Potassium-graphite Intercalation Compounds Research Articles

An attempt to produce graphene by pulverizing graphite intercalation compounds

Stage 1 and 2 potassium-graphite intercalation compounds (GICs) suspended in N-methylpyrrolidone were ground and sonicated. After centrifuging, the supernatant solutions were analyzed photochemically. Absorption and emission spectra of the supernatant suggest that the solute is a mixture of aromatic hydrocarbons with some benzene nuclei. Graphene layers in GICs are supposed to be torn during the process including sonication. This finding is expected to provide some information on the isolation methods of graphene, which has been attracting much attention due to its fascinating physical properties.

Electrochemical intercalation of potassium into graphite in KF melt

Electrochemical intercalation of potassium into graphite in molten potassium fluoride at 1163 K was investigated by means of cyclic voltammetry, galvanostatic electrolysis and open-circuit potential measurements. It was found that potassium intercalated into graphite solely between graphite layers. In addition, the intercalation compound formed in graphite bulk in molten KF was quite unstable and decomposed very fast. X-ray diffraction measurements indicate that a very dilute potassium–graphite intercalation compound was formed in graphite matrix in the fluoride melt. Analysis with scanning electron microscope and transmission electron microscope shows that graphite was exfoliated to sheets and tubes due to lattice expansion caused by intercalation of potassium in molten KF.

Sorption of molecules by alkali metal-graphite intercalation compounds

Several aspects of sorption behavior of alkali metal-graphite intercalation compounds were described. Dependence of hydrogen-sorption capacity of potassium-graphite intercalation compounds (K-GICs) on the potassium concentration of KGICs was found to be analogous to those of resistivity and conduction electron spin resonance (CESR) g-value of the K-GICs. Host effect on the hydrogen-sorption behavior of CsC24 was also described. The saturated amount of sorption increased with heat-treatment temperature (HTT) of the host carbons. It was confirmed that H2 was able to be sorbed even for CsC24 prepared from less graphitized carbons such as HTT-1300. The change of electrical resistivity (conductivity) and CESR spectra of alkali metal-graphite intercalation compounds (AM-GICs) during sorption of ammonia was also described.

Hydrogen-sorption capacity and transport properties of potassium-graphite intercalation compounds

Potassium-graphite intercalation compounds with 7 different compositions ranging from KC300 to KC24 were prepared from exfoliated graphite sheet (Grafoil). Resistivity, magnetoresistance and hydrogen-sorption capacity of these compounds were determined. KC300 and KC250 showed semi-conductive temperature dependence of the resistivity as well as host Grafoil, while KC168 and more rich compounds showed metallic behavior. Dependence of the resistivity and magnetoresistance on the potassium concentration was divided into two regions with a critical concentration at about KC60-70. Both of these properties decreased considerably with increasing potassium concentration below this critical concentration, and then became almost constant above it. A remarkable correlation between transport properties and hydrogen-sorption capacity was suggested.

Hydrogen in potassium-doped carbon nanotubes studied by positron annihilation

The positron lifetime for potassium-doped carbon nanotubes (K-NTs) ascribed to positrons in trapping centers is shortened upon exposure of the K-NTs to hydrogen. Thermal desorption of hydrogen from hydrogenated K-NTs shows a peak around 510 K, where a hydrogenated potassium-graphite intercalation compound (K-GIC) shows a peak. The absorbed hydrogen is bonded with K-NTs in the same manner as found in hydrogenated K-GIC, and their positron-trapping centers are located in a more negatively charged region, such as a two-dimensional structure of hydride ions.

The Reaction of Potassium-Graphite Intercalation Compounds with Halogens

The Reaction of Potassium-Graphite Intercalation Compounds with Halogens

On the interaction between the potassium—GIC and unsaturated hydrocarbons

KC 8 and KC 24 were allowed to react with several unsaturated hydrocarbons. After contact with isoprene, styrene or 1,3-butadiene, potassium—graphite intercalation compounds (GICs) expanded slowly along the c-axis direction. Molecules of unsaturated hydrocarbons are considered to be introduced into the interlayer spacing of GICs and polymerized progressively. When acrylonitrile, 1-butene or isobutene is used as the unsaturated hydrocarbon, the interaction between GICs and hydrocarbons is quite different. Monomer molecules are polymerized only in the vicinity of the sample edge, which enhances the stability of potassium—GICs in water. Two other kinds of interaction between alkali metal—GICs and unsaturated hydrocarbons, i.e. intercalation with and without oligomerization, are also reported elsewhere by several authors. We classified the interaction into four categories mainly according to the reactivity of unsaturated hydrocarbon for polymerization.

