Graphene-based transmission dynamically adjustable flexible frequency selective absorber

Abstract

The present invention discloses a graphene-based transmission dynamically adjustable flexible frequency selective absorber, which belongs to the technical field of microwave devices, and includes a graphene-metal mixed layer, a first dielectric layer, a graphene-electrolyte-graphene sandwich layer, a second dielectric layer, and a metal frequency selective surface layer stacked from top to bottom. Graphene is used to achieve a passband with small insertion loss, and there are two absorption bands with high absorbency on the left and right of the passband. Since the present invention has no lumped elements, it can be applied to a wide wavelength region ranging from microwave to terahertz. In addition, through the graphene sandwich structure, adjustable transmission on the passband is achieved, so that our frequency selective absorber has the function of continuous spatial energy manipulation, which shows that our antenna cover has great application prospects in electromagnetic stealth, compatibility and protection.

Description

Technical Field

[0001] The invention belongs to the technical field of microwave devices, and in particular relates to a graphene-based transmission dynamically adjustable flexible frequency selective absorber. Background Art

[0002] A radome is a cover placed over an antenna to protect the radiating element from the external environment, such as wind, rain, ice and sand. In addition to physical protection, the radome should be transparent to radio frequencies from an electromagnetic point of view so that it does not degrade the electrical performance of the enclosed antenna. Conventional radomes are usually composed of frequency selective surfaces because of their attractive frequency filtering properties. Most of the time, a bandpass FSS is used to allow waves to pass without much passband loss, however, there is a strong reflection passband that greatly increases the radar cross section. The ideal way to reduce the out-of-band bistatic radar cross section is to adopt lossy materials in the design and manufacture of the radome to absorb the incident waves, so a structure called frequency selective absorber (FSR) has been proposed.

[0003] In general, existing FSRs can be divided into two categories: two-dimensional FSRs and three-dimensional FSRs. Although three-dimensional FSRs show the advantages of smaller unit size and stable oblique-incidence performance, the thickness is greatly increased, thus limiting its practical application, so only two-dimensional FSRs are discussed and studied in our work. In the past, two-dimensional FSRs were usually realized by placing a lossy layer above the lossless bandpass FSS, and the absorption function was provided by the lossy layer, such as a resistor square ring, a resistor patch, and a resistor ring. These structures have the weakness of relatively low transmittance in the transmission band, which leads to the inevitable insertion loss of the purely resistive lossy layer. Afterwards, attempts were made to improve the transmission performance of FSRs by loading lumped resistors in metal components. The added metal components provide a strong LC resonance in the transmission window, resulting in high impedance on the lossy layer, so the transmittance can be improved. However, too many lumped components are used, which can lead to quite complicated design and manufacturing, and the PCB-based manufacturing technology limits their application in flexible or conformal situations. In addition, these designs fail at high frequencies (such as the terahertz band) because it is difficult to use lumped components at such high frequencies.

[0004] For the absorbers mentioned above, the operating frequency band is transparent to in-band signals, so high-intensity radiated fields (HIRF) and electromagnetic pulses (EMP) can enter the system and cause irreparable damage to the electronic equipment. To address this issue, we studied FSRs with switchable passbands. The existing switchable properties lie in the change of the diode state driven by a biased gate. These PIN diode-based structures also suffer from complex manufacturing processes and limited applicable frequency ranges. More importantly, only binary functions of the transmission properties, i.e., "on" and "off" states, can be achieved. The value of the threshold field strength is mainly limited by the intrinsic characteristics of the PIN diode. In this case, continuous regulation of the incident wave power remains an unresolved challenge. Summary of the invention

[0005] Purpose of the invention: In order to solve the problems existing in the prior art, the present invention provides a graphene-based transmittance dynamically adjustable flexible frequency selective absorber, which realizes an adjustable passband between two adjacent absorption bands, and a bandpass characteristic with high permeability and high absorptivity between the two adjacent absorption bands, and can realize continuously adjustable transmittance on the transmission window through active dynamic regulation, so that the frequency selective absorber has the functions of energy manipulation and arbitrary controllability.

