STRUCTURE AND GRAPHITIZATION 231
HTT), evidence was found for a microfibrillar structure on the scale of 300-500 A, believed to be derived from the rayon starting material. In the "graphite" fibers transmission microscopy revealed a well-developed micropore structure with adjacent pores separated by N60 A. In high modulus graphite fibers, these micropores were aligned parallel with the fiber axis. They formed a honeycomb structure, with ribbon-shaped graphite layer packets forming the cell walls. l This research was sponsored in part by the Air Force Materials Laboratory, Nonmetallic Materials Division, Fibrous Materials Branch. 148. X-ray studies otr the preferred orientation of carbon fibres.* W. Ruland (Union Carbide European Research Associates, Brussels, Belgium).
The determination of the preferred orientation in carbon fibres is complicated by the highly imperfect structure of these materials. A method is developed to overcome these difficulties and the applicability of the method is demonstrated in a study of the carbonization of cellulose fibres. It is shown that the preferred orientation in the starting material is destroyed with the destruction of the cellulose structure; only a very small orientation effect remains and stays nearly constant up to about 1000Β°C HTT. Further heating produces a reorientation which can attain and even surpass the degree of preferred orientation in the starting material.