Minimum Reflection Loss Value Research Articles

Optimization of electromagnetic wave absorption properties of CoNi/MoSe2 composites with 3D flower-like by controlling the Co/Ni molar ratio

Developing composite materials with a rational composition structure has been a critical strategy to obtain superior electromagnetic (EM) wave absorption materials. CoNi/MoSe2 composites were synthesized by a two-step method in this work. Firstly, the flower-like MoSe2 was synthesized by hydrothermal method, and then the CoNi alloy was loaded on the MoSe2 surface by an electroless plating method. It is worth noting that in this paper, PdCl2 was not used for activation during the electroless plating process, and the flower-like structure of MoSe2 was not destroyed. In order to investigate the effect of different Co/Ni molar ratios on the absorption performance of the CoNi/MoSe2 composites, three samples with Co to Ni molar ratios of 1:3, 1:1, 3:1 and two other samples with only one metal (Ni or Co) were prepared. The analysis results show that the minimum reflection loss(RL) value of Ni/MoSe2 is −18.01 dB and the maximum effective absorption bandwidth(EAB) of Co/MoSe2 is 2.16 GHz. In comparison, the CoNi/MoSe2 composite exhibits the best EM wave absorption performance when the Co/Ni ratio is 1:3, and the minimum RL can reach −48.6 dB at a matching thickness of only 1.8 mm. And when the matching thickness is 1.4 mm, the broadest EAB is 3.76 GHz. Therefore, this work provides a new research idea for the design of broadband and efficient EM wave absorbers.

Facile synthesis of heterogeneous Co/MnO@C nanocapsules with dual-cores: Achieving strong and broadband microwave absorption by magnetic-dielectric synergy

Nowadays, constructing a high-efficiency microwave absorber with strong absorption strength and wide absorption bandwidth is urgently demanded for practical applications in the microwave absorption (MA) field. In this work, the Co/MnO@C nanocapsules with magnetic/dielectric dual cores were fabricated through in-situ carbon reduction of the MnCo2O4 nanoparticles. The effects of calcination temperature on electromagnetic parameters and MA ability are investigated in detail. Notably, the sample obtained at 900 ℃ exhibits the minimum reflection loss (RL) value of −97.5 dB at 10.6 GHz, while the effective absorption bandwidth (EAB, RL ≤ −10 dB) is up to 8.1 GHz (9.9–18 GHz) at 2.1 mm thickness, covering half of X-band and whole Ku-band. The excellent MA performance is attributed to the core–shell structure, multiple heterogeneous interface polarizations, synergistic effects between magnetic and dielectric components, as well as promoted impedance matching. Importantly, the electric and magnetic field distributions are simulated by ANSYS software, and the polarization and magnetic coupling behaviors are deeply analyzed. The simulated radar cross-section (RCS) results confirm that the optimized Co/MnO@C nanocapsules can efficaciously attenuate more electromagnetic energy in the practical environment. It is believed that the as-prepared Co/MnO@C nanocapsules are considered to be promising high-efficiency microwave absorbers for practical applications.

Interfacial Polarization-Dominated Dielectric Loss in SnO2@rGO Electromagnetic Wave Absorbers

Interfacial polarization is generally a major cause of dielectric loss, but its exact contribution to the electromagnetic wave (EMW) absorption capacity of absorbers remains to be elucidated. In this work, SnO2@rGO composite (S2) with tight interfaces formed by chemical bonds and SnO2/rGO mixture (S3) were synthesized by a simple chemical route followed by further calcined in argon, respectively. Compared with pure SnO2 (S1) and S3, S2 exhibits much better EMW-dissipation ability, with a smaller minimum reflection loss (RLmin) value of −20.5 dB at a matched thickness of 5 mm and a larger effective absorption bandwidth (fe) value of 5.8 GHz (from 11 GHz to 16.8 GHz) at 3.2 mm. By comprehensively comparing the defects, dipoles, and interfaces in S2 and S3, it is concluded that the excellent EMW absorption capacity of S2 is mainly caused by strong dielectric loss dominated by interfacial polarization as well as suitable impedance matching. This study provides an insight into the exact contribution of interfacial polarization to the EMW-dissipation ability of absorbers, showing that the EMW absorption of graphene-based composites can be effectively promoted by constructing well-connected interfaces between graphene and absorbers.

