Hideaki Sano

Effect of low-level boron doping on oxidation behavior of polyimide-derived carbon films

Abstract The oxidation behavior of polyimide-derived carbon films doped with boron in low levels (up to 1.23×10−2 B/C) was investigated by means of thermogravimetric analysis (TGA). It was observed that boron-doping less than 7.09×10−3 B/C in the carbon film formed at 1500°C had a catalytic effect on the oxidation of the carbon. Based on a kinetic analysis, it is revealed that, four factors — the catalytic effect and the inhibiting effect of substitutional boron, the improvement of graphitization due to boron doping, and the formation of B2O3 during oxidation — may contribute to the appearance of the oxidation process. The catalytic effect would be dominant up to some level of carbon burn-off when less boron is doped; the contribution of the inhibiting effect(s) may rapidly become dominant when more doped boron is employed.

Formation and oxidation resistance of the coating formed on carbon material composed of B4C-SiC powders

Abstract The formation and oxidation resistance of surface oxide layers formed on carbon-B 4 C-SiC composites with two different B 4 C-SiC ratios were investigated at elevated temperatures in dry and moist atmospheres. From SEM observation, X-ray diffraction analysis, EPMA, and ESCA measurements of the samples before and after oxidation, it was found that the oxidation resistance depended on the oxidation temperature, the presence of moisture, and the B 4 C-SiC ratio. The surface oxide layer was very weak for oxidation at a temperature range of 800 °–1000 °C, while a dense and homogeneous surface layer formed on the composite with higher B 4 C-SiC ratio on oxidation at 1200 °C. This coating was found to be a suitable protective SiO 2 -rich borosilicate layer with a self-healing property that was remarkably stable against oxidation, even in moist air of about 900 °C.

A TEM study of microstructure of carbon fiber/polycarbosilane-derived SiC composites

The structures of two types of mesophase pitch-based cal bon fibers (M30 and M70) reinforced SIC composites, prepared by the polycarbosilane impregnation-pyrolysis process, were investigated using transmission electron microscopy (TEM). It was found that M70 possessed a highly-ordered graphite structure despite occasional misorientation of some crystallites. However, the skin of M70 was less ordered than the interior of M70. The structure of M30 was uniform throughout, and was less ordered than that of M70. The fiber and matrix in M70/SiC bonded weakly, whereas the fiber and matrix in M30/SiC bonded tightly and locked together. This difference in. the interface feature originates from the difference of the surface crystalline structures of M30 and M70, and is formed during the first impregnation-pyrolysis cycle of polycarbosilane

The properties of carbon fibre/SiC composites fabricated through impregnation and pyrolysis of polycarbosilane

Unidirectional carbon fibre reinforced SiC composites were prepared from four types of carbon fibres, PAN-based HSCF, pitch-based HMCF, CF50 and CF70, through nine cycles or twelve cycles of impregnation of polycarbosilane and subsequent pyrolysis at 1200°C. The polycarbosilane-derived matrix was found to be β-SiC with a crystallite size of 1.95 nm. The mechanical properties of the composites were evaluated by four-point bending tests. The fracture behavior of each composite was investigated based on load-displacement curves and scanning electron microscope (SEM) observation of fracture surfaces of the specimens after tests. It was found that CF50/SiC and CF70/SiC exhibited high strength and non-brittle fracture mode with multiple matrix cracking and extensive fibre pullout, whereas HSCF/SiC and HMCF/SiC exhibited low strength and brittle fracture mode with almost no fibre pullout. The differences in the fracture modes of these carbon fibre/SiC composites were thought to be due to differences in interfacial bonding between carbon fibres and matrix. Values of flexural strengths of CF70/SiC and CF50/SiC were 967 MPa and 624 MPa, respectively, which were approximately 75% and 38% of the predicted values. The relatively lower strength of CF50/SiC, compared with CF70/SiC, was mainly attributed to the shear failure of CF50/SiC during bending tests.

