A method for preparing a high-precision low-line-expansion thermal metamaterial spatial mirror

Abstract

This invention relates to a method for fabricating a high-precision, low-linear-expansion thermal metamaterial space mirror. The method utilizes 3D design software combined with finite element simulation software for structural optimization design. Based on the designed structural model, 3D printing is employed, along with sintering and magnetron sputtering, to achieve the integrated fabrication of a metamaterial structure with high surface accuracy, high resolution, and a low coefficient of linear expansion space mirror. The structural parameters, printing parameters, sintering process, and film thickness control involved in this invention have all undergone repeated experiments and optimizations, effectively ensuring the dimensional accuracy and printing speed of the structure, the structural stability of the sintered sample, and the surface accuracy after polishing.

Description

Technical Field

[0001] This invention belongs to the field of additive manufacturing technology, and particularly relates to a method for preparing a high-precision, low-linear-expansion thermal metamaterial space mirror. Background Technology

[0002] As a crucial precision component of spaceborne antennas, space reflectors ensure satellites can perform remote control and telemetry, satellite-to-ground data transmission, and inter-satellite communication functions. They are often used in the non-uniform temperature fields of space. To prevent the high-precision structure from losing its original design accuracy under drastic temperature changes, to reduce the separation of multiple structures due to uncoordinated thermal expansion that could compromise structural integrity, and to achieve higher reflectivity and higher resolution, the reflector structure needs to have a low coefficient of thermal expansion or even zero expansion.

[0003] Chinese patent CN 112277123 A discloses a method for producing low thermal expansion and high modulus ceramic thermal metamaterials. The method utilizes 3D printing technology to obtain various thermal metastructures with low coefficients of thermal expansion. Furthermore, silicon carbide substrates, silicon carbide whiskers, and nepheline powder introduced through chemical vapor deposition, vacuum impregnation, and impregnation pyrolysis further reduce the coefficient of thermal expansion of the structures and enhance their mechanical properties. However, these metamaterial structures lack large-area continuous surfaces, making them difficult to apply to space mirrors. The literature “Gdoutos E., et al. Thin and Thermally Stable Periodic Metastructures. Experimental Mechanics. 2013; 53(9):1735-1742” discloses a thin-film superstructure composed of a bimetallic system. The thermal expansion coefficient of the superstructure is tuned by adjusting the ratio of the thermal expansion coefficients of the bimetallic system. The bimetallic superstructure, composed of an aluminum core and an outer titanium frame, exhibits a low coefficient of thermal expansion. However, this bimetallic system cannot be molded as a single piece, and problems such as stress concentration and unstable thermal properties are prone to occur at the connection points between the core and the frame, which limits its actual production and application. Summary of the Invention

[0004] To overcome the shortcomings of existing technologies, this invention proposes a method for fabricating a high-precision, low-linear-expansion thermal metamaterial space mirror. By optimizing the three-dimensional structure, the special thermal properties are tuned, and additive manufacturing technology is used to achieve the integrated fabrication of the metamaterial, ultimately obtaining a space mirror with high precision, low linear expansion coefficient, and good polishing performance.

[0005] The above-mentioned objective of this invention is achieved through the following technical solution:

[0006] A method for fabricating a high-precision, low-linear-expansion thermal metamaterial space mirror includes the following steps:

[0007] Step 1: 3D Structural Design. Using modeling software, the minimum repeating elements of the superstructure are initially designed, generating and exporting a 3D model, typically in STP format. This 3D model is then imported into finite element simulation software. Material properties are set, boundary conditions are input, meshing is performed, and simulation calculations are conducted to obtain the thermal strain of the minimum repeating elements under temperature loads. By adjusting the structural parameters of the minimum repeating elements, a structure with a theoretically low coefficient of linear expansion is obtained. The minimum repeating elements are arranged in an array along the x and y directions to obtain a 3D model of a metamaterial space mirror with dimensions ranging from 10 to 500 mm, based on practical application requirements.

[0008] The modeling software includes, but is not limited to: SolidWorks, CreO, etc.

