Microwave absorbers with layered structures that can provide abundant interfaces are highly desirable for enhancing electromagnetic absorbing capability and decreasing the thickness. The atomically thin layers of two-dimensional (2D) transition-metal carbides (MXenes) make them a convenient precursor for synthesis of other 2D and layered structures. Here, laminated carbon/TiO2 hybrid materials composed of well-aligned 2D carbon sheets with embedded TiO2 nanoparticles were synthesized and showed excellent microwave absorption. Disordered 2D carbon layers with an unusual structure were obtained by annealing multilayer Ti3C2 MXene in a CO2 atmosphere. The minimum reflection coefficient of laminated carbon/TiO2 composites reaches −36 dB, and the effective absorption bandwidth ranges from 3.6 to 18 GHz with the tunable thickness from 1.7 to 5 mm. The effective absorption bandwidth covers the whole Ku band (12.4–18 GHz) when the thickness of carbon/TiO2/paraffin composite is 1.7 mm. This study is expected to pave the way to the synthesis of carbon-supported absorbing materials using a large family of 2D carbides.Keywords: carbon; exfoliation; laminated structure; microwave absorption; MXene;

In this work, mesoporous carbon hollow microspheres (PCHMs) with designable mesoporous shell and interior void are constructed by a facile in situ stöber templating approach and a pyrolysis-etching process. The PCHMs are characterized by scanning electron microscopy, transmission electron microscopy, X-ray photoelectron spectra, Raman spectroscopy, and nitrogen adsorption and desorption system. A uniform mesoporous shell (pore size 4.7 nm) with a thickness of 55 nm and a cavity size of 345 nm is realized. The composite of paraffin mixed with 20 wt % PCHMs exhibits a minimum reflection coefficient (RCmin) of −84 dB at 8.2 GHz with a sample thickness of 3.9 mm and an effective absorption bandwidth (EAB) of 4.8 GHz below −10 dB (>90% electromagnetic wave is attenuated). Moreover, the composite of phenolic resin mixed with 20 wt % PCHMs exhibits an ultrawide EAB of 8 GHz below −10 dB with a thinner thickness of 2.15 mm. Such excellent electromagnetic wave absorption properties are ascribed to the large carbon–air interface in the mesoporous shell and interior void, which is favorable for the matching of characteristic impedance as compared with carbon hollow microspheres and carbon solid microspheres. Considering the excellent performance of PCHMs, we believe the as-fabricated PCHMs can be promising candidates as highly effective microwave absorbers, and the design philosophy can be extended to other spherical absorbers.Keywords: carbon hollow microspheres; carbon-air interface; dielectric properties; electromagnetic wave absorption; mesoporous shell;

Three-dimensional (3D) macroscopic covalently bonded carbon nanowires/graphene (CNWs/G) architectures can achieve the extraordinary properties based on theoretical work. A series of 3D structures have been recently fabricated. However, these architectures are far from theoretical model because CNWs are difficult to perfectly connect with the graphene substrate which has extremely low surface energy and complex internal structure. Here, we highlight a bioinspired approach based on polydopamine interface buffer for the fabrication of 3D macroscopic CNWs/G sponge composite. CNWs uniformly grow on graphene substrate through covalent CC bonding. The defects of CNWs and junction interface between CNWs with graphene greatly influence the electronic transport, leading to the strong polarization and electromagnetic wave attenuation under alternating electromagnetic field. Owing to this unique 3D architecture, the CNWs/G composite attains ultralight density and outstanding electromagnetic attenuation capability. CNWs/G/poly(dimethyl siloxane) composite exhibits the electromagnetic interference shielding effectiveness of 36 dB in X-band (8.2–12.4 GHz) and the composite density is 97.1 mg/cm3. The macroscopic 3D CNWs/G architecture overcomes the drawbacks of presently available 3D graphene products and opens up a wide horizon for structural, electronic, thermal transport, intramolecular junction, heterogeneous catalysis, electrochemical energy storage, and etc.

Herein, Ti2CTx MXene and its derivatives with various heterogeneous structures were constructed via etching and a facile oxidation treatment. The effect of different oxidation conditions on their structural evolution and phase composition was studied in detail. Compared with that of pristine Ti2CTx MXene, the improvement in the electromagnetic wave absorption capability of the as-prepared Ti2CTx/TiO2 and C/TiO2 nanocomposites was attributed to their enhanced polarization loss and stronger conductivity loss. The enhanced polarization loss is caused by the generated heterogeneous interfaces and higher specific surface area, and the stronger conductivity loss is due to the completely exfoliated carbon layers. Additionally, the remaining multilayered structure after exfoliation of the carbon layers favors energy dissipation. The C/TiO2 nanocomposites attain a minimum reflection coefficient of −50.3 dB at 7.1 and 14.2 GHz, and an effective absorption bandwidth of 4.7 GHz (covering the whole X-band) with a matching thickness of 2.1 mm; this indicates their excellent electromagnetic wave absorption properties. We believe that these nanocomposites with a heterogeneous structure also hold great promise for application in the fields of photocatalysis, lithium batteries, water purification, etc.

•The closed pores in alumina ceramics were achieved using core-shell structured C@Al2O3 microspheres as pore-forming agent.•The closed pores significantly decreased the thermal conductivity to 3.8 W·m− 1·K− 1 at 1200 °C.•The closed pores decreased the real permittivity from 9.74 to 7.48 with a slight increase of the imaginary permittivity.The porous alumina ceramics with closed pores were fabricated using C@Al2O3 microspheres as pore-forming agent. The closed pores with a diameter about 5 μm were successfully obtained in the dense matrix, owing to the additive C@Al2O3 microspheres. Different amount of C@Al2O3 microspheres was added to investigate the effects of closed pores on thermal, dielectric and mechanical properties. With the increasing microspheres amount, the closed porosity of alumina ceramics ranges from 5.82 to 14.5%, the thermal conductivity decreases continuously at the temperature ranging from 30 to 1200 °C, and the flexural strength and fracture roughness decrease. As the closed porosity increases, the real permittivity decreases, while the tangent loss has a slight increase. When the mass ratio of C@Al2O3 microspheres is 10 wt.%, the porous alumina ceramics with the thermal conductivity (13.07–3.8 W·m− 1·K− 1), the flexural strength (97.05 ± 18 MPa), and the fracture roughness (2.65 ± 0.13 MPa·m1/2) can be obtained.Download high-res image (307KB)Download full-size image

