Enhanced microwave-absorption properties of polymer-derived SiC/SiOC composite ceramics modified by carbon nanowires

Novel microwave-absorbing SiOC composite ceramics with dual nanowires (carbon nanowires (CNWs) and SiC nanowires) with high performances were fabricated by using the polymer-derivation method and heat treatment in Ar atmosphere. The introduction of CNWs in the amorphous SiOC ceramics promotes the ceramic crystallization into SiC nanoparticles and SiC nanowires at lower annealing temperatures, which leads to multi-phases and multiple nano heterogeneous interfaces. The distinctive architectures largely increase the interfacial and dipole polarizations of the composite ceramics. The CNWs/SiC/SiOC composite ceramics exhibit excellent microwave-absorption properties in the Ku band (12.4–18 GHz). The minimum reflection coefficient (RC) is -24.5 dB at a thickness of 1.8 mm, while the maximum effective absorption bandwidth (EAB, the corresponding frequency band in which RC is smaller than -10 dB) is 4.8 GHz at a thickness of 1.9 mm, which make the CNWs/SiC/SiOC composite ceramics promising electromagnetic-wave-absorbing materials.     

Fabrication and properties of lightweight SiOC modified carbon-bonded carbon fiber composites

SiOC modified carbon-bonded carbon fiber composites (CBCFs) with densities of 0.38, 0.61, 0.94 g cm⁻³ were prepared by precursor infiltration and pyrolysis method using dimethoxydimethylsilane and methyltrimethoxysilane as precursors. The densification behavior was investigated by analyzing the microstructure of CBCF-SiOC (CS) composites with different densities. The mechanical properties and oxidation resistance of the CS composites were studied. Results indicate that the CS composites with the density of 0.94 g cm⁻³ exhibit better mechanical and anti-oxidation properties.     

Preparation and mechanical behaviors of SiOC-modified carbon-bonded carbon fiber composite with in-situ growth of three-dimensional SiC nanowires

Carbon-bonded carbon fiber (CBCF) composites are novel and important high-temperature insulation materials owing to their light weight, low thermal conductivity and high fracture tolerance. To further improve the mechanical property of CBCF composite, we propose a three-dimensional (3D) SiC nanowires structure, which is in situ grown on a CBCF matrix via directly annealing silicon oxycarbide (SiOC) ceramic precursor. The synthesized multiscale reinforcements including microscale SiOC ceramics and nanoscale SiC nanowires are mainly attributed to the initial phase separation of SiOC phase and subsequent solid-phase reaction of SiO and C phases. Compared to SiOC/CBCF composite, the resulting 3D SiC nanowires/SiOC/CBCF hybrid structure exhibited high flexural/tensile strength and fractured strain due to the pull-out and bridging behavior of SiC nanowires. This one-step process supplied a feasible way to synthesize 3D SiC nanowires to reinforce and toughen SiOC-modified CBCF composite.     

Multifunction properties of SiOC reinforced with carbon fiber and in-situ SiC nanowires

Herein, the SiC nanowires were successfully fabricated via chemical vapor infiltration (CVI) into carbon fiber felts (CFs) and then the SiOC/SiCnws/CFs composites were synthesized by precursor infiltration and pyrolysis (PIP) processes. Results indicated that the lightweight composites possessed enhanced mechanical performance, low thermal conductivity, and excellent electromagnetic wave absorption properties. Detailedly, the compressive strength reached to 22.0 MPa and 9.6 MPa after two PIP processes cycles in z and x/y directions, respectively. Meanwhile, the composites exhibited tailored electromagnetic wave absorption performance with the effective absorption bandwidth of 3.06 GHz, and the minimum reflection loss (RL_min) was -48.2 dB with a thickness of 3.6 mm. The present work has a guidance to prepare and design multifunction properties for application in harsh environment.     

Subcritical crack growth models for static fatigue of Hi-NicalonTM-S SiC fiber in air and steam

