Compressive strength of three-dimensionally reinforced carbon/carbon composite

Compressive behavior of three-dimensionally reinforced carbon/carbon composite (3D-C/C) was examined from room temperature to elevated temperatures up to about 3000 K. Three-dimensionally reinforced C/C was found to have an inclination to induce kinks at the ends of specimens due to extremely low shear strength. In order to avoid this type of premature fracture and to conduct high-temperature tests, discussion was made on specimen geometry and testing procedure, and the combination of a dumbbell-shape specimen and test configuration without a supporting jig were found to be suitable for the present study. Using this set-up, the compressive strength of a 3D-C/C was evaluated as a function of temperature up to about 3000 K. The compressive strength of the 3D-C/C monotonically increased with the increase in temperature up to 2300 K, but decreased above this temperature. The strength enhancement was suggested to be caused by improvement in the fiber/matrix interfacial bonding, and the degradation over 2300 K was by softening of the matrix at high temperatures.     

The preparation and performance of short carbon fiber reinforced adhesive for bonding carbon/carbon composites

A short carbon fiber reinforced adhesive for bonding carbon/carbon composites was developed. We found that when the thickness of the bonding layer was 80 μm, the concentration of short carbon fiber was 0.2 wt.%, and the heat-treatment temperature was 1000 °C, the adhesive could operate below 1700 °C and endure 20 times of thermal shock circles at 1500 °C. Finite element and micrograph analysis indicated that the bonding strength was larger than the interlaminar shear strength of carbon/carbon substrate, so that the fracture did not occur in the bonding layer but the carbon/carbon substrate. Weibull distribution analysis results showed that the Weibull modulus was 21.56 and the bonding strength was 11.43 MPa. We investigated that short carbon fiber could advance the tensile strength and thermal shock resistance of the adhesive, release residual stress and inhibit extension of micro-crack in the bonding layer.     

Low temperature mechanical behavior of 1D-Cf/SiO2 composite

Unidirectional carbon fiber reinforced fused silica (1D-C_f/SiO₂) composite was prepared by slurry infiltration and hot-pressing. The flexural strength and the coefficient of thermal expansion (CTE) at room and liquid nitrogen temperature (77 K) were investigated. The flexural strength of the composite tested at 77 K was 878 MPa, higher than that 667 MPa at room temperature. Moreover, the CTE of the composite at 77 K was higher than that at room temperature. Due to the difference of CTE between the matrix and fiber, gaps appeared at the fiber/matrix interface of as-prepared specimens. However, they may be healed up because of the thermal expansion of carbon fiber at 77 K. It led to a higher interfacial sliding resistance and changed the weak fiber/matrix interfacial bonding. Thus, it was helpful for the load transfer from matrix to fiber.     

A hot-pressing reaction technique for SiC coating of carbon/carbon composites

A hot-pressing reactive sintering (HPRS) technique was explored to prepare SiC coating for protecting carbon/carbon (C/C) composites against oxidation. The microstructures of the coatings were analyzed by X-ray diffraction and scanning electron microscopy. The results show that, SiC coating obtained by HPRS has a dense and crack-free structure, and the coated C/C lost mass by only 1.84 wt.% after thermal cycles between 1773 K and room temperature for 15 times. The flexural strength of the HPRS-SiC coated C/C is up to 140 MPa, higher than those of the bare C/C and the C/C with a SiC coating by pressure-less reactive sintering. The fracture mode of the C/C composites changes from a pseudo-plastic behavior to a brittle one after being coated with a HPRS-SiC coating.     

