《复合材料 Composites》课程教学资源（学习资料）第二章 增强体_carbon fiber_THE EFFECT OF PROCESSING ON THE STRUCTURE AND PROPERTIES OF CARBON FIBERS

Carbon vol.36,No.4,pp.345-362,1998 Printed in great britain. al 0008-6223/98519.00+0.00 P:006901 THE EFFECT OF PROCESSING ON THE STRUCTURE AND PROPERTIES OF CARBON FIBERSK D. D. EDiE Center for Advanced Engineering Fibers and Films and Department of Chemical Engineering. Clemson University, Clemson, SC 29634-0909, USA (Received 25 August 1997) Abstract Even though the same three process steps (hiber formation, stabilization, and carb are used t e both polyacrylonitrile-based (PAN-based )and pitch-based carbon fibers, properties difFer significantly. This is a direct result of the precursors used to produce thes Formation, creating fibers with a high degree of molecular orientation, whereas polymers ith less ordered, fibrillar structures. Carbon fibers with high degrees of molecula light moduli and thermal conductivities. By contrast, cai bon fibers with iture ordered, fibrillar structures tend to develop higher tensile strengths. Thus it is not that PAN based carbon fibers have become the preferred reinforcement for high-strength of pitch-based carbon fibers can be improved significantly. Alternatively, linearizing the molecular Researchers now realize that understanding and controlling structure during the fiber formation step critical if the properties of carbon fibers are to be optimized. Controlling the structure during fiber ormation can also permit milder conditions to be used during subsequent process steps. As a result. recursor fiber formation olTers the best opportunity for improving properties and reducing production costs for both PAN-based and pitch-based carbon fibers, o 1998 Elsevier Science Ltd. Al Key words-A, Carbon fibers, fiber formation, structure/properties, PAN, A. mesophase. 1 INTRODUCTION the conditions used to form the precursor fiber, Post Carbon fibers are, perhaps, the most successful new treatment steps(in this case crosslinking and carbon carbon product to be commercialized in the past ization) nerely refine and perfect the as-spun struc 35 years. Their high strength and stifness, combined ture. This is not to say that fiber properties cannot with their light weight, make these fibers attractive be dramatically altered during post-treatment However, the fundamental fiber structure needed to for high-volume applications ranging from sporting develop high strength or high thermal conductivity oods to aircraft structures. Today, carbon fibers are also being developed for a new class of applications. must be created during the initial fiber formation step. This paper details the processes used to form ial carbon fibers must exhibit consistent mechanical the two dominant classes of carbon fibers, polyacry lonitrile-based(PAN-based and mesophase pitch and transport properties, and the optimum properties based(pitch-based ) Then, the relationship between may differ for each application. Because of this, the control of structure and the interaction between the structure and propertics for both classes of carbon fibers is discussed. Finally, recent breakthroughs in structure and properties has been extensively studied the control of structure for pitch-based fibers will be since high performance fibers were first commercial- ized by Union Carbide in the 1960s reviewed and the possible implications for PAN Nearly all commercial carbon fibers are produced based fiber producers discussed by first converting a carbonaceous precursor into derable progress has been made fiber form. The precursor fiber then is crosslinked in over the past 35 years. the origin and development order to render it infusible. Finally. the crosslinked f structure as well as the relationship between precursor fiber is heated at temperatures from 1200 structure and properties are still not completely o ca 3000C in an inert atmosphere to drive off understood for carbon fibers. Therefore. this review nearly all of the noncarbon elements, converting the article indicates areas where researchers are in general greement and points out phenomena that are still not created caracteristics of Based Carbon o7 at The PAn precursor fibers differ from those of mesophase Pennsylvania State Univ 3-18 July 1997. pitch precursor fibers. Because of this

