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Carbon fibers, though important constituent as reinforcements for high performance carbon/carbon composites, are shadowed by their oxidation in air at temperatures beginning 450 °C. Owing to tailorable properties of carbon fibers, efforts are underway to explore structural modification possibilities to improve the oxidation resistance of the fibers and their composites. The pitch based ribbon shape carbon fibers are found to have highly preferential oriented graphitic structure resulting in high mechanical properties and thermal conductivity. In the present work oxidation behaviour of ribbon shape carbon fibers and their composites heat treated to 1000–2700 °C has been studied. SEM examination of these composites exhibits development of graphitic texture and ordering within the fibers with increase in heat treatment temperature. Oxidation studies made by thermogravimetric analysis in air show that matrix has faster rate of oxidation and in the initial stages the matrix gets oxidized at faster rate with slower rate of oxidation of the fibers depending on processing conditions of fibers and composites.

A novel photocrosslinkable and melt processible terpolymer precursor for carbon fiber has been successfully synthesized and characterized. The terpolymer was synthesized by an efficient emulsion polymerization route and has a typical composition of acrylonitrile/methyl acrylate/acryloyl benzophenone in the mole ratio, 85/14/1. It has been characterized by FTIR, NMR, intrinsic viscosity and GPC molecular weights. The composition of the monomer repeat units in the terpolymer was determined by NMR, and was almost identical to the molar feed ratios of the monomers used for polymerization. The Tg of the terpolymers, were somewhat a function of molecular weight, but were in the range 77‐91 8C. The fibers were spun from the terpolymer melts unlike the conventional solution spinning method. The terpolymers when stabilized with boric acid afforded a stable melt for about 30 min at 200‐220 8C, which was empirically found to be sufficiently long to spin fibers. The terpolymer with the highest molecular weight (Mn, w48,000) was not melt processible, apparently because the melt viscosity was very high and the terpolymer degraded fast. However, terpolymers, which had an intrinsic viscosity !0.6 dL/g (NMP, 25 8C) were invariably melt processible. The initial carbon fibers produced from these terpolymer fibers upon complete carbonization exhibited good mechanical properties for proposed automotive applications; the tensile strength of the best fibers generated thus far was in the range 450‐700 MPa with a strain to failure of w0.4%. The diameter of the carbon fibers was of the order of 7 mm. q 2006 Elsevier Ltd. All rights reserved.

In this exploratory study, polyacrylonitrile (PAN)-based carbon fiber tows are coated with a commercial, heat-crosslinkable polyetherimide (Ultem®) powder. Then an electrical current is applied across the powder-coated fiber tows (towpreg) to melt the thermoplastic powder and form rigidized composites. These consolidated composite samples are characterized by scanning electron microscopy (SEM) and conventional tensile testing. Measuring the applied current and the degree of consolidation yielded the power requirement for successful wetting of the carbon fiber tows by Ultem melt, both on earth and in outer space. The results show that resistive heating is a feasible route for the rigidization of inflatable composite structures in space.

Activated carbon fibers (ACFs) prepared from petroleum isotropic pitch and the same containing silver and cobalt nitrate, cobalt and palladium acetylacetonates, as well as mixtures of two salts with a total metal or metal mixture content of 1 wt% have been studied. The processing parameters are summarized for activated carbon fibers containing individual metals and metal mixtures. The results suggest that the generation of metal-containing particles and the formation of pore structure depend on many different factors including the composition of the metal and pitch precursors, the interaction of the metal and pitch precursors during the fiber production process, and the interaction between the two metal precursors when more than one metal salt is used. The addition of silver and cobalt in the form of nitrate salts enlarges the micropores and generates small mesopores with a narrow range of sizes. The addition of palladium as an acetylacetonate salt leads to the formation of both small micropores and larger mesopores. The cobalt additive as an acetylacetonate salt catalyzes the activation process, creating large mesopores and macropores. Mixing of two different metal precursors affects the particle composition and size. This, in turn, controls the pore structure of the final activated fibers. During activation, the two metal precursors can act independently (Ag/Co mixture). However, in other cases their effect can be additive (Co/Pd mixture), or even synergistic (Ag/Pd mixture).

A two-phase self-consistent model for large deformation stress–strain behavior and strain-induced crystallization in polymers at temperatures above the glass transition temperature is proposed. In this model, a composite framework is utilized to deal with the presence of the two phases, crystalline and amorphous, after the onset of strain-induced crystallization. The plastic behavior of each phase is approached by a widely used viscoplastic power law. The crystallization rate is expressed following a non-isothermal phenomenological expression based on the modified Avrami equation. Our predicted results are compared to the upper and lower bound estimates and to the existing experimental results in which good agreement is found with these experiments.

The rheology of a liquid crystalline mesophase pitch is reported. Over the range of shear rates and temperatures investigated, the steady state shear viscosity and the first normal stress difference display region 1 and region 2 flow behavior typical of many liquid crystalline systems. A kink in the transition between regions 1 and 2 is also observed. The first normal stress difference is negative at low shear rates, but transitions to positive values in the shear rate region of the kink in the viscosity. These phenomena are explained by change in the molecular orientation of the sample. The evolution of the shear stress in transient startup and relaxation experiments is measured. There is a distinct overshoot in the startup of shearing, and a rapid decay of stress upon cessation of flow. As expected, the magnitude of the overshoot depends on the rest time between shearing. The estimated elastic constant is consistent with other work.

A low-cost route for producing PAN-based carbon fibers is being developed. The approach involves forming polyacrylonitrile terpolymers that can be melt-spun into fibers. The fibers are then stabilized and carbonized to yield carbon fibers. Melt-processibility, however, precludes direct thermal stabilization of these polymeric fibers. Therefore, a precursor terpolymer containing acryloyl benzophenone (ABP) is used. The UV sensitivity of ABP moiety enhances the UV crosslinkability of the precursor fibers. After a brief exposure to UV radiation, the melt-spun terpolymer fibers can be oxidatively stabilized at 320 °C without melting and subsequently carbonized. UV–visible and ATR-IR spectroscopic analyses suggest that UV radiation induces the formation of free radicals which, in turn, cyclize the PAN. Cyclized PAN was characterized by a strong absorbance in UV–visible region (300–500 nm) due to conjugated >C C C N– bonds which were also detected by infrared spectroscopy.

Pure and silver-containing carbon fibers were prepared from isotropic pitch precursors supplied by Conoco, Inc., and a Korean research team and activated in carbon dioxide to varying degrees of burn-off. The specific activation rates for the carbon fibers were measured as well as the nitrogen adsorption characteristics of the activated carbon fibers. Scanning electron microscopy was used to investigate the surface morphology and the behavior of silver particles during the activation process. Molecular composition of the two pitch precursors was determined using a gas chromatograph mass spectrometer and a MALDI TOF mass spectrometer. Results showed that specific surface area increased with the burn-off, and the trends were similar for the pure and silver-containing fibers formed from both isotropic pitch precursors. However, the catalytic behavior of silver during activation, the activation rate, and even the pore characteristics of the activated fiber were found to be dependent on the molecular composition of the precursor pitch.

A constitutive model for large deformation stress–strain behavior and strain-induced crystallization in poly(ethylene terephthalate), at temperatures above the glass transition temperature, is proposed. In this model, the intermolecular resistance is treated in a composite framework where the crystalline and amorphous phases are considered as two separate resistances coupled through two different analog representations leading to the upper and the lower bound approaches. The crystallization rate is expressed following a non-isothermal phenomenological expression based on the modified Avrami equation. Our predicted results are compared to existing experimental results and good agreement is found.

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.

The objective of this work was to provide a comprehensive rheological and structural study of AR mesophase pitch. The shear viscosity of AR mesophase pitch was found to be qualitatively similar to results from prior investigations of mesophase pitch and followed the typical trend for liquid crystalline fluids. However, a distinct hesitation, or kink, was observed in the shear viscosity curve near the shear thinning to constant viscosity transition. This behavior has been seen previously for some low molecular weight and polymeric liquid crystals and was thought to be due to a transition between tumbling and steady alignment of the uniaxial director. Optical studies of mesophase pitch revealed an orientation change of the poly-domain structure at shear rates where the kink was observed. This orientation change results in a high viscosity to low viscosity transition. This viscosity transition, and not tumbling, is responsible for the kink phenomenon. The optical studies also reaffirmed that shear flow reduces the size of the poly domain structure and elongates the domains along the flow direction. Also under quiescent conditions, the poly-domain size increased with increasing relaxation time.

The objective of this research was to evaluate the effectiveness of several different methods for controlling the pore size and pore size distribution in activated carbon fibers. Variables studied included fiber shape, activation time, and the addition of small amounts of silver nitrate. Pure isotropic pitch and the same isotropic pitch containing 1 wt.% silver were melt spun to form fibers with round and trilobal cross sections. These fibers were then stabilized, carbonized, and activated in carbon dioxide. Field emission scanning electron microscopy (FE SEM), electron dispersive spectra (EDS), and wavelength dispersive spectra (WDS) were used to monitor the size and distribution of the silver particles in the fibers before and after activation. Each of these analyses showed that the distribution of silver particles was extremely uniform before and after activation. The fibers were also weighed before and after activation to determine the percent burn-off. The BET specific surface areas of the activated fibers were determined from N2 adsorption isotherms measured at −196 °C. The results showed that round and trilobal fibers with equivalent cross-sectional areas yielded similar burn-off values and specific surface areas after activation. Also, activation rates were found to be independent of CO2 flow rate. The porosity of the activated fibers depended on the total time of activation and the cross-sectional area of fibers. The N2 adsorption measurements showed that the activated fibers had extremely high specific surface areas (greater than 3000 m2/g) and high degrees of meso- and macro-porosity. FE SEM was also used to investigate surface texture and size of pore openings on the surfaces of the activated fibers. The photos showed that silver particles generated surface macro- and mesopores, in agreement with the inferences from N2 adsorption measurements.

Flow-induced orientation of discotic mesophase precursor typically leads to a center-origin, radial microstructure in the resulting carbon fibers. In the present study, we report the microstructure of carbon fibers that were derived from mesophase pitch that was modified using carbon multi-wall nanotubes (MWNTs). For an MWNT content as low as 0.1 wt.%, the microstructure was found to be fairly random (rather than radial).

Poly(acrylonitrile)(PAN) solutions in dimethylformamide(DMF) were electrospun to prepare webs consisting of 400nm ultra-fine fibers. The webs were oxidatively stabilized, activated by steam and resulted to be activated carbon fibers(ACFs). The specific surface area was , which showed a trend of a decrease of the surface area with an increase in activation temperature, showing opposite behavior to the other ACFs. The activation energy of the stabilized fibers for the steam activation was determined as 29.2 kJ/mol to be relatively low indicating the easier activation than that of other carbonized fibers. The ACF webs were characterized by pore size and specific surface uea which would be related to the specific capacitance of the electrical double layer capacitor (EDLC). The specific capacitances measured were 27 F/g, 25 F/g, 22 F/g at the respective activation temperature of , showing similar trend with the specific surface area i.e., the higher activation temperature was, the lower specific capacitance resulted.

Mixtures of two mesophase pitches derived from different raw material sources were investigated for producing high-performance carbon fibers. In particular, a high-melting, supercritically extracted (SCE) mesophase that was hard to spin but produced fibers with excellent properties was combined with AR mesophase, a relatively low-melting material. Mixtures of the SCE and AR mesophases ranging from 25 to 75% SCE were melt-spun into fiber form. The addition of 25% AR to the SCE mesophase dramatically improved its spinnability to the point that fiber yields were similar to those obtained for 100% AR. Surprisingly, fibers produced from the mesophase mixtures all stabilized more rapidly than those produced from either of the pure mesophases, with the fastest weight gain being obtained for the 25% SCE–75% AR mixture. DRIFTS was used to follow the stabilization reactions and confirmed these results. In general, final fiber properties of the mesophase mixtures were also superior to those of the pure mesophases. For example, significant improvements in both the thermal conductivity and Young’s modulus of AR mesophase were obtained with the addition of 25% SCE mesophase.

The composites of tomorrow will need to be developed in advanced engineering environments. This transformation to computer-based materials design requires a transformation of traditional engineering education programs. Students need training in systems design, simulation and visualization technologies, teamwork, and communication to function effectively in distributed, diverse, collaborative work environments. The programs being coordinated and implemented by CAEFF are designed to meet these challenges and constitute a paradigm shift in engineering education.

Previous work at Clemson University has shown that ribbon-shaped mesophase pitch-based carbon fibers graphitized at only 2400°C can develop thermal conductivities comparable with those of commercial round-shaped pitch-based carbon fibers graphitized at temperatures above 3000°C. The thermal and electronic transport properties (i.e. thermal conductivity and electrical resistivity) of ribbon-shaped carbon fibers produced at Clemson University are being studied. In addition, the structure of these fibers is being analyzed by electron microscopy and X-ray diffraction techniques. This paper will discuss the relationships between processing conditions, fiber structure and fiber properties.

The present work reports a comparative study of alternative models to describe the tensile strength of carbon fibers. The theoretical models were validated using data determined for a wide range of materials. Fibers derived from PAN and pitch, with different geometries (circular and ellipsoidal cross section), with and without surface treatment, were tensile tested. A mixed-mode Weibull distribution function, adapted for the length dependence of fiber strength, was used to model the tensile data, assuming the weakest link approximation. This function is capable of describing the effect of flaws and crystalline misalignment on fiber tensile strength. Additionally, the ‘end-effect’ model was employed to distinguish between these failures and those that result from the test method itself, namely the influence of the machine clamps. It was observed that the relative importance of end-effects increases as the length or the aspect ratio of the fiber decreases. For most fibers studied at longer gauge lengths, a two-parameter Weibull distribution could adequately fit the strength data. Thus, in these cases, a dominant flaw population may be responsible for fiber failure. Based on the results of this study, criteria are proposed for selecting the most appropriate statistical models to predict the tensile strength of carbon fibers at small gauge lengths.

The use of mesophase pitch-based carbon fibers in composite materials has been limited by their poor oxidation resistance at high temperatures. Ribbon-shaped mesophase fibers with excellent axial thermal conductivity were spun at Clemson University and coated with SiC at the University of Erlangen-Nurnberg using chemical vapor deposition (CVD) in an effort to protect them from high-temperature oxidation. Using the microbond technique with a newly developed microfixture to apply an axisymmetric load to each specimen, the failure modes and interfacial shear stress (IFSS) were investigated for untreated ribbon fibers and ribbon fibers with 0.75-μm and 1.2-μm thick SiC coatings. For microbond testing, an Epon® 828-based epoxy was used as a model thermoset matrix material. The three distinct failure modes found for SiC-coated fibers included microdrop debonding, fiber breakage, and coating failure. The uncoated fibers exhibited the debonding failure mode for approximately 90% of the samples. However, the fibers with a 0.75-μm thick coating exhibited all three failure modes with uniform frequency, while the fibers with a 1.2-μm thick coating exhibited coating failure for 70% of the samples. The SiC coating was found to increase the IFSS by approximately 40%. However, the coating thickness had no effect on the increase of the IFSS.

The thermal conductivity of ribbon-shaped fibers produced at Clemson University and graphitized at 2400 degrees C and of commercial round fibers graphitized at temperatures above 3000 degrees C were measured by two different methods. One instrument, located at BP Amoco in Alpharetta, GA, measures a fiber sample's thermal response to an oscillating heat input. This device was used to measure room-temperature thermal conductivity of the fibers. A second apparatus, located at the University of Louvain-la-Neuve in Belgium, measures a fiber sample's thermal response to a controlled thermal gradient. This device was used to measure the temperature dependence of the thermal conductivity of selected fiber sets. The results will be presented and the accuracy of the two methods compared. (C) 2000 Elsevier Science Ltd. All rights reserved.

Towpregs are a new class of pre-impregnated materials that can be used to obtain structural composites by compression molding. Consolidation is an important part of this process. In this work, a mathematical model was developed to predict the evolution of the compression pressure during the isothermal consolidation of thermoplastic matrix towpregs. The model relates the applied pressure to operational variables (speed and temperature) and material variables (viscosity, particle size, and polymer volume fraction). Tests were carried out at different temperatures and platen closing speeds, using carbon fiber-reinforced polycarbonate towpregs. The data obtained was compared with theoretical calculations, demonstrating the applicability of the model.

A finite-element model has been developed to predict the thermal conductivities, parallel and transverse to the fiber axis, of unidirectional carbon/carbon composites. This versatile model incorporates fiber morphology, matrix morphology, fiber/matrix bonding, and random distribution of fibers, porosity, and cracks. The model first examines the effects of the preceding variables on the thermal conductivity at the microscopic level and then utilizes those results to determine the overall thermal conductivity. The model was able accurately to predict the average thermal conductivity of standard pitch-based carbon/carbon composites. The model was also used to study the effect of different composite architectures on the bulk thermal conductivity. The effects of fiber morphology, fiber/matrix interface, and the ratio of transverse fiber conductivity to matrix conductivity on the overall composite conductivity was examined.

Silver-containing pitch-based carbon fibers were prepared and activated in steam. SEM and TEM were used to investigate the surface morphology and the behavior of the silver particles in fibers. Physical properties such as density, tensile strength, and electrical resistivity were measured. The SEM photos of fibers containing silver (at initial concentrations of 1000 and 10 000 ppm) were similar to those of non-activated carbon fibers at high level burn-off. Silver particles accelerate the activation rate. However, the specific surface areas of silver-containing activated carbon fibers were similar to those of non-silver containing activated carbon fibers. The apparent density and the tensile strength of the 10 000 ppm silver-containing carbon fibers were 1.677g/cm3 and 24 kgf/mm2, and these decreased to 0.795 g/cm3 and 6 kgf/mm2, respectively, at 69% burn-off. The electrical resistivity of isotropic pitch-based carbon fiber was 97 μΩ m. By comparison, as the initial silver content of the fiber was increased to 1000 and 10 000 ppm, the resistivity decreased to 69 and 57 μΩ m, respectively. These resistivities depended on the total pore volume and increased exponentially with increasing specific surface area of fibers.

A commercial impregnating coal-tar pitch was thermally treated at 430°C for 2–5 h in nitrogen. Resultant pitches, which contained mesophase ranging from 10 to 46 vol.%, were filtered to separate the isotropic phases, in order to study their contribution to the rheology of the whole pitches. The parent pitch, thermally treated pitches and the individual isotropic phases were characterized by measuring their elemental composition, softening point, carbon yield, solubility in toluene and in N-methyl-2-pyrrolidone, mesophase content (by optical microscopy) and X-ray diffraction parameters. The rheological behavior of the pitches and the isotropic phases was studied using steady shear rheometry. Results showed that not only had the whole pitches, which were partially anisotropic, a non-Newtonian behavior, but also the single isotropic phases.

Naphthalene-based mesophase ribbon fibres were studied in the present work. The fibres were surface treated in a plasma reactor and their mechanical and surface characteristics determined before and after the surface oxidation. The dispersive and non-dispersive components of the surface energy were obtained by contact angle measurements using a tensiometric method, and the surface oxygen content was measured by XPS. The mechanical characterisation was made by single filament tensile tests. The interactions between the ribbon fibres and a polycarbonate matrix were assessed by means of fragmentation tests. The results were compared with those obtained with pitch-based fibres with circular cross-sections.

A powder coating technique was used to produce carbon fiber towpregs with low fiber content. Unidirectional composites obtained from this material were subjected to mechanical testing. The tensile strength of the carbon fibers was determined from experimental data, using the Weibull and log-normal distribution functions. The properties of the composites are comparable with those of a commercial Sheet Molding Compound (SMC), and could be predicted using the Classical Lamination Theory. It is shown that the tensile strength of the composites is more sensitive than the modulus to processing derived defects.

Chemical impurities (principally sulfur) derived from petroleum-based precursor materials can impact the graphitization process by volatilizing during high temperature thermal treatment. In an attempt to eliminate the crystal damage caused by the evolution of sulfur-bearing gases from petroleum-based fibers, the present research utilizes a chemically pure precursor synthetically derived from naphthalene. Structural and chemical changes were then monitored as a function of heat treatment temperature with X-ray diffraction and spectroscopic techniques in an attempt to better understand how the graphitic crystal structure develops in these materials. As is the case with fibers produced from alternative precursors, gas evolution during the thermolysis region (i.e.

PBO [poly(p-phenylenebenzobisoxazole)] fiber has been shown to convert to an ordered carbon fiber without the need for stabilization. This article presents the first detailed analysis of the carbonization and graphitization behavior of this unique material. The carbonization process was modeled as a series of free-radical reactions, and thermogravimetric analysis was used to determine an activation energy of 76 ± 6 kcal/mol for the thermal initiation of free radicals. The initiation reaction data then were applied to determine the temperature dependence of the termination reaction. Additionally, the development of long-range order in the graphitizing fiber was examined. The spacing between graphene planes was shown to decrease with increasing treatment temperature and soak duration. Carbonized PBO fibers developed more long-range order than carbon fibers produced from other polymers, which may partially explain why these PBO-based fibers display excellent lattice-dependent properties. Finally, an Arrhenius analysis found the activation energy for graphitization to be 120 ± 17 kcal/mol.

Poly p-phenylene benzobisoxazole (PBO) converts directly to carbon fiber without stabilization. However, the applicability of PBO-based carbon fibers is limited by their low tensile strength. This paper represents the first study of the high-temperature conversion of PBO to carbon fiber and examines some factors leading to this low tensile strength. A mixed-mode Weibull analysis implies that the flaws present in the precursor fiber persist throughout carbonization and cause the tensile failure of the carbonized fiber. Improvements in the PBO spinning process resulted in increased tensile strengths for the carbonized fibers. Further, the tensile strengths of the carbonized fibers appear to be related to the release of nitrogen during the onset of crystallite growth. The rapid heating rates associated with continuous carbonization were shown to minimize the negative effects of this nitrogen release. Fibers heated to 1600 °C in a continuous operation displayed nearly double the tensile strength of those produced in a batch operation. Ultimately, this study showed that, while the tensile strengths of PBO-based carbon fibers can be increased by using a continuous process, enhancements in the solution spinning process of the polymer will be required for PBO to become a commercially viable carbon fiber precursor.

The textures and microstructures of three different experimental mesophase pitch-based ribbon fibers were studied using the complementary techniques of scanning and transmission electron microscopy and wide angle X-ray diffraction. The effect of mesophase pitch precursor and capillary entry design on the structure and heat transfer capabilities of these ribbon-shaped fibers was investigated. It was found that ribbon fibers melt-spun from a synthetic, naphthalene-based mesophase had higher degrees of preferred orientation and graphitization, larger crystallite sizes and lower electrical resistivities than those of fibers melt-spun from a petroleum-based, heat-soaked mesophase. However, capillary entry design proved to be critical to the development of a more linear transverse texture and a structure that one might expect to be more conducive to axial heat transfer.

Mesophase pitch-based carbon fibers are ideally suited for applications where the rapid dissipation of heat is important (structural composites, printed circuit substrates, etc.). However, control of texture is essential in order to optimize fiber properties, especially thermal conductivity. Because ribbon-shaped mesophase pitch-based carbon fibers possess a highly linear texture, these fibers are very promising for high thermal conductivity applications. The observed texture of these ribbon-shaped fibers and the radial texture commonly observed in circular fibers can be predicted from liquid-crystal flow theory, where the precursor molecules are modeled as rigid disks. In order to determine the orientation of flowing mesophase pitch, a capillary rheometer was modified to quench capillaries during extrusion. Aluminum capillaries were constructed with rectangular channels down the center (with a 9:1 aspect ratio). The capillaries containing the mesophase samples were mounted in low-viscosity epoxy resin and cured. The desired mesophase surface (transverse or longitudinal) was isolated by grinding and polishing the samples to 3 gmm. Then the pitch surface was observed optically under cross-polarized light. The transverse surfaces of mesophase pitch indicated an orientation pattern where the aromatic rings lie perpendicular to the channel walls. This result is consistent with that predicted by the liquid-crystal flow theory, as well as with the textures of carbonized fibers.

Analytical solutions to the Leslie-Ericksen equations are discussed for flow through a circular capillary and a rectangular channel. The analysis allows for a director component out of the flow-gradient plane. The resulting orientation profiles match the commonly observed textures of mesophase pitch-based carbon fibres, specifically the radial and line-origin textures. Thus, this formulation would appear to be correct for the flow of discotic mesophase pitches. Furthermore, the velocity profile for capillary flow suggests the possibility of measuring the Leslie coefficient α4 for a mesophase pitch using a capillary rheometer.

Mesophase pitch-based carbon fibers with circular and non-circular (ribbon, C-shaped) cross sections were coated with a thin, approximately 100–150 nm SiC-layer, using a CVD technique. The surface free energy of the uncoated and SiC-coated carbon fibers was determined by measuring the contact angle of a variety of liquids with known polar and dispersive components of their total surface free energy. It was shown that the polar and dispersive components of the total surface free energy of the carbon fibers depends strongly on both the SiC-layer as well as on the shape of the cross section.

A continuous powder coating process was used to produce flexible, preimpregnated towpreg from a heat-treated Mitsubishi AR mesophase pitch powder (AR-120) and three different carbon fibers: T300 PAN-based fiber, P55 pitch-based fiber, and an experimental high thermal conductivity pitch-based ribbon fiber. The towpreg was hot-pressed into unidirectional composites, carbonized at 1100 °C, oxidized and then graphitized at 2400 °C. As expected, the PAN-based fibers developed strong fiber/matrix bonding and the pitch-based fibers developed poor fiber/matrix bonding. This resulted in high flexural strengths (841 MPa) in the graphitized composites reinforced with the T300 fibers and low flexural strengths (196 MPa) in the graphitized composites reinforced with the P55 fibers. In addition, it was found that during consolidation the ribbon fibers oriented normal to the pressing direction. The thermal conductivity (parallel to the fibers) of the graphitized T300/AR-120 and P55/AR-120 composites was 80.5 and 135.5 W/m · K, respectively. These results, along with X-ray analysis, indicated a significant development of preferred crystalline order (parallel to the fibers) upon graphitization at 2400 °C. The composites reinforced with ribbon fibers exhibited three-dimensional anisotropy, with a thermal conductivity (transverse to the fibers) of 213.5 W/m · K, higher than that parallel to the fibers (145 W/m · K). These results indicated that fiber shape can affect matrix properties in carbon/carbon composites. Finally, the towpreg was woven into a two-dimensional fabric, demonstrating that towpreg can be used to produce preimpregnated multidimensional composite preforms. Towpreg may provide a low-cost route for producing carbon/carbon composites.

Poly p-phenylene benzobisoxazole (PBO) fiber appears to display unusual characteristics during its conversion to carbon fiber. This introductory study focused on specific aspects of the carbonization behavior that differentiate PBO from other materials. Results showed that oxidative stabilization did not affect the tensile properties of carbonized fiber produced from PBO precursor fiber. Unlike other polymeric precursors, unstabilized PBO fibers could be directly converted to carbon fibers with promising mechanical and thermal properties. The carbonization characteristics of unstabilized PBO were found to be similar to those of stabilized PAN-based fibers. However, the PBO-based carbon fibers exhibited a radial texture similar to pitch-based fibers. Surprisingly, the PBO-based carbon fibers exhibited electrical resistivities in the range of commercial pitch-based fibers, implying that their thermal conductivities also will be similar. Low-temperature carbonization studies showed that PBO fiber can be carbonized at rapid rates without adversely affecting the tensile properties of the carbonized material. Its ability to be directly carbonized, combined with the unusual thermal properties of the final carbon fiber, may make PBO an attractive precursor for some carbon fiber applications.

Their extremely high thermal conductivity, combined with their relatively low density, make mesophase pitch-based carbon fibers attractive for many applications where heat transfer is critical. Although many thermal management applications could create large markets for mesophase fibers, the current high cost of these fibers makes their use uneconomical. The objective of the present research is to produce low-cost, mesophase pitch-based carbon fibers with high mechanical and thermal properties. In the current study, circular and ribbon fibers were produced from a naphthalene-based mesophase. After stabilization and carbonization, their mechanical and electrical properties were compared to fibers produced at similar conditions, but using a heat-soaked mesophase precursor. The ribbon fibers produced from the naphthalene-based mesophase exhibited higher moduli and electrical resistivities than round fibers formed from the same precursor. Also, the mechanical and electrical properties of the naphthalene-based ribbon fibers were superior to ribbon fibers previously produced using a heat-soaked mesophase and heat treated at equivalent conditions. At carbonization temperatures of only 2250°C, the ribbon fibers produced from naphthalene-based mesophase developed electrical resistivities as low as 2.68 μohm · m, a factor of three lower than those previously produced from the heat-soaked mesophase. Thus, ribbon fibers formed from naphthalene-based mesophase should exhibit higher thermal conductivities than either round fibers formed from the same precursor or ribbon fibers formed from a heat-soaked precursor. An additional benefit is that fibers formed from naphthalene-based mesophase develop excellent properties at relatively low carbonization temperatures.

Their high thermal conductivity and low density make pitch-based carbon fibers an attractive alternative to conventional metals in heat transfer applications. Already, thermal conductivities of up to 1100 W/m-°K, about three times that of copper, have been reported. These high conductivities are possible because of the excellent phonon conduction in the two-dimensional graphite layer plane. Thus, the perfection of the graphitic structure to a large extent determines the thermal conductivity of a carbon fiber. In this study, circular fibers exhibiting radial transverse texture and ribbon-shape fibers of linear texture were melt spun from a mesophase pitch precursor. After equivalent oxidation and carbonization treatments, the fibers were characterized by single filament tensile and electrical resistivity tests. The strong inverse correlation of electrical resistivity and thermal conductivity allows the use of electrical resistivity measurement as a reliable predictor of thermal properties. In addition, differential scanning calorimetry (DSC) and wide angle X-ray diffraction techniques were used to determine whether fiber texture can influence graphitization kinetics. The results indicated that linear textures of the ribbon-shaped fibers allow them to exhibit a lower electrical resistivity than circular fibers of equivalent tensile moduli. The electrical resistivity of the ribbon-shape fibers decreased with increasing aspect ratio, carbonization temperature, and dwell time during carbonization. Thus, ribbonshape fibers with a linear texture should exhibit higher thermal conductivities than circular fibers, and their thermal conductivities may increase further with higher aspect ratios.

Graphitization of high-sulfur content mesophase pitch fibers is complicated by the evolution of sulfur-bearing gases at high temperatures. Removal of these gases leads to appreciable strain and crystal misorientation if large structural units are present. The most obvious result of gas removal is “puffing,” an increase in the fiber cross-sectional area, accompanied by a decrease in lattice-dependent properties such as elastic modulus and conductivity. In this study, the effect of sulfur removal on fiber crystal structure is addressed. X-ray diffraction and differential scanning calorimetry are used to characterize the structure resulting from various heating rates, maximum temperatures, and dwell times. High heating rates during the sulfur removal period (from 1600 to 2000°C) were found to lessen the severity of fiber damage. When followed with a slow, high-temperature annealing treatment, a well-ordered graphitic structure was produced. Finally, a heat treatment strategy is outlined to improve the elastic properties of fibers produced from a high sulfur content mesophase.

Short beam shear, double cantilever beam, and torsional dynamic mechanical (DMA) tests were performed on composite materials systems to characterize the interfacial bond strength. The composite materials were unidirectional flat laminate with different fiber surface treatments and sizings. The results indicated that the DMA test was superior to the short beam shear and double cantilever beam test for measuring fiber/ matrix bonding. The DMA tests detected the effect of surface treatment with much greater accuracy than the standard mechanical tests. Also, DMA tests readily detected the differ ence in fiber/matrix bonding of PAN-based and pitch-based fibers. The DMA test had the additional feature of being able to isolate fiber/matrix interface effects from matrix and fiber effects.

Naphthalene-based mesophase, supplied by Mitsubishi Gas Chemical Co., Inc., (under the commercial name “AR” mesophase) and a heat-soaked mesophase pitch were melt-spun into fiberform using a pilot-scale extruder. The oxidation conditions of the as-spun fibersformed from each mesophase were optimized (based on tensile strength), and all fibers were graphitized at 2400°C. The naphthalene-based mesophase was found to be more spinnable, despite the fact thatits viscosity was more temperature-dependent than that of the heat-soaked mesophase. Also, fibers formed from the naphthalene-based mesophase were found to stabilize more rapidly than those formed from the heatsoaked mesophase. After carbonization, the naphthalene-based mesophase fibers exhibited much higher tensile strengths and moduli than equal-sized fibers produced from heat-soaked mesophase. The naphthalene-based mesophase fibers also exhibited lower electrical resistivities. Wide angle X-ray diffraction showed that the naphthalene-based mesophase fibers developed a smaller average interplanar spacing of the graphite basal planes and a larger average crystallite size than the heat-soaked mesophase pitch-based fibers.

A mathematical model was developed for the torsional dynamic response of unidirectional composites with imperfect bonding between the fibers and matrix. The model is basically an assemblage of homogeneous isotropic cylinders with different dynamic properties. The interfacial region where the fibers and matrix are chemically and mechanically bonded was modeled as a nonuniform interphase. The interphase, containing both a stiff region and a soft region, was incorporated into a three-phase model. To study the effects of the interphase on the dynamic response of the composites, the dynamic modulus of the interphase was varied to simulate fiber/matrix bonding conditions. A low storage modulus and high loss modulus of the interphase, corresponding to poor interfacial bonding, were found to result in more damping. The torsional dynamic response for a pitch-based carbon fiber/epoxy composite system with different fiber volume fractions was evaluated with the current models. The dynamic response of the composites predicted by the three-phase model agreed well with values measured by torsional dynamic mechanical test, while the two-phase model with perfect bonding between the fibers and matrix not only underestimated the loss modulus but failed to predict the trend of the loss tangent of the composites. These observations indicated that a viscoelastic interphase was significant and necessary to obtain good correlation with experimental results.

Pitch-based carbon fibers are formed by melt spinning a liquid crystal, mesophase precursor into fiber form. This fiber is then heat treated to develop its modulus and strength. In the present study, a two-stage heat treatment process was evaluated and found to yield an excellent mesophase precursor for carbon fibers. In one variation of the process, a reflux was applied during an initial heat treatment of the isotropic pitch, and then the pitch was sparged with nitrogen during a final stage of pyrolysis. In the second variation, the isotropic pitch was heat treated under a moderate pressure prior to a final stage where, again, nitrogen was sparged through the pitch during pyrolysis. Both of these two-stage processes produced mesophase which could be readily spun at approximately the same temperature as mesophase produced by solvent extraction. Also, the physical properties of the final heat treated fibers were quite comparable to those of the solvent extraction mesophase fibers.

A continuous powder coating system was developed for coating carbon fiber with LaRC-TPI (Langley Research Center-Thermoplastic Polyimide), a high-temperature thermoplastic polymide invented by NASA-Langley. The coating line developed used a pneumatic fiber spreader to separate the individual fibers. The polymer was applied within a recirculating powder coating chamber then melted using a combination of direct electrical resistance and convective heating to make it adhere to the fiber tow. The tension and speed of the line were controlled with a dancer arm and an electrically driven fiber wind-up and wind-off. The effects of heating during the coating process on the flexibility of the prepreg produced were investigated. The uniformity with which the fiber tow could be coated with polymer also was examined. Composite specimens were fabricated from the prepreg and tested to determine optimum process conditions. The study showed that a very uniform and flexible prepeg with up to 50 percent by volume polymer could be produced with this powder coating system. The coating line minimized powder loss and produced prepeg in lengths of up to 300 m. The fiber spreading was found to have a major effect on the coating uniformity and flexibility. Though test results showed low composite tensile strengths, analysis of fracture surfaces under scanning electron microscope indicated that fiber/matrix adhesion was adequate.

The properties of carbon/carbon composites like others depend on the properties of the reinforcing carbon fibers, fiber architecture and matrix microstructure. The processing of these composites with alternative reinforcements and their influence on matrix microstructure are of significant interest. Carbon/carbon composites have been prepared using PAN & Pitch based stabilized fibers with PFA and binder pitch as matrix precursors. These composites have been heat treated to different temperatures in the range 550°-3000°C. Optical texture, SEM and XRD examination have been used to study development of graphitic texture and ordering in these composites as function of heat treatment. It has been observed that PAN based fibers have higher surface activity resulting in high thermal stresses in the heat treated composites than pitch based fibers. On heat treatment to >2700°C latter composites exhibit better graphitic ordering than former composites, both the composites exhibit high degree of graphitization. However, the maximum contribution is coming from fibers themselves.

The carbon nanoparticles obtained from either arcing of graphite under water or thermal annealing of nanodiamonds are commonly called carbon nano onions (CNOs), or spherical graphite, as they are made of concentric fullerene cages separated by the same distance as the shells of graphite. A more careful analysis reveals some dramatic differences between the particles obtained by these two synthetic methods. Physicochemical methods indicate that the CNOs obtained from nanodiamonds (N-CNOs) are smaller and contain more defects than the CNOs obtained from arcing (A-CNOs). These properties explain the enhanced reactivity of the N-CNOs in cycloaddition and oxidation reactions, as well as in reactions involving radicals. Given the easier functionalization of the N-CNOs, they are the most obvious choice for studying the potential applications of these multi-shelled fullerenes.

Efficient storage of hydrogen for use in fuel cell-powered vehicles is a challenge that is being addressed in different ways, including adsorptive, compressive, and liquid storage approaches. In this paper we report on adsorptive storage in nanoporous carbon fibers in which palladium is incorporated prior to spinning and carbonization/activation of the fibers. Nanoparticles of Pd, when dispersed in activated carbon fibers (ACF), enhance the hydrogen storage capacity of ACF. The adsorption capacity of Pd-ACF increases with increasing temperature below 0.4 bar, and the trend reverses when the pressure increases. To understand the cause for such behavior, hydrogen uptake properties of Pd with different degrees of Pd-carbon contact (Pd deposited on carbon surface and Pd embedded in carbon matrix) are compared with Pd-sponge using in situ XRD under various hydrogen partial pressures (Rietveld refinement and profile analysis of diffraction patterns does not show any significant changes in carbon structure even under 10 bar H2. Pd forms β PdH0.67 under 10 bar H2, which transforms to α PdH0.02 as the hydrogen partial pressure is decreased. However, the equilibrium pressure of transition (corresponding to a 1:1 ratio of α and β phases) increases with increasing the extent of Pd-carbon contact. This pressure is higher for Pd embedded in carbon than for Pd deposited on carbon surface. Both these Pd-carbon materials have higher H2 desorption pressure than pure Pd, indicating that carbon “pumps out” hydrogen from PdHx and the pumping power depends on the extent of Pd-carbon contact. These results support the spillover mechanism (dissociative adsorption of H2 followed by surface diffusion of atomic H).

Ultraviolet radiation-based stabilization routes were explored to produce carbon fibers from melt-processible PAN-based copolymers. An acrylonitrile /methyl acrylate (AN/MA) copolymer was melt-spun into fibers that were crosslinked using UV radiation. The fibers could then be stabilized by oxidative heat treatment, and subsequently carbonized. Physical and mechanical testing was performed to determine the degree of stabilization and the properties of the stabilized and carbonized fibers., U.S.A. Department of Energy