Solid state electronic structure of potassium graphite intercalation compounds; the systems KC24 and KC8

AbstractThe solid state electronic structures of the potassium graphite intercalation compounds (GIC) KC24 and KC8 are investigated by acrystal orbital (CO) approach of the intermediate neglect of differential overlap (INDO) type. KC24 crystallizes in a hexagonal and KC8 in an orthorhombic lattice. The CO results derived for the potassium GIC are compared with CO results of pristine graphite and potassium‐doped C60 fullerides. An incomplete K‐to‐graphite charge transfer (CT) of about 50% is predicted for both GIC studied. This incomplete CT in GIC is correlated with the incomplete CT in potassium‐doped fullerides predicted in a recent study. The present results can be considered as an a posteriori corroboration of our findings in the case of alkali‐doped fullerides. The acceptor capability of graphite is smaller than the one of the C60 soccerball with its pentagon defects. Modifications in the electronic density of states (DOS) distribution in the potassium GIC relative to pristine graphite are analyzed. The influence of theexcess electrons on the graphite electronic structure is described by a parameter which measures the ‘number of covalent bonds’ per CC contact.

The stability of potassium-graphite deintercalation compounds

We have synthesized K-GICs (potassium-graphite intercalation compounds) by the modified two-bulb method. The compounds were kept under a stabilization agent (liquid paraffin). The stage transition of K-GDICs (graphite deintercalation compounds), owing to the slow diffusion of water, was observed by X-ray diffraction measurements. The effect of the formation of higher-stage K-GDICs by oxidation was discussed with the change of d 001 values and stage-number. We can make assumptions for the mechanism of stage transition by considering the mechanism of deintercalation of intercalants between carbon layers. As a model for the deintercalation process, two kinds of stage transformation could be considered: statistical deintercalation and irregular deintercalation of K-GDICs with the presence of the disordered stages. The stage stabilities and the kinetics in K-GDICs have been investigated by UV/VIS spectrophotometry.

Interaction of potassium-graphite intercalation compounds with hydrogen at pressures up to 2000 atm

The equilibria in KC4n-H2 (n = 1 or 2) systems were experimentally studied at hydrogen pressures up to 2000 atm in the temperature range 78–700 K. The maximum content of hydrogen in the samples was achieved at 78 K under pressure higher than 500 atm and it was close to the stoichiometry KC8H3.5–4. The rate of hydrogen interaction is very high and there is no hysteresis for the studied reactions even at liquid-nitrogen temperature. The shape of the absorption-desorption isotherms at room temperature and at 361 K may be considered as almost ideal parabolic, corresponding to the linear dependence of hydrogen content in hydride on P1/2. However, a considerable change in isotherm shape and a sharp increase in hydrogen capacity at 78 K occur compared with higher temperature conditions. After a hydrogen pressure decrease the sample always has the composition KC8H0.5 in spite of the previous treatment. X-ray diffraction data obtained under ambient conditions for the samples are close to that known for KC8H0.66–0.8 stage 2 compounds.

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 (KC x ; 8 ≤ x ≤ 36) is investigated. The hydrogen-physisorption behavior depends largely on the HTT of MCMB. Particularly, KC x compounds prepared from MCMB with a HTT of 1500 °C (KC x (HTT-1500)) show anomaly in the composition range from KC 8 to KC 12; the saturated amount of physisorbed hydrogen is remarkably large compared to that observed in the usual KC x compounds prepared from graphite, and the H 2 D 2 separation coefficient is also very large. It is found by X-ray diffraction measurement that KC x (HTT-1500; 8 ≤ x ≤ 11) compounds have a stage-1 structure. This is in contrast to KC x (HTT-2600; 8 ≤ x ≤ 11) compounds, which have a mixture of stages 1 and 2 as is the case with KC x prepared from graphite. This means that the density of the potassium layer in KC x (HTT-1500; 8 < x ≤ 11) is smaller than that of usual stage-1 KC 8. 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.

New type of pressure-induced phase transition in potassium graphite intercalation compounds

New stage 1 potassium-graphite intercalation compound was recently reported. This compound differs from those already known by its structure, which includes two metal layers in the same gallery. It has roughly KC4 stoichiometry. This compound was studied under high quasi-hydrostatic pressures up to 25 kbar. From the volume vs pressure measurements and X-ray diffraction studies, it follows that under high pressure the sample overgoes first-order phase transition accompanied by the formation of dense single layers of potassium with the same (KC4) stoichiometry. The compound with such a structure and stoichiometry was known before, but it was obtained by the direct high-pressure synthesis from metal and graphite, and is not observable at ambient conditions.

Poly(methylphenylsilane) with an enhanced isotactic content prepared using the graphite intercalation compound, C8K

Poly(methylphenylsilane) of Mw approaching 105, formed in association with the solid phase in the reaction of the potassium–graphite intercalation compound, C8K, with dichloromethylphenylsilane in THF at low temperatures has a significantly higher isotactic content than that formed using the conventional alkali metal reductive-coupling reaction.

Liquid phase interaction of lithium with potassium-mercury graphite intercalation compounds

The liquid phase interactions of lithium with potassium-mercury graphite intercalation compounds (GICs) of the first and second stages are considered in detail. The final product has two phases in both cases: the first compound is more thermodynamically preferable potassium GIC of the first stage and the second one is a GIC stable to the influence of the melt. Thus the last compound screens C 8K from the intercrystalline alloy. The mechanisms of reactions are discussed from the point of view of the Daumas-Herold model.

