Shiushichi Kimura

Graphitization of carbon fibre/ glassy carbon composites

Abstract Carbon fibre/glassy carbon composites were prepared by aligning unidirectionally in furfuryl alcohol condensate the PAN-based carbon fibres treated at different temperatures and with different degrees of stretching. The graphitization of the composites was found to start at the boundary between the fibres and glassy carbon matrix, and to proceed into the matrix. This is considered to be due to the stress accumulation at the boundary caused by a large shrinkage of the matrix. The carbon fibres remain nongraphitized even after a heat-treatment at 2800°C. The composites heat-treated at high temperature (2800°C) are found to show a high overall degree of graphitization, unexpected on basis of the known graphitization behavior of carbon fibres and of glassy carbon, and a high degree of uniaxial preferred orientation of crystallites.

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.

Negative creep and recovery during high-temperature creep of MgO single crystals at low stresses

High-temperature creep equipment with very high precision has been used to measure the creep of MgO single crystals above 1948 K and stresses lower than 4 MPa. A transition in exponent,n, from 3 at stresses higher than 2 MPa to almost unity at lower stress region was observed. Since in a single crystal deformation can only occur by the generation and movement of dislocations, the transition in stress exponent from high to low stress region cannot be interpreted in terms of a change from dislocation to diffusional creep processes. Decreasing the stress by a small amount during steady-state creep resulted in an incubation period of zero creep rate before creep commenced at lower stress. However, large stress reduction led to a period of negative creep during which the dislocation substructure coarsens and the subgrain cell boundaries straighten. On the basis of dislocation substructure studies, it is proposed that the kinetics of backflow are thought to be based on the local network refinement caused by the reverse movement of dislocations and that recovery is necessary before further movement of dislocation can occur. It is shown that the network theory proposed by Davis and Wilshire can satisfactorily account for all stress reduction observed during forward creep.

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.

Degradation of fracture mechanics properties of reactor graphite due to burn-off

Abstract This paper evaluates the change in various mechanical and fracture mechanics properties of two kinds of graphite for high temperature gas-cooled reactors due to burn-off by air oxidation. Thermal shock resistance and thermal shock fracture toughness of the burn-off graphite are also quantitatively determined by using an arc discharge heating method. These results are expressed as a function of burn-off, B , by an empirical formula of the form; S = S 0 exp (−nB) , where S 0 and n denote respectively the initial value and the degradation exponent of the material. The empirical formulas for thermal shock are found to agree reasonably well with those calculated from the results of individually examining the effect of burn-off on the associated properties.

Statistical analysis of the effect of surface grinding on the strength of alumina using Weibull's multi-modal function

Statistical analysis of the effect of surface grinding on the fracture strength of ceramics was performed and a new distribution function based on multi-modal Weibull distribution function has been suggested. The experimental verifications were carried out on alumina specimens ground in various directions using diamond wheels which have different abrasive diameters. The theory agreed well with the experimental results.

STM study on surfaces of HOPG modified by implantation and heat treatment

Abstract High-energy ions, such as argon, copper and tungsten ions were implanted in pyrographite and the damages on surfaces, i.e. hillocks and craters, were studied mainly under scanning tunneling microscopy (STM). The number and area of the structural defects increase in function of atomic weight of implanted ions, while ion radius seems not to be a dominant parameter. A change of the inter-atomic distance between carbon atoms on hillocks was highlight by filtering STM images. The heat treatments of ion-implanted samples were performed, leading to an increase in volume of the pre-existing hillock in function of the temperature, i.e. no new hillock was created on the treated surfaces. Therefore, the diffusion of trapped-atoms (argon, copper and tungsten) is supposed to be weak and to occur towards the surfaces of pyrographite. In the proximity of the surfaces of irradiated sample, an easier diffusion of trapped-atoms is allowed by bond disrupted and interstitials, due to residual strain as shown on X-ray diffraction patterns and Raman spectra. A rearrangement of carbon atoms may also occur in pyrographite and on its surface, where the dot arrangement seems to be bend and focus only on some hillocks.

Stochastic analysis on crack path of polycrystalline ceramics based on the difference between the released energies in crack propagation

The crack path of polycrystalline ceramics has been theoretically analysed with a stochastic model based on the difference between the released energies in intergranular and transgranular crack propagation. Assuming that the path with the lowest released energy should be realized as the actual crack path, the expected values of the fraction of transgranular fracture on fracture surface and the fracture toughness of polycrystalline ceramics were formulated as functions of grain size and the critical energy release rates of grain and grain boundary. By comparison between the theory and the experimental results it was shown that the stochastic model proposed here expressed the change of the crack path and the fracture toughness of polycrystalline Al2O3, relative to grain size.

Carbonization of fractionated pitches under pressure

Abstract Seven fractions of a coal tar pitch having different molecular weights were carbonized up to 650°C under a pressure of 30 MPa in closed and open gold tubes. Carbon yield and microscopic texture of the resultant products were investigated. The graphitizability was evaluated through measurements of interlayer spacing and crystallite size after heat treatment at 3000°C. Carbons prepared from benzenesoluble fractions showed a coarse mosaic texture and high graphitizability, while the carbon yield depended not only on the molecular weight of the fraction but also on the condition of carbonization. The benzene-insoluble/pyridine-soluble fraction led to an isotropic texture and a small value of the crystallite size under the open system, but to a coarse flow-type texture and a high graphitizability under the closed system. A similar trend was observed more remarkably for the pyridine-insoluble fraction. The carbon yield of the original pitch was found to be a weighted sum of those of the constituent fractions. The presence of low molecular weight fractions seemed to have a strong effect on the texture of the resultant carbons, particularly under the closed system.

Neutron irradiation effects on dimension and mechanical properties of carbon fiber/carbon composite

Abstract A study has been conducted of the effects of neutron irradiation on dimensions and mechanical properties of a carbon fiber/carbon composite. The matrix was derived from a thermosetting resin and the reinforcement was a PAN-based carbon fiber. Irradiation up to 4.5 × 10 24 n / m 2 at 240°C gave rise to shrinkage parallel to the fibers and expansion perpendicular to them, while irradiation to 6.0 × 10 24 n / m 2 at 640°C produced the opposite (i.e., parallel expansion and perpendicular shrinkage). The mechanical strength, failure strain and fracture energy increased after neutron irradiation, while Poissons ratio decreased to about one half the value before irradiation. These effects are correlated to structural changes in the matrix and/or interface induced by the irradiation.