Broadband Wave Research Articles

Construction of cobalt zinc metal-organic frameworks derived Co/CoO/C composites toward broadband and high-efficiency electromagnetic wave absorption

The fabrication of novel carbon-based electromagnetic wave (EMW) absorbers with broad absorption bandwidth, strong absorption strength, low filler loading ratio and thin matching thickness remains a challenging task. Metal-organic frameworks (MOFs) are widely recognized as ideal self-sacrificial templates for the construction of carbon-based EMW absorbers. In this work, bimetallic CoZn-MOFs derived Co/CoO/C composites were prepared by combining magnetic stirring with subsequent pyrolysis treatment. The results showed that the morphology of carbon skeletons evolved from an irregular agglomerate to a uniform hexagonal star shape and then to an agglomerate again by reducing the molar ratio of Co2+ to Zn2+. Remarkably, excellent EMW absorption capacity in the Ku-band was achieved through carefully regulating the molar ratio of Co2+ to Zn2+. When the filling ratio was 20wt.%, the prepared Co/CoO/C composite exhibited the best EMW absorption performance with the minimum reflection loss of -64 dB at a thickness of 1.79mm and the maximum effective absorption bandwidth of 6GHz at a thickness of 2.14mm. Furthermore, the radar cross section in the far-field was simulated by computer simulation technique. In addition, the possible EMW attenuation mechanism was proposed. Therefore, the results of this paper could be helpful for developing carbon-based magnetic composites derived from MOFs as broadband and high-efficiency EMW absorbers.

Study on the influence of heave plate on energy capture performance of central pipe oscillating water column wave energy converter

This paper proposes a novel central pipe oscillating water column (OWC) wave energy converter (WEC) with a heave plate and investigates the influence of heave plate on energy capture performance of the OWC WEC through numerical methods. First, dynamic equations are established for floater and water column. Then, energy capture performance of the WEC is computed by solving relative motion equations using numerical methods. Wave tank experiments are conducted to validate the proposed numerical models. Finally, the influences of heave plate opening diameter and connecting column length on energy capture performance are investigated based on the proposed numerical models. The results indicate that increasing heave plate opening diameter decreases both the maximum capture width ratio (CWR) and response period. And increasing connecting column length widens the response bandwidth. When the heave plate opening diameter decreases from 350 mm to 0, the maximum CWR increases from 30.05% to 41.68% and the response period increases from 1.28 s to 1.72 s. When the connecting column length increases from 225 mm to 600 mm, the response band extends by 0.1 s. The proposed device and the obtained analysis results are of great significance for broadband and efficient wave energy generation.

Fabrication of core–shell nickel ferrite@polypyrrole composite for broadband and efficient electromagnetic wave absorption

Herein, nickel ferrite@polypyrrole (NiFe2O4@PPy) composite was prepared by a simple two-step route of solvothermal synthesis and in-situ oxidation polymerization reaction. The results of micromorphology analysis showed that the obtained NiFe2O4@PPy composite had a unique core–shell structure, good magnetic performance and electrical conductivity. Furthermore, the as-prepared magnetic/conductive NiFe2O4@PPy composite exhibited notably enhanced electromagnetic wave (EMW) absorption performance than pure NiFe2O4 and PPy. When the filling ratio was 17.5 wt% and the matching thickness was 2.24 mm, the optimal minimum reflection loss (RLmin) of −56.25 dB and a broad effective absorption bandwidth (EAB) of 5.2 GHz were achieved for NiFe2O4@PPy composite. With slightly increasing the matching thickness to 2.6 mm, the maximum EAB of 7.12 GHz could be reached. Additionally, the probable EMW absorption mechanism was elucidated. Therefore, this work could provide a valuable reference for the preparation of magnetic ferrite/conductive polymer composites as broadband and high-efficiency EMW absorbers.

Synergistically magnetic and dielectric properties of two dimensional Fe3Al@PPy lamellae exhibiting broadband and strong electromagnetic wave absorption

Addressing the issue of low impedance characteristics is essential to improve the electromagnetic wave absorption performance of magnetic materials. Herein, two dimensional Fe3Al@polypyrrole (PPy) lamellae with synergistic magnetic and dielectric properties are fabricated by a mechanical alloying, ordering transformation and polymerization process, which exhibits excellent electromagnetic wave absorption performance. By carefully controlling the thickness of the PPy shell, the optimized Fe3Al@PPy lamellae show a minimum reflection loss of −45.6 dB and an effective absorption bandwidth of 9.1 GHz at a thickness of only 1.5 mm. The conformal growth of Fe3Al@PPy lamellae can induce strong interfacial polarization, dipole polarization, multiple scattering effect and magnetic loss behaviors for the attenuation of electromagnetic waves. This study demonstrates a facile strategy for the development of efficient Fe3Al@PPy composite absorbents showing great potential for practical applications.

