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The tensile strength of carbon/carbon composites (C/Cs) was examined as a function of the density in the range above ρ=1.6 g/cm3. Three processing routes of preformed yarn, resin char, and HIP processes were adopted to densify the C/C composites. The density was increased to 1.95 g/cm3 through these routes. The interfacial strength between the fiber and matrix was significantly and unexpectedly varied by repeating the HIP process, though the detailed source mechanisms could not be identified. The tensile strength of the densified C/Cs was shown to be dependent on the processing routes. In particular, opposite tendencies were obtained as a function of ρ, i.e. the monotonic strength increases for the C/Cs with the preformed yarn and resin char treatments, while a monotonic decrease was observed using the HIP process. However, after the tensile strength was re-examined, the ultimate tensile strain was found to be expressed by a monotonic decreasing function of the interfacial strength. This implies that weakening fiber interfaces is a key factor for obtaining C/C composites with the higher tensile strength.

A highly porous carbon/carbon composite, known as carbon bonded carbon fiber (CBCF) and used as thermal insulation, was densified by chemical vapor infiltration (CVI). The erosion resistance, thermal conductivity and thermal expansion coefficient were measured with interest to utilization of the CVI densified composite as erosion protection in furnaces that employ inert gas quenching. It was found that the erosion rate decreased exponentially as bulk density increased from 0.18 to 1.00 Mg·m−3 and this was related to the Ryshkewitch and Duckworth relationship which relates mechanical properties to the porosity of ceramic materials. The thermal conductivity of the CVI densified composite both parallel and perpendicular to the fiber array was greater than that of CBCF in the measured range between 1000 and 2000°C. The measured values of thermal conductivity showed reasonable agreement with values calculated from a model structure. For CBCF and CVI densified composite the contribution of solid conduction was calculated to be dominant compared to the contribution of radiative heat transfer and heat transfer taking a path through the gas present in the pores of the composite. As a function of temperature thermal conductivity decreased for the CVI densified composite whereas an increase was found for CBCF. This opposite behavior is a result of the intrinsic properties of the dominant component in the composites: fibers in CBCF and pyrolytic carbon deposit in the case of CVI densified composite. Both materials exhibited anisotropy in the thermal conductivity; the thermal conductivity parallel to the fiber array being about 2.8 times that in the perpendicular direction in the case of CBCF. The thermal expansion coefficient in both directions increased as CBCF was densified by CVI; the increase was greater in the direction perpendicular to the fiber array which increased from 1.8×10−6 K−1 for CBCF to 5.3×10−6 K−1 for the CVI densified composite (1.10 Mg·m−3). The thermal expansion anisotropy was consistent with a c-axis radial orientation of the pyrolytic carbon around the fibers in the CVI densified composite.

Tantalum(V) chloride was intercalated from the gas phase into highly oriented pyrolytic graphite and into natural graphite. Second stage intercalation compounds were produced. The TaCl 5 –graphite intercalation compounds were heated in a hydrogen atmosphere for 1 week at 200°C. The flakes showed no exfoliation, but reduction was insufficient at this temperature. A reduction for 1 week at 1000°C produced extensively exfoliated samples. After 1 week at 1000°C in a hydrogen atmosphere, it was still possible to detect chlorine in some regions by energy-dispersive X-ray analysis. However, transmission electron microscope images showed the occurrence of nanometer-size particles within the graphite host lattice, located near the prismatic edges and also at the center of the particles. Different morphologies of the particles were observed, namely two-dimensional platelets and numerous one-dimensional `chains', with particle sizes between 10 and 300 nm. Selected-area electron diffraction patterns of the particles gave evidence as to their chemical nature. Metallic tantalum was formed by the reaction, the patterns gave also evidence that a small amount of TaCl 5 had still survived.