Carbon/ceramic composites are promising candidates as electromagnetic interference (EMI) shielding materials used at various harsh environments. The aim of present work is to prepare and investigate two kinds of composite ceramics reinforced with carbon nanowires (CNWs) and nanowires-nanotubes (CNWs-CNTs) hybrid, respectively. Results indicate that CNWs is highly curved and multi-defected, and CNWs-CNTs hybrid shows the best crystal structure at an optimal catalyst concentration of 5 wt%. When CNWs accounts for 5.15 wt%, the total shielding effectiveness (SE) of CNWs/Si3N4 reaches 25.0 dB with absorbed SE of 21.3 dB, meaning that 99.7% incident signal can be blocked, while it reaches 25.4 dB for CNWs-CNTs/Si3N4 as the carbon loading only increasing to 3.91 wt%. By contrast, CNWs/Si3N4 exhibits better electromagnetic attenuation capability with stronger absorption, mainly due to the unique microstructure of CNWs. Both of two composite ceramics have great potential to be designed as structural and multi-functional materials.

The tensile strength and microwave absorbing properties of the amorphous silicon carbide fiber (Tyranno-ZMI) annealed at different temperatures were studied. The tensile strength of the as-received ZMI fiber tows was 1.1 GPa; and the average real and imaginary parts of permittivities of the as-received ZMI/resin samples were 11.3 and 10.5 respectively. The major dielectric loss mechanism of the fibers was conduction loss, which was due to high electrical conductivity of the enriched carbon in ZMI fibers. The 2.0 mm thick ZMI/resin composites could absorb 80% microwave energy in X band, indicating good microwave absorbing property. After heat treatment, fibers degraded gradually and permittivities increased, which were mainly attributed to the decomposition of amorphous SiCxOy and the growth of the SiC nanocrystals and free carbon nanodomains.

The evaluation and optimization of EMW absorbing properties have been widely studied, but little research focused on EMI shielding properties predicted by complex permittivity. Based on the transmission-line theory, shielding effectiveness (SE) of a dielectric composite was evaluated by the reflection coefficient (Г) and transmission coefficient (T) which were calculated by the complex permittivity. SiCf/SiCN composites containing different content of CVI SiCN matrix are attractive for their tunable dielectric properties, which may vary from EMW absorption to EMI shielding. Therefore, SiCf/SiCN composites are typical dielectric composites used for experimental verification, and the results indicate that the dielectric composites without CVI SiCN phase have good EMW absorbing properties, while they exhibit good EMI shielding effectiveness with CVI SiCN phase. This work builds a relationship between the EMI shielding effectiveness and the complex permittivity, and obtains the optimized complex permittivity for excellent EMI shielding effectiveness.

Thinner absorbents with high dielectric loss usually cannot meet the requirement of impedance match, and multi-layer absorbents with excellent performance usually cannot be thin. Thus, it is a challenge to balance strong dielectric loss and impedance matching. An impedance matching interface layer can provide abundant interfaces, which are highly desirable for enhancing electromagnetic absorbing capability and decreasing surface reflection. In this study, cobalt tetrapyridinoporphyrazine (CoTAP) was assembled on the surface of multi-walled carbon nanotubes (MWCNTs) as a shell via a coordination bond; this produced a heterostructure and enhanced interfacial polarization loss at the hetero-interface. The impedance matching characteristic of the CoTAP–CNT hybrid can be optimized by the CoTAP shell with an intermediate conductivity. Contact resistance between CNTs can be increased via insulation owing to the CoTAP shell, which decreases surface EM reflection. When the CNT content of the CoTAP–CNTs hybrid is 30 wt% and the thickness of the absorber is 2.1 mm, the minimum value of the reflection coefficient and the corresponding frequency are −54.7 dB and 9.8 GHz, respectively. The combination of CNTs and the intermediate dielectric loss CoTAP in a core–shell hybrid can overcome the contradiction of strong dielectric loss and impedance matching of traditional materials; this can be considered as an effective route for designing high-performance EM absorbing materials.

Ti3C2Tx MXenes modified with in situ grown carbon nanotubes (CNTs) are fabricated via a simple catalytic chemical vapor deposition (CVD) process. The as-prepared Ti3C2Tx/CNT nanocomposites show that one-dimensional (1D) carbon nanotubes are uniformly distributed in the interlayers of two-dimensional (2D) Ti3C2Tx MXene flakes. Compared with the pristine Ti3C2Tx MXenes, the hierarchical sandwich microstructure makes a contribution to the excellent electromagnetic wave absorption performance in the frequency range of 2–18 GHz, including higher absorption intensity (the minimum reflection coefficient reaches −52.9 dB, ∼99.999% absorption), broader effective absorption bandwidth (4.46 GHz), lower filler loading (35 wt%) and thinner thickness (only 1.55 mm). In addition, with the adjustment of thickness from 1.55 to 5 mm, the effective absorption bandwidth can reach up to 14.54 GHz (3.46–18 GHz). Different absorption mechanisms mainly based on polarization behaviors and conductivity loss are discussed. This work not only proposes the design of a novel electromagnetic wave absorber, but also provides an effective route for extending further the applications of 2D MXene materials in the field of electromagnetic wave absorption.

Nano SiC modified silicon oxycarbide (n-SiC/SiOC) ceramics were prepared through the pyrolysis of a mixture of liquid polysiloxane and n-SiC with an average grain diameter of 30 nm. After adding n-SiC, palingenetic SiC nanograins with an average grain diameter smaller than 10 nm, and nanosized free carbon were gradually separated from the amorphous SiOC phase when the annealing temperature increased from 1100 °C to 1450 °C. The various interfaces among n-SiC, in situ formed SiC nanograins, nanosized carbon and amorphous SiOC phases can obtain interfacial scattering. Eventually, the electric dipole polarization and interfacial scattering enhanced the absorption properties. The minimal reflection coefficient (RCmin) of the n-SiC/SiOC ceramics annealed at 1400 °C (n-SiC/SiOC-1400) reached −61 dB at 8.6 GHz. The widest effective absorption bandwidth (EAB) reached 3.5 GHz in the X-band, which indicates that the n-SiC/SiOC ceramics can be considered as high-performance microwave absorbing materials because of the strong absorption capability and wide absorption bandwidth.

