THREE-DIMENSIONAL PRINTING OF MULTILAYER CERAMIC MISSILE RADOMES BY USING INTERLAYER TRANSITION MATERIALS

Abstract

Production of multilayered ceramic missile radomes with wide frequency band and high electromagnetic permeability through three-dimensional printing technology and the use of glass inter-layer materials to minimize defects caused by thermo-mechanical incompatibility of adjacent layers during sintering are provided. The three dimensional printing of the multilayered ceramic missile radomes provide an automated, operator-independent and repeatable manufacturing technique to produce wide band ceramic missile radomes.

Claims

1. A method using 3D printing technology to produce multilayer ceramic/glass-ceramic radomes with CTE-compatible layers by the use of inter-layer transition materials providing an electromagnetic permeability in a wide frequency band, comprising the steps of: (i) preparing a feed material to print by mixing predetermined compositions of at least a ceramic/glass-ceramic powder selected for each layer with organic binders enhancing a particle packing and by filling the each layer into single containers of a multi-nozzle 3D printing machine, (ii) repeating step (i) for an inter-layer transition material, wherein the inter-layer transition material is a glass or other glassy materials. (iii) preparing a computer-aided design file of a three-dimensional model of a desired radome and transferring the computer-aided design file to the multi-nozzle 3D printing machine, (iv) initiating a multi-nozzle extrusion printing process in the multi-nozzle 3D printing machine in accordance with a printing order of ceramic and transition layers, (v) debinding a green body printed in the ceramic and transition layers, (vi) machining the green body to bring an object closer to a near-net shape after firing, (vii) sintering the green body printed.

2. The method according to claim 1, further comprising the step of using glass transition elements to prevent cracks caused by Coefficient of Thermak Expansion (CTE) mismatch between printed ceramic/glass-ceramic layers.

3. The method according to claim 1, further comprising the step of machining the green body after step (v).

4. The method according to claim 1, wherein a debinding process is performed at temperatures below 500° C. and at heating rates of less than 1° C./min for removal of the organic binders.

5. The method according to claim 1, wherein the ceramic and transition layers are selected from ceramic/glass-ceramic materials to form a multilayered radome with a sandwich structure, wherein inner and outer layers of the multilayered radome are thin and have a high dielectric constant, and a middle layer of the multilayered radome is thick and has a relatively low dielectric constant.

6. The method according to claim 1, wherein the ceramic and transition layers are selected from ceramic/glass-ceramic materials to form a multilayered radome with a sandwich structure, wherein inner and outer layers of the multilayered radome are thick and have a low dielectric constant, and a middle layer of the multilayered radome is thin and has a relatively high dielectric constant.

7. The method according to claim 1, wherein the ceramic and transition layers are selected from ceramic/glass-ceramic materials to form a multilayered radome with a functionally-graded material structure, wherein a density/dielectric constant of each layer of the multilayered radome are vary.

8. The method according to claim 1, wherein the ceramic and transition layers are selected from ceramic/glass-ceramic materials to form a multilayered radome, wherein each layer of the multilayered radome is selected from different segments vertically according to a position of an RF seeker head.

9. The method according to claim 1, wherein ceramic/glass-ceramic materials are selected from the group consisting of SiO.sub.2 (Silicon dioxide), Si.sub.3N.sub.4 (Silicon nitride), RBSN (Reaction Bonded Silicon Nitride), Al.sub.2O.sub.3 (Aluminum oxide), SiAlON (Silicon alumina nitride), LAS (Lithium Aluminum Silicate), and MAS (Magnesium Aluminum Silicate).

10. The method according to claim 9, wherein the LAS is a glass-ceramic material composed of Lithium-Aluminum-Silicate oxides in varying proportions around a principal composition 1Li.sub.2O.sub.3.1Al.sub.2O.sub.3.2SiO.sub.2.

11. The method according to claim 9, wherein the MAS is a glass-ceramic material composed of Magnesium-Aluminum-Silicate oxides in varying proportions around a principal composition 2MgO.2Al.sub.2O.sub.3.5SiO.sub.2.

12. The method according to claim 1, wherein glass inter-layer elements are selected from the group consisting of silicate glass oxides, borate glass oxides, compositions of the silicate glass oxides with modifying oxides from groups 1A and 2A of the periodic table, and intermediate oxides.