Tantalum(V) chloride (TaCl5) was intercalated into highly oriented pyrolytic graphite and into natural graphite flakes. X-ray diffraction measurements show that second stages were obtained. The fresh intercalation compounds were investigated by scanning tunneling microscopy in air and by electron diffraction. The commensurate ( 7 × 7 ) superlattice of tantalum(V) chloride shows a rotation of ±19.1° against the graphite lattice. Additional superstuctures of longer dimensions were detected by scanning tunneling microscopy. A Moire pattern and some bands were observed. The graphite intercalation compounds were immersed in water for two hours at room temperature and dried at 100°C in air. X-ray diffraction showed that the original stages were preserved after exposure to water. The scanning tunneling micrographs of samples exposed to water show a pattern similar to that of pristine graphite, the superlattice had disappeared. Electron diffraction studies on such samples gave evidence that the intercalate is rearranged and forms amorphous layers. In the case of water-immersed samples, no surface structures of the intercalate were observable with scanning tunneling microscopy.

In the presence of chlorine gas, BiCl3 was intercalated from the gas phase into highly oriented pyrolytic graphite; second-stage compounds were obtained. The surface structure in air was determined with a scanning tunneling microscope. A long-periodical rectangular structure was observed with an a-axis of 1429 pm and a b-axis of 1841 pm. This commensurate structure fits very well with X-ray diffraction data for a superlattice in BiCl3 graphite intercalation compounds published by another group.

Unidirectional carbon fiber reinforced carbon composites (CC composites) with different matrix textures were prepared from six matrix precursors. Two composites with an optically isotropic matrix showed tensile strength and modulus values that increased on heat treatment of the samples at a high temperature. Composites with an anisotropic matrix, having preferred orientation parallel to fiber surface, were deformed due to shrinkage as the graphitic structure developed on heat treatment. This caused the fibers to be exfoliated from the anisotropic matrix, as a result of residual stresses generated by restriction of matrix shrinkage. The strengths of the composites with an anisotropic matrix were reduced after graphitization. On oxidation in a wet atmosphere, the composites with an anisotropic matrix showed higher durability than those with isotropic matrices. The oxidation of the former composites was localized at the fibermatrix interphase, but this effect was not observed with isotropic matrix composites.

The torsional behavior, together with the tensile behavior, of PAN-based and mesophase pitchbased carbon fibers was studied by means of single-filament test methods, followed by scanning electron microscope examination. The torsional modulus decreased with increasing Young's modulus for both fibers. And the mesophase pitch-based carbon fibers exhibited lower torsional moduli than those of PAN based carbon fibers, in particular markedly lower moduli at higher Young's modulus. The torsional modulus depended to a large extent on the orientation structure of the graphite crystallites in the cross-section, but also upon the linking force between neighboring graphitic-layer planes. The torsional strength of both the carbon fibers increased with increasing tensile strength and that of the PAN-based carbon fibers increased as the torsional modulus increased. The spiral face of torsional fracture surfaces of the PAN based carbon fibers and the very long longitudinal faces which the torsional-fracture surfaces of the mesophase pitch-based carbon fibers have, were also considered to be associated with the orientation structures.

Stage 2 FeCl3-graphite intercalation compound (GIC) was chemically reduced with lithium-naphthalene complex in THF and 1-MB. The GIC was also reduced electrochemically in THF, PC and EC+DEC, containing 1 mol/dm3 of LiClO4. When the reduction was performed in THF chemically or electrochemically, fine iron particles were obtained in the graphite matrix. The mean diameter of the particles is in the range of 11-13nm in this case, while 4nm for 1-MB. For PC and EC+DEC, no iron particle was observed in the product. The difference would be related to the co-intercalation of the solvent. In the reduction process, THF is incorporated together with lithium in the interlayer spacing of FeCl3-GIC. The co-intercalated THF molecules facilitate the reduction of FeCl3 by lithium in the graphite gallery probably because FeCl3 is soluble in THF, and relatively large iron particles are obtained.

Transverse sections of carbon fibers prepared from two different lots of dehydrated polyvinyl alcohol fibers, with and without graphitizing treatment, were observed by both polarized light and scanning electron microscopy. Several different types of transverse layer plane preferred orientation were found in the graphitized fibers: layers aligned parallel with the section major axis in the central zone of elliptical cross sections; parallel with the fiber surface in concave regions of an irregular cross section shape; and parallel with the fiber surface in a highly crystallized fiber with an irregular cross section shape. These graphitic layer plane configurations are considered in connection with the lamellar structure in crystalline regions of the dehydrated polyvinyl alcohol fibers.