Synthesis of nitrogen-doped graphene-based binary composite aerogels for ultralightweight and broadband electromagnetic absorption

The development of ultralightweight and broadband electromagnetic wave (EMW) absorbing materials remains a big challenge. In this work, porous magnesium ferrite microspheres decorated nitrogen-doped reduced graphene oxide (NRGO/MgFe₂O₄) composite aerogels were prepared by a two-step route of solvothermal synthesis and hydrothermal self-assembly. Results of microscopic morphology characterization showed that NRGO/MgFe₂O₄ composite aerogels had a unique hierarchical porous structure. Moreover, the influence of additive amounts of graphene oxide on the electromagnetic parameters and EMW absorption properties of NRGO/MgFe₂O₄ composite aerogels was explored. Remarkably, the attained binary composite aerogel with the content of NRGO of 70.21 wt% exhibited the best EMW absorption performance. The minimum reflection loss reached up to −55.7 dB, and the corresponding effective absorption bandwidth was as large as 5.36 GHz at a thin matching thickness of 1.98 mm. Furthermore, when the matching thickness was slightly increased to 2.29 mm, the widest effective absorption bandwidth was enlarged to 7.1 GHz, covering the entire Ku-band. The magnetodielectric synergy and unique hierarchical porous structure in NRGO/MgFe₂O₄ composite aerogels not only improved the impedance matching, but also greatly enhanced the EMW absorption capacity. It was believed that the results of this work could be helpful for the preparation of graphene-based magnetic composites as broadband and efficient EMW absorbers.     

Construction of core-shell ZnO@ZnO/FeNi microrods with improved impedance matching and electromagnetic wave absorption performance

Functional core-shell heterostructure, which can integrate the characteristics of multiple components to achieve synergistic effects, have been widely explored in electromagnetic wave (EMW) absorption materials. In this work, core-shell ZnO@ZnO/FeNi microrods (MRs) derived from ZnO@ZnFeNi hydroxide (ZnFeNi OH) are prepared by a simple hydrothermal reaction and subsequent pyrolysis process. The introduction of FeNi alloy helps to optimize the impedance matching of ZnO, thus improving the EMW absorption performance. The different impedance matching properties of core-shell ZnO@ZnO/FeNi MRs are realized by adjusting the ZnO/FeNi shell thickness by changing the hydrothermal reaction time. When the hydrothermal time is 10 h, the core-shell ZnO@ZnO/FeNi MRs supplies the optimal EMW absorption performance with the minimum reflection loss of −53.7 dB and the widest absorption bandwidth of 5.3 GHz at a filler content of 33%. The synergistic effect of ZnO–FeNi interfacial polarization and the strong dielectric-magnetic loss are responsible for its superior EMW absorption performance. This work provides a valuable strategy for constructing core-shell dielectric@ magnetic composites to obtained high efficiency absorber.     

Synthesis and tunable electromagnetic shielding and absorption performance of the three-dimensional SiC nanowires/carbon fiber composites

Silicon-carbide nanowires (SiCnws) have been considered as dielectric loss materials for application in the field of electromagnetic wave (EMW) attenuation. In this study, SiCnws/carbon fiber (CF) composites were fabricated using precursor infiltration and pyrolysis process for the in-situ growth of SiCnws. The SiCnw fraction of the SiCnws/CF composites could be adjusted using various catalysts. At a small SiCnw fraction (38 wt%), the composites exhibited excellent EMW absorption performance with the minimum reflection loss of ? 18.3 dB when their thickness was only 1.2 mm and can cover the entire X and Ku bands by adjusting the material thickness. They transformed from EMW absorption performance to electromagnetic interference (EMI) shielding property with the increase in SiCnw fraction from 38 wt% to 73 wt%, reaching an EMI shielding effectiveness of 31.25 dB. In addition, the density of the SiCnws/CF composites was only 0.31–0.41 g/cm³, and their compressive strength can reach 0.61–0.99 MPa with excellent high-temperature stability. Therefore, the SiCnws/CF composite presents a promising EMW absorption and EMI shielding material that can be applied in harsh environments.     

