Graphene replaces metal in 5G wireless communications.
For effective information transmission and communication, 5G and 6G networks require more antennas, greater bandwidth, and higher base station density. As a result, the demand for RF electronics will soar in the coming years. For example, 5G base stations worldwide are expected to reach 65 million by 2025. With the explosive growth of radio frequency electronic equipment, especially mobile terminals, electromagnetic pollution is also a problem that needs to be solved urgently.
RF electronics and electromagnetic shielding materials have typically been made from metal-based structures for decades. However, with the growing demand for flexibility, high integration, lightweight manufacturing processes, and operation at higher frequencies, structures based on metal materials are difficult to meet the development needs of radio frequency electronic devices. In addition, as the number of electronic products increases, e-waste impacts the environment, making the sustainable development of next-generation consumer electronics products critical.
Since they were first developed, conductive materials in wireless communications and electromagnetic interference (EMI) shielding devices have been primarily made of metallic structures. He Daping's Wuhan University of Technology team has proposed a graphene assembly film (GAF) that can replace copper in such practical electronic products.
Advantages of GAF
GAF-based antennas have strong anti-corrosion properties.
Figure 1: GAF as a dipole antenna and its anti-corrosion properties. (A) Digital photos of GAF and copper dipole antennas, (B) Measured gains across bandwidths for different material antennas. ( C ) Gain of antennas of different materials at 865 MHz. ( D ) 3D radiation pattern simulation of GAF antenna. ( E ) The radiation pattern of E-plane and H-plane of GAF and copper antenna. (F and G) are the measurement environments of the E-plane and H-plane radiation patterns, respectively. (H) Comparison of dipole antenna gain from ref. and results from this work. (J) Digital photographs of GAF and copper dipole antennas after 1 and 2 weeks of salt spray treatment. ( K ) Measurements and simulations |S 11 | GAF antenna, initial, 1 and 2 weeks after salt spray. ( L ) Measurements |S 11 |Copper antenna, initial, after 1 week and 2 weeks salt spray. ( M ) Gain of GAF and copper antenna measured at 865 MHz under initial salt spray, after 1 week, and after 2 weeks of salt spray.
After two weeks of salt spray treatment, electrical properties such as gain and reflection coefficient remained unchanged, while the copper antenna was damaged by corrosion.
Higher operating bandwidth
The GAF ultra-wideband antenna covers the frequency range from 3.7 GHz to 67 GHz, with a bandwidth (BW) of 63.3 GHz, approximately 110% higher than the copper foil antenna. Compared with copper antennas, GAF fifth-generation (5G) antenna arrays have wider bandwidth and lower side-lobe levels. We also demonstrate that GAF metamaterials act as flexible frequency-selective surfaces, exhibiting promising frequency-selective properties and angular stability.
Higher conductivity
The conductivity of GAF is as high as 2.58×106 S/m. After 200,000 bending tests (bending radius is 1.5mm), the graphene-assembled film can maintain its high flexibility and conductivity without causing structural damage. To study the capabilities of graphene-assembled films in 5G technology and flexible electronics, they demonstrated the performance stability of graphene-based flexible coplanar waveguide transmission lines and resonators under various twisting conditions. Graphene-based dipole antennas have good reflection coefficients and high-gain performance comparable to copper antennas.
Better electromagnetic shielding performance
Figure 2: GAF is used for EMI protection. ( A – F ) EMI SE of 15 μm and 50 μm thickness GAF and 10 μm and 50 μm thickness Cu in the frequency range 2.6 GHz to 0.32 THz (same legend), ( A ) 2.6 to 18 GHz (rectangular waveguide method), (B) 18 to 26.5 GHz (rectangular waveguide method), (C) 26.5 to 40 GHz (rectangular waveguide method), (D) 40 to 67 GHz (free space method), (E) 75 to 110 GHz ( free space method), and ( F ) 0.22 to 0.3235 THz (free space method).
GAF also has better EMI shielding effectiveness (SE) than copper, up to 127 dB in the 2.6 GHz to 0.32 THz frequency range, with an SE per unit thickness of 6,966 dB/mm. Better than copper material of the same thickness.
