SiCnw@SiC foam prepared based on vapor-solid mechanism for efficient microwave absorption

A SiC_nw@SiC foam with highly efficient microwave absorption (MA) performance was successfully synthesized based on Vapor-Solid (V–S) growth mechanism. SiC nanowire (SiC_nw) and SiC foam skeleton efficiently form a double network coupling structure, which gives additional interface polarization and dielectric loss for the SiC foam, significantly enhancing the MA capacity of the foam. In this study, the SiC_nw@SiC foam has a minimum reflection loss (RL_min) of ?86.31 dB and an effective absorption bandwidth (EAB) of 12.55 GHz in room-temperature environment. However, the MA performance of SiC_nw@SiC foam decreases with increasing temperature, which may be due to the thickening of the SiO₂ layer in the SiC_nw at high temperature. At 600 °C, it has no effective absorption bandwidth, while at 1000 °C, the EAB and RL_min were 0.6 GHz and ?13.04 dB, respectively. As the temperature reaches 1000 °C, the defects in the material further increase, leading to a recovery in the MA performance.     

Ultra-light 3D bamboo-like SiC nanowires felt for efficient microwave absorption in the low-frequency region

Nonmagnetic ceramics are ideal microwave absorbing materials used in high-temperature and oxidizing environments. However, low-frequency absorbing properties of this material are rarely reported because low-frequency absorbing requires nonmagnetic materials to have much higher permittivity. In this research, a series of three-dimensional architectures formed by SiC nanowires with different microstructures felt were fabricated to address this issue. The morphology of the SiC_nw (linear, bamboo-shaped, and worm-like) dominated by the VLS growth mechanism can be manipulated by the silicon vapor concentration, which is governed by the vaporization temperature of the mixed silicon source (Si and SiO₂) in different sintering processes. The spontaneously overlapped bamboo-shaped SiC nanowires in these felt enhance the permittivity and conductivity loss and produce multiple scattering effects on the incident EM waves, thus increasing the low-frequency wave absorption ability. The RL_min of the bamboo-shaped SiC_nw felt reaches ?44.3 dB at 3.85 GHz with the corresponding EAB of 0.64 GHz (3.6–4.24 GHz) at a thickness of 3.5 mm. The density of the SiC_nw felt is as low as 0.022 g/cm³ due to the high porosity (99.3%) of 3D networks, which fulfills lightweight requirements and highly efficient electromagnetic wave absorption.     

Adjustable iron-containing SiBCN ceramics with high-temperature microwave absorption and anti-oxidation properties

Quaternary siliconboron carbonitride (SiBCN) ceramics show excellent high-temperature stability and oxidation resistance, indicating great potential as high-temperature electromagnetic (EM) wave absorbing materials. In this contribution, an efficient and facile method was developed to prepare bulk iron-containing SiBCN (Fe–SiBCN) ceramics with remarkable EM wave absorption at high temperature by pyrolyzing boron and iron containing precursors (PSZV-B–Fe). The introduction of boron and iron not only improves the high-temperature stability but also influences the complex permittivity and EM wave absorption. The minimum reflection coefficient (RC_min) is −61.05 dB, and the effective bandwidth absorption (EAB) is 3.35 GHz (9.05–12.4 GHz). The RC_min will be decreased to −52.3 dB at 600°C as well as the EAB covers more than 67% of the X band (2.8 GHz). The high-temperature stable Fe–SiBCN ceramics with adjustable dielectric properties can be utilized as high-performance EM wave-absorbing materials in high-temperature and harsh environments.     

Effect of preparation conditions on mechanical,dielectric and microwave absorption properties of SiC fiber/mullite matrix composite

