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C/SiC is a promising structural-functional material with excellent mechanical and electromagnetic (EM) interference shielding properties. In order to investigate its evolution of mechanical and shielding properties during intermediate temperature oxidation, C/SiC composites were oxidized at 700 °C for different times in this study. The oxidation of C/SiC at 700 °C leads to the consumption of pyrocarbon (PyC) interphase and carbon fibers. As the content of PyC-coated carbon fibers decreases from 41.7 vol.% to 0 vol.%, the volume fraction of carbon fibers that can effectively load decreases. The abilities of C/SiC both in reflecting and absorbing EM waves also decrease due to the decline of electrical conductivity. Consequently, the flexural strength, fracture toughness and total shielding effectiveness of C/SiC decrease by 95.5 %, 97.4 % and 44.2 %, respectively. The decrease of mechanical properties is sensitive to the oxidation of PyC interphase. The decrease of shielding properties depends on the continuity and content of carbon fibers. The much higher retention ratio of shielding properties than mechanical properties is attributed to the high density stacking faults (SFs) in columnar SiC crystals. These SFs are formed by embedding 2H–SiC segments in 3C–SiC. Dipole polarization loss occurs at 3C–SiC/2H–SiC interfaces. Image 108639 [ABSTRACT FROM AUTHOR]

In order to optimize the dielectric performance of polymer derived SiCN ceramics (PDCs-SiCN), carbon nanowires (CNW) were deposited in SiCN by catalytic chemical vapor deposition (CCVD). Microstructure evolutions, dielectric property and electromagnetic (EM) wave absorption capacity of CNW/SiCN were investigated. Results show that carbon nanowires had plentiful pits/defects on their roughened surface and formed hierarchical network in SiCN which benefited the impedance match and generated strong conductivity and polarization loss, enhancing the absorption ability of CNW/SiCN. When CNW accounted for 5.61 wt%, RC min reached −51 dB with EAB of 3.0 GHz at 2.7 mm in thick, showing excellent microwave absorbing performance. The favorable microwave absorption ability could be ascribed to three aspects including enhanced conductivity loss derived from the excellent conductivity of CNW, polarization loss generated by defects, and multiple reflection loss enhanced by hierarchical network. By comparing the variation tendency between defect concentration and electrical conductivity in CNW/SiCN, it is rational to conclude that the conductivity loss dominated the dielectric loss while the polarization loss and repeated multi-reflection simultaneously worked. This work can be further extended to study regarding the effect of heat-treatment temperature since CNW have the potential to promote the crystallization process of amorphous PDCs thereby improving their dielectric properties. [ABSTRACT FROM AUTHOR]

Designing pure carbonaceous microwave absorbers with strong absorption capability and broad effective absorption bandwidth (EAB) is still a challenge for efficient microwave absorption. Here, a kind of bimetallic metal-organic frameworks derived porous amorphous carbon particle with hollow graphene spheres confined in it (HGS@PAC), was prepared by the controlled annealing process. The thickness of hollow graphene spheres could be tuned from 9 graphene layers to 24 graphene layers by adjusting the annealing time, and thereby tuning their dielectric properties. The minimum RC of the optimized HGS@PAC reaches −56 dB at 3.5 GHz with a thickness of 6.75 mm, indicating its strong microwave absorption performance. In addition, the widest EAB reaches 5.6 GHz (12.4–18 GHz) with a thickness of only 1.85 mm. It is important to highlight that the EAB can cover the whole measured bandwidth (2–18 GHz) with the sample thickness ranging from 1.85 mm to 10 mm. The enhanced microwave attenuation ability can be ascribed to the enhanced interfacial polarization, dipole polarization, and conductive loss, resulting from the designed unique architectures. HGS@PAC with tunable heterogeneous interface prepared in this work, exhibit excellent microwave absorption performance with an RC min lower to −56 dB and an EAB reaches 5.6 GHz with a certain thickness of only 1.85 mm. Besides, the EAB can covers the whole measured bandwidth from 2 GHz to 18 GHz with the sample thickness ranges from 1.85 mm to 10 mm. Image 1069008 [ABSTRACT FROM AUTHOR]

Porous SiO 2 /3Al 2 O 3 ·2SiO 2 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 SiO 2 /3Al 2 O 3 ·2SiO 2 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-SiO 2 /3Al 2 O 3 ·2SiO 2 composite ceramics can be tuned. As expected, the CNWs-SiO 2 /3Al 2 O 3 ·2SiO 2 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-SiO 2 /3Al 2 O 3 ·2SiO 2 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. [ABSTRACT FROM AUTHOR]

