Microstructures and mechanical properties of carbon/carbon composites reinforced with carbon nanofibers/nanotubes produced in situ

Using ferrocene as catalyst and toluene as the liquid precursor, carbon/carbon (C/C) composites were prepared by chemical liquid-vapor infiltration at 850-1100 °C. The microstructures and properties of C/C composites obtained with different ferrocene contents were studied. The results show smooth laminar and isotropic pyrocarbon are obtained after adding ferrocene to the precursor. Carbon nanofibers can be formed as the catalyst content is 0.3-1 wt.%. When the ferrocene content is 2 wt.%, multi-walled carbon nanotubes with the diameter about 20-90 nm are obtained together with carbon-encapsulated iron nanoparticles. After adding ferrocene to the precursor, the fracture modes of the composites change from brittle facture to tough fracture. The flexural strength of the composites is a maximum for 0.3 wt.% ferrocene in the precursor, higher than for ferrocene contents of 0, 0.5, 1 and 2 wt.%. The flexural modulus of the composites decreases after adding ferrocene to the precursor.     

Preparation of in situ grown silicon carbide nanofibers radially onto carbon fibers and their effects on the microstructure and flexural properties of carbon/carbon composites

Silicon carbide nanofibers (SiCNFs) used as the second reinforcements of carbon/carbon composites were grown radially on the carbon fiber surface. The microstructure of SiCNFs and their effects on the microstructure and flexural properties of C/C composites were investigated. Results show that there are many defects such as twin crystals and stacking faults in SiCNFs which were grown by catalytic chemical vapor deposition. During the same process, the skin region of carbon fiber has changed. Several SiC layers are formed and the arrangement of the graphite layers around SiC layers is more orderly. In the next chemical vapor infiltration, due to the induction of SiCNFs, the middle textural pyrocarbon were formed firstly and then is the high textural pyrocarbon. The existence of SiCNFs also contributes to the three-phase interface between pyrocarbon, SiCNFs and carbon fibers, resulting in a good bond between carbon fiber and matrix. Those structural changes lead the better flexural properties of SiCNF–C/C composites compared with C/C composites.     

Mechanical and physical properties of epoxy composites reinforced by vapor grown carbon nanofibers

Epoxy/vapor grown carbon nanofiber composites (VGCF) with different proportions of VGCF were fabricated by the in situ process.The VGCFs were well dispersed in both of the low and high viscosity epoxy matrices, although occasional small aggregates were observed in a high viscosity epoxy of 20 wt.%. The dynamic mechanical behavior of the nanocomposite sheets was studied. The storage modulus and the glass transition temperature (T_g) of the polymer were increased by the incorporation of VGCFs.The electrical and mechanical properties of the epoxy-VGCFs nanocomposite sheets with different weight percentages of VGCFs were discussed. The results were that both had maximum tensile strength and Young’s modulus at 5 wt.% for both materials and reduced the fracture strain with increasing filler content. The electrical resistivity was decreased with the addition of filler content. Mechanical, electrical and thermal properties of low viscosity epoxy composites were resulted better than that of the high viscosity composites.     

Transmission electron microscopy study of the microstructure of unidirectional C/C composites fabricated by catalytic chemical vapor infiltration

Unidirectional carbon/carbon (C/C) composites were fabricated by catalytic chemical vapor infiltration, using electroless Ni–P as catalyst. Transmission electron microscopy (TEM) investigations indicate that the catalyst particles (100–800 nm) in the pyrocarbon (PyC) matrix are composed of Ni₃P and Ni phases, but only the Ni₃P phase was observed in the tiny catalyst particles (<50 nm) in carbon fibers. The catalyst particles in the matrix were encapsulated by high-textured PyC shells, in which openings were observed. The thicknesses of the medium-textured PyC in the composites (720–850 nm) are greater than in conventional C/C composites (660–740 nm), but have no significant difference in texture degree. Catalysts were partially extruded out of the PyC shells and migrated into the carbon fibers, leading to the catalytic graphitization of the carbon fibers, and their structural homogeneity was destroyed. Based on the TEM observation, a dissolution/precipitation mechanism was proposed for the catalytic graphitization of carbon fibers, and a dissolution/precipitation/encapsulation/fracture/extrusion mechanism was proposed for the encapsulation of catalyst particles.     