Silicon carbide fiber-reinforced mullite matrix (SiC_f/Mu) composites were fabricated via an infiltration and sintering method. Effects of sintering parameters on microstructure, mechanical, dielectric and microwave absorption properties of SiC_f/Mu composites have been investigated. The flexural strength is significantly improved with increasing sintering temperature, and the highest flexural strength of 213?MPa is obtained in vacuum at 1000?°C for 2?h. The performances of composites with different holding time are further studied at 1000?°C. The flexural strengths of composites sintered at 1000?°C for 2 and 4?h reach 213 and 219?MPa, respectively. The failure displacement of the composite sintered at 1000?°C for 4?h reaches 0.39?mm. The excellent microwave absorption properties are achieved for the composite sintered at 1000?°C for 2?h. The minimum reflection loss (RL) reaches ?38?dB with a thickness of 2.9?mm?at 12?GHz and the effective absorbing bandwidth (RL?≤??10?dB) with a thickness of 3.4?mm covers the whole X?band, which indicate that SiC_f/Mu composite is a good candidate for microwave absorbing materials. These results provide valuable solutions to obtaining structural-functional materials for microwave absorption applications in civil and military areas.     

Preparation and wave-absorbing properties of rhombic cross-sectional FeSiAlp/PP magnetic fibers

ABSTRACT

Adjusting fibers shape can further optimize absorbing properties of the FeSiAl_p/PP fibers composites. 80 wt% FeSiAl_p/PP rhombic cross-sectional fibers were successfully prepared by liquid phase blending and extrusion drawing. The strength of the fibers is about 6 ~ 8 MPa. The effective absorption bandwidth (EAB, RL < ?10 dB) of the resin matrix composites modified by rhombic cross-sectional fibers is 3.9 GHz and the optimal RL reaches ?18.43 dB at 4.5 GHz. Compared with the resin matrix composites modified by circular cross-sectional fibers, the EAB increases by 1.8 GHz and there is a second absorption peak, which has better broadband absorption characteristics.     

Impedance matching optimization of SiCf/Si3N4–SiOC composites for excellent microwave absorption properties

SiC fiber reinforced ceramic matrix composites (SiC_f-CMCs) are considered to be one of the most promising materials in the electromagnetic (EM) stealth of aero-engines, which is expected to achieve strong absorption and broad-band performance. Multiscale structural design was applied to SiC_f/Si₃N₄–SiOC composites by construction of micro/nanoscale heterogeneous interfaces and macro double-layer impedance matching structure. SiC_f/Si₃N₄–SiOC composites were fabricated by using SiC fibers with different conductivities and SiOC–Si₃N₄ matrices with gradient impedance structures to improve impedance matching effectively. Owing to its unique structure, SiC_f/Si₃N₄–SiOC composites (A3-composites) achieved excellent EM wave absorption performance with a minimum reflection coefficient (RC_min) of ?25.1 dB at 2.45 mm and an effective absorption bandwidth (EAB) of 4.0 GHz at 2.85 mm in X-band. Moreover, double-layer SiC_f/Si₃N₄–SiOC with an improved impedance matching structure obtained an RC_min of ?56.9 dB and an EAB of 4.2 GHz at 3.00 mm, which means it can absorb more than 90% of the EM waves in the whole X-band. The RC is less than ?8 dB at 2.6–2.8 mm from RT to 600 °C in the whole X-band, displaying excellent high-temperature absorption performance. The results provide a new design opinion for broad-band EM absorbing SiC_f-CMCs at high temperatures.     

Microstructure and electromagnetic wave absorption property of reduced graphene oxide-SiCnw/SiBCN composite ceramics

In this account, RGO-SiC_nw/SiBCN composite ceramics were fabricated using polymer derived ceramic (PDC) combined with chemical vapor infiltration (CVI) technology. Dielectric property of as-obtained RGO-SiC_nw/SiBCN composite ceramics was significantly enhanced thanks to established conductive pathway through overlapped nanoscale SiC_nw and micro-sized RGO. The minimum RC of composite ceramics with 0.5 wt% GO and 2.29 wt% SiC_nw at thickness of 3.6 mm reached -42.02 dB with corresponding effective absorption bandwidth (EAB) of 4.2 GHz. As temperature rose from 25 to 400 °C, permittivity increased with enhanced charge carrier density and then it decreased due to oxidation process of RGO from 400 to 600 °C. The minimum reflection coefficient (RC) was recorded as -39.13 dB and EAB covered the entire X-band at 600 °C. EMW absorption ability was evaluated after high-temperature oxidation experiment under protective effect of wave-transparent Si₃N₄ coating. RGO-SiC_nw/SiBCN composite ceramics maintained outstanding EMW absorption ability with minimum RC of -10.41 dB after oxidation at 900 °C, indicating RGO-SiC_nw/SiBCN composite ceramics with excellent EMW absorption characteristic even at high temperatures and harsh environments.     