Pergamon PII: MOOS-6223(97)00185-l Carbon Vol. 36, No. 4, pp. 3455362, 1998 0 1998 Elsevier Science Ltd Printed in Great Britain. All rights reserved 0008.6223/98 $19.00 + 0.00 THE EFFECT OF PROCESSING ON THE STRUCTURE AND PROPERTIES OF CARBON FIBERS* D. D. EDIE Center for Advanced Engineering Fibers and Films and Department of Chemical Engineering, Clemson University, Clemson, SC 29634-0909, USA (Received 25 August 1997) 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 PAN￾based 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. Q 1998 Elsevier Science Ltd. All rights rcscrved. Key Words A. Carbon fibers, fiber formation, structure/properties, PAN, A. mesophase. 1. INTRODUCTION Carbon fibers are, perhaps, the most successful new carbon product to be commercialized in the past 3.5 years. Their high strength and stiffness, combined with their light weight, make these fibers attractive for high-volume applications ranging from sporting goods to aircraft structures. Today, carbon fibers are also being developed for a new class of applications, thermal management. Like other products, commer￾cial carbon fibers must exhibit consistent mechanical and transport properties, and the optimum properties may differ for each application. Because of this, the control of structure and the interaction between the structure and properties has been extensively studied since high performance fibers were first commercial￾ized by Union Carbide in the 1960s. Nearly all commercial carbon fibers are produced by first converting a carbonaceous precursor into fiber form. The precursor fiber then is crosslinked in order to render it infusible. Finally, the crosslinked precursor fiber is heated at temperatures from 1200 to cu 3000°C in an inert atmosphere to drive off nearly all of the noncarbon elements, converting the precursor to a carbon fiber. Like other commercial fiber processes, the final properties are, to a great extent, determined by the material, the process, and *Based on Plenary lecture given at Curbon ‘97 at The Pennsylvania State University, 13-~18 July 1997. the conditions used to form the precursor fiber. Post￾treatment steps (in this case crosslinking and carbon￾ization) merely refine and perfect the as-spun struc￾ture. This is not to say that fiber properties cannot be dramatically altered during post-treatment. However, the fundamental fiber structure needed to develop high strength or high thermal conductivity must be created during the initial fiber formation step. This paper details the processes used to form the two dominant classes of carbon fibers, polyacry￾lonitrile-based (PAN-based) and mesophase pitch￾based (pitch-based). Then, the relationship between structure and properties for both classes of carbon fibers is discussed. Finally, recent breakthroughs in the control of structure for pitch-based fibers will be reviewed and the possible implications for PAN￾based fiber producers discussed. Although considerable progress has been made over the past 35 years, the origin and development of structure as well as the relationship between structure and properties are still not completely understood for carbon fibers. Therefore, this review article indicates areas where researchers are in general agreement and points out phenomena that are still subject to debate. Unlike people, all carbon fibers are not created equal. The fundamental structural characteristics of PAN precursor fibers differ from those of mesophase pitch precursor fibers. Because of this, certain proper- 345

D. D. EDiE ties are easier to develop in PAN-based carbon fibers, 2.2 Production of PAN precursor fibers while other properties are easier to develop in pitch Although PAN fibers can be produced by either based carbon fibers. Therefore. to understand struc- wet or dry spinning processes, wet spinning is used ture and properties of these two classes of carbon to produce nearly all precursor fibers used in commer fibers, one must begin by detailing the materials and cial PAN-based carbon fiber processes. The solution processes used to form the precursor fibers from 10 to 30% by weight of PAN or(PAN copoly mer)dissolved in a polar solvent, such as sodium PAN-BASED CARBON FIBERS isocyanate, nitric acid or dimethylacetamide. Th Nearly all commercial fibers are produced using solution is first filtered and then extruded through a one of three techniques; melt spinning: wet spinning spinnerette into a coagulation bath [3, 4](scc Fig. 1) or dry spinning. In melt spinning the precursor is The coagulation bath can contain various solutions, merely melted and extruded through a spinneret ranging from water and sodium thiocyanate or ontaining numerous small capillaries. As the precur dimethylacetamide to ethylene glycol and dimethyl sor emerges from these capillaries, it cools and solidi cetamide or dimethylformamide. The rate of fiber fics into fibcr form. In wct spinning a concentrated formation is controlled by adjusting parameters such solution of the precursor is extruded through a as the solution concentration, the concentration of spinneret into a coagulation bath. The solvent is the coagulation bath, the bath temperature, the