Three subcritical crack growth (SCG) laws were used to model strain-rates and failure times for static fatigue of Hi-Nicalon^TM-S SiC fiber tows in air and Si(OH)₄(g)-saturated steam. Models were fit to tow failure times ( t_f ) and steady-state strain rates ( ἑ ) for brittle creep measured at 700 to 1100°C under initial applied stresses ( σ_A ) of 260 to 1260 MPa. A power law, a reaction-rate law, and a bond-energy law were used to describe SCG that caused sequential filament failure, and ultimately tow failure. Two versions of each model were developed. One allowed access of chemisorbed species to flaws throughout the fiber (mode 1) and another only allowed access to flaws at the SiC-SiO₂ interface (mode 2). The stress increase on intact filaments as others fractured and as filaments oxidized, and the increase in stress intensity geometric factors ( Y ) as crack size increased were incorporated in the models. The fits to data were compared for the different models by using both simple regression analysis and orthogonal distance regression (ODR) analysis. Faster convergence and more consistent results were achieved using ODR analysis. Regression analyses found parameters for all models with similar error in data fits, so validity of a model could not be distinguished by regression analysis alone. For all models, the stress dependence of SCG rates was much stronger in steam than in air, and for most models activation energies were between 300 and 420 kJ/mol, regardless of environment. For the steam environment, the bond-length parameter ( δ ) for the bond-energy model was very close to the lattice parameter of β-SiC (.436 nm), but in air it was significantly lower at 0.25-0.26 nm, but still larger than the Si-C bond length of 0.189 nm. Other factors suggest that either a bond-energy based law or a modified version of a reaction-rate law are the best choices for a SCG law. Filament strength distributions initially described by Weibull distributions could not be described by such distributions after application of the models. SCG mechanisms are discussed.     

SiC fiber strength after low pO2 oxidation

Hi‐Nicalon?‐S SiC fiber was heat treated for 1 hour at 1300°C, 1400°C, and 1500°C in argon with pO₂ of 3.7, 10, 20, 50, 100, and 200 ppm. Fiber strengths were measured by 30 single‐filament tensile tests. Fiber microstructure and surface morphology were characterized by TEM. Active oxidation occurred in all cases except at 1500°C with 200 ppm pO₂, 1400°C with 100 ppm pO₂ or higher, and 1300°C with 50 ppm pO₂ or higher. When active oxidation did not occur, a glass SiO₂ scale formed at 1300°C and 1400°C, and a cristobalite scale formed at 1500°C. The thickness of these scales was much larger than that predicted by linear dependence of oxidation rate on pO₂. Fiber strengths were lowest after heat treatment at 1300°C and a pO₂ of 3.7 ppm, 1400°C and a pO₂ of 20 ppm, and 1500°C and a pO₂ of 200 ppm. Active oxidation caused fiber surface roughening, but no obvious changes to the internal fiber microstructure. Decreased fiber strength correlated with increased fiber surface roughness, but roughness magnitudes were not large enough to explain the amount by which strength was degraded. Fiber strengths, surface roughness, scale thicknesses, and the passive‐active oxidation transition for SiC are compared with previous observations. Possible strength degradation mechanisms are discussed.     

Stress rupture of SiC/SiC composite tubes under high-temperature steam

SiC-fiber–reinforced SiC matrix composite cladding for light water reactor fuel elements must withstand high-temperature steam oxidation in a loss-of-coolant accident scenario (LOCA). Current composite designs include an outer monolithic SiC layer, in part, to increase steam oxidation resistance. However, it is not clear how such a structure would behave under high-temperature steam in the case when the monolithic layer cracks and carbon interphases and SiC fibers are exposed to the environment. To fill this knowledge gap, stress-rupture tests of prototypic SiC composite cladding at 1000°C under steam and inert environments were conducted. The applied stress was ∼120 MPa, which was beyond the initial cracking stress. The failure lifetime under steam was 400–1300 s, while 75% of the composite specimens did not fail after 3 h of total exposure under inert gases. Microstructural observations suggest that steam oxidation activated slow crack growth in the fibers, which led to failure of the composite. The results from this study suggest that stress rupture in steam environments could be a limiting factor of the cladding under reactor LOCA conditions.     

Enhancement of the microwave absorption properties of PyC-SiCf/SiC composites by electrophoretic deposition of SiC nanowires on SiC fibers

The employment of coating technique on the silicon carbide fibers plays a pivotal role in preparing SiC fiber-reinforced SiC composites (SiC_f/SiC) toward electromagnetic wave absorption applications. In this work, SiC nanowires (SiCNWs) are successfully deposited onto the pyrolytic carbon (PyC) coated SiC fibers by an electrophoretic deposition method, and subsequently densified by chemical vapor infiltration to obtain SiCNWs/PyC-SiC_f/SiC composites. The results reveal that the introduction of SiCNWs could markedly enhance the microwave absorption properties of PyC-SiC_f/SiC composites. Owing to the increasing of SiCNWs loading, the minimum reflection loss of composites raises up to −58.5 dB in the SiCNWs/PyC-SiC_f/SiC composites with an effective absorption bandwidth (reflection loss ≤ −10 dB) of 6.13 GHz. The remarkable enhancement of electromagnetic wave absorption performances is mainly attributed to the improved dielectric loss ability, impedance matching and multiple reflections. This work provides a novel strategy in preparing SiC_f/SiC composites with excellent electromagnetic wave absorption properties.     

