Abstract In the present study, the effect of nickel nitrate addition as a catalytic precursor for the in situ formation of Ni nanoparticles during the heating process has been investigated on the modification of microstructure and graphitization of amorphous carbon resulting from pyrolysis of phenolic resin. For this purpose, the prepared resin samples were cured in carbon substrate with and without additives at temperatures of 800, 1000, and 1250 °C. XRD, FESEM, and TEM studies were performed to investigate the phase and microstructural changes in the samples during the heating process. In addition to phase and microstructural studies, thermodynamic calculations of the reactions performed for the in situ formation of nickel nanoparticles and their effective factors during the curing process were performed. The results indicated that nickel nitrate is transformed to nickel nanoparticles of different sizes during the reduction process in a reduced atmosphere. The in situ formation of nickel nanoparticles and its catalytic effect led to the graphitization of carbon resulting from the pyrolysis of phenolic resin at a temperature of 800 °C and above. By increasing temperature, the morphology of the formed graphite changed and hollow carbon nanotubes, carbon cells, and onion skin carbon were formed in the microstructure. It was also observed that by increasing the temperature and the amount of additive, carbon nanotubes and their size are increased. A noteworthy point from thermodynamic calculations during the formation of nickel nanoparticles was that the nickel nanoparticles themselves acted as accelerators of nickel oxide reduction reactions and the formation of nickel nanoparticles. This increases the amount of amorphous carbon graphitization resulting from the pyrolysis of phenolic resin which leads to the formation of more carbon nanotubes at higher temperatures.

Commercial ultra-high-strength PAN-based carbon fibers (T1000G) were heat-treated at the temperature range of 2300– 2600 °C under a constant stretching of 600 cN. After continuous high-temperature graphitization treatment, microstructures, mechanical properties and thermal stability of the carbon fibers were investigated. The results show that the T1000G carbon fibers present the similar round shape with a smooth surface before and after graphitization, indicating the carbon fibers are fabricated by dry–wet spinning. In comparison, the commercial high-strength and high-modulus PAN-based carbon fibers (M40JB and M55JB) present elliptical shapes with ridges and grooves on the surface, indicating the carbon fibers are fabricated by wet spinning. After graphitization treatment from 2300 to 2600 °C under a constant stretching of 600 cN, the Young’s modulus of the T1000G carbon fibers increases from about 436 to 484 GPa, and their tensile strength decreases from about 5.26 to 4.45 GPa. The increase in Young’s modulus of the graphitized T1000G carbon fibers is attributed to the increase in the crystallite sizes and the preferred orientation of graphite crystallites along the fiber longitudinal direction under a constant stretching condition. In comparison with the M40JB and the M55JB carbon fibers, the graphitized T1000G carbon fibers are easier to be oxidized, which can be contributed to the formation of more micropores and defects during the graphitization process, thus leading to the decrease in the tensile strength.

Graphite fibers are materials with a high specific modulus that have attracted much interest in the aerospace industry, but their high manufacturing cost and low yield are still problems that prevent their wide applications in practice. This paper presents a laser-based process for graphitization of carbon fiber (CF) and explores the effect of laser radiation on the microstructure of CF. The obtained Raman spectra indicate that the outer surface of CF evolves from turbostratic structures into a three-dimensional ordered state after being irradiated by a laser. The X-ray diffraction data revealed that the growth of crystallite was parallel to the fiber axis, and the interlayer spacing d002 decreased from 0.353 to 0.345 nm. The results of scanning electron microscopy revealed that the surface of irradiated CFs was rougher than that of the unirradiated ones and there were scale-like small fragments that had peeled off from the fibers. The tensile modulus increased by 17.51% and the Weibull average tensile strength decreased by 30.53% after being irradiated by a laser. These results demonstrate that the laser irradiation was able to increase the graphitization degree of the CFs, which showed some properties comparable to graphite fibers.

Chop molding composites and 2D carbon/carbon composites were manufactured by hot press molding method. Phenol resin of novolac type was used for matrix precursor and PAN-based carbon, PAN-based graphite and pitch-based carbon fiber were used for reinforcement and boron oxide was used for oxidation retardant. All of the composites were treated by 2000℃ and 2400℃ graphitization process, respectively. After graphitization process, amount of a boron residue in carbon/carbon composites is much according to irregularity of used raw materials. Under the presence of boron in carbon/carbon composites, catalytic effect of boron was a little at 2000℃ graphitization temperature. However, it was quite at 2400℃ graphitization.

Carbon/carbon composites were developed using PAN based carbon fibres and phenolic resin as matrix in different volume fractions and heat treated to temperatures between 1000℃ to 2500℃. Although both the starting precursors are nongraphitizing hard carbons individually, their composites lead to very interesting properties e.g. x-ray diffractograms show the development of graphitic phase for composites having fibre volume fractions of 30~40%. Consequently the electrical resistivity of such composites reaches a value of 0.8 mΩcm, very close to highly graphitic material. However, it was found that by increasing the fibre volume fraction to 50~60%, the trend is reversed. Optical microscopy of the composites also reveals the development of strong columnar type microstructure at the fibre (matrix interface due to stress graphitization of the matrix. The study forcasts a unique possibility of producing high thermal conductivity carbon/carbon composites starting with carbon fibres in the chopped form only.

Raman spectroscopy has been used to investigate the structure of coal tar pitch heat-treated up to 3000℃ by using 514.5 run Ar ion laser line. Four critical temperature ranges were found on pyrolyzing coal tar pitch, which correspond to four distinct processes from disordered carbons to the well-ordered graphite structure. The range of heat treat temperature (HTT) below 1000℃ corresponds to gas evolution during the pyrolysis of coal tar pitch. Above the HTT are correlated to rearrangements of enlarged molecules, growth of the molecules along the direction of plane, finally stacking in the normal direction of the plane, in the respective HTT ranges of 1000-2000, above 2000 and 2500-3000℃.