Gas flow through highly porous graphite matrices

Highly porous graphite matrices with 20-200 kg m⁻³ of bulk densities have been characterized using helium pycnometer, mercury porosimeter, and fluid flow methods, i.e. both gas permeation and diffusion measurements. The gas permeability is ranging from 10⁻¹² to 10⁻¹⁵ m² and decreases as the bulk density of the graphite matrix increases. According to the Carman-Kozeny correlation, the evolution of the gas permeability with respect to the bulk density is very well correlate with, on the one hand, the increase of the tortuosity, and on the other hand, the decrease of the porosity as well as the pore diameter with the bulk density of the graphite matrix.     

Effect of preparation conditions on the characteristics of exfoliated graphite

Exfoliated graphite (EG) was prepared from graphite intercalation compounds (GICs) synthesized electrochemically with different electricity consumption from 10.83 to 40.00 A h/kg. Effects of electricity consumption on the synthesis of GICs and of exfoliation temperature on different parameters of EG, i.e. exfoliation volume, volatile content, specific surface area and pore volume measured by mercury porosimetry, length and width of worm-like particles, and distance between neighboring balloons based on the zigzag model for worm-like particles of EG, were studied. These parameters were found to depend strongly on the electricity consumption and also on exfoliation temperature. Exfoliation volume, volatile content, specific surface area and pore volume on EG prepared at 1000 °C increased with increasing electricity consumption, but the distance between neighboring balloons was found to decrease. These results reveal marked development of pores in EG samples. Raising exfoliation temperature increased exfoliation volume, specific surface area and pore volume up to 800 °C. Above this temperature these parameters tended to be stable.     

Densification of expanded graphite

A brief theoretical investigation of the behavior of exfoliated graphite (EG) undergoing compaction is presented. Simple arguments and assumptions allow one to calculate the density of the initial individual worm-like particles of EG for any final porosity of the resultant consolidated blocks. These results as well as excluded volume considerations are used to derive the mean disorientation angle of the graphite sheets within the blocks of compressed EG. X-ray experiments were performed on such autoconsolidated samples. The overall mosaic spreads thus obtained are shown to be in a very good agreement with the model developed in this paper.     

Fracture mechanism of flexible graphite sheets

The fracture behavior of flexible graphite sheets (FGSs) was investigated by loading along and perpendicular to the sheet surface. The vertical strength is lower than the tensile strength by two orders of magnitude, at only 0.03 MPa. The fracture processes of a notched specimen of FGS and compressed graphite worms were also studied directly and by in-situ observation by SEM. From observations of the fractured surfaces, structural units for the fracture of FGS are assumed to be interlocked micro-discs composed of orientated graphite layers, which originate from the worm-like particles of the original exfoliated graphite. During the formation of FGS by compression and rolling, micro-discs in the different worms interlock and become structural units. When a static tensile load was applied to FGS, some structural units rotated, cleaved and produced microcracks. When the load was increased, slip between the structural units of FGS started and the structural units slipped away from each other and the FGS was broken. Therefore, The tensile strength of FGS originates from the frictional force between the structural units.     

Aqueous electrochemical intercalation of bromine into graphite fibers

For the first time, graphite fibers have been electrochemically intercalated with Br⁻ that have the same structure and properties as those intercalated from vapor phase Br₂. This was accomplished by intercalating pitch-based Thornel^® K-1100 graphite fibers at low temperature (near 0 °C) and high currents (2 A) for long times (6 h). The mechanism appears to be that Br⁻ is oxidized to aqueous Br₂ which, when sufficient local concentration builds up, intercalates the fiber. This was confirmed by intercalating K-1100 fiber in a saturated aqueous Br₂ solution without passing an electrical current. The applied voltage does apparently lower the activation energy of the reaction as evidenced by the observation that P-120 and P-100 fibers will not intercalate in aqueous Br₂ unless a voltage is applied.     

