Noboru Akuzawa

Effect of crystallite size on the chemical compositions of the stage 1 alkali metal-graphite intercalation compounds

Abstract The relation between the compositions of alkali metal-graphite intercalation compounds and the crystallite size of pristine graphite was mathematically analyzed taking into account the inplane structure and the stacking sequence of the graphite intercalation compounds. It has been found that the atomic ratio of carbon and alkali metal in GICs, C M (M = Li, K, Rb, and Cs) was approximated by a function of the crystallite size (La and Lc) and the interlayer spacing (d002) of the pristine graphite. The derived equation shows good agreement with the experimental results previously reported.

Stability of potassium-graphite intercalation compounds in an oxygen atmosphere

Abstract The stability of potassium-graphite intercalation compounds (K-GICs) in an oxygen atmosphere was examined at temperatures from 0 to 60 °C. After introducing a fixed amount of dry oxygen gas onto the K-GIC sample, which was held at constant temperature, the pressure change of the oxygen gas was measured continuously by a mercury manometer. The time dependence of the oxygen consumption by the reaction with K-GICs was found to be parabolic over the temperature range investigated. The apparent activation energy for the oxidation of KC 8 was determined to be 0.27 eV, while that for KC 24 was found to vary for different temperature ranges, namely, 0.29 eV at 20–60 °C and 0.09 eV at 0–20 °C. By X-ray diffraction measurements of K-GICs before and after oxidation, it was observed that the samples were transformed to higher-stage compounds by the oxidation. These facts suggest that the reaction of K-GIC with oxygen gas is rate-determined by a diffusion process: potassium atoms could diffuse out to the surface from the intercalate sites, where they would react to form KO 2 .

Preparation and characterization of sodium-graphite intercalation compounds

Preparation and characterization of sodium-graphite intercalation compounds were investigated. It was confirmed that the resulting compound (NaC x ) had the stage 8 structure with identity period of 2.8 nm. The Raman spectroscopy showed that the G-band signal of NaC x was observed at 1582 cm −1 and at 1608 cm −1 , which is a typical indication of the formation of intercalation compounds. The g-value of NaC x was determined to be 2.0018 by the ESR measurement, which was very close to the reported values for the other alkali metal-graphite intercalation compounds. The electrical resistivity of NaC x was about 1/3 of that of host graphite at room temperature and showed metallic temperature dependence.

Characterization of unsaturated organic molecule - alkali metal - ternary graphite intercalation compounds

Abstract Raman scattering and X-ray diffraction measurements were made to identify stage structure of CsC24(C2H4)n and CsC24(C2H3CN)n, which were exposed to air for a long time. Their electric conductivities were also measured during the long exposure to air. Raman spectra suggest that the gradual degradation of their electric conductivities in air is caused by the slow decomposition of the ternary GICs from the periphery region, although results of X-ray diffraction for all samples indicate the retention of the initial stage structure. Among the samples, CsC24(C2H4)1.4 which absorbed large amount of ethylene, was found to be exceptionally stable both in its stage structure and electric conductivity.

A mixed compound of the KNH3C ternary system having marked stability in air

Abstract A newly synthesized stage 1 KNH3C ternary compound with Ic ≅ 5.4A, coexisting with a minor component with Ic ≅ 8.3 A, was found to be unusually stable in air, oxygen atmosphere and acid solutions. The compound preparation procedures were as follows: 1. (1) The well known compounds of K(NH3)2C12 or K(NH3)2C28 were prepared. 2. (2) The KNH3GICs thus obtained were submitted to deammoniation treatment by evacuation at temperature between 16 and 400 °C. 3. (3) The deammoniated compound was again allowed to react with potassium vapor by the two-bulb method. On exposure to air, this compound was transformed very slowly to higher stage compounds. After 3 months of exposure, a considerable amount of the stage 1 compound still remained, accompanied by higher stage compounds and a small amount of graphite.

Electrical conductivity, magnetoresistance, and hall coefficient in the course of bromination of graphite and ammonia absorption by potassium-graphite intercalation compounds

Abstract Electrical conductivity, transverse magnetoresistance, and Hall coefficient, both of graphite in the course of bromination and of potassium-graphite intercalation compounds (K-GICs) in the course of ammoniation, were determined. Concentrations and mobilities of holes and electrons ( n h , n e , μ h , μ c ) for graphite-bromine intercalation compounds were determined as a function of the bromine content, Br C . As Br C increases, both n h and μ e increase, while n c and μ h decrease. The value of the product n h μ h increases with Br C , but n c μ e decreases. The increase of conductivity of graphite by bromination is confirmed to be a reflection of the increase in n h , in agreement with the previous investigation by Marchand and Mathur. The conductivity and Hall coefficient of KC 8 were also determined as a function of NH 3 K , but the magnetoresistance was too small to be detected. The mean carrier concentration, n , and the mean carrier mobility, \ gm, were estimated as a function of NH 3 K : n increases with the increase of NH 3 K , while \ gm decreases with NH 3 K . It was found that the effect of the decrease of n is larger than that of the increase of \ gm, resulting in the decrease of conductivity found with KC 8 as it is ammoniated. These results were attributed to backdonation of free electrons form the carbon π ∗ band to the intercalated layers, caused by the increase of the interlayer distance when potassium ions are solvated by ammonia molecules.