Optimal particle distribution induced interfacial polarization in hollow double-shell composites for electromagnetic waves absorption performance

Tunable designs of polymorphic structured transition metal dichalcogenide (TMDC) demonstrate promising applications in the field of electromagnetic wave absorption (EMW). However, it remains a technical challenge for achieving a balanced relationship between well-matched impedance characteristics and dielectric losses. Therefore, the co-modification strategies of polydopamine coating and wet impregnation are chosen to construct CoS2 magnetic double-shell microspheres with phase component modulation to achieve the optimized performance. Dopamine hydrochloride forms a coating on the surface of CoS2 microspheres by self-polymerization and forms a double-shell structure during the pyrolysis process. Then the different metal is doped to generate heterogeneous components in the process of heat treatment. The results show that the cobalt doped double-shell microspheres have an ultra-high electromagnetic wave absorption absorption capacity with an effective absorption bandwidth of 5.04 GHz (1.98 mm) and a minimum reflection loss value of −48.90 dB. The double-shell layer structure and metal ion hybridization can improve the interfacial polarization and magnetic loss behavior, which provides an explicit inspiration for the development of transition metal dichalcogenide and even transition metal compounds with tunable absorption properties.

Electromagnetic wave absorption properties of the FeSiAl/PLA and FeSiAl-MoS2-Graphene/PLA double-layer absorber formed by fused deposition modeling

The FeSiAl/PLA and FeSiAl-MoS2-Graphene (GR)/PLA composite filaments were prepared through mechanical mixing and melt extrusion process, and the double-layer absorbers while the material of layer 1 is FeSiAl/PLA and the material of layer 2 is FeSiAl-MoS2-GR/PLA were prepared by fused deposition modeling (FDM) 3D printing technology. The results show that the FeSiAl/PLA and FeSiAl-MoS2-GR/PLA double-layer absorber can effectively improve impedance matching. The electromagnetic wave absorption performance is improved under the synergistic effect of multi-loss mechanisms. The minimum reflection loss (RLmin) value of the double-layer absorber is −52.50 dB at 17.76 GHz with the thickness of FeSiAl/PLA layer is 0.6 mm and the total thickness of 1.7 mm. The effective absorption bandwidth (EAB) of the double-layer absorber reaches 5.92 GHz (12.08 ∼ 18 GHz) with the thickness of the two layers is 0.8 mm and 1.2 mm, respectively. It is 29.8 % wider than the EAB (4.56 GHz) of the single-layer FeSiAl-MoS2-GR/PLA composite with the same thickness.

Order effect of nitrogen and phosphorus co-doping carbon nanofibers for enhancing electromagnetic wave absorption

In this work, N, P co-doped carbon nanofibers (CNF-NP and CNF-PN) were successfully synthesized by a two-step pyrolysis, and the effect of doping orders on microwave absorption (MA) was investigated for the first time. It was found that graphitic-N produced from the first N-doping suppressed the formation of C3PO due to the competition of doping sites for CNF-NP. Therefore, extra P atoms combined with oxygen-containing groups formed lots of C-OPO3 during the subsequent P-doping. However, for CNF-PN large amounts of C3PO were easily formed when P atoms were first doped, which limited the production of graphitic-N and resulted in the ratio improvement of pyrrolic-N and pyridinic-N. It was found that the ratios of different doping sites obviously influenced the MA performance. CNF-PN exhibited the minimum reflection loss (RLmin) value of −55.0 dB at 2.3 mm and effective bandwidth (EAB) of 6.6 GHz, attributing to the dielectric loss increased by an appropriate ratio of C-OPO3 and a high ratio of pyrrolic-N and pyridinic-N. In contrast, RLmin of CNF-NP was only −30.8 dB at 1.7 mm with the EAB of 3.3 GHz, due to the impedance mismatching from too much C-OPO3. This work sheds new light on adjusting MA properties by doping orders, which is of great significance for the development of novel absorbers based on carbon nanofibers.