dray more soluble in the coagulation fluid than it is in the down rate, and the rate of extrusion [5] precursor. Therefore, as the solution emerges from Mass transfer at the fiber/liquid interface is rela the spinneret capillaries, the precursor precipitates tively slow in the wet-spinning process. The reason into fiber form. Dry spinning also involves spinning is that the solvent concentration of the coagulation concentrated solution through a spinneret bath is relatively high. Because of this the solvent However, in dry spinning the solution is extruded can diff use radially through the solidifying fiber faster nto a drying chamber. Here, the solvent evaporates than it can diffuse from the fiber surface. As and the precursor precipitates into fiber form. result, the solvent concentration is relatively uniforn Because melt spinning converts a pure recurse across the fiber's cross-section during solidification directly into fiber form and does not involve the Therefore, the fiber shrinks uniformly in the radial added expense of solvent recycling and recovery, direction, giving the circular cross-section that is is the preferred fiber formation process. However. characteristic of wetspun PAN. H if the pol either wet or dry spinning must he employed if the precursor degrades at or near its melting temperature. relatively rigid fiber skin can also form in this process before the center of the fiber has solidified, yielding 2.1 Production of PAN precursor PAN is an atactic, linear polymer containing highly As the PAN solution is forced through the spin polar nitrile pendant groups. Because of its highly neret capillaries. the shear field tends to orient the olar nature, pure PAN has a glass transition temper- solidifying polymeric structure parallel to the direc- ature of ca 120 C and tends to decompose before it tion of flow. In fact, various studies have found that melts. Therefore, PAN precursor fibers must be pro- a solvent call decrease the entanglement of polymers duced by either wet- or dry-spinning processes using during extrusion and enhance orientation. Like many a highly polar solvents. Actually, PAN homopolymer other polymers, PAN tends to precipitate into fibril rarely, if ever, used as a carbon fiber precursor. form. Various processing parameters, such as coag Commercial PAn precursor fibers normally contain lation bath temperature, solvent concentration and from G to 9% of other monomers, such as itaconic stretch. can influcncc thc fibrillar structure and its cid, acrylic acid, methacrylic acid, methyl acrylate, orientation within the as-spun PAN fiber(see Fig. 2) vinyl bromide, etc. [1, 2]. These additions lower the [5]. In other words, wet-spinning yields a precursor glass transition temperature and affect the reactivity fiber in which the PAn molecules are organized into of the polymer structure. Both of these changes can fibrils which, in turn, are generally oriented parallel dramatically influcncc subscquent process steps to the fiber axis. Electron micrographs of as-spun Storage tank Dry and heated-draw Spinnerette Coagulation bath Wash bath Wash bath Nind-up Fig. I. Schematic of wet-spinning process used to produce PAN precursor fibers [41

346 D. D. EDK ties are easier to develop in PAN-based carbon fibers, while other properties are easier to develop in pitch￾based carbon fibers. Therefore, to understand struc￾ture and properties of these two classes of carbon fibers, one must begin by detailing the materials and processes used to form the precursor fibers. 2. PAN-BASED CARBON FIBERS Nearly all commercial fibers are produced using one of three techniques: melt spinning; wet spinning; or dry spinning. In melt spinning the precursor is merely melted and extruded through a spinneret containing numerous small capillaries. As the precur￾sor emerges from these capillaries, it cools and solidi￾fies into fiber form. In wet spinning a concentrated solution of the precursor is extruded through a spinneret into a coagulation bath. The solvent is more soluble in the coagulation fluid than it is in the precursor. Therefore, as the solution emerges from the spinneret capillaries, the precursor precipitates into fiber form. Dry spinning also involves spinning a concentrated solution through a spinneret. However, in dry spinning the solution is extruded into a drying chamber. Here, the solvent evaporates and the precursor precipitates into fiber form. Because melt spinning converts a pure precursor directly into fiber form and does not involve the added expense of solvent recycling and recovery, it is the preferred fiber formation process. However, either wet or dry spinning must be employed if the precursor degrades at or near its melting temperature. 