Three-Dimensionally Printed K-Band Radar Stealth Lightweight Material with Lotus Leaf Structure.

K-band radar waves have high penetration and low attenuation coefficients. However, the wavelength of this radar wave is relatively short; thus, designing and preparing both broadband and wide-angle radar wave absorbers in this band presents considerable challenges. In this study, a resin-based K-band radar wave absorber with a biomimetic lotus leaf structure was designed and formed by UV curing. Here, microscale lotus leaf papillae and antireflection structures were prepared using a DLP 3D printer, and the contact angle between the material and water droplets was increased from 56° to 130°. In addition, the influence of the geometric parameters of the lotus leaf antireflection structure on the electromagnetic absorption performance and mechanical strength was investigated. After simulation optimization, the maximum electromagnetic loss of the lotus leaf structure 3D-printed sample was -32.3 dB, and the electromagnetic loss was below -10 dB in the 20.8-26.5 GHz frequency range. When the radar incidence angle was 60°, the maximum electromagnetic loss was still less than -10 dB. The designed lotus leaf structure has a higher mechanical energy absorption per unit volume (337.22 KJ/m3) and per unit mass (0.55 KJ/Kg) than commonly used honeycomb lightweight structures during the elastic deformation stage, and we expect that the designed structure can be used as an effective lightweight material for K-band radar stealth.

Heterogeneous interface engineering of spindle shaped Fe/C through optimizing impedance response for enhanced microwave absorption and thermal conductivity

Heterogeneous interface design is pivotal in creating lightweight, broadband, and high-efficiency electromagnetic wave (EMW) absorption materials. These materials enhance wave absorption by adaptively matching impedance. In this study, spindle-shaped Fe/C nanocomposites with heterogeneous interfaces and diverse surface microtopographies were synthesized by calcining iron-supported metal-organic frameworks (MOFs). During controlled pyrolysis, Fe3+ was reduced by carbon and conversely induced the graphitization of carbon, which is essential for the distribution and appearance of Fe and graphitized carbon as well as for modulating impedance matching. The porous structure of the Fe/C composites and the polarization loss and ferromagnetic resonance from the defective carbon and Fe magnetic particles synergistically enhance the wave absorption performance. The Fe/C-700 nanocomposite achieves a reflection loss (RL) of −55.9 dB at 12.48 GHz with an effective absorption bandwidth (EAB, RL ≤ −10 dB) of 3.9 GHz at 1.8 mm. The Fe/C-900 nanocomposite demonstrates a significant RL of −63.5 dB at 15.8 GHz with an EAB of 4.4 GHz between 13.6 GHz and 18 GHz at 1.5 mm, and an EAB of 5.5 GHz at 1.63 mm. These improvements are attributed to the impedance matching facilitated by the introduction of Fe magnetic particles and graphitized carbon. Additionally, the thermal conduction pathway formed among the Fe/C nanocomposites significantly enhances the thermal conductivity to 0.502 W m−1 K−1. Thus, this work offers a novel approach to the design of advanced wave-absorbing and thermal conductive materials by optimizing impedance through heterogeneous interfaces.

Modulational instability of nonuniformly damped, broad-banded waves: Applications to waves in sea ice

Modulational instability of nonuniformly damped, broad-banded waves: Applications to waves in sea ice

Terahertz-absorbing,dual-polarization converting supersurface structures based on the phase-change material VO2

Dynamically tunable multifunctional hypersurface structures have attracted the interest of researchers due to their good control of electromagnetic waves. Based on this, we have designed a dynamically tunable multifunctional hypersurface structure using the phase change material vanadium dioxide. By changing the state of vanadium dioxide, three functions of efficient broadband wave absorption, linear polarization conversion (LPC) and linear circular polarization conversion (LCPC) can be achieved. At room temperature, the hypersurface is used as a polarization converter to achieve broadband linear polarization conversion in the 0.903 THz range and line-circular polarization conversion with a total bandwidth of 0.608 THz in the 0.522 THz, 0.964 THz and 1.448 THz bands. When the temperature is raised to 68 °C, the hypersurface structure can be used as a broadband absorber, absorbing more than 90% of the incident wave in a wide frequency range of 1.348 THz and showing a large tolerance to variations in the angle of incidence and polarization. The electromagnetic field distribution is used to illustrate the absorption mechanism, and Stokes analysis is introduced to evaluate the polarization transition state. The designed structure can realize three electromagnetic modulation functions by changing the excitation conditions and can cover a wide range of frequency bands. Finally, we used coding to simulate the effect of loss on the wave absorption results in real applications and verified the stability of the structure. Our design provides new ideas for the development of terahertz communication and multifunctional integrated devices.

