Effects of pressure on the pore formation of carbon/carbon composites during carbonization

In this study, porosity and graphitizability of coal tar pitch with the treatment pressure were investigated. 4-directional carbon/carbon composites (4D C/C) were made from the matrix precursor of coal tar pitch through the process of impregnation and carbonization. Then the effects of applied pressure during the densification on the composites were observed. The matrix pitch which had 600 bar applied during the carbonization process had one and a half times less pore area ratio than that treated at 1 bar. When the pitch was heat treated up to 2300°C after the high pressure carbonization, the degree of graphitization was improved on a small scale and the crystal size tended to reduce. As the applied pressures to 4D C/C composites increased from 1 to 600 bar, the densification ratio was greatly improved. In the pore size distribution of the 4D C/C composites, the macropore portion was decreased while the mesopore portion increased, when high pressures were applied.     

Influence of carbon fibre treatment and filler addition on porosity and structure change of carbon fibres reinforced phenolic matrix composites during carbonization

The transformation during pyrolysis of ex-cellulose carbon fibres/phenolic matrix 2D composites is investigated. The amount and the dimensions of pores and micro-cracks created after heat treatment are characterized and described from 600 °C (resin pyrolysed matrix) to 1,000 °C (carbonized matrix). Characterization of samples was performed using mercury intrusion porosimetry at different stages of thermal treatment. Three different composites were prepared in order to study the influence of fibre surface properties and the role of carbon filler addition in matrix, on the porosity emergence. Inter-laminar shear strengths of these materials were recorded. On the investigated materials, the lowest fibre/matrix bonding is found to preserve material cohesion by promoting interfacial debonding and thus limiting consequences of shrinkage during matrix carbonization.     

Structural changes during pitch-based carbon granular composites carbonisation

This article deals with the study of carbon composites behavior during their carbonization. Composites were prepared using four granular carbons (graphite, anthracite, green petroleum coke, and foundry coke) and four pitches (a commercial impregnating coal-tar pitch, an air-blown and two thermally treated pitches). The evolution of the optical microstructure, porosity, volume, and weight of carbon composites was monitored at different intermediate carbonization temperatures (350, 500, 700, and finally 1000 °C). The porosity of composites increases with carbonization due to volume changes and weight loss of pitches. Weight loss of carbon composites during their carbonization mainly depends on the pitch characteristics and it was slightly influenced by the presence of granular carbon. On the other hand, carbon composites with the commercial coal-tar pitch and foundry coke, anthracite, or graphite deform in the initial stages of carbonization (<350 °C) probably due to the lower porosity of the green pellets and the high amount of low-molecular weight compounds of the pitch. Carbon composites with green petroleum coke underwent important dimensional changes during their carbonization, expanding initially and then shrinking at temperatures above 700 °C. The type of granular carbon strongly influenced the microstructure of the final carbon composite, as a result of its effect on the development of mesophase. Graphite, anthracite and foundry coke delays mesophase development, whereas green petroleum coke accelerates mesophase formation.     

Fabrication of silicon carbide composites with carbon nanofiber addition and their fracture toughness

The authors have examined the fabrication conditions of SiC composites containing carbon nanofiber, i.e., vapor-grown carbon nanofiber (VGCF), to enhance the fracture toughness. Commercially available ultrafine SiC powder (specific surface area: 47.5 m² g⁻¹) was mixed with VGCF and sintering aid in the Al₄C₃–B₄C system. Approximately 1.5 g of the mixture was uniaxially pressed at 50 MPa to obtain a compact with a diameter of 20 mm and a thickness of approximately 1.5 mm. The resulting compact was hot-pressed at 1800 °C for 1 h in Ar atmosphere under a pressure of 62 MPa. The relative density of hot-pressed SiC composite decreased from 98.0 to 96.3%, whereas the fracture toughness was enhanced from 3.8 to 5.2 MPa m^1/2, as the amount of VGCF increased from 0 to 6 mass%. Furthermore, an acid treatment of VGCF was conducted to enhance its dispersibility within the SiC matrix, owing to the formation of COO⁻ groups on the VGCF surface. As a result of this treatment, the relative density and fracture toughness of hot-pressed SiC composite with 6 mass% acid-treated VGCF addition increased to 99.0% and 5.7 MPa m^1/2, respectively.     