Microstructure evolution and microwave absorbing properties of novel double-layered SiC reinforced SiO2 aerogel

In this paper, a novel double-layered SiC reinforced SiO2 aerogel (SSA) was successfully synthesized by a facile route. The porous double-layered SiC microstructure was constructed by chemical vapor deposition (CVD) technology, and SiO2 aerogel was introduced by sol-gel method to adjust the dielectric properties. The microstructures of the interconnected double-layered SiC structure remained intact after SSA formation. The porous SiO2 aerogel particles were filled between the double-layered SiC structure, and the stem segments of the double-layered SiC structure were wrapped by SiO2 aerogel. In addition, SiO2 aerogel spread into the hollow pores of the double-layered SiC, forming a large number of interfaces. As expected, SSA could be used as an absorber. The minimum reflection loss (RL) value of SSA achieved − 22.80 dB at 15.77 GHz at the thickness of 3.57 mm. The optimal effective absorption bandwidth (fE) of 6.8 GHz was recorded at a 4.0 mm absorber thickness. The absorber thickness of 3.5–5.0 mm was easy to obtain wider fE and smaller RL, which provided practical guidance for the application of SSA. Finally, the microwave absorbing mechanism of SSA was studied deeply. The importance of attenuation constant (α) was more significant than the impedance matching ratio (Zr) for the microwave absorption performance of SSA. The higher Zr ascertained that a large proportion of the electromagnetic waves entered the SSA absorber, and then the α supported the microwave absorbing mechanisms.

Effect of B4C content and annealing on complex permittivity and microwave-absorption properties of B4C/Al2O3 coatings

The B4C/Al2O3 coatings were fabricated by air plasma spraying technology, and their complex permittivity and microwave absorption properties in the X-band were investigated before and after annealing (500 °C/2 h). Both the real and imaginary parts of the complex permittivity of the coatings decreased after annealing, which can be attributed to the weakening of polarization relaxation intensity and the reduction of electrical conductivity caused by the escape of carbon atoms. In addition, the density of B4C/Al2O3 coatings decreased from 3.01 to 2.16 g/cm3 with increasing B4C content. The B4C/Al2O3 coatings exhibit a minimum reflection loss (RL) value of −39.58 dB and the effective absorption bandwidth (RL＜−10 dB, EAB) covers 1.9 GHz at a thickness of 1.6 mm. After annealing, the above coatings still had an EAB of 1 GHz. Therefore, the B4C/Al2O3 coatings can be considered as a promising microwave-absorption candidate with good high-temperature microwave-absorbing performance and low density.

1D-2D heterostructured silicon carbide fibers@WS2 with high efficiency and broad bandwidth for microwave absorption performance

In this study, 1D@2D hetero-structured silicon carbide fibers coated by tungsten disulfide (SiCf@WS2) was successfully synthesized, and the microwave absorption performance was explored. It was found that the minimum reflection loss value reached −72.76 dB at 12.74 GHz with the thickness of 2.86 mm for 15 wt% loading of SiCf@WS2/wax. The effective absorbing bandwidth (EAB, <−10 dB) could reach up to 7.7 GHz with the thickness of 2.84 mm. The SiCf provided a wide surface for WS2 nanosheets to grow on, where the WS2 played a leading role for microwave absorption. WS2 film created more interfaces not only between SiCf and WS2 sheets but among the WS2 nanosheets themselves, which resulted in more interfacial polarization and dielectric loss enhancing the microwave absorbing abilities. In addition, the electronic properties of hetero-interface were calculated by density functional theory (DFT). The results showed that with the electronic charges transferring through the heterointerface, the interfacial polarization and dipole polarization were hugely improved. All above indicated that the hybrid of SiCf@WS2 was a promising microwave absorbing material through a facile synthesis process with low cost, which can be excellent candidate for the efficient microwave absorbing applications.

