FABRICATION OF RF-TRANSPARENT CERAMIC COMPOSITE STRUCTURES BY COMPOSITIONAL GRADING

Abstract

A method is provided and suggests grading of a CMC (Ceramic Matrix Composite) structure as a function of dielectric constant by altering the solid loading (SL) ratio of the individual composite layers. The slurry is applied either by impregnation into the ceramic fabrics or by coating on ceramic fibers. The final structure is prepared by piling up prepregs or weaving ceramic fibers with specific SL ratio, drying and firing.

Claims

1. A method for making dielectrically-graded ceramic matrix composite structures exhibiting broadband radio frequency (RF) transparency, comprising the process step of preparing single layers each exhibiting a dielectric constant through ceramic fabrics and fibers impregnated by ceramic slurries of a solid loading (SL) ratio, wherein the SL ratio varies between 10-90% by weight in a broader range and between 30-80% by weight in a tighter process window; wherein one type of ceramic slurry material is configured for all of the layers, wherein the one type of ceramic slurry material assures the CTE-compatibility between the layers.

2. The method according to claim 1, wherein the ceramic fabrics woven from ceramic fibers and ceramics fabrics are impregnated by ceramic slurries of an SL ratio.

3. The method according to claim 1, wherein the ceramic slurry comprises quartz, silica, alumina, mullite, a mixture of alumina, boric oxide and silica, a mixture of alumina and yttria, zirconia and as such dielectric oxide ceramics.

4. The method according to claim 1, wherein the coated ceramic fiber comprises E-glass, quartz, silica, alumina, mullite, a mixture of alumina, boric oxide, silica, a mixture of alumina and yttria, zirconia and as such dielectric oxide ceramic fibers.

5. The method according to claim 1, wherein each impregnated fabric layer is pressed.

6. The method according to claim 1, wherein each layer of the dielectrically-graded ceramic matrix composite structure is prepared by weaving the ceramic slurry impregnated ceramic fiber around cylindrical or tubular molds for a fabrication of cylindrical or conical objects in a wet state, the dielectrically-graded ceramic matrix composite structure is then dried and fired.

7. Graded ceramic matrix composite structures comprising radomes produced by the method according to claim 1.

8. The graded ceramic matrix composite structures according to claim 7, wherein in the method, the ceramic fabrics woven from ceramic fibers and ceramics fabrics are impregnated by ceramic slurries of an SL ratio.

9. The graded ceramic matrix composite structures according to claim 7, wherein in the method, the ceramic slurry comprises quartz, silica, alumina, mullite, a mixture of alumina, boric oxide and silica, a mixture of alumina and yttria, zirconia and as such dielectric oxide ceramics.

10. The graded ceramic matrix composite structures according to claim 7, wherein in the method, the coated ceramic fiber comprises E-glass, quartz, silica, alumina, mullite, a mixture of alumina, boric oxide, silica, a mixture of alumina and yttria, zirconia and as such dielectric oxide ceramic fibers.

11. The graded ceramic matrix composite structures according to claim 7, wherein in the method, each impregnated fabric layer is pressed.

12. The graded ceramic matrix composite structures according to claim 7, wherein in the method, each layer of the dielectrically-graded ceramic matrix composite structure is prepared by weaving the ceramic slurry impregnated ceramic fiber around cylindrical or tubular molds for a fabrication of cylindrical or conical objects in a wet state, the dielectrically-graded ceramic matrix composite structure is then dried and fired.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0030] FIG. 1 shows the relationship between the density of slip cast fused silica samples with different solid loading ratios sintered (all samples are sintered at the same temperature).

[0031] FIGS. 2A-2C show the simulation of insertion losses (s21) of virgin, A-sandwich and graded silica. The losses over the entire frequency range is below 1 dB for the graded silica (red dotted line represents the 1 dB loss level).

