Enhanced microwave absorption properties of Zn-substituted SrW-type hexaferrite composites in the Ku-band

Enhanced microwave absorption properties were successfully achievable from SrFe_2-xZn_xFe₁₆O₂₇ (SrFe_2-xZn_xW; x = 0.0, 0.5, 1.0, and 2.0) hexaferrite filler-epoxy resin matrix composites. The composite samples were fabricated with the filler volume fractions (V_f) of 30, 50, 70, and 90%. Compared with fully Zn-substituted SrZn₂W composite (x = 2.0), unsubstituted and partially Zn-substituted SrFe_2-xZn_xW (x = 0.0, 0.5, and 1.0) composites exhibited much higher real and imaginary parts of complex permittivity (ε_r), which is attributable to higher electron hopping between Fe²⁺ and Fe³⁺ ions, and also slightly higher real and imaginary parts of complex permeability (μ_r) due to higher saturation magnetization (M_s). Among all samples, a 2.8 mm-thick SrFe_1·5Zn_0·5W (x = 0.5) composite with the V_f of 90% exhibited the most appropriate for application in the region of 3.4–3.8 GHz, having the minimum reflection loss (RL_min) of ?46 dB at 3.6 GHz with the bandwidth of 0.43 GHz (3.38–3.81 GHz) below ?10 dB, while a 2.15 mm-thick SrFeZnW (x = 1.0) composite with the V_f of 70% showed the most appropriate for application in the region of 5.9–7.1 GHz, possessing the RL_min value of ?23.7 dB at 6.6 GHz with the bandwidth of 1.38 GHz (5.85–7.23 GHz) below ?10 dB. Consequently, partially Zn-substituted SrW-type hexaferrites are very promising microwave absorbers for 5G mobile communications in the Ku band (0.5–18 GHz).     

Dimensionality determined microwave absorption properties in ferrite/bio-carbon composites

Composition and structural design play a very influential role in the microwave absorption (MA) manipulation of ferrite/carbon composites. Here, by carefully choosing the dimensionality of the bio-carbon materials, the interfacial geometries and MA properties of ferrite/bio-carbon composites have been controlled effectively. The one dimensional (1D), two dimensional (2D), and three dimensional (3D) biomass-based carbon materials decorated with ZnFe₂O₄ (ZFO) particles were obtained respectively from carbon fibers (1D), tree leaves (2D), wheat straw (2D), peanut shell (3D) and orange peel (3D) by a simple two-step synthesis method. With increasing the bio-carbon's dimensionality from 1D, 2D to 3D, the ferrite/carbon composite's MA properties are promoted and the minimum reflection loss is enhanced from −9 dB to −45 dB. By changing the ZFO/3D-bio-carbon samples' thickness, a broad absorption range from 4 to 18 GHz can be covered. Moreover, the effective absorption bandwidth for ZFO/3D-bio-carbon can be modified up to 7.1 GHz, which covers the whole Ku band. These observations identified the important roles of the ferrite/carbon interface and dimensionality of carbon materials and provided an effective and low-cost route to design microwave absorption materials based on biomass-industrial waste composites.     

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.     

Enhancement on high-temperature microwave absorption properties of TiB2–MgO composites with multi-interfacial effects

TiB₂–MgO microwave absorbing materials with TiB₂ as the absorber, MgO as the matrix are prepared by spark plasma sintering (SPS). The influences of commercial TiB₂ content and sintering temperature on dielectric and microwave absorption (MA) properties are studied. Besides, to optimize the MA performance, TiB₂–MgO composite containing TiB₂ synthesized by the carbonthermal process is prepared. Meanwhile, its high-temperature dielectric and MA properties are investigated. Indeed, both the commercial TiB₂ content and sintering temperature play key roles in dielectric and MA properties, as they reaching 8 wt% and 1400 °C, the composite presents the optimal MA performance. For composite with synthetic TiB₂ as the absorber, the temperature has a positive effect on dielectric and MA properties. The enhanced high-temperature MA properties with minimum reflection loss (RL_min) of ?52.11 dB at 1.6 mm under 500 °C and effective absorption bandwidth (EAB, RL < ?5 dB) of 4.2 GHz at 1.4–1.6 mm under 800 °C are obtained, which is mainly attributed to the temperature-dependent interfacial polarization compared to the temperature-insensitive conductivity. The excellent mechanical properties (flexural strength = 212.48 MPa), thin absorbing layer (d < 2 mm), enhanced thermal stability and high-temperature MA properties indicate that the TiB₂–MgO composites can be considered as new candidates for high-temperature structure microwave absorbing materials.     