Behavior of unsaturated organic molecules in the nanospace of stage-2 cesium-graphite intercalation compounds

Intercalation reaction of benzene (C6H6) into CsC24 and the characteristics of resulting ternary graphite intercalation compound (GIC) have been investigated, in comparison with the similar CsC24–ternary GICs with ethylene (C2H4) and acrylonitrile (C2H3CN), and the behavior of these organic molecules within the interlayer nanospace has been discussed. The stability of these ternary GICs under exposure in air, in both stage structure and electrical conductivity was found to be in the order of C2H4–, C6H6– and C2H3CN–GICs. This result was reasonably explained as follows: the C2H4 molecules in the interlayer space oligomerize considerably and form network preventing diffusion of the Cs atoms in the interlayer, while the C6H6 molecules form only dimers or trimers and unable to form network, and the C2H3CN molecules, in spite of their strong tendency to polymerize, apparently do not form oligomers in the interlayer space, and its ternary GIC tends to degrade easily. By the electrical and galvanomagnetic measurements, it is revealed that the dominant carrier of these ternary GICs is electrons, even when the electrical conductivity decreases considerably after prolonged exposure in air, while the host graphite, Grafoil, shows two-carrier conduction. This fact indicates that the degradation of the ternary GICs occurs mainly at the periphery or surface layer of the crystallite, and at the inside of the crystallite, the ternary stage structure is retained unchanged.

Host Effect on the Properties of AM-GICs

Abstract Carbon materials (A-1, A-2 and A-3 derived from pitch cokes) with different graphitization degree, were allowed to react with potassium. The hydrogen-sorption behavior at 77 K, electrical resistivity. ESR and Raman spectra of the resulting compounds with the composition of KC60 were determined. The sorbed amount at saturation, (n H2/n K)sat, was 0.55 and 1.43 for KC60s prepared from A2 (d 002 = 0.3377 nm) and A3 (d 002 = 0.3361 nm), respectively. No H2 sorption was observed for KC60 from A-1 (d 002=0.342 nm). Temperature dependence of the resistivity of KC60s from A-2 and A-3 showed metallic behavior, contrary to semiconductive one for the host materials. However, KC60 (A-1) showed semiconductive temperature dependence, similarily to the host material. Raman spectra of KC60s from A-2 and A-3 showed doublet structure, similarly to that of K-GICs from HOPG, characteristic for graphite intercalation compounds with stage n > 2. On the contrary, KC60 (A-1) gave single peak at around 1597 cm−1. Those facts suggest that potassium exists in the interlayer spaces for KC60s (A-2, A-3), but not for KC60 (A-1). It was also shown that g-factor of ESR spectra of KC60 can be useful to predicting the H2-sorption behavior.

Hydrogen physisorption by potassium-graphite intercalation compounds prepared from mesocarbon microbeads

Potassium is intercalated into mesocarbon microbeads (MCMBs) with different heat-treatment temperatures (HTTs) and the hydrogen physisorption of the resulting compounds (KCx; 8 ≤ x ≤ 36) is investigated. The hydrogen-physisorption behavior depends largely on the HTT of MCMB. Particularly, KCx compounds prepared from MCMB with a HTT of 1500 °C (KCx(HTT-1500)) show anomaly in the composition range from KC8 to KC12; the saturated amount of physisorbed hydrogen is remarkably large compared to that observed in the usual KCx compounds prepared from graphite, and the H2D2 separation coefficient is also very large. It is found by X-ray diffraction measurement that KCx(HTT-1500; 8 ≤ x ≤ 11) compounds have a stage-1 structure. This is in contrast to KCx(HTT-2600; 8 ≤ x ≤ 11) compounds, which have a mixture of stages 1 and 2 as is the case with KCx prepared from graphite. This means that the density of the potassium layer in KCx(HTT-1500; 8 < x ≤ 11) is smaller than that of usual stage-1 KC8. It is considered that nanospaces of this ‘dilute’ stage-1 structure can accommodate large amounts of hydrogen and that it is suitable for the hydrogen-isotope separation.

Decomposition of rubidium- and cesium-graphite intercalation compounds in oxygen atmosphere

Abstract The decomposition rate of rubidium-graphite intercalation compounds (Rb-GICs) and cesium-graphite intercalation compounds (Cs-GICs) in pure oxygen gas was investigated by measuring the pressure change of the fixed volume of oxygen gas in contact with the GIC sample. The behavior of Rb-GICs was similar to that of K-GICs, but Cs-GICs showed quite complicated decomposition characteristics. It was found that the decomposition rate of Rb-GICs was faster than that of K-GICs, and the decomposition rate of stage 2 Rb-GIC was faster than that of stage 1 Rb-GIC. The decomposition of stage 1 Rb-GIC(RbC8) showed a parabolic time dependence and the apparent activation energy, EA, was determined to be 0.20 eV, while the decomposition of stage 2 Rb-GIC(RbC24) followed a cubic time dependence.