Flexible double-layer frequency selective surface and preparation method thereof

Abstract

The present invention provides a flexible double-layer frequency selective surface and a preparation method thereof, comprising a plurality of frequency selective surface unit cells arranged in a matrix; the frequency selective surface unit cell comprises an elastic substrate, a lower PI copper-clad film is provided in the center of the elastic substrate, an upper PI copper-clad film is provided above the lower PI copper-clad film, the lower PI copper-clad film and the upper PI copper-clad film are arranged in parallel, and a gap is left between them; the lower PI copper-clad film and the upper PI copper-clad film both comprise a lower polyimide film and an upper copper layer; the polyimide film and the copper layer are fixedly connected. The frequency selective surface of the present invention has mechanical flexibility and can achieve perfect light and transparent attachment on the surface of a target object. Since the Ecoflex substrate and the PI copper-clad film used are both flexible materials, they have the properties of being stretchable and bendable.

Description

Technical Field

[0001] The invention belongs to the technical field of spatial filters, and in particular relates to a flexible double-layer frequency selective surface and a preparation method thereof. Background Art

[0002] Frequency Selective Surface (FSS) is usually called a spatial filter. It is a periodic structure composed of metal dielectric structural units such as the same metal patch and aperture on the metal screen arranged in a certain pattern. It has frequency-selective electromagnetic scattering characteristics for electromagnetic waves of different frequencies, different incident angles and polarization states. It mainly includes two types: the patch type in which the conductive unit is periodically attached to the dielectric substrate, and the aperture type formed by periodic openings directly on the conductive substrate. With the increasing complexity of the electromagnetic environment, the demand for electromagnetic control is becoming more and more urgent. Many people have proposed active control frequency selective surfaces such as electrical, optical, and field control types. Multilayer frequency selective structures are also used in many scenarios due to their own wide advantages. With its efficient filtering and wave transmission characteristics, frequency selective surfaces have received widespread attention in various fields, especially in the military, and are often used in the design of intelligent stealth antenna covers, new generation phased array antennas and other fields.

[0003] Traditional frequency selective surfaces are mostly passive two-dimensional periodic resonant unit arrays made of pure metal materials. Metal sheets are attached to dielectric substrates, or periodic holes are directly opened on metal materials. They have a hard texture. Multiple metal sheets can be combined with multiple dielectric substrates to form a multi-layer cascade frequency selective surface, which has better filtering performance and better mechanical strength than single-layer frequency selective surfaces. In addition, although single-layer and double-layer frequency selective surfaces can achieve the regulation of electromagnetic waves, once they are manufactured and processed, the electromagnetic characteristics and structure are fixed after molding, and the frequency response characteristics cannot be changed, which has certain limitations.

[0004] Traditional frequency selective surfaces can no longer adapt to the increasingly complex external electromagnetic environment and changing working conditions, and have the following disadvantages:

[0005] It is difficult to stretch and bend, and cannot conform to the curved surface. Traditional frequency selective structures cannot be bent or extended, and need to be combined with flexible electronic technology to further expand the realization and application of frequency selective surfaces.

[0006] A single-layer frequency selective surface has a single resonance point, few adjustable parameters, and a limited range of achievable phase shift. A single-layer frequency selective surface has reflection or transmission characteristics at its resonant frequency, but can usually only produce one resonance point, which results in a narrow working bandwidth and is not flat enough, thus limiting its application scenarios.

[0007] Although multi-layer cascading can improve frequency selection performance, the structure is bulky, and there are certain requirements and restrictions on the size of the filter structure in practical applications. Summary of the invention

[0008] In view of the above technical problems, the present invention provides a flexible double-layer frequency selective surface and a preparation method thereof. By placing a paper-cut structure on an elastic base material, the structure can produce a buckling response due to the release of the internal stress of the elastic base, resulting in a large deformation, and the frequency selection unit is then converted from a two-dimensional structure to a three-dimensional structure. Then, a metal unit array is placed in advance on the elastic base and the paper-cut structure to realize a new type of flexible double-layer frequency selective surface. It has the following characteristics:

[0009] (1) Using a flexible dielectric substrate, the frequency selective surface can be conformal to complex curved surfaces.

