40207_09 - CARBON FIBERS Khalid L afdi a nd Maurice A...

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CARBON FIBERS 9 Khalid Lafdi and Maurice A. Wright 9.1 INTRODUCTION Carbon fibers exhibit truly outstanding prop- erties. As shown in Table 9.1, their strength, 0, competes with the strongest steels; they can have stiffness, E, greater than any metal, ceramic or polymer; and they can exhibit ther- mal and electrical conductivities that greatly exceed those of competing materials. If the strength or stiffness values are divided by the low density, p, - 1800-2200 kg m-3, then their huge specific properties (o/p, E/p) make this class of materials quite unique. All carbon fibers sold commercially are fab- ricated from polyacrylonitrile (PAN) or from a coal, petroleum or synthetic pitch. PAN-based fibers are produced from a solubilized mixture that is wet or dry spun to produce a fiber, ostensibly for use in the textile industry. This fiber is stabilized and carbonized to produce a carbon fiber. Aerospace grade material can be obtained in tows that contain between 3000 and 12 000 fibers. Lower performance materi- als are usually formed using larger tows that contain up to 320 000 fibers. PAN-based car- bon fibers are cheaper when produced from larger tows. Pitch fibers are melt spun products obtained in small tow sizes varying from 2000 to 4000 fibers. They are usually of larger diameter (- 10-15 pm) than fibers formed from PAN. The spinning process is controlled by the pitch-based carbon fiber producer; thus, Handbook of Composites. Edited by S.T. Peters. Published in 1998 by Chapman & Hall, London. ISBN 0 412 54020 7 microstructural features developed in the spin- ning process that also influence the stiffness, strength and the thermal and electrical proper- ties of pitch can be optimized. Nevertheless, the tensile strength of PAN-based fibers has always been superior to pitch-based fibers; since PAN fibers were developed before pitch fibers, most structural materials and compo- nents use PAN-based fibers. The most important mechanical and physi- cal properties exhibited by carbon fibers are the elastic modulus, tensile strength and the electri- cal thermal conductivities. These properties are sensitive to the crystallite size and perfection of the graphene layers devel- oped within the carbon fiber and depend for the most part on the degree of molecular align- ment with respect to the fiber axis. Growth and alignment of such layers occur within the pre- cursor and within the solid carbon fiber when it is heated to high temperature. However, it can be argued that the most extensive structural rearrangement occurs when the basic structural units (BSUs) present in the original precursor are large and plate-like so that the shear stresses generated during spinning can align these large areas more easily. Heating the fiber to high tem- perature after spinning relaxes the spinning stresses and allows the oriented regions to grow. Compared to PAN, the basic structural units in an original mesophasic pitch are much larger in area and length and are not twisted to the same degree. The resulting ease of graphiti- zation and alignment of the graphene layers and the development of large crystallites, pro- duces a large elastic modulus and electrical and
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This note was uploaded on 03/16/2010 for the course MECHANICAL ME765401 taught by Professor Prof.sulis during the Spring '10 term at Institut Teknologi Bandung.

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40207_09 - CARBON FIBERS Khalid L afdi a nd Maurice A...

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