Preparation,mechanical, dielectric and microwave absorption properties of hierarchical porous SiCnw-Si3N4 composite ceramics

Herein, hierarchical porous SiC_nw-Si₃N₄ composite ceramics with good electromagnetic absorption properties were prepared. A porous Si₃N₄ matrix with different pore structures was first prepared by gelcasting-pressureless sintering (G-PLS) and gelcasting combined with particle stabilized foam-pressureless sintering (G-PSF-PLS). SiC_nw was then introduced by catalytic chemical vapor deposition (CCVD). An increase in solid loading (25–40 vol%) decreased apparent porosity (47.7–41.3%) and improved flexural strength (142.19–240.36 MPa) and fracture toughness (2.25–3.68 MPa·m^1/2). The addition of foam stabilizer propyl gallate (PG, 0.5–1.0 wt%) significantly increased apparent porosity (73.2–86.4%) and realized large-sized spherical pores, reducing flexural strength (58.23–38.56 MPa) and fracture toughness (0.75–0.41 MPa·m^1/2). High apparent porosity and large-sized pores facilitated the introduction of SiC_nw. The 25 vol% sample exhibited a reflection loss of ? 14.67 dB with an effective absorption bandwidth of 3.47 GHz, suggesting a development potential in the electromagnetic wave absorption field.     

Enhanced high-temperature dielectric and microwave absorption properties of SiC fiber-reinforced oxide matrix composites

Because of outstanding performances of the SiC fiber-reinforced ceramic matrix composites in aircraft/aerospace systems, two silicon carbide fiber-reinforced oxide matrices (SiC_f/oxides) composites have been prepared by a precursor infiltration and sintering method. Results indicate that the flexural strength of the SiC_f/Al₂O₃–SiO₂ composite reaches 159 MPa, whereas that of the SiC_f/Al₂O₃ composite is only 50 MPa. The high-temperature microwave absorption properties of the composite are significantly enhanced due to choosing Al₂O₃ and SiO₂ as the hybrid matrices. Particularly, the minimum reflection loss (RL) value of the SiC_f/Al₂O₃–SiO₂ composite reaches −37 dB in the temperature of 200 °C at 8.6 GHz, and the effective absorption bandwidth (RL ≤ −5 dB) is 4.2 GHz (8.2–12.4 GHz) below 400 °C. The superior microwave absorption properties at high temperatures indicate that the SiC_f/Al₂O₃–SiO₂ composite has promising applications in civil and military areas. © 2018 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2019 , 136, 47097.     

Tunable design of ZnFe2O4@C@BPC hybrid with rich heterogeneous interfaces as a hydrophobic electromagnetic wave absorber

Environmentally friendly microwave absorbers with superior electromagnetic wave absorption, lightweight and hydrophobic ability have received considerable attention in practical applications. However, addressing the above-mentioned characteristics is simultaneously a tremendous challenge. Along these lines, in this work, a lightweight and efficient hybrid material was fabricated by employing simple self-assembly of core-shell ZnFe₂O₄@C nanospheres embedded within longan shell-derived honeycomb-like porous carbon. The results display that the carbon skeleton not only improves the conduction loss, but also promotes the reflection and scattering of EM wave. In addition, the core-shell ZnFe₂O₄@C microspheres are conducive to enhancing the ability of interface polarization and magnetic loss, and further improving the synergistic effect between the dielectric loss and magnetic loss. Furthermore, the unique structure of the ZnFe₂O₄@C@BPC endows it excellent hydrophobicity and avoids water vapor contamination in practical applications. Precisely, at a thickness of 3.4 mm, the minimum reflection loss (RL) is up to ?58.6 dB at 12.9 GHz. Notably, the effective absorption bandwidth (EAB) is as wide as 9.1 GHz (8.9–18.0 GHz), covering almost the entire X and Ku bands. Consequently, this outstanding performance renders the ZnFe₂O₄@C@BPC composite quite attractive for a broad range of applications in lightweight, hydrophobic microwave absorption materials.     