13. The method according to claim 12, wherein the silicate glass oxide is SiO.sub.2 (Silicon dioxide).

14. The method according to claim 12, wherein the borate glass oxide is B.sub.2O.sub.3 (Boron trioxide).

15. The method according to claim 12, wherein the modifying oxides are Na.sub.2O (Sodium oxide), K.sub.2O (Potassium oxide), Li.sub.2O (Lithium oxide), CaO (Calcium oxide), MgO (Magnesium oxide), BaO (Barium oxide) or PbO (Lead oxide).

16. The method according to claim 12, wherein the intermediate oxides are Al.sub.2O.sub.3 (Aluminium oxide), Bi.sub.2O.sub.3 (bismuth III oxide), or TeO.sub.2 (Tellurium dioxide).

17. The method according to claim 12, wherein the glass inter-layer elements are PbO—B.sub.2O.sub.3—SiO.sub.2 (PBS), ZnO—B.sub.2O.sub.3 (ZB), BaO—ZnO—B.sub.2O.sub.3 (BZB), La.sub.2O.sub.3—B.sub.2O.sub.3—ZnO (LBZ), BaO—Al.sub.2O.sub.3—SiO.sub.2 (BAS), Li.sub.2O—B.sub.2O.sub.3—SiO.sub.2 (LBS), Li.sub.2O—B.sub.2O.sub.3—SiO.sub.2—CaO—Al.sub.2O.sub.3 (LBSCA), or BaO—B.sub.2O.sub.3—SiO.sub.2 (BBS).

18. A multilayer ceramic and glass-ceramic radome produced by the method according to claim 1.

19. The multilayer ceramic and glass-ceramic radome according to claim 18, wherein the multilayer ceramic/glass-ceramic radome is used in missile radomes operating at super and hypersonic speeds and in a wide/narrow frequency band, or used for a high-speed aircraft and/or components of the high-speed aircraft, or in electromagnetic windows and caps.

Description

BRIEF DESCRIPTION OF THE FIGURES

[0052] FIG. 1 is a general view of the typical missile and radome structure.

[0053] FIG. 2A is the cross-section view of the A-sandwich radome structure that can be produced by three-dimensional printing.

[0054] FIG. 2B is the cross-section view of the B-sandwich radome structure that can be produced by three-dimensional printing.

[0055] FIG. 2C is a cross-sectional view of the FGM (Functionally Graded Material) radome structure that can be produced by three-dimensional printing. (The properties of material A (density, dielectric constant) gradually change in the thickness direction (A′, A″) accordingly.)

[0056] FIG. 2D Is a cross-sectional view of a multi-segment (A, B, C) radome structure that can be produced by three-dimensional printing.

REFERENCE NUMBERS

[0057] 1 Missile [0058] 10 Radome [0059] 20 Radar [0060] 30 Flange [0061] A, ceramic/glass-ceramic radome material with a dielectric constant higher than B [0062] A′, ceramic/glass-ceramic radome material with different dielectric constant/density from A [0063] A″, ceramic/glass-ceramic radome material with different dielectric constant/density from A or A′ [0064] B, ceramic/glass-ceramic radome material with a dielectric constant lower than A [0065] C, ceramic/glass-ceramic radome material with a dielectric constant different from A or B

DETAILED DESCRIPTION OF THE INVENTION

[0066] In this detailed description, three-dimensional printing of the multilayer ceramic missile radomes of the invention are explained only for a better understanding of the subject matter and without any restrictive effect.

[0067] FIG. 1 shows a typical missile (1) image showing the radar (20) protected by a radome (10) and the flange (30) structure it is connected to. The ceramic radome (10) is one of the most critical components of the missiles flying at high speeds (1). This is mainly due to the temperatures resulting from the aerodynamic frictions that can rise up to 1000° C. in the nose of the radome (10) in a very short time, and to the acceleration loads resulting from the sudden maneuvers over the radome (10). To protect the radar and the electronic circuitry and to guarantee the desired RF performance of the missile, the radome (10) is produced by using the optimum design, materials, and manufacturing technique.

[0068] Slip casting is a standard production technique used for making large, asymmetric and complex designed ceramics which cannot be prepared by molding, extrusion, pressing, or hot pressing. For this reason, it is often used in the production of ceramic missile radomes. In this technique, the ceramic powder is first prepared in an aqueous solution with optimized rheology, which is then poured into plaster molds. When the water of the slurry is filtered from the porous gypsum, the ceramic accumulates on the walls of the gypsum and reaches a certain thickness. After a period of time, which is determined by empirical methods, the cast ceramic is removed from the mold, dried and then sintered. Following this process, machining and polishing operations are performed on and under the radome surface in order to attain the desired geometric tolerances.