[0006] Technical solution: In order to achieve the above-mentioned invention object, the present invention adopts the following technical solution:

[0007] A graphene-based transmission dynamically adjustable flexible frequency selective absorber comprises a stacked graphene-metal mixed layer, a first dielectric layer, a graphene-electrolyte-graphene sandwich structure, a second dielectric layer, and a metal frequency selective surface layer; wherein the graphene-metal mixed layer comprises a multi-layer graphene layer and a metal arm, the multi-layer graphene layer is a ring structure, and the number of the metal arms is four and is equidistantly arranged on the periphery of the multi-layer graphene layer; and the metal arms are respectively connected to the interior of the multi-layer graphene layer.

[0008] Furthermore, the graphene metal mixed layer and the metal frequency selective surface layer are both formed by laminating PVC on a copper foil, then printing a pattern on the laminated copper foil using a laser printer, etching the printed laminated copper foil in an aqueous nitric acid solution and using the printed toner as an etching mask, and the laminated copper under the toner is not etched to produce a pattern made of the laminated copper foil.

[0009] Furthermore, the metal frequency selective surface layer is a copper sheet etched into a complementary circular ring-shaped periodic structure.

[0010] Furthermore, the multi-layer graphene layer is grown on a pre-patterned nickel foil, and by controlling the growth temperature to 903 degrees Celsius, a fan-shaped structure with a square resistance of 70 ohms / sq is obtained.

[0011] Furthermore, the graphene sandwich structure includes two single-layer graphenes and a separator paper arranged between the two single-layer graphenes; the separator paper is soaked in electrolyte, and each of the single-layer graphenes is respectively connected to a bias voltage.

[0012] Furthermore, the electrolyte layer is a mixture of diethyl methyl phthalate, trifluoromethanesulfonimide and Tf2N ion liquid.

[0013] Furthermore, the outer diameter r1 of the multilayer graphene layer is 3 mm, the inner diameter r2 is 2 mm, the length l of each metal arm is 4.97 mm, the gap width w1 is 0.5 mm, and the arm length width excluding the gap width is w2=1 mm; the metal frequency selective surface layer is a 10-micron thick copper sheet etched into a complementary circular ring periodic structure, with an inner diameter r3=5.14 mm and an outer diameter r4=4.14 mm; the first dielectric layer and the second dielectric layer are both foam boards, the foam board material is polyethylene terephthalate, and the thickness is 3 mm respectively; the graphene-electrolyte-graphene sandwich structure is 190 microns thick; the graphene-electrolyte-graphene sandwich structure is processed by laying a single layer of graphene on a PVC film substrate, and the thickness of the PVC film substrate is 60 microns.

[0014] Furthermore, the absorber has a thickness of 6 mm, a size of 27 cm×27 cm, and includes 18×18 square ring units; wherein the size of each square ring unit is 15 mm×15 mm.

[0015] Principle of the invention: A graphene-based transmission dynamically adjustable flexible frequency selective absorber, including a graphene-metal mixed layer, a first dielectric layer, a graphene-electrolyte-graphene sandwich structure, a second dielectric layer, and a metal frequency selective surface layer. When an electromagnetic wave is incident on the device, the fan-shaped multilayer graphene in the graphene-metal mixed layer and the metal frequency selective surface of the bottom layer form two absorption peaks, and the surface current can flow through the fan-shaped patch at f1 and f3, so the multilayer graphene absorbs electromagnetic waves in two adjacent absorption bands due to ohmic loss. On the contrary, resonance occurs at the four metal arms at f2 and the metal frequency selective surface, which prevents the surface current from flowing along the lossy patch, so that a transmission window with low loss can be achieved, allowing electromagnetic waves to pass through. Through the middle graphene electrolyte interlayer, its conductivity can be dynamically adjusted by an applied voltage, affecting the surface impedance of the graphene sandwich layer, thereby affecting the transmittance of the passband.