Carbon-encapsulated Co7Fe3 nanocomposites with high intensity and ultra-wideband microwave absorption

Composites composed of ferromagnetic alloy/dielectric material have been widely studied and applied. The alloying of the two metals endows these composites with escalated magnetic loss capacity, but they are usually micro-sized, which is not conducive to further improving the absorption performance. Herein, we develop an efficient approach to synthesize Co7Fe3@C nanocomposites with particle sizes ranging from 32 to 42 nm based on the in-situ carbon coating on the FeCo2O4 precursor and subsequent carbothermal reduction procedure. The thickness of carbon coating can be effectively tailored by controlling the reaction conditions. The minimum reflection loss (RL) value of the Co7Fe3@C nanocomposite with a carbon coating of 2.5 nm goes up to −100.8 dB at only 1.7 mm thickness, and the effective absorption bandwidth (EAB) also reaches 9.9 GHz (8.1–18 GHz) at 2.0 mm. Moreover, as the carbon coating increases to 4.2 nm, the bandwidth escalates to 11.5 GHz (6.5–18 GHz) at 2.0 mm, which covers the whole X and Ku bands and half of the C band. The remarkable performance originates from the strong magnetic loss capability of the nano-sized Co7Fe3 core as well as the good impedance matching brought by appropriate carbon coating. Accordingly, the Co7Fe3@C nanocomposite is convinced to be a competent candidate among absorbers with high absorption intensity and ultra-wide broadband.

A two-dimensional semiconductive Cu-S metal-organic framework for broadband microwave absorption

Electromagnetic radiation and pollution are urgent and crucial problems to be solved. Metal-organic framework (MOF) derivates play an important role in the attenuation of electromagnetic waves efficiently. However, limited by the complex pyrolysis process, the chemical composition and crystal structure of MOF were totally destroyed. Inspired by the electrical window (10−6–10 S·cm−1) of microwave absorption materials (MAMs), semiconductive MOF with precise crystal structure is potential to be an ideal microwave absorbent. This work reports a two-dimensional (2D) semiconductive MOF (Cu-S-MOF) as broadband high-performance MAMs. The effective absorption bandwidth (EAB) of the Cu-S-MOF can reach 6.72 GHz (9.68–16.4 GHz) at a thickness of 1.5 mm, and a minimum reflection loss (RLmin) value of −52.8 dB at 1.69 mm. Due to the absence of the pyrolysis process, a well-defined single-crystal structure of Cu-S-MOF can be obtained. A density functional theory (DFT) calculation was performed to illuminate the microwave absorption mechanism. Resistance loss and interfacial polarization loss between the Cu-S layers mainly contribute to the Cu-S-MOF MAMs. The semiconductive MOF towards MAMs provides a facile and effective strategy to tune microwave absorption performance, which can be applied in the electromagnetic protection field.

Comprehensive study of electromagnetic wave absorption properties of GdMnO3-MoSe2 hybrid composites

Recently, the public does not avoid the use of RF/microwave communication and non-communication devices, which means they will be constantly exposed to radiation from those devices, which will worsen their health in the long term, as well as there will be interference and coupling effects between devices. In this paper, a novel and high-performance absorber material is created and analyzed. The microscopic and macroscopic properties of Gadolinium Manganite (GdMnO3), Molybdenum Diselenide (MoSe2), and three mixtures of GdMnO3-MoSe2 (GdMo) with 20, 30, and 40% filler loading were investigated. The procedure for producing the five materials was explained and described in detail. The mixture of GdMo with 20, 30, and 40 wt% filler loading has the features of GdMnO3 and MoSe2. For macroscopic analysis, the measured relative complex permittivity and permeability of the five synthesized materials are modeled using the Lorentz dispersion model. Among the five study materials, GdMo with 40 wt% filler loading shows the highest electric and magnetic losses with loss tangent, tan δe ≈ 0.228 and tan δm ≈ 0.38 where the absorption performance is better compared to its pristine components, namely GdMnO3 and MoSe2. Based on the transmission theory of metal plate backing materials, the GdMo with 40 wt% filler loading and thicknesses ranging from 0.0014 m to 0.0021 m, which is terminated by a metal plate on the back side, exhibited the optimum microwave absorption performance with a minimum reflection loss value better than −20 dB at X-band operating frequency. Without a metal backing, a 0.008 m thickness of GdMo with 40 wt% can achieve reflection loss, RL of −10 dB from 9.2 GHz to 12.4 GHz, and RL of −20 dB from 11.1 GHz to 12.4 GHz.

