The influence of primary QI on the oxidation behaviour of pitch-based C/C composites

Three unidirectional C/C composites were prepared from PAN-based carbon fib res and three coal-tar pitches: obtained from the same parent coal-tar pitc h but differing in their primary QI content. The effect of the amount of pr imary QI present in pitches on the air reactivity of pitch-cokes and C/C co mposites was studied by thermogravimetric analysis. The oxidation of pitch- cokes was performed at 10 degreesC min(-1) up to 1000 degreesC. The air rea ctivity of C/C composites was assessed from the weight loss curves obtained under isothermal conditions in the temperature range of 500 to 575 degrees C for 3 h. The topography of the oxidized composites was examined by scanni ng electron microscopy. The results showed that pitch-cokes were more react ive than carbon fibres because the presence of QI particles increased their reactivity. With the increase of primary QI content in the pitches, C/C co mposites showed a higher percentage of carbon matrix, with a finer optical texture, fewer microcracks and less closed porosity. The air reactivity of the composites was affected by both chemical and physical factors, the exte nt of oxidation determining which of these exerted the greater influence. A t low conversions the reactivity was decided by both QI content and the opt ical texture of the matrix. At high conversions the porosity of the composi te was the determining factor. (C) 2001 Elsevier Science Ltd. All rights re served.