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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]

Since the last century, a variety of reactive synthesis methods has been extensively studied and developed to derive ceramic composite materials, which show superior mechanical and physical properties for numerous applications. Melt casting/infiltration is one of the preferred techniques to fabricate ceramic‐metal composites. In this article, different reactive processing routes and the properties of the synthesized ceramic‐metal composites are reviewed. The reaction forming techniques regarding raw material properties and processing parameters are discussed. [ABSTRACT FROM AUTHOR]

Random composites with negative permittivity or negative permeability have developed rapidly while few were accomplished using ceramic materials as the functional fillers. In this work, titanium nitride/polyphenylene sulfide (TiN/PPS) composites with different TiN content were prepared by hot-pressing method. The electrical percolation threshold phenomenon occurred and the conductive mechanism transferred from hopping conduction to metal-like conduction when TiN’s content increased. Negative permittivity in composite with high TiN content was derived from low frequency plasmonic state of free electrons and the plasm-like negative permittivity spectra was analyzed by Drude model. Besides, it’s indicated that the dielectric loss was dominated by conduction loss and polarization loss in composites with high TiN content. Equivalent circuit model was utilized to investigate the impedance properties and suggested that parallel inductances played critical role in achieving negative permittivity. [ABSTRACT FROM AUTHOR]

Abstract: Electromagnetic (EM) absorbing and shielding materials have attracted great interests due to the increasing electromagnetic pollutions in the past years. Microstructure plays a crucial role in determining the performance of the above materials. Herein, a scale‐like structure based on Ti3C2 Mxenes is proposed to approach improved EM absorption properties. For the first time, graphite/TiC/Ti3AlC2 (G/TiC/Ti3AlC2) hybrids are fabricated in a molten salts bath and graphite/TiC/Ti3C2 (G/TiC/Ti3C2) hybrids are obtained after Al atoms are etched from G/TiC/Ti3AlC2. In G/TiC/Ti3C2, Ti3C2 sheets are perpendicular to the plane of G/TiC, which like a bionic structure of fish scale. The scale‐like G/TiC/Ti3C2 hybrids are dispersed in paraffin matrix to evaluate the EM properties. Owing to the structure‐induced EM absorption mechanism, G/TiC/Ti3C2 show much enhanced EM absorption ability than those materials without structure design, e.g., G/TiC/Ti3AlC2, pure Ti3C2, G/TiC, and the simple mixture of G/TiC with Ti3C2 (G/TiC+Ti3C2). The minimum reflection coefficient (RC) of G/TiC/Ti3C2 with the sample thickness of 2.1 mm reaches −63 dB and the effective absorption bandwidth (the frequency where RC is lower than −10 dB) is more than 3.5 GHz. The results indicate that the scale‐like structure can greatly improve the EM absorption ability. [ABSTRACT FROM AUTHOR]

The mechanical behavior and oxidation resistance of SiC/SiC-SiBC composites were studied in this work. According to the debonding criterion of He and Hutchinson, the debonding could occur at the BN interphase, which insures that the fibers can well play the strengthening and toughening performance. The oxidation resistance of SiC/SiC-SiBC composites consisting of SiC fibers with thermal expansion coefficients (CTE) of 5.1 × 10 −6 K −1 and 4.0 × 10 −6 K −1 was compared. The composites consisting of SiC fibers with higher CTE show slight weight changes at 800, 1000, and 1200 °C, and the corresponding strength retention ratios are 109.6%, 103.2% and 102.9%, exhibiting excellent oxidation resistance. The CTE of composites consisting of SiC fibers with higher CTE matches well with the CTE of SiC coating, so rarely no cracks can be formed in the coating, which inhibits the inward diffusion of oxidizing medium and leads to high strength retention ratios after oxidation tests. [ABSTRACT FROM AUTHOR]

Novel single-source-precursors (SSPs), namely carbon nanotube modified poly (methylvinyl) silazane (CNTs-HTT 1800), were synthesized via amidation reaction of poly (methylvinyl) silazane (HTT 1800) with carboxylic acid functionalized carbon nanotubes (CNTs-COOH) at the assistance of ZnCl2 catalyst, which was confirmed by means of Fourier transform infrared spectra ( FT IR) and transmission electron microscopy ( TEM). Besides, the TEM results unambiguously show the homogeneous distribution of the CNTs in the matrix of SSPs while serious aggregation of the CNTs in the matrix of physically-blended-precursor. Crack-free monolithic silicon carbonitride modified by carbon nanotubes ceramic nanocomposites ( CNTs-Si CN) were prepared through pyrolysis of the obtained SSP green bodies at 1000°C. Due to the strong influence of polymer structure on the microstructure of final ceramics, the SSP-derived CNTs-Si CN nanocomposites clearly show the homogeneous distribution of the CNTs in the Si CN matrix while the physically-blended-precursor derived CNTs-Si CN nanocomposites exhibit serious aggregation and entangling of the CNTs in the Si CN matrix. With the same CNT content in the feed, the SSP-derived CNTs-Si CN nanocomposites possess significant improvements of electromagnetic ( EM) absorbing properties compared to those from physically-blended-precursors, due to the quality of the dispersion of CNTs in the ceramic matrices. [ABSTRACT FROM AUTHOR]

Dense SiC/SiC-based composites were fabricated by introducing Ti 3 Si(Al)C 2 -based ceramics into Chemical Vapor Infiltration (CVI) fabricated porous SiC/SiC composites. For the purpose of reducing the damage to SiC fiber, the low-temperature melt infiltration of Al–Si alloy was utilized. The microstructure was observed, showing that the inter-bundle pores were filled by reaction-formed new phases and the porosity of SiC/SiC-based composites decreased to less than 5%. Owing to the densification of the matrix, higher thermal diffusivity was obtained compared with CVI fabricated SiC/SiC composites. The mechanical properties were also analyzed. The compressive strength and in-plane shear strength were 10.9% and 11.3% higher than those of CVI fabricated SiC/SiC composites, respectively. [ABSTRACT FROM AUTHOR]

Two different matrices (silicon and Ti 3 SiC 2 ) were introduced into two-dimensional woven C/SiC reinforced with T-300™ carbon fiber (40 vol.%) by liquid silicon infiltration, and their mechanical and thermophysical properties were studied compared with those of C/SiC fabricated by chemical vapor infiltration. C/SiC–Si shows higher tensile strength than C/SiC and C/SiC–Ti 3 SiC 2 due to the existence of low thermal residual stress (TRS), while the low interlaminar shear strength and three-point bending strength of C/SiC–Si can be attributed to the brittleness of dense Si–SiC matrix. The introduction of Ti 3 SiC 2 in a dense matrix can cause toughening mechanisms and inhibit the crack propagation by various micro-deformations of Ti 3 SiC 2 grains. Therefore, C/SiC–Ti 3 SiC 2 shows higher Young’s modulus, interlaminar shear strength and thermal conductivity than C/SiC and C/SiC–Si. The low tensile strength is due to the existence of high TRS in C/SiC–Ti 3 SiC 2 . [ABSTRACT FROM AUTHOR]

Abstract: In order to enhance the dielectric and electromagnetic interference shielding (EMI) properties, the SiC/C composite ceramics were fabricated by pyrolysis of ferrocene-modified polycarbosilane. The microstructure evolutions, dielectric properties, EMI and microwave absorption properties of SiC/C composite ceramics were investigated. The increases of both ferrocene contents and annealing temperatures led to the increases of crystallizations of SiC and carbons. Crystallized carbons including carbon nanowires, turbostratic carbons, onion-like carbons and graphene-like carbons were obtained in the materials. The carbon nanowires were longest when the 5wt.% ferrocene-modified polycarbosilane was annealed at 1250°C. These carbons played a more important role than SiC in the increases of dielectric and EMI properties. The average real and imaginary permittivities of materials increased from 4.4 and 0.7 to 38.9 and 39.6, respectively. The materials exhibited high total shielding effectiveness, high absorption shielding effectiveness and low reflection shielding effectiveness, which were 36.6, 30.1 and 6.5dB, respectively. [Copyright &y& Elsevier]

Abstract: Silicon carbide (SiC) or boron carbide (B4C) ceramic filler was introduced into a three-dimensional needled carbon fiber integrated felt by unidirectional pressure slurry infiltration–filtration to prepare ceramic modified carbon/carbon (C/C) composites. The contents of SiC and B4C introduced by a one-shot infiltration were about 18wt% and 16wt% respectively. The fade of coefficient of friction (COF) in seawater conditions for C/C composites was significantly reduced by ceramic modification. SiC modified C/C composite was superior to C/SiC for its excellent stable friction without any fade in seawater conditions. The weak hygroscopicity and the ability to produce fresh debris continuously in seawater conditions were the primary factors for ceramic modified C/C composites to reduce and even avoid the fade. The double lubrication action from water film and silicon oxide film increased the COF fade for C/SiC. [Copyright &y& Elsevier]

Abstract: Porous Si3N4–SiC composite ceramic was fabricated by infiltrating SiC coating with nano-scale crystals into porous β-Si3N4 ceramic via chemical vapor infiltration (CVI). Silica (SiO2) film was formed on the surface of rod-like Si3N4–SiC grains during oxidation at 1100°C in air. The as-received Si3N4–SiC/SiO2 composite ceramic attains a multi-shell microstructure, and exhibits reduced impedance mismatch, leading to excellent electromagnetic (EM) absorbing properties. The Si3N4–SiC/SiO2 fabricated by oxidation of Si3N4–SiC for 10h in air can achieve a reflection loss of −30dB (>99.9% absorption) at 8.7GHz when the sample thickness is 3.8mm. When the sample thickness is 3.5mm, reflection loss of Si3N4–SiC/SiO2 is lower than −10dB (>90% absorption) in the frequency range 8.3–12.4GHz, the effective absorption bandwidth is 4.1GHz. [Copyright &y& Elsevier]

A T300 carbon fiber and a SiC-coated C/SiC composite made from the same fiber were studied in atomic oxygen environment. The carbon fiber shows significant degradation while the erosion rate of SiC-coating of C/SiC is about 50 times lower. Evidence shows that Si is preferentially etched from the SiC surface. And XPS information showed that amorphous carbon and diamond-like carbonare periodically generated on the tested composite surface. Statistical analysis shows that the C/SiC specimens have no significant change in flexural properties after 1-year fluence AO treatment. [ABSTRACT FROM AUTHOR]

Abstract: In this paper carbon fiber reinforced carbon–boron nitride binary matrix composites (C/C–BN) were prepared by chemical vapor infiltration (CVI). The infiltration of BN in the CVI process was controlled by the diffusion of BCl3, and BN matrix was distributed homogeneously in the porous carbon fiber reinforced carbon matrix composites (C/C) due to the good infiltration ability of BN. The as-received C/C–BN composites were composed of 92vol% C and 8vol% BN. Both the friction coefficient and wear rate of C/C composites decreased significantly by the incorporation of BN. After heat-treated at 1600°C, the interlayer spacing of CVI BN decreased to 3.36Å, and CVI BN with high crystalline degree displayed the excellent lubricating effect, leading to the decrease of friction coefficient and wear rate. The improvement of the tribological properties also was partially attributed to the improved oxidation resistance and the formation of friction film by the incorporation of BN matrix. [Copyright &y& Elsevier]

Abstract: SiC was infiltrated into porous yttria-stabilized zirconia (YSZ) felt by chemical vapor infiltration (CVI), and continuous SiC matrix layer was formed around YSZ fibre. When 86.9wt.% SiC is introduced into the porous YSZ felt, the mean values of the real part of the permittivity and dielectric loss tangent of porous YSZ felt increase from 1.16 and 0.007 to 8.2 and 1.31, respectively. The electromagnetic interference (EMI) shielding efficiency (SE) increases from 0.069dB to 16.2dB over the frequencies ranging from 8.2GHz to 12.4GHz. The reflection loss of the composites with a thickness of 5mm at 8–18GHz is smaller than −6.5dB, and the bandwidth below −10dB is 5GHz at room temperature, which increases to 5.9GHz at 800°C. The considerable increases in EMI SE and microwave absorption properties are attributed to the formation of continuous SiC matrix layer composed of SiC nanocrystals in the porous YSZ felt, which is beneficial for the production of induced electric current and the enhancement of dielectric loss. [Copyright &y& Elsevier]

Three-dimensional needled carbon/carbon (C/C) preforms were achieved after one cycle of polymer infiltration and pyrolysis (PIP). Phenolic resin or the mixture of phenolic resin and 30 wt% graphite was used as the carbon precursor. Carbon/silicon carbide (C/SiC) composites were prepared by liquid silicon infiltrating into the C/C preforms with or without high-temperature treatment (HTT). The graphitization degree of C/C preforms was improved by HTT or graphite addition, the improvement being more outstanding by the latter, which influenced phase compositions as well as distribution and facilitated the formation of friction films for the resulting C/SiC composites. The friction studies show that higher the graphitization degree, lower the wear rate and higher the coefficient of friction as well as more stable the friction curve, with a lower sensitivity of friction and wear to the velocity-pressure conditions being due to the differences in the topography and stability of friction films. [ABSTRACT FROM AUTHOR]

Silicon carbide (SiC) matrix composite reinforced by both SiC and carbon fibers ([SiC-C]/pyrolytic carbon [PyC]/SiC) was fabricated by chemical vapor infiltration for reducing matrix microcracks. Microstructure, mechanical properties, and oxidation resistance of the composite were compared with those of C/PyC/SiC and C/PyC/SiC composites in which PyC interphase was untreated and heat-treated at 1800°C in argon, respectively. Compared with the C/PyC/SiC composite, (SiC-C)/PyC/SiC composite shows considerable improvements in flexural strength, fracture toughness, and oxidation resistance. The different mechanical behaviors of the three composites were analyzed based on the He and Hutchinson's model. [ABSTRACT FROM AUTHOR]

Abstract: In order to reduce residual silicon in carbon fibre reinforced carbon–silicon carbide binary matrix composite fabricated by liquid silicon infiltration, the aqueous slurry of titanium carbide (TiC) powder was used for introducing TiC into porous carbon/carbon composite (C/C) by slurry infiltration. With increasing the infiltration times, the amount of TiC in C/C increased. During liquid silicon infiltration, silicon reacted with TiC and part of carbon matrix, leading to the formation of a carbon fibre reinforced carbon–silicon carbide–titanium silicon carbide hybrid matrix composite (C/C–SiC–Ti3SiC2). When liquid silicon was infiltrated into the C/C filled with 4vol% TiC particles, the composite attained 21vol% SiC and 4vol% Ti3SiC2. The flexural strength of the composite was improved from 125 to 200MPa and fracture toughness from 8 to 9MPam1/2. During flexural testing, Ti3SiC2 grains exhibited characteristics of delamination, deformation, kinking, layered fracture and intergranular cracking, beneficial to improving the mechanical properties of the composites. [Copyright &y& Elsevier]

Abstract: In order to prepare a structural/functional material with not only higher mechanical properties but also lower dielectric constant and dielectric loss, a novel process combining oxidation-bonding with sol–gel infiltration-sintering was developed to fabricate a porous Si3N4–SiO2 composite ceramic. By choosing 1250°C as the oxidation-bonding temperature, the crystallization of oxidation-derived silica was prevented. Sol–gel infiltration and sintering process resulted in an increase of density and the formation of well-distributed micro-pores with both uniform pore size and smooth pore wall, which made the porous Si3N4–SiO2 composite ceramic show both good mechanical and dielectric properties. The ceramic with a porosity of 23.9% attained a flexural strength of 120MPa, a Vickers hardness of 4.1GPa, a fracture toughness of 1.4MPam1/2, and a dielectric constant of 3.80 with a dielectric loss of 3.11×10−3 at a resonant frequency of 14GHz. [Copyright &y& Elsevier]

A broadband microwave absorbing composite with a multi-scale layered structure is proposed, in which a reduced graphene oxide (RGO) film sandwiched between two layers of epoxy glass fiber laminates serves as the frequency selective surface (FSS). RGO films with the desired electrical properties were synthesized directly by hydrothermal reaction, vacuum filtration, and heat treatment without subsequent processing. With the novel layer-by-layer structure ranging from micro to macro scale, the optimized composite exhibits excellent microwave absorption performance with a total thickness of 3.2 mm. Its reflection coefficient (RC) is less than −10 dB in the entire X and Ku band, reaching a minimum value of −32 dB at 10.2 GHz and an average RC of −22.8 dB from 8 to 18 GHz. Enhanced microwave absorption of the composites is achieved through the optimization of layer thickness in the sandwich structure to promote destructive interference. Improved impedance matching by the introduction of FSS along with the polarization and conduction loss of layered graphene films also contribute to the increased absorption. [ABSTRACT FROM AUTHOR]

In this work, the self-healing Si-B-C matrix-modified ZMI-SiC fiber/SiC (SiC/SiC-SiBC) composite was prepared by a combined process of chemical vapor infiltration, slurry infiltration, and liquid silicon infiltration. The weight changes of the as-fabricate composite after oxidation at 800, 1000, and 1200 °C for 100 h were −0.41%, −0.06%, +0.05%, respectively. The corresponding strength retention ratios were 75%, 99%, 100%, respectively. With the increase of oxidation temperature, coating cracks would be closed. A dense oxidation film was formed with the oxidation of the SiBC matrix, preventing the oxidation of interphase and fiber and leading to excellent oxidation resistance at 1000 and 1200 °C. [ABSTRACT FROM AUTHOR]

Chemical vapor infiltration has been introduced for preparing z-pinned joint, which is made of two-dimensional carbon fiber reinforced silicon carbide composite. The effects of oxidation on the shear properties of the joint were investigated. The results showed that the joint strength increases with the increase of oxidation temperature, which is consistent with the oxidation consumption of the carbon phases. An exponential relationship is presented between the weight loss and the joint strength. In contrast, linear relationships are presented between the weight loss and the mechanical properties of the composite. The exponential relationship results from the coupled shear and bending stress states of the pin, according to the failure mechanisms of the joint. It is observed that in-plane and intra-layer cracks are formed under the shear stress. And these cracks are bridged by the fibers under the bending stress. Accordingly, the fiber bridging mechanism contributes to the joint strength before and after oxidation. For the conditions of this study, the joint strength can be roughly estimated as the plus of the in-plane shear strength and the tensile matrix cracking stress. [ABSTRACT FROM AUTHOR]

A dense alumina fiber reinforced silicon carbide matrix composites (Al 2 O 3 /SiC) modified with Ti 3 Si(Al)C 2 were prepared by a joint process of chemical vapor infiltration, slurry infiltration and reactive melt infiltration. The conductive Ti 3 Si(Al)C 2 phase introduced into the matrix modified the microstructure of Al 2 O 3 /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 Al 2 O 3 /SiC and Ti 3 Si(Al)C 2 modified Al 2 O 3 /SiC were investigated over the frequency range of 8.2–12.4 GHz. The EMI SE of Al 2 O 3 /SiC-Ti 3 Si(Al)C 2 exhibited a significant increase from 27.6 to 42.1 dB compared with that of Al 2 O 3 /SiC. The reflection and absorption shielding effectiveness increased simultaneously with the increase of the electrical conductivity. [ABSTRACT FROM AUTHOR]

Abstract: SiC nanoparticles with different contents (5–20wt%) 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 8.2–12.4GHz 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 15wt% of SiC was measured at temperatures of 293–773K. 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. [Copyright &y& Elsevier]

Abstract A three-dimensional carbon/silicon carbide (C/SiC) composite was achieved by infiltrating liquid Si into a carbon/carbon composite which was produced by one cycle of infiltration of resin slurry containing graphite particles, hot-pressing curing and pyrolysis. The specific heats of the graphite-modified C/SiC, which increased from 0.92 to 2.33 J K−1 g−1 as the temperature increased from 30 to 1300 °C, were higher than those of the unmodified C/SiC, which increased from 0.74 to 1.89 J K−1 g−1. The through-thickness thermal conductivities of the graphite-modified C/SiC (40.03–55.53 W m−1 K−1) were higher than those of the unmodified C/SiC (20.93–35.46 W m−1 K−1) in the range of 30–1300 °C. The coefficient of thermal expansion of the graphite-modified C/SiC was lower than that of the unmodified C/SiC at each temperature, with average values of 1.69 × 10−6 and 2.56 × 10−6 K−1 for the graphite-modified and unmodified C/SiC composites, respectively, along the carbon cloth layer in the range of 25–1300 °C. Highlights • This is a simple, fast and low-cost manufacturing solution for 3D C/SiC composites. • Adding graphite improved the specific heats of the C/SiC composite. • Adding graphite greatly improved the thermal conductivity of the C/SiC composite. • Adding graphite reduced the thermal expansion coefficient of the C/SiC composite. [ABSTRACT FROM AUTHOR]

Abstract: The effects of stitch spacing on the mechanical properties of the carbon fiber reinforced silicon carbide composites fabricated by chemical vapor infiltration (CVI) were investigated. The results showed that the stitch spacing had little effect on the density and open porosity of the composites. The stitches improved the shear and tensile strengths of all stitched C/SiC composites, by suppressing and delaying the delamination. However, flexural strength was not changed for the composites with stitch spacing of 10mm or 5mm, but was slightly reduced for densely stitched composites with spacing of 2.5mm. The optimum stitch spacing was 5mm, which improved apparently the interlaminar shear strength and tensile strength of the composites with little detrimental effect on the flexural strength. [Copyright &y& Elsevier]