Static friction properties of carbon/carbon composites

Preforms were made from 1K PAN plain carbon cloth and densified using the rapid directional diffusion chemical vapor infiltration (RDD CVI) processes. Four carbon/carbon (C/C) composite specimens treated at 1800 (specimen A), 1800+2000 (B), 2000 (C), and 2300 °C (D), respectively, were prepared, then machined into ring-on-ring specimen configurations. The influence of high-temperature heat treatment and the test temperatures on the static friction properties of the C/C composites has been researched. The results show that the high-temperature heat treatment processes has an important impact on the static friction behaviors of RDD CVI C/C composites. With raising the treatment temperature, the interlayer spacing of the matrix carbon in them becomes small, and the crystallite width as well as height increase. Under the test at room temperature, the static friction coefficients (FC) of the specimen treated at 1800 °C (A) are the lowest, but become very big and the highest under the test temperature of 260 °C due to desorption of the water absorbed on the friction surface. The composites treated at 2000 °C (C) exhibit enough high static friction coefficients under room temperature owing to their absorption of less water and the difficult delamination of the matrix carbon. However, in the test temperature of 260 °C or after the dynamic friction tests, their static FC is low. The specimen treated at 2300 °C (D) has a low FC at 260 °C heat condition, but its FC is higher than that of A and C under room temperature and largest after dynamic friction tests. No matter how high the heated temperatures are, the static FC of C/C composites decreases with an increase of the brake-specific pressure. When the specific pressure is very high and exceeds a certain value, the static FC is almost the same for specimens B, C, and D.     

The effect of zirconium addition on the microstructure and properties of chopped carbon fiber/carbon composites

Carbon/carbon composites containing zirconium were prepared using chopped carbon fiber, mesophase pitch and Zr powder by the traditional process including molding, carbonization, densification and graphitization. The influence of Zr on the microstructure and properties of the composites were investigated. Results show that Zr can improve the interface bonding, promote more perfect and larger crystallites and enhance the conductive/mechanical properties of the composites. The high in-plane thermal conductivity of 464 W/(m K) and excellent bending strength of 83.6 MPa was obtained for a Zr content of 13.9 wt% at heat treatment temperature(HTT) of 2500 °C. However the conductive/mechanical properties of the composites decrease dramatically for an higher HTT of 3000 °C. SEM micrograph of the fracture surface for the composites shows that lower disorder crystallite arrangement of fiber and carbon matrix come into being in the composites during HTT of 3000 °C, which should be responsible for the low properties. Correlation between the content of Zr and the microstructure and properties are discussed.     

中间相沥青基碳/碳复合材料的组织与性能

以3K PAN基碳纤维为增强体,以中间相沥青为基体前驱体,采用压力浸渍-碳化工艺制备出2D中间相沥青基碳/碳复合材料.研究分析了材料的偏光组织结构、弯曲性能及弯曲断口形貌,结果表明:基体碳的组织结构随碳化压力的不同而变化,低压时以小域组织为主,高压时以广域流线型组织为主;材料的抗弯强度、密度随碳化压力的增加而增高,最高抗弯强度可达278MPa;断裂特征与材料的密度、界面结合状况有关,密度较高、界面结合适中时,弯曲断口以纤维断裂、纤维拔出为主,材料具有韧性断裂特征.     

Study on the thermal expansion properties of C/C composites

Two different woven (2D and 3D) carbon/carbon composites (C/C) and a block carbon have been prepared by chemical vapor infiltration (CVI). The effects of the density and porosity of composites, preform architectures and heat treatment on the thermal expansion properties of the C/C composites were investigated. It is revealed that the coefficient of thermal expansion (CTE) of C/C composites is negative below 100 °C, and the CTE values are inversely proportion to its porosity. Comparing with 2D C/C composites, 3D C/C composites have a better thermal stability. Heat treatment can increase the thermal stability of composites by changing interfacial thermal stress. The thermal expansion behavior of C/C composites is considered as the result of interaction between fibers and matrix.     