2.1 Production of PANprecursor PAN is an atactic, linear polymer containing highly polar nitrile pendant groups. Because of its highly polar nature, pure PAN has a glass transition temper￾ature of ra 120°C and tends to decompose before it melts. Therefore, PAN precursor fibers must be pro￾duced by either wet- or dry-spinning processes using a highly polar solvents. Actually, PAN homopolymer is rarely, if ever, used as a carbon fiber precursor. Commercial PAN precursor fibers normally contain from 6 to 9% of other monomers, such as itaconic acid, acrylic acid, methacrylic acid, methyl acrylate, vinyl bromide, etc. [1,2]. These additions lower the glass transition temperature and affect the reactivity of the polymer structure. Both of these changes can dramatically influence subsequent process steps. Storage tank A 2.2 Production of PANprecursorJibers Although PAN fibers can be produced by either wet or dry spinning processes, wet spinning is used to produce nearly all precursor fibers used in commer￾cial PAN-based carbon fiber processes. The solution used in a wet spinning process normally consists of from 10 to 30% by weight of PAN or (PAN copoly￾mer) dissolved in a polar solvent, such as sodium thiocyanate, nitric acid or dimethylacetamide. This solution is first filtered and then extruded through a spinnerette into a coagulation bath [3,4] (see Fig. 1). The coagulation bath can contain various solutions, ranging from water and sodium thiocyanate or dimethylacetamide to ethylene glycol and dimethyla￾cetamide or dimethylformamide. The rate of fiber formation is controlled by adjusting parameters such as the solution concentration, the concentration of the coagulation bath, the bath temperature, the draw￾down rate, and the rate of extrusion [ 51. Mass transfer at the fiber/liquid interface is rela￾tively slow in the wet-spinning process. The reason is that the solvent concentration of the coagulation bath is relatively high. Because of this the solvent can diffuse radially through the solidifying fiber faster than it can diffuse away from the fiber surface. As a result, the solvent concentration is relatively uniform across the fiber’s cross-section during solidification. Therefore, the fiber shrinks uniformly in the radial direction, giving the circular cross-section that is characteristic of wetspun PAN. However, if the poly￾mer concentration in the spinning solution is low, a relatively rigid fiber skin can also form in this process before the center of the fiber has solidified, yielding a dogbone-shape fiber. As the PAN solution is forced through the spin￾neret capillaries, the shear field tends to orient the solidifying polymeric structure parallel to the direc￾tion of flow. In fact, various studies have found that a solvent can decrease the entanglement of polymers during extrusion and enhance orientation. Like many other polymers, PAN tends to precipitate into fibril form. Various processing parameters, such as coagu￾lation bath temperature, solvent concentration and stretch, can influence the fibrillar structure and its orientation within the as-spun PAN fiber (see Fig. 2) [5]. In other words, wet-spinning yields a precursor fiber in which the PAN molecules are organized into fibrils which, in turn, are generally oriented parallel to the fiber axis. Electron micrographs of as-spun Dry and heated-draw Coagulation bath Wash bath P Fig. 1. Schematic of wet-spinning process used to produce PAN precursor fibers [4]

Efect of processing on carbon fibers (a)0°C (b)30°C (c)50C ig. 2. Effect of coagulation bath temperature on the fibril and pore structure of wet-spun PAN fiber after 6 x stretch [5] Tonsion Control PAN Supply Creel Oxidized Fiber Wind-uip Fig. 3. Schematic of commercial PAN stabilization oven [2] fibers show that these fibrils are joined together in a relaxation and chain scission during the final carbon three-dimensional network [6] ization step [7, 8]. In most commercial processes, the After being spun into fibers, the orientation within PAn precursor fiber is stabilized by ng it to the PAn is enhanced by stretching. Although the ir at temperatures ranging from 230 to 280C, maximum degree of crystallinity within the PAN fiber Tension must be applied during this step to limit is only 50%6, this step is cssential for producing a relaxation uf the polymer structure(see Fig 3).Most final carbon fiber with adequate strength and modu- would agree that both cyclization and dehydrogena lus. Like most polymeric fiber processes, stretching tion can occur during the stabilization step does not greatly increase the crystallinity or the Cyclization, in particular, is highly exothermic, but molecular order within the PAN; rather, it enhances the exotherm is reduced when PAN copolymer pre- the