Oxidation microstructure studies of reinforced carbon/carbon

Laboratory oxidation studies of reinforced carbon/carbon (RCC) are discussed with particular emphasis on the resulting microstructures. This study involves laboratory furnace (500-1500 °C) and arc-jet exposures (1538 °C) on various forms of RCC. RCC without oxidation protection oxidized at 800 and 1100 °C exhibits pointed and reduced diameter fibers, due to preferential attack along the fiber edges. The 800 °C sample showed uniform attack, suggesting reaction control of the oxidation process; whereas the 1100 °C sample showed attack at the edges, suggesting diffusion control of the oxidation process. RCC with a SiC conversion coating exhibits limited attack of the carbon substrate at 500, 700 and 1500 °C. However samples oxidized at 900, 1100, and 1300 °C show small oxidation cavities at the SiC/carbon interface below through-thickness cracks in the SiC coating. These cavities at the outer edges suggest diffusion control. The cavities have rough edges with denuded fibers and can be easily distinguished from cavities created in processing. Arc-jet tests at 1538 °C show limited oxidation attack when the SiC coating and glass sealants are intact. When the SiC/sealant protection system is damaged, attack is extensive and proceeds through cracks, creating denuded fibers in and along the cracks. Even at 1538 °C, where diffusion control dominates, attack is non-uniform with fiber edges oxidizing preferentially.     

Transmission electron microscopy of propene-derived pyrolytic carbon coatings

The microstructures of a series of pyrocarbon coatings prepared from a 20 per cent propene atmosphere in the temperature range 1250 to 2000°C have been examined by transmission electron microscopy. The structures vary markedly with temperature, and the structural changes can be correlated with density and microporosity measurements. At temperatures between 1400 and 2000°C, the structure can be described in terms of two components; the relative amount of each component is dependent on the temperature.     

Formation of graphitic rods in carbon/carbon composites reinforced with carbon nanotubes

The formation of graphitic rods with a carbon nanotube (CNT) in the center was observed in CNT-reinforced phenolic resin-based carbon/carbon composites heat treated at 2000 °C. TEM characterization indicated that the carbon surrounding the CNT has a much better degree of graphitization compared to the carbon in most of the matrix. The formation temperature (2000 °C) of the graphitic rod is lower than for stress graphitization and normal graphitization of phenolic resin.     

Synthesis and mechanical properties of polybenzimidazole nanocomposites reinforced by vapor grown carbon nanofibers

Polybenzimidazole (PBI) nanocomposites containing 0.5–5 wt% vapor grown carbon nanofibers (VGNFs) were successfully synthesized by solvent evaporation method. Fracture morphology examination confirmed the uniform dispersion of VGNFs in the matrix. The mechanical properties of neat PBI and the nanocomposites were systematically measured by tensile test, dynamic mechanical analysis (DMA), hardness measurement, and friction test. Tensile tests revealed that Young's modulus increased by about 43.7% at 2 wt% VGNFs loading, and further modulus growth was observed at higher filler loadings. DMA studies showed that the nanocomposites have higher storage modulus than neat PBI in the temperature range of 30–350°C, holding storage modulus larger than 1.54 GPa below 300°C. Outstanding improvement of hardness was achieved for PBI upon incorporating 2 wt% of VGNFs. The results of friction test showed that coefficient of friction of PBI nanocomposites decreased with VGNFs content compared with neat PBI. POLYM. COMPOS., 2010. © 2009 Society of Plastics Engineers     

Isotactic polypropylene–vapor grown carbon nanofibers composites: Electrical properties

Nanocomposites have been obtained by dispersing various amounts of vapor grown carbon nanofibers within isotactic polypropylene. Thermal investigations done by differential scanning calorimetry and dynamic mechanical analysis revealed the effect of the vapor grown carbon nanofibers on the melting, crystallization, α, and β relaxations. Direct current electrical features of these nanocomposites have been investigated and related to the thermal features of these nanocomposites. The effect of the loading with carbon nanofibers on the electrical properties of these nanocomposites is discussed within the percolation theory. The percolation threshold was estimated at about 5.5% wt carbon nanofibers. The temperature dependence of the direct current conductivity is analyzed in detail and it is concluded that the electronic hopping is the dominant transport mechanism. A transition from one‐dimensional hopping towards a three‐dimensional hopping was noticed as the concentration of carbon nanofibers was increased from 10% wt to 20% wt carbon nanofiber. The possibility of a differential negative resistivity is suggested. © 2017 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2017 , 134, 45297.     