[0069] Missile radomes are also manufactured using LAS (Lithium Aluminum Silicate) and MAS (Magnesium Aluminum Silicate) based glass ceramics. These materials are prepared by melting, casting and then firing of the glass. The firing process consists of nucleation and crystallization steps through which, the amorphous glass is gradually converted to the crystalline structure by devitrification.

[0070] In both methods of radome production, the control of the technical parameters is difficult, efficiency is limited, and tool/process losses in post-casting machining operations are high. For these reasons, three-dimensional printing is emerging as a suitable technique for radome production at high efficiency and yield. Through this technique, it is possible to develop multilayer sandwich structures that provide high electromagnetic permeability in a wide frequency band. Accordingly, [0071] 1. embodiments comprising the inner and outer layers of which materials are thin and having high dielectric constant (A); and comprising the middle layer of which material is thick and having relatively low dielectric constant (B) (FIG. 2A), [0072] 2. embodiments comprising the inner and outer layers of which materials are thick and having low dielectric constant (B); and comprising the middle layer of which material is thin and having relatively high dielectric constant (A) (FIG. 2B), [0073] 3. embodiments being formed functionally-graded materials of which the density/dielectric constant properties of each layer vary (A, A′, A″) (FIG. 2C), [0074] 4. embodiments being formed vertically different segments (A, B, C) materials according to the position of the RF seeker (FIG. 2D),
can be developed with three-dimensional printing.

[0075] The points that the three-dimensional printing of ceramic missile radomes are basically separated from the slip casting technique can be summarized as follows: [0076] The design is transferred directly from CAD (Computer-aided design) file to the printer without the need for any tools. For this reason, changes and improvements to the product are quickly performed on the computer. This provides additional advantages in the assembly of the radome with other components (flange, etc.). [0077] It is an automated process and is independent of the operator. It is therefore highly reproducible. [0078] The costly and time-consuming design and production of the mold/negative-mold components are not required. [0079] According to the nature of the binder used, it allows the printed substrate to be machined in the green state, in other words before sintering, which is much faster to accomplish compared to machining of the sintered structure. In this way, the product is obtained with tolerances close to the desired values after firing. In this way, it provides a production method in which the additive and subtractive processes can be used together. [0080] It is an ideal production method to produce complex shapes such as pits, protrusions, recesses. [0081] A material can be printed on top of another material using a multi-nozzle tip. [0082] It provides mass customization by printing the multiple designs of the radome on the same device platform simultaneously. Accordingly, this allows for fast testing of different product designs (as a dummy or in final version). [0083] Time to market has been shortened. [0084] There is no loss of material properties in comparison with conventionally manufactured products. [0085] The energy and material loss are minimized, and waste is reduced.

[0086] When considered as a production method, the highest resolution is achieved by lithography technique in the three-dimensional printing of ceramics. In this method, the radome material, that is a ceramic or glass ceramic powder, is mixed with a photocurable organic binder at a certain proportion. The determination and optimization of the rheology of the mixture is an important process. The binder in the mixture has two basic functions: (1) Keeping ceramic powder and organic binders together; (2) converting the mixture into solid “green body” consistency by the photo-initiator in its composition. The most important parameters in the forming process are the thickness of the printed layer, the intensity of the light source used and the duration of exposure to light.

[0087] The production process is initiated as the energy from the light source activates the photo initiator in the binder. In this way, new radicals are formed directly or through the reaction with other molecules. This process is called photo polymerization. After each layer is printed, photo-curing is applied, and the process is repeated until the print object is complete. The object printed in layers becomes ready for sintering after being dried.

[0088] The sintering process is one of the most fundamental steps in three-dimensional printing. The debinding and degassing of the organic binder in the structure is performed at low temperatures (<500° C.) and at sensitive heating rates (<1° C./min). The purpose of doing so is to prevent cracks that may occur during debinding process. For this reason, analytical methods such as dilatometry, TGA (Thermo Gravimetric Analysis) and DSC (Differential Scanning Calorimetry) must be used to determine the critical processing temperatures and heating profile. The other critical temperatures in firing are the sintering temperature, duration, and atmosphere in which ceramic material gains its properties. At this temperature the material reaches high density and the resulting microstructure determines the properties that the material will have in application. Although the sintered material is in the “near net shape” dimensions, it is forwarded to machining to comply with the final tolerances.

