Ichitaro Ogawa

Oxidation resistance of carbon-ceramics composite materials sintered from ground powder mixtures of raw coke and ceramics

Carbon-SiC-B4C composite materials were prepared from ground powder mixtures of petroleum raw coke, SiC and B4C by powder sintering, without the use of any special binder and hot-pressing process. Dense composites with a fine microtexture were obtained. Oxidation tests were carried out on the composites at temperatures from 1000 to 1300° C under an air flow. The oxidation resistance depended strongly on the SiC/(SiC + B4C) ratio and total contents of SiC and B4C in the composites, which determined the compositions of B2O3 and SiO2 in the protective film formed at the surface of the composite block during oxidation. In optimum ratios, from 63 to 87%, the composites showed such a high oxidation resistance that they were comparable with Si3 N4 at 1200° C.

Carbon/ceramics composites — Preparation and properties

Abstract Fabrication methods for carbon/ceramics composites were established by using two different processes of hot-pressing and pressureless sintering without any binder phase. In the hot pressing method, some boron compounds were found to be an effective aid for sintering and graphitization of coke powder above 2000°C under some pressure. When the content of boron compound such as B4C was high, graphite/B4C composites could be fabricated. If some other ceramic powder such as NbC, TiC or TaC was mixed in addition to the B4C, three component composites with graphite matrix could be obtained. In pressureless sintering method, raw coke carbon powder was ground for a long time to be transformed in to a sinterable and non-graphitizing-type carbon powder. From a mix of ceramic powders such as SiC or B4C with the ground coke powder, the composites of carbon/SiC or carbon/SiC/B4C systems could be fabricated by heat-treatment under normal pressure. Some properties of the graphite samples and carbon/ceramic composites were investigated. It was found that their mechanical properties were much better than those of conventional graphite samples and the resistance to oxidation and corrosion was also excellent. It is suggested that the composites could be applied as bearing or mechanical seals both for use in high temperature environments and as machine parts in contact with some molten metals.

Influence of grinding on the graduation graphitization and densification of coke powder

Petroleum coke powder, made by the delayed coking method at about 500° C, was ground for various times from 15 min to 44 h. Carbon solids were made from the ground coked powder compacts and heat-treated at 1000 to 2800° C without the use of binder materials. The coke particles ground for a considerable time had a spherical appearance and an amorphous structure, and showed non-graphitizability. These ground powders were easily densified and hard carbon solid could be obtained by heat-treatment. However, if the coke powder was pre-heat treated above about 600° C before grinding, no densification occurred and the powder graphitized as well as non-ground ones. The hard carbon solids made from powder ground for 44 h had a bulk density of 1.71 g cm−3, Shore hardness of 120 and bending strength of about 700 kg cm−2 at a heat-treatment temperature of 1000° C. These values changed with increasing heat-treatment temperature to a bulk density of 1.93 g cm−3, Shore hardness of 90 and bending strength of 500 kg cm−2 at a heat-treatment temperature of 2800° C.

Pressure carbonization of pitch/phenolic resin mixtures

Phenolic resins and the benzene-soluble fraction of a coal tar pitch were mixed in a solvent (pyridine) and carbonized at 600° C in a gold tube under a pressure of 30 MPa. Yields, optical textures and graphitizabilities of the carbons were studied. Large carbon yields (>80%) were obtained from sealed tubes under pressure (closed system). In open tubes under pressure (open system), only slight improvements in carbon yields were observed. As the resin content in the starting mixtures increased, the optical texture of the resultant carbons decreased from coarse mosaic to isotropic through intermediates with a gradual decrease in size of mosaic units. These intermediate optical textures occurred with a wider range of resin content under pressure than under atmospheric pressure, especially from closed systems. Changes in structural parameters of the carbons after the 2800° C treatment corresponded to the changes in optical texture with resin content.

Influence of quinoline soluble component on the sintering behavior of ground raw coke

Abstract An attempt was made to clarify, from the viewpoint of the behavior of the quinoline soluble component, the reason for the promotion of the sinterability of raw coke powder caused by long grinding. Raw coke powder manufactured at 460°C was ground for 44 hr by an automatic mortar. The ground powder was then treated with quinoline to remove the soluble component. Two powders, as-ground and quinoline-treated, were compressed into cylindrical compacts 20 mm in dia., and then heat-treated up to 2800°C. Three points were clarified; 1. (1) the amount of the quinoline soluble component was not increased by grinding; 2. (2) in ground raw coke powder compact, the quinoline soluble component existed as a boundary phase, and transformed into a porous carbon phase by successive heat-treatment; 3. (3) the ground raw coke powder treated with quinoline showed relatively better sinterability at elevated temperatures, that is, the quinoline soluble component in ground raw coke hindered the sintering.

Effects of boron addition on some properties of hard-type carbons

The boron-containing hard-type (HT) carbons were prepared by heating the raw coke compacts with 1.6 wt% boron at temperatures ranging from 1000 to 2800° C. Some physical and mechanical properties of boron-doped HT carbons have been measured and compared with those for boron-free materials. It was confirmed that the boron enters the HT carbon at a relatively low temperature of 1400° C and enhances the densification process of compacts during heat-treatment above 1800° C. The addition of boron caused increases in Youngs modulus and thermal conductivity, and decreases in hardness and electrical conductivity of HT carbons. The effects are discussed, and compared with those for graphitizable carbons.

Densification of raw coke powder compacts

The effects of grinding on densification of a petroleum raw coke were examined. The compact of the powder ground for a short time showed large puffing during calcination below 400 °C, and no densification by subsequent heat treatment. It was found that the puffing could be suppressed by long grinding of the powder. In the compact of the powder subjected to prolonged grinding, the pore volume decreased in two temperature ranges of calcination, at 400–500 and 600–700 °C. The first decrease seems to be attributed to softening and volume reduction due to carbonization of quinoline-soluble components among the coke grains. The second decrease suggests that the sintering due to solid state material transport, such as viscous flow and plastic deformation, takes place in addition to gas-phase material transport. The increase in bulk density at temperatures over 1000 °C was mainly due to contraction of constituent grains in which micropores decreased, not a decrease in pore volume of the compact. The most remarkable effect of grinding on densification of the raw coke was the suppression of puffing during the calcination process.