Comparison of 2D and 3D carbon/carbon composites with respect to damage and fracture resistance

The static mechanical responses of two- and three-dimensionally reinforced carbon/carbon composites (2D- and 3D-C/Cs) were compared. The mechanical properties examined included tensile and shear stress-strain (S-S) relations, and fracture behavior using compact tension and double edge notch configurations. Compared with 2D-C/Cs, 3D-C/Cs were shown to possess a similar tensile S-S relation, lower shear strength, higher ultimate deformation in shear, and much higher fracture resistance. The differences in shear and fracture resistance were shown to be derived from a weaker fiber/matrix interface and weaker bonding between fiber bundles in the 3D-C/Cs. These weak interface characteristics of 3D-C/Cs are due to the high value of residual stresses caused by the three-dimensional fiber constraint of 3D-C/Cs.     

Effects of high-temperature heat treatment on the mechanical properties of unidirectional carbon fiber reinforced geopolymer composites

Unidirectional carbon fiber reinforced geopolymer composite (C_uf/geopolymer) is prepared by a simple ultrasonic-assisted slurry infiltration method, and then heat treated at elevated temperatures. Effects of high-temperature heat treatment on the microstructure and mechanical properties of the composites are studied. Mechanical properties and fracture behavior are correlated with their microstructure evolution including fiber/matrix interface change. When the composites are heat treated in a temperature range from 1100 to 1300 °C, it is found that mechanical properties can be greatly improved. For the composite heat treated at 1100 °C, flexural strength, work of fracture and Young's modulus reach their highest values increasing by 76%, 15% and 75%, respectively, relative to their original state before heat treatment. The property improvement can be attributed to the densified and crystallized matrix, and the enhanced fiber/matrix interface bonding based on the fine-integrity of carbon fibers. In contrast, for composite heat treated at 1400 °C, the mechanical properties lower substantially and it tends to fracture in a very brittle manner owing to the seriously degraded carbon fibers together with matrix melting and crystal phases dissolve.     

Effect of carbon fiber surface functional groups on the mechanical properties of carbon-carbon composites with HTT

The present investigation describes the quantitative measurement of surface functional groups present on commercially available different PAN based carbon fibers, their effect on the development of interface with resol-type phenol formaldehyde resin matrix and its effect on the physico-mechanical properties of carbon-carbon composites at various stages of heat treatment. An ESCA study of the carbon fibers has revealed that high strength (ST-3) carbon fibers possess almost 10% reactive functional groups as compared to 5.5 and 4.5% in case of intermediate modulus (IM-500) and high modulus (HM-45) carbon fibers, respectively. As a result, ST-3 carbon fibers are in a position to make strong interactions with phenolic resin matrix and HM-45 carbon fibers make weak interactions, while IM-500 carbon fibers make intermediate interactions. This observation is also confirmed from the pyrolysis data (volume shrinkage) of the composites. Bulk density and kerosene density more or less increase in all the composites with heat treatment up to 2600 °C. It is further observed that bulk density is minimum and kerosene density is maximum upon heat treatment at 2600 °C in case of ST-3 based composites compared to HM-45 and IM-500 composites. It has been found for the first time that the deflection temperature (temperature at which the properties of the material start to decrease or increase) of flexural strength as well as interlaminar shear strength is different for the three composites (A, B and C) and is determined by the severity of interactions established at the polymer stage. Above this temperature, flexural strength and interlaminar shear strength increase in all the composites up to 2600 °C. The maximum value of flexural strength at 2600 °C is obtained for HM-45 composites and that of ILSS for ST-3 composites.     

Densification and properties of AlN ceramic bonded carbon

To obtain light and tough materials with high thermal conductivity, AlN ceramic bonded carbon (AlN/CBC) composites were fabricated at temperatures from 1600 to 1900 °C in a short period of 5 min by the spark plasma sintering technique. All AlN/CBCs (20 vol% AlN) have unique microstructures containing carbon particles of 15 μm in average size and continuous AlN boundary layers of 0.5-3 μm in thickness. With an increase in sintering temperature, AlN grains grow and anchor into carbon particles, resulting in the formation of a tight bonding layer. The AlN/CBC sintered at 1900 °C exhibited a light weight (2.34 g/cm³), high bending strength (100 MPa), and high thermal conductivity (170 W/mK).     