Electromagnetic (EM) absorbing and shielding composites with tunable absorbing behaviors based on Ti3C2 MXenes are fabricated via HF etching and annealing treatment. Localized sandwich structure without sacrificing the original layered morphology is realized, which is responsible for the enhancement of EM absorbing capability in the X-band. The composite with 50 wt % annealed MXenes exhibits a minimum reflection loss of −48.4 dB at 11.6 GHz, because of the formation of TiO2 nanocrystals and amorphous carbon. Moreover, superior shielding effectiveness with high absorption effectiveness is achieved. The total and absorbing shielding effectiveness of Ti3C2 MXenes in a wax matrix with a thickness of only 1 mm reach values of 76.1 and 67.3 dB, while those of annealed Ti3C2 MXenes/wax composites are 32 and 24.2 dB, respectively. Considering the promising performance of Ti3C2 MXenes with the modified surface, this work is expected to open the door for the expanded applications of MXenes family in EM absorbing and shielding fields.Keywords: dielectric; electromagnetic shielding; microwave absorption; mxenes; surface modification

Three types of SiC fiber reinforced SiC matrix composites (SiCf/SiC) with BN interphase were fabricated by a chemical vapor infiltration (CVI) method to investigate the effect of heat treatment (HT) on the mechanical properties of the prepared materials. The different mechanical behaviors of the three composites were analyzed based on the He and Hutchinson׳s model. The mechanical properties of SiCf/SiC composites were improved (366–377 MPa in flexural strength, 13.9–16.2 MPa m1/2 in fracture toughness) by the HT after the infiltration of SiC matrix through increasing the crystallization degree of BN interphase. As a comparison, the mechanical properties of SiCf/SiC composites were decreased (366–134 MPa in flexural strength, 13.9–5.2 MPa m1/2 in fracture toughness) by the HT before the infiltration of SiC matrix mainly due to the degradation of SiC fibers during HT process without the protection of SiC matrix.

Core/shell structured C/ZnO nanoparticles were synthesized by a two-step process based on hydrothermal method. The experimental results show that ZnO nanoparticles attach on the surface of carbon spheres through the surfacial functional groups. The core/shell structure enhances the electromagnetic wave attenuation capability owing to defects, multiple interfaces and optimal impedance match. Different mass percentages of C/ZnO nanoparticles were mixed in paraffin wax to investigate the electromagnetic wave absorbing and shielding performance. When the filler loading is 40 wt%, the composite shows a minimum reflection coefficient of −52 dB at 11 GHz with a sample thickness of 1.75 mm. When the mass ratio is 50 wt%, the sample has an electromagnetic shielding performance of 14.85 dB dominated by absorption. Compared with pure carbon spheres and ZnO hollow spheres, the core/shell structure of C/ZnO composites exhibits a promising route to design electromagnetic wave absorbing materials with high dielectric loss and moderate impedance match.

In this work, the oxidation behavior of C/SiC-Ti3SiC2 from 800 to 1300 °C has been studied compared with that of C/SiC. C/SiC-Ti3SiC2 shows severe weight loss and low flexural strength after oxidation of 10 h at 800 °C. With the oxidation temperature increasing to 1000 °C, the coating cracks can be healed by silica and the matrix cracks can be healed by the oxidation product of Ti3SiC2, so the strength retention ratios of C/SiC-Ti3SiC2 reach to above 90% after oxidation of 10 h at 1000 and 1200 °C, revealing better oxidation resistance than C/SiC at the temperature range from 1000 to 1200 °C. With the oxidation temperature increasing to 1300 °C, the flexural strength decreases after oxidation of 10 h due to the oxidation of outer carbon fibers.

A high-performance electromagnetic wave absorbing composite based on graphene and polysiloxane-derived SiOC ceramic is realized via the polymer pyrolysis process. Hierarchical architecture consisting of two-dimensional graphene and one-dimensional SiC nanowire in ceramic matrix is achieved owing to the heterogeneous nucleation of SiC nanowires promoted by graphene at lower temperature. The dielectric and microwave absorption properties of the composites were studied at 293–673 K. When graphene oxide loading is 3 wt%, the composite attains a minimum reflection loss value of −69.3 dB at 10.55 GHz with a thickness of 2.35 mm. With the increase of temperature, the composite exhibits better absorbing performance that the effective absorption bandwidth reaches 3.9 GHz at 673 K. The hierarchical networks with graphene/SiC nanowires achieved in SiOC matrix provide a feasible process for the realization of efficient electromagnetic wave absorption in ceramic-based composites at high temperature.

A dense alumina fiber reinforced silicon carbide matrix composites (Al2O3/SiC) modified with Ti3Si(Al)C2 were prepared by a joint process of chemical vapor infiltration, slurry infiltration and reactive melt infiltration. The conductive Ti3Si(Al)C2 phase introduced into the matrix modified the microstructure of Al2O3/SiC. The refined microstructure was composed of conductive phase, semiconductive phase and insulating phase, which led to admirable electromagnetic shielding properties. Electromagnetic interference shielding effectiveness (EMI SE) of Al2O3/SiC and Ti3Si(Al)C2 modified Al2O3/SiC were investigated over the frequency range of 8.2–12.4 GHz. The EMI SE of Al2O3/SiC-Ti3Si(Al)C2 exhibited a significant increase from 27.6 to 42.1 dB compared with that of Al2O3/SiC. The reflection and absorption shielding effectiveness increased simultaneously with the increase of the electrical conductivity.

Design and development of advanced materials for electromagnetic applications and bringing these materials into use is one of the most challenging tasks of materials engineering. Silicon-based polymer derived ceramics (PDCs) are natural candidates for these demanding applications due to their very attractive microstructure and properties. Compared with sintered technical ceramics, such as SiC, Si3N4, Al2O3 or ZrO2, polymer derived ceramics offer the possibility of flexible plastic-technical processing, for instance by means of injection molding or extrusion without the employment of additional binder systems. The chemical synthesis permits a purposeful optimization of the polymers with respect to workability, ceramic yield and composition by the substitution of different elements in the basic structure as well as the organic side groups. In many cases the microstructure of PDCs characterized by homogeneous distribution of semiconducting or conducting nano-phases in the amorphous matrix. This can lead to the good microwave absorbing properties. Thus, those materials may not only satisfy the impedance matching but also rapidly attenuate electromagnetic waves. The absorption properties of PDCs can be easily tailored by the design of the molecular precursor, changes in morphology, and volume fraction of the filler particles. Different classes of preceramic polymers are briefly introduced and their absorption properties with adjustable phase compositions and microstructures are presented in this review.

