Graphite Bromine Research Articles

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

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.

Kinetics and thermodynamics of intercalation of bromine in graphite—II. Theory

A theory that describes the diffusion of an intercalate within graphite in a multiphase situation is presented. The theory was applied to the intercalation of bromine in graphite. Single-phase diffusion represents the intercalation of bromine in graphite as well as multiphase diffusion due to the dominance of the final stage.

124. Model of intercalation of bromine in graphite

124. Model of intercalation of bromine in graphite

Electrical conductivity measurements on fluorosulphate-containing graphite intercalation compounds

The variation in the electrical conductance as a function of composition has been investigated for graphite fluorosulphates (C nSO 3F) and graphite bromine (I) fluorosulphates (C nBrSO 3F). Among the first-stage compounds of these materials, C 7.2SO 3F exhibits a rather low planar conductance σ σ g =2.1 but its derivatives obtained by solvolysis with HSO 3CF 3 or SbF 5 or the addition of AsF 5 show a significant increase σ σ g =6.0−9.5 in their conductance. The observed conductance values are discussed together with the structural information on these compounds.

Thermodynamics Of Intercalation of Bromine in Graphite

ABSTRACTThe thermodynamics of intercalation of bromine in highly oriented pyrolytic graphite had been studied by determining the pressure-temperature equilibrium diagram for stages 2–4. The standard heat and entropy of reaction for the transformation from stage n to stage n−l (n=5, 4, 3) were found to be roughly the same, though the enthalpy of reaction became slightly more negative as the stage number increased. The heat and entropy of formation from pure graphite were thus found to be −10.9 kcal mol-1 Br2 and −30.4 cal mol−1 Br2 K−1 respectively for stage 2, −11.32 kcal mol−1 Br2 and −30.6 cal mol−1 Br2 K−1 respectively for stage 3, and −11.5 kcal mol−1 Br2 and −30.6 cal mol−1 Br2 K−1 respectively for stage 4.

Thermal gravimetric analysis of graphite-bromine compounds

Thermal gravimetric analysis was applied for the first time to the study of graphite intercalation compounds. The gravimetric measurements were carried out on graphite-bromine compounds in the temperature range from room temperature to 140 °C. Intercalate desorption was found to be thermally activated. Its rate is anomalously high near the intralayer intercalate position order-disorder transformation temperature (100 °C) and the exfoliation temperature. This rate was also found to increase with increasing intercalate concentration. The mechanism of desorption depends on the intercalate concentration. At high intercalate concentrations, desorption appears to be dominated by a diffusion process and has an activation energy of 14 kcal (mol Br 2) −1, in good agreement with the activation energy for the diffusion of bromine in graphite. As desorption ends, the intercalate concentration approaches a minimum value which depends on the desorption temperature. Much of the retained intercalate is ordered. Below the intralayer order-disorder transformation temperature an initially saturated graphite-bromine sample has a minimum concentration of 18 – 19 wt.% Br 2. Above the order-disorder transformation temperature the minimum concentration is smaller. The energy of the desorption process is utilized mainly for the vaporization of the intercalate, as suggested by our differential scanning calorimetry results.

The solid state 13C-NMR spectra of some intercalation compounds

The 13C-nuclear magnetic resonance spectra of graphite bromine (C 8Br) and two graphite potassium compounds (C 8K and C 24K) have been obtained in the solid state. A similar spectrum is observed for all three compounds. The major feature in all three compounds is a very broad upfield resonance. These spectra are interpreted as indicative of an interaction of the intercalate with the graphite π system leading to more aliphatic-like carbon atoms.

High temperature resistivity of pyrolytic graphite bromine residual compounds

The electrical resistivity of pyrolytic graphite (PG) deposited at 2300°C decreases in both the a- and c-axis directions as the temperature is increased up to 1000°C, but the high temperature resistivity of PG-bromine residual compounds differ somewhat from that of the original PG. When the temperature is increased, the a-axis resistivity remains nearly constant at the lower temperature range but increases rapidly between 300 and 500°C. Above 500°C the value of the resistivity approaches that of the original PG. On cooling, the change of the electrical resistivity of PG-bromine residual compounds is the same as that on heating, but its value is larger between 500 and 100°C. At room temperature, the a-axis resistivity of the compound before and after the cyclic heat-treatment is the same. This behaviour is closely related to the c-axis thermal expansion and is well explained by the same bromine vaporisation model. As to the c-axis direction, the electrical resistivity of PG-bromine residual compounds show a different behaviour. However, the factors affecting the electrical conduction in this case are complicated and cannot be resolved.

Crystal structure of graphite bromine lamellar compounds

The isotherm of bromine on pyrolytic graphite was studied at 20°C by stepwise bromination, and several phases of graphite bromide were identified, which have compositions of C 8Br, C 12Br, C 16Br and C 20Br. X-ray studies show that these possess the socalled 2nd, 3rd, 4th and 5th stage structures respectively. At bromine concentrations between these definite compositions, X-ray diffraction patterns show mixtures of the higher stage and the lower stage structures. The density of the intercalated bromine molecules in a layer is believed to stay unchanged with the change of the composition. During the course of the reaction with the saturated vapor of bromine, an intermediate metastable lamellar compound structure was found in which the bromine layers are intercalated into every second layer spacing but for which the bromine concentration is believed to be much lower than that of C 8Br.

