Monolithic Ceramic Truss Structure

Technical Field

The present invention is directed to monolithic ceramic truss structures, particularly monolithic ceramic truss structures useful as mirror substrates.

Background Art

The development of space-based imaging technologies has created a need for dimensionally stable, stiff, low mass, high natural frequency, precision mirrors. Such mirrors are also useful for ground-based research, such as laser research, and in commercial imaging technologies where dimensional stability and low weight are important considerations. Mirrors used for these applications should have highly efficient substrates, that is, substrates with low areal densities and high structural stiffness. Areal density is the mirror's weight per unit area of reflective surface.

Traditionally, most precision mirrors have been made from glass or glass-ceramics because these materials are easy to shape and polish into optical surfaces. Moreover, glass and glass-ceramics can have low coefficients of thermal expansion, which give them the thermal stability needed for optical applications. These materials, however, have poor structural properties. As a result, mirrors made from these materials must often have a large mass to provide adequate structural support to the mirror's reflective surface. This is evident from the typical areal density of a glass mirror, which can range from about 20 kg/m ² to several hundred kg/m ² .

Mirrors with a large mass can create many problems, both on the ground and in space. On the ground, the weight of these mirrors poses significant handling problems. Moreover, the force of gravity acting on the mirrors can distort or deform their surfaces, making them dimensionally unstable. In space, where weight is not important, a large mass creates inertia that makes accurate and rapid positioning difficult.

Mirrors with a lightweight metal substrate, such as beryllium (Be) , are one alternative to glass mirrors. Such a substrate can support an attached reflective surface or can itself be polished to form a reflective surface. Unlike glass, Be has structural properties suitable for low mass, precision mirrors, such as a high modulus and specific stiffness. For example, Be mirrors made with current manufacturing methods can have areal densities as low as about 15 kg/m ² .

Silicon carbide (SiC) substrates are another alternative to glass mirrors. Like Be, SiC has a high modulus and specific stiffness and other properties suitable for low mass, precision mirrors. The combination of a low coefficient of thermal expansion and high thermal conductivity, however, gives SiC better thermal dimensional stability than Be and low thermal expansion glass and glass-ceramics. Moreover, SiC manufacturing techniques permit SiC mirror substrates to be much more structurally efficient than Be substrates. For example, prior art SiC mirror substrates can have areal densities as low as about 4 kg/m ² . These substrates typically have a honeycomb of solid-walled reinforcing cells sandwiched between a face sheet, which supports the mirror's reflective surface, and a back sheet. The reinforcing cells have polygonal cross-sections, such as square, hexagonal, or triangular cross-sections. Several different processes, including chemical vapor deposition

and slip casting, are available to make SiC mirrors. One particularly useful slip casting method is disclosed in commonly assigned U.S. Patent No. 4,975,255 to Vivaldi et al. Although SiC mirror substrates with the honeycomb sandwich design have low areal densities, some precision mirror applications require mirror substrates that provide adequate stiffness, but are significantly lighter than the honeycomb sandwich design.

Therefore, what is needed in the industry is a dimensionally stable mirror substrate with an ultralow areal density that is suitable for supporting a precision mirror.

Disclosure of the Invention

The present invention is directed to a dimensionally stable mirror substrate with an ultralow areal density that is suitable for supporting a precision mirror.

One aspect of the invention includes an open truss structure that has a plurality of ceramic truss members integrally cast in the form of a three-dimensional, lightweight, monolithic truss.

Another aspect of the invention includes a ceramic mirror substrate that has a continuous ceramic face sheet and an integral ceramic truss extending from the face sheet and integrally cast with the face sheet. The integral truss provides sufficient stiffness to permit the mirror substrate to support a precision mirror surface.

Another aspect of the invention includes a method of making a dissolvable core suitable for slip casting a plurality of integral truss members of a monolithic ceramic truss by forming a plurality of internal passages in a suitably-shaped piece of a nonporous material. The nonporous material becomes the dissolvable core and the internal passages form the integral truss members when

filled with a liquid-containing ceramic slip. The nonporous material used to make the dissolvable core is capable of dissolving at a temperature below the freezing point of the liquid in the ceramic slip. These and other features and advantages of the present invention will become more apparent from the following description and accompanying drawings.

Brief Description of the Drawings

Figure 1 is a perspective view of a ceramic mirror substrate of the present invention that has a continuous face sheet and an integral truss with lateral and backing members.

Figure 2 is a perspective view of a ceramic mirror substrate, similar to that shown in Fig. 1, in which the backing members are replaced with an integral, continuous back sheet.

Figure 3 is a perspective view of a planar, ceramic truss structure of the present invention.

Figure 4 is a cross-section view of a tool used to make a dissolvable core that creates the internal geometry of the mirror substrates or truss structures of Figs. 1-3.

Figure 5 is a cross-section view of a mold used to make the mirror substrates or truss structures of Figs. 1-3.

Best Mode for Carrying Out the Invention

The monolithic, ceramic, truss mirror substrates of the present invention can serve as structural supports for precision mirrors that are suitable for many space- and ground-based applications. The truss by itself can be used as a structural member for applications that require light weight and stiffness.