•Doped Sr promotes the phase transformation from hexagonal phase to monoclinic phase.•Doped Sr decreases the dielectric constant of BSAS from 6.03 to 5.37.•Doped Sr decreases the infrared emissivity of BSAS from 0.78 to 0.54.Strontium-doped barium aluminosilicate ceramics were prepared by the sol-gel and hot-pressing processes. The experimental results show that doped Sr promotes the hexacelsian-to-celsian phase transformation of barium aluminosilicate ceramics. When the mass ratio of Sr increases, the dielectric constant of the samples decreases from 6.03 to 5.37, which is accompanying by the slight increase of tangent loss. These are attributed to the lower polarizability of Sr ion and its oxides. The infrared emissivity presents a tread of first increasing and then decreasing, owing to the effects of doped Sr on crystal structure and phase transformation of barium aluminosilicate ceramics.

•The combination of 3DP and PIP was first employed to fabricate Si3N4–SiC in this work.•Twinned SiC nanowires were formed in the porous Si3N4 using PIP.•We report a novel Si3N4–SiC ceramic that has superior absorption properties than already-reported PDCs.Near net- and complex shaped porous silicon nitride (Si3N4) composites reinforced with in-situ formed twinned silicon carbide (SiC) nanowires (NWs) were successfully fabricated by 3d-printing (3DP) followed by polymer precursor infiltration and pyrolysis (PIP) up to 1400 °C. An increase of the PIP cycle number of the printed bodies resulted in a homogeneous distribution of SiC NWs in the fabricated composites. An increase of SiC NW content in the fabricated composites led to the growth of both the real and the imaginary parts of permittivity. The formation of twinned SiC NWs with high electrical conductivity led to a minimal electromagnetic wave RC of −57 dB, demonstrating that Si3N4–SiC ceramics with the in-situ formed SiC NWs have a superior microwave absorbing ability.

Graphene-wrapped ZnO hollow spheres were synthesized by a two-step process, which combined a hydrothermal reaction with surface modification. The experimental results show that reduced graphene oxide sheets adhere entirely to the surface of the ZnO hollow spheres consisting of nanoparticles. The unique structure effectively decreases the density of the composite without sacrificing the contact between graphene and the nanoparticles. Different mass ratios of graphene to ZnO hollow spheres mixed in a paraffin wax matrix (50 wt%) were prepared to investigate the electromagnetic wave absorption properties in the X-band region. When the mass ratio of graphene oxide to ZnO is 12:88, the composite exhibits a maximum absorption of −45.05 dB at 9.7 GHz with a sample thickness of only 2.2 mm. The fundamental mechanism based on electrical conductivity and the polarization between the graphene sheets and ZnO nanoparticles is discussed. The hierarchical structure of graphene-wrapped ZnO hollow spheres exhibits a promising designable approach to lightweight electromagnetic wave absorbing materials.

Carbon nanotube reinforced carbon fiber/pyrolytic carbon composites were fabricated by precursor infiltration and pyrolysis method and their electromagnetic interference shielding effectiveness (EMI SE) was investigated over the frequency range of 8.2–12.4 GHz (X-band). Carbon nanotubes (CNTs) were in situ formed through catalyzing hydrocarbon gases evaporating out of phenolic resin with nano-scaled Ni particles. The content of CNTs increased with the increase of Ni loadings (0.00, 0.50, 0.75 and 1.25 wt.%) in phenolic resin. Thermal gravimetrical analysis results showed that the carbon yield of phenolic resin increased with the addition of Ni catalyst. With the formation of CNTs, the EMI SE increased from 28.3 to 75.2 dB in X-band. The composite containing 5.0 wt.% CNTs showed an SE higher than 70 dB in the whole X-band.

By using a catalytic growth procedure, carbon nanotubes (CNTs) are in situ formed on reduced graphene oxide (RGO) sheet at 600 °C. CNTs growing on RGO planes through covalent C–C bond possess lower interfacial contact electrical resistance. As a hybrid structure, the CNTs/graphene (CNT/G) are well dispersed into poly (dimethyl siloxane). The hybrid combining electrically lossy CNTs and RGO, which disperses in electrically insulating matrix, constructs an electromagnetic wave (EM) absorbing material with ternary hierarchical architecture. The interfacial polarization in heterogeneous interface plays an important role in absorbing EM power. When the filler loading is 5 wt.% and thickness of absorber is 2.75 mm, the minimum value of reflection coefficient and the corresponding frequency are −55 dB and 10.1 GHz, and the effective absorption bandwidth reaches 3.5 GHz. Therefore, combining the CNTs and graphene sheet into three-dimensional structures produces CNT/G hybrids that can be considered as an effective route to design light weight and high-performance EM absorbing material, while the effective EM absorption frequency can be designed.

A dense carbon fiber reinforced silicon carbide matrix composites modified by SiBC matrix (C/SiC-SiBC) was prepared by a joint process of chemical vapor infiltration, slurry infiltration and liquid silicon infiltration. The effects of pyrolytic carbon (PyC) interphase thickness on mechanical properties and oxidation behaviors of C/SiC-SiBC composites were evaluated. The results showed that C/SiC-SiBC composites with an optimal PyC interphase thickness of 450 nm exhibited flexural strength of 412 MPa and fracture toughness of 24 MPa m1/2, which obtained 235% and 300% improvement compared with the one with 50 nm-thick PyC interphase. The enhanced mechanical properties of C/SiC-SiBC composites with the increase of interphase thickness was due to the weakened interfacial bonding strength and the decrease of matrix micro-crack amount associated with the reduction of thermal residual stress. With the decrease in matrix porosity and micro-crack density, C/SiC-SiBC composites with 450 nm-thick interphase exhibited excellent oxidation resistance. The residual flexural strength after oxidized at 800, 1000 and 1200 °C in air for 10 h was 490, 500 and 480 MPa, which increased by 206%, 130% and 108% compared with those of C/SiC composites.