Oxidation resistance and thermal stability of the SiC-ZrB2 composite ceramic fibers

In this study, continuous SiC-ZrB₂ composite ceramic fibers were synthesized from a novel pre-ceramic polymer of polyzirconocenecarbosilane (PZCS) via melt spinning, electron beam cross-linking, pyrolysis, and finally sintering at 1800°C under argon. The ZrB₂ particles with an average grain size of 30.7 nm were found to be uniformly dispersed in the SiC with a mean size of 59.7 nm, as calculated using the Scherrer equation. The polycrystalline fibers exhibit dense morphologies without any obvious holes or cracks. The tensile strength of the fibers was greater than 2.0 GPa, and their elastic modulus was ~380 GPa. After oxidation at 1200°C for 1 hour, the strength of the fibers did not decrease despite a small loss of elastic modulus. Compared to the advanced commercial SiC fibers of Tyranno SA, the fibers exhibited improved high-temperature creep resistance in the temperature range 1300-1500°C.     

Model for SiC fiber strength after oxidation in dry and wet air

The strengths of oxidized SiC fibers were modeled from the effects of SiO₂ scale residual stress on fracture. Surface tractions from scale residual stress were determined for SiC surface flaws. The residual stress was the sum of the growth stress from oxidation volume expansion, thermal stress from SiO₂-SiC thermal expansion mismatch, and stress from phase transformations in crystallized scale. The partial relaxation of tensile residual stress from scale cracking was also calculated. Scale thicknesses were determined using Deal-Grove oxidation kinetics for glass and crystalline scales. Kolmogorov-Johnson-Mehl-Avrami (KJMA) kinetics was used to determine scale crystallization rates. Strengths of fibers with glass and with crystalline scales formed by oxidation in dry and wet air between 600° and 1400°C were modeled. The effects of partially crystallized scales were calculated using Weibull statistical methods. Modeled strengths were compared with measurements. Slight strength increases after glass scale formation, large decreases that accompany scale crystallization, and some differences between dry and wet air oxidation were accurately modeled. This suggests that under some conditions the scale residual stress dominates the changes in strength after SiC fiber oxidation. However, modeled strengths were significantly higher than those measured for some fibers oxidized in wet air, which suggests another degradation mechanism is active for these conditions. Modeling assumptions and implications for SiC fiber strength after oxidation for long times are discussed.     

Na2SO4 deposit-induced hot corrosion of SiC fibers relevant for SiC CMCs

Hi Nicalon, Hi Nicalon S, Sylramic, and Sylramic iBN SiC fibers were exposed to ~60 μg/cm² of Na₂SO₄ in a 0.1% SO₂/O₂ gaseous environment for times between 0.75 and 24 h at 1000°C. After exposure, the corrosion products were characterized using SEM, EDS, ICP-OES, TEM, and EFTEM to determine their high-temperature resistance to Na₂SO₄ and key reaction mechanisms. All SiC fiber types tested in this work exhibited little resistance to Na₂SO₄ deposit-induced attack relative to their behavior in dry O₂ environments. It was found that Hi-Nicalon displayed the least resistance to Na₂SO₄ deposit-induced attack due to excess carbon content resulting in the formation of a highly porous crystalline oxide and promotion of basic corrosion conditions. All fiber types formed a crystalline SiO₂ reaction product, either cristobalite or tridymite. Sylramic and Sylramic iBN formed a crystalline SiO₂ reaction layer containing TiO₂ needles due oxidation of TiB₂ particles. Additionally, Na₂SO₄ deposits resulted in pitting of all fiber surfaces.     

3D printed crack-free SiOC(Fe) structures with pyrolysis-induced carbon nanowires for enhanced wave absorption performance

High-performance SiOC(Fe) wave-absorbing ceramics, containing a large number of carbon nanowires, were successfully prepared using a combination of photopolymerization 3D printing technology and the polymer-derived ceramic pyrolysis method. By employing an optimized segmented slow heating scheme with extended holding time, the pyrolysis of SiOC(Fe) ceramics at 1000 °C facilitated the growth of carbon nanowires, Fe₃C and SiO₂ grains. These carbon nanowires were interlaced and interconnected within the samples, forming abundant conductive networks. This highly conducive network efficiently converted electromagnetic energy into thermal energy, effectively dissipating electromagnetic waves, and consequently enhancing the microwave absorption performance of ceramics. Moreover, this approach not only reduced ceramic cracks but also improved the dielectric loss performance of the materials, achieving a minimum reflectivity value of −35.72 dB. The SiOC(Fe) ceramics added with 5 wt% VcFe effectively enhanced the magnetic loss of the material, reduced the difference between the relative complex permeability (μ_r) and the relative complex dielectric constant (ε_r), and improved the impedance matching between the material surface and air, thereby further improving its microwave absorption performance. This resulted in an increase in the maximum effective absorption bandwidth of the material to 12.7 GHz at 5 mm. This study offers a promising solution for the preparation of ceramic matrix composite materials incorporating carbon nanowires, magnetic particles and ceramic precursors, which would be potentially valuable for radar detection and sensor applications.     