[0010] (2) Control the release of the flexible substrate to achieve the adjustment of the frequency selective surface filtering characteristics.

[0011] (3) The volume of the paper-kirigami structure is significantly smaller than the elastic substrate, realizing a miniaturized three-dimensional frequency selective structure.

[0012] The specific technical solutions are:

[0013] A flexible double-layer frequency selective surface, including a plurality of frequency selective surface unit cell matrix arrangements;

[0014] The frequency selective surface unit cell comprises an elastic substrate, a lower PI copper-clad film is provided in the center of the elastic substrate, an upper PI copper-clad film is provided above the lower PI copper-clad film, the lower PI copper-clad film and the upper PI copper-clad film are arranged in parallel, and a gap is left between them;

[0015] The lower PI copper-clad film and the upper PI copper-clad film both include a lower polyimide film and an upper copper layer; the polyimide film and the copper layer are fixedly connected.

[0016] The lower PI copper-clad film is fixed on the elastic substrate, and an extended part is arranged around the polyimide film of the upper PI copper-clad film, and the end of the extended part is fixed on the elastic substrate.

[0017] The PI copper-clad film is fixed to the elastic substrate by glue.

[0018] The method for preparing a flexible double-layer frequency selective surface comprises the following steps:

[0019] (A) Preparation of elastic substrate and underlying PI copper film array

[0020] Preparation of elastic substrate: Pour the raw material of Ecoflex into a square flat bottom mold and place it at room temperature for 4 hours.

[0021] Pre-stretching the elastic substrate; placing the cured Ecoflex elastic substrate on the stretching instrument platform, and stretching the four sides of the cube elastic substrate at equal distances;

[0022] Making a lower PI copper-clad film array; cutting the lower PI copper-clad film to form a square ring array composed of a copper layer;

[0023] Transfer and paste: apply a small amount of glue on the polyimide film surface of each lower PI copper-clad film, discard the part that does not belong to the copper layer, and transfer the lower PI copper-clad film to the pre-stretched elastic substrate;

[0024] Let it stand for 24 hours. Place a heavy object on the elastic substrate to which the lower PI copper film has been pasted.

[0025] (B) Preparation of PI copper-clad film;

[0026] Firstly, copper is etched on the PI copper-clad film by using a photosensitive dry film method, leaving an array of copper layers on the polyimide film dielectric substrate, and then the upper PI copper-clad film array is obtained by cutting with a laser cutting machine.

[0027] (C) Fabrication of flexible double-layer frequency selective surface

[0028] The upper PI copper-clad film is pasted on the elastic substrate to form a planar frequency selective surface;

[0029] The stress of the elastic substrate is released to form a flexible double-layer frequency selective surface; the elastic substrate is controlled to change the layer height through the four-side recovery distance, thereby adjusting the filtering performance of the three-dimensional frequency selective surface.

[0030] The present invention aims to solve the three problems that the current frequency selective surfaces cannot be conformally attached to the curved surface, the filtering performance cannot be adjusted after forming, and the multi-layer cascaded frequency selective surfaces are large in size. Based on the elastic matrix guiding the compression and buckling of the paper-cut structure, a flexible double-layer frequency selective surface is invented.

[0031] The frequency selective surface has mechanical flexibility and can achieve perfect light and transparent adhesion on the surface of the target object. Since the Ecoflex substrate and PI copper-clad film are both flexible materials, they have the properties of stretchability and bendability.

[0032] An extended part is provided around the polyimide film to form a cross-shaped paper-cut structure, which can undergo compression buckling during the stretching and releasing process of the Ecoflex elastic substrate, thereby achieving a change in the relative height between the upper and lower metal units and a change in the filtering performance.