Synthesis of PPy/Ni/RGO and enhancement on its electromagnetic wave absorption performance

The ternary composites of hollow tubular polypyrrole (PPy) and Ni particles based on reduced graphene oxide (RGO) were synthesized by a low-energy method. PPy and Ni particles were uniformly distributed on the surface of RGO. Specifically, multilayer interfaces existed in the hybrid, residual defects and folded structures of RGO and the hollow tubular structures of PPy made a significant contribution to electromagnetic (EM) wave attenuation. The final results showed that the strongest reflection loss (RL) value of PNR-2 sample could achieve ??47.32?dB at 5.76?GHz. The PNR-3 sample's RL value was ??18.21?dB at 15.92?GHz with the thickness of only 1.5?mm, and its corresponding effective absorption bandwidth reached 4.32?GHz (13.68–18?GHz). Hence, with the changes of PPy mass ratio, the impedance matching and attenuation were regulated to realize remarkable EM wave absorption performance including lightweight, thin thickness, wide bandwidth and strong absorption.     

Morphology-controlled preparation and tunable electromagnetic wave absorption performance of manganese dioxide nanostructures

The development of electromagnetic wave (EMW) absorption materials with lightweight, wide absorption bandwidth, thin thickness, and strong EMW absorption performance has become a hotspot. Herein, the morphology-controlled preparation of α-manganese dioxide (α-MnO₂) was successfully obtained via a facile hydrothermal method, and the EMW absorption performance of α-MnO₂ was investigated in detail. The results indicated the as-obtained Mn-1.0-120 possessed the best EMW absorption performance with minimum reflection loss of −53.43 dB at about 5.2 GHz with a thickness of 4.1 mm originated from the synergistic effects of multiply scattering, dielectric loss, and magnetic loss. This contribution demonstrates that the MnO₂ has promising candidates with a tunable EMW absorption performance for applications in the electromagnetic field in the future.     

The heterointerface of graphene in-situ growth for enhanced microwave attenuation properties in La-doped SiBCN ceramics

The electromagnetic wave (EMW) absorbing materials are widely applied to attenuate the useless and harmful EMW generated from wireless communication and 5G networks, which could protect the human health and electronic device safety. In this study, La-doped SiBCN ceramics with broadband EMW absorption capability were prepared via generating abundance of heterointerfaces, as graphene were in-situ grown by La₂O₃ catalyzing. The graphene in-situ formed in the ceramics can be attributed to the La atom decreasing the potential energy of the free carbon ring nucleation from −760.9 Ha to −8984.3 Ha. Consequently, the electrical conductivity of the SiBCN ceramics improved from 12.360 S/m to 18.025 S/m, the minimum reflection loss (RL_min) obtained was −26.48 dB at 7.2 GHz and the effective absorption bandwidth (EAB) was 6.32 GHz (11.68–18.00 GHz) at a thickness of 1.7 mm. At 700 °C, the RL_min and EAB values reached −43.18 dB and 4.2 GHz, respectively. The enhanced EMW absorbing capability can be attributed to the rationally tailor the heterointerfaces to improve the polarization loss and conduction loss of the SiBCN ceramics. The interfaces between graphene and amorphous phases generate built-in electric fields and space chare regions to strengthen the interface polarization, while the electrons migrating rapidly in the graphene and other crystals improved the electrical conductivity. The positive effect of heterointerfaces regulation of graphene in-situ growth improved the dielectric loss capacity of the SiBCN ceramics; therefore, this study provides a feasible method to enhance the EMW absorption capability of polymer-derived ceramics.     

Continuous growth of carbon nanotubes on the surface of carbon fibers for enhanced electromagnetic wave absorption properties

As electromagnetic wave (EMW) pollution has become a serious problem in daily life, lightweight, efficient, and mass-produced EMW-absorbing materials are urgently needed. Herein, we developed a novel method for the continuous growth of carbon nanotubes (CNTs) on the surface of polyacrylonitrile (PAN)-based carbon fibers (CFs) by chemical vapor deposition (CVD), which can be applied to mass production. The obtained CF/CNT composites demonstrate outstanding EMW absorption capability, exhibiting a -58.75 dB reflection loss (RL) at a thickness of 1.54 mm. An effective absorption bandwidth (RL < -10 dB) of 4.24 GHz (13.76–18.00 GHz) was achieved at a thickness as low as 1.25 mm, which almost covers the entire Ku band. The excellent EMW-absorbing performance can be attributed to the 3D conductive network constructed by the CNT forest, which effectively promotes multiple reflections and scattering, and further favors dipole and interface polarizations. The mechanical properties of CF, CF-electrochemical anodic oxidation (EAO), and CF/CNT composites were examined, the results showed that the single-filament tensile strength of CF/CNT@0.07 and CF/CNT@0.09 was effectively improved. Our work suggests that the novel CF/CNT composite is a promising material for EMW absorption and strength enhancement owing to its light weight, high strength, low thickness, and good scale-up ability.     

