The effect of carbonization temperature on the structure and properties of carbon fibers prepared from poly(m-phenylene isophthalamide) precursor

Abstract

Carbon fibers with diameters ranging from 8.1 and 12.7 µm were produced using a commercially available meta-aramid precursor after oxidation and carbonization steps. The carbonization process was performed using a single-step procedure between 500 and 1100 °C. The effect of temperature on the structure and properties of carbon fibers was investigated in detail. The process of carbonization was examined using several characterization techniques including fiber diameter, mass yield, density, elemental analysis, mechanical property testing and electrical property measurements. Structural transformations were followed and monitored using X-ray diffraction, IR and Raman spectroscopy techniques. The results suggested that the mass yield reached a value of 40.4 % at a heating temperature of 1100 °C. Density, carbon content, mechanical properties and electrical conductivity values increased and hydrogen and nitrogen contents decreased with an increase in temperature. Analysis of the X-ray diffraction traces of carbon fibers suggested broadening of the (002) and (100) diffraction planes which was attributed to the formation of an amorphous carbon structure. The IR spectra showed, at temperatures of 500 °C and above, initial weakening and eventual disappearance of the major amide bands (amide I, II, III and IV) due to the loss of hydrogen bonds between the polymer chains indicating the partial removal of nitrogen, hydrogen and oxygen atoms during the carbonization reactions. The analysis of Raman spectra demonstrated that the positions and the peak widths of the G- and D-bands showed great dependence on treatment temperature. The mechanical properties of the carbon fibers showed strong dependence on heat-treatment temperature, porosity and gage length. Temperature and gage length showed a significant effect on the tensile strength values obtained after each treatment temperature. A marked increase was observed in tensile strength values after extrapolation to 1 mm, which increased from 186 to 589 MPa with carbonization up to 1100 °C. Tensile modulus values were affected by both temperature and porosity and reached a value of 81 GPa at 1100 °C. Porosity correction caused an enhancement in tensile modulus values between 21 and 33.5 %. SEM images of carbon fibers demonstrated the presence of structural imperfections confirming the results obtained from the porosity measurements.