The effects of nanoparticulate SiC upon the oxidation behavior of C-SiC-B4C composites

C-SiC-B4C composite materials were fabricated from pitch based needle coke, SiC and B4C with or without SiC nanoparticulate additive by grinding and then pressureless sintering. Anisothermal and isothermal oxidation tests of the composites obtained were carried out in oxygen Bow at temperature increasing continuously from 773 to 1573 K. Self-protection behavior of the composites with nanoscaled SiC additive was observed, in contrast to those without nanoscaled SiC additives. The morphology and constitution of the protective him that encapsulated the inner composites were investigated with scanning electron micrography and X-ray photon spectroscopy. A general model on the self-protection against oxidation was proposed

The effect of grinding on the sintering of raw petroleum coke

Preparation of dense carbon materials without use of any binder was attempted, effect of grinding treatment on the characteristics of raw petroleum needle coke and on the properties of the sintered carbon compacts was investigated, and the sintering mechanizm of the coke after grinding was discussed. It was found that the coke particle size and crystalline size was decreased and the specific surface area of the coke was increased with grinding, and the influence was obvious up to 20 hours grinding treatment. The interlayer spacing of the coke after graphitization at 2773 K was gradually increased and the a-axis constant was marginally reduced with the grinding time. Results from the sintering experiment show that carbon materials can be sintered from the ground coke, and the density and flexural strength of the sintering compacts reached stable values of 1.8 g cm(-3) and about 70 MPa, respectively, after 20 hours grinding of the coke. The sintering ability of the coke after grinding was considered to result from the increase of surface energy, lattice distortion energy and fraction of volatile matters of the coke, which were induced by grinding treatment

Oxidation behaviours of carbon/carbon composite with multi-coatings of LaB6-Si/polycarbosilane/SiO2

The influence of multi-coatings of LaB6-Si/polycarbosilane/SiO2 on the oxidation behaviour of carbon/carbon composite materials was investigated in the temperature range from 500 to 1400 °C. The additives of LaB6-Si offered lower oxidation rates and accelerated increases in oxidation rates at temperatures below 900 °C. The coating of polycarbosilane (PCS) improved the compatibility of the coating on the carbon/carbon composite and lowered the oxidation rates of the LaB6-Si coated composite below the transition temperature. With the SiO2 coating, the cracks of the LaB6-Si/PCS coating was sealed and a good oxidation resistance of the LaB6-Si/PCS/SiO2 coated composite was found at temperatures up to 1300 °C.

Oxidation kinetics and mechanisms of ceramic-carbon composites I. Modeling for the nonreactive ceramic layer type

Abstract The decarbonization and oxidation processes in ceramic-carbon composites during oxidation in gaseous oxygen have been studied theoretically. The influences of temperature, duration of time, content of ceramic in the composites and other factors on the oxidation are discussed kinetically. According to the model, the recession rate of the carbon matrix will slow down rapidly with decrease of ceramic particle size to nano scale, predicting a possible approach to design oxidation-resistant new ceramic-carbon composites.

A study of film thickness dependence of the graphitizability of PMDA–ODA polyimide-derived carbon film

Abstract PMDA–ODA (pyromellitic dianhydride–4,4′-oxydianiline) polyimide (PI) films with thickness of 2–26 μm were synthesized and heat-treated at 2200–2500°C. The effect of film thickness on both the graphitizability of the PI-derived carbon film and the crystalline organization of the initial PI film was investigated by X-ray diffraction. The degree of graphitization of the resultant carbon film was observed to increase with decreasing initial PI film thickness, corresponding to the film thickness dependence of the crystalline ordering in the initial PI films. Structural factors related to PI film thickness are discussed for understanding the graphitizability of the resultant carbon.

Effect of Boron Addition on Oxidation Resistance of Carbon Fiber Polycarbosilane–derived SiC Composites

Nagasaki Univ, Dept Mat Sci & Engn, Nagasaki 8528521, Japan. Acad Sinica, Inst Met Res, Shenyang 110015, Peoples R China.;Zheng, GB (reprint author), Nagasaki Univ, Dept Mat Sci & Engn, Bunkyo Machi 1-14, Nagasaki 8528521, Japan