[0009] The superstructure includes, but is not limited to, polygonal topologies and torsional topologies. In the case of a polygonal topology, the minimum repeating unit structural parameters include the cell diagonal length parameter L1, the cell gap width parameter L2, the cell gap length parameter L3, the maximum length of the internal hole parameter L4, and the triangular gap position parameter L5, where L2 = 0.0005–0.002L1, L3 = 0.25–0.5L1, L4 = 0.25–0.45L1, and L5 = 0.25–0.45L1. In the case of a torsional topology, the minimum repeating unit structural parameters include the cell diagonal length parameter L1, the cell gap width parameter L2, the cell gap length parameter L3, and the cell internal solid radius parameter L6, where L2 = 0.0005–0.002L1, L3 = 0.25–0.5L1, and L6 = 0.25–0.45L1.

[0010] The finite element simulation software includes, but is not limited to: COMSOL Multiphysics, Abaqus, etc.

[0011] The material properties include, but are not limited to: density, elastic modulus, Poisson's ratio, thermal conductivity, constant pressure heat capacity, and coefficient of thermal expansion.

[0012] The boundary conditions include, but are not limited to, solid mechanics, solid heat transfer, etc.

[0013] The meshing methods include, but are not limited to: free tetrahedral mesh, swept mesh, etc.

[0014] Step 2: 3D Printing. Import the 3D model of the metamaterial space mirror designed in Step 1 into the control software of the 3D printing equipment, and set the printing parameters: single layer thickness 25-100μm, bottom layer exposure time 15-75s, bottom layer rest time 2-20s, upper layer exposure time 2-15s, upper layer rest time 2-10s, current intensity 5-50mJ / cm². 2 Separation speed: 30-60 mm / min;

[0015] The 3D printing processes include, but are not limited to: digital light processing photopolymerization 3D printing, stereolithography photopolymerization 3D printing, selective laser melting 3D printing, selective laser sintering 3D printing, fused deposition modeling 3D printing, and direct writing 3D printing.

[0016] Step 3: Sinter the printed blank from Step 2 using a tube furnace, chamber furnace, or laser sintering furnace. The sintering atmosphere is air or inert gas, with a gas flow rate of 100-300 mL / min; the sintering heating rate is 0.5-5℃ / min, employing a two-stage heating and two-stage holding process. The first stage involves heating to 200-600℃ and holding for 60-300 min; the second stage involves heating to 900-1500℃ and holding for 60-300 min, followed by cooling to room temperature to complete the sintering.

[0017] The inert atmosphere includes nitrogen, argon, etc.

[0018] Step 4, Surface Treatment. The surface of the metamaterial mirror sintered in Step 3 is initially polished until the surface roughness is 0.5-2μm; then, a thin film of 0.1-3μm is deposited on its surface using measurement and control sputtering technology to improve polishing performance; finally, the mirror is finely polished until the roughness is 8-20nm.

[0019] The thin film materials include, but are not limited to, amorphous silicon, silicon dioxide, etc.

[0020] The fabrication method of this invention utilizes 3D design software combined with finite element simulation software for structural optimization design, effectively providing guidance for actual production and fabrication. Through 3D printing technology combined with magnetron sputtering technology, an integrated fabrication of a metamaterial structure, a space mirror with high surface accuracy, high resolution, and low coefficient of linear expansion is achieved. The structural parameters, printing parameters, sintering process, and film thickness control involved in this invention have all undergone repeated experiments and optimizations, effectively ensuring the dimensional accuracy and printing speed of the structure, the structural stability of the sintered sample, and the surface accuracy after polishing. Attached Figure Description

[0021] Figure 1 This is a flowchart illustrating the method of the present invention.

[0022] Figure 2 This is a schematic diagram of the minimum repeating unit of the polygonal topological metamaterial structure in the method of the present invention: where L1 is the diagonal length parameter of the cell, L2 is the width parameter of the gap between cells, L3 is the length parameter of the gap between cells, L4 is the maximum length parameter of the internal hole of the cell, and L5 is the position parameter of the triangular gap.

[0023] Figure 3 This is a schematic diagram of the minimum repeating unit of the torsional topological metamaterial structure in the method of the present invention: where L1 is the diagonal length parameter of the cell, L2 is the width parameter of the gap between cells, L3 is the length parameter of the gap between cells, and L6 is the radius parameter of the solid inside the cell.

[0024] Figure 4 A 3D-printed polygonal topological metamaterial specimen (Φ100mm×10mm) sintered according to the method of the present invention is shown.