Porous SiCN ceramics were successfully fabricated by pyrolysis of a kind of polysilazane. The effects of annealing temperature on the microstructure evolution, direct-current electrical conductivity, dielectric properties, and microwave absorption properties of SiCN in the frequency range 8.2–12.4 GHz (X-band) were investigated. With the increase of annealing temperature, SiC, Si3N4 and free carbon nanodomains are gradually formed in the SiCN. Both the SiC and free carbon nanodomains lead to the increases of the complex relative permittivity and loss tangent of SiCN. With the increase of the annealing temperature, the average real permittivity, imaginary permittivity and loss tangent increase from 4.4, 0.2 and 0.05 to 13.8, 6.3 and 0.46, respectively. The minimum reflection coefficient and the frequency bandwidth below −10 dB for SiCN annealed at 1500 °C are −53 dB and 3.02 GHz, indicating good microwave absorption properties.

SiC nanoparticles with different contents (5–20 wt%) were mixed with liquid polyborosilazane. The compound was used to prepare SiC nanoparticle/polymer-derived SiBCN ceramics (SiC/PDCs-SiBCN). Thermal gravity tests (25–1400 °C) in air and helium atmosphere were used to investigate the thermal stability of SiC/PDCs-SiBCN. Dielectric and microwave-absorption properties of SiC/PDCs-SiBCN were determined at frequencies of 8.2–12.4 GHz by waveguide method. Results show that the addition of SiC nanoparticles increased the thermal stability of SiBCN ceramics. The permittivity, dielectric loss and absorption coefficient of ceramics increased as an elevated SiC content, resulting from the increase of carrier concentration. To understand the high-temperature dielectric property of SiC/PDCs-SiBCN, the permittivity of SiBCN ceramics with 15 wt% of SiC was measured at temperatures of 293–773 K. The composite ceramics were found to have a visible increase in the permittivity and dielectric loss, indicating their great potential as the high-temperature microwave absorption materials.

Dense Ti3Si(Al)C2-based ceramics were synthesized using reactive melt infiltration (RMI) of Al70Si30 alloy into the porous TiC preforms. The effects of the infiltration temperature on the microstructure and mechanical properties of the synthesized composites were investigated. All the composites infiltrated at different temperatures were composed of Ti3Si(Al)C2, TiC, SiC, Ti(Al, Si)3 and Al. With the increase of infiltration temperature from 1050 °C to 1500 °C, the Ti3Si(Al)C2 content increased to 52 vol.% and the TiC content decreased to 15 vol.%, and the Vickers hardness, flexural strength and fracture toughness of Ti3Si(Al)C2-based composite reached to 9.95 GPa, 328 MPa and 4.8 MPa m1/2, respectively.

•The existence of carbon promotes the formation of Ti3SiC2.•Al plays a catalytic role and promotes the formation of Ti3SiC2.•A diffusion–competition process exists between Al and Si melts.•Si prefers to inward diffusion due to the more reaction activity with TiC than Al.In this paper, Ti3SiC2-based materials were fabricated by reactive melt infiltration (RMI), and the effect of carbon and Al on the formation of Ti3SiC2 was discussed. In the infiltration process of Si melt, the existence of carbon in the initial preform is beneficial to the formation of Ti3SiC2. Carbon can react with TiSi2 to form new TiC with more carbon vacancies than the initial TiC, promoting the formation of TiC twins and Ti3SiC2. In the infiltration process of Al–Si alloy, the existence of Al can effectively decrease the twin boundary energy of TiC grains, leading to the formation of TiC twins and nucleation of Ti3SiC2. There is a diffusion–competition process existing between Si and Al melts, leading to a higher Si/Al ratio in the final samples than in the Al–Si alloy. A possible formation mechanism was proposed to explain the final products.

SiO2 reinforced with both multi-wall carbon nanotubes (MWCNTs) and ZnO particles was prepared. Owing to the consumption of an amorphous carbon layer on the outer surface of MWCNTs and the generation of oxygen vacancies in ZnO during sintering, the contact resistance between MWCNTs is lowered and a higher concentration of charge carriers is produced in ZnO. The permittivity of the composite is improved by both changes. The composite containing 15 wt% ZnO particles and 3 wt% MWCNTs exhibits a wider effective absorption bandwidth and lower minimum reflection coefficient than both SiO2 reinforced with 15 wt% ZnO particles and SiO2 reinforced with 3 wt% MWCNTs.

Pyrolytic carbon-Si3N4 ceramics (PyC-Si3N4) with gradient PyC distribution (Gradient-PyC-Si3N4) is fabricated by directional oxidation. After directional oxidation for 1.0 h, the electromagnetic absorptivity of Gradient-PyC-Si3N4 increases significantly from 0.8 to 50.1% with the obvious reduction of electromagnetic reflectivity from 99.2 to 43.8%. The Gradient-PyC-Si3N4 is a good electromagnetic absorbing material meeting the requirement of self-concealing technology.

•A novel SiCN ternary phase was introduced into porous Si3N4 ceramics by CVI.•SiCN was composed of crystalline SiC nano-particles and amorphous Si3N4 matrix.•Si3N4–SiCN ceramics realized a minimal EM reflection coefficient of −42.6 dB.A novel SiCN ternary phase was introduced into porous Si3N4 preform by chemical vapor infiltration from SiCl4–C3H6–NH3–H2–Ar, which was used to fabricate Si3N4–SiCN composite ceramics. SiCN was composed of amorphous Si3N4 matrix and crystalline SiC nano-particles (with a homogeneous size of ~5 nm) uniformly distributed in amorphous matrix. This distinctive microstructure made the Si3N4–SiCN ceramics not only maintain a high flexural strength of 227 MPa but realize a low dielectric constant and a high dielectric loss, which led to a minimal electromagnetic wave reflection coefficient of −42.6 dB, ranking the best level in the already-reported structural and wave-absorbing ceramic materials for high-temperature applications.