[0089] In the open literature, three-dimensionally printed materials using the Lithography-based Ceramic Manufacturing (LCM) method, are Al.sub.2O.sub.3 (Aluminum oxide), ZrO.sub.2 (Zirconium dioxide), and Si.sub.3N.sub.4 (Silicon nitride). It is stated that these materials made of high purity raw materials have over 99% of their theoretical densities and their mechanical and electrical properties are comparable to or even superior to those of the same materials produced by other methods. However, these are relatively small structures.

[0090] Considering the size of ceramic missile radomes, the lithography technique developed for smaller objects, is expected to be a more comprehensive solution only in the medium/long term. The production of such structures with extrusion is a more appropriate approach for prototyping large ceramic radomes, despite the lower resolution of the print. In this technique, ceramic slurry with optimized rheology is printed three dimensionally with a semi-automatic system from the nozzle. The object is then dried and sintered. Multiple extrusion nozzle can be used for printing multiple materials on top of each other. The printing device is fed with special cartridges or tubes for each desired material. Each cartridge/tube can be connected to a single nozzle and activated by applying high pressure by the machine according to the printing order of the layers.

[0091] The greatest obstacle to the printing of multilayer ceramic structures is the formation of delamination and cracks between the layers due to the mismatch of the thermal expansion coefficients. This problem is often seen in multilayer ceramic structures such as capacitors, piezo-actuators, ceramic modules, fuel cells and thick-film sensors that are simultaneously fired at elevated temperatures.

[0092] Molten SiO.sub.2 (Silicon dioxide), Si.sub.3N.sub.4 (Silicon nitride), RBSN (Reaction Bonded Silicon Nitride), Al.sub.2O.sub.3 (Aluminum oxide), SiAlON (Silicon alumina nitride), LAS (Lithium Aluminum Silicate) (1Li.sub.2O.sub.3.1Al.sub.2O.sub.3.2SiO.sub.2), MAS (Magnesium Aluminum Silicate) (2MgO.2Al.sub.2O.sub.3.5SiO.sub.2) and similar materials are the examples of ceramic/glass ceramic radome materials discussed within the present invention. To ensure broadband high electromagnetic permeability, these materials must be printed as multilayers. (FIGS. 2A, 2B, 2C, 2D). Thermo-mechanical compatibility between layers during sintering is possible by using transition materials (buffers) those do not impair electromagnetic, thermal, mechanical, thermo-mechanical performance requirements expected from the radome. The formulation of these materials, material purity, particle size and distribution, form factors (powder, wax, plate), designs (single/multi-line printing, different patterns), print thicknesses, temperature, humidity, corrosion resistance should be carefully optimized in accordance with the matrix material.

[0093] The present invention involves the use of glass as a transition material compensating the mismatch of CTE (Coefficient of Thermal Expansion) between ceramic layers. Glass is an effective transition material as an inter-layer material since it can be formulated and prepared in different properties and form factors (powder, paste, melt) to adopt the neighboring layers.

[0094] The glasses used in RF applications are produced by mixing the network former oxides with network modifier oxides. The network former oxides are SiO.sub.2 (Silicon dioxide-silicate glass) with high melting point and viscosity, and B.sub.2O.sub.3 (Boron trioxide-borate glass) with low viscosity. In addition, network modifier oxides from 1A and 2A groups of the periodic table [Na.sub.2O (Sodium oxide), K.sub.2O (Potassium oxide), Li.sub.2O (Lithium oxide), CaO (Calcium oxide), MgO (Magnesium oxide), BaO (Barium oxide)] and PbO (Lead oxide) participate into SiO.sub.2, into B.sub.2O.sub.3 or into the composition of both oxides together. The modifier oxides facilitate the structure to be opened up by creating oxygen sites that are not connected to the glass, thereby increasing CTE and ionic conductivity at the same time. Apart from these, there is also an oxide group in the glass composition called intermediate oxides (Al.sub.2O.sub.3 (Aluminium oxide), Bi.sub.2O.sub.3 (bismuth(III) oxide), TeO.sub.2 (Tellurium dioxide)) which contribute as a network former or as a network modifier according to the composition of the glass.

[0095] By using the glasses in the aforementioned groups, unlimited number of new glass compositions with attractive features can be obtained. The important thing is the compatibility of the selected glass with the thermo-mechanical and chemical properties of the bulk radome layers to be printed. It is also preferred that the glass has a small CTE value for its high thermal shock resistance. Table 1 shows the variation of the values of T.sub.s (Softening Temperature), CTE, dielectric constant (ε), dielectric loss (tg δ) for PbO—B.sub.2O.sub.3—SiO.sub.2 system, as a function of Pb—B—Si oxides [1].