Microstructure of carbon/carbon composites reinforced with pitch-based ribbon-shape carbon fibers

Carbon/carbon composites were prepared with ribbon-shape pitch-based carbon fibers serving as reinforcement and thermosetting PFA resin and thermoplastic pitch as matrix precursors. The composites were heat treated to 1000, 1600 and 2700 °C. Microstructural transformations taking place in the reinforcement, carbon matrix, and the interface were studied using polarized optical and scanning electron microscopy. The fiber/matrix bond and ordering of the carbon matrix in heat-treated composites was found to vary depending on the heat treatment temperature of the fibers. Stabilized fiber cleaved during carbonization of resin-derived composites. In contrast, fibers retain their shape during carbonization of pitch matrix composites. Optical activity was observed in composites made with carbonized fibers; the extent decreases with increased heat treatment of the fibers. Studies at various heat treatment temperatures indicate that ribbon-shape fibers developed ordered structure at 1600 °C when co-carbonized with thermosetting resin or thermoplastic pitches.     

Cure kinetic study of carbon nanofibers/epoxy composites by isothermal DSC

An investigation was carried out into the cure kinetics of carbon nanofibers (CNF)/epoxy composites, composed of tetraglycidyl‐4,4′‐diaminodiphenylmethane (TGDDM) resin and 4,4′‐diaminodiphenylsulfone (DDS) as a curing agent. The experimental data for both neat system and CNF/epoxy composites revealed an autocatalytic behavior. Analysis of DSC data indicated that the presence of carbon nanofibers had only a negligible effect on the cure kinetics of the epoxy. Kinetic analysis was performed using the phenomenological model of Kamal and two diffusion factors were introduced to describe the cure reaction in the latter stage. Activation energies and kinetic parameters were determined by fitting experimental data. Comparison between the two diffusion factors was performed, showing that the modified factor was successfully applied to the experimental data over the whole curing temperature range. © 2005 Wiley Periodicals, Inc. J Appl Polym Sci 96: 329–335, 2005     

Temperature and strain rate dependences of yield stress of polypropylene composites reinforced with carbon nanofibers

Polypropylene (PP) nanocomposites filled with 0.1, 0.3, 0.5, and 1.0 wt% carbon nanofiber (CNF) were prepared via melt compounding in a twin‐screw extruder followed by injection molding. The effects of CNF additions on the structure, mechanical and tensile yielding behavior of PP were investigated. TEM and SEM observations showed that CNFs were dispersed uniformly within PP matrix. Tensile test showed that the yield strength and Young's modulus of PP were improved considerably by adding very low CNF loadings. The reinforcing effect of CNF was also verified from the dynamic mechanical analysis. Impact measurement revealed that the CNF additions were beneficial to enhance the impact toughness of PP. The yield stress of the PP/CNF nanocomposites was found to be strain rate and temperature dependent. The yielding responses of PP/CNF nanocomposites can be described successfully by the Erying's equation and a reinforcing index n. The structure and mechanical property relationship of the nanocomposites is discussed. POLYM. COMPOS., 2009. © 2008 Society of Plastics Engineers     

Effect of in situ grown SiC nanowires on microstructure and mechanical properties of C/SiC composites

Hexagonal-shaped SiC nanowires were in situ formed in C/SiC composites with ferrocene as catalyst in the densification process of polymer impregnation and pyrolysis. The effect of SiC nanowires on microstructure and properties of the composites were studied. The results show that the in situ formed SiC nanowires were hexagonal, mostly with diamer of about 250 nm, and grew by the vapor–liquid–solid (VLS) mechanism. The C/SiC composite with nanowires shows higher bulk density and flexural strength than the one with no SiC nanowires, and the high temperature flexural strength behavior of C/SiC composites with SiC nanowires was evaluated.     

Combustion synthesis of SiC-based ceramics reinforced by discrete carbon fibers with in situ grown SiC nanowires

This study proposes a combustion-based ceramic matrix composite processing technique intended on single-step in situ deposition of single-crystal SiC nanowires (SiC_nw) on the surface of carbon fibers (C_f) and formation of SiC_nw–reinforced SiC matrix. This was accomplished by Ta-catalyzed combustion of poly-(C₂F₄)-containing reactive mixtures with pre-mixed chopped C_f. Depending on the combustion conditions, carbon fiber surface is subjected either to formation of diffusion layers, ceramic particle incrustation or growth of continuous arrays of carbon-coated single-crystal SiC_nw with a nearly defect-free lattice, 10–50 nm diameter and 15–20 μm length. Thermodynamics, phase and structure formation mechanisms are explored, and the optimal conditions are outlined for reproducible C_f/in situ SiC_nw dual reinforcement of SiC-based ceramics. Hot pressing at 1500 °C produced C_f/in situ SiC_nw-reinforced ceramic SiC–TaSi₂ specimens with a relative density of 97%, 19 GPa Vickers hardness, 3-point flexural strength σ = 420 ± 70 MPa and fracture toughness K_1C = 12.5 MPa m^1/2.     