Potassium-graphite intercalation compounds from thin carbon films prepared between montmorillonite lamellae

An attempt was made to synthesize first stage potassium graphite intercalation compounds (K-GIC) from carbon films with thickness of only about 40 nm, which was prepared in the two-dimensional space between the lamellac of montmorillonite. For comparison, K-GIC from natural graphite flakes were also synthesized. The compounds thus obtained were characterized with X-ray diffractometer and their chemical stability was investigated. The process of potassium intercalation was discussed in relation to the change in average crystal size L c , due to the intercalation.

Band structure and charge transfer of the stage-2 potassium graphite intercalation compound.

The de Haas--van Alphen (dHvA) effect of the stage-2 potassium graphite intercalation compound (${\mathrm{C}}_{24}$K) has been measured. Two dominant dHvA frequencies, 286 T and 2570 T, were observed. These frequencies were analyzed in terms of the two-dimensional rigid-band model of Blinowski et al.$---- The charge transfer derived from the band model, ${\mathit{f}}_{\mathit{k}}$=0.87, is in good agreement with the experimental result.

Fermi surface of the stage-1 potassium graphite intercalation compound.

de Haas--van Alphen (dHvA) experiments with angular-dependence measurements for the stage-1 potassium graphite intercalation compound ${\mathrm{C}}_{8}$K have been done. Two dHvA frequencies, 3126 and 4250 T, were measured with the magnetic field parallel to the c axis. The angular dependence of the dHvA frequencies for directions within \ifmmode\pm\else\textpm\fi{}18\ifmmode^\circ\else\textdegree\fi{} of the c axis showed that there are both three-dimensional and two-dimensional parts of the Fermi surface in ${\mathrm{C}}_{8}$K. When the c axis is parallel to the magnetic field, the three-dimensional Fermi surface has a cross-sectional area corresponding to the dHvA frequency of 3126 T and the two-dimensional cylinder gives rise to the frequency of 4250 T. The charge transfer per potassium atom measured directly from the dHvA effect is 0.97. This implies that the potassium is ionized completely. In addition, the corresponding effective masses 0.86${\mathit{m}}_{0}$ and 0.92${\mathit{m}}_{0}$ were obtained for ${\mathrm{C}}_{8}$K from the temperature dependence of the dHvA amplitude.

Enrichment of Low Level Tritium from Hydrogen Using Potassium-Graphite Intercalation Compound (KC24)

A sorption-desorption process using potassium-graphite intercalation compound (K-GIC) was studied to verify its applicability to hydrogen isotope separation. Lowly concentrated tritium (7.55 MBq/mol-H2) contained in about 1 l of H2 gas was successfully enriched using 1 g of KC24. Three cycles of the sorption-desorption process applied to the H2)-HT gas mixture proved to enhance the tritium concentration by a factor of 3.9, with a concomitant reduction of volume to 1/14. The positive result obtained in this study promises for this type of sorption-desorption process using K-GIC very good prospects of useful application to hydrogen isotope separation. The experimental process was simulated by calculation based on the material balance of the two components, using data of equilibrium in single stage. The calculated values agreed very well with experimentally determined data, thus attesting the applicability of the above simulation procedure to designing K-GIC hydrogen isotopes separators.

Production of fine flaky ground particles of potassium graphite intercalation compounds and potassium exfoliated graphite

A new grinding method for obtaining fine flaky graphite particles was investigated by using an intercalation reaction. The grinding characteristics of potassium graphite intercalation compounds and exfoliated graphite in a vacuum were studied and compared with those of natural graphite. The grinding of potassium graphite intercalation compounds in a vacuum for 12 h provided fine flaky particles with a mean size of 0.4–0.5 μm. The particle shape of the ground product of the potassium intercalation compound in a vacuum was flaky compared with those of natural graphite. The ground product of exfoliated potassium graphite was also found to be flaky.

Enrichment of Low Level Tritium from Hydrogen Using Potassium-Graphite Intercalation Compound (KC24).

A sorption-desorption process using potassium-graphite intercalation compound (K-GIC) was studied to verify its applicability to hydrogen isotope separation. Lowly concentrated tritium (7.55 MBq/mol-H2) contained in about 1 l of H2 gas was successfully enriched using 1 g of KC24. Three cycles of the sorption-desorption process applied to the H2)-HT gas mixture proved to enhance the tritium concentration by a factor of 3.9, with a concomitant reduction of volume to 1/14. The positive result obtained in this study promises for this type of sorption-desorption process using K-GIC very good prospects of useful application to hydrogen isotope separation. The experimental process was simulated by calculation based on the material balance of the two components, using data of equilibrium in single stage. The calculated values agreed very well with experimentally determined data, thus attesting the applicability of the above simulation procedure to designing K-GIC hydrogen isotopes separators.