Fabrication of hierarchical PANI@W-type barium hexaferrite composites for highly efficient microwave absorption

Hexagonal barium ferries is a promising and efficient microwave (MW) absorbing material, but the low dielectric loss and poor conductivity have limited their extensive applications. In this work, a simple tactic of coating conductive polymer PANI on hexaferrite BaCo₂Fe₁₆O₂₇ is presented, wherein the dielectric properties are customized, and more significantly, the electromagnetic loss is greatly enhanced. As displayed from structural characterizations, PANI were coated equably on the surface of hexaferrite grains by an in-situ polymerization process. The outcomes exhibit the as-prepared PANI@hexaferrite composite has remarkable electromagnetic wave absorption capacity. When the thickness is 6.0 mm, the minimal RL of ?40.4 dB was achieved at 2.9 GHz. The effective absorption bandwidth (RL < ?20 dB) of 0.65 GHz, 0.53 GHz, 0.65 GHz, 0.52 GHz, 0.46 GHz and 0.39 GHz was achieved separately when the thickness ranges from 4 to 9 mm. The highly efficient MW absorbing performance of PANI@hexaferrite composite were the consequence of multiple loss mechanisms and perfect impedance matching. It is demonstrated that the PANI@hexaferrite composite with excellent MW absorption performance is expected to be potential MW absorbers for extensive applications.     

Tunable microwave absorption performance of carbon fiber-reinforced reaction bonded silicon nitride composites

The hybrid network of Si₃N₄ whiskers and conducting carbon fiber has great potential for microwave absoprtion applications. The high electrical conductivity of the carbon fiber helps to transform the microwave transparent Si₃N₄ into microwave absorbing materials. Herein, the microwave absorption performance of 5–20 vol % of carbon fiber reinforced reaction bonded Si₃N₄ (C_f-RBSN) composites have been discussed in detail. The C_f reinforcement tuned the X-band dielectric properties of the RBSN composites. The 5 vol % C_f-RBSN composite exhibit a minimum reflection loss (RL_min) of ?36.16 dB (99.998% microwave absorption) at 11.89 GHz and a high specific reflection loss of 920 dB. g^?1 for 5.9 mm thickness, while 20 vol % C_f-RBSN composites resulted in RL_min of ?22.86 dB at 11.56 GHz with a low thickness of 1.5 mm. Thus, the superior microwave absorption performance of the prepared lightweight composites results from the multiple interfacial polarization, dipole polarization, and conduction loss due to the 3D network of interconnected Si₃N₄ whiskers and C_f.     

Sandwich structure SiCf/Si3N4–SiOC–Si3N4 composites for high-temperature oxidation resistance and microwave absorption

SiC fiber reinforced ceramic matrix composites (SiC_f-CMCs) have been widely used as structural-functional materials at high temperatures. However, their mechanical and electromagnetic wave (EMW) absorbing properties will deteriorate due to high-temperature oxidation. Therefore, unique sandwich structure, consisting of inner Si₃N₄ impedance layer, middle porous SiOC loss layer and dense oxidation-resistant Si₃N₄ layer, was proposed to enhance multiple material properties in oxidation environment. Herein, SiC_f/Si₃N₄–SiOC–Si₃N₄ composites was fabricated by alternating chemical vapor infiltration (CVI) and polymer infiltration pyrolysis (PIP) methods. For these composites, SiC fiber is used as both reinforcing phase and electromagnetic (EM) absorber. CVI Si₃N₄ matrix was distributed in inner and outer layer of the SiC_f/Si₃N₄–SiOC–Si₃N₄ composites. While inner Si₃N₄ layer between BN interphase and SiOC matrix forms nano-heterogeneous interphase to consume EM energy and enhance mechanical properties of composites, outer dense and oxidation-resistant CVI Si₃N₄ coating serves to maintain properties. PIP SiOC matrix exhibits porous structure that can effectively deflect cracks and achieve multiple scattering of EMW. SiC_f/Si₃N₄–SiOC–Si₃N₄ composites with sandwich structure demonstrated excellent EMW absorbing properties and mechanical performance in high-temperature oxidation environments.     

