NITROGEN DISTRIBUTION IN HIGH-TENSILE STRENGTH CARBON-FIBERS

It is already established that a nitrogen content in carbon fibres induces poor high tensile strength properties of the fibres. Up to now the nitrogen localization in the fibre, particularly along the diameter, was not known, due to the small size of the fibres (7 μm) and the insufficient sensitivity of the usual analytical techniques, using X-rays or ion beam for instance, for such low atomic number elements (nitrogen and carbon). By using electron Energy Loss Spectroscopy (EELS) in a Philips EM 400T microscope equipped with a V.G. electron spectrometer, we have determined this distribution for the Toray 300 carbon fibre. The measurements have been made by using the carbon and nitrogen K edge intensity when EEL spectra correspond to thin enough samples, thickness ≤ 500 Å. The accuracy of this determination is on the order of 10%. In this way, we have measured a N C atomic concentration varying from 2 to 7%. For all the Toray 300 fibres a similar variation of the ratio is observed. The nitrogen content is low in the core of the fibre, about 2%. It remains practically constant in the whole center of the fibre and grows rapidly to 6-7% in the periphery. These measurements reveal a skin core concentration gradient effect in the high tensile strength fibres. They confirm that it is necessary to avoid this structure if one desires ultra-high mechanical properties for the fibres. © 1990.