[0016] The present invention designs and manufactures a two-dimensional FSR, in which the lossy layer is based on multilayer graphene (MLG) integrated with metal arms. The composite structure has a bandpass characteristic of high permeability and absorptivity between two adjacent absorption bands, and the structure has the advantages of easy fiberization and wide frequency applicability. In order to overcome the inflexibility of traditional FSR, ultra-thin polyvinyl chloride (PVC) is selected as the substrate for the lossy layer and the lossless layer, and the manufactured prototype has good flexibility and low weight. It is worth noting that by using a graphene sandwich structure (GSS) ("Graphene-enabled electrically switchable radar-absorbing surfaces, Nature Commun, 6, 6628, 2015"), a continuously adjustable transmittance on the transmission window is achieved, so that our frequency-selective absorber has the functions of energy manipulation and arbitrary controllability. Beneficial effects: Compared with the prior art, the graphene-based transmittance dynamically adjustable flexible frequency selective absorber of the present invention combines voltage-dynamically regulated graphene technology and frequency selective surface electromagnetic absorbing surface, and uses graphene to achieve a passband with small insertion loss. The composite structure has a bandpass characteristic of high permeability and absorptivity between two adjacent absorption bands, and the structure has the advantages of easy fiberization and wide frequency applicability. To overcome the inflexibility of traditional FSR, ultra-thin polyvinyl chloride (PVC) is selected as the substrate for lossy and lossless layers, and the manufactured prototype has good flexibility and low weight. It is worth noting that by using a graphene sandwich structure (GSS), continuously adjustable transmittance on the transmission window is achieved, which makes our frequency selective absorber have the functions of energy manipulation and arbitrary controllability, indicating that our antenna cover has great application prospects in electromagnetic stealth, compatibility and protection. BRIEF DESCRIPTION OF THE DRAWINGS

[0017] Figure 1 This is a side view of a transmission-tunable flexible frequency-selective absorber based on electrolyte-controlled graphene;

[0018] Figure 2 is a schematic diagram of the graphene metal hybrid layer pattern structure;

[0019] Figure 3 is a schematic diagram of a metal pattern structure of a metal frequency selective surface layer;

[0020] Figure 4 It is a schematic diagram of the equivalent circuit of the designed electromagnetic absorbing surface's absorbing principle;

[0021] Figure 5 is the frequency distribution diagram of dynamically regulated transmittance;

[0022] Figure 6 is the frequency distribution diagram of the dynamically controlled absorption rate;

[0023] Figure 7 is the frequency distribution diagram of dynamically regulated reflectivity;

[0024] Figure numerals: 1 - graphene-metal mixed layer, 2 - first dielectric layer, 3 - graphene-electrolyte-graphene sandwich structure, 4 - second dielectric layer, 5 - metal frequency selective surface layer, 11 - multi-layer graphene layer and 12 - metal arm. DETAILED DESCRIPTION

[0025] The present invention is further described below in conjunction with the accompanying drawings and specific implementation examples. It should be understood that these embodiments are only used to illustrate the present invention and are not intended to limit the scope of the present invention. After reading the present invention, modifications to various equivalent forms of the present invention by those skilled in the art all fall within the scope defined by the appended claims of the present application.

[0026] like Figure 1 As shown, the transmission-adjustable flexible frequency selective absorber based on electrolyte-regulated graphene includes a five-layer structure stacked from bottom to top: a graphene-metal mixed layer 1, a first dielectric layer 2, a graphene-electrolyte-graphene sandwich structure 3, a second dielectric layer 4, and a metal frequency selective surface layer 5; the second dielectric layer 4 is arranged on the metal frequency selective surface layer 5, the graphene-electrolyte-graphene sandwich structure 3 is arranged on the second dielectric layer 4, the first dielectric layer 2 is arranged on the graphene-electrolyte-graphene sandwich structure 3, and the graphene-metal mixed layer 1 is arranged on the first dielectric layer 2. The two graphene layers in the graphene-electrolyte-graphene sandwich structure 3 are both processed by laying a single layer of graphene on a PVC substrate.