Electrically Conductive Carbon Nanofiber Composites with High-Density Polyethylene and Glass Fibers

Carbon nanofibers are being investigated for incorporation into composites to improve mechanical, thermal, and electrical properties. The difficulties in making such composites are issues of dispersion of the nanofiber and wetting of the nanofibers by the matrix. The processing methods developed to date tend to be complex, involving multiple steps. This paper reports on a study to make electrically conductive composites with small volume fraction of vapor-grown carbon nanofibers (VGCF). The matrix is a high-density polyethylene (HDPE); the effect of adding glass fibers to this composite is also studied. Certain types of the VGCF fibers did not produce conductive composites with standard mixing techniques; however, VGCF nanofibers heat treated with a post-processing surface treatment produced conductive composites without extensive or vigorous dispersion techniques. The results indicate that surface treatments and dispersion methods are important factors in producing conductive composites. It is demonstrated that small volume fractions of nanofiber can be used to produce conductive composites without extensive processing steps.     

Ablation,mechanical and thermal conductivity properties of vapor grown carbon fiber/phenolic matrix composites

The ablation, mechanical and thermal properties of vapor grown carbon fiber (VGCF) (Pyrograf III™ Applied Sciences, Inc.)/phenolic resin (SC-1008, Borden Chemical, Inc.) composites were evaluated to determine the potential of using this material in solid rocket motor nozzles. Composite specimens with varying VGCF loadings (30–50% wt.) including one sample with ex-rayon carbon fiber plies were prepared and exposed to a plasma torch for 20 s with a heat flux of 16.5 MW/m² at approximately 1650°C. Low erosion rates and little char formation were observed, confirming that these materials were promising for rocket motor nozzle materials. When fiber loadings increased, mechanical properties and ablative properties improved. The VGCF composites had low thermal conductivities (approximately 0.56 W/m-K) indicating they were good insulating materials. If a 65% fiber loading in VGCF composite could be achieved, then ablative properties are projected to be comparable to or better than the composite material currently used on the Space Shuttle Reusable Solid Rocket Motor (RSRM).     

Carbon fibre reinforced carbon produced by polymer impregnation and pyrolysis

Carbon fibre reinforced carbon (C/C) is an attractive material for intermediate and high temperature applications due to its specific properties like low density, high strength and chemical stability. Unfortunately the material oxidizes, so that in an oxidative environment a protective coating has to be applied. Polymer impregnation and pyrolysis is a cost effective production technique to produce C/C materials. In the present work, an abstract of a research program funded by the German Science Foundation (DFG), the mechanical properties of C/C as a function of processing temperature and test temperature have been described. In the program the behaviour of two-dimensionally reinforced (2D) material and unidirectional reinforced (1D) materials has been investigated. All materials experience a strength reduction as a result of carbonization of the polymer matrix at temperatures up to 1000°C. An additional heat treatment above 1000°C causes a partial recovery of the strength. The 1D C/C material shows up to testing temperatures of 1800°C a 10 % loss of strength, whereas for the 2D C/C the strength increases by 10 % at 1500°C in comparison with the room temperature results.     

SiC<Subscript>f</Subscript>/SiC composites reinforced by randomly oriented chopped fibers prepared by semi-solid mechanical stirring method and hot pressing

SiC short fibers, with an average diameter of 13 μm, length of 300–1,000 μm and chopped from SiC continuous fibers, were surface modified by the semi-solid mechanical stirring method to produce a discrete coating of aluminum particles. Then the starting mixtures, which consist of SiC short composite fibers, aluminum powder less than 50 μm and α-SiC powder of an average diameter of 0.6 μm, were mechanically mixed in ethanol for about 3 h, dried at 80 °C in air, and hot pressed under 30 MPa pressure at 1,650, 1,750 and 1,850 °C with 1 h holding time to prepare SiC_f/SiC composites. Volume fraction of SiC short fibers in the starting powder for SiC_f/SiC composites was about 25 vol.%. The composites were characterized in terms of bulk density, phase composition, and mechanical properties at room temperature. In addition, the distribution of SiC short fibers in the matrix and the cracking pattern in the composites were examined by optical microscope. Fracture surface of the composites were performed by a scanning electron microscope (SEM). The effect of hot-pressing temperature on bulk density and mechanical properties was investigated. The results indicated that SiC short fibers were uniformly and randomly distributed in the matrix, bending strength and bulk density of the composites increased with increasing sintering temperature. The composite, hot-pressed at 1,850 °C, exhibited the maximum bulk density and bending strength at room temperature, about 3.01 g/cm³ and 366 MPa, respectively. SEM analyses showed that there were a few of fiber pullout on the fracture surface of samples sintered at 1,650 °C and 1,750 °C, which was mainly attributed to lower densities. But few of fiber pullout was observed on the fracture surface of sample sintered at 1,850 °C, the combined effects of high temperature and a long sintering time were considered as a source of too severe fiber degradation because of the large amount of oxygen in the fibers.     