A new seismic metamaterial design with ultra-wide low-frequency wave suppression band utilizing negative Poisson's ratio material

In this paper, we present a new seismic metamaterial (SM) for the suppression of Lamb waves and surface waves utilizing the negative Poisson's ratio (NPR) foam. The periodic cell of the metamaterial is composed of a square cylinder made of steel in the center and NPR foam connecters at the corners. Utilizing the unique resonance properties of the NPR material, an ultra-low-frequency broadband wave suppression is achieved. First, the characteristics of bandgaps (BGs) of the proposed structure with different materials were analyzed using the finite element (FE) method. It shows that the application of the NPR foam can significantly broaden the BGs compared with the case using traditional rubber. The vibration modes were analyzed to investigate the mechanism of BG generation, and the results show that the broad BG is achieved via the unique local resonance mechanism of the NPR foam. Subsequently, the frequency-domain analysis was carried out to verify the shielding effect of the designed SM on Lamb waves, using a periodic structure consisting of 10 × 6 unit cells. It shows that the designed SM can effectively attenuate Lamb waves in the range of 3–44 Hz with attenuation up to −590 dB at 32 Hz. A parametric analysis shows that the NPR foam with small thickness, high modulus, low mass density, and low Poisson's ratio can broaden the first complete BG. In addition, the acoustic cone method was combined with the frequency-domain analysis to calculate the BGs of the proposed structure against the surface wave, using a periodic structure consisting of 20 unit cells. It shows that the surface wave can be significantly attenuated within the BG. Finally, real seismic excitations were applied for time-domain analyses, and the simulation results show that the proposed structure can effectively shield the low-frequency seismic surface waves in the range of 0.1–20 Hz. The design scheme proposed in this paper is simple in structure and easy to implement, and is believed to possess bright application potentials in seismic protection.

Hierarchical porous SiCnws/SiC composites with one-dimensional oriented assemblies for high-temperature broadband wave absorption

The research on high-performance electromagnetic wave absorption materials with high-temperature and oxidative stability in extreme environments is gaining popularity. Herein, the lightweight silicon carbide nanowires (SiCnws)/SiC composites are fabricated with in-situ SiC interface on one-dimensional oriented SiCnws skeleton, which collaborative configuration by 3D printing and freeze casting assembly. The constructed porous structure optimizes the impedance matching degree and scattering intensity, the maximum effective absorption bandwidth (EABmax) of 5.9 GHz and the minimum reflection loss (RLmin) of −41.4 dB can be realized. Considering the inherent oxidation resistance of SiC, the composites present well-maintained absorption performance at 600 °C. Even at 1100 °C, the EABmax of 4.9 GHz and RLmin of −30.4 dB also demonstrate the high-temperature absorption stability of the composites, indicating exceptional wave absorption properties and thermal stability. The slight attenuation can be attributed to the decrease in impedance matching capability accompanying the elevated dielectric constant. This work clarifies the impact of structure and component synergy on wave absorption behavior, and offers a novel approach to producing high-performance and high-temperature resistance ceramic-based electromagnetic wave absorption materials suitable for extreme environments.

Development of generic no-scale inflation

We develop generalized no-scale supergravity models of inflation, and then study the corresponding cosmological predictions as well as the formation of primordial black holes (PBHs) and scalar-induced gravitational waves (SIGWs). With a new parameter 0 < a ≤ 1, the generalized no-scale supergravity provides the continuous connections among the generic no-scale supergravity from string theory compactifications. The resulting prediction of the CMB, spectrum index ns , and tensor-to-scalar ratio r can be highly consistent with the latest Planck/BICEP/Keck Array observations. Notably, the models with a ≠ 1 give a smaller ratio r ≤ 10-3, which is flexible even under the anticipated tighter observational constraints at the future experiments. Additionally, these models have the potential to generate a broad-band stochastic gravitational wave background, and thus explain the NANOGrav 15yr signal. Furthermore, they predict the formation of PBHs with various mass scales, which could account for a significant portion of dark matter relic density in the Universe.