Lightweight waterproof magnetic carbon foam for multifunctional electromagnetic wave absorbing material

Achieving multifunctionality to broaden the application scenarios is the development direction of electromagnetic wave (EMW) absorbing materials in the future. Here, a waterproof magnetic carbon foam (CF) was successfully prepared using a simple high-temperature pyrolysis and dip coating process, where the magnetic Fe3O4 nanoparticles were homogeneously and tightly encapsulated on the carbon skeleton of CF with the help of low surface energy PDMS, achieving the expected high-efficient EMW absorption performance. Except the 3D porous structure that contributes stronger EMW absorption through multi-reflection and scattering, the coexistence of conductive carbon skeleton and magnetic Fe3O4 can supply the dielectric-magnetic dual-loss, which is also beneficial for better impedance matching. In addition, the amounts of heterointerfaces and abundant crystalline defects and doped N atoms in graphitic carbon can also generate additional polarization loss. Furthermore, high-efficient EMW absorption capacity was achieved by tuning the pyrolysis temperature, of which the CF pyrolyzed at a temperature of 900 °C (PDMS/Fe3O4/CF-900) exhibited a minimum reflection loss (RLmin) value of −47.36 dB with 3.15 mm thickness and a maximum effective absorption bandwidth (EABmax) up to 9.76 GHz with 4.2 mm thickness. What's more, combining with the merits of high conductivity, hydrophobicity, and special 3D porous structure, the expected multifunctionality was also successfully achieved for the prepared PDMS/Fe3O4/CF-900, where it displays the excellent thermal management performance including a high Joule heating temperature of 161 °C at only 5 V and reliable heating capacity over 1500 s, high-efficient oil-water separation capacity such as high absorption amount (45.58 times), rapid absorption (1.14 s), and stable cyclic adsorption-desorption performances, and great thermal insulation property with a temperature difference up to 219.9 °C that make it to be promising infrared stealth material. This work provides a feasible strategy for the rational design and application of novel lightweight multifunctional high-performance EMW absorbing materials.

Construction of SiCnws/carbon foam composites with multiple loss mechanisms for excellent electromagnetic wave absorption

Composition optimization and delicate microstructure construction are considered to be critical factors to achieve outstanding electromagnetic (EM) wave absorption performance. SiC nanowires/carbon foam (SiCnws/CF) composites were prepared by the in-situ growth of SiCnws inside carbon foam pores using an internal growth method. After self-foaming of the silicon powder and mesophase pitch mixture, a carbonizing process was adopted subsequently to synthesis SiCnws/CF composites with excellent EM wave absorption performance. With increasing silicon content, the amount of SiCnws generated increases, the distribution of SiCnws becomes uniform, and the diameter decreases continuously. The prepared SiCnws were composed of β-SiC coated with an amorphous Si layer on the surface, which were conducive to the reception of EM wave and impedance matching to free space. Multiple loss mechanisms involving the dipole polarization generated by SiCnws, the interface polarization generated by multi heterogeneous interfaces and the conductive loss provided by CF, which caused high effective energy loss of incident EM wave, hence, improved the EM wave absorption performance significantly. After the addition of silicon, the SiCnws/CF composites obtained excellent impedance matching. When the silicon content was 15 wt %, the prepared SiCnws/CF composite showed the best wave absorption performance, the minimum reflection loss value of the 3.11 mm thick sample was − 55.27 dB, and the effective absorption bandwidth was 2.08 GHz, which provides a new method toward the high-performance EM wave absorbing materials with multiple loss mechanisms.

Constructing hierarchical Ti3SiC2 layer and carbon nanotubes on SiC fibers for enhanced electromagnetic wave absorption

To enhance the absorption performance of silicon carbide fiber (SiCf), hybrid fibers with a double shell structure (Ti3SiC2 and carbon nanotubes (CNTs)) on the SiCf (CNT@Ti3SiC2@SiCf) were successfully synthesized by the combination of molten salt method and floating catalytic chemical vapor deposition. A series of 10% weight fraction fibers reinforced paraffin samples was prepared to study the double coating influences on the electromagnetic wave (EMW) absorption performances. Coated by Ti3SiC2 and CNTs, the dielectric permittivity of hybrid fibers could be modulated in a quite wide range. The CNT@Ti3SiC2@SiCf with a thickness of 3.8 mm showed a minimum reflection loss value of −53 dB at 6.57 GHz, and the CNT@Ti3SiC2@SiCf with a thickness of 2.5 mm presented a wide effective absorption bandwidth of 5.6 GHz (from 9 to 14.6 GHz). The highly improved EMW absorption performance of CNT@Ti3SiC2@SiCf was attributed to the combination of conductive loss and dielectric loss aroused by interfaces. The excellent absorption performance provided the modified SiCf with a high potential in the application of EMW absorbers.