Effect of moisture absorption on the mechanical behavior of carbon fiber/epoxy matrix composites

A carbon fiber/epoxy unidirectional laminated composite was exposed to a humid environment and the effect of moisture absorption on the mechanical properties and failure modes was investigated. The composites were exposed to three humidity conditions, namely, 25, 55, and 95 % at a constant temperature of 25 °C. The carbon fiber–epoxy laminated composites for two different carbon fiber surface treatments were used. The results showed that the mechanical properties differ considerably for each fiber surface treatment. The application of a coupling agent enhanced the fiber-matrix adhesion and reduced dependence of the properties on humidity. The damage mechanism observed at micromechanical level was correlated to acoustic emission signals from both laminated composites. The untreated carbon fiber failure mode was attributed to fiber-matrix interfacial failure and for the silane-treated carbon fiber reinforced epoxy laminate attributed to matrix yielding followed by fiber failure with no signs of fiber-matrix interface failure for moisture contents up to 1.89 %.     

Lightweight carbon-bonded carbon fiber composites with quasi-layered and network structure

Lightweight carbon-bonded carbon fiber (CBCF) composites were fabricated with chopped carbon fibers and dilute phenolic resin solution by pressure filtration, followed by carbonization at 1000 °C in argon. The as-prepared CBCF composites had a homogenous fiber network distribution in xy direction and quasi-layered structure in z direction. The pyrolytic carbon derived from phenolic resin was mainly accumulated at the intersections and surfaces of chopped carbon fibers. The composites possessed compressive strengths ranged from 0.93–6.63 MPa in xy direction to 0.30–2.01 MPa in z direction with a density of 0.162–0.381 g cm^− 3. The thermal conductivity increased from 0.314–0.505 to 0.139–0.368 Wm^− 1 K^− 1 in xy and z directions, respectively. The experimental results indicate that the CBCF composites prepared by this technique can significantly contribute to improve the thermal insulation and mechanical properties at high temperature.     

Investigation of microstructure and mechanical properties of nano MgO reinforced Al composites manufactured by stir casting and powder metallurgy methods: A comparative study

In the present work, Al–nano MgO composites using A356 aluminum alloy and MgO nanoparticles (1.5, 2.5, and 5 vol.%) have been fabricated via stir casting and powder metallurgy (PM) methods. Different processing temperatures of 800, 850, and 950 °C for stir casting and 575, 600, and 625 °C for powder metallurgy were considered. Powder metallurgy samples showed more porosity portions compare to the casting samples which results in higher density values of casting composites (close to the theoretical density) compare to the sintering samples. Introduction of MgO nanoparticles to the Al matrix caused increasing of the hardness values which was more considerable in casting samples. The highest hardness value for casting and sintering samples have been obtained at 850 and 625 °C respectively, in 5 vol.% of MgO. Compressive strength values of casting composites were higher than sintered samples which were majorly due to the more homogeneity of Al matrix, less porosity portions, and better wettability of MgO nanoparticles in casting method. The highest compressive strength values for casting and sintered composites have been obtained at 850 and 625 °C, respectively. Scanning electron microscopy images showed higher porosity portions in sintered composites and more agglomeration and aggregation of MgO nanoparticles in casting samples which was due to the fundamental difference of two methods. Generally, the optimum processing temperatures to achieve better mechanical properties were 625 and 850 °C for powder metallurgy and stir-casting, respectively. Moreover, casting method represented more homogeneous data and higher values of mechanical properties compare to the powder metallurgy method.     