Enhanced microwave absorption properties of flake-shaped FeCo/BaFe12O19 composites

In this work, flake-shaped FeCo/BaFe₁₂O₁₉ composites were successfully prepared via a facile two-step process involving ball milling and sol-gel treatment. Compared with commonly used FeCo alloys, FeCo/BaFe₁₂O₁₉ composite material can achieve remarkable microwave absorption performance, and this is largely because of the flake-shaped structure and dielectric relaxation processes. Furthermore, changing mass fractions of BaFe₁₂O₁₉ enable flexible control of applicable frequency ranges of FeCo/BaFe₁₂O₁₉ composites. It was found that when the BaFe₁₂O₁₉ mass fraction is as high as 20%, minimum reflection loss can reach ?51.22 dB with effective loss of < –10 dB and effective bandwidth of 6.24 GHz even at a thin thickness of 1.45 mm. It is highly believed that these significant achievements will arouse interest from researchers for further practical exploration of microwave absorbers.     

Effect of SiC interphase on the mechanical,high-temperature dielectric and high-temperature microwave absorption properties of the SiCf/SiC/Mu composites

In this study, SiC interphase was prepared via a precursor infiltration-pyrolysis process, and effects of dipping concentrations on the mechanical, high-temperature dielectric and microwave absorption properties of the SiC_f/SiC/Mu composites had been investigated. Results indicated that different dipping concentrations influenced ultimate interfacial morphology. The SiC interphase prepared with 5 wt% PCS/xylene solution was smooth and homogeneous, and no bridging between the fiber monofilament could be observed. At the same time, SiC interphase prepared with 5 wt% PCS/xylene solution had significantly improved mechanical properties of the composite. In particular, the flexural strength of the composite prepared with 5 wt% PCS/xylene solution reached 281 MPa. Both ε′ and ε′′ of the SiC_f/SiC/Mu composites were enhanced after preparing SiC interphase at room temperature. The SiC_f/SiC/Mu composite prepared with 5 wt% PCS/xylene solution showed the maximum dielectric loss value of 0.38 at 10 GHz. Under the dual action of polarization mechanism and conductance loss, both ε′ and ε′′ of the SiC_f/SiC/Mu composites enhanced as the temperature increased. At 700 °C, the corresponding bandwidth (RL ≤ ?5 dB) of SiC_f/SiC/Mu composites prepared with 5 wt% PCS/xylene solution can reach 3.3 GHz at 2.6 mm. The SiC_f/SiC/Mu composite with SiC interphase prepared with 5 wt% PCS/xylene solution is expected to be an excellent structural-functional material.     

Dielectric regulation of high-graphitized fine ash wrapped cube-like ZnSnO3 composites with boosted microwave absorption performance

Intrinsic dielectric properties and tuning conductivity play important roles in microwave absorption. Novel multi-interfaced ZnSnO₃@ fine ash (ZSFA) composite was successfully synthesized by coating cube-like ZnSnO₃ particles with highly graphitized gasification fine ash. After hydrothermal reaction and Ostwald ripening process, fine ash was tightly wrapped around the assembly of ZnSnO₃ particles. Related electromagnetic parameters and dielectric dissipation ability were discussed with different mass additions. Owing to the strong polarization relaxation, special conductive network, and multi-interface structural design, the as-synthesized ZSFA exhibited adjustable dielectric loss behaviors and efficient microwave absorption ability. When 50% mass added, the maximum reflection loss value of the obtained ZSFA-2 is ?47.8 dB at 2.5 mm thickness, showing the enhanced dielectric loss ability. Meanwhile, the widest effective absorption bandwidth (RL ≤ ?10 dB) can cover 7.0 GHz (11.0–18.0 GHz) at a thickness of only 2.2 mm, which included the entire Ku band. This unique pure dielectric composite exhibited high-performance electromagnetic wave attenuation property and broadband frequency response, thereby providing a new approach to the production of a superior microwave absorber.     