[0027] The patterns of the graphene metal hybrid layer 1 and the metal frequency selective surface layer 5 are both obtained by laminating PVC on a 10 micron copper foil, and then printing the pattern on the laminated copper foil using a laser printer. The printed laminated copper foil is then etched in a nitric acid aqueous solution and the printed toner is used as an etching mask, and the laminated copper foil under the toner is not etched to produce a pattern made of the laminated copper foil.

[0028] like Figure 2 As shown, the graphene metal hybrid layer 1 includes a multi-layer graphene layer 11 and a metal arm 12. The multi-layer graphene layer 11 is a ring structure. The metal arms 12 are four and are arranged at equal intervals on the periphery of the multi-layer graphene layer 11 and are respectively connected to the inside of the multi-layer graphene layer 11. The specific parameters are: outer diameter r1 = 3 mm, inner diameter r2 = 2 mm, length l of each metal arm = 4.97 mm, gap width w1 = 0.5 mm, and arm length width excluding the gap width is w2 = 1 mm. The multi-layer graphene is grown on a pre-patterned nickel foil, and the square resistance is a fan-shaped structure of 70 ohms / sq by controlling the growth temperature to 903 degrees Celsius. The size of each unit is 15mm×15mm.

[0029] like Figure 3 As shown, the metal frequency selective surface layer 5 is a 10 micron thick copper sheet etched into a complementary circular ring periodic structure, with an inner diameter r3 = 5.14 mm and an outer diameter r4 = 4.14 mm. The overall absorber structure has a size of 27 cm × 27 cm, including 18 × 18 square ring units; wherein the size of each square ring unit is 15 mm × 15 mm.

[0030] The first dielectric layer 2 and the second dielectric layer 4 are both polyethylene terephthalate foam boards with a thickness of 3 mm respectively. They are mainly used to separate the graphene-metal mixed layer 1 and the graphene-electrolyte-graphene sandwich structure 3 and the graphene-electrolyte-graphene sandwich structure 3 and the metal frequency selective surface layer 5 to prevent voltage leakage to the metal surface. The first dielectric layer 2 and the second dielectric layer 4 are both light in weight, thin in thickness and have good isolation.

[0031] The graphene-electrolyte-graphene sandwich structure 3 is made of a single-layer graphene laid on a PVC substrate, and the thickness of the graphene-electrolyte-graphene sandwich structure 3 is 190 microns. The thickness of the PVC film substrate is 60 microns. The electrolyte layer is a mixture of diethyl methylphthalate, trifluoromethanesulfonyl imide and Tf2N (1-ethyl-3-methylimidazolium bistrifluoromethanesulfonyl imide salt) ionic liquid. Figure 6-7 V in the graphene layer refers to the voltage of the electrodes applied to the two layers of graphene, ranging from 0 volts to +4 volts. The electrolyte layer is 1-butyl-3-methylimidazolium hexafluorophosphate of Aiwei Scientific Research brand; CAS: 174501-64-5; ≥99%; [BMIM] PF6.

[0032] Figure 4 This is an equivalent circuit diagram of the principle of a transmission-adjustable flexible frequency selective absorber based on graphene. The specific principle is as follows: the frequency selective surface layer is equivalent to a parallel circuit of inductor L2 and capacitor C2 in this design, and the graphene-metal hybrid layer is equivalent to an RLC circuit. Within the working frequency band, the fan-shaped multilayer graphene in the graphene-metal hybrid layer forms two absorption peaks with the metal frequency selective surface of the bottom layer, and the surface current can flow through the fan-shaped patch at f1 and f3, so the multilayer graphene absorbs electromagnetic waves in two adjacent absorption bands due to ohmic loss. On the contrary, resonance occurs at the four metal arms at f2 and the metal frequency selective surface, which prevents the surface current from flowing along the lossy patch, so a transmission window with low loss can be achieved, allowing electromagnetic waves to pass through.

