1 FORMATION OF MICROSTRUCTURE IN MESOPHASE CARBON FIBERS J.L.White',B.Fathollahi,and X.Bourrat 1 Introduction Mesophase carbon fiber was invented in the 1970s,independently and simultaneously,from our viewpoint today,by Leonard Singer in the US(Singer,1978)and by Sugio Otani in Japan (Otani,1981).Both based their concepts on the role of the liquid-crystalline car- bonaceous mesophase described by Brooks and Taylor in 1965.Both recognized two key steps:flow of the anisotropic liquid in the shear-stress field of the spinneret to align the disk- like molecules,and oxidation thermosetting to stabilize the shape and microstructure of the fiber prior to carbonization. These inventions led to high expectations in the carbon materials community for the rapid attainment of fiber with superior properties at the low costs anticipated for a pitch product. Vigorous research activities ensued,many under conditions of proprietary secrecy.An impor- tant advance,with potential for many carbon materials besides fiber,was the development of more satisfactory mesophase pitches,fully transformed to the liquid-crystalline state and of low viscosity (Lewis and Nazem,1987;Mochida et al.,1988;Sakanishi et al.,1992). However for mesophase carbon fiber,the results fell short of expectations.The mechan- ical properties were not competitive with PAN-based fiber,except for some high-modulus grades.Low costs were never achieved,apparently due to the lengthy process of stabiliza- tion and the early 1990's saw downsizing and abandonment of research programs,with only a few products commercialized. Nevertheless the prospects for a carbon fiber spun in the liquid-crystalline state contin- ued to fascinate carbon scientists.Comprehensive microstructural studies initiated by Hamada et al.in 1987 demonstrated the remarkable flow memory of viscous mesophase in a simple spinneret.Then Bourrat et al.(1990a-c)showed that the nanostructure of as-spun filaments can be described in terms of the basic microstructural features of liquid crystals: bend,fold,splay,and disclinations. Some practical results of manipulating the flow of mesophase in the spinneret became apparent.In a 1990 patent,Hara et al.showed that a fine-weave screen placed across the mesophase stream flowing to the spinneret can profoundly alter the microstructure of as-spun fiber.In 1993,Taylor and Cross reported their study of screened flow prior to spinning;their observations were rationalized in terms of an array of fine mesophase cylin- ders,leading directly to the concept of a filament comprised of a linear composite of near- nanotubes.Then rheologists entered the scene to study the flow instabilities of a discotic liquid crystal under the flow conditions in a spinneret(Singh and Rey,1995,1998;Didwania et al.,1998),thus providing basic guidance for spinning experiments. In memorial of Jack White deceased in 2002. ©2003 Taylor&Francis
1 FORMATION OF MICROSTRUCTURE IN MESOPHASE CARBON FIBERS J. L. White† , B. Fathollahi, and X. Bourrat 1 Introduction Mesophase carbon fiber was invented in the 1970s, independently and simultaneously, from our viewpoint today, by Leonard Singer in the US (Singer, 1978) and by Sugio Otani in Japan (Otani, 1981). Both based their concepts on the role of the liquid-crystalline carbonaceous mesophase described by Brooks and Taylor in 1965. Both recognized two key steps: flow of the anisotropic liquid in the shear-stress field of the spinneret to align the disklike molecules, and oxidation thermosetting to stabilize the shape and microstructure of the fiber prior to carbonization. These inventions led to high expectations in the carbon materials community for the rapid attainment of fiber with superior properties at the low costs anticipated for a pitch product. Vigorous research activities ensued, many under conditions of proprietary secrecy. An important advance, with potential for many carbon materials besides fiber, was the development of more satisfactory mesophase pitches, fully transformed to the liquid-crystalline state and of low viscosity (Lewis and Nazem, 1987; Mochida et al., 1988; Sakanishi et al., 1992). However for mesophase carbon fiber, the results fell short of expectations. The mechanical properties were not competitive with PAN-based fiber, except for some high-modulus grades. Low costs were never achieved, apparently due to the lengthy process of stabilization and the early 1990’s saw downsizing and abandonment of research programs, with only a few products commercialized. Nevertheless the prospects for a carbon fiber spun in the liquid-crystalline state continued to fascinate carbon scientists. Comprehensive microstructural studies initiated by Hamada et al. in 1987 demonstrated the remarkable flow memory of viscous mesophase in a simple spinneret. Then Bourrat et al. (1990a–c) showed that the nanostructure of as-spun filaments can be described in terms of the basic microstructural features of liquid crystals: bend, fold, splay, and disclinations. Some practical results of manipulating the flow of mesophase in the spinneret became apparent. In a 1990 patent, Hara et al. showed that a fine-weave screen placed across the mesophase stream flowing to the spinneret can profoundly alter the microstructure of as-spun fiber. In 1993, Taylor and Cross reported their study of screened flow prior to spinning; their observations were rationalized in terms of an array of fine mesophase cylinders, leading directly to the concept of a filament comprised of a linear composite of nearnanotubes. Then rheologists entered the scene to study the flow instabilities of a discotic liquid crystal under the flow conditions in a spinneret (Singh and Rey, 1995, 1998; Didwania et al., 1998), thus providing basic guidance for spinning experiments. † In memorial of Jack White deceased in 2002. © 2003 Taylor & Francis