[0033] In addition, the filtering performance of the double-layer frequency selective surface is creatively regulated by guiding the deformation of the paper-cut structure through compression and buckling, achieving a breakthrough application of the paper-cut structure on the frequency selective surface, so that a small-volume single-layer frequency selective surface can be easily transformed into a double-layer frequency selective surface without increasing the volume. BRIEF DESCRIPTION OF THE DRAWINGS

[0034] Figure 1 It is a schematic diagram of the structure of the present invention;

[0035] Figure 2a is a schematic cross-sectional view of a double-layer frequency selective surface unit cell structure of the present invention;

[0036] Figure 2b is a top view schematic diagram of a double-layer frequency selective surface unit cell structure of the present invention;

[0037] Figure 3a It is a schematic diagram of the lower PI copper-clad film structure of the present invention;

[0038] Figure 3b It is a schematic diagram of the cross-sectional structure of the lower PI copper-clad film of the present invention;

[0039] Figure 4a This is one of the schematic diagrams for preparing the upper PI copper-clad film of the present invention;

[0040] Figure 4b This is the second schematic diagram of preparing the upper PI copper-clad film of the present invention;

[0041] Figure 4c This is the third schematic diagram of the preparation of the upper PI copper-clad film of the present invention;

[0042] Figure 4d This is the fourth schematic diagram of the preparation of the upper PI copper-clad film of the present invention;

[0043] Figure 5a Schematic diagram of the cross-sectional structure of the frequency-selective surface for a double layer with pre-stretching;

[0044] Figure 5b A schematic diagram of the top view structure of a double-layer frequency selective surface with pre-stretching;

[0045] Figure 6 It is a schematic diagram of various shapes of the copper layer of the present invention. DETAILED DESCRIPTION

[0046] The specific technical solution of the present invention is explained in conjunction with embodiments.

[0047] The flexible double-layer frequency selective surface is divided into two parts: one is the paper-cut structure, which includes the upper metal array unit and its dielectric substrate, and the other is the elastic matrix, which includes the lower metal array unit and its dielectric substrate. Figure 1As shown, the filtering characteristics are achieved through the upper and lower copper layers. Figure 2a , 2b Schematic diagram of a double-layer frequency selective surface unit cell structure. The frequency selective surface unit cell comprises an elastic substrate 1, a lower PI copper film is provided in the center of the elastic substrate 1, an upper PI copper film is provided above the lower PI copper film, the lower PI copper film and the upper PI copper film are arranged in parallel with a gap between them;

[0048] The lower PI copper-clad film and the upper PI copper-clad film both include a lower polyimide film 3 and an upper copper layer 4; the polyimide film 3 and the copper layer 4 are fixedly connected.

[0049] The lower PI copper-clad film is fixed on the elastic substrate 1 , and the polyimide film 3 of the upper PI copper-clad film is provided with an extended portion around its periphery, and the end of the extended portion is fixed on the elastic substrate 1 .

[0050] The PI copper-clad film is fixed on the elastic substrate 1 by glue 2 .

[0051] The copper layer 4 is in different combinations, such as annular or Figure 6 The frequency selection unit shape shown is, for example, upper solid-lower ring, upper combination-lower center connection, etc., that is, any upper and lower layer unit combination that can achieve frequency selection can be selected.

[0052] This embodiment uses an open square copper layer.

[0053] Preparation Usually PI copper-clad film is used as a raw material to make the upper and lower unit cells, and the upper polyimide (abbreviated as PI) is a double-layer film composed of a polyimide film 3 and a lower copper layer 4; it is composed of a polyimide film layer, glue, and a copper layer. The polyimide film 3 and the copper layer 4 are firmly bonded by the glue layer when they leave the factory, and the glue layer does not affect the technical effect, so Figure 2a The glue layer between the polyimide film 3 and the copper layer 4 is not shown, and it is believed that the copper layer 4 and the polyimide film 3 are directly and firmly bonded.

