Hisashi Imai

The reaction of nuclear graphites with hydrogen

Abstract The corrosion reactions of various nuclear graphites with hydrogen were studied in the temperature range 970–1090°C at hydrogen partial pressures between 7 and 48 torr in helium stream of total pressure of 1 atm. Methane was the only gaseous product found in this reaction. The rates of corrosion reaction were found to be 10−1 ~ 10−2 μg/cm2 hr on the basis of methane yield at 28 torr hydrogen pressure at 1025°C. The influence of the hydrogen partial pressure on the methane production rate is expressed as: R=K(P H 2 ) 3 2 . The apparent activation energy for the reaction was found to be 22 kcal/mol, independent of the impurity content of the graphite. No clear correlations were found between the production rate of methane and the total ash content in the graphite, its degree of crystallinity (determined by X-ray diffraction) and its degree of graphitization (infered from butanol immersion density).

High temperature heat capacities and electrical conductivities of boron carbides

Abstract The heat capacities and the electrical conductivities of B x C ( x = 3, 4, 5) were measured by means of direct heating pulse calorimetry in the temperature range from 300 to 1500 K. The heat capacities of B x C increased with increasing x value. This increase in the heat capacity is probably related to the change of the lattice vibration mode originated from the reduction of the stiffness of the intericosahedral chain accompanied with a change from C-B-C to C-B-B chains. A linear relationship between the logarithm of σT (σ is the electrical conductivity and T is the absolute temperature) of B x C and the reciprocal temperature was observed, indicating the presence of small polaron hopping as the predominant conduction mechanism. The electrical coductivity of B x C also increased with increasing x value (from 4 to 5) due to an increase of the polaron hopping of holes between carbon atoms at geometrically nonequivalent sites, since these nonequivalent sites of carbon atoms were considered to increase in either B 3 C icosahedra or in icosahedral chains with increasing x . The electrical conductivity of B 3 C was higher than that of B 4 C, which is probably due to the precipitation of high-conducting carbon. The thermal conductivity and the thermodynamic quantities of B 4 C were also determined precisely from the heat capacity value.

Changes in young's modulus and electrical conductivity of nuclear grade graphites oxidized with air

Abstract Five kinds of nuclear grade graphites were oxidized to study thermal oxidation effects on Youngs modulus and electrical conductivity. The property changes were measured on specimens which were oxidized uniformly throughout their whole volume in the temperature range 500–600°C in air. The following relations were derived as a function of the bulk density of the graphites: E/E0 = (ρ/ρ0)nE and R0/R = (ρ/ρ0)nR, where E, R and ρ are Youngs modulus, specific electrical resistivity and bulk density, respectively, and subscript zero refers to the initial unoxidized condition. The exponents nE and nR were found to be dependent on both the kind of graphite and the oxidation temperature, and the dependences were discussed in connection with selective oxidation in the graphite texture. It was also tried to relate the property changes with oxidation rate.