Effect of heat treatment on the tribological behavior of 2D carbon/carbon composites

The effect of high temperature heat treatment on the tribological behavior of carbon/carbon (C/C) composites has been investigated. C/C composite preforms were made from 1K PAN plain carbon cloth, and densified using rapid directional diffusion (RDD) CVI processes. Four specimens treated at 1800, 1800+2000, 2000, and 2300 °C, respectively, were prepared. A ring-on-ring specimen configuration was used to simulate aircraft brakes. The brake initial angular velocity ranged from 1800 to 7500 rpm (6.2-26.0 m s⁻¹ average linear sliding velocity). The specific pressure and moment of inertia were 392-784 kPa and 0.25-0.31 kg m², respectively (1.9-42.3 MJ m⁻² kinetic energy loading per unit friction surface area). The results showed that the stability of the brake moment-time curves increased with increasing heat treatment temperature (HTT) for the four composites, and those treated at 2300 °C possessed the lowest initial brake moment peak ratio values (from 1.1 to 1.3). The high degree of graphitization and low shear forces of the matrix carbon resulting from the high HTT could allow friction films to develop and reduce those values under the present brake conditions. The friction coefficients of four RDD CVI C/C composites decreased with an increase in specific pressure. The resulting changes in the friction coefficient of the four composites due to the specific pressure changes have basically nothing to do with the interface temperature under those conditions. According to the practical brake conditions, the friction properties of RDD CVD C/C composites could be improved by regulating the structure of the brake discs, changing the specific pressure exerted on the discs and the heat treatment. The linear wear rates of the four materials increased with increasing HTT. The composites treated at 2000 °C had both high enough friction coefficients and the lower linear wear rates. The different heat treatment methods at 2000 °C had no obvious effect on the friction and wear properties of RDD CVI C/C composites.     

Effects of infiltration conditions on the densification behavior of carbon/carbon composites prepared by a directional-flow thermal gradient CVI process

Multiple quasi three-dimensioned carbon fibre preforms with disk-like shape were simultaneously densified by a directional flow thermal gradient CVI process. The effects of infiltration conditions, including temperature (ranging from 850 to 1050 °C), temperature gradient (5 and 10 °C/mm), pressure (2.5, 5.0, 7.5 and 9.5 kPa) and the type of carrier gas (N₂ or H₂), on the densification behavior of the resultant carbon/carbon composites were investigated. The results showed that lower temperatures (below 900 °C), a larger temperature gradient and higher pressure are favorable for higher average bulk density and homogeneous infiltration. Carbon/carbon composites disks with an average bulk density of 1.78 g/cm³ were achieved in one CVI cycle at a total pressure of 9.5 kPa. It was also found that adding N₂ carrier gas has no pronounced influence on the densification of the preforms. As compared to N₂, H₂ had positive effects on the densification of the preforms for temperatures above 900 °C, but it had negative effects on the densification when the control temperature was as low as 850 °C.     

Densification of a 2D carbon fiber preform by isothermal, isobaric CVI: Kinetics and carbon microstructure

Chemical vapor infiltration of a 2D carbon fiber preform with a 0/0/90/90° fiber architecture and a fiber volume fraction of 22.5% was investigated as a function of methane pressure at various temperatures as well as a function of infiltration time at constant pressure. Inside-outside densification was obtained at the most attractive temperature of 1095 °C up to 29 kPa resulting in a maximum bulk density of 1.84 g cm⁻³ and a matrix density of 2.17 g cm⁻³, which corresponds to high-textured carbon. Texture formation can be perfectly explained with the earlier proposed particle-filler model. Studies at increasing infiltration times suggest a recrystallization of carbon deposited in the early stages of the infiltration.     

Lignin-based carbon fibers: Oxidative thermostabilization of kraft lignin

The thermostabilization of lignin fibers used as precursors for carbon fibers was studied at temperatures up to 340 °C at various heating rates in the presence of air. The glass transition temperature (T_g) of the thermally treated lignin varied inversely with hydrogen content and was found to be independent of heating rate or oxidation temperature. A continuous heating transformation (CHT) diagram was constructed from kinetic data and used to predict the optimum heating rate for thermostabilization; a heating rate of 0.06 °C/min or lower was required in order to maintain T_g > T during thermostabilization. Elemental and mass analyses show that carbon and hydrogen content decrease during air oxidation at constant heating rates. The hydrogen loss is sigmoidal, which is consistent with autocatalytic processes. A net increase in oxygen occurs up to 200-250 °C; at higher temperatures, oxygen is lost. Spectroscopic analyses revealed the oxidation of susceptible groups within the lignin macromolecule to ketones, phenols and possibly carboxylic acids in the early stage of the reaction; the later stage involving the loss of CO₂ and water and the formation of anhydrides and possibly esters. Slower heating rates favored oxygen gain and, consequently, higher glass transition temperatures (T_g) as opposed to faster heating rates.     

High temperature rearrangement of disordered nanoporous carbon at the interface with single wall carbon nanotubes

Composites of nanoporous carbon and single wall carbon nanotubes were heat treated in vacuum at temperatures ranging from 1200 to 2000 °C. The resultant interface between the two allotropes of carbon was characterized using high resolution transmission electron microscopy and Raman spectroscopy. At the interface between the nanoporous carbon and the nanotube, the nanotube served as a template for ordering and orientation of the normally disordered nanoporous carbon along the nanotube axis during high temperature treatment. When annealed at 2000 °C, the nanoporous carbon transformed to graphitic nanoribbon which in turn crushed the nanotube to form a nanoscale carbon “bulb”. This result is interesting since at these temperatures, the native nanoporous carbon is well known to resist ordering and is therefore referred to as being a “non-graphitizing” carbon. That the nanotube should act as a template for the incipient graphitization suggests that bonding and strength for load transfer may be developed at these interfaces.     

Amorphous carbon nanostructures from chlorination of ferrocene

The chlorination of ferrocene at different temperature conditions yields several carbon nanostructures, which were studied by means of transmission and scanning electron microscopies. Amorphous carbon nanotubes (α-CNTs) up to 10 μm long with thick walls and ∼15 nm of internal diameter were observed in a sample treated at 200 °C during 30 min. They consisted on ∼90% of carbon, while the remaining 10% consists on iron and chlorine. At this temperature, amorphous carbon bags and open-ended branches were also found. When chlorinating ferrocene at the same temperature but with longer reaction time (180 min), no α-CNTs were formed. At higher temperature (300 °C, 30 min), amorphous carbon bags were found, with lower content of residual chlorine and iron, and presenting thinner walls. In the sample treated at even higher temperature (900 °C, 30 min) the carbon nanobags (wall thickness ∼12 nm) were almost spherical and more graphitic, and without impurities.     

Raman spectroscopy studies of the high-temperature evolution of the free carbon phase in polycarbosilane derived SiC ceramics

The Raman spectra of a number of SiC ceramics synthesized from polycarbosilane at 1200 °C and annealed at 1400, 1600, 1800 and 2000 °C have been recorded using laser excitation wavelength of 532 nm. The peak positions, their intensities (I_D/I_G) and full width at half maximum (FWHM) were used to obtain information about the degree of disorder in the free carbon phases. The increasing ordering with annealing temperature was confirmed by lower FWHM values and G-peak positions obtained from the SiC ceramics annealed at higher temperature. However, the I_D/I_G has shown to be the highest point at 1600 °C, which illustrates that the temperature is one critical point of the microstructure evolution of the free carbon phase changing amorphous to turbostratic with increasing temperatures. Obviously, the oxidation behaviors of the SiC ceramics are significantly affected by the microstructures of the free carbon phases. In the SiC ceramics with above 1600 °C annealing, the oxidation temperatures of the SiC phases are postponed more than 100 °C, because they are surrounded by the free carbon phases.     

Titanium carbide derived nanoporous carbon for energy-related applications

High surface area nanoporous carbon has been prepared by thermo-chemical etching of titanium carbide TiC in chlorine in the temperature range 200-1200 °C. Structural analysis showed that this carbide-derived carbon (CDC) was highly disordered at all synthesis temperatures. Higher temperature resulted in increasing ordering and formation of bent graphene sheets or thin graphitic ribbons. Soft X-ray absorption near-edge structure spectroscopy demonstrated that CDC consisted mostly of sp² bonded carbon. Small-angle X-ray scattering and argon sorption measurements showed that the uniform carbon-carbon distance in cubic TiC resulted in the formation of small pores with a narrow size distribution at low synthesis temperatures; synthesis temperatures above 800 °C resulted in larger pores. CDC produced at 600-800 °C show great potential for energy-related applications. Hydrogen sorption experiments at −195.8 °C and atmospheric pressure showed a maximum gravimetric capacity of ∼330 cm³/g (3.0 wt.%). Methane sorption at 25 °C demonstrated a maximum capacity above 46 cm³/g (45 vol/vol or 3.1 wt.%) at atmospheric pressure. When tested as electrodes for supercapacitors with an organic electrolyte, the hydrogen-treated CDC showed specific capacitance up to 130 F/g with no degradation after 10 000 cycles.     

Ternary solubility of mono- and di-tert-butyl ethers of glycerol in supercritical carbon dioxide

Using a continuous flow apparatus, the ternary solubility of mono- and di-tert-butyl ethers of glycerol (MTBG and DTBG, respectively) in supercritical carbon dioxide was measured at the temperatures of 313.15, 333.15, and 348.15 K; a pressure range of 80-200 bar; and an expanded gas flow rate of 180 ± 10 mL min⁻¹ at average laboratory temperature of 300.15 K and pressure of 0.89 bar. The ternary solubility of the ethers at the constant temperatures of 333.15 and 348.15 K increased with increasing pressure up to the crossover point (i.e., 152 bar for MTBG and 170 bar for DTBG). MTBG exhibited a higher solubility than DTBG in scCO₂. The experimental data for the ternary solubility of MTBG and DTBG were correlated using the Bartle equation.     

Effect of synthesis temperature on hardness of carbon phases prepared from C60 and nanosized diamonds under pressure

Vicker’s hardness and Raman scattering spectra have been studied for carbon phases prepared from C₆₀ fullerene and nanosized diamonds at high temperatures and a pressure of 6 GPa. It was found that the hardness dependence on annealing temperature has a maximum near ∼1100 K for both fullerene and nanosized diamonds as initial materials. This temperature is only slightly higher than the temperature at which the C₆₀ cage collapses, and appears to correspond to the termination of intercluster bonding in the case of nanosized diamonds. The hardness maximum is interpreted as a result of competition between an increase in intercluster/intercage bonding and local instability for graphitic-like ordering.     

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.     

Mechanical properties up to 1100 °C of Al2O3–MgO-extruded graphite pellets castables reinforced with steel fibres

The influences of adding 19, 25 and 50 mm carbon steel and 19 mm stainless steel fibres in the range of 0–3 volume percentages on the mechanical properties of alumina–magnesia-extruded graphite pellet castables have been studied at intermediate temperatures between 800° and 1100 °C, in argon atmosphere, as well as at room temperature. Wedge splitting test results at room temperature have shown that both carbon and stainless steel fibres increases work of fracture. Same tests at 1100 °C have shown decrease in strength with all length and volume percentage of carbon steel fibre while improving work of fracture values as 50% over fibre-free castables. Strength degradation is caused by the defect generation in the castable structure.