[0054] The preparation method is:

[0055] (A) Preparation of lower frequency selective unit (elastic substrate and metal unit array on it)

[0056] 1. Preparation of elastic substrate 1: Pour Ecoflex A and B agents into a square flat-bottomed mold with a length and width of 80 mm in a 1:1 weight ratio, pour about 5 mm in height, and place at room temperature for 4 hours.

[0057] 2. Pre-stretching the elastic substrate 1; placing the cured Ecoflex elastic substrate on the stretching instrument platform, and stretching the four sides of the cube elastic substrate equidistantly to ensure that a deformation of about 20% is generated.

[0058] 3. Make the lower metal unit array. Take the 4×4 array as an example, the dotted line is the unit cell boundary line, and the laser cutting machine is used to cut the lower metal unit array. Figure 3a The open square ring shape shown cuts the lower PI copper film to form a square ring array composed of a copper layer 4. The thickness of the polyimide film 3 in the lower PI copper film is about 30 microns, and the copper layer 4 is about 20 microns; the size of each frequency selective surface unit cell is about 20mm×20mm; the plane size of each square ring is about 5mm*5mm.

[0059] 4. Transfer and paste. Apply a small amount of glue on the polyimide film 3 of each lower PI copper-clad film, discard the part that does not belong to the copper layer 4, and transfer the lower PI copper-clad film to the pre-stretched elastic substrate 1. During the transfer, the relative positions of the copper layers 4 cannot be changed. The cross-sectional view of the frequency selective surface unit cell is shown in the figure. Figure 3b shown.

[0060] 5. Let it stand for 24 hours. Place a heavy object on the elastic substrate 1 to which the lower PI copper film has been attached.

[0061] (B) Preparation of upper frequency selection unit (paper-kirigami structure and its metal unit array)

[0062] The preparation of the lower frequency selective unit first uses the photosensitive dry film method (step 1-2) to etch most of the copper on the PI copper-clad film, leaving a copper layer 4 array on the polyimide film 3 dielectric substrate, and then uses a laser cutting machine to cut the PI film to form a paper-cut structure array to obtain the upper PI copper-clad film, and finally paste it on the elastic substrate 1 to release the stress and transform it into a three-dimensional frequency selective surface.

[0063] 1. Film pasting and exposure. Paste the photosensitive film (two layers of film, including a photoresist layer and a PET layer. The photoresist layer can be cured by ultraviolet light to protect the attached surface, and the PET layer is a transparent film) on the clean copper surface of the PI copper film ( Figure 4a Copper layer), Figure 4a The pattern shown on the left is printed with film (a transparent film on which black patterns can be printed). After printing, only the square ring of the film is transparent, which is used as a mask and pressed against the surface of the photosensitive film. Then, in a dark environment, ultraviolet light is evenly exposed to the photosensitive film for about 5 minutes, and the copper film is left to cool.

[0064] 2. Development, etching, and demolding. The photosensitive film developer and water are prepared into a developer at a weight ratio of 1:10. After peeling off the PET film, the cooled copper-clad film is placed in the developer, and the unexposed photoresist layer is detached and dissolved; FeCl3 and water are prepared into an etching solution at a weight ratio of 1:1, and the developed copper-clad film is placed in the etching solution. The metal copper that is not protected by the cured photoresist layer on the photosensitive film is etched away from the PI surface; then the photosensitive film release agent and water are prepared into a demolding solution at a weight ratio of 1:10, and the etched copper-clad film is placed in the demolding solution. All photosensitive films protecting the copper layer 4 are detached, forming a film as shown in the following figure. Figure 4b 4 arrays of copper layers.

[0065] 3. Make the upper metal unit array. Figure 4c The left pattern is laser cut to form a cross-shaped paper-cut structure array. The cross-shaped unit cell is the upper PI copper film.