Electromagnetic wave absorbing properties of Cr2AlB2 powders and the effect of high-temperature oxidation

The electromagnetic (EM) wave absorbing properties of Cr₂AlB₂ powders and those after high-temperature oxidation were investigated. Coupling of magnetic and dielectric loss enables Cr₂AlB₂ with good absorption properties. The minimum reflection loss (RL) value is −44.9 dB at 8.5 GHz with a thickness of 2.7 mm, and the optimized effective absorption bandwidth (E_AB) is 4.4 GHz (13.0-17.4 GHz) with a thickness of 1.6 mm. After oxidation at 750, 900, and 1000°C for 2 h, the minimum RL values, respectively, are −23.9 dB (17.5 GHz, 1.5 mm), −41.4 dB (16.5 GHz, 1.5 mm), and −39.5 dB (8.0 GHz, 3.0 mm); and the corresponding E_AB values, respectively, are 3.8 GHz (13.6-17.4 GHz, 1.7 mm), 4.1 GHz (13.5-17.6 GHz, 1.6 mm), and 4.4 GHz (13.0-17.4 GHz, 1.7 mm). With an absorber thickness of 1.5-4.0 mm, the E_AB with a RL value of less than −10 dB can be tuned in a broad-frequency range 5.0-18.0 GHz, which basically covers C (4-8 GHz), X (8-12 GHz), and Ku (12-18 GHz) bands. These results demonstrate that Cr₂AlB₂, as a high-efficient and oxidation-resistant absorber, is a promising candidate for microwave absorption applications and can retain good EM wave absorbing properties after high-temperature oxidation.     

Enhanced microwave absorption properties of Y-Co2Z/PANI hexaferrites composites in the frequency range of 0.1–18 GHz

We prepared Ba_3−xY_xCo₂Fe₂₄O₄₁ (Y-Co₂Z, x = 0, 0.2, and 0.4) by the solid-state reaction method. Y-Co₂Z and polyaniline (PANI) composites (named as Y-P0, Y-P2, and Y-P4) were prepared by using the in-situ polymerization method. The Y-doping played an important role in the variation of lattice parameters, a and c. The combination of Y-doping and PANI modified the magnetic properties of the composites, which could be observed by the changing of the saturation magnetization and coercivity. This combination had also affected the electromagnetic properties of composites through the measurements of complex permittivity and permeability. Using the transmission line theory, we calculated refection loss (RL) of composites with the variation thickness of 1.00–2.50 mm. Our composites tuned the minimum RL from the X band (RL = −29.6 dB at 11.4 GHz for Y-P2) to Ku band (RL = −16.3 dB at 15.7 GHz for Y-P4 and RL = −26.4 dB at 16.6 GHz for Y-P4). For maximum effective bandwidth, our composites covered a huge range from the S and C bands (Y-P0 with 3.9 GHz in the range of 3.4–7.3 GHz) through the X band (Y-P2 with 3.9 GHz in the range of 9.0–12.9 GHz) to the Ku band (Y-P4 with 4.0 GHz in the range of 13.8–17.8 GHz). Those properties proved that the composites could act as promising absorbers in the S, C, X, and Ku bands.     

Carbon nanowires reinforced porous SiO2/3Al2O3·2SiO2 ceramics with tunable electromagnetic absorption properties

Porous SiO₂/3Al₂O₃·2SiO₂ ceramics decorated with carbon nanowires (CNWs) were manufactured by catalytic chemical vapor deposition (CCVD) for wide absorption band and high absorption application. The results show that the as-prepared CNWs grow uniformly in porous SiO₂/3Al₂O₃·2SiO₂ ceramics to form the three dimensional(3D) structure. The content of CNWs can be effectively controlled by changing the weight of catalyst, and the dielectric properties of CNWs-SiO₂/3Al₂O₃·2SiO₂ composite ceramics can be tuned. As expected, the CNWs-SiO₂/3Al₂O₃·2SiO₂ composite ceramics possess wonderful electromagnetic (EM) absorption properties due to the effects of conductive loss, impedance match and dipoles polarization. It can be concluded that the minimum reflection coefficient (RC_min) of CNWs-SiO₂/3Al₂O₃·2SiO₂ composite ceramics reaches ?31?dB?at 9.1?GHz (which means 99.9% of EM wave powers are absorbed) and the effective absorption bandwidth (EAB) covers the whole X-band (8.2–12.4?GHz) at the thickness of 5.0?mm.     