[0033] like Figure 5-7As shown, no matter how the voltage changes, the transmittance, absorbance and reflectance at f1:4GHz-10GHz, f3:14GHz-20GHz do not change significantly, and the absorbance is stable at more than 80%. The absorbance function is realized in the f1 and f3 frequency bands, and the transmittance in the f2:10-14GHz frequency band will change significantly due to the change of the applied voltage. When the applied voltage changes from 0 to 4V, the graphene surface impedance is adjusted to change from 1250 to 280Ω, and the transmittance of the frequency selective absorber at the f2 center frequency band changes from 0.67 to 0.35. When no graphene is added, the transmittance is greater than 80% in the center frequency band, so that most waves can be transmitted. The reflectance is very small in the three frequency bands and does not change significantly. Therefore, the present invention realizes a frequency selective absorber with dynamically adjustable transmission, realizes high absorbance at the f1 and f3 frequency bands, and can make the transmittance dynamically adjustable at the f2 frequency band through the graphene sandwich structure.

Claims

1. A graphene-based transmission dynamically adjustable flexible frequency selective absorber, characterized by: The invention comprises a graphene-metal hybrid layer (1), a first dielectric layer (2), a graphene-electrolyte-graphene sandwich structure (3), a second dielectric layer (4), and a metal frequency selective surface layer (5); wherein the graphene-metal hybrid layer (1) comprises a multi-layer graphene layer (11) and a metal arm (12); the multi-layer graphene layer (11) is a ring structure; the metal arms (12) are four and are arranged at equal intervals on the periphery of the multi-layer graphene layer (11); the metal arms (12) are respectively connected to the inside of the multi-layer graphene layer (11); the metal frequency selective surface layer (5) is a copper sheet etched into a complementary circular ring periodic structure; the multi-layer graphene layer (11) is grown on a pre-patterned nickel foil, and by controlling the growth temperature to 903 degrees Celsius, a fan-shaped structure with a square resistance of 70 ohms / sq is obtained.

2. The graphene-based transmission dynamically adjustable flexible frequency selective absorber according to claim 1, characterized in that: The graphene metal hybrid layer (1) and the metal frequency selective surface layer (5) are both formed by laminating PVC on a copper foil, then printing a pattern on the laminated copper foil using a laser printer, then etching the printed laminated copper foil in a nitric acid aqueous solution and using the printed toner as an etching mask, and the laminated copper under the toner is not etched to produce a pattern made of the laminated copper foil.

3. The graphene-based transmission dynamically adjustable flexible frequency selective absorber according to claim 1, characterized in that: The graphene sandwich structure (3) comprises two single-layer graphenes and a separator paper arranged between the two single-layer graphenes; the separator paper is soaked in electrolyte, and each of the single-layer graphenes is respectively connected to a bias voltage.

4. The graphene-based transmission dynamically adjustable flexible frequency selective absorber according to claim 3, characterized in that: The electrolyte layer is formed by mixing diethyl methyl phthalate, trifluoromethanesulfonimide and Tf2N ion liquid.

5. The graphene-based transmission dynamically adjustable flexible frequency selective absorber according to claim 1, characterized in that: The outer diameter r1 of the multi-layer graphene layer (11) is 3 mm, the inner diameter r2 is 2 mm, and the length of each metal arm (12) is l =4.97 mm, the gap width w1=0.5 mm, and the arm length width minus the gap width is w2=1 mm; the metal frequency selective surface layer (5) is a 10-micron thick copper sheet etched into a complementary circular ring-shaped periodic structure, with an inner diameter r3=5.14 mm and an outer diameter r4=4.14 mm; the first dielectric layer (2) and the second dielectric layer (4) are both foam boards, the foam board material is polyethylene terephthalate, and the thickness is 3 mm respectively; the graphene-electrolyte-graphene sandwich structure (3) is 190 microns thick; the graphene-electrolyte-graphene sandwich structure (3) is processed by laying a single layer of graphene on a PVC film substrate, and the thickness of the PVC film substrate is 60 microns.

6. The graphene-based transmission dynamically adjustable flexible frequency selective absorber according to any one of claims 1 to 5, characterized in that: The absorber has a thickness of 6 mm, a size of 27 cm×27 cm, and includes 18×18 square ring units; wherein the size of each square ring unit is 15 mm×15 mm.

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