axial oricntation of the pan fibrils. This fibrillar ursors are employcd. Evidently, the e network appears to be the precursor of the graphene as an initiator for the stabilization reaction [91 network that develops during final heat treatment Numerous studies [10, 11] have shown that the rate of oxidative stabilization is affected by the copolyme 2.3 Stabilization of PAN precursor fibers The primary function of the stabilization step is to to composition of the Pan precursor fiber, the temper- ature and even the applied tension. Although most slink this as-spun structure, insuring that both researches agree that a ladder polymer forms during he molecular and the fibrillar orientation will not be this process, its exact structure lost during final heat treatment. To accomplish this, recent review article, Bashir [12 It that the either the inherent stiffness of the pan molecul cyclization reaction may be ste cific. In fact. a lust be increased or the molecules must be "tied study by Colman et al. [13] suggests that cyclization together in order to eliminate, or at least limit would occur preferentially in isotactic sequences

Effect of processing on carbon fibers 341 (a) 0°C (b) 30°C (c) 50°C Fig. 2. Effect of coagulation bath temperature on the fibril and pore structure of wet-spun PAN fiber after 6 x stretch [5]. Tension Coti Exhaust Gases Oxiied Fiber Wind-up Tension Contml Fig. 3. Schematic of commercial PAN stabilization oven [2]. fibers show that these fibrils are joined together in a three-dimensional network [6]. After being spun into fibers, the orientation within the PAN is enhanced by stretching. Although the maximum degree of crystallinity within the PAN fiber is only 50%, this step is essential for producing a final carbon fiber with adequate strength and modu￾lus. Like most polymeric fiber processes, stretching does not greatly increase the crystallinity or the molecular order within the PAN; rather, it enhances the axial orientation of the PAN fibrils. This fibrillar network appears to be the precursor of the graphene network that develops during final heat treatment. 2.3 Stabilization of PANprecursorfibers The primary function of the stabilization step is to crosslink this as-spun structure, insuring that both the molecular and the fibrillar orientation will not be lost during final heat treatment. To accomplish this, either the inherent stiffness of the PAN molecules must be increased or the molecules must be “tied” together in order to eliminate, or at least limit relaxation and chain scission during the final carbon￾ization step [7,8]. In most commercial processes, the PAN precursor fiber is stabilized by exposing it to air at temperatures ranging from 230 to 280°C. Tension must be applied during this step to limit relaxation of the polymer structure (see Fig. 3). Most would agree that both cyclization and dehydrogena￾tion can occur during the stabilization step. Cyclization, in particular, is highly exothermic, but the exotherm is reduced when PAN copolymer pre￾cursors are employed. Evidently, the comonomer acts as an initiator for the stabilization reaction [9]. Numerous studies [lO,ll] have shown that the rate of oxidative stabilization is affected by the copolymer composition of the PAN precursor fiber, the temper￾ature and even the applied tension. Although most researches agree that a ladder polymer forms during this process, its exact structure is still in doubt. In a recent review article, Bashir [ 121 pointed out that the cyclization reaction may be stereospecific. In fact, a study by Colman et al. [ 131 suggests that cyclization would occur preferentially in isotactic sequences

D. D. EDIE However, Chen et al. [14] found that syndiotactic minimized by applying tension during heat treatment sequences were equally capable of cyclization. The 18]. By contrast, the degree of preferred orientation problem may be that the polymer chains within the within the fiber and thus. the modulus of the pan fibril form an irregular rod-like helix due to the based carbon fibers increase continuously as heat intramolecular repulsion of the nitrile groups. These treatment temperature is increased [17]. Because of fibrils, in turn, contain both crystalline and amo this, the various grades uf PAN-based carbon fiber ta and Harrison [15] found that available from a particular manufacturer are, nor tramolecular reactions within the rod-like heli mally, the result of changes in heat treatment dominate at lower temperatures(below ca 290"C) whereas intermolecular reactions between adjacent helices occur between 300 and 380.C. Perhaps both 2.5 Structure of PAN-based carbon fibers Colman et al. and Chen et al. are correct-intramolec Pioneering studies by Diefendorf and Tokarsky