Ni-MOF/Ti3C2Tx derived multidimensional hierarchical Ni/TiO2/C nanocomposites with lightweight and efficient microwave absorption

Benefiting from its large specific surface area, abundant defects and functional groups, two-dimensional (2D) laminated Ti₃C₂T_x MXene is a kind of electromagnetic wave (EMW) absorber with great potential. However, the impedance mismatch caused by the excessive conductivity, inappropriate permittivity and lack of magnetic loss seriously hinders the application of MXene to EMW absorption. Herein, multidimensional hierarchical Ni/TiO₂/C nanocomposites composed of three-dimensional (3D) hydrangea-like Ni/C microspheres and well-arranged 2D carbon sheets embedded with TiO₂ nanoparticles were successfully fabricated from a Ni-based trimellitic acid framework (Ni-BTC) and Ti₃C₂T_x MXene via facile in-situ solvothermal assembly and annealing processes. As expected, excellent EMW absorption properties were obtained only by changing the annealing temperature. The minimum reflection loss (RL_min) value of -45.6 dB and the effective absorption bandwidth (EAB) of 3.40 GHz (14.6–18.0 GHz) with a layer thickness of only 1.5 mm is obtained by annealing the sample at 700 °C. The outstanding ternary multilayer structure and the optimization of magnetoelectric synergy in impedance matching jointly create its remarkable EMW absorption performance. This work is expected to provide a simple and effective method to design MXene-based EMW absorbing materials possessing high absorption intensity, light weight, wide EAB and thin thickness.     

EMW absorption properties of in-situ growth seamless SiBCN-graphene hybrid material

A three-dimensional graphene/SiBCN hybrid composite was prepared by mixing and annealing Polyborosilazanes and sugar at 1200?°C. In this regard, the interface between the ceramic and the graphene formed a stable chemical bonding such as C-B-N, not physically absorbed as traditional meaning. Compared with the purity SiBCN ceramic, the hybrid material possessed a significant improvement on electromagnetic wave (EMW) absorption properties. When the thickness of the hybrid composites fixed on 2.5?mm, the maximum reflection coefficient (RC) increased from 1.36 to 24.24?dB and the bandwidth over 10?dB increased from 0 to 5.2?GHz. In addition, the bandwidth over 10?dB covered the whole X band when the thickness reached 3?mm. In short, this simple method was deemed to be a useful technology to fabricate a unique structure with excellent EMW absorption properties.     

Core-shell MXene/nitrogen-doped C heterostructure for wide-band electromagnetic wave absorption at thin thickness

MXenes have been utilized to fabricate electromagnetic wave (EMW) absorbers owning to large aspect ratio, high electronic conductivity, and favorable hydrophilicity. In this work, the core-shell MXene/nitrogen-doped (N-doped) C heterostructure was firstly prepared via HCl and LiF etching, in-situ polymerization, and carbonization. When mixed with paraffin at a low filler loading of 30 wt%, the MXene/N-doped C hybrid reached a wide effective absorption bandwidth of 5.0 GHz (13.0 GHz–18.0 GHz) at a thin thickness of 1.72 mm. The stronger ability of attenuating EMWs promoted the absorption performance of MXene/N-doped C, overcoming the deficiency in the characteristics of impedance compared with its counterparts. This work provides a new insight in manufacturing MXene-based absorbers to alleviate EMW pollution by delicate structural design and effective multi-component strategy.     