Constructing 3D Tent-Like frameworks in melamine hybrid foam for superior microwave absorption and thermal insulation

Taking serious electromagnetic pollution and practical application requirements into consideration, it’s imperative to develop multifunctional microwave absorption materials. In this work, the phenol–formaldehyde resin (PF) successfully adheres to melamine–formaldehyde foam (MF) as three-dimensional (3D) porous frameworks, and subsequently the substrate anchors few-layered MXene nanosheets by dripping-squeeze-drying procedures. According to the results, the hollow tent-like structure of MXene/PF@MF (MPM) shows a significant effect on microwave absorption capability, including extending heterogeneous interfaces for polarization relaxation and prolonging microwave propagation paths for energy dissipation. Based on the synergy of multiple components and rational structure, MPM exhibits a minimum reflection loss (RLmin) value of −58.8 dB at 2.2 mm and achieves an effective absorption bandwidth (EAB) of 6.74 GHz covering the entire Ku band. The microwave absorption mechanism reveals that the superior microwave absorption capability results from high conduction loss, polarization relaxation, multiple attenuations, and appropriate impedance matching. Furthermore, the MPMs possess lightweight features (32.7–48.7 g cm−3) and low thermal conductivity (0.0309–0.0318 W m−1 K−1) benefitting the unique 3D porous structure. Meanwhile, the low thermal conductivity endows MPMs with infrared stealth, which broadens the application in military fields. This work proposes a facile strategy for constructing highly efficient microwave absorbers with multiple functions.

Lightweight 3D interconnected porous carbon with robust cavity skeleton derived from petroleum pitch for effective multi-band electromagnetic wave absorption

Ingenious porous skeleton design is highly desired for advanced carbonaceous electromagnetic wave absorbers (EWAs) to address the issues of impedance mismatch and enhance attenuation ability. Herein, a controllable structure of petroleum pitch-derived coral-like 3D interconnected porous carbon (PC) framework is constructed in one step by a hard-template method. The well-designed air-filled 3D interconnected cavity structure provides ideal impedance matching for absorbers and facilitates the multiple scattering of electromagnetic waves, while continuous robust carbon skeleton exhibits a satisfied electrical conductivity (≈553 S/m), greatly boosting the electrical energy loss capability of PCs. Therefore, even under an ultralow filler loading of only 6 wt%, the corresponding minimum reflection loss (RL) values of multiple thicknesses are all below −20 dB in the range of 1.5–5.0 mm, and the strongest RL of PCs reaches −50.04 dB (matching thickness is 2.37 mm), the widest effective absorption bandwidth is up to 4.4 GHz. The overall performance outperforms most of previously reported carbon-based EWAs. This synthetic strategy presents a broad application prospect due to its simple preparation method and lost-cost raw materials, and provides a new route for the design of advanced lightweight absorbers from abundant inferior heavy oil. Lightweight 3D interconnected porous carbon material with robust cavities skeleton is prepared by a facile template method and yields excellent multi-band electromagnetic wave absorption at an ultralow filler loading. • 3D porous carbon framework with robust cavity skeleton is prepared by a facile salt template strategy. • 3D interconnected cavity structure endows porous carbon with strong conduction loss and excellent impedance matching. • Porous carbon yields a high electromagnetic wave absorption property (−50.04 dB) at an ultralow filler loading of 6 wt%. • Highly-efficient multi-band electromagnetic wave absorption has been achieved by simply tuning the thickness. • This work offers a new route for the design of advanced carbon-based absorbers from low-cost petroleum pitch.