Thus the stage is being set for a new class of mesophase carbon fibers,with designed microstructures produced by manipulating flow of the anisotropic liquid in the spinneret. 2 Microstructural approach Consider first the microstructure of manufactured mesophase carbon fiber,keeping in mind that such fiber has been processed through stabilization,carbonization,and graphitization after spinning of the viscous mesophase."Nanostructure"may seem more suitable to describe the architecture of graphitic layers in a filament whose diameter is near ten microns.The scanning electron micrograph(SEM)of Fig.1.1 offers an example of fibers (a) 3.gkx76k4:29m (b) Figure 1.Fracture surface of a mesophase carbon filament(a)manufactured by DuPont(E35). The schematic diagram (b)outlines an oriented core bounded on each side by a+m super-disclination and with wavy and rippled layers leading from the core to zigzag bands at the rim.From Bourrat(2000). ©2003 Taylor&Francis
Thus the stage is being set for a new class of mesophase carbon fibers, with designed microstructures produced by manipulating flow of the anisotropic liquid in the spinneret. 2 Microstructural approach Consider first the microstructure of manufactured mesophase carbon fiber, keeping in mind that such fiber has been processed through stabilization, carbonization, and graphitization after spinning of the viscous mesophase. “Nanostructure” may seem more suitable to describe the architecture of graphitic layers in a filament whose diameter is near ten microns. The scanning electron micrograph (SEM) of Fig. 1.1 offers an example of fibers +π +π (a) (b) Figure 1.1 Fracture surface of a mesophase carbon filament (a) manufactured by DuPont (E35). The schematic diagram (b) outlines an oriented core bounded on each side by a super-disclination and with wavy and rippled layers leading from the core to zigzag bands at the rim. From Bourrat (2000). © 2003 Taylor & Francis
available in the 1980s.Such fibers,manufactured by Union Carbide and by DuPont,were studied extensively by Fitzgerald,Pennock,and Taylor(1991,1993).Although there is some variability in structural detail,even in filaments from the same tow,the sketch outlines trans- verse features that are generic to most mesophase carbon fibers.This filament exhibits an oriented core with the surrounding layers in radial orientation.As a function of increasing radius,three zones may be seen in which the radial layers waver,ripple,and corrugate increasingly to form zigzag bands near the rim.Finally there is a thin skin,finely structured and highly corrugated. Figure 1.2 includes a transmission electron micrograph(TEM)of the same type of fiber. The diffraction contrast defines the oriented core as well as the +m super-disclination that accommodates the parallel layers of the oriented core to the radial layers in the surrounding zone.The term"super-disclination"is used to distinguish a new disclination imposed on an oriented mesophase body or stream that already may carry many disclinations from its pre- vious history of coalescence and flow.An example is the formation of+2 super-disclinations by passage of mesophase pitch through a screen,described later in some detail. The higher-magnification TEM micrographs of Fig.1.3 define layer orientations within the bands located near the rim of a DuPont fiber,here again in the as-spun condition. 1 ORIENTED CORE 2 RADIALZONE 18K0X20,8e81mND8 ZIZAG BAND FINE-TEXTURED SKIN Figure 1.2 An SEM of a carbonized DuPont E35 filament superposed on a TEM dark-field image of the same type of fiber at the as-spun stage (a).The four zones observed in the carbonized filament are evident in the as-spun state (b).The elliptic shape is due to the angle of cutting of a circular filament.From Bourrat(2000). ©2003 Taylor&Francis
available in the 1980s. Such fibers, manufactured by Union Carbide and by DuPont, were studied extensively by Fitzgerald, Pennock, and Taylor (1991, 1993). Although there is some variability in structural detail, even in filaments from the same tow, the sketch outlines transverse features that are generic to most mesophase carbon fibers. This filament exhibits an oriented core with the surrounding layers in radial orientation. As a function of increasing radius, three zones may be seen in which the radial layers waver, ripple, and corrugate increasingly to form zigzag bands near the rim. Finally there is a thin skin, finely structured and highly corrugated. Figure 1.2 includes a transmission electron micrograph (TEM) of the same type of fiber. The diffraction contrast defines the oriented core as well as the super-disclination that accommodates the parallel layers of the oriented core to the radial layers in the surrounding zone. The term “super-disclination” is used to distinguish a new disclination imposed on an oriented mesophase body or stream that already may carry many disclinations from its previous history of coalescence and flow. An example is the formation of 2 super-disclinations by passage of mesophase pitch through a screen, described later in some detail. The higher-magnification TEM micrographs of Fig. 1.3 define layer orientations within the bands located near the rim of a DuPont fiber, here again in the as-spun condition. Figure 1.2 An SEM of a carbonized DuPont E35 filament superposed on a TEM dark-field image of the same type of fiber at the as-spun stage (a). The four zones observed in the carbonized filament are evident in the as-spun state (b). The elliptic shape is due to the angle of cutting of a circular filament. From Bourrat (2000). +π 1 2 3 4 +π ORIENTED CORE RADIAL ZONE RADIAL ZONE ADIAL ZONE ZIZAG BAND ZIZAG BAND FINE-TEXTURED SKIN b © 2003 Taylor & Francis