As shown in Fig. 1, a mirror substrate 2 of the present invention has a thin, continuous, ceramic face sheet 4 that is integrally joined to a three-dimensional, ceramic truss 8. The face sheet 4 has a mirror surface 6 on one side. The truss 8 makes the mirror substrate 2 stiff enough to limit mirror surface deflections to a fraction of a micrometer. The mirror surface deflections can be caused by mechanical and thermal loads. The truss 8 can have any conventional three-dimensional configuration, such as the tetrahedral configuration shown. In this configuration, integral lateral members 10 extend from face sheet stiffening ribs 14 to form a plurality of tetrahedrons, the basic structural elements of the truss 8. The face sheet stiffening ribs 14 are integral parts of the face sheet 4. Integral backing members 12 connect the individual tetrahedrons previously described to form inverted tetrahedrons, completing the three-dimensional truss.

Many different configurations based on the Fig. 1 design are possible. To stiffen the mirror substrate 2 further, multiple layers of integral trusses can be used. For example, a truss layer of small tetrahedrons can be positioned adjacent to the face plate to provide local stiffening to the face plate. This layer can be backed up with another integral layer of the same size or larger tetrahedrons to stiffen the entire structure. As many integral truss layers in whatever sizes are needed to provide the desired stiffness can be used. Another way to further stiffen the mirror substrate 2 is to replace or supplement the backing members 12 with a thin, integrally cast, continuous, ceramic back sheet 16, as shown in Fig. 2. In another configuration, both the face sheet 4 and back sheet 16 can be omitted to form an open truss structure 18, shown in Fig. 3. The open truss structure 18 has a plurality of backing members 12 and

front members 20 that provide enough strength and stiffness for the truss structure 18 to be used for structural applications. Depending on the application, the truss structure 18 can be a planar sheet, as shown in Fig. 3, or some other useful shape, such as a cone or ellipsoid.

The areal densities of open truss structures and mirror substrates of the present invention will vary with the dimensions and construction of these articles. In general, though, these articles will have areal densities lower than comparable prior art honeycomb sandwich articles. For example, an open truss structure can have an areal density of less than about 1 kg/m ² . For mirrors with reflective surfaces of about 0.25 m (10 inches) in diameter or less, substrates with areal densities less than about 2 kg/m ² are possible. For example, a mirror with a reflective surface about 0.25 m in diameter can have a 0.5 mm (0.020 inch) thick face sheet and a 25 mm (1 inch) thick truss with members that are 0.75 mm (0.030 inch) in diameter. Such a substrate should have an overall areal density of 1.8 kg/m ² based on a face sheet with an areal density of 1.4 kg/m ² and a truss with an areal density of 0.4 kg/m ² . A comparable mirror with a honeycomb sandwich substrate designed to current manufacturing limits can have 0.5 mm thick face and back sheets and an 11.7 mm (0.461 inch) thick core of 12.7 mm (0.5 inch) square cells that have 0.5 mm walls. Such a substrate will have an overall areal density of 4.2 kg/m ² based on face and back sheets with areal densities of 1.4 kg/m ² each and a core with an areal density of 1.4 kg/m ² .

The mirror substrate 2 and truss structure 18 can be made from any ceramic material that provides the desired weight and stiffness characteristics. Suitable ceramics include SiC, silicon nitride, boron carbide, and similar materials. The preferred ceramic is SiC. Both the

mirror substrate 2 and truss structure 18 can be made by any conventional method used to make similar ceramic articles. For example, the structures of the present invention can be made by chemical vapor deposition or slip casting. Commonly assigned U.S. Patent No.

4,975,255 to Vivaldi et al. describes a suitable slip casting method. If a slip casting method is used, the mirror substrate 2 and truss structure 18 also can be made from metal powders, as is known in the slip casting art.

Slip casting uses a slip of ceramic or metal powders dispersed in a liquid, usually water, to form the desired article. The slip also may contain additional materials, such as a nucleating agent, as is known in the art. Any slip capable of forming an article with the desired properties can be used with the present invention. For example, a suitable SiC slip may contain about 40 weight percent (wt%) to about 60 wt% of a F-320 mesh SiC powder, about 30 wt% to about 45 wt% of a 1.0 μm SiC powder, about 7 wt% to about 15 wt% water, about 0.05 wt% to about 0.55 wt% sodium silicate binder, and about 0.3 wt% to about 2.5 wt% of a nucleating agent, such as urea or dimethyl sulfoxide. These materials are commercially available from numerous sources. To form the desired article, the slip is injected into a mold 36 that contains a dissolvable core 22, as shown in Fig. 5. The dissolvable core 22 has a plurality of internal passages 23 and a suitable external geometry that form the truss members 10, 12, 20 of the three-dimensional truss 8. The mold 36 forms the external geometry of the face sheet 4 and back sheet 16, if there is one.