lar reactions are stereospecific, but intermolecular [191, Johnson [20] and others showed that the struc reactions are not ture of PAN-based carbon fiber is fibrillar in nature Thus, one might expect a stereoregular precurso mimicking the fundamental structure of the poly lymer to offer advantages. Howcvcr, to date, no meric precursor fiber. Diefendorf and Tokarsky also research in this area has been reported showed that the amplitude of the undulation in the fibrillar structure was highest in the center and lowest 2.4 Carbonization of stabilized PAN fibers near the surface of the PaN-based carbon fibers Once stabilized. the pan fiber is carbonized at This indicated that the modulus of a PAN-based 1000 1500C in an inert atmosphere [2, 8]. During carbon fiber varies throughout its cross-section. In a this step most of the noncarbon elements within the recent study Huang and Young [21] confirmed this hydrogen cyanide, water, carbon monoxide, carbon fibers using Raman spectroscopy, Both Johnson [201 ioxide, ammonia and various other gases [6.16]. and Endo [22]employed wide-angle X-ray diffraction The evolution of these compounds decreases the mass to show that the layer planes of PAN-based carbon of the fiber by from 55 to 60 wt%. As a result the fibers have no regular three-dimensional order. Also, fiber shrinks in diameter. Therefore, in a typical Pan by subjecting longitudinal and transverse sections to cess, the precursor fiber might begin with an small-angle X-ray difiraction and transmission as-spun diameter of 35 um and then be stretched to electron microscope(TEM) analysis, Johnson [20] a diameter of 10.5 um. Finally, shrinkage during showed that needle-shaped voids exist between crys carbonization yields a carbon fiber with a diameter tallies in the outer skin of the fiber and that in this of 7 um. In other words, small carbon fiber diamete region, the layer planes are essentially parallel to the are a characteristic of the high weight loss of PAl surface. However, in the core region, Johnson found during processing. Even though the diameter of that the layer planes were folded extensively, often PAN-based carbon fibers is smaller than that of most through angles of 180. Based on these results. itch-based carbon fibers, the as-spun diameter of Johnson developed the three-dimensional schematic the precursor fiber is actually significantly greater representation of the microstructure of PAN-based Initially, increasing the final heat treatment ter carbon fiber shown in Fig. 5. Guigon et ul. [23]have ature es tensile strength. However, as Fig 4 proposed that the microtexture of PAN-based carbon shows, the tensile strength suddenly drops when heat fibers is even more complicated-crumpled sheets treatment temperatures exceed 1600C [17]. Fitzer form these fibrils claims that this decrease is associated with the release of niTrogen and tliat the reduction in strength can be · sAF230一270c40min Final Heat Treatment Temperature(C) Fig 4. Influence of final heat treatment temperature on the Fig. 5. Microstructure of PAN-based carbon fiber proposed ensile strength of PAN-based carbon fiber [17

348 D. D. EIIII: However, Chen et al. [14] found that syndiotactic sequences were equally capable of cyclization. The problem may be that the polymer chains within the fibril form an irregular rod-like helix due to the intramolecular repulsion of the nitrile groups. These fibrils, in turn, contain both crystalline and amor￾phous regions. Gupta and Harrison [ 151 found that intramolecular reactions within the rod-like helix dominate at lower temperatures (below ca 290°C) whereas intermolecular reactions between adjacent helices occur between 300 and 380°C. Perhaps both Colman et al. and Chen et al. are correct - intramolec￾ular reactions are stereospecific, but intermolecular reactions are not. minimized by applying tension during heat treatment [ 181. By contrast, the degree of preferred orientation within the fiber and, thus, the modulus of the PAN￾based carbon fibers increase continuously as heat treatment temperature is increased [17]. Because of this, the various grades of PAN-based carbon fiber available from a particular manufacturer are, nor￾mally, the result of changes in heat treatment temperature. Thus, one might expect a stereoregular precursor polymer to offer advantages. However, to date, no research in this area has been reported. 