(a) (b) 15752准.即2520m 05732湘.T25288m (c) (d) 摄渊 Figure 1.3 (a,b,c)Three TEM dark-field image at 45 rotation of diffraction vector show the zigzag bands in the rim of the Dupont filament in the as-spun condition.The bar in each micro- graph defines the orientation of mesophase layers that appear bright.(d)Structural sketch illustrating the corrugated layer orientation. 2003 Taylor Francis
(a) (b) (c) (d) Figure 1.3 (a,b,c) Three TEM dark-field image at 45º rotation of diffraction vector show the zigzag bands in the rim of the Dupont filament in the as-spun condition. The bar in each micrograph defines the orientation of mesophase layers that appear bright. (d) Structural sketch illustrating the corrugated layer orientation. © 2003 Taylor & Francis
The zigzag bands,only a fraction of a micron in width,consist of well-aligned layers within each band,and the boundaries are sharply defined.The zigzag angle,referred to later as the ripple angle,appears not to be fixed,but tends to 90 in the outer bands.Note the presence of many dots and short dashes appearing in reverse contrast to the bands in which they occur.These appear to be +/disclination loops(Zimmer and White,1982)inherited from the mesophase pitch as it enters the spinneret;the contrast is due to the local rotation of mesophase molecules in the disclination loop.The density of dots is much higher in the skin,which may reflect the shear experienced briefly at the capillary wall as the stream exits the spinneret. In 1990,Bourrat et al.(1990a)published observations by high-resolution electron microscopy(HREM)to identify +m and-m wedge disclinations on transverse sections of mesophase fiber heat-treated to 1600C.Later these authors (Bourrat et al.,1990b,c) demonstrated the presence of +2 and-2m as well as +m and-m disclinations,along with bend,splay,and folding,in mesophase filaments in the as-spun condition.The presence of these liquid-crystalline structural features in finished fiber was confirmed by Pennock et al.(1993).Figure 1.4 is a lattice-fringe image of an Amoco P25 mesophase carbon filament (Bourrat et al.,1990c);the structural diagram locates the +m,-m,and -2 disclinations in the micrograph.From these observations,mesophase carbon fibers may be viewed as carbonized fossils of highly oriented mesophase streams with non-equilibrium microstructures frozen in place as each stream is swiftly drawn to a filament. Although an extensive patent literature has come to exist for mesophase carbon fiber, little information was published on the formation of microstructure within the spinneret until Hamada and co-workers at Nippon Steel undertook their comprehensive micrographic (a)】 (b) 10nm Figure 1.4(a)A high resolution lattice-fringe of an Amoco P25 mesophase carbon filament.(b)The structure diagram locates +m disclinations by U and-T disclinations by Y.From Bourrat et al.(1990c). ©2003 Taylor&Francis
The zigzag bands, only a fraction of a micron in width, consist of well-aligned layers within each band, and the boundaries are sharply defined. The zigzag angle, referred to later as the ripple angle, appears not to be fixed, but tends to 90 in the outer bands. Note the presence of many dots and short dashes appearing in reverse contrast to the bands in which they occur. These appear to be / disclination loops (Zimmer and White, 1982) inherited from the mesophase pitch as it enters the spinneret; the contrast is due to the local rotation of mesophase molecules in the disclination loop. The density of dots is much higher in the skin, which may reflect the shear experienced briefly at the capillary wall as the stream exits the spinneret. In 1990, Bourrat et al. (1990a) published observations by high-resolution electron microscopy (HREM) to identify and wedge disclinations on transverse sections of mesophase fiber heat-treated to 1600 C. Later these authors (Bourrat et al., 1990b,c) demonstrated the presence of 2 and 2 as well as and disclinations, along with bend, splay, and folding, in mesophase filaments in the as-spun condition. The presence of these liquid-crystalline structural features in finished fiber was confirmed by Pennock et al. (1993). Figure 1.4 is a lattice-fringe image of an Amoco P25 mesophase carbon filament (Bourrat et al., 1990c); the structural diagram locates the , , and 2 disclinations in the micrograph. From these observations, mesophase carbon fibers may be viewed as carbonized fossils of highly oriented mesophase streams with non-equilibrium microstructures frozen in place as each stream is swiftly drawn to a filament. Although an extensive patent literature has come to exist for mesophase carbon fiber, little information was published on the formation of microstructure within the spinneret until Hamada and co-workers at Nippon Steel undertook their comprehensive micrographic (a) (b) 10 nm Figure 1.4 (a) A high resolution lattice-fringe of an Amoco P25 mesophase carbon filament. (b) The structure diagram locates disclinations by U and disclinations by Y. From Bourrat et al. (1990c). © 2003 Taylor & Francis