[0096] Apart from this, by combining the components in the ZnO—B.sub.2O.sub.3, BaO—ZnO—B.sub.2O.sub.3, La.sub.2O.sub.3—B.sub.2O.sub.3—ZnO, SiO.sub.2—BaO—Al.sub.2O.sub.3, Li.sub.2O—B.sub.2O.sub.3—SiO.sub.2, Li.sub.2O—B.sub.2O.sub.3—SiO.sub.2—CaO—Al.sub.2O.sub.3, BaO—B.sub.2O.sub.3—SiO.sub.2 glass groups in different compositions, new glasses compatible with the bulk radome layers can be produced [1]. The glass should be developed carefully considering its composition, thickness, shape, and its impact on environment.

TABLE-US-00001 TABLE 1 Material Properties Based on The Glass Composition Material Ts CTE tg δ (Vol. %) (° C.) (ppm/K) ε (@ 1 MHz) PbO—B.sub.2O.sub.3—SiO.sub.2 (70:20:10) 348 −155 19.57 0.020 PbO—B.sub.2O.sub.3—SiO.sub.2 (60:20:20) 312 −124 15.32 0.018 PbO—B.sub.2O.sub.3—SiO.sub.2 (50:40:10) 408 −98 13.78 0.012 PbO—B.sub.2O.sub.3—SiO.sub.2 (40:40:20) 449 −69 12.74 0.009 PbO—B.sub.2O.sub.3—SiO.sub.2 (40:20:40) 442 −31 12.11 0.010 PbO—B.sub.2O.sub.3—SiO.sub.2 (30:60:10) 492 −15 9.06 0.011

[0097] Glass ceramic radome materials can be printed in multiple layers using a suitable glass or by changing the proportions of the components in their composition (without requiring any extra glass). For example, Li.sub.2O—Al.sub.2O.sub.3—SiO.sub.2 based LAS glass ceramic can be prepared by using MgO, ZnO, K.sub.2O, Na.sub.2O, P.sub.2O.sub.5, TiO.sub.2, ZrO.sub.2 and As.sub.2O.sub.2,5 additions at different ratios, or can be developed with different physical, mechanical, thermal, electrical properties only by varying the process parameters in nucleation and crystallization processes. It is possible to print the layers from multiple extruder nozzles by changing the glass composition to produce either a functionally-graded structure (FIG. 2C) or a segmented one (FIG. 2D).

[0098] In the light of previous explanations, the invention is a method using 3D printing technology to produce multilayer ceramic/glass-ceramic radomes providing high electromagnetic permeability in a wide frequency band, comprising the steps of; [0099] preparing the feed material to print by mixing the predetermined compositions of at least a ceramic/glass-ceramic powder selected for each layer with adequate organic binders that enhances particle packing and by filling each mixture (layer) into the single containers (cartridge, tube, etc.) of the multi-nozzle 3D printing machine, [0100] repeating step (i) for inter-layer transition material, which is stated as glass in here, but can be extended to other glassy materials. [0101] preparing a computer-aided design file of the three-dimensional model of the desired radome and transferring the file to the 3D printing machine, [0102] initiating multi-nozzle extrusion printing process in the 3D printing machine in accordance with the printing order of the ceramic and transition layers, [0103] drying of the green body printed in layers, [0104] machining of the green body to bring the object closer to the near-net shape after firing, [0105] sintering of the printed green body.
and involves the use of glass inter-layer elements to prevent cracks caused by CTE (Coefficient of Thermal Expansion) mismatch between said layers.

[0106] The printing of multilayer ceramic/glass-ceramic radomes by the multi-nozzle extrusion process mentioned in this invention and the use of glass inter-layer elements to prevent cracks caused by CTE mismatch between layers can be considered and improved for different applications. Missile radomes operating at super and hypersonic speeds and in the wide/narrow frequency band, constructions required for high-speed aircraft or their components, electromagnetic windows and caps can be given as examples.

REFERENCES

[0107] [1] M. T. Sebastian, H. Jantunen, Low Loss Dielectric Materials for LTCC Applications: A Review, International Materials Reviews, 2008, vol. 53 [2], 57-90. [0108] [2] M. I. Ojovan, Viscosity and Glass Transition in Amorphous Oxides, Advances in Condensed Matter Physics, 2008, [817829], 1-24.