D.D. Edie

The effect of processing on the structure and properties of carbon fibers

Abstract Even though the same three process steps (fiber formation, stabilization, and carbonization) are used to produce both polyacrylonitrile-based (PAN-based) and pitch-based carbon fibers, their final properties differ significantly. This is a direct result of the precursors used to produce these two types of carbon fibers (polymeric versus liquid-crystalline). Liquid-crystalline materials readily orient during fiber formation, creating fibers with a high degree of molecular orientation, whereas polymers form fibers with less ordered, fibrillar structures. Carbon fibers with high degrees of molecular orientation exhibit high moduli and thermal conductivities. By contrast, carbon fibers with more discontinuous and less ordered, fibrillar structures tend to develop higher tensile strengths. Thus, it is not surprising that PANbased carbon fibers have become the preferred reinforcement for high-strength composites. However, recent studies have proven that, by disrupting molecular orientation during fiber formation, the strengths of pitch-based carbon fibers can be improved significantly. Alternatively, linearizing the molecular orientation during fiber formation can yield pitch-based fibers with enhanced thermal conductivities. Researchers now realize that understanding and controlling structure during the fiber formation step is critical if the properties of carbon fibers are to be optimized. Controlling the structure during fiber formation can also permit milder conditions to be used during subsequent process steps. As a result, research into precursor fiber formation offers the best opportunity for improving properties and reducing production costs for both PAN-based and pitch-based carbon fibers.

Modeling of deformation behavior and strain-induced crystallization in poly(ethylene terephthalate) above the glass transition temperature

Abstract 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.

Melt spinning pitch-based carbon fibers

Abstract Although pitch-based carbon fibers can be melt spun, the flow characteristics of mesophase pitch make it extremely difficult to process. Mesophase pitch, like many fluids, can be non-Newtonian, but its viscosity is considerably more temperature sensitive than most melt-spun materials. In the present study, energy and force balances are applied to demonstrate the influence of process variables and material properties on the spinability of mesophase. The results indicate that the extreme temperature dependence of the viscosity of mesophase can create filament stresses during melt spinning that are near the ultimate tensile strength of the filament. Therefore, the control of both temperature and heattransfer rate is extremely critical during the formation of pitch-based carbon fibers.

Melt-spun non-circular carbon fibers

Abstract Carbon fibers are commercially produced from both PAN and petroleum pitch-based processes. PAN-based fibers are usually circular or dogbone-shaped in cross section due to the nature of the precipitation processes used in their production. Pitch-based fibers are melt spun and thus (like other melt-spun fibers) can be extruded into a wide variety of non-circular shapes. In this investigation trilobal and octalobal carbon fibers were melt spun using a precursor produced from petroleum pitch. These non-circular fibers were oxidized and carbonized at conditions typically used in processing circular carbon fibers. Then the fiber modulus, strength and structure was compared to that of circular fibers produced as a control. Surprisingly, the non-circular carbon fibers, which possess a unique structure, have both a higher strength and modulus than the circular carbon fibers.

UV stabilization route for melt-processible PAN-based carbon fibers

Abstract 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.

Flow behavior of mesophase pitch

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.

Ribbon-shape carbon fibers for thermal management

Abstract 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.

Model of stabilization for pan-based carbon fiber precursor bundles

Abstract A combination of theoretical modeling and confirmation experiments has resulted in a model of the complex stabilization process in the manufacture of carbon fibers from polyacrylonitrile precursors. The model describes the temperatures and compositions inside large bundles during stabilization. The bundle is modeled as a homogeneous solid, with the many chemical reactions grouped into the three most important: cyclization of the nitrile groups, dehydrogenation of saturated carbon-carbon bonds, and oxidation. Experiments showed that the resulting mathematical model describes well the measured temperatures and compositions for bundles with 12,000 to 60,000 filaments. The heat-transfer and reaction-kinetics parameters required by the model were estimated experimentally. The model and experiments show that the fiber bundle temperature can be as much as 15°C above the stabilization oven temperature. A series of case studies with the model demonstrates key points about the stabilization process and shows how oven temperature profiles may be used to reduce the time required for stabilization.

High thermal conductivity ribbon fibers from naphthalene-based mesophase

Abstract 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.

Finite-element modeling of heat transfer in carbon/carbon composites

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.