Carbon. Vol. 30, No. 4. pp. 55 l-567, I992 Printed in Great Britam. ooO8-6223/92 $5.00 + .oO Copyright 0 1992 Pergamon Press Ltd. MECHANISM OF FORMATION OF THE TEXTURE AND MICROTEXTURE IN AS-SPUN THEN OXIDIZED ANISOTROPIC PITCH-BASED CARBON FIBERS, PART I : RADIAL WITH WEDGE TYPE 11 K. LAFDI, S. BONNAMY’ and A. OBERLIN Laboratoire Marcel Mathieu UMR 124 CNRS-DRET-Universite, 2 av. du Prkident P. Angot, Htlioparc, 64000 Pau, France ‘CRSOCI-lb rue de la Fkrollerie. 4507 1 Orltans Cedex 2 (Received 27 March 199 1; accepted in revised,form 4 November 199 I) Abstract-An anisotropic pitch made ofa mixture ofisotropic phase 1 (containing BT mesophase spheres) and a two-phase-disperse system 2.3 (edge-to-edge type of gel) was used for spinning fibers (radial with wedge type I I). The as-spun, then oxidized fibers were studied by optical microscopy (OM) and transmis- sion electron microscopy (TEM). The sample preparation technique was thin sectioning (cross, oblique, and longitudinal) for TEM and thin-sectioning residues for OM. The microtexture of a fiber is that of an edge-to-edge type ofgel elongated uniaxially by spinning. The structure finally obtained is highly ordered. It is a combination of two orthogonal orientations of the aromatic layers. A dominant radial texture 2 (containing radial aromatic layers) is constantly prolonged parallel to the fiber axis. It is associated to sin- uous channels 3, running radially in the fiber cross-sections. Inside them, the aromatic layer orientation is concentric. The sinuous channels themselves are constantly prolonged along the fiber axis as dotted slits, with well-aligned dots. The isotropic phase of the initial pitch is elongated as fibrils I. A central pole is occupied by another fibril I. The other fib& are increasingly deformed when approaching the periphery of the fiber. Key Words-Anisotropic pitch-based carbon libers, as-spun radial with wedge carbon fibers, oxidized radial with wedge carbon fibers. zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFEDCBA 1. INTRODUCTION In another paper, the anisotropic pitch precursor of the carbon fibers here studied (pitch B), was charac- terized by OM and TEM[ 11. It was then studied with a hot stage[2]. Since the microtexture and the behav- ior of this pitch clear up the microtexture of the as- spun and oxidized fibers, it should thus be valuable to consult refs.[ 1,2,3] before reading the present paper. Previous works of the laboratory summarized in Ref. [4] have shown that among the numerous pre- cursors studied (i.e., pitches, tars, asphaltenes . . .) all of them were made of nanometric elementary units (basic structural units of BSU) distributed at random in a medium isotropic in TEM. Other studies[5,6] have shown that a BSU follows the models of dico- ronene or diekacoronene[7]. in the direction of P progressively occur, allowing LMO in 002DF to be in evidence. As secondary car- bonization and then graphitization are produced. the BSU happen to be coherently associated face-to-face (establishment of stacking order), then they coalesce into distorted layer stacks. As distortions further anneal, the smooth curvatures change into flat poly- edral pore walls. Each grain is the flat part of a pore wall separated from the adjacent one by a sudden change in direction and a defective grain boundary. This process is absolutely comparable to the polygon- ization of carbon blacks[ lo]. Whatever the stage further reached after LMO oc- currence, the BSU contained within are strongly bonded by their edges, so that the material becomes irreversibly a brittle solid which will become a crystal at the end. During quiescent primary carbonization, BSU as- In the anisotropic pitches, precursors of carbon fi- sociate edge-to-edge (see part II, Fig. 7a) without co- bers, the association between BSU is also edge-to- herence between faces since, as is seen on the figure, edge (see part II, Fig. 7b) but much more flexible, along the thickness of the parallelepiped, two succes- since between BSU, approximately parallel to P, are sive BSU are not exactly parallel. The misorientation introduced a non-neglibible amount of largely and (by tilt and twist) is about Ifr 20” in most of the sam- even totally misoriented units. The local LMO is re- ples studied[8,9]. This misorientation is also respon- placed by a long-range statistical orientation or sible for the relatively weak edge-to-edge bonding, SO[ l-31, similar to the very long-range flow orienta- forbidding coalescence of BSU. tions produced in tars submitted to stresses[ 1 11. The whole parallelepiped of part II, Fig. 7 corre- sponds to an homogeneous and marked orientation parallel to P. The total extension of a given oriented volume was called local-molecular orientation or grain. From one grain to the next, important changes The SO of anisotropic pitches is destroyed either by heat treatment (thermolropic materials) or by dis- persion into a suitable reagent (tar). In the latter case, it is accompanied by considerable swelling. These phenomena are reversible. The disappearance of an- CAR 30:4-B 551