DETAILED DESCRIPTION OF THE EMBODIMENTS

[0032] Ceramics are widely used building blocks of RF-transparent airborne components such as missile radomes, nosecones, RF caps and windows moving at super/hypersonic velocities. This does not preclude alternative material options such as organic/inorganic/filler-added polymers applicable in this regime. However, ceramics possess inherently strong intermolecular bonds giving them significantly improved mechanical strength, chemical and thermal stability and abrasion resistance. Moreover, they can be used both in oxidizing and reducing atmospheres depending on their chemistry. These are attractive features sought especially when the surface temperature of the aforementioned structures exceeds 1.000 C. under severe environmental conditions such as chemical attack, rain/dust/sand erosion, etc.

[0033] The traditional ceramic manufacturing route consists of well-known steps: raw material preparation for processing, shaping and firing followed by post processes such as machining (grinding, polishing, lapping) and alternatively by coating to further extend material's endurance against thermal, abrasive and environmental impacts. Among several techniques, slip casting and glass melt spinning are the most-widely used to manufacture big ceramic structures such as missile radomes operating in the super/hypersonic regime. The former technique relies on the capillary effect to compact and shape the ceramic powder dispersed in an aqueous slip when placed in a gypsum mold. The latter uses hot molding and/or hot spinning to shape the molten glass-ceramic poured on a spinning mold. Both techniques have been used for manufacturing of commercial missile radomes for decades. There are advantages and disadvantages of each technique. But from a broader perspective, both techniques have significant limitations: [0034] The monolithic bulk ceramic is inherently fragile. Fracture is catastrophic (immediate and complete) [0035] Shaping process is limited. Complex structures with low tolerances are achieved only by post processes. [0036] Process yield in both techniques is quite low. The production yield for both techniques is approximately 40-50%. [0037] Multi-layering for broadband characteristic is practically impossible due to very finite layers of high dielectric constant materials, which need to be integrated to the thicker low dielectric constant layers. [0038] Physical, chemical, thermal and thermo-mechanical (CTE) mismatch between different layers lead to delamination, fracture or malfunctions even if the extremely thin high dielectric constant layer is attached to the thicker low dielectric constant layer.

[0039] O/O CMC's (Oxide/Oxide CMC) can address the aforementioned shortcomings of monolithic bulk ceramics. These materials are composed of an oxide fiber (network) and an oxide matrix. The traditional oxide ceramic fiber material is alumina (Al.sub.2O.sub.3). However, alumina suffers grain growth and hence, creeps at high temperatures. Therefore, it is usually mixed with SiO.sub.2 and B.sub.2O.sub.3 to delay/prevent creep behavior. Another motive to mix these oxides with Al.sub.2O.sub.3 is to improve the oxidation and the alkaline resistance of the composite [2-4]. The matrix, which is the other part of the composite, is an oxide ceramic such as alumina, silicate, mullite, zirconia compatible with the ceramic fiber. It is prepared as a slurry, which is a mixture of the ceramic powder, solvent, surfactant, binder and similar functional components. Each of these ingredients has a specific function; the ceramic powder is the functional element giving the physical, thermal, mechanical and electrical properties of the composite together with the fibers; the solvent is the carrier of the powder and it determines the rheology of the mixture by dissolving the binder, whereas the surfactant enhances the reactivity of the powder by modifying its surface properties.

[0040] The ceramic powder represents the solid content of the slurry and it forms the matrix of the composite. The other solids in the slurry are additives oxidized at much lower temperatures. Therefore, the SL ratio is the ceramic powder weight percent or ratio in the slurry. SL ratio is a critical slurry parameter: When the powder is homogenously dispersed in the slurry, the number of particle to particle contacts per unit volume is higher for a slurry with higher SL. This indicates an increase in the green density of the material, which also improves the sintered density due to the enhanced necking and material diffusion through particle contacts during sintering.

