L.M. Manocha

Characterization of oxidised pan fibres

Abstract A comprehensive study of mechanical properties of PAN fibres, oxidised for various times, is made. The decrease in strength and secondary modulus of oxidised fibres with oxidation time shows that the oxidation reaction is intramolecular and not intermolecular. The increase in formation of ladder polymer with oxidation time is confirmed by X-ray studies. The variation of strength of carbon fibres (H.T.T. 1000°C) with change in preoxidation time is reported and it is shown that the strength of carbon fibres starts decreasing after a certain optimum pre-oxidation time. Carbon fibres with slow heating rate during preoxidation stage are found to be stronger than those with quick heating during preoxidation. The implications are discussed.

Effect of carbon fiber surface-treatment on mechanical properties of C/C composites

Abstract Carbon fiber/carbon composites were prepared with thermosetting resin-derived carbon matrix and two groups of carbon fibers (i.e., surface treated and non-surface treated) with and without sizing treatment. These composites were heat treated at 1000°C and 3000°C. Fracture behavior and flexural strength of the composites were studied at ambient temperature. Surface treatment of carbon fibers played an important role on fracture behavior and strength of the composites. The composite with surface-treated carbon fibers heat treated at 1000°C showed low strength and a catastrophic fracture pattern, whereas those heat treated at 3000°C showed a pseudo-plastic fracture pattern. However, the behavior was just the opposite in the composite with non-surface-treated carbon fibers. Graphitization of composites with two series of carbon fibers showed an altogether different matrix microstructure. Composites with surface treated fibers showed a columnarlike carbon matrix whereas those made with non-surface-treated fibers possessed a lamellar type carbon matrix well oriented around carbon fibers.

Influence of carbon fiber type and weave pattern on the development of 2D carbon-carbon composites

Abstract 2D Carbon-carbon composites have been developed using a multiple impregnation-carbonization technique. It has been found that the weave pattern of the carbon fabric as well as the type of the carbon fibers used plays a significant role in densification as well as on the mechanical properties of the carbon-carbon composites. Studies on fractured faces along with stress-strain analysis have revealed that the mechanical properties of the carbon-carbon composites are controlled by the fracture pattern of the composites which, in turn, has been found to depend on the type of reinforcement and heat-treatment temperature of the composites. Implications have been discussed.

Mechanical behaviour of carbon-carbon composites made with surface treated carbon fibers

Abstract Unidirectional carbon-carbon composites have been made with type I carbon fibers surface treated with HNO3 for different durations and phenolic, polyfurfuryl alcohol, and coal tar pitch as matrix precursors. It has been found that fiber-matrix bonding in polymer composites has great influence on the pyrolysis shrinkage, macrostructure, flexural strength and fracture behaviour of the final carboncarbon composites. The stronger the fiber-matrix bonding in polymer composites, the higher is the shrinkage of the composites during carbonisation and graphitisation and stronger are the fiber-matrix interactions in carbonised composites. Carbonised composites made with surface-treated carbon fibers, having strong fiber-matrix interactions, are found to fail catastrophically at low load. However, on graphitisation, these composites are found to exhibit increased mechanical properties with mixed mode fracture. These observations are found to hold true for all matrix precursors under study. Implications have been discussed.

Effect of preoxidation conditions on mechanical properties of carbon fibres

Abstract A detailed study of the temperature and the duration of oxidation of polyacrylonitrile fibres has been made. The importance of choosing the correct duration and temperature of preoxidation has been demonstrated. Shrinkage during carbonization versus preoxidation time was studied. An empirical method is described which can be used for the estimation of optimum preoxidation time at any temperature. By the use of oxygen instead of air for oxidation, optimum duration of oxidation has been reduced to just 5 hr as compared to 20 hr reported in the literature. The implications are discussed.