Effect of vapor‐grown carbon nanofibers on the sliding friction of natural rubber composites

The friction properties of vapor‐grown carbon nanofibers (VGCFs) reinforced natural rubber (NR) composites were investigated with the ball‐on‐plate sliding test. A mechanism was proposed on the basis of the viscoelastic properties, morphology and hardness of the composites, determined using dynamic mechanical analysis, optical microscopy, field emission scanning electron microscopy, transmission electron microscopy and a hardness‐testing device. The friction behavior of NR/VGCF composites showed three different stages: an increment trend at first stage, a decrement trend at second stage and a stable state at third stage. The peak values of friction coefficient were similar, and the peak shifted to a smaller cycle with increased VGCF content. The eventual friction coefficient decreased with increased VGCF content due to accelerated formation of abrasion patterns in the NR/VGCF composites. Moreover, the arranged VGCFs contributed to the self‐lubrication of NR/VGCF composites and the NR/20 wt% VGCF composite had the smallest friction coefficient. POLYM. COMPOS., 2011.© 2011 Society of Plastics Engineers     

A TEM study of microstructure of carbon fiber/polycarbosilane-derived SiC composites

The structures of two types of mesophase pitch-based carbon fibers (M30 and M70) reinforced SiC composites, prepared by the polycarbosilane impregnation-pyrolysis process, were investigated using transmission electron microscopy (TEM). It was found that M70 possessed a highly-ordered graphite structure despite occasional misorientation of some crystallites. However, the skin of M70 was less ordered than the interior of M70. The structure of M30 was uniform throughout, and was less ordered than that of M70. The fiber and matrix in M70/SiC bonded weakly, whereas the fiber and matrix in M30/SiC bonded tightly and locked together. This difference in the interface feature originates from the difference of the surface crystalline structures of M30 and M70, and is formed during the first impregnation-pyrolysis cycle of polycarbosilane.     

Mechanical properties of vapor grown carbon nanofibers

Individual as-fabricated, high temperature heat-treated and graphitized/surface oxidized vapor grown carbon nanofibers (VGCNFs), with average diameter of 150 nm were tested for their elastic modulus and their tensile strength by a MEMS-based mechanical testing platform. The elastic modulus increased from 180 GPa for as-fabricated, to 245 GPa for high temperature heat-treated nanofibers. The nominal fiber strengths followed Weibull distributions with characteristic strengths between 2.74 and 3.34 GPa, which correlated well with the expected effects of heat treatment and oxidative post-processing. As-fabricated VGCNFs had small Weibull modulus indicating a broad flaw population, which was condensed upon heat treatment. For all VGCNF grades, the nanofiber fracture surface included the stacked truncated cup structure of the oblique graphene layers comprising its backbone and cleavage of the outer turbostratic or thermally graphitized layer.     

碳纳米管增强陶瓷基复合材料研究进展

鉴于其优良的力学性能、独特的物理和化学性能,碳纳米管被认为是最理想的增韧材料。本文主要从制备方法和力学性能两方面综述了碳纳米管增韧陶瓷基复合材料的研究现状,并针对研究中存在的问题,提出了相应的设想和发展趋势。     

Reactivity and related microstructure of 3d carbon/carbon composites

Four carbon/carbon composites fabricated with either PAN fibers or coal tar pitch fibers were examined. Detailed analysis of composite properties and structure included total surface area by Kr adsorption at 77 K, active surface area, porosity, crystallite parameters, as well as SEM and optical microscopic observations. Rates of composite gasification were measured at 1123 K in 3.1 kPa of steam. Under these experimental conditions the composites fabricated with PAN fibers are roughly three times as reactive as those fabricated with pitch fibers. Microscopic examination of the composites provides detail on two different microstructures for each fiber and respective composite. Binder associated with the fibers is influenced by the fiber microstructure, and a continuation of structure is developed throughout the composite body. Even though the fiber fraction is only approximately 50% of the composite by weight, it is clear that fiber microstructure influences overall composite microstructure and, hence, composite physical properties and subsequent composite gasification behavior.