[0041] Density and SL relation of slip cast fused silica (SCFS) samples prepared at 50, 60, 70 and 80 percent SL ratios fired at the same sintering temperature is presented in FIG. 1. The strong correlation between the two parameters (R 2=0,9958) is evident. The relationship between the SL ratio and the dielectric constant is directly proportional but relatively supressed; the effect of 30% variation in SL ratio results in a change of 10% only in dielectric constant (Table 1). Moreover, the tg at 60% SL ratio exhibits an increased value, which is ascribed to possible contamination during processing. To sum up, the major idea behind dielectric grading disclosed in this work is accomplished by preparing the single layers of the composite with a specific SL ratio.

TABLE-US-00001 TABLE 2 SL Ratio, Density Dielectric Constant and Loss of SCFS SL Ratio Density (g/cm.sup.3) tg 50 1.71 3.02 0.001 60 1.80 3.04 0.005 70 1.86 3.34 0.001 80 1.94 3.40 0.001

[0042] The slurry can be prepared from oxide ceramics such as Al.sub.2O.sub.3, SiO.sub.2, mixture of Al.sub.2O.sub.3 and SiO.sub.2 mixture of Al.sub.2O.sub.3, SiO.sub.2, B.sub.2O.sub.3, ZrO.sub.2, mixtures of Al.sub.2O.sub.3, ZrO.sub.2, mixtures of Y.sub.2O.sub.3 and Al.sub.2O.sub.3, etc. The binary or ternary compositions of these and other metal oxides can be prepared by mixing the constituents at different ratios to optimize the material characteristics further. The purity, the particle size and distribution, the specific surface area and the morphology of the ceramic powder are critical factors, which directly impact the sintering behavior and the dielectric response of the composite. The SL ratio of the slurry should be selected in a specific range; it should neither be too low leading to an extremely weak inter particle bonding nor too high resulting in a highly segregated microstructure. Usually, 10% to 90% by weight should work with appropriate additives, whereas, 30% to 80% is a safer range for the ceramic systems discussed.

[0043] The starting point for dielectric grading is preparation of slurries with different SL ratio. The composite structures can be fabricated by using ceramic fiber networks (fabrics) or continuous ceramic fiber bundles. For planar composites, ceramic fabrics impregnated with slurries of desired dielectric constant are piled up together in wet state, pressed, dried and fired. Alternatively, the bundles of ceramic fibers can be immersed into the slurry baths with specific dielectric constant, dried, wrapped around the cylindrical molds, removed from the mold and fired. The process of piling up of fabrics or wrapping of fibers can be repeated with as many different slurries (with specific SL ratio) as desired to fulfill the RF design. It is important to re-mention that the slurry material discussed here is of one material only (like silica or alumina) and the dielectric constant of this single material is tuned by varying its SL ratio per composite layer.

[0044] Dielectric grading of an O/O CMC structure by this technique leads to an improved broadband characteristic compared to sandwich structures with dissimilar materials. FIGS. 2A-2C show the insertion loss (s21) parameter simulation of 3 silica samples: The first sample is silica with 90% relative density, whereas the second one is an A-type sandwich composed of silica as low and another material as high dielectric constant (3 times of silica) material. The thickness of silica for this design is approximately 5 times that of the high dielectric constant skin layer. The third design is composed of equivalently-thick 4 silica layers, each layer varying in density by approximately 10%. The reflection loss for these 3 structures is simulated between 0,50-40 GHz. As it is clearly observed in FIGS. 2A-2C, the graded silica shows a loss less than 1 dB over the entire frequency spectrum, whereas the sandwich and the virgin samples exhibit losses over 1 dB at certain frequency intervals.

REFERENCES

[0045] 1 D. C. Chang, Comparison of Computed and Measured Transmission Data for the AGM-88 HARM Radome, 1993, MSc Thesis, Naval Postgraduate School. [0046] 2 B. Klauss, B. Schawallar, Modern Aspects of Ceramic Fiber Development, 2006, Advances in Science and Technology, Vol. 50, 1-8. [0047] 3 B. Clauss, Fibers for Ceramic Matrix Composites, Chapter 1, Ceramic Matrix Composites. Edited by Walter Krenkel, WILEY-VCH Verlag GmbH & Co. KGaA, 2008, 1-20. [0048] 4 Nextel Application Brochure, 1-16.