Electromagnetic wave absorbing properties of In–Sn/rGO composites supported by impurity defect

With the fast development of E-communication technology, effective electromagnetic wave absorbing materials are highly needed to address the growing electromagnetic pollution. Herein, Indium doped tin microsphere/reduced graphene oxide (In–Sn/rGO) composites with rich impurity defects were synthesized via the sol-gel and hydrothermal method. The excellent microwave absorption of In–Sn/rGO composites can be attributed to the modifications of electronics status and Fermi energy level after In doping. This can significantly increase the carrier mobility between In–Sn microspheres and rGO sheets to strike a superior interfacial polarization loss. As a result, the maximum absorptivity can reach ?51.16 dB at 8.73 GHz (thickness: 3.5 mm) with a lower filler loading of 10 wt%. Meanwhile, the synthesized In–Sn/rGO composites also exhibit an ultra-wide absorbing frequency range of 13.84 GHz (within the X band, Ku band, and most of the C band). This research provides a new idea for the synthesis of effective microwave absorbing material by introducing impurity defects.     

Metal ions induced dielectric and magnetic loss in Co3O4 for electromagnetic wave absorption

Despite Co₃O₄ has been widely applied in electromagnetic wave (EMW) absorbers, single Co₃O₄ doesn't have excellent EMW absorbing performance. Modification of Co₃O₄ with other metal ions addition is an effective way to improve its impedance matching and EMW attenuation. Herein, CuO/Cu_0.3Co_2.7O₄/Co₃O₄ and NiCo₂O₄/Co₃O₄ composites have been obtained via a facile two-stage strategy, and the influence of Cu²⁺ and Ni²⁺ on the high-frequency and low-frequency EMW absorbing performance of the composites has been investigated as well. The electromagnetic parameters of samples are regulated by adding different metal ions to achieve optimum impedance matching. Dipole polarization and magnetic resonance are the main loss mechanisms. The composite with Cu²⁺ and Ni²⁺ additions exhibits the best EMW absorption with an effective absorption bandwidth (EAB) of 10.8–18.0 GHz for 2.1 mm thickness at high-frequency and 4.5–8.5 GHz for 4.9 mm thickness at low frequencies, respectively. This work offers an effective method for preparing composite materials with multicomponent broadband absorption of oxides.     

Polyaniline-based networks combined with Fe3O4 hollow spheres and carbon balls for excellent electromagnetic wave absorption

Polyaniline (PANI)-based networks combined with Fe₃O₄ hollow spheres and carbon balls (FCP) for improved electromagnetic wave (EMW) absorption were investigated using an easy-to-industrialize solvothermal and physical method. Hollow structure Fe₃O₄ spheres with a lower density than that of the common solid sphere were prepared. As a thin and light magnetic material, Fe₃O₄ hollow spheres generate magnetic loss, carbon balls and PANI networks generate dielectric loss. The magnetic and conductive parts play appropriate roles in achieving complementarity in the EMW absorption. The relatively high specific surface area introduced by PANI networks promotes interfacial polarization and further supports dielectric loss. In conclusion, the above reasons provide multiple attenuation mechanisms. Samples FCP1 (?65.109 dB, at 12.800 GHz, 1.966 mm, from 5.6 to 18.0 GHz) and FCP2 (?61.033 dB, at 8.480 GHz, 3.328 mm, from 4.3 to 18.0 GHz) demonstrated a wide bandwidth, a small thickness, a minimum reflection loss (R_L), and a low loading ratio (25%) in paraffin-based composites. Specifically, their loading ration of 25% is much lower than the loading ratio of conventional materials (usually 50% and above). In addition, the bandwidth is excessively wide, above 12 GHz, possessing good absorption performance in continuous intervals with different thicknesses. Such excellent characteristics have rarely been reported in literature.     

Polymer-derived Co2Si@SiC/C/SiOC/SiO2/Co3O4 nanoparticles: Microstructural evolution and enhanced EM absorbing properties

In this work, porous core-shell structured Co₂Si@SiC/C/SiOC/SiO₂/Co₃O₄ nanoparticles were fabricated by a polymer-derived ceramic approach. The in situ formation of mesopores on the shell, microstructural, and phase evolution of resulting nanoparticles were investigated in detail. The obtained nanoparticles-paraffin composites possess a very low minimum reflection coefficient (RC_min) −60.9 dB, broad effective absorption bandwidth 3.50 GHz in the X-band and 15.5 GHz in the whole frequency range (from 2.5 to 18 GHz). The results indicate outstanding electromagnetic wave (EMW) absorbing performance among all the reported cobalt-based nanomaterials, due to the reasons as follows: (a) The unique core-shell structure as well as complex phase composition of SiC/C/SiOC/SiO₂/Co₃O₄ in the shell, result in a large number of heterogeneous interfaces in the nanoparticles; (b) Nanoparticles have both dielectric and magnetic loss; (c) Mesopores in the shell prolong the propagation path of EMW, thereby increasing the absorption/reflection ratio of EMWs. Thanks to the material structure design, the resulting core-shell structured cobalt-containing ceramic nanoparticles have great potential for thin and high-performance EMW absorbing materials applied in harsh environment.     

