Norio Iwashita

Potential change with intercalation of sulfuric acid into graphite by chemical oxidation

Abstract Potential of graphite during the intercalation of sulfuric acid by chemical oxidation with either nitric acid or potassium permanganate in H2SO4 of 18 mol/dm3 was found to increase to positive direction in stepwise. On potential change, a distinct plateau corresponding to the transformation from stage two to one compound and a faint one for the formation of stage three were observed, exactly the same as those observed during galvanostatic oxidation. The intercalation of sulfuric acid into graphite by chemical and electrochemical oxidations was shown to be fundamentally the same processes; potential of graphite increases with oxidation and the intercalation of sulfuric acid proceeds by showing potential plateaux. On the change of potential of graphite by chemical oxidation, there was found to be an upper limit of saturated potential, depending strongly on the oxidizer used and also on the concentration of sulfuric acid even if enough amounts of oxidizer was used. The relation between the threshold potential for stage transformation in H2SO4-GICs, increasing with dilution of sulfuric acid, and the upper limit of saturated potential by chemical oxidation, decreasing with dilution of the acid, governed the intercalation reaction by chemical oxidation in sulfuric acid with various concentrations.

Potential survey of intercalation of sulfuric acid into graphite by chemical oxidation

Abstract The intercalation reaction of sulfuric acid into graphite was surveyed by measuring the potential of the host graphite. The potential of host graphite during the intercalation of sulfuric acid by chemical oxidation was found to increase to positive direction in stepwise, corresponding to the formation of each stage structure of intercalation compounds, and to be saturated at a certain value. The threshold potential for the stage transformation in H 2 SO 4 _GICs remarkably increases with dilution of sulfuric acid, and the upper limit of saturated potential by the addition of excess amount of oxidizer remarkably decreases with dilution of sulfuric acid. It is the relation between these two potentials that governs the intercalation of sulfuric acid into graphite in various sulfuric acid solutions.

Electrochemical synthesis of graphite intercalation compounds with nickel and hydroxides

Abstract Graphite intercalation compounds (GIGs) with nickel and iron hydroxides were synthesized from the corresponding GICs with metal chlorides by galvanostatic oxidation and also by repeated charge-discharge cycles in an alkaline secondary battery with KOH aqueous solution. The stage-one structure in the original chloride-GICs was found to disappear after the first charge-discharge process and the compounds kept the stage-two structure. A gradual increase in the capacity of the battery with charge-discharge repetition suggests a slow replacement of chloride ions by hydroxide ones in the graphite interlayer space.

Discharge performance on the electrode of ternary intercalation compounds of graphite with metal hydroxides

Abstract The graphite intercalation compounds with metal hydroxides (metal hydroxide—GICs) were used as the cathode materials of an alkaline secondary battery. The performance of charge and discharge reactions on the electrode was characterized by activation energies for both reactions. The activation energy for the discharge reaction on the GIC electrode was lower than that on the conventional nickel hydroxide electrode. for a battery with a hydroxide—GIC electrode, it was shown tha the discharge curve had a well-defined plateau and also the overvoltage was small even in the discharge under a large current. The lowering of activation energy would strongly be related with the structure of the GIC, in which nickel hydroxide was held in between conductive graphite layer planes and charges transfer occured easily between intercalate and host graphite.

Effect of interface control on fracture behaviour of woven C/C composites

Effects of fibre/matrix interface and matrix microstructure on the strength and fracture mechanism of C/C composite materials were investigated by coating bismaleimide-triazine co-polymer (BT-resin) on the surface of the carbon fibre and changing the heat-treatment temperature. Here, the high-modulus type carbon fibre was used to avoid the change of fibre properties during heat treatment. For the case of carbonized C/C composites (heat-treatment temperature=1600°C), the notched strength of composites without BT-resin coating was smaller than the net section stress failure criterion. The notched strength of the wider specimen was smaller than the narrower specimens. Thus, the composites indicated clear notch sensitivity. On the other hand, the notched strength of C/C composites with BT resin coating was well estimated by the net section stress criterion. For the case of graphitized C/C composites (heat-treatment temperature=2500°C), the notched strength was well estimated by the net strength criterion without respect to BT-resin coating. The change of the fracture mechanism in microscopic scale was well correlated to the change of the fracture criterion.

Measurement of the distribution of cross sectional shape along the axis of carbon fibers and its effect on the mechanical properties

Mechanical tests of a single carbon fiber are difficult because of its small size. The diameter or the shape of the cross section is very important for a precise evaluation of the properties. However, most size evaluations use a measurement of fiber diameter in only one direction and its effect on the results has not been studied sufficiently. In order to investigate the apparent diameter variation of carbon fibers, we have developed a new apparatus with two functions. One is that we can accurately control the measurement position along the fiber axis and the other is that we can rotate a single fiber around its axis. Measurements were carried out for two types of carbon fibers. Though the cross section of one of them was far from circular, its shape was almost the same along the fiber axis and its orientation was rotating randomly. For both fibers, an assumption of an elliptical cross section gave a smaller coefficient of variation for torsional modulus and tensile properties.

Synthesis of nickel- and iron-hydroxide graphite intercalation compounds and their application to alkaline secondary battery

Recently one of authors developed new process to synthesize different graphite intercalation compounds(GICs) of metal chlorides by using their molten salts[1,2]. Ternary GICs of NiCl2 and FeCl3 were obtained from the molten salts of NiCl2 and FeCl3 with different ratios at 400°C after 3 days without any intentional addition of chlorine gas. The compounds were found to be stable in air and even in boiling water.