2.4 Carbonization of stabilized PANJibers Once stabilized, the PAN fiber is carbonized at 1000~1500°C in an inert atmosphere [2,8]. During this step most of the noncarbon elements within the fiber are volatilized in the form of methane, hydrogen, hydrogen cyanide, water, carbon monoxide, carbon dioxide, ammonia and various other gases [6,16]. The evolution of these compounds decreases the mass of the fiber by from 55 to 60 wt%. As a result the fiber shrinks in diameter. Therefore, in a typical PAN process, the precursor fiber might begin with an as-spun diameter of 35 pm and then be stretched to a diameter of 10.5 ,nm. Finally, shrinkage during carbonization yields a carbon fiber with a diameter of 7 pm. In other words, small carbon fiber diameters are a characteristic of the high weight loss of PAN during processing. Even though the diameter of most PAN-based carbon fibers is smaller than that of most pitch-based carbon fibers, the as-spun diameter of the precursor fiber is actually significantly greater. Initially, increasing the final heat treatment temper￾ature increases tensile strength. However, as Fig. 4 shows, the tensile strength suddenly drops when heat treatment temperatures exceed 1600°C [ 171. Fitzer claims that this decrease is associated with the release of nitrogen and that the reduction in strength can be 2.5 Structure of PAN-based carbonJibers Pioneering studies by Diefendorf and Tokarsky [ 191, Johnson [20] and others showed that the struc￾ture of PAN-based carbon fiber is fibrillar in nature, mimicking the fundamental structure of the poly￾meric precursor fiber. Diefendorf and Tokarsky also showed that the amplitude of the undulation in the fibrillar structure was highest in the center and lowest near the surface of the PAN-based carbon fibers. This indicated that the modulus of a PAN-based carbon fiber varies throughout its cross-section. In a recent study Huang and Young [21] confirmed this skin-core structural difference in PAN-based carbon fibers using Raman spectroscopy. Both Johnson [20] and Endo [22] employed wide-angle X-ray diffraction to show that the layer planes of PAN-based carbon fibers have no regular three-dimensional order. Also. by subjecting longitudinal and transverse sections to small-angle X-ray diffraction and transmission electron microscope (TEM) analysis, Johnson [ 201 showed that needle-shaped voids exist between crys￾tallites in the outer skin of the fiber and that. in this region, the layer planes are essentially parallel to the surface. However, in the core region, Johnson found that the layer planes were folded extensively, often through angles of 180”. Based on these results, Johnson developed the three-dimensional schematic representation of the microstructure of PAN-based carbon fiber shown in Fig. 5. Guigon rt al. [23] have proposed that the microtexture of PAN-based carbon fibers is even more complicated~“crumpled sheets” form these fibrils. 40 - I ,.., I I ,, I I I * , I,, , , I,, 500 ,000 1500 2000 2500 Final Heat Treatment Temperature (“C) Fig. 4. Influence of final heat treatment temperature on the tensile strength of PAN-based carbon fiber [ 171. Fig. 5. Microstructure of PAN-based carbon fiber proposed by Johnson [20]

Effect of processing on carbon fibers Misoriented crystallite linking two Tensile stress causes basal Catastrophic failure occurs if crystallites parallel to fiber axis. plane rupture in La direction crystallite size> critical flaw size (b) Fig. 6. Reynolds and Sharp mechanism for tensile failure of carbon fibers [20] Each of these show that the PAN-based failure mode explains the difference between the flaw extensively folded and sensitivity(and, therefore, the tensile strength) of the interlinked turbostratic layers of carbon with nongraphite, fibrillar PAN-based carbon fiber and interlayer spacings considerably larger than those of graphitic, mesophase pitch-based fiber. Recent work raphite. As a result, PAN-based carbon fibers have by Dobb er al. [24] indicates that inter-crystalline a low degree of graphitization. The turbostratic layers and intra-crystalline disorder, most likely caused by within PAN-based carbon fibers appear to follow the the fibrillar structure of the PAN-based carbon fiber. original fibril structure of the PAN precursor fiber. is responsible for the superior compressive strength Although the turbostratic layers within these fibrils of this class of carbon fiber tend to be oriented parallel to the fiber axis, they are Obviously, the fundainental fibrillar structure of not highly aligned. As first proposed by Johnson PAN-based carbon fibers is created during initial [20]. it is this fibrillar structure that makes PAN fiber formation. However, little if any research into based fibers less prone to faw-induced failure. He this fiber formation process has been done since based his argument on the brittle-failure mechanism PAN's adoption as a carbon fiber precursor. Instead, proposed by Reynolds and Sharp. As discussed as the above revicw bove, the crystallites within PAN-based carbon based carbon fiber research has