[0025] Figure 5 A polygonal topological metamaterial specimen (Φ100mm×10mm) is shown after precision polishing following magnetron sputtering deposition of an amorphous silicon thin film according to the method of the present invention.

[0026] Figure 6 The results of high-temperature thermal expansion coefficient test of 3D printed polygonal topological metamaterial standard are shown.

[0027] Figure 7 The results of low-temperature thermal expansion coefficient tests on 3D-printed polygonal topological metamaterial mirrors are shown.

[0028] Figure 8 The test results of the mirror surface shape of the 3D printed polygonal topological metamaterial are shown (laser wavelength λ = 632.8 nm). Detailed Implementation

[0029] To better understand the purpose, technical solution, and advantages of this invention, the invention will be further described in detail below with reference to the accompanying drawings and embodiments.

[0030] Artificially designed structures with tunable thermal properties exhibit unique thermal expansion behaviors not found in natural structures. Under temperature loads, the coefficient of thermal expansion can be tuned. Optimizing structural parameters using finite element simulation allows for the efficient design of mirror structures with low linear expansion coefficients. 3D printing of thermal metamaterial mirror surfaces offers significant forming advantages and excellent stability, making it an effective method for mirror structure fabrication. High-polishability films prepared by magnetron sputtering are an effective means of achieving high-precision surfaces through precision polishing. Therefore, this invention aims to achieve the integrated fabrication of a space mirror with high surface accuracy, high resolution, and a low linear expansion coefficient, enabling effective tuning of the coefficient of thermal expansion without altering the intrinsic properties of the material. Figure 1The flow chart of the method of the present invention is shown.

[0031] Example 1: 3D Printed Polygonal Topological Thermal Metamaterial Space Mirror

[0032] The original design of the minimum repeating element of the polygonal topological metamaterial structure was performed using SolidWorks software, such as... Figure 2 As shown. The dimension L1 is set to 30mm, and the parameters of L2, L3, L4, and L5 are adjusted based on L1. The parameters are: L2: 0.02L1, 0.01L1, 0.005L1, 0.002L1, 0.001L1 (5 levels); L3, L4, L5: 0.25L1, 0.3L1, 0.35L1, 0.4L1, 0.45L1 (5 levels). The smallest repeating element with each dimension parameter is imported into COMSOL software for finite element simulation. Input material properties: density 3.21g / cm³. 3 The parameters are: elastic modulus 460 GPa, Poisson's ratio 0.18, thermal conductivity 490 W / (m·K), constant pressure heat capacity 690 J / (kg·K), and coefficient of thermal expansion 2.5 ppm / K. Boundary conditions are set: solid mechanics includes linear elastic free deformation, initial values ​​set to zero, and periodic boundary conditions; solid heat transfer includes initial values ​​set to zero, ambient temperature load, external natural convection, and temperature periodic boundary conditions. Mesh generation is performed using physics field control to generate a refined mesh. Simulation calculations are conducted by setting the output step size and range for transient calculations and starting the calculation. Based on the simulation results of thermal stress and coefficient of linear expansion of the minimum repeating element under temperature loads from room temperature to 120℃, L2 = 0.001L1, L3 = 0.3L1, L4 = 0.3L1, and L5 = 0.4L1 are selected as the structural parameters of the minimum repeating element. This structure theoretically has a low level of thermal stress and coefficient of linear expansion. By continuously arraying the minimum repeating element in the x and y directions, metamaterial space mirror models of various sizes can be obtained.

[0033] The designed 3D structure was imported into the control software of the 3D printing equipment, and the printing parameters were set as follows: single layer thickness 50μm, bottom layer exposure time 25s, bottom layer rest time 5s, upper layer exposure time 5s, upper layer rest time 3s, and current intensity 15mJ / cm². 2 The separation speed is 48 mm / min, and the designed structure is formed using digital light processing photopolymerization 3D printing equipment and resin slurry.

[0034] The preform printed using a tube furnace was sintered in an argon atmosphere at a flow rate of 150 mL / min. The heating rate was 1 °C / min, employing a two-stage heating and two-stage holding process. The first stage involved heating to 450 °C and holding for 240 min; the second stage involved heating to 900 °C and holding for 300 min, followed by cooling to room temperature to complete the sintering. The resulting 3D-printed polygonal topological metamaterial prototype is shown below. Figure 4 As shown in the image.