The effect of electrolyte temperature on the passivity of solid electrolyte interphase formed on a graphite electrode

The effect of electrolyte temperature on the passivity of solid electrolyte interphase (SEI) was investigated in 1 M LiPF₆-ethylene carbonate/diethyl carbonate (50:50 vol.%) electrolyte, using galvanostatic charge-discharge experiment, and ac-impedance spectroscopy combined with Fourier transform infra-red spectroscopy, and high resolution transmission electron microscopy (HRTEM). The galvanostatic charge-discharge curves at 20 °C evidenced that the irreversible capacity loss during electrochemical cycling was markedly increased with rising SEI formation temperature from 0 to 40 °C. This implies that the higher the SEI formation temperature, the more were the graphite electrodes exposed to structural damages. From both increase of the relative amount of Li₂CO₃ to ROCO₂Li and decrease of resistance to the lithium transport through the SEI layer with increasing SEI formation temperature, it is reasonable to claim that, due to the enhanced gas evolution reactions during transformation of ROCO₂Li to Li₂CO₃, the rising SEI formation temperature increased the number of defect sites in the SEI layer. From the analysis of HRTEM images, no significant structural destruction in bulk graphite layer was observed after charge-discharge cycles. This means that solvated lithium ions were intercalated through the defect sites in the SEI, at most, into the surface region of the graphite layer.     

Preparation and characterization of graphite nanosheets from ultrasonic powdering technique

Natural flake graphite was exfoliated into exfoliated graphite via an acid intercalation procedure. The resulting exfoliated graphite was a worm-like particle composed of graphite sheets with thickness in the nanometer scale. Subjecting it to ultrasonic irradiation, the exfoliated graphite was effectively further foliated into isolated graphite nanosheets. SEM, TEM, SAD, laser counting, and BET measurements revealed that the graphite nanosheets prepared with 10 h irradiation were about 52 nm in thickness and 13 μm in diameter. FTIR examination showed that there were oxygen-containing groups presented on the surface of the exfoliated graphite. This result substantiated the statement reported in the literature that acid treatment could result in oxidization of carbon bonds on graphite surface.     

Hydrated Mn(IV) oxide-exfoliated graphite composites for electrochemical capacitor

Commercially available low cost exfoliated graphite (EG, nominal diameter 130 μm) was used as a conductive substrate for electrochemical capacitor of hydrated Mn(IV) oxide, MnO₂·nH₂O. The MnO₂·nH₂O-EG composites were prepared by addition of EG to potassium permanganate solution, followed by 1 h stirring and then slow addition of manganese(II) acetate solution. By this procedure submicrometer or smaller sized MnO₂·nH₂O particles having mesopores of 6-12 nm in diameter were formed on the graphite sheets of EG. Although EG alone showed only about 2 F g⁻¹, the composites showed good rectangular cyclic voltammograms at 2-20 mV s⁻¹ in 1 mol L⁻¹ Na₂SO₄. The capacitance per net amount of MnO₂ increased proportionally with EG content, that is, utilization ratio of MnO₂ increased with EG content. The composites of MnO₂·nH₂O and smaller diameter of EG (nominal diameter 45 μm) or artificial graphite powder (average diameter 3.7 μm) showed fairly good performance at 2 mV s⁻¹, but with increasing potential scan rate the rectangular shape was distorted and capacitance decreased drastically. The results implies that sheet-like structure is more effective than small particles as conductive materials, when the formation procedure of composite is the same. Large sized EG may be a promising conductive material for electrochemical capacitors.     

Synthesis of submicrometer-sized β-SiC particles from the precursors composed of exfoliated graphite and silicone

Utilizing micro-spaces of exfoliated graphite (EG), a process to synthesize fine β-SiC particles was developed. Two types of low molecular weight silicone and a catalyst were impregnated into EG by sorption, and then heated in air to cure the compounds. Formed precursors were black flakes of a few millimeters in diameter. Only by the heat treatment of precursors at 1500 °C for 5 h or at 1550-1600 °C for 1 h in Ar, β-SiC of a few tens to hundreds nanometers in size was obtained with the yield around 40 mass%. In the present process, EG play two important roles; one is as reaction spaces and the other is as a reductant that functions at elevated temperatures. Initially the cured silicone is coating the graphite sheets of EG as thin films of less than 1 μm, and above 1300 °C they start to decompose and form small particles of a few tens to 100 nm in diameter on the graphite sheets. These intermediate particles are composed of the Si-C-O composites and SiO₂ and they spontaneously decompose to β-SiC from around 1400 °C, and between 1400 and 1500 °C the reduction of remaining Si-C-O intermediates by graphite sheets occur and form well-crystallized fine β-SiC particles. The process is simple and raw materials are not expensive so that it is promising for industrial applications.     