In addition to complete electromagnetic shielding, selective shielding of electromagnetic fields is also important in many cases to ensure normal transmission in other frequency bands. As a member of metamaterials, FSS is composed of structural units periodically arranged and can selectively absorb, reflect and transmit electromagnetic waves. It is an effective way to achieve frequency selection. After testing, the frequency selection performance of GAF FSS remains stable within the bending angle range of 0°~40°. It maintains good frequency selection performance within the electromagnetic wave incident angle range of ±30°. Has high angular stability.
Made by GAF
Figure 3: GAF characterization. (A) TEM image of a typical large-sized GO sheet with a lateral dimension of 108 μm. (B) Optical microscopy image of LGO sheets prepared by drop-casting LGO solution on the SiO surface. Inserted is a statistical study of the corresponding LGO size distribution. ( C ) XRD pattern and Raman spectrum of GAF. ( D ) Summary of electrical conductivity of GAF, graphite, highly oriented pyrolytic graphite (HOPG), and typical sheet-sized GO assembled films (TGF), with SAXS patterns inset. (E) The resistance change of GAF after 200,000 repeated bending tests demonstrates long-lasting flexibility and stability. (F) The 860 MHz skin depth is related to the conductive layer thickness of GF, graphene ink, carbon nanotubes (CNT), and MXene prepared by chemical reduction, vacuum filtration, thermal reduction GO membrane, and other methods. (G and H) Simulated (G) and measured (H) transmission coefficients of MTL. (I and J) Conductor losses at different frequencies (I) and conductivities (J). ( K ) Flexible GAF FCPW TL bent in diameters of 60 mm, 40 mm, 20 mm and twisted 180°. ( L ) Transmission coefficient of FCPW TL under different conditions in the frequency band between 10 MHz and 40 GHz. ( M ) Flexible GAF λ/4 short-circuit resonator under different twisting conditions: untwisted, twisted 90°, 180°, 360°, and 540°. ( N ) Reflection coefficient results of the resonator under different torsion conditions between the 10 MHz and 12 GHz frequency bands.
Two main approaches have been adopted to achieve high conductivity in graphene-based laminates. First, the research team maximized the size of the graphene crystallites, which allowed the research team to reduce the impact of contact resistance. Secondly, the research team introduced a secondary annealing process and special assembly technology, which can assemble such graphene crystallites into a continuous film with a high degree of lamination and defect-free.
First, GO was prepared by a modified Hummers method. LGO was separated, collected, and used as a precursor for film fabrication. LGO was separated from the GO suspension (3 wt.%) after seven repeated centrifugations (the bottom 30% GO was collected each time). Statistical study of LGO lateral size through optical microscopy: 74% of LGO lateral size >75 μm, and 54% of LGO >100 μm. Typical flake-sized graphene oxide (TGO) was used during synthesis and as a control experiment. LGO-assembled films are prepared by roller transfer coating of pre-metered LGO hydrogel on a self-peeling substrate such as polyethylene terephthalate (PET) film.
Rapid self-assembly of ultrathin graphene oxide film and application to silver nanowire flexible transparent electrodes - RSC Advances (RSC Publishing)
Subsequently, the LGO hydrogel on the substrate was heated (70 to 80°C) for drying. After that, the soft, dark brown, free-standing paper-like film (LGO film) can be easily peeled off from the PET substrate. In this step, the anisotropic liquid crystalline behavior of the LGO hydrogel can produce pre-aligned oriented structures after force-directed rolling transfer.
This highly ordered laminate can be converted into meter-long dimensions of virgin glass fibers through a high-temperature graphitization process. The LGO film was thermally annealed at 1,300°C for 2 hours and 2,850°C for 1 hour in an Ar atmosphere between two graphite plates for reduction and graphite crystallization. The coalescence of adjacent reduced LGO sheets forms the huge crystalline graphite domain size in GAF.
After initial high-temperature annealing, GF is fully graphitized, allowing dangling bond-free graphene nanosheets to tile each other through large-area planar contacts. Rolling compression with a pressure of 300 MPa was further introduced to obtain the final GAF. Subsequent rolling compression helps eliminate interlayer gaps and contact resistance and results in excellent flexibility.
After rolling compression, a secondary high-temperature annealing process at 2850°C is performed in an Ar atmosphere to further eliminate structural damage during the rolling process and improve electrical conductivity. The same method was used, but TGF was obtained from TGO.
Source From Carbontech
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