环氧树脂/改性SiC复合材料的制备及性能

采用硅烷偶联剂KH-560和丙烯酰胺对SiC进行表面改性,将其添加到环氧树脂中制备环氧树脂/改性SiC复合材料.采用傅里叶变换红外光谱仪、X射线衍射仪以及接触角测试仪探究改性SiC的性能,并对复合材料的性能进行测试.结果表明:SiC表面带有憎水基团,与环氧树脂相容性提高;SiC用量为环氧树脂质量的20％时,拉伸强度和弯...     

Oxidation Effects on the Mechanical Properties of a SiC-Fiber-Reinforced Reaction-Bonded Si3N4 Matrix Composite

The room-temperature mechanical properties of a SiC-fiberreinforced reaction-bonded silicon nitride composite were measured after 100 h treatment in nitrogen and oxygen environments to 1400°C. The composite heat-treated in nitrogen to 1400°C showed no appreciable loss in properties. In contrast, composites heat-treated in oxygen from 600° to 1000°C retained ∼65% and 35% of the matrix fracture and ultimate strength, respectively, of the as-fabricated composites, and those heat-treated from 1200° to 1400°C retained greater than 90% and 65% of the matrix fracture and ultimate strength, respectively, of the as-fabricated composites. For all nitrogen and oxygen treatments, the composite displayed strain capability beyond the matrix fracture strength. Oxidation of the fiber surface coating, which caused degradation of bond between the fiber and matrix and reduction in fiber strength, appears to be the dominant mechanism for property degradation of the composites oxidized from 600° to 1000°C. Formation of a protective silica coating at external surfaces of the composites at and above 1200°C reduced oxidation of the fiber coating and hence degrading effects of oxidation on their properties.     

Integrative improvement on thermophysical properties and ablation resistance of laminated carbon/carbon composites modified by in situ grown HfC nanowires onto carbon fiber cloths

HfC nanowires modified carbon fiber cloth laminated carbon/carbon (HfC_nw-C/C) composites were fabricated by in situ growth of HfC nanowires on carbon cloths via catalytic CVD, followed with lamination of the cloths and densification by pyrolytic carbon (PyC). Morphologies, thermal conductivity, coefficient of thermal expansion (CTE), and ablation resistance of the composites were investigated. Due to the loading of HfC nanowires, the matrix PyC with low texture was obtained; the thermal conductivity of the composites in the Z direction was enhanced from 100℃ to 2500℃; CTE along the X–Y direction also decreased in the range of 2060 ℃ – 2500 ℃, which reaches the maximum of 24 % at 2500℃. Moreover, the 20s-ablation-resistance of HfC_nw-C/C composites exhibits mass and linear ablation rates of 5.3 mg/s and 21.0 μm/s, which are 40 % and 37 % lower than those of pure C/C composites, respectively. Our work shows laminated HfC_nw-C/C composites are a promising candidate for high-temperature applications.     

Strengths of Ceramic Fibers at Elevated Temperatures

Tensile strengths were measured and Young's moduli were estimated for two SiC-based and three oxide ceramic fibers for temperatures from 25° to 1400°C. The SiC-based fibers were stronger but less stiff than the oxide fibers at room temperature and retained more of both strength and stiffness to high temperatures. High-temperature strengths of the SiC-based fibers were limited by internal void formation and oxidation; those of the oxide fibers were limited by softening of an intergranular glassy phase.     

