0008-6223/9651 0086223(95)00172-7 MICROSTRUCTURE OF MESOPHASE PITCH-BASED CARBON FIBER AND ITS CONTROLN I MOCHIDA, S.H. YooN, N. TAKANo, F FoRtiN, Y KoRAI and K. YoKogawa Institute of Advanced Material Study, Kyushu University 86, Kasuga, Fukuoka 816, Japan uasi-onion. High resolution SEM clarified the domains to have typical shapes of linear, bent, multi-fold ent and loop. The shapes and alignment of the domains depended upon the spinning conditions and which defined the texture in a rather complex manner, varying in th ansverse locations of the fibers of any overall textures. Each domain appeared to possess micro-domain which were arranged to define the size and shape of the domain. the shape of the micro-domains was unit grew during carbonization and graphitization within the micro-domain. Such crystal e g row tr apitis about shrinkage of the micro-domain to change the size of the domains, strongly influencing overall structure and the properties of the carbon and graphitized fibers. The unique pleat shaped structural fibers, were found by high resolution SEM and STM. Such structures were formed in the spinning ster with the fiber of liquid crystal polymers and mesophase pitch-based carbon fiber. Neither PAN, isotropic tch-bascd fiber necdle coke exhibited such a structure, The size and alignment in the pleat like structure were also dependent upon the nature of mesophase pitch and spinning conditions, although the Formation mechanism is not clarified yet. Such overall structure is briefly compared to the properties of he carbon fiber to find guidelines for the preparation of carbon fibers with better pe Key Words--mesophase pitch, pitch-based carbon fiber, micro-structure, domain, micro-domain, pleat In the pre dy, four mesoph Mesophase pitch-based carbon fibers have been rec. carbon fibers with representative transverse align- gnized as one of the key materials for the next century because of their excellent performances in polarized optical microscope (OM), scanning mechanical, electrical and thermal applications [1]. electron microscope EM)of conventional method- The cost and quality are problems which need to be improved [2]. The present authors have introduced scope(HR-SEM), transmission electron microscope novel types of mesophase pitches which are derived (TEM), wide angle X-ray reflection diffractometer from aromatic hydrocarbons by the aid of HF/BF, (XRD)and scanning tunneling microscope(STM) [3-5]. The high yield and structural control of the [ioj, in their transverse and longitudinal sections to mesophase pitch appear to offer a solution to these clarify their microscopic, mesoscopic, and macro- problems. Control of the fiber structure is now a scopic views. Ihe structure of the carbon fibers was major aim in order to improve the performance of compared to the properties of mesophase pitches to the resultant carbon fiber. Unfortunately, the struc- discuss how such overall structure is influenced or ture has not been fully defined from microscopic as controlled well as macroscopic views. The SEM of limited magnification classifies the transverse texture of the fiber as radial, onion, random, and their mixture [6] 2. EXPERIMENTAL TEM has revealed units of graphitic layers and their 2.1 Materials and preparation of mesophase orientation [7-9]. However, it has not yet been pitch-based carbon fibers solved how the units compose the texture. Hence, Table 1 summarizes some properties of mesophase objective fiber structure for the best performance is pitches used in this study. Mesophase pitches were not clarified prepared from aromatic hydrocarbons, such as naph Presented at the 1994 American Carbon s thalene and methylnaphthalene, at Mitsubishi Gas orkshop on Carbon Materials for Advanced Technologies Chemical Co. using HF/ BF3 as a catalyst [11, 12] ak Ridge, Tennessee, May 19 Naphthalene, methylnaphthalene derived meso- chugoku National Industrial Research Institute Kure, phase pitches, and their mixtures were spun under irushima 737-01, Japar nitrogen pressure through a circular shaped spinningCarbon Vol. 34, No. 8, pp. 941-956,1996 Copyright 0 1996 Elsevier Science Ltd Printed in Great Britain. All rights reserved 0008-6223/96 $15.00 + O._oO 0008-6223(95)00172-7 MICROSTRUCTURE OF MESOPHASE PITCH-BASED CARBON FIBER AND ITS CONTROL* I. MOCHIDA, S.