Pyrolytic carbon (PyC) was infiltrated into silicon nitride (Si3N4) ceramics by precursor infiltration and pyrolysis (PIP) of phenolic resin, and Ni nanoparticles were added into the phenolic resin to change the electric conductivity of Si3N4–PyC composite ceramics. Dielectric permittivity, electromagnetic interference (EMI) shielding and absorption properties of Si3N4–PyC composite ceramics were studied as a function of Ni content at 8.2–12.4 GHz (X-band). When Ni nanoparticles were added into phenolic resin, the electric conductivity of the prepared composite ceramics decreased with increasing Ni content, which was attributed to the decrease of graphitization degree of PyC. The decrease in electric conductivity led to the decrease in both permittivity and EMI shielding effectiveness. Since too high permittivity is harmful to the impendence match and results in the strong reflection, the electromagnetic wave absorption property of Si3N4–PyC composite ceramics increases with increasing Ni content. When the content of Ni nanoparticles added into phenolic resin was 2 wt%, the composite ceramics possessed the lowest electric conductivity and displayed the most excellent absorption property with a minimum reflection loss as low as −28.9 dB.

The dielectric and electromagnetic properties of two types of SiC fibres with different compositions were investigated. The permittivity and electromagnetic shielding effectiveness (SE) of SiC fibre bundles were measured in 8.2–12.4 GHz by waveguide method. The reflection coefficient (RC) of unidirectional SiC fibre laminates was determined in 8–18 GHz using naval research laboratory (NRL)-arc method. Results showed that the electromagnetic wave (EMW) absorbing properties of SiC fibres were correlated with their composition, microstructure and instinct performance of electrical resistance. SiC fibres with higher content and greater size of nano-scale β-SiC showed higher permittivity, conductivity, SE and lower RC, which resulted in their better EMW absorbing ability, i.e. the lower reflection to EMW.

Graphene is highly desirable as an electromagnetic wave (EM) absorber because of its large interface, high dielectric loss, and low density. Nevertheless, the conductive and electromagnetic parameters of pure graphene are too high to meet the requirement of impedance match, which results in strong reflection and weak absorption. In this paper, we report a facile solvothermal route to synthesize reduced graphene oxide (RGO) nanosheets combined with surface-modified γ-Fe2O3 colloidal nanoparticle clusters. The obtained two-dimensional hybrids exhibit a relatively low EM reflection coefficient (RC) and wide effective absorption bandwidth, which are mainly attributed to the unique microstructure of colloidal nanoparticle clusters assembled on RGO. The nanoparticle clusters have more interfaces. The interfacial polarization within nanoparticle clusters and conductivity loss of RGO plays an important role in absorbing EM power. The minimum RC reaches −59.65 dB at 10.09 GHz with a matching thickness of 2.5 mm. The special integration of some metal oxide semiconductor crystals assembled on RGO sheets provides an effective avenue to design metal oxide semiconductor/carbon hybrids as EM absorbing materials.

Considering the widespread presence of electromagnetic interferences (EMI), it is necessary to develop new electromagnetic wave (EM) absorbing materials with low reflection coefficient and large operating frequency band. The well-known EM absorbing materials have a microstructure combining a low permittivity phase with a high electrical conductivity phase. In the present work, a phase in nanoscale with medium permittivity is added into the well-known EM absorption materials to obtain an EM absorption material with low EM reflection coefficient and wide absorption band. Composite powders with special microstructure have been synthesized via sol–gel process, which are composed of submicrometer-sized ZnO acting as electrically lossy phase and ZnAl2O4 nanograins acting as a medium permittivity phase. When the composite powders are mixed with paraffin, the as-received materials exhibit appropriate permittivity and electrical conductivity, which can be attributed to the high carrier concentration and mobility at the interfaces in nanoscale. The high absorption coefficient, small reflection coefficient, and wide absorption band can be obtained. Absorption coefficient per unit thickness increases from 0.01 to 0.13/mm, the minimum reflection coefficient reaches −25 dB, and the effective absorption bandwidth covers the whole X-band (8.2–12.4 GHz). The ZnO/ZnAl2O4 composite materials exhibit excellent EM absorption properties.

To obtain better electromagnetic wave absorbing property, it is vitally necessary to develop novel ceramics with not only high dielectric loss but also low dielectric constant. Si3N4–SiBC, a composite ceramic with such dielectric properties, was fabricated by infiltrating SiBC into porous Si3N4 ceramic via low pressure chemical vapor infiltration. The high dielectric loss and the low dielectric constant are attributed to the unique microstructure of SiBC, which also leads to a very excellent wave-absorbing property of Si3N4–SiBC ceramic, attaining a minimal reflection coefficient of −28 dB. Besides, the Si3N4–SiBC ceramic also shows a high mechanical property. Therefore, the Si3N4–SiBC ceramic exhibits great potential as an excellent functional and structural ceramic.

In the present paper, friction and wear behaviors of a carbon fiber reinforced carbon–silicon carbide–titanium silicon carbide (C-SiC–Ti3SiC2) hybrid matrix composites fabricated by slurry infiltration and liquid silicon infiltration were studied for potential application as brake materials. The properties were compared with those of C/C-SiC composites. The composites containing Ti3SiC2 had not only higher friction stability coefficient but also much higher wear resistance than C/C-SiC composites. At an initial braking speed of 28 m/s under 0.8 MPa pressure, the weight wear rate of the composites containing 5 vol% Ti3SiC2 was 5.55 mg/cycle, which was only one-third of C/C-SiC composites. Self-lubricious film-like debris was formed on the composites containing Ti3SiC2, leading to the improvement of friction and wear properties. The effect of braking speed and braking pressure on the tribological properties of modified composites were investigated. The average friction coefficient was significantly affected by braking speed and braking pressure, but the wear rate was less affected by braking pressure.Graphical abstractHighlights► Residual silicon is harmful to friction and wear properties of C/C-SiC composites. ► The residual silicon is replaced by the in situ formed Ti3SiC2. ► The wear resistance and friction stable coefficient are improved. ► The film-like debris of Ti3SiC2 leads to the excellent wear resistance.

In the present paper, the devitrification kinetics of silica powder heat-treated in air and in nitrogen were investigated. The heat-treated powders were confirmed to partially crystallize into cristobalite by X-ray diffraction analysis. The devitrification kinetics data of silica powder fitted the Avrami equation very well. The measured value of n was ascertained to be 1.63 and 1.60 when the silica powder was heat-treated in air and in nitrogen, respectively, corresponding to activation energies of 408 kJ/mol and 529 kJ/mol, respectively. The effect of phenolic resin-derived carbon on crystallization of silica powder was also studied. By adding phenolic resin into silica powder, the devitrification of silica powder was fully restrained up to 1500 °C in flowing nitrogen. High concentrations of oxygen vacancies may retard the nucleation of cristobalite on the surface of silica powder. Phenolic resin-derived pyrolysis carbon restrained the nucleation of cristobalite, because it prevented the reactions of oxygen vacancies with H2O and O2 molecules.

