Eiichi Yasuda

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.

A role of charge-transfer complex with iodine in the modification of coal tar pitch

Abstract Iodine introduced into coal tar pitch (CTP) can drastically alter its reology and carbonization behavior. Here, the mechanism of interaction between iodine and CTP has been investigated by using various spectroscopic methods such as 127I-NMR, EPR and FT–IR. It is shown that some iodine molecules infiltrated into the CTP and form charge transfer complexes with the relative large aromatic components of the CTP. Hyperfine sublevel correlation spectroscopy (HYSCORE) revealed the molecular size of cation radicals, which contain more than ten benzene rings, and the location of iodine anion that is incorporated at ca. 0.3 nm from the aromatic cation radicals. The elemental H/C ratio decreased and the viscosity of CTP increased with the density of the charge-transfer complexes, which was also increased by the iodine treatment. These results strongly suggest that dehydrogenative polymerization of CTP occurs during the iodine treatment. The cation radicals in the charge-transfer complexes accelerate the dehydrogenative polymerization and result in a high carbon yield.

Carbonization and graphitization behavior of iodine-treated coal tar pitch

Abstract The authors investigated carbonization and graphitization behavior of coal tar pitch (CTP) treated with iodine. The behavior of weight decrease and the microstructural development with increase of heat treatment temperature were different between non-treated and iodine-treated CTP. The carbon yield of CTP increased from 40 to 70% by the iodine treatment. Flow type texture was obtained from non-treated specimen and isotropic texture was obtained from the iodine-treated one. Gas mass spectrometric analysis showed that dehydrogenative reactions were accelerated by iodine at the temperature range from 200 to 450 °C. The iodine treatment was mainly effective for the benzene soluble fraction of the CTP. The X-ray diffraction on graphitized specimens showed that the graphitizability was able to be delayed by the iodine treatment.

Fracture behavior of CFRPs impacted by relatively high-velocity steel sphere

Abstract Carbon fiber reinforced plastics (CFRPs) of 2-mm thickness were made with different interfacial strengths, properties, and sequences of reinforcing fibers. A 5-mm diameter steel sphere was impacted upon the CFRPs at 150– 314 m / s . By energy absorption measurement and in situ morphological observation, the effects of interfacial strength and properties on the fracture behavior of the CFRPs were investigated. On impact at 150 m / s , a weak interface resulted in high energy absorption. The rear layer should consist of high-strength fiber for higher energy absorption in this velocity range.

Surface graphitization of furan-resin-derived carbon

Furan-resin-derived carbon generally produces a glass-like carbon having entangled graphene layers (graphite structure) after high temperature heat-treatments. However, Raman spectroscopy reveals that it produces well-graphitized thin skins on surfaces. The graphitization is promoted on the faces that are formed at lower heat-treatment temperatures. Fractured faces of specimens pre-heat-treated at 1000°C result in well-developed structures in graphitization after re-heat-treatment at 3000°C. It is considered that the surfaces, i.e. free faces, play an important role in the graphitization.

Divorced eutectic and interface characteristics in a solidified YAG-spinel composite with spinel-rich composition

Solidified microstructures of YAG (Y3Al5O12)-spinel (MgAl2O4) composite with spinel-rich composition (YAG : spinel = 1 : 30 molar ratio) were investigated. Intergranular YAG divorced eutectic exhibiting fine-grained structures was observed in the solidified composite. Formation of the divorced eutectic was attributed to the metastable growth of primary phase spinel during solidification, which was verified by the presence of in situ spinel precipitates. TEM-SAD technique and crystal structure analysis reveal that a set of specific crystallographic orientation relationships are present between {640} plane of YAG and {111} plane of spinel crystals. The presence of low-energy YAG/spinel interfaces is believed to be responsible for the transgranular fracture characteristics exhibited for the solidified composite.

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.

THE MICROSTRUCTURE AND CREEP DEFORMATION OF HOT-PRESSED SI3N4 WITH DIFFERENT AMOUNTS OF SINTERING ADDITIVES

Compressive creep deformation of hot-pressed silicon nitride with different amounts of grain boundary glassy phase was investigated at 1300–1400 °C under 30–100 MPa. The stress exponent of the creep rate was determined to be nearly unity. The apparent activation energy of silicon nitride with a larger amount of glassy phase was measured to be about 700 kJ/mole, and that with a smaller amount of glassy phase was found to be 400 kJ/mole. In addition, the microstructural observation found that no cavity appeared and grain boundary glass was recrystallized during creep test. Thus, the rate-limiting steps in solution/precipitation creep mechanism change from the solution-reprecipitation of Si 3 N 4 grains to the diffusion through the grain boundary with increasing the amount of glassy phase.

Anisotropy of creep deformation rate in hot-pressed Si3N4 with preferred orientation of the elongated grains

Compressive creep deformation of hot-pressed silicon nitride with two different preorientations of grain was investigated at temperatures in the range of 1300–1400 °C under 30–100 MPa. The stress exponent of the creep rate was determined to be nearly unity of the apparent activation energy of creep rate was found to be about 500 kJ mol-1. It means the creep deformation is due to diffusion controlled solution/precipitation. Creep rate of specimen with creep loading direction in parallel to the hot-pressing axis was determined to be higher than that in perpendicular to the hot-pressing axis. In addition, microstructural observation revealed that no cavity appeared and grain boundary glass was recrystallized during creep. X-ray diffraction (XRD) analysis confirms that needle-like Si3N4 grains were reoriented during creep test. These results indicate that the anisotropy of creep rate results from the disparity in the rate of solution–reprecipitation of grains rather than that in diffusion through the grain boundary, which is dependent on the preferred orientation of the needle-like grains.

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.