The dissolvable core 22 can be made by creating the internal passages in a suitably-shaped piece of a nonporous material. The nonporous material may be any nonporous material that is suitable for use with a slip

casting method and that can be readily dissolved at temperatures below the freezing point of the liquid in the slip. Suitable, nonporous materials include polystyrene. A tool 24, shown in Fig. 4, that has a top plate 26, a bottom plate 28, and a spacer 30 can be used to make the dissolvable core 22. The spacer 30 separates the two plates 26, 28 and forms a closed cavity in which the core 22 is made. Holes 32 in the plates 26, 28 permit pins 34 to slide into the tool 24 to form the passages 23 in the desired tetrahedral pattern. The plates 26, 28 also have ribs 35 that form the backing members 12 and face sheet stiffening ribs 14. The core 22 can be made from polystyrene beads that are placed in the tool 24 and expanded by injecting steam into the tool, heating the tool in a water bath, or by any other means. The pins 34 are then removed from the tool 24, the tool is opened, and the core 22 is removed. The core 22 can also be made by machining the internal passages 23 into a suitably-shaped piece of the nonporous material. Regardless of how the core 22 is made, it should have internal passages 23 and a suitable external geometry so it can form all the integral structures on the mirror support 2 or truss structure 18, such as the face sheet 4, lateral members 10, backing members 12, face sheet stiffening ribs 14, back sheet 16, and front members 20. Multiple integral truss layers can be made by stacking more than one core 22 so the passages 23 connect with each other.

After the core 22 is formed, it is placed in a nonabsorbent mold 36 shown in Fig. 5. The mold 36 may be aluminum, plastic, or some other nonabsorbent material. Air in the mold 36 can be displaced by filling the mold 36 with water to which a small amount of wetting agent, such as PHOTO-FLO ^® (Eastman Kodak Company, Rochester, NY) , is added. A suitable ceramic slip is then injected

into the mold 36 through a fill port 38 to displace the water through vent ports 40 and form the mirror substrate 2, including the face sheet 4 and truss 8. To be sure the slip completely fills the mold 36 and core 22, the mold 36 may be shaken or vibrated. The mold 36 is then cooled to a temperature below the freezing point of the liquid in the slip to freeze the slip. Preferably, the mold will be cooled to a temperature below about -50°C to freeze the slip rapidly. The frozen slip 42, which contains the dissolvable core 22, is removed from the mold 36 and held at a suitable temperature, for example, below about -50"C, to permit its temperature to equilibrate. The frozen slip 42 is then immersed in a bath of solvent capable of dissolving the core 22 for a time sufficient for the core 22 to dissolve completely. If the core 22 is polystyrene, the solvent can be methylene chloride. The solvent bath should be cold enough to prevent the slip from thawing while the core 22 dissolves. The frozen slip 42, without the core 22, is removed from the solvent bath and freeze-dried to sublimate the water in the slip and form a "green body." The green body is vacuum dried to remove any remaining volatiles and sintered in an argon atmosphere in a vacuum furnace at a suitable temperature, such as about 2050°C, to form a porous sintered body. The porous sintered body is densified by exposing it to molten silicon (Si) in an argon atmosphere in a vacuum furnace at a suitable temperature, such as about 1750°C. The Si wicks into the sintered body to fill open pores. The random orientation of the sintered SiC and the uniform distribution of the Si filler make the articles of the present invention substantially isotropic.

Articles made by this method are monolithic, high stiffness, lightweight structures suitable for use as precision mirror substrates or structural supports. If

the article is a mirror substrate, the face sheet and back sheet, if there is one, can be machine ground to improve flatness to less than 0.025 mm (0.001 inch). The SiC/Si can be polished to a surface roughness between 5θA and lOoA RMS, adequate for many optical applications. If a smoother optical surface is required, a layer of silicon metal of about 0.05 mm (0.002 inch) to about 0.075 mm (0.003 inch) thick can be deposited on one of the faces. The silicon layer can be polished to a roughness of less than 5JI RMS. If desired, the polished surface can be coated with a thin layer of a reflective metal, such as gold, to enhance reflectivity.

The articles of the present invention offer several advantages over the prior art. First, the integral truss used in mirror substrates of the present invention produces articles that have lower areal densities than comparable prior art mirror substrates. Although the mirror substrates of the present invention are lighter than prior art substrates, they can be as stiff as the prior art substrates. Therefore, they are suitable for precision mirrors used in advanced space- and ground- based applications.

Second, because the truss is integrally formed, it has a uniform coefficient of thermal expansion that makes it dimensionally stable. Moreover, the SiC/Si material used to make the trusses is isotropic. By contrast, the adhesive or mechanical joints used to assemble prior art trusses that are sometimes used to support mirrors can create thermal expansion mismatches between the truss members and joints. As a result, prior art trusses distort when exposed to temperature changes. In addition, adhesive and mechanical joints make the prior art trusses anisotropic, which can make then unsuitable for precision mirror applications. Moreover, the mechanical joints that are sometimes used on prior art

trusses can increase the weight of the trusses, making them less desirable for weight-critical applications.

Third, the integral truss requires less labor to make than prior art trusses because it is cast in a single piece. By contrast, many prior art trusses require extensive labor to assemble them piece by piece.

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

I claim: