SiC/Si3N4 composite nanofibers with in situ embedded graphite, which show highly efficient electromagnetic (EM) wave absorption performance in gigahertz frequency, were prepared by electrospinning with subsequent polymer pyrolysis and annealing. By means of incorporating graphite and Si3N4 into SiC, the EM wave absorption properties of the nanofibers were improved. The relationship among processing, fiber microstructure, and their superior EM wave absorption performance was systematically investigated. The EM wave absorption capability and effective absorption bandwidth (EAB) of nanofibers can be simply controlled by adjusting annealing atmosphere and temperature. The nanofibers after annealing at 1300 °C in Ar present a minimum reflection loss (RL) of −57.8 dB at 14.6 with 5.5 GHz EAB. The nanofibers annealed in N2 at 1300 °C exhibit a minimum RL value of −32.3 dB at a thickness of 2.5 mm, and the EAB reaches 6.4 GHz over the range of 11.3–17.7 GHz. The highly efficient EM wave absorption performance of nanofibers are closely related to dielectric loss, which originated from interfacial polarization and dipole polarization. The excellent absorbing performance together with wider EAB endows the composite nanofibers potential to be used as reinforcements in polymers and ceramics (SiC, Si3N4, SiO2, Al2O3, etc.) to improve their EM wave absorption performance.Keywords: broad-band; dielectric polarization; electromagnetic wave absorption; electrospinning; graphite/SiC/Si3N4 composite nanofibers;

In this work, 1D carbon/SiC nanocomposites were fabricated by the electrospinning of polycarbosilane (PCS) and polyvinylpyrrolidone (PVP) with subsequent polymer pyrolysis and annealing treatment. The dielectric properties and electromagnetic (EM) wave absorption performance of the nanocomposites were studied over the frequency range of 2–18 GHz. By controlling the weight ratio of precursor PVP, both the dielectric loss ability and microwave absorption performance of the nanocomposites were improved. The nanocomposites obtained from 4.3 wt% PVP exhibited excellent EM performance, their optimal reflection loss (RL, in dB unit) value reached as high as −57.8 dB at a coating thickness of 1.9 mm. In addition, the nanocomposites displayed an ultrawide effective absorption bandwidth (EAB, RL below −10 dB, 90% EM attenuation) of 7.3 GHz at an absorber coating thickness of 2.7 mm. It was found that the carbon defects, SiC nanocrystals, and heterojunction interfaces among carbon, SiC, and amorphous SiOxCy contribute to the nanocomposite's outstanding EM performance. The low RL values and wide absorption bandwidth of the carbon/SiC nanocomposites make it a good candidate as highly efficient radar wave absorbers under harsh conditions.Download high-res image (397KB)Download full-size image

Chemical vapor infiltration (CVI) is a prominent process for fabricating carbon fiber/silicon carbide (C/SiC) composites. However, the preparation of enclosed-structure or thick-section C/SiC composites/components with CVI remains a challenge, since the difficulty of densification increases. Here, machining-aided CVI (MACVI) is designed, in which infiltration-assisting holes are utilized (machined) to increase matrix deposition. To validate the approach, thick-section (10 mm thick) C/SiC composites were fabricated by MACVI. Porosity analysis and microstructure characterization were performed on the fabricated MACVI C/SiC composites and their CVI counterparts, showing a density increase up to 12.7% and a porosity decrease up to 32.1%. The mechanical behavior of the fabricated MACVI C/SiC composites was characterized, showing an increase of flexural strength by a factor of 1.72 at most. Besides, the toughness also largely increases. Both the porosity decrease and the strength and toughness increase brought by MACVI demonstrate its effectiveness for fabricating stronger and tougher enclosed-structure or thick-section ceramic matrix composites/components.

•The SiC nanofibers were fabricated by electrospinning with subsequent annealing.•The SiC nanofibers show superior electromagnetic wave absorption performance.•The dipoles polarization are main loss mechanism for SiC nanofibers.A method combined electrospinning and preceramic polymer pyrolysis, annealing has been developed to fabricate SiC nanofibers. In this work, the effects of polycarbosilane (PCS) mass ratio (8.7 wt% ∼ 11.8 wt%) in precursor solution on microstructure and electromagnetic (EM) absorption properties of SiC nanofibers are systematically studied. The phase composition of SiC nanofibers are mainly inclusive of abundant SiC nanocrystallines, a small quantity of graphitic-like carbon coated on fibers surface and randomly distributed amorphous SiOxCy phase. When PCS mass ratio is 10 wt%, the SiC nanofibers present a minimum reflection loss (RL) value of −57.8 dB at 14.6 GHz with a absorber coating thickness of 1.9 mm and the effective absorption bandwidth (RL < −10 dB, 90% EM wave absorbed) covers from 6 to 18 GHz. As PCS mass ratio increasing, the EM absorption performance of SiC nanofibers gradually becomes poor. Multi-reflection among SiC nanofibers gradually weaken the incident EM wave. Moreover the interfacial polarization originated from heterogeneous interfaces which exist among SiC nanocrystallines, graphite carbon, amorphous SiOxCy dissipates EM wave energy contributed to high EM performance of SiC nanofibers. This work demonstrates a new kind of nano SiC EM absorption materials that has potential to be applicated in harsh environments.

•Tape casting and reactive hot pressing were used to prepare laminated ceramics.•Soft and hard interbedded dense layers with an in-situ weak interface were obtained.•The dynamic pseudoplastic behavior was observed for the first time.•Research the loading pattern by the derivatives of the true stress–strain curves.Laminated ZrO–Zr2CN/Si3N4 ceramics with soft and hard interbedded dense layers were prepared via tape casting and reactive hot pressing. The dynamic compressive behavior was investigated at various high strain rates ranging from 1000 to 5000 s−1 using a split Hopkinson bar, and the dynamic loading pattern was analyzed. With increasing the strain rate, the dynamic strength obtained a peak value, while the failure strain increased continuous. The compressive stress–strain curves exhibited universal four deformation regions, including inelastic deformation region, rapid loading region, pseudoplastic deformation region and failure region. The dynamic pseudoplastic behavior of the laminated ceramics was similar to that of some metal matrix composites.The dynamic pseudoplastic behavior and dynamic loading pattern of laminated ZrO–Zr2CN/Si3N4 ceramics at various strain rates.

To understand the quasi-static and dynamic compressive mechanical behavior of two-dimensional SiC fiber-reinforced SiC composites (2D-SiCf/SiC), their compressive behavior at room temperature was investigated at a strain rate from 10−4 to 104 /s, and the fracture surfaces and damage morphology were observed. The results show that the dynamic failure strength of 2D-SiCf/SiC obeys the Weibull distribution, and the Weibull modulus is 5.66. Meanwhile, 2D-SiCf/SiC presents a transition from brittle to tough with a decrease of strain rate, and 2D-SiCf/SiC has a more significant strain rate sensitivity compared to the 2D-C/SiC composites. The failure mode of 2D-SiCf/SiC depends upon the strain rate.

The effects of SiC coating and heat treatment on the emissivity were investigated for 2D C/SiC composites prepared by CVI in the 6–16 μm range. SiC coating had an obvious effect on the spectral emissivity of the composites but caused just 5% difference in the total emissivity. A radiation transport model was applied to explain those changes caused by SiC coating. Heat treatment affected the thermal radiation properties of the composites through the microstructure evolution. Base on the complementary analytical techniques, the changes in the emissivity were attributed to a good graphitization degree of carbon phases, large β-SiC grain sizes and high α-SiC content resulting in high emissivity.

SiC and SiCw/SiC coatings were prepared on two-dimensional carbon fiber reinforced silicon carbide ceramic matrix composites (2D C/SiC), and strengthening/toughening of the composite by the coatings was investigated. After coating, the density of the C/SiC composites was increased effectively and the mechanical properties were improved significantly. Compared with SiC coating, SiCw/SiC coating showed the more significant effect on strength/toughness of the composites. Coatings had two effects: surface strengthening and matrix strengthening. The latter was the dominant effect. The surface strengthening can increase the crack initiation stress, while the matrix strengthening can enhance the crack propagation resistance. The former effect increased the strength and the latter effect increased the toughness.

Atomic oxygen (AO) is considered the most erosive particle to spacecraft materials in low earth orbit (LEO). Carbon fiber, carbon/carbon (C/C), and some modified C/C composites were exposed to a simulated AO environment to investigate their behaviors in LEO. Scanning electron microscopy (SEM), AO erosion rate calculation, and mechanical property testing were used to characterize the material properties. Results show that the carbon fiber and C/C specimens undergo significant degradation under the AO bombing. According to the effects of AO on C/C-SiC and CVD-SiC-coated C/C, a condensed CVD-SiC coat is a feasible approach to protect C/C composites from AO degradation.

The advanced high temperature composites have been considered as structural or functional material in complex and harsh environments, which usually comprise of various oxidizing particles such as molecular oxygen (MO), atomic oxygen (AO) and molecular water. This study aimed to investigate C/SiC behaviors in single factor or combination environments. In the MO oxidation, an inert surface is formed on SiC. At the AO oxidation, the Si of SiC is preferentially etched off leaving a certain depth of C layer on the sample. Two types of coupled oxidations defined as mode A (first MO and then AO oxidation), and mode B oxidation (first AO and then MO oxidation) were investigated systematically in this study, and found that the prior oxidative effect majorly determined the corresponding coupled oxidation behavior. From microscopic observation and fractural test, the mode B oxidation induced a more serious degradation upon the material than that of the mode A, which is illustrated by the proposed erosion mechanisms for the modes A and B.Highlights► A C/SiC was exposed in O2, AO, mode A (first O2 and then AO) and mode B (first AO and then O2) oxidation. ► The O2 produced SiO2 layer on the specimens, but the AO can preferentially etch off the Si of SiC. ► The mode B oxidation can induce more serious degradation for specimen. ►The mechanisms of the coupled oxidations were proposed in this study.

A two-dimensional axisymmetrical mathematical model taking into account the transport phenomena of momentum, energy and mass in conjunction with the infiltration induced changes of the preform structure were implemented to simulate the effects of the reactor dimension on the isothermal CVI process of C/SiC composites. The effects of the reactor inlet dimension and the reactor diameter on the concentration distribution and the time-dependent densification behaviors of C/SiC composites were studied. The reactor inlet dimension was found to impose trivial effects on the ICVI process of C/SiC composites. Calculation results show that the mean MTS molarity in preform and the global density increase firstly and then decrease with the elevating reactor diameter at any given infiltration time though the MTS molarity gradients drops gradually with the elevating reactor diameter. The preferable reactor diameter is implied to be 90 mm for the preform taken. Calculation results of the densification behavior of C/SiC composites during the ICVI process imply that the reactor diameter does not evidently influence both the rules of densification and the absolute values of the density.