Dual gradient direct ink writing of functional geopolymer-based carbonyl-iron/graphene composites for adjustable broadband microwave absorption

As a novel type of moulding technology, additive manufacturing (AM) can realise the rapid manufacturing of complex structures. This research applied dual gradient direct ink writing (DGDIW) with in-situ dispersion to fabricate multi-materials in an extensive gradient range. Functionally graded materials (FGM) have emerged as intelligent composites with peculiar advantages in wave absorption applications. An FGM geopolymer containing carbonyl-iron and graphene (CIG) powders with graded structure was successfully fabricated to enhance the wave-absorbing property via DGDIW three-dimensional (3D) printing. Compared with the non-structured homogeneous geopolymer composites, FGM composites performed more consecutive and effective absorbing from 2 to 18 GHz, resulting in a significantly increased electromagnetic (EM) wave absorption property. Combining with the gradient content of CIG fillers, the impedance matching and electromagnetic attenuation of 3D printed gradient lattice structure geopolymer composites has been significantly improved, the minimum reflection loss can reach ?46.47 dB at 17.58 GHz with a broadband absorption of 14.62 GHz (3.38–18.00 GHz). The results provided a promising strategy for fabricating functional graded ceramic composites with great potential as an absorption device in wave absorption applications, especially in protective structures, radiation-proof equipment/architecture, and defence constructions.     

Comparative study of microwave absorption properties of Ni–Zn ferrites obtained from different synthesis technologies

Spinel ferrites are widely used for electromagnetic wave (EMW) absorption applications. In this study, spinel Ni–Zn ferrites with excellent microwave absorption properties were synthesized. Their EMW absorption characteristics and interaction mechanisms were studied to lay the foundation for the study of the role of Ni–Zn ferrite as a magnetic substrate for composites. Herein, Ni_0·5Zn_0·5Fe₂O₄ was prepared by the hydrothermal method (H-NZFO) and the sol–gel auto-combustion method (S-NZFO); both samples exhibited distinct microwave absorption properties. The S-NZFO absorber (thickness = 3.72 mm) demonstrated the best dual-zone microwave absorption with two strong reflection loss peaks at 5.1 and 10.5 GHz. The corresponding effective absorption bandwidth (EAB) reached 9.0 GHz, which covered part of the S-band and all of the C- and X-bands. These results were attributed to the high saturation magnetization, outstanding complex permeability, and multiple magnetic loss channels of S-NZFO. The H-NZFO sample exhibited excellent absorption capability and matching thickness. At a thickness as low as 1.71 mm, the minimum reflection loss (RL_min) of the H-NZFO absorber reached -60.2 dB at 13.1 GHz. The maximum bandwidth corresponding to RL below -10 dB was 4.6 GHz. These results can be attributed to small particle size, high complex permittivity, and multiple dielectric loss channels of H-NZFO. The observed wide effective absorption bandwidth of S-NZFO and strong microwave absorption capability of H-NZFO suggest the potential of both materials as substrates for efficient microwave absorbers in military as well as civilian absorption applications.     

Electromagnetic wave absorption properties of polymer-derived magnetic carbon-rich SiCN-based composite ceramics