Electromagnetic shielding capacity of carbon matrix composites made from nickel-loaded black rice husk

Non- and nickel-loaded carbon matrix composites were made from rice husk to measure their electromagnetic shielding (EMS) capacities in the range of 30–1500 MHz. The results showed that the EMS capacity was dependent on carbonization temperature, content of nickel loading, molding pressure and pore size diameter. The combination of carbonization at 1200°C, molding at 20 MPa and loading of 6 wt% nickel can provide 80–60 dB EMS capacity over the measuring frequency range.     

Vapor grown carbon fiber composites with epoxy and poly(phenylene sulfide) matrices

Vapor grown carbon fibers (VGCF, Pyrograf III™ from Applied Sciences, Inc.), with 100–300 nm diameters and ∽10–100 μm lengths, were formulated in various fiber volume fractions into epoxy (thermoset) and into poly(phenylene sulfide) (thermoplastic) composites. Increases in stiffness were observed as with previous VGCF/organic matrix composites. Large increases in flexural strengths were achieved in both systems demonstrating for the first time that discontinuous randomly oriented Pyrograf III™ can give strength increases and has substantial potential as a reinforcement in composites. Here-to-fore, addition of VGCF caused strength decreases. Voids, residual thermal strains (as the fiber surface area is ∽35 times greater than 7 μm-diameter PAN fiber), or uncertainties about fiber strength, fiber–matrix bonding and the degree of fiber dispersion, could cause losses of strength. Thermal conductivity properties of VGCF/ABS (acrylonitrile–butadiene–styrene from GE Plastics) and VGCF/epoxy composites with various fiber volume fractions were measured. Thermal conductivity increased with an increase in fiber volume fraction. However, these increases were not significant enough to make these VGCF fiber/organic matrix composites candidates for thermally conductive materials.     

Influence of heat treatment temperature on interfacial shear strength of C/C

In order to investigate the influence of heat treatment temperature on the interfacial shear strength of C/C composites, a pull-out test was carried out on a microcomposite heat-treated at various temperatures which was composed of seven filaments. It was confirmed from SEM observation and Raman spectroscopy that shear fracture proceeded through both interface and interphase. Interfacial shear strength was calculated taking into account the effect of stress distribution. It was found that interfacial shear strength decreased by carbonization until 1200°C. However, it increased by graphitization at 2000°C. At the same time, exfoliation of the matrix from some fibers occurred by graphitization and those samples showed a very low level shear fracture force. Therefore, strengthened parts and weakened parts were mixed in the graphitized C/C composite.     

The preparation and characterization of SiC-AlN-Fe2Si composites with high porosity from allophane

The reaction products of an allophane heated with carbon at 850–1600 °C in the stream of nitrogen for a given time were characterized by X-ray diffractometry. As a result, it was found that cristobalite and mullite were stable phases at 850–1300 °C, β-Si₃N₄ and α-Al₂O₃ at 1300–1500 °C, and SiC-AlN-Fe₂Si at temperatures higher than 1500 °C. SiC-AlN-Fe₂Si composites with high porosity of about 50% were easily prepared by a heat treatment at a temperature higher than 1500 °C with carbon in a stream of nitrogen. The formation mechanism of the composites is kinetically discussed from a viewpoint of small-pore shrinkage and large-pore expansion by volume diffusion during heating. The resultant microstructure of the composites obtained is also discussed.     

Thermal expansion coefficient and thermal fatigue of discontinuous carbon fiber-reinforced copper and aluminum matrix composites without interfacial chemical bond

Fully dense carbon fiber-reinforced copper and aluminum matrix (Cu–CF and Al–CF) composites were fabricated by hot press without the need for an interfacial chemical compound. With 30 vol% carbon fiber, the thermal expansion coefficients (TECs) of pure Cu and Al were decreased to 13.5 × 10^?6 and 15.5 × 10^?6/K, respectively. These improved TECs of Cu–CF and Al–CF composites were maintained after 16 thermal cycles; moreover, the TEC of the 30 vol% Cu–CF composite was stable after 2500 thermal cycles between ?40 and 150 °C. The thermal strain caused by the TEC mismatch between the matrix and the carbon fiber enables mechanical enhancement at the matrix/carbon fiber interface and allows conservation of the improved TECs of Cu–CF and Al–CF composites after thermal cycles.