Production of natural graphite particles with high electrical conductivity by grinding in alcoholic vapors

Natural graphite particles with a high crystallinity, which were sieved to obtain particles less than 63 μm, were ground with a ball mill under a dry atmosphere and various alcoholic vapors such as