Electrical Conductivity of Graphite Bromine Lamellar Compounds

Abstract Polycrystalline graphite was allowed to react with bromine vapor, and the change of its electrical conductivity during the course of bromination and debromination was studied. The graphite was found to behave in two different ways depending on the process of bromination. When the vapor pressure of bromine was changed stepwise, following the equilibrium line of bromine absorption of graphite, the electrical conductivity showed a reversible change. On the other hand, when a graphite sample was brominated with the saturated vapor, an increase of the electrical conductivity steeper than that of the former case was observed. These results were discussed in relation to the structural analysis of the brominated graphite. A metastable phase of graphite bromide, the electrical conductivity of which remained unchanged in spite of a considerable increase of absorbed bromine, was found. The electrical conductivity of the residue compounds was also investigated, and a steep decrease was observed at high concentrations of bromine.

A Study on the bromination of graphite by X-ray diffractometry

A Study on the bromination of graphite by X-ray diffractometry

Thermal Expansion of Pyrolytic Graphite-Bromine Residual Compound

Investigations have been made on the thermal expansion behavior in the c-direction of pyrolytic graphite bromine residual compounds (PG (Br), 0.9∼5.5 weight percent bromine) within the temperature range of 20∼1000°C. The thermal expansion consists of three stages: the first stage below about 120°C, the second stage 120∼300°C, and the third stage above about 300°C. The thermal expansion coefficient in the third stage increases linearly with the amount of bromine. The expansion under heating-cooling cycles shows a thermal hysteresis which relates to the existence of bromine. The observed behavior is explained, using a model which considers the bromine to be ionically bonded to carbon near room temperature (1st stage) and to vaporize with increasing temperatures to make gaseous bubbles within the lamellar structure of the graphite crystal (2nd stage). After 2nd stage PG(Br) expands according to the ideal gas law (3rd stage). The calculation of the thermal expansion coefficients is based on the Griffith's crack model employed by Martin and Brocklehurst. The experimental results are in good agreement with the calculated values.

126. The effect of sample shape on the bromination of graphite

126. The effect of sample shape on the bromination of graphite

The effect of sample shape on the bromination of graphite

Weight and length measurements made during the bromination of three pyrolytic graphites in the form of cylinders of various sizes show that intercalation starts in the spaces adjacent to the outer two basal planes and proceeds to the central layer. This splits the layer system into a number of thin discs at a rate that increases with the degree of alignment of the layer planes. These discs then acquire Br 2 up to the maximum of C 16Br 2 at a rate which for heat treated samples follows Fick's Law of radial diffusion in a cylinder with about the same diffusion coefficient as for natural graphite. For a non heat treated sample the rate is greater because it is determined by both axial and radial diffusion. For non heat treated pyrolytic graphite the threshold pressure required to start disc production increases with the number of layer planes in the sample. For heat treated material the threshold pressure is lower and shows much less dependence on the number of layers.

The mechanism of the bromination of graphite

The rate of intercalation of graphite by bromine has been measured for natural crystals over a range of 500 fold in area and 10 fold in thickness. The results have been interpreted in terms of Fick's Law for radial diffusion in a cylinder and a mechanism in which all layers are open to intercalation as soon as bromine is admitted at high relative pressure.

Thermal Expansion Measurements on Graphite–Bromine Compounds

The effect of residual bromine, formed in graphite by exposure to bromine vapor and subsequent desorption at room temperature, on the thermal expansion of graphite was investigated at 0 to 700 deg C. Measurements were made on two types of artificial extruded graphites, cut parallel and perpendicular to the extrusion axis, and on a pressed natural graphite compact cut parallel to the pressing direction. The results show a rapid increase in the thermal expansion coefficient above 300 deg C. (D.L.C.)

Magnetic Susceptibility and Free Energy of Graphite Bromide

The magnetic susceptibility of graphite bromide residue compounds has been measured by a Faraday method. The addition of bromine decreased the diamagnetism of graphite. Fair agreement was obtained between the measured diamagnetism and values calculated by assuming an undistorted graphite band structure depleted in electrons due to the dissociation and ionization of two out of eleven bromine molecules. It has also been shown that the transfer of electrons from the graphite bands to bromine can account for the observed pressure dependence of the graphite bromine equilibrium.

The Properties of the Interstitial Compounds of Graphite. III. The Electrical Properties of the Halogen Compounds of Graphite

The presence of lamellar or of residual bromine in artificial graphite reduces the resistance, the absolute magnitude of its temperature coefficient near room temperature, and the magneto resistance. The Hall coefficient increases and eventually changes from negative to positive values. The changes resemble very closely those obtained in the oxidation of graphite in sulfuric acid. This similarity in properties suggests that the electron acceptors in these compounds are only weak scattering centers for conduction electrons. Since the electrical properties of graphite bisulfate and of graphite bromide are very similar, it may be assumed that both compounds are of a similar type. Since the bisulfate compound is ionic in nature, the bromine in graphite bromide is also probably ionized. On this assumption the graphite bromides have the formulas CnBr·3 Br2 for the lamellar and CnBr·2.25 Br2 for the residue compounds. Chlorine reacts with graphite at low temperatures only. The properties of the compounds of chlorine and graphite resemble in all respects those of the compounds of bromine and graphite. Attempts to react iodine and graphite have failed. Iodine chloride, however, reacts rapidly with graphite.

39. Some physical properties associated with “aromatic” electrons. Part III. The pseudo-metallic properties of potassium–graphite and graphite–bromine

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