[0035] The surface of the sintered metamaterial mirror was initially polished until the surface roughness was 1 μm. Subsequently, a 1.5 μm layer of amorphous silicon was deposited on its surface using measurement-controlled sputtering technology to improve polishing performance. Finally, the mirror was finely polished until the roughness was 10 nm. The resulting polygonal topological metamaterial prototype, after precision polishing following magnetron sputtering of the amorphous silicon thin film, is shown below. Figure 5 As shown in the image.

[0036] Example 2: 3D Printed Twisted Topological Thermal Metamaterial Space Mirror

[0037] The original design of the minimum repeating element of the torsion topological metamaterial structure was carried out using SolidWorks software, such as... Figure 3 As shown. The L1 dimension was set to 30mm, and the parameters of L2, L3, and L4 were adjusted based on L1. The parameters were set as follows: L2: 0.02L1, 0.01L1, 0.005L1, 0.002L1, 0.001L1 (5 levels); L3 and L6: 0.25L1, 0.3L1, 0.35L1, 0.4L1, 0.45L1 (5 levels). The smallest repeating element with each dimension parameter was imported into COMSOL software for finite element simulation. Input material properties: density 3.21g / cm³. 3 The parameters are: elastic modulus 460 GPa, Poisson's ratio 0.18, thermal conductivity 490 W / (m·K), constant-pressure heat capacity 690 J / (kg·K), and coefficient of thermal expansion 2.5 ppm / K. Boundary conditions are set: solid mechanics includes linear elastic free deformation, initial values ​​set to zero, and periodic boundary conditions; solid heat transfer includes initial values ​​set to zero, ambient temperature load, external natural convection, and temperature periodic boundary conditions. Mesh generation is performed using physics field control to generate a refined mesh. Simulation calculations are conducted by setting the output step size and range for transient calculations and starting the calculation. Based on the simulation results of thermal stress and coefficient of linear expansion of the minimum repeating element under temperature loads from room temperature to 120℃, L2 = 0.001L1, L3 = 0.3L1, and L6 = 0.35L1 are selected as the structural parameters of the minimum repeating element. This structure theoretically has a low level of thermal stress and coefficient of linear expansion. By arranging the minimum repeating element in a continuous array in the x and y directions, metamaterial space mirror models of various sizes can be obtained.

[0038] The designed 3D structure was imported into the control software of the 3D printing equipment, and the printing parameters were set as follows: single layer thickness 50μm, bottom layer exposure time 25s, bottom layer rest time 5s, upper layer exposure time 5s, upper layer rest time 3s, and current intensity 15mJ / cm². 2 The separation speed is 48 mm / min, and the designed structure is formed using digital light processing photopolymerization 3D printing equipment and resin slurry.

[0039] The blanks printed using a tube furnace were sintered in an argon atmosphere with a flow rate of 150 mL / min. The heating rate was 1 °C / min, and the process involved two stages of heating and two stages of holding. The first stage involved heating to 450 °C and holding for 240 min. The second stage involved heating to 900 °C and holding for 300 min. The blanks were then cooled to room temperature to complete the sintering process.

[0040] The surface of the sintered metamaterial mirror is initially polished until the surface roughness is 1 μm; then, a 1.5 μm amorphous silicon thin film is deposited on its surface using measurement and control sputtering technology to improve the polishing performance; finally, the mirror is finely polished until the roughness is 10 nm.

[0041] Figure 6 The results of high-temperature thermal expansion coefficient test of 3D printed polygonal topological metamaterial standard sample are shown. The structure can obtain a low thermal expansion coefficient of <0.2ppm / K below 201.6℃. Figure 7 The results of thermal expansion coefficient tests on 3D-printed polygonal topological metamaterial mirror surfaces at both low and high temperatures are shown. The average thermal expansion coefficient of the mirror structure is 0.24 ppm / K in the low-temperature region, 0.38 ppm / K in the room-temperature region, and 0.87 ppm / K in the high-temperature region. Figure 8 The test results of the mirror surface shape of the 3D printed polygonal topological metamaterial (laser wavelength λ = 632.8 nm) are shown. The surface has high flatness and the surface RMS value is less than λ / 50.