3D-printed multi-material pyramids for broadband electromagnetic wave absorption

Designing and manufacturing broadband electromagnetic wave-absorbing materials represents one of the significant approaches to address electromagnetic pollution issues. This study utilizes three different PLA-based composite absorbing materials to prepare multilayer pyramid structures employing 3D printing technology. Results indicate that with material composition Z1-GT6-N8, material layer height (h) of 6 mm, and pyramid base length (L) of 36 mm, the fabricated structure achieves a minimum reflection loss of −24.19 dB with an effective absorption bandwidth of 14.21 GHz, demonstrating outstanding absorption performance. It is noteworthy that regardless of the incident angle increasing from 0° to 50°, the equivalent absorption bandwidth (EAB) for both transverse electric (TE) and transverse magnetic (TM) polarizations remains greater than 10 GHz, indicating excellent angle stability. The broadband and efficient electromagnetic wave (EMW) absorption characteristics exhibited by this absorber can be attributed to its outstanding impedance matching performance and the integration of multiscale loss mechanisms.

Three-dimensional phononic crystals with self-similar structures

Acoustic metamaterials have the advantages of designability, strong pertinency, small size and good effect, and have good application value in solving the problem of sound insulation and noise reduction. Phononic crystals with wide bandgap and multi-bandgap can inhibit elastic wave propagation to some extent. In this study, a three-dimensional phononic crystal model with self-similar properties is designed by using fractal method. First, an initial unit is constructed, then the arm of the initial unit is replaced with the structure itself to form a self-similar structure. The self-similar model can block sound waves in the wide band and multi-band range. By changing the structure shape and size of phononic crystal, the sound wave blocking in different frequency range is also studied. At the same time of continuous optimization of the structure, the variation rules of the model band structure under different parameters are summarized. To find the good parameters of broadband and multi-band sound wave blocking, so as to achieve the effect of vibration isolation and noise reduction. The finite element method is used to simulate the vibration of the model to verify the existence of elastic wave bandgap. Phononic crystals have a good prospect in the field of sound insulation and noise reduction.

Design of millimeter wave broadband polarization reconfigurable horn antenna based on elliptical waveguide polarizer

AbstractA broadband polarized reconfigurable horn antenna for K‐band and Ka‐band is designed using a circular polarizer in the form of an elliptical waveguide and a circular waveguide flange based on a gap structure. The designed noncontact circular waveguide flange with gap structure can switch the antenna excitation signal to different modes of electromagnetic waves by mechanical rotation. When excited by different modes of electromagnetic waves, the designed elliptical waveguide circular polarizer can switch between linear polarization, left‐handed circular polarization, and right‐handed circular polarization. The antenna is analyzed in principle, modeled and simulated, and processed by three‐dimensional printing process. The measured results show that S11 ≤ −15dB, axial ratio ≤ 3 dB, gain ≥14 dBi in 37.8% relative bandwidth range (22.5–33 GHz). The proposed antenna is broadband, multipolar, and reconfigurable, which makes it have broad application prospects in the field of satellite communication and millimeter‐wave communication.

Construction of 1D heterogeneous PPy@FeCo@PPy nanotubes with broadband and strong electromagnetic wave absorption

Developing nanomaterials with efficient electromagnetic wave absorption to eliminate increasingly serious electromagnetic pollution is of great significance. The peculiar electromagnetic characteristics of one-dimensional dielectric-magnetic heterostructures bring a promising strategy for designing high-efficiency absorber. Herein, a hierarchical PPy@FeCo@PPy nanotube was fabricated through a process of oxidative polymerization and electroless plating. In the product, the FeCo nanoparticles are encapsulate in the middle of the polypyrrole (PPy) dual conductive layers, which facilitates the deep construction of heterointerfaces and simultaneously prevents magnetic agglomeration. Furthermore, the hollow structure and suitable dielectric-magnetic component optimize impedance matching. Profiting from the reasonable structural design and the strong synergistic loss mechanism of dielectric-magnetic, the coaxial multi-shell PPy@FeCo@PPy nanotube exhibits prominent electromagnetic wave absorption performance, with a minimum reflection loss of −50.5 dB and the effective absorption bandwidth of 5.7 GHz at 2.0 mm. This work offers a gentle strategy for preparing ideal microwave absorbers by constructing dielectric-magnetic heterogeneous interfaces.

Doped Porous Carbon Spheres with Controllable Vesicle Structure: Preparation and the Effects of Pore Size on Electromagnetic Wave Absorption Properties.