Tunable high-performance microwave absorption of cobalt nanoparticles wrapped in N-self-doped carbon nanofibers at ultralow filler loadings

Manufacturing lightweight, ultralow filler loading, broad bandwidth and highly efficient microwave absorbers is still a challenge. Herein, we have synthesized Co nanoparticles decorated N-self-doped carbon nanofibers (Co@NCNFs) composite by electrospinning followed by a two-step heat treatment and investigated its electromagnetic (EM) and microwave absorption performances under different filler contents (from 2.5 to 7.5 wt%) in paraffin wax matrix. It is found that the Co@NCNFs composite has strong ferromagnetism and moderate graphitization degree as well as abundant polar groups and defects, and Co nanoparticles encapsulated by graphitic carbon layers with sizes around 45–115 nm are uniformly dispersed in the NCNFs. With increasing the filler loading, the EM attenuation ability is gradually strengthened while the impedance matching level is just the opposite. The EM absorbing properties of the composites are more superior within 5.0 wt% Co@NCNFs loading owing to the synergy of improved impedance matching and satisfactory EM dissipation ability. Under an ultralow filler loading of 2.5 wt%, the optimal reflection loss (RL) can reach as strong as −49.3 dB at 17.83 GHz with a thin thickness of 1.61 mm and the maximum effective absorption bandwidth (EAB, RL<−10.0 dB) is as wide as 5.44 GHz (12.56–16.99 GHz) at 1.92 mm. When the filler loading is only 5.0 wt%, a minimum RL value of −52.3 dB is achieved at 6.36 GHz with the thickness of 3.03 mm and the maximum EAB is 4.64 GHz (13.36–18.00 GHz) at 1.42 mm. Compared to other Co/C composite absorbers, the as-prepared Co@NCNFs composite with well-designed constitute and structure demonstrates the better comprehensive absorption performance at a much lower filler loading. Therefore, the present Co@NCNFs may be more promising for practical applications as lightweight and high-performance EM wave absorbers especially in the medium and high frequency range.

Microwave absorption properties of porous activated carbon-based palm oil empty fruit bunch

The increased growth of electronics and wireless communication systems causes the urge to find microwave absorbing materials that are lightweight, inexpensive, and environmentally friendly. In this study, we successfully synthesize activated carbon materials from the oil palm empty fruit bunch (OPEFB) as future microwave absorbing material candidates with a porous structure using different kalium hydroxide (KOH) concentrations during the activation process followed by physical carbonization at 800 °C under an inert atmosphere. The prepared porous activated carbon (PAC)-based OPEFB was characterized by several techniques, including x-ray diffraction, scanning electronic microscopy-energy dispersive x-ray analysis, and Brunauer–Emmett–Teller analysis. The microwave absorption properties of the PAC-based OPEFB were measured using one port rectangular waveguides at the X-band frequency range (from 8 to 12 GHz). A minimum reflection loss value of −32.21 dB (more than 99.9% absorption) at 10.64 GHz is obtained for the PAC-based OPEFB with 20% of KOH concentration. The SEM observations confirm the porous structure of activated carbon, and the BET analysis measures the surface area of the PAC-based OPEFB with 20% of KOH concentration of about 693.06 m2/g. The improvement of reflection loss of the PAC-based OPEFB compared to its raw materials is attributed to the better impedance matching, and multiple reflections occur in the porous structure of activated carbon. This study suggests that the PAC-based OPEFB can become a future potential material for microwave absorbers used in electronics and wireless communications devices.

Rational design of carbon-rich silicon oxycarbide nanospheres for high-performance microwave absorbers

Polymer-derived-ceramics (PDCs) are considered as a promising candidate for excellent microwave absorption under harsh environments due to their controllable microstructure at the molecular level. Herein, a series of silicon oxycarbide (SiOC) nanospheres are successfully fabricated through a PDC method and then pyrolyzed under argon atmosphere at different temperatures and the evolutions of microstructure and their microwave absorption properties are investigated simultaneously. It is found that the microwave absorption properties mainly depend on carbon content. With a relative carbon content of 38.13%, the carbon-rich SiOC nanospheres-1 pyrolyzed at 1400 °C possess excellent microwave absorption properties with a minimum reflection loss (RL) value of −56.28 dB at 6.8 GHz with a thickness of 2.93 mm. Electromagnetic analysis demonstrates that proper impedance matching, intrinsic attenuation ability, and the multiple reflection are responsible for the excellent RL. This work will provide a novel approach to obtain the PDCs with excellent microwave absorption properties.