High-performance carbon nanotubes (CNTs) with adjustable electro-conductivity have been widely used as electromagnetic (EM) waves absorption materials to achieve stealth of weapons and equipment. Chemical vapor deposition technology is used for the simple preparation of CNTs on porous Sc 2 Si 2 O 7 matrix. As expected, well-matched impedance and EM dissipation of CNTs/Sc 2 Si 2 O 7 ceramics are synthesized. Curls of CNTs formed a three-dimensional network structure and large amounts of interfaces, resulting in multiple reflections and scattering of EM waves. The defect concentrations can be optimized by tuning reaction times. The existing defects will produce dipole polarization and affect the band gap of CNTs as well. Using density functional theory as first principle calculations, we calculate and simulate the relationship between band gap and vacancy-defects of CNTs. The results show that the appropriate contents of vacancy-defects will increase the band gap of CNTs, which regulate the conductivity loss of CNTs/Sc 2 Si 2 O 7. Hence, the excellent microwave absorption performance of CNTs/Sc 2 Si 2 O 7 (loading content of 1.56 wt%) is RC min of −33.5 dB at the thickness of 2.85 mm, achieving an effective absorbing bandwidth of 4.2 GHz covering the whole X-band. The exploration results provide a useful reference to EM wave absorption materials with strong absorption, wide bandwidth and thin thickness. Image 102 [ABSTRACT FROM AUTHOR]

High electrical conductivity and the layered structure features are desired for high-performance electromagnetic interference (EMI) shielding materials. Although 2D graphene has proven to be a promising EMI shielding material, the complex preparation, including the necessary reduction process accompanied by the sacrifice of functional groups, and subsequent structural design inevitably limit its practical application. Herein, conductive 2D Ti 2 CT X MXene with native sandwich-like structure and abundant surface functional groups is synthesized via a simple chemical etching process. The Ti 2 CT X MXene exhibits excellent EMI shielding performance which exceeds 70 dB at only 0.8 mm in X-band, surpassing most of reported graphene-based EMI shielding composites. The multilayered structure obviously strengthens the internal EM attenuation and greatly improves the absorption effectiveness, while the high electrical conductivity ensures sufficient EM reflection on the surface. In addition, the high conductivity, unique structure, and abundant surface functional groups make Ti 2 CT X MXene a promising candidate in energy storage and multifunctional composites application beyond EMI shielding. Image 1 [ABSTRACT FROM AUTHOR]

Abstract In this study, in order to improve the tensile strength and toughness of dense C/SiC-SiBC, the thickness of PyC interphase (e PyC) was increased from 200 nm to 400 nm. C/SiC (e PyC ≈200 nm) was also fabricated as a benchmark comparison. C/SiC-SiBC (e PyC ≈200 nm) exhibited higher axial thermal residual stress (TRS) than C/SiC (e PyC ≈200 nm). Increased axial TRS resulted in increased interfacial shear strength (τ) through fiber bending, which caused lowered mechanical properties of C/SiC-SiBC (e PyC ≈200 nm). By increasing the thickness of PyC interphase from 200 nm to 400 nm, the axial TRS in C/SiC-SiBC decreased. Accordingly, the radial stress induced by fiber bending decreased, leading to a reduced τ in C/SiC-SiBC (e PyC ≈400 nm). Decreased axial TRS and τ are beneficial to the effective loading of fiber bundles and pull out of fibers. Therefore, C/SiC-SiBC (e PyC ≈400 nm) performed excellent tensile strength (250 ± 11 MPa) and fracture toughness (23.7 ± 0.5 MPa·m1/2). [ABSTRACT FROM AUTHOR]

Combination of lightweight and superior electrical conductive performance is charming for the application of macroscopic graphene foam composite. A porous carbon nanotubes/reduced graphene oxide (CNTs/RGO) foam composite are prepared by freeze-drying and in-situ catalytic grown methods. The CNTs/RGO foam composite consists of interconnected RGO nanosheets as the 3D frame and in-situ growth CNTs as the electromagnetic wave (EM) absorbing reinforcement, which grow on graphene substrate. The in-situ grown CNTs on graphene nanosheets lead to the enhancement of conductive and polarization loss, which results in the enhancement of absorption shielding performance. The CNTs/RGO foam composites with different CNT loading are prepared to investigate their EM shielding properties in X-band. The EM shielding effectiveness (SE) of CNTs/RGO foam composite reaches 31.2 dB with 2 mm thickness, especially the specific EMI shielding effectiveness reaches 547 dB cm3/g with an ultralight density of 57 mg cm−3, the absorption is the primary shielding mechanism. Furthermore, SE value reaches 49 dB with thickness of 3.1 mm. Owing to the unique 3D foam hierarchical architecture, light density and outstanding SE performance, our work shows a promising designable approach of preparing CNTs/RGO foam composite to lightweight and absorption type EMI shielding materials. Image 1063 [ABSTRACT FROM AUTHOR]