Flexible and high-performance electromagnetic absorbing materials of three-dimensional (3D) hierarchical reduced graphene oxide (RGO) foams decorated with in-situ grown ZnO nanowires (ZnOnws) were realized by a direct freeze-drying and hydrothermal process. The unique structure not only effectively reduces agglomeration of RGO and the density of composites, but also makes great contributions to impedance match, dielectric loss and inner scattering, achieving enhanced microwave absorption performance. When RGO foam is 0.8 mg/mL, the ZnOnws/RGO foam/PDMS composite with 3.3 wt% absorber loading attains a minimum reflection loss value of −27.8 dB at 9.57 GHz with a thickness of 4.8 mm, and a wide effective absorption bandwidth of 4.2 GHz covering the whole X band (8.2–12.4 GHz). The fundamental microwave absorption mechanism of the composites is discussed. These results demonstrate a promising method to fabricate an economical, lightweight, broadband and highly efficient microwave absorption material.Hierarchical ZnOnws/RGO foam/PDMS composite of 3.3 wt% absorber loading shows an excellent EM absorbing property with a wide EAB covering the X band.

In this contribution, we design a novel strategy to synthesize SiOC ceramics by pyrolysis of hyperbranched ferrocene-containing polysiloxane (HBPSO-VF) which are synthesized by the reaction of polysiloxane (PSO) with 1,1′-Bis(dimethylvinylsilyl)ferrocene (VF). This SiOC ceramics show much lower crystallization temperature because of the capability of HBPSO-VF to incorporate metallic iron into the backbone of PSO. The usage of HBPSO-VF offers enhanced ceramic yield of 83 wt% at 1200 °C due to the deep cross-linking of hydrosilylation. Nano-sized SiC and turbostratic carbons are separated from amorphous SiOC phase when it is annealed at 1100 °C, while crystallization temperature is 1400 °C when PSO is used as polymer precursors. The minimum reflection coefficient (RCmin) of this nanocrystal-containing ceramic reaches −46 dB, exhibiting a promising prospect as a kind of electromagnetic wave (EMW) absorbing materials. This method also can be further extended to develop other functional Si-based polymer derived ceramic (PDC) systems for EMW absorption and shielding applications.

•The variation in microstructures and electromagnetic properties of carbon/Si3N4 composites with CVD temperatures.•Different growth mechanisms of carbon nanowires and nanotubes.•Four-state transformation process from solid wires into hollow tubes.•Reduction of surface energy during tubular formation at high temperature.Polycrystalline carbon nanowires (p-CNWs) has been synthesized via the CVD strategy using C2H4 as carbon source and Ni particles distributed in porous Si3N4 ceramic as growth seeds at 700 °C. As-received p-CNWs showed the solid core and the basal planes of graphite nanosheets were perpendicular to the wire axis. Elevated growth temperatures encouraged the formation of hollow carbon nanotubes (CNTs) at 750 °C and amorphous carbon above 800 °C, which in turn altered the electrical conductivity and dielectric properties of carbon reinforced Si3N4 composite ceramics. The growth mechanisms and microstructures of solid wires and hollow tubes were discussed and compared detailedly. It is energetically favorable for p-CNWs, which shows high surface energy, to convert itself into crystallized CNTs during the annealing at 1000 °C. The four-state transition process was accompanied by the diffusion of carbon atoms and the rearrangement and growth of nanosheets. The present research strategies and results are not only beneficial to investigate the growth and graphitization behavior of carbon nanoscale materials, but also probably extended to investigate other nanowire-nanotube systems.Solid carbon nanowires (CNWs), hollow nanotubes (CNTs) and amorphous carbon (a-C) were synthesized by CVD at different temperatures.Figure optionsDownload full-size imageDownload high-quality image (161 K)Download as PowerPoint slide

In order to enhance dielectric properties of polymer derived SiCN ceramics (PDCs-SiCN), nano-structured carbons were in-situ fabricated in PDCs-SiCN by pyrolysis of ferrocene-modified polysilazane. Microstructure evolutions, dielectric and microwave absorption properties of PDCs-SiCN decorated with nano-structured carbons were investigated. Nano-structured PDCs-SiCN ceramics are composed of carbon nanowires as well as interpenetrating graphene-like free carbons, SiC nano-crystals, Si3N4 nano-crystals and amorphous SiCN. Relative complex permittivities of nano-structured PDCs-SiCN increase with increasing ferrocene contents and annealing temperatures. Free carbons in PDCs-SiCN play a dominating role on the improved dielectric properties. Polarization loss is the primary dielectric loss. Loss tangent of PDCs-SiCN exceeding 0.7 is obtained when free carbons are only 2.57% in weight. Nano-structured PDCs-SiCN exhibit good microwave absorption property. The reflectivity is smaller than −14 dB in the whole X band when material is composed of both impedance and microwave absorption materials.

•The designed two-layer periodic stepped absorbing structure has more than 90% absorption from 2.64 to 40 GHz frequency.•Resonances between adjacent unit cells and edge diffraction effects lead to a great absorption.•The two-layer periodic stepped absorbing structure can achieve great absorption when incident angles is less than 45°.This paper presents a novel ultra-broadband two-layer periodic stepped radar absorbing structure (PSRAS). The optimal PSRAS shows more than 90% absorption in the frequency range from 2.64 to 40.0 GHz by using a conventional α Fe reinforced epoxy resin composite. The greatly enhanced radar absorption property of designed PSRAS is a result of the multi-scale effect by combing effects of microscopic and mesoscopic scales. The designed PSRAS makes the effective impedance match to that of the free space in a wide frequency: most of the incident energy is dissipated by the simultaneous incorporation of multiple λ/4 resonances, strong resonances between adjacent unit cells and edge diffraction effects. Based on the geometrical parameters optimization for the two-layer PSRAS, it is specified that size of unit cell obeys a simple rule of the Golden Section approximately. Additionally, the incident angle dependent absorption properties are simulated and discussed. It is expected that the proposed PSRAS composite has great potentials in stealth technologies and electromagnetic interferences.Download high-res image (328KB)Download full-size image

The microstructure and electromagnetic (EM) properties of near-stoichiometric SiC fibres (with C/Si ratio of 1.125) were analyzed and evaluated in detail. The SiC fibres consisted of β-SiC nanocrystallines and free carbon, and exhibited a uniquely specific skin-core structure with thin carbon layer of 5 nm on their surfaces. The relative complex permittivity increased with the increasing fibre volume fraction from 13 vol% to 27.5 vol%. The imaginary part of permittivity increased from 1.36 to 2.13 at 10 GHz, due to more SiC nanocrystallines and interfaces generating. The EM wave absorption properties were enhanced by the increasing fibre volume fraction and the effective absorption bandwidth was approximately 2.6 GHz when the fibre volume fraction was 27.5 vol%.