Simultaneously improving mechanical and microwave absorption properties of a novel SiCf/SiOC & mullite hybrid ceramic matrix composite

For enhanced mechanical and microwave absorption properties at the same time, the SiC_f/hybrid matrix composites were fabricated by precursor infiltration and pyrolysis (PIP) method with polysiloxane (PSO) ethanol solution, alumina sols and silica sols. As the first layer of the hybrid matrix, the SiOC ceramic was pyrolyzed from PSO solution. The remained hybrid matrix was mullite, which sintered from alumina sols and silica sols. The effects of different content of PSO solution on the morphologies, flexure strength and reflection loss values of composites were studied. Additionally, the XRD patterns, Fourier Transform Infrared (FTIR) and Raman spectrum of hybrid matrix were also investigated. With the increasingly content of PSO solution from 0% to 10 % and 20%–60% in the first infiltration-pyrolysis process, the flexure strength of composites was increased from 175.18 MPa to 301.94 MPa and decreased from 263.33 MPa to 221.30 MPa, respectively. The complex permittivity was increased with the increasing content of PSO solution from 0%–40% due to the free carbon conductive network from excessive SiOC. Moreover, the complex permittivity of SiC_f/hybrid matrix composites with 50 % and 60 % content of PSO solution was reduced due to more open porosity and broken free carbon conductive network. Additionally, the maximum reflection loss values of SiC_f/hybrid matrix composite with 50 % PSO solution were over -60 dB and the effective absorption bandwidth (EAB) of this composite reaches 3.89 GHz in the X band.     

Interface evolution of a C/ZnO absorption agent annealed at elevated temperature for tunable electromagnetic properties

Excellent electromagnetic (EM) wave absorption agents that attenuate EM waves by mechanisms based on impedance matching, conductive loss and polarization loss instead of destructive interference are urgently needed but remain challenging. Here, an EM wave absorption agent with a tailorable heterogeneous interface is designed and prepared by the in situ growth of ZnO nanoparticles on the surface of mesoporous carbon hollow microspheres (PCHMs@ZnO) via hydrothermal synthesis followed by annealing. A controlled interface evolution associated with abundant heterogeneous interfaces plays a crucial role in optimized impedance matching and enhanced interfacial polarization loss. With this method, targeted EM wave absorption agents with an excellent absorption ability that is derived mainly from polarization loss and conductive loss rather than destructive interference are successfully obtained. When the PCHMs@ZnO annealed at 700°C were combined with paraffin, the effective absorption bandwidth of the resultant composites covers the whole X band, and the mean value of reflection loss (RL) reaches −12 dB, exceeding those of other reported ZnO-based materials. When the thickness of the composites varies from 3.3 to 4.3 mm, the value of the RL is lower than −8 dB in the whole X band. This work provides a promising model for preparing high-performance EM wave absorption agents.     

Electromagnetic wave absorption and mechanical properties of silicon carbide fibers reinforced silicon nitride matrix composites

It is difficult for ceramic matrix composites to combine good electromagnetic wave (EMW) absorption properties (reflection coefficient, RC less than -7 dB in X band) and good mechanical properties (flexural strength more than 300 MPa and fracture toughness more than 10 M P·m^1/2). To solve this problem, two kinds of wave-absorbing SiC fibers reinforced Si₃N₄ matrix composites (SiC_f/Si₃N₄) were designed and fabricated via chemical vapor infiltration technique. Effects of conductivity on EM wave absorbing properties and fiber/matrix bonding strength on mechanical properties were studied. The SiC_f/Si₃N₄ composite, having a relatively low conductivity (its conduction loss is about 33% of the total dielectric loss) has good EMW absorption properties, i.e. a relative complex permittivity of about 9.2-j6.4 at 10 GHz and an RC lower than ?7.2 dB in the whole X band. Its low relative complex permittivity matches impedances between composites and air better, and its strong polarization relaxation loss ability help it to absorb more EM wave energy. Moreover, with a suitably strong fiber/matrix bonding strength, the composite can transfer load more effectively from matrix to fibers, resulting in a higher flexural strength (380 MPa) and fracture toughness (12.9 MPa?m^1/2).     