In this paper, the mixture of Fe and Ni nanoparticles (abbreviated as FeNi) was added to liquid polysilazane (PSZ) as a magnetic source, to prepare a series of magnetic carbon-rich SiCN-based composite ceramics by adjusting the mass ratio of FeNi through the polymer derivation method. The phase composition, microstructure, conductivity, electromagnetic wave (EMW) absorption performance and mechanism of composite ceramics prepared were discussed. The analysis shows that the introduction of magnetism has adjusted the impedance matching and improved the magnetic loss performance of composite ceramics on the whole, and the dielectric loss ability of composite ceramics has been strengthened benefiting from the formation of conductive path of CNTs precipitated by FeNi catalysis in the matrix. Therefore, the addition of magnetic particles improves the EMW absorption peak intensity and effective absorption bandwidth (EAB) of composite ceramics. When the addition amount of FeNi was 5 wt%, the sample 5# exhibited the best comprehensive EMW absorption performance: Its minimum reflection loss (RL_min) was ?18 dB and the EAB was 2.5 GHz when the thickness was 1 mm, the EAB covering the C, X and Ku bands can be obtained by adjusting the thickness from 1.0 mm to 4.0 mm. Through calculation, the EAB (EAB_tf) of 5# with a thickness of 1 mm and a filling rate of 1 wt% can reach 50, which is significantly higher than that of a series of SiCN-based composite ceramics previously reported. In addition, the density of 5# was 2.3 g/cm³, and its compressive strength (CS) can reach 337 MPa. The data shows that the composite ceramic 5# prepared in this experiment has the merits of light weight, excellent comprehensive EMW absorption performance and good compression resistance, and is expected to be one of the promising materials in the field of new-generation EMW absorbers.     

Ultralight and efficient microwave absorption of SiC/SiO2 ceramic aerogels derived from biomass

Rational multicomponent regulation and microstructure design have proven to be effective strategies for achieving high performance electromagnetic wave (EMW) absorbers. Herein, the ultralight hierarchically porous SiC/SiO₂ aerogels (HPSA) were successfully synthesized by an ingenious one-step method to achieve carbonization and carbothermal reduction. The composition of the HPSA and the quantity of SiC/SiO₂ fibers grown by in situ reaction can be controlled by adjusting the amount of silicon source introduced. The results indicate that the composition of HPSA and the quantity of fibers have a significant effect on the EMW absorption properties. When the introduced silicon source concentration was 0.7 mol/L, the HPSA exhibited excellent EMW absorption performance, with a minimum reflection loss (RL_min) of -55.01 dB at 6.00 GHz and a maximum effective absorption bandwidth (EAB_max) of 6.16 GHz. The highly interconnected porous SiC/SiO₂ skeleton structure significantly contributes to the multiple reflection-absorption effect of EMW and provides available pathways for electron conduction losses. The in situ reaction generates SiC/SiO₂ fibers with a large number of stacking faults and heterojunctions, which further promote the dissipation of EMW. In addition, the maximum radar cross section of HPSA under far-field conditions is reduced to 20.21 dB m² compared to the PEC conductive layer, which implies a much lower probability of detection by radar. In brief, this work provides a reference for the use of highly efficient EMW absorbers and electromagnetic stealth materials.     

Optimization of microwave absorption properties of C/NiP microfiber composites

Core-shell C/NiP microfiber composites were fabricated via electroless plating in this work. Their microstructure and electromagnetic properties were adjusted by annealing treatment and different phosphorus (P) content in the NiP coating. The C/NiP microfibers composites with 6 wt % P content in the NiP coating annealed at 300 °C remarkably owns ?46.7 dB strong reflection loss at 10.0 GHz with the thickness of 2.0 mm, and the effective bandwidth (RL ≤ ?10 dB) reaches 8.1 GHz (3.1–11.2 GHz). This work shows that the annealing conditions and different P concentrations in the coating layers for C/NiP microfibers composites are the effective way to optimize the electromagnetic wave absorption performance.     

Dielectric and electromagnetic wave absorption properties of reduced graphene oxide/barium aluminosilicate glass–ceramic composites

BaAl₂Si₂O₈ (BAS) glass–ceramic powders were prepared by sol–gel method. Graphene oxide (GO)/BAS mixture powders were prepared by a simple mixing process of GO and BAS. Dense and uniform reduced graphene oxide (RGO)/BAS composites were fabricated by the hot-pressing of GO/BAS, which was accompanied by the in-situ thermal reduction of GO. Microstructure, phase composition, dielectric and electromagnetic wave (EM) absorption properties of RGO/BAS were investigated. The results reveal that RGO can promote the hexacelsian-to-celsian phase transformation of BAS. In the frequency range from 8 GHz to 12 GHz, the complex permittivity of RGO/BAS increases with increasing RGO content. The composite with 1.5 wt% of RGO shows good EM absorbing ability. When the sample thickness is 2.1 mm, the minimum reflection coefficient (RC) reaches −33 dB, and the effective absorption bandwidth is more than 3.1 GHz.