The thermal and microstructure stability of Nextel 610 fibers has great influence on high-temperature application of Nextel 610 fiber-reinforced ceramic matrix composites. In this work, Nextel 610 fibers were heat treated at 500–700 °C in vacuum and 800–1100 °C in Ar atmosphere, respectively. The sizing agent on Nextel 610 fiber surface could be decomposed into pyrolytic carbon, SiC and gaseous little molecules at lower temperatures, otherwise it was decomposed mainly in the form of gaseous little molecules at higher temperatures, so that the complex permittivity firstly increased and then decreased with the increasing of temperatures. The results showed that the annealed Nextel 610 fiber (T>900 °C) could be regarded as electromagnetic wave transparent fibers, while the tensile strength had declined by half when the temperature increased to 1100 °C. Therefore, Nextel 610 fibers after being annealed at higher temperatures could be further used as reinforcement to prepare high temperature ceramic matrix composites for electromagnetic wave absorption and transparent applications.

This work investigated electromagnetic wave (EMW) absorption and mechanical properties of silicon carbide (SiC) fibers with and without boron nitride (BN) coating by chemical vapor infiltration (CVI). The dielectric property and EM shielding effectiveness of SiC fiber bundles before and after being coated by BN were measured by wave guide method. The EM reflection coefficient of SiC fiber laminates with and without BN coating was determined by model calculation and NRL-arc method, respectively. Tensile properties of SiC fiber bundles with and without BN coating were tested at room temperature. Results show that SiC fibers with BN coating had a great improvement of EMW absorbing property because the composites achieved the impedance matching. BN with the low permittivity and dielectric loss contributed to the enhancive introduction and reduced reflection of EMW. The tensile strength and Weibull modulus of SiC fiber bundles coated by BN increased owing to the decrease of defects in SiC fibers and the protection of coating during loading.Highlights► BN coating was deposited on SiC fibers by CVI. ► After coated by BN, the tensile strength of SiC fiber bundles increased by 1.389%. ► After coated by BN, SiC fiber laminates had a great improvement of wave-absorbing ability with a minimum RC of −16.41 dB at 15.38 GHz.

Ti3C2Tx MXenes modified with in situ grown carbon nanotubes (CNTs) are fabricated via a simple catalytic chemical vapor deposition (CVD) process. The as-prepared Ti3C2Tx/CNT nanocomposites show that one-dimensional (1D) carbon nanotubes are uniformly distributed in the interlayers of two-dimensional (2D) Ti3C2Tx MXene flakes. Compared with the pristine Ti3C2Tx MXenes, the hierarchical sandwich microstructure makes a contribution to the excellent electromagnetic wave absorption performance in the frequency range of 2–18 GHz, including higher absorption intensity (the minimum reflection coefficient reaches −52.9 dB, ∼99.999% absorption), broader effective absorption bandwidth (4.46 GHz), lower filler loading (35 wt%) and thinner thickness (only 1.55 mm). In addition, with the adjustment of thickness from 1.55 to 5 mm, the effective absorption bandwidth can reach up to 14.54 GHz (3.46–18 GHz). Different absorption mechanisms mainly based on polarization behaviors and conductivity loss are discussed. This work not only proposes the design of a novel electromagnetic wave absorber, but also provides an effective route for extending further the applications of 2D MXene materials in the field of electromagnetic wave absorption.

Nano SiC modified silicon oxycarbide (n-SiC/SiOC) ceramics were prepared through the pyrolysis of a mixture of liquid polysiloxane and n-SiC with an average grain diameter of 30 nm. After adding n-SiC, palingenetic SiC nanograins with an average grain diameter smaller than 10 nm, and nanosized free carbon were gradually separated from the amorphous SiOC phase when the annealing temperature increased from 1100 °C to 1450 °C. The various interfaces among n-SiC, in situ formed SiC nanograins, nanosized carbon and amorphous SiOC phases can obtain interfacial scattering. Eventually, the electric dipole polarization and interfacial scattering enhanced the absorption properties. The minimal reflection coefficient (RCmin) of the n-SiC/SiOC ceramics annealed at 1400 °C (n-SiC/SiOC-1400) reached −61 dB at 8.6 GHz. The widest effective absorption bandwidth (EAB) reached 3.5 GHz in the X-band, which indicates that the n-SiC/SiOC ceramics can be considered as high-performance microwave absorbing materials because of the strong absorption capability and wide absorption bandwidth.

Graphene-wrapped ZnO hollow spheres were synthesized by a two-step process, which combined a hydrothermal reaction with surface modification. The experimental results show that reduced graphene oxide sheets adhere entirely to the surface of the ZnO hollow spheres consisting of nanoparticles. The unique structure effectively decreases the density of the composite without sacrificing the contact between graphene and the nanoparticles. Different mass ratios of graphene to ZnO hollow spheres mixed in a paraffin wax matrix (50 wt%) were prepared to investigate the electromagnetic wave absorption properties in the X-band region. When the mass ratio of graphene oxide to ZnO is 12:88, the composite exhibits a maximum absorption of −45.05 dB at 9.7 GHz with a sample thickness of only 2.2 mm. The fundamental mechanism based on electrical conductivity and the polarization between the graphene sheets and ZnO nanoparticles is discussed. The hierarchical structure of graphene-wrapped ZnO hollow spheres exhibits a promising designable approach to lightweight electromagnetic wave absorbing materials.