath, or by other methods. Expanded polystyrene beads may also be used.

The pin 24 is then removed from the tooling 24, the tooling is opened, and the core is removed. removed. The core 22 may also be inserted into a suitably shaped member made of non-porous material. The passage 23 may also be created by machining. In what way does core 22 Regardless of whether the Therefore, all integral structures in the mirror support 2 or the truss structure 18, Mari surface sheet 4, horizontal member 10, rear member 12, surface sheet reinforcement rib 14, back surface It is necessary to form the seat 16 and the front member 20. Passages 23 will connect to each other By stacking one or more cores 22, multiple integral truss layers can be created. may be formed.

After core 22 is formed, it is placed in a non-absorbable mold 36 as shown in FIG. be done. The mold 36 is made of aluminum, plastic, or other non-absorbent material. It is fine as long as it is a fee. The air inside mold 36 is PH0TO-PLO' (Eastman Koda Fill mold 36 with water containing a small amount of physicochemical agent such as It can be eliminated by Then filled with suitable ceramic slip injected into the mold 36 through port 38 and expelling water through discharge boat 40; Then, a mirror base material having the top sheet 4 and the truss 8 is formed. Ensures no slipping Mold 36 may be shaken or vibrated to completely fill mold 36 and core 22. mold 36 is then heated to a temperature below the freezing point of the liquid in the slip to freeze the slip. cooled to a certain degree.

Preferably, the mold is cooled to below about -50°C to quickly freeze the slip. is good. The frozen slip 42 containing the soluble core 26 is removed from the mold 36 and e.g. For example, it is maintained at an appropriate temperature so that the temperature of the object is maintained at about -50℃ or less. . The frozen slip 42 is then allowed to cool for a period of time sufficient for the core 22 to completely thaw. Thus, the core 22 is bathed in a solvent that can dissolve it. Core 22 is polystyrene. Alternatively, the solvent may be methylene chloride. The solvent bath melts the core 22 but does not It needs to be cold enough so that it doesn't melt. Frozen space with core 22 removed The lip 42 is removed from the solvent bath and freeze-dried to sublimate the moisture in the slip. Let's make the ``base''. This substrate was vacuum dried to remove any remaining volatiles. , in an argon atmosphere in a vacuum furnace at a suitable temperature, e.g. Sintered to form a sintered body. This porous sintered body is heated to a temperature of about 1750°C. It is immersed in molten silicon (St) and densified in a vacuum furnace with a Rougon atmosphere. child The silicon enters the sintered body to fill the open pores. Lander of sintered SIC The uniform arrangement and constant distribution of the silicone filler make the product of the invention substantially isotropic. become something.

Products made in this way are suitable for use as substrates and structural supports for precision mirrors. It has a one-piece structure that is highly rigid and lightweight. This product is a mirror base material, It can be a face sheet and a back sheet, or it has a flatness of 0.02 A machine base material for improving seeds to less than 5 mm (0,001 inch), Also good.

SiC/Si has a surface roughness of 50 to 100 RMS suitable for many optical applications. It can be polished to If a smoother optical surface is required, approximately 0.05mm ( 0,002 inch) to approximately 0.075+am (0,003 inch) thick. A metal layer may be deposited on the surface. This silicon layer has a roughness of less than 5 RMS. Can be polished.

If desired, a polished surface can be coated with a thin layer of reflective metal such as gold to increase reflectivity. It may be coated with a layer.

The product according to the invention offers several advantages over the prior art. Firstly , the integrated truss structure used in the mirror base material of the present invention has a smaller area density than conventional mirror base materials. Make products of degrees. Although the mirror base material of the present invention is lighter than conventional base materials, It can have similar stiffness to the base material. Therefore, these are advanced Suitable for precision mirrors used for various purposes.

Second, since the truss is integrally formed, the coefficient of thermal expansion is uniform, resulting in dimensional stability. Qualitative results can be obtained. Additionally, the SiC/St material used to make the truss is isotropic. has. In contrast, it is sometimes used to support mirrors in conventional truss installations. In adhesives and mechanical joints, the truss material and the bonding material may have different coefficients of thermal expansion. A collision occurs.

As a result, prior art trusses buckle when subjected to thermal changes. In addition, adhesives and Due to mechanical joints, traditional trusses are no longer monolithic, making them unsuitable for precision mirror applications. Become something. Furthermore, mechanical joints that are sometimes used on conventional trusses are This increases weight, making it increasingly unsuitable for applications with weight restrictions.

Third, since the integral two truss is cast in a single member, it It can be made with less effort. In contrast, many conventional trusses are made by assembling parts together. It requires a lot of effort to maintain.

The invention is not limited to the particular embodiments shown and described herein. Complain Various changes and modifications can be made without departing from the spirit and scope of the invention.

, N+ PCT/US 92109235

Claims (15)

[Claims] 1. Three-dimensional, lightweight, monolithically cast to form a monolithic truss An open truss structure with multiple ceramic truss members. 2. The truss structure of claim 1, wherein the structure has - more truss layers. 3. The truss member is made of glue made of silicon carbide, silicon nitride, and boron carbide. The truss structure of claim 1, comprising a material selected from the group consisting of: 4. The truss structure according to claim 1, wherein the truss member contains metal powder. 5. The truck of claim 1, wherein said structure has an areal density of less than about 1 kg/m2. structure. 6. (a) a continuous ceramic facesheet; and (b) extending from said facesheet and an integral ceramic truss integrally cast on the surface sheet; The monolithic ceramic truss is such that the mirror base material is sufficiently rigid to support the precision mirror surface. Ceramic mirror base material that provides unique properties. 7. 7. The mirror substrate of claim 6, wherein said substrate has more than one truss layer. 8. 7. The mirror substrate of claim 6, wherein the substrate has an areal density of less than about 2 kg/m2. 9. further comprising a backsheet integrally cast into the integral truss, said backsheet The mirror base material according to claim 6, wherein the mirror base material further increases the rigidity of the mirror base material. 10. The base material is made of a groove made of silicon carbide, silicon nitride, and boron carbide. 7. A mirror substrate according to claim 6, containing the selected material. 11. The mirror base material according to claim 6, wherein the base material contains a metal cover. 12. Slip multiple integral truss members in a monolithic ceramic truss A method of manufacturing a meltable core for casting, wherein the truss is a liquid-containing ceramic. obtained from a slip, in which a plurality of internal parts are inserted into a member of non-porous material of suitable shape. forming a partial passageway in which the non-porous material becomes a soluble core and the ceramic said internal passageway forming said integral truss member when filled with slip; At the same time, the non-porous material has a temperature lower than the Isoma point of the liquid in the ceramic slip. A method that can be melted at low temperatures. 13. The internal passage is (a) assembling a tool having removable pins arranged in the form of a truss; (b) filling the tool with a non-porous material to form a meltable core; (c) removing the pin from the jig; and (d) dissolving the pin from the jig. formed by removing the core, the removable pin thereby forming an internal passageway within the dissolvable core; 13. The method according to claim 12. 14. The internal passageway may be machined from a piece of non-porous material having a suitable shape. 13. The method of claim 12, wherein the method is formed by: 15. Method. 13. The non-porous material of claim 12, wherein the non-porous material is polystyrene.