Matrix modification by graphite powder additives in carbon fiber/carbon composite with thermosetting resin precursor as a matrix

Abstract Carbon fiber/thermosetting resin matrix precursor modified by graphite powder heat treated at 1000°C and at 3000°C was prepared. Flexural strength and fracture pattern of the composites were observed. Small addition (5–10%) of graphite powder to matrix precursor was effective on the strength of both types of carbonized composites, and graphitized ones with non-surface-treated fiber but was not so effective on that of the graphitized composite with surface-treated carbon fiber. Small addition of graphite powder gave ductility to the carbonized matrix and decreased interfacial gaps in the graphitized composite with non-surface-treated fibers. However, a large amount of graphite powder addition was less effective, due to the matrix inhomogeniety.

Oxidation behaviour of pitch based carbon fibers

Abstract Oxidation behaviour of a variety of carbon fibers made from isotropic as well as mesophase pitch has been studied. The microstructure of these carbon fibers has been studied using SEM Optical Microscope, and x-ray diffractometer. After graphitization, the carbon fibers made from isotropic pitch did not exhibit any structural pattern under SEM or the optical microscope. However, the crystallite parameters did improve significantly. Some mesophase pitch based carbon fibers are found to possess sheath and core type microstructure, the sheath being made of sheet-like graphitic layers laid parallel to the fibers axis while others exhibit uniform structure wherein the graphitic sheets are parallel to the radius or diameter of the fibers and run along the length of the fibers. The oxidation behaviour of the fibers has been correlated with their microstructure and crystallite parameters. Carbonized fibers made from isotropic pitch exhibit the least oxidation resistance. In the case of fibers having a sheath and core microstructure, the oxidation of the core starts at a relatively lower temperature than that of the sheath. Carbon fibers having higher La value are found to have higher oxidation resistance. The reaction rates of different carbon fibers are not affected much with burn-off up to 80% at a particular temperature. The apparent activation energies vary from 112 kJ/mole to 205 kJ/mole for different carbon fibers.

Abnormal behaviour during graphitisation of carbon/carbon composites made with pitch based carbon fibers

Abstract Carbon/carbon composites have been made with coal tar pitch as matrix precursor and mesophase pitch based carbon fibers with different microstructures as reinforcements. Composites are made with and without intermediate graphitisation steps. It has been found that composites made with carbon fibers having parallel sheet-like microstructure exhibit expansion in fiber direction and increase in flexural strength during first graphitisation, whereas those made with radial and mixed microstructure exhibit no change in fiber direction but decrease in flexural strength after first graphitisation. Studies on structural parameters as evaluated from x-ray diffraction patterns of the fibers and composites have revealed that in former composites the fiber/matrix interactions lead to stretching of the fibers during graphitisation of the composites resulting in increase in length, flexural strength, and structural parameters. This phenomenon is inhibited in latter composites due to mixed microstructure of the fibers.

Influence of matrix precursor on the oxidation behavior of carbon-carbon composites

Abstract Carbon-carbon composites have been made using PAN- and pitch-based carbon fibers as reinforcement, and the carbon matrix has been derived from phenolic resin (R) and coal tar pitch (P) and a mixture of the two. The temperature of initiation of oxidation in the case of carbonized composites is about 150–200°C lower than that of their graphitized counterparts. Likewise, for 100% weight loss, the temperature required is 150–200°C higher for graphitized samples compared to carbonized samples. Comparing the different matrix systems (R + R, R + P, and P + P) and the same fiber, whether PAN or pitch, the temperature of initiation of oxidation decreases in the order R + R >R + P >P + P. The initiation of oxidation has been related to crystallite dimensions, porosity, and its accessibility to the oxidizing atmosphere and microstructure of the composites. Composites having pitch as the matrix leading to open porosity accessible to the oxidizing atmosphere show anisotropy even in the carbonized stage, which results in a lamellar-type microstructure on graphitization; whereas composites made with resin as the matrix leading to amorphous carbon having closed porosity show an isotropic microstructure at the carbonization stage, which converts into a columnar-type microstructure upon graphitization. The columnar-type microstructure is more oxidation resistant than the lamellar-type microstructure. Therefore, composites made with resin as matrix should be more oxidation resistant than composites made with pitch as the matrix; this was confirmed by TGA results. It is concluded that the microstructure derived from the matrix, the porosity, and its accessibility to the oxidizing atmosphere are the factors which control the reactivity of the carbon-carbon composites.