Porous graphite matrix for chemical heat pumps

Expanded graphite powders were prepared by rapid heating of expandable graphite powders intercalated with sulfuric acid at different heat treatment temperatures (HTT). Porous graphite matrices with 100–400 kg m⁻³ of bulk density were fabricated by pressing expanded graphite powders in order to use as heat conductive media. They were characterized using an C/S analyzer, inductively coupled plasma spectroscopy, X-ray diffraction, scanning electron microscopy, Fourier transform infrared (FTIR), nitrogen adsorption, optical microscopy and helium pycnometer before and after heat treatment. Gas permeability and thermal conductivity were measured for porous graphite matrices with different HTT and bulk densities. Chemical analysis and FTIR showed that as the HTT of expandable graphite powders increase, the residual sulfur content decreased remarkably. Nitrogen adsorption experiments for expanded graphite powders showed that specific surface area and total pore volume increased with HTT. Helium penetration results showed that porous graphite matrices with different HTT have noticeably different open porosities which were attributed to the different degrees of expansion of graphite layers. The gas permeability of porous graphite matrices was in the range of 10⁻¹²–10⁻¹⁵ m² and exhibited higher values with low HTT. Thermal conductivity values in the axial and the radial directions were in the range of 4.1–20.0 and 4.6–42.3 W mK⁻¹, respectively. A semi-empirical model was developed that can be used to correlate with the thermal conductivity of graphite matrix on the basis of solid conductivity, bulk density and porosity.     

Surface area of compressed expanded graphite

In a previous work [Carbon (2002) in press], the densification of expanded graphite was modelled. This very light material consists of worm- or accordion-like particles in a loose packing. Worm-like particles are made up of thin disc-shaped graphite sheets, and the average disorientation angle of the thin discs was estimated based on the excluded volume concept. Then, using a simple picture of particle densification, the mosaicity of compressed expanded graphite was calculated and compared with the results of XRD experiments. In the present paper, the same model and assumptions are taken into account for predicting the surface area variations of two kinds of expanded graphite undergoing compaction. The constitutive graphite sheets are assumed to bend and to make flat contacts with their neighbours, thus decreasing the surface of the material. It is worth noting that the proposed modelization is free of any adjustable parameter. The calculations are shown to be in good agreement with experimental results obtained from krypton adsorption at liquid nitrogen temperature.     

The electrochemical behaviour of an exfoliated graphite electrode in simulated seawater containing oil

An exfoliated graphite (EG) electrode was prepared and the electrochemical response to oil in simulated seawater was studied by means of the potential step technique and electrochemical impedance spectroscopy (EIS). The capacitance and other electrochemical characteristics of the electrode were affected by the presence of oil. The effects of temperature and salinity on the electrochemical behaviour of the EG electrode in NaCl solution containing oil were investigated by EIS. The results showed that the higher the temperature or the salinity, the higher the double layer capacitance of the electrode.     

A simplified preparation of mesoporous carbon and the examination of the carbon accessibility for electric double layer formation

Mesoporous carbon was prepared from resol-type phenol-formaldehyde resin using mesoporous silica as template. By filling the resin into the pores of the template, followed by resin carbonization and template dissolution, mesoporous carbon preparation can be significantly simplified. Small-angle X-ray diffraction reflected the long-range ordering of the pores in the carbon. TEM and N₂-adsorption analysis showed that the carbon contained mesopores of different sizes and a high proportion of micropores. Electrochemical cyclic voltammetry was conducted in H₂SO₄ to examine the surface accessibility of the carbon for double layer formation. Microporous activated carbon, also from the resol resin, was prepared for comparison. Although the pore sizes are different, the double-layer capacitances per unit area for both carbons are similar at low potential sweep rates. However, the capacitance decline with the sweep rate was less significant for the mesoporous carbon. Upon gasification of the carbons to increase their surface area, the ultimate capacitance per unit carbon area was enhanced and the enhancement was slightly larger for the mesoporous carbon. It is suggested that the presence of mesopores has facilitated the electrolyte migration into carbon interior. A two-electrode capacitor assembled with the mesoporous carbon was shown to have a small resistance and still exhibited a capacitive behavior at high potential sweep rates.     

Flexible graphite as a heating element

Flexible graphite is an effective heating element. It provides temperatures up to 980 °C (though burn-off occurs in air at 980 °C), response half-time down to 4 s, and heat output at 60 s up to 5600 J. The electrical energy for heating by 1 °C is 1-2 J in the initial portion of rapid temperature rise. The temperature and heat output increase with decreasing thickness and with increasing power.