concentrated on fibers are not perfectly aligned, and misoriented abilization and carbonization, by contrast, research itively common (see Fig. 6(a) in pitch-based carbon fibers has concentrated on When a stress is applied parallel to the fiber axis, the perfecting precursor chemistry and the development ystallites align until their movement is restricted by of structure during fiber formation. As will be seen, a disclination in the structure( Fig. 6(b)). If the stress pitch researchers appear to have chosen the more is sufficient, the misoriented crystallite will rupture critical area for control and optimization of fiber and relieve the stress within the fiber(Fig. 6(c)). properties When the size of the ruptured crystallite(perpendicu lar to the fiber axis)is larger than the critical faw size,a catastrophic failure occurs, and the fiber 3. PITCH-BASED CARBON FIBERS breaks. Even if the rupture crystallite is smaller than Its highly condensed aromatic structure the critical flaw size, catastrophic failure can occur if phase pitch (the precursor for pitch-ba he crystallites surrounding the disclination are con- fibers) relatively good thermal stability ous enough to allow a crack to propagate inte this, mesophase pitch precursor fibers are me/t sn,or boring crystallites. According to Johnson, this As previously mentioned, melt spinning is the pre-

Effect of processing on carbon fibers 349 Misoriented crystallite linking two crystallites parallel to fiber axis. (4 ! I Tensile stress causes basal plane rupture in La direction. (b) Catastrophic failure occurs if crystallite size 5 critical flaw size. cc> Fig. 6. Reynolds and Sharp mechanism for tensile failure of carbon fibers [20]. Each of these studies show that the PAN-based carbon fibers contain extensively folded and interlinked turbostratic layers of carbon with interlayer spacings considerably larger than those of graphite. As a result, PAN-based carbon fibers have a low degree of graphitization. The turbostratic layers within PAN-based carbon fibers appear to follow the original fibril structure of the PAN precursor fiber. Although the turbostratic layers within these fibrils tend to be oriented parallel to the fiber axis, they are not highly aligned. As first proposed by Johnson [20], it is this fibrillar structure that makes PAN￾based fibers less prone to flaw-induced failure. He based his argument on the brittle-failure mechanism proposed by Reynolds and Sharp. As discussed above, the crystallites within PAN-based carbon fibers are not perfectly aligned, and misoriented crystallites are relatively common (see Fig. 6(a)). When a stress is applied parallel to the fiber axis, the crystallites align until their movement is restricted by a disclination in the structure (Fig. 6(b)). If the stress is sufficient, the misoriented crystallite will rupture and relieve the stress within the fiber (Fig. 6(c)). When the size of the ruptured crystallite (perpendicu￾lar to the fiber axis) is larger than the critical flaw size, a catastrophic failure occurs, and the fiber breaks. Even if the rupture crystallite is smaller than the critical flaw size, catastrophic failure can occur if the crystallites surrounding the disclination are con￾tinuous enough to allow a crack to propagate into neighboring crystallites. According to Johnson, this failure mode explains the difference between the flaw sensitivity (and, therefore, the tensile strength) of the nongraphite, fibrillar PAN-based carbon fiber and graphitic, mesophase pitch-based fiber. Recent work by Dobb ef al. [24] indicates that inter-crystalline and intra-crystalline disorder, most likely caused by the fibrillar structure of the PAN-based carbon fiber, is responsible for the superior compressive strength of this class of carbon fiber. Obviously, the fundamental fibrillar structure of PAN-based carbon fibers is created during initial fiber formation. However, little if any research into this fiber formation process has been done since PAN’s adoption as a carbon fiber precursor. Instead, as the above review indicates, nearly all recent PAN￾based carbon fiber research has concentrated on stabilization and carbonization. By contrast, research in pitch-based carbon fibers has concentrated on perfecting precursor chemistry and the development of structure during fiber formation. As will be seen, pitch researchers appear to have chosen the more critical area for control and optimization of fiber properties. 3. PITCH-BASED CARBON FIBERS Its highly condensed aromatic structure gives meso￾phase pitch (the precursor for pitch-based carbon fibers) relatively good thermal stability. Because of this, mesophase pitch precursor fibers are melt spun. As previously mentioned, melt spinning is the pre-