This work reports on the preparation of uniform vesicle-structural carbon spheres doped with heteroatoms of N, P, and S, with the pore sizes strictly controlled by the hard templates of monodisperse submicron SiO2 spheres. The uniformly doped vesicular carbon microspheres are obtained in three steps: Stöber hydrolysis for the SiO2; in situ polymerization for the immobilization; and alkaline etching after carbonization. The size of the vesicles can be easily adjusted by regulating the particle size of the submicron SiO2 spheres, which has a significant effect on its electromagnetic wave (EMW) absorption performance. Compared with microspheres with pore sizes of 180 and 480nm, when the vesicle aperture is 327nm, with only 5.5wt.% filling load and 1.9mm thickness, the material shows the best EMW absorption behavior with the effective absorption bandwidth (EAB) covers the entire Ku band (6.32GHz) and the minimum reflection loss (RLmin) of -36.10dB, suggesting the optimized pore size of the microspheres can significantly improve the overall impedance matching of the material and achieve broadband wave absorption. This work paves the way for the enhancement of EMW absorption properties of porous material by optimizing the pore size of uniform apertures while maintaining their composition.

Giant Terahertz Birefringence in an Ultrathin Anisotropic Semimetal.

Manipulating the polarization of light at the nanoscale is key to the development of next-generation optoelectronic devices. This is typically done via waveplates using optically anisotropic crystals, with thicknesses on the order of the wavelength. Here, using a novel ultrafast electron-beam-based technique sensitive to transient near fields at THz frequencies, we observe a giant anisotropy in the linear optical response in the semimetal WTe2 and demonstrate that one can tune the THz polarization using a 50 nm thick film, acting as a broadband wave plate with thickness 3 orders of magnitude smaller than the wavelength. The observed circular deflections of the electron beam are consistent with simulations tracking the trajectory of the electron beam in the near field of the THz pulse. This finding offers a promising approach to enable atomically thin THz polarization control using anisotropic semimetals and defines new approaches for characterizing THz near-field optical response at far-subwavelength length scales.

Structural Engineering of Hierarchical Magnetic/Carbon Nanocomposites via In Situ Growth for High-Efficient Electromagnetic Wave Absorption

HighlightsHierarchical Fe3O4-Fe@CNFs/Al-Fe3O4-Fe nanocomposites were constructed by in situ growth, vacuum-assisted filtration, and self-reduction methods.The carbon framework, with in situ grown magnetic nanoparticles, supports two-dimensional magnetic nanosheets, achieving excellent electromagnetic performance and good impedance matching.Excellent reflection loss value (− 59.3 dB), broadband wave absorption (5.6 GHz at 2.2 mm thickness), and low radar cross-section value were achieved.

Aperiodic Two-Dimensional Acoustic Black Holes for Broadband Vibration Attenuation in a Strip

Abstract Purpose Acoustic black holes (ABHs) are promising for vibration control in lightweight structures as proven for one- or two-dimensional periodic arrangements. Here, we explored the effects of spatial disorder and heterogeneous designs of ABHs to broaden an intrinsically limited attenuation bandwidth of periodic counterparts. Method We proposed several strategies to introduce non-periodic arrangements and/or different ABH profiles by solving a maximization problem for the attenuation bandwidth of a plate strip decorated by five ABHs. These strategies allow for finding appropriate dimensions and positions of the ABHs by analyzing a small design subset and are verified experimentally. Results The identified periodic heterogeneous ABHs enable greatly extending the attenuation bandwidth, while disordered identical ABHs allow for increasing the attenuation intensity as compared to the corresponding periodic configurations. The mechanisms underlying the wave attenuation enhancement were clarified by tracing the evolution of the wave transmission and structural vibration modes at each design step. We have found that the broadened wave attenuation attributes to the activation of strongly localized modes at broadband frequencies in aperiodic scenarios. These abundant modes are multi-frequency local resonances in ABHs that are sensitive to both the ABH profile and their spatial arrangement. Conclusion We prove that relaxing the periodicity requirement on multiple two-dimensional ABHs can extend the vibration attenuation to broadband regimes below the ABH characteristic frequency, numerically and experimentally. Aperiodic designs of ABHs thus enlarge the design space by enabling a broadband wave mitigation with attenuation intensity comparable to that of periodic counterparts without increasing the structural mass.

A magnetic metal composite biomass carbon material with high efficiency and broadband wave absorbing property

A magnetic metal composite biomass carbon material with high efficiency and broadband wave absorbing property