Oxidation kinetics strength of Hi‐NicalonTM‐S SiC fiber after oxidation in dry and wet air

Hi‐Nicalon?‐S SiC fiber strengths and Weibull moduli were measured after oxidation for up to 100 hours between 700°C and 1400°C in wet and dry air. SiO₂ scale thickness and crystallization extent were measured by TEM. The effect of furnace environment on trace element levels in the SiO₂ scales was characterized by secondary ion mass spectroscopy. Crystallization kinetics and Deal‐Grove oxidation kinetics for glass and crystalline scale, and the transition between them, were modeled and determined. Crystallization retards oxidation kinetics, and scale that formed in the crystalline state was heavily deformed by the growth stress accompanying SiC oxidation volume expansion. Glass scales formed in dry air slightly increased fiber strength. Glass scales formed in wet air did not increase strength, and in some cases significantly decreased strength. Scales more than 200 nm thick were usually partially or completely crystallized, which degraded fiber strength. Contamination of scales by trace impurities such as Al and Ca during heat treatment inhibited crystallization. The oxidation kinetics and the strengths of oxidized Hi‐Nicalon?‐S fibers are compared with previous studies on SiC fibers, bulk SiC, and single‐crystal SiC. Empirical relationships between oxidation temperature, time, scale thickness, and strength are determined and discussed.     

Boosting biotribological and biological properties of carbon fiber composites by grafting SiC nanowires and hydroxyapatite for hip joint application

Carbon fiber composites have great potential application in hip joints, where excellent biotribological and biological properties are required. In this work, SiC nanowires (SiCnws) and hydroxyapatite (HA) were grafted into carbon fiber composites by carbothermal reduction and electrochemical deposition method to obtain carbon fiber-SiCnws-hydroxyapatite-carbon composites (CHS). Results show that SiCnws are grown on the surface of carbon fibers and fill the gaps between carbon fibers. Particle-shaped HA covers both carbon fibers and SiCnws completely. The introduction of SiCnws improves the biotribological properties of CHS. Compared with carbon fiber-hydroxyapatite-carbon composites without SiCnws, the friction coefficient of CHS increased from 0.196 to 0.269, while the wear rate decreased from 0.032 × 10^?14 m³/(N·m) to 0.016 × 10^?14 m³/(N·m). Biological tests show that CHS could promote cell proliferation and differentiation effectively. With these improved biotribological and biological properties, synthesized CHS composites exhibits a potential to be used as hip joints.     

Preparation and properties of SiOC ceramic modified carbon fiber needled felt preform composites

SiOC ceramic modified carbon fiber needled felt preform composites (C_f/SiOC) with densities of 0.4 and 0.7 g/cm³ were prepared by precursor infiltration and pyrolysis (PIP) method using methyltrimethoxysilane (MTMS) and dimethyldimethoxysilane (DMDMS) as precursors. The densification behavior was investigated through scanning electron microscopy (SEM) analysis of microstructure of C_f/SiOC composites undergoing different PIP times. The results indicate that with increase of PIP times, a great amount of SiOC ceramic was introduced into the preform, completely covering on the carbon fibers and occupying the open pores. The thermal performance, mechanical properties, and oxidation resistance of the composites were studied via various tests. The results illustrate that after two-time PIP procedure, thermal conductivities of the composites are 0.41–2.54 and 1.28–4.04 W/(m·K) in z direction and x/y plane, respectively, at RT-1500 °C. The compressive strengths of the composite arrive at 2.1 MPa in z direction and 7.8 MPa in x/y plane, which are almost 3.5 times and 6.5 times, respectively, counterparts of the raw preform. The incorporation of SiOC ceramic can remarkably improve anti-oxidation ability of the composites at 600 °C. The oxidation weight loss is merely 2.1 wt% after 60-min oxidation at 600 °C.     

Nano‐carbon black filled Lyocell fiber as a precursor for carbon fiber

In this work, Lyocell fibers filled with various amounts of carbon black were prepared. Wide angle X‐ray diffraction (WAXD) results showed that carbon black filled Lyocell fibers still had a cellulose II crystal structure and kept the characteristic peak of carbon black at the same time. The results of mechanical properties showed a slight reduction in the carbon black filled Lyocell fiber. Moreover, the heat stabilities of the carbon black filled Lyocell fibers showed no obvious change. The residue of carbon black filled Lyocell fiber at 1000°C was higher than that of Lyocell fiber, implying higher carbon yield could be obtained for the carbon black filled Lyocell precursor. Scanning electron microscopy (SEM) experiments showed that the surface and the cross section of carbon black filled Lyocell fiber were smooth and round, which are consistent with the carbon fiber precursor. The WAXD pattern of carbon black filled Lyocell‐based carbon fiber was different from that of Lyocell‐based carbon fiber. The addition of carbon black transfers the diffraction peak of carbon fiber while keeping the characteristic structure of carbon black. The results of mechanical properties of carbon fibers show that, if an appropriate amount of carbon black was chosen, carbon fiber with better properties than Lyocell‐based carbon fiber could be obtained by using the carbon black filled Lyocell fibers as the precursor. © 2005 Wiley Periodicals, Inc. J Appl Polym Sci 99: 65–74, 2006