Recent advances in carbon nanotubes-based microwave absorbing composites

With the blossom of information industry, electromagnetic wave technology shows increasingly potential in many fields. Nevertheless, the trouble caused by electromagnetic waves has also drawn extensive attention. For instance, electromagnetic pollution can threaten information safety in vital fields and the normal function of delicate electronic devices. Consequently, electromagnetic pollution and interference become an urgent issue that needs to be addressed. Carbon nanotubes (CNTs) have become a potential candidate to deal with these problems due to many advantages, such as high dielectric loss, remarkable thermodynamic stability, and low density. With the appearance of climbing demands, however, the carbon nanotubes combining various composites have shown greater prospects than the single CNTs in microwave absorbing materials. In this short review, recent advances in CNTs-based microwave absorbing materials were comprehensively discussed. Typically, we introduced the electromagnetic wave absorption mechanism of CNTs-based microwave absorbing materials and generalized the development of CNTs-based microwave absorbers, including CNTs-based magnetic metal composites, CNTs-based ferrite composites, and CNTs-based polymer composites. Ultimately, the growing trend and bottleneck of CNTs-based composites for microwave absorption were analyzed to provide some available ideas to more scientific workers.     

Dielectric and microwave absorption properties of FeSiAl/Al2O3 composites containing FeSiAl particles of different sizes

Here in, the effects of FeSiAl particle size on the dielectric and microwave absorption properties of FeSiAl/Al₂O₃ composites were studied. FeSiAl/Al₂O₃ composites containing 18–25 μm, 25–48 μm, and 48–75 μm FeSiAl particles were prepared by hot-pressed sintering based on uniformly mixed FeSiAl and Al₂O₃ powders. Results show that the real permittivity and the imaginary permittivity are significantly promoted with increasing FeSiAl particle size, which is ascribed to the enhanced interfacial polarization and conductance loss. In addition, the favorable matching impedance and suitable attenuation coefficient enabled the composite containing 25–48 μm FeSiAl powder to show a minimum reflection loss of ?34.4 dB at 11.7 GHz and an effective absorption bandwidth (<-10 dB) of 1.4 GHz in 11.0–12.4 GHz, when the thickness is 1.1 mm. By adjusting the thickness to 1.4 mm, the effective absorption bandwidth of the composite reaches a maximum value of 2.0 GHz in the 8.3–10.3 GHz range, indicating tunable, strong, and highly efficient microwave absorption performance.     

Ultralight and superelastic polyvinyl alcohol/SiC nanofiber/reduced graphene oxide hybrid foams with excellent thermal insulation and microwave absorption properties

Being in the strategic direction of next-generation absorbers, multifunctional microwave absorbing materials possess great application value in military and commercial fields. However, the stringent requirements for performance necessitate the combination of multiple functions in such type of composites, which is still a challenge. This work aims to develop a foam-type absorber composed of multi-dimensional organic and inorganic materials, in which reduced graphene oxide sheets and polyvinyl alcohol membranes serve as the framework and crosslinker to form a three-dimensional skeleton. Meanwhile, SiC nanofibers as a reinforcing component can effectively suppress the over-stacking of reduced graphene oxide and enhance the conductivity and mechanical strength of cell walls. Among the remarkable microwave absorbing properties of the obtained foam, there are the ultra-light (9.85 mg cm^-3), broadband (7.04 GHz), and strong absorption (reflection loss of -61.02 dB), all combined in the ultra-thin (2.5 mm). In addition, the foam possesses superelastic and excellent heat-insulating characteristics that ensure shock resistance, heat preservation, and infrared stealth. The remarkable versatility benefits from the porous structure, as well as from the synergistic effect of multi-dimensional organic and inorganic constituents of the foam. Therefore this study lays the foundation for the design of new-generation microwave absorbers with broad application potential.     