Abstract 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·m1/2). To solve this problem, two kinds of wave-absorbing SiC fibers reinforced Si 3 N 4 matrix composites (SiC f /Si 3 N 4) 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 /Si 3 N 4 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▪m1/2). [ABSTRACT FROM AUTHOR]

Abstract Ceramic matrix composites are potential electromagnetic (EM) wave absorbing materials for high temperature applications, and the critical problem is to obtain temperature insensitive absorption characteristics in broadband. In this study, the double layer SiC fiber reinforced composite with ZMI-SiC f /Si 3 N 4 as lossy layer and SLF-SiC f /Si 3 N 4 as impedance matching layer, has been constructed via the chemical vapor infiltration method. The designed SiC f /Si 3 N 4 composite with the thickness of 4.2 mm, which processes uniformly continuous microstructure and hierarchical permittivity, shows greatly enhanced EM wave absorbing performance of measured reflection loss less than −8 dB in whole X and Ku band at the room temperature. The thickness dependent absorbing properties of the SiC f /Si 3 N 4 composite are investigated, which are found to be highly correlated to the combining mode of the lossy and matching layers. Particularly, the temperature dependent absorption characteristics indicate that the fabricated SiC f /Si 3 N 4 composite keeps relatively reliable high temperature absorbing performances up to 600 °C. The temperature insensitive absorption characteristics should be mostly attributed to the enhanced compensating effect of the decreased interfacial polarization loss with increasing temperature induced by multiple interfaces, as well as the unchanged geometric structure of the designed composite. [ABSTRACT FROM AUTHOR]

Abstract The development of high-performance carbonaceous microwave absorbers to achieve full X band (8.2–12.4 GHz) efficient microwave absorption with lower filler loading ratio is highly desirable but remains challenging. Here, a kind of hollow graphene nano-spheres uniformly confined in porous amorphous carbon particles (HGS@PAC) are fabricated via a facile pyrolysis of bi-metal organic framework (bi-MOF). The successful construction of hollow graphene nano-spheres (HGS) by the in situ catalysis of Co in the framework of bi-MOF significantly improve the interface polarization and conductive loss of HGS@PAC, which ensures the high enough absorption intensity with lower filler loading ratio when used as microwave absorbers. The optimal HGS@PAC demonstrate excellent microwave absorption performance, achieving an effective absorption bandwidth of 4.2 GHz which covers the whole X band with 10 wt.% loading content and the minimum reflection coefficient reaches −32.43 dB at 9.19 GHz. In addition, compared with other MOF derived absorbers (such as Co/C, C/TiO 2 and CNTs/Co), the HGS@PAC exhibit broader effective absorption bandwidth and stronger absorption intensity. Therefore, this work provides a promising method for the design and synthesis of HGS uniformly confined in PAC with high-performance microwave absorption. Graphical abstract Hollow graphene nano-spheres with dozen nanometer in size confined in porous amorphous carbon matrix, which possess enhanced interface polarization and conductive loss from highly graphitization HGS and well impedance match from the amorphous state of bulk porous carbon, exhibit excellent microwave absorption performance with an RC min lower to −32.43 dB and effective absorption bandwidth covers the whole X band with a filler loading of only 10 wt.%. Image 1 [ABSTRACT FROM AUTHOR]

Abstract For enhancing the absorption ability of dielectric and electromagnetic wave (EMW), C-rich SiC NWs /Sc 2 Si 2 O 7 ceramics are successfully fabricated through in-situ growth of SiC nanowires (NWs) into porous Sc 2 Si 2 O 7 ceramics by precursor infiltration and pyrolysis (PIP) at 1400 °C in Ar. SiC NWs are in-situ formed in the pore channels via a vapor-liquid-solid (VLS) mechanism, the relative complex permittivity increases notably with the content of absorber (C-rich SiC NWs), which tune the microstructure and dielectric property of C-rich SiC NWs/Sc 2 Si 2 O 7 ceramics. Meanwhile, the minimum reflection coefficient (RC) of C-rich SiC NWs/Sc 2 Si 2 O 7 ceramic decreases from −9.5 dB to − 35.5 dB at 11 GHz with a thickness of 2.75 mm, and the effective absorption bandwidth (EAB) covers the whole X band (8.2–12.4 GHz) when the content of absorber is 24.5 wt%. The results indicate that Sc 2 Si 2 O 7 ceramics decorated with SiC NWs and nanosized carbon have a superior microwave-absorbing ability, which can be contributed to the Debye relaxation, interfacial polarization and conductivity loss enhanced by in-situ formed SiC NWs and nanosized carbon phases. The C-rich SiC NWs /Sc 2 Si 2 O 7 ceramics can be a promising microwave absorbing materials within a broad bandwidth. [ABSTRACT FROM AUTHOR]