-H. YOON, N. TAKANO, F. FORTIN, Y. KORAI and K. YOK~GAWA~ Institute of Advanced Material Study, Kyushu University 86, Kasuga, Fukuoka 816, Japan (Received 15 May 1994; accepted in revisedform 12 September 1995) Abstract-The macro as well as microscopic structure of four mesophase pitch-based carbon fibers of typical textures is observed using optical, scanning electron, transmission electron, and scanning tunneling microscopes. The four fibers had transverse textures of radial, radial-skin/random-core, random and quasi-onion. High resolution SEM clarified the domains to have typical shapes of linear, bent, multi-fold bent and loop. The shapes and alignment of the domains depended upon the spinning conditions and mesophase pitch properties, which defined the texture in a rather complex manner, varying in the transverse locations of the fibers of any overall textures. Each domain appeared to possess micro-domains which were arranged to define the size and shape of the domain. The shape of the micro-domains was basically unchanged by carbonization and graphitization, while the carbon plane cluster, or graphitic unit grew during carbonization and graphitization within the micro-domain. Such crystal growth brings about shrinkage of the micro-domain to change the size of the domains, strongly influencing overall structure and the properties of the carbon and graphitized fibers. The unique pleat shaped structural units, which are aligned in a zig-zag manner in the longitudinal section of the carbon and graphitized fibers, were found by high resolution SEM and STM. Such structures were formed in the spinning step with the fiber of liquid crystal polymers and mesophase pitch-based carbon fiber. Neither PAN, isotropic pitch-based fibers nor needle coke exhibited such a structure. The size and alignment in the pleat-like structure were also dependent upon the nature of mesophase pitch and spinning conditions, although the formation mechanism is not clarified yet. Such overall structure is briefly compared to the properties of the carbon fiber to find guidelines for the preparation of carbon fibers with better performances. Copyright 0 1996 Elsevier Science Ltd Key Words-mesophase pitch, pitch-based carbon fiber, micro-structure, domain, micro-domain, pleat shaped structure. 1. INTRODUCTION Mesophase pitch-based carbon fibers have been rec￾ognized as one of the key materials for the next century because of their excellent performances in mechanical, electrical and thermal applications [ 11. The cost and quality are problems which need to be improved [Z]. The present authors have introduced novel types of mesophase pitches which are derived from aromatic hydrocarbons by the aid of HF/BF, [3-51. The high yield and structural control of the mesophase pitch appear to offer a solution to these problems. Control of the fiber structure is now a major aim in order to improve the performance of the resultant carbon fiber. Unfortunately, the struc￾ture has not been fully defined from microscopic as well as macroscopic views. The SEM of limited magnification classifies the transverse texture of the fiber as radial, onion, random, and their mixture [ 61. TEM has revealed units of graphitic layers and their orientation [7-91. However, it has not yet been solved how the units compose the texture. Hence, objective fiber structure for the best performance is not clarified. *Presented at the 1994 American Carbon Society Workshop on Carbon Materials for Advanced Technologies, Oak Ridge, Tennessee, May 1994. +Chugoku National Industrial Research Institute, Kure, Hiroshima 737-01, Japan. In the present study, four mesophase pitch-based carbon fibers with representative transverse align￾ments prepared in our laboratory were observed by the polarized optical microscope (OM), scanning electron microscope (SEM) of conventional method￾ology and high resolution scanning electron micro￾scope (HR-SEM), transmission electron microscope (TEM), wide angle X-ray reflection diffractometer (XRD) and scanning tunneling microscope (STM) [lo], in their transverse and longitudinal sections to clarify their microscopic, mesoscopic, and macro￾scopic views. The structure of the carbon fibers was compared to the properties of mesophase pitches to discuss how such overall structure is influenced or controlled. 2. EXPERIMENTAL 2.1 Materials and preparation of mesophase pitch-based carbonjbers Table 1 summarizes some properties of mesophase pitches used in this study. Mesophase pitches were prepared from aromatic hydrocarbons, such as naph￾thalene and methylnaphthalene, at Mitsubishi Gas Chemical Co. using HF/BF, as a catalyst [ 11,121. Naphthalene, methylnaphthalene derived meso￾phase pitches, and their mixtures were spun under nitrogen pressure through a circular shaped spinning 941