Influence of different matrices on the mechanical and microwave absorption properties of SiC fiber-reinforced oxide matrix composites

Fiber-reinforced ceramic matrix composites have excellent mechanical and microwave absorption properties, but still present considerable challenges. We prepared a SiC_f/mullite-SiO₂ composite (composite A) and a SiC_f/Al₂O₃-SiO₂ composite (composite B) by a precursor infiltration and sintering (PIS) process. Compared with the composite B, the composite A was easily densified. The flexural strength of the composite A reached 216 MPa, whereas that of the composite B was 159 MPa. The imaginary part of permittivity for composites A and B, which was determined by the contents of matrix and porosity, varied in the range of 2.5–3.5 and 3.6–5, respectively. The microwave absorption properties of the composite A were significantly enhanced in the range of 8.2–12.4 GHz. The results indicate that an optimal reflection loss of ?44 dB was reached at 12 GHz with a thickness of 2.9 mm for the composite A. These SiC fiber-reinforced oxide matrix materials have promising applications in microwave absorption, especially at high temperatures.     

Electrical conductivity,dielectric and microwave absorption properties of graphene nanosheets/magnesia composites

Herein, a novel microwave absorbing material with Graphene nanosheets (GNSs) as microwave absorbing filler and magnesia (MgO) as matrix were prepared by hot-pressing sintering. The composites were highly dense with a homogeneous distribution of GNSs. Electrical conductivity, dielectric and microwave absorption properties in X-band were investigated. The results revealed that the electrical conductivity of the GNSs/MgO composites showed a typical percolation-type behavior with a percolation threshold of 3.34 vol%. With GNSs content increased to 3 vol%, the real permittivity, imaginary permittivity and dielectric loss tangent of the composites increased from ～9, ～0 and ～0 to 26–43, 23–28 and 0.55–0.96, respectively. By adjusting the GNSs content, thickness and frequency, the 2.5 vol% GNSs/MgO composite shows the minimum reflection loss of ?36.5 dB at 10.7 GHz and the reflection loss below ?10 dB (90% absorption) ranges from 9.4 to 11.4 GHz with 1.5 mm thickness, exhibiting excellent microwave absorption properties.     

Simultaneously enhancing mechanical and microwave absorption properties of Cf/SiC composites via SiC nanowires additions

C_f/SiC composite is an excellent structural and functional material, silicon carbide nanowires (SiCnws) are not only a toughening material but also a great application in the field of microwave absorption. In this study, SiCnws are grown on the surface of carbon fiber (C_f) by polymer impregnation and pyrolysis, and the SiC matrix was prepared by chemical vapor osmosis method. The SiCnws are introduced to enhance the mechanical and microwave absorption properties simultaneously. After 3 impregnations, the flexural strength of the composite was 107.35 ± 10 MPa. When the thickness is 1.86 mm, the minimum reflection loss value is ?41.08 dB, and the effective absorption bandwidth (RL ≤ ?10 dB) is 3.86 GHz. Furthermore, the microwave absorption mechanism of the material is discussed. This work provides a new method to prepare lightweight, stable and high-performance microwave absorption materials, and these materials are expected to be used in high temperature environments.     

Enhanced high-temperature dielectric properties and microwave absorption of SiC nanofibers modified Si3N4 ceramics within the gigahertz range

Si₃N₄ ceramics modified with SiC nanofibers were prepared by gel casting aiming to enhance the dielectric and microwave absorption properties at temperatures ranging from 25?°C to 800?°C within X-band (8.2–12.4?GHz). The results indicate that the complex permittivity and dielectric loss are significantly increased with increased weight fraction of SiC nanofibers in the Si₃N₄ ceramics. Meanwhile, both complex permittivity and dielectric loss of SiC nanofibers modified Si₃N₄ ceramics are obviously temperature-dependent, and increase with the higher test temperatures. Increased charges mobility along conducting paths made of self-interconnected SiC nanofibers together with multi-scale net-shaped structure composed of SiC nanofibers, Si₃N₄ grains and micro-pores are the main reason for these enhancements in dielectric properties. Moreover, the calculated microwave absorption demonstrates that much enhanced microwave attenuation abilities can be achieved in the SiC nanofibers modified Si₃N₄ ceramics, and temperature has positive effects on the microwave absorption performance. The SiC nanofibers modified Si₃N₄ ceramics will be promising candidates as microwave absorbing materials for high-temperature applications.     