α-MnO2 nanorods-polyaniline (PANI) nanocomposites synthesized by polymer coating and grafting approaches for screening EMI pollution

In this work, α-MnO₂ nanorods-polyaniline nanocomposites were synthesized using polymer coating and grafting approaches. The synthesized nanocomposites were characterized by XRD, FESEM, EDAX, TEM, TG-DTA and FT-IR techniques. The Electromagnetic properties of prepared samples were measured using vector network analyzer in the 8–18 GHz (X and Ku-Band) frequency region. The α-MnO₂-NH₂-PANI nanocomposite synthesized by grafting approach showed better electrical conductivity, excellent dielectric loss with superior microwave absorption ability. In comparison with pure MnO₂, the microwave absorption characteristics of α-MnO₂-NH₂-PANI nanocomposite display considerable improvements, with an effective absorption band at 10.8 GHz and 14.5 GHz under ?10 dB and minimum reflection loss (RL) of ?30.79 dB at 14.5 GHz. The α-MnO₂-NH₂-PANI sample also showed considerable shielding effectiveness (SE) i.e. ?20.85 dB in the 8–18 GHz frequency region. The observed value of RL and SE surpasses the required value for being utilized at a commercial level. These results are surely helpful to explore the microwave absorption study of different combinations of organic/inorganic nanocomposite materials particularly for shielding and microwave absorption applications.     

Controllable synthesis of porous CxNy nanofibers with enhanced electromagnetic wave absorption property

Porous C_xN_y nanofibers are controllably synthesized by a simple two-step method. The prepared samples possess uniform micropores and a chemical composition of C_0.73 N_0.27 with a surface area of 329 m² g⁻¹. The obtained C_xN_y nanofibers exhibit remarkable electromagnetic (EM) wave absorption properties when compared with conventional one-dimensional carbon materials. The minimum reflection loss (RL) reaches −36 dB at 2.7 GHz when the ratio of the C_xN_y absorbent added in paraffin matrix is only 1:3. The bandwidth of the RL below −10 dB covers 7.7 GHz (8.1–15.8 GHz) at the sample thickness of 2.5 mm. A possible EM wave loss mechanism was proposed in detail. The multiple reflection and dielectric loss could govern the excellent EM absorption leading the product to a probable application in stealth materials.     

Enhanced microwave absorption properties of Fe-doped SiOC ceramics by the magnetic-dielectric loss properties

The combination of multiple loss characteristics is an effective approach to achieve broadband microwave wave absorption performance. The Fe-doped SiOC ceramics were synthesized by polymer derived ceramics (PDCs) method at 1500 °C, and their dielectric and magnetic properties were investigated at 2–18 GHz. The results showed that adding Fe content effectively controlled the composition and content of multiphase products (such as Fe₃Si, SiC, SiO₂ and turbostratic carbon). Meanwhile, the Fe promoted the change of the grain size. The Fe₃Si enhanced the magnetic loss, and the SiC and turbostratic carbon generated by PDCs process significantly increased the polarization and conductance loss. Besides, the magnetic particles Fe₃Si and dielectric particles SiO₂ improved the impedance matching, which was beneficial to EM wave absorption properties. Impressively, the Fe-doped SiOC ceramics (with Fe addition of 3 wt %) presented the minimum reflection coefficient (RC_min) of ?20.5 dB at 10.8 GHz with 2.8 mm. The effective absorption bandwidth (EAB, RC < ?10 dB) covered a wide frequency range from 5 GHz to 18 GHz (covered the C, X and Ku-band) when the absorbent thickness increased from 2 mm to 5 mm. Therefore, this research opens up another strategy for exploring novel SiOC ceramics to design the good EM wave-absorbing materials with broad absorption bandwidth and thin thickness.     

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.