[0066] 4. Apply glue. Figure 4d The mask shown on the left is attached to the lower surface of the polyimide film 3 on the upper PI copper film. A small amount of glue 2 is applied to each white square, and the mask is peeled off to form a Figure 4d The PI copper film is shown on the right.

[0067] (C) Preparation of flexible double-layer frequency selective surface (bonding and stress release process)

[0068] 1. Paste the upper and lower layers. Figure 4d Array Pasted on Figure 3b On the elastic substrate 1, a Figure 5a-5b Frequency selective surface shown.

[0069] 2. Release the stress of the elastic substrate 1 to form a flexible double-layer frequency selective surface, see Figure 1 , Figure 2a-2b By controlling the recovery distance of the four sides of the elastic substrate 1, the layer height can be changed, thereby adjusting the filtering performance of the three-dimensional frequency selective surface.

Claims

1. Flexible double-layer frequency selective surface, characterized in that: including a plurality of frequency selective surface unit cell matrix arrangements; The frequency selective surface unit cell comprises an elastic substrate (1), a lower PI copper-clad film is provided in the center of the elastic substrate (1), an upper PI copper-clad film is provided above the lower PI copper-clad film, and the lower PI copper-clad film and the upper PI copper-clad film are arranged in parallel with a gap between them; The lower PI copper-clad film and the upper PI copper-clad film both comprise a lower polyimide film (3) and an upper copper layer (4); the polyimide film (3) and the copper layer (4) are fixedly connected; The lower PI copper clad film is fixed on an elastic substrate (1) pre-stretched on four sides, and the polyimide film (3) of the upper PI copper clad film is provided with an extended portion around its periphery, and the ends of the extended portions are fixed on the elastic substrate (1) pre-stretched on four sides. The extended portions form a cross-shaped paper-cut structure, which can be compressed and buckled during the stretching and releasing process of the elastic substrate, thereby realizing the change in the relative height between the lower PI copper clad film and the upper PI copper clad film.

2. The flexible double-layer frequency selective surface according to claim 1, characterized in that: The PI copper-clad film is fixed on the elastic substrate (1) by means of glue (2).

3. The method for preparing the flexible double-layer frequency selective surface according to claim 1 or 2, characterized in that: The following steps are involved: (A) Preparation of elastic substrate (1) and underlying PI copper film array The elastic substrate (1) is manufactured; Pour the Ecoflex raw material into a square flat-bottom mold and leave it to set at room temperature for 4 hours; The elastic substrate (1) is pre-stretched; Place the cured Ecoflex elastic matrix on the tensile tester platform and stretch the four sides of the cube elastic matrix at equal distances; Making a lower PI copper-clad film array; cutting the lower PI copper-clad film to form a square ring array composed of a copper layer (4); Transfer and paste; apply a small amount of glue on the polyimide film (3) surface of each lower PI copper-clad film, discard the part that does not belong to the copper layer (4), and transfer the lower PI copper-clad film to the pre-stretched elastic substrate (1); Allow to stand for forming; place a heavy object on the elastic substrate (1) to which the lower PI copper-clad film has been attached, and allow to stand for 24 hours; (B) Preparation of PI copper film; Firstly, copper is etched on the PI copper-clad film by using a photosensitive dry film method, leaving an array of a copper layer (4) on the polyimide film dielectric substrate, and then the upper PI copper-clad film array is obtained by cutting with a laser cutting machine; (C) Preparation of flexible double-layer frequency selective surface The upper PI copper-clad film is adhered to the elastic substrate (1) to form a planar frequency selective surface; The stress of the elastic substrate (1) is released to form a flexible double-layer frequency selective surface; the recovery distance of the four sides of the elastic substrate (1) is controlled to change the layer height, thereby adjusting the filtering performance of the three-dimensional frequency selective surface.

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