[0042] The fabrication method of this invention utilizes finite element simulation to effectively tune the linear expansion coefficient of the mirror structure. The use of 3D printing technology in conjunction with fine surface polishing after coating further enables the practical production and application of high-precision optical components. Furthermore, the designed polygonal and torsional topological metamaterials, under thermal loads, can compensate for overall thermal expansion through the gaps between cells, thus achieving a low linear expansion coefficient. This invention utilizes 3D modeling software and finite element software to collaboratively optimize the thermal metastructure, enabling customizable design of the structural thermal expansion coefficient. The 3D printing technology employed in this invention can fabricate complex and intricate structures, avoiding the low forming accuracy drawbacks of traditional molding methods. Building upon the aforementioned methods, this invention combines magnetron sputtering technology for mirror surface coating, improving the polishing performance of the printed preform and resulting in a high-precision optical mirror during fine polishing.

Claims

1. A method for fabricating a high-precision, low-linear-expansion thermal metamaterial space mirror, comprising the following steps: Step 1: Three-dimensional structural design The minimum repeating unit of the superstructure is designed and a three-dimensional model of the minimum repeating unit is generated and exported. Finite element simulation calculation is performed on the three-dimensional model for the thermal strain of the minimum repeating unit under temperature load. The structural parameters of the minimum repeating unit are adjusted according to the simulation results to obtain a structure with a theoretical low coefficient of linear expansion. The minimum repeating unit is arranged into an array in the x and y directions to obtain a three-dimensional model of the metamaterial space mirror. Step 2: 3D printing The 3D model of the metamaterial space mirror obtained in step 1 was used for 3D printing to obtain a 3D printed blank. The 3D printing parameters were as follows: single layer thickness 25–100 μm, bottom layer exposure time 15–75 s, bottom layer rest time 2–20 s, upper layer exposure time 2–15 s, upper layer rest time 2–10 s, and energy density 5–50 mJ / cm³. 2 The separation speed is 30–60 mm / min; Step 3: Sintering the 3D printed blank The 3D printed blank from step 2 is sintered in an air or inert gas atmosphere with a gas flow rate of 100–300 mL / min. The sintering heating rate is 0.5–5 °C / min, and a two-stage heating and two-stage holding process is adopted. The first stage is to heat up to 200–600 °C and hold for 60–300 min. Second stage: Heat to 900-1500℃, hold for 60-300 min, then cool to room temperature to complete sintering; Step 4: Surface Treatment The surface of the metamaterial mirror sintered in step 3 is initially polished. Then, a thin film of 0.1–3 μm is deposited on the surface using magnetron sputtering technology. Finally, the mirror is finely polished until the roughness is 8–20 nm. The superstructure is either a polygonal topology or a twisted topology. In the case of a polygonal topology, the structural parameters of the minimum repeating unit include the cell diagonal length parameter L1, the inter-cell gap width parameter L2, the inter-cell gap length parameter L3, the maximum length of the internal hole parameter L4, and the triangular gap position parameter L5, where L2 = 0.0005–0.002 L1, L3 = 0.25–0.5 L1, L4 = 0.25–0.45 L1, and L5 = 0.25–0.45 L1. In the case of a twisted topology, the structural parameters of the minimum repeating unit include the cell diagonal length parameter L1, the inter-cell gap width parameter L2, the inter-cell gap length parameter L3, and the internal solid radius parameter L6, where L2 = 0.0005–0.002 L1, L3 = 0.25–0.5 L1, and L6 = 0.25–0.45 L1. To determine the thermal strain of the minimum repeating element under temperature load, the finite element simulation calculation of the three-dimensional model includes importing the three-dimensional model of the minimum repeating element into the finite element simulation software, setting material properties, inputting boundary conditions, generating a mesh, and performing simulation calculations to obtain the thermal strain of the minimum repeating element under temperature load. The material properties include density, elastic modulus, Poisson's ratio, thermal conductivity, constant pressure heat capacity, and coefficient of thermal expansion. The boundary conditions include solid mechanics and solid heat transfer; The surface roughness achieved by the preliminary polishing is 0.5–2 μm; The material of the thin film in step 4 is amorphous silicon or silicon dioxide; The 3D printing used is digital light processing photopolymerization 3D printing.

2. The method according to claim 1, wherein the size of the metamaterial space mirror is 10 to 500 mm.

3. The method according to claim 1, wherein the meshing is performed using a free tetrahedral mesh or a swept mesh.

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