Rapid one-pot synthesis of SiO2@SiO2/Carbon@Carbon core-shell composites with efficient microwave absorption in a short time

A carbon-silica nanomaterial for electromagnetic wave (EW) absorption was synthesized using a modified one-pot method. The unique hydrolysis-polymerization process forms a SiO₂@SiO₂/Carbon@Carbon core-shell structure. The growth process of the material was studied by transmission electron microscope (TEM) and thermogravimetric analysis (TGA). Nanoparticles were successfully synthesized with a core-shell structure after 6 h of reaction, and the composite material showed excellent EW absorption performance with a thickness of 3.8 mm. The minimum reflection loss (RL_min) was ?56.3 dB, and the broadest effective absorption bandwidth (EAB) (RL ≤ 10 dB, 90% absorption) covered 5.15 GHz (12.48–17.63 GHz) with 1.8 mm of thickness. There is no significant difference in the EW absorption performance with increasing reaction time. Thus, this study provides a method for synthesizing EW-absorbing materials with shorter reaction time, simpler process, and excellent absorption properties, possibly a candidate for further application.     

Dielectric and microwave absorption properties of CB doped SiO2f/PI double-layer composites

Carbon black (CB) with contents of 5.5?wt% and 15?wt% filled quartz glass fiber reinforced polyimide (SiO_2f/PI) composite were designed and prepared. A double-layer absorbing material was designed using the two composites materials as a matching layer and an absorption layer, respectively. The microwave absorption property of single-layer and double-layer composites is calculated according to transmission line theory. The results show that the microwave absorbing property of double-layer composite is better than that of single-layer at the same thickness. When the 5.5?wt%CB doped SiO_2f/PI composite is used as the matching layer with a thickness of 0.7?mm and 15?wt%CB doped SiO_2f/PI composite is used as the absorption layer with a thickness of 0.9?mm, the RL (reflection loss) of the composite reaches a minimum value of ?46.18?dB at 16.07?GHz. Meanwhile, the bandwidth of RL?≤??5?dB is 5.87?GHz and the bandwidth of RL?≤??10?dB is 3.95?GHz.     

Effect of microwave absorption properties and morphology of manganese dioxide on catalytic oxidation of toluene under microwave irradiation

A large number of studies had shown that the morphology of the sample had a significant effect on the microwave absorption properties and catalytic activity of the sample. Manganese dioxide with different morphologies was synthesized by hydrothermal method through different precursors. The effects of sample morphology and microwave absorption properties on the catalytic activity of the sample in conventional thermal and microwave fields were studied. The results indicated that compared with the conventional thermal field, the catalytic activity of the samples in microwave field were obviously improved, and the activation energy of the reaction were decreased. Compared with the conventional thermal field, the conversion of toluene in microwave thermal field of MnO₂(Ac), MnO₂(S) and MnO₂(N) increased by 59%, 42% and 12%, and the mineralization rate increased by 36%,11% and 2%, respectively, when the catalytic temperature was 150 °C. Compared with the traditional thermal field, the activation energy of the sample MnO₂(Ac) in the microwave field was reduced by 88.3 KJ. A series of characterization results showed that the sample MnO₂(Ac) had good catalytic activity in the microwave field was due to: MnO₂(Ac) had proper microwave absorption properties, large amount of surface functional groups, large specific surface area and rich pore structure. The analysis results of electromagnetic parameters showed that: the reason that the sample MnO₂(Ac) had good microwave absorption performance was that the MnO₂(Ac) had proper impedance matching, high attenuation constant and Debye dipole relaxation effect.