Hisayoshi Yoshida

Oxidative degradation of carbon blacks with nitric acid. (I) : Changes in pore and crystallographic structures

Abstract Furnace and acetylene blacks were oxidized with nitric acid in order to obtain some insight into the mechanism of oxidative degradation of carbon blacks. Changes in the pore and crystallographic structures of the carbon blacks in the course of oxidation were examined by nitrogen adsorption, X-ray diffraction, scanning tunneling microscopy (STM) and transmission electron microscopy (TEM). A furnace black with an undeveloped crystallographic structure was easily oxidized, and at an early oxidation stage many pores were formed in the particles, leading to a high surface area. As the oxidation proceeded the surface area became negligible because the carbon black particles changed into masses resulted from coagulation of crystallites and carbon layers broken up by oxidation. For an acetylene black with a developed crystallographic structure, oxidation proceeded at the peripheries of the crystallites, and many crevasses were formed and developed into the inner parts of particles. Furthermore, the oxidative degradation proceeded preferentially in the inside of the particles with a less developed carbon structure and left a hollow structure in which the holes were opened to the outside through the crevasses formed in the shells. The presence of the holes and the crevasses were observed with TEM and STM, respectively.

Preparation and characterization of swelling porous carbon beads

Porous carbon beads have been prepared by two replica methods, Ester and CVD methods. The Ester method consists of esterification of silica gel beads with alcohol or phenol, pyrolysis of the esterified silica gel, and dissolution of the silica gel with HF solution, yielding porous carbon beads with specific surface area ranging 1100–2000 m2 g−1. On the other hand the CVD method consists of pyrolysis of benzene vapor on silica gel beads and dissolution of the silica gel with HF solution, yielding porous carbon whose specific surface area is 640 m2g−1. Porous carbon beads made from 4-phenyl-1,2-dihydroxybenzene by the Ester method showed enormously large expansion on adsorbing benzene (their apparent volumes doubled) and contracted to the original size on desorption. These dimensional changes were reproducible, and expansion and contraction completed in a few tens of seconds. The extent of dimensional changes was influenced by the kinds of adsorbates and decreased in the following order: toluene > xylene, benzene > chloroform > carbon tetrachloride > 1-butanol > 1-propanol, octane > heptane > cyclohexane, ethanol, hexane > pentane > methanol > water. On the contrary the porous carbon made by the CVD method did not show any dimensional change. A mechanism for the dimensional change of the porous carbon beads was proposed based on strain energy of carbon films, capillary force at menisci in the pores, and interfacial energy of carbon films.

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.

Pore structure and swelling character of porous carbon beads prepared from esterified silica gels

Porous carbon beads have been prepared by a replica method using five silica gels with surface areas ranging from 70 to 765 m2 g−1. The replica method consists of esterification of silica gel with 4-phenyl-1,2-dihyroxybenzene, pyrolysis of the esterified silica gel, dissolution of the silica gel, and drying of the porous carbon from water or, in some cases, from benzene. Porous carbon beads, at maximum, shrank to about 33% of the size of silica gel beads used as a substrate during drying in the preparation. The extent of shrinkage was increased with decreasing surface area of the silica gels. The porous carbons prepared from different silica gels showed a similar pore size distribution, although the distribution became somewhat broader when a silica gel with a small surface area was used. The percentage of expansion of porous carbon beads on adsorbing benzene varied to some extent with the kind of silica gel, and the porous carbon prepared from the silica gel with the surface area of 470 m2 g−1 showed a maximum expansion, 31%. Porous carbon beads dried from benzene showed about 10% higher percentages of expansion on average compared with that from water. Furthermore, in the carbon beads from benzene, a small number of carbon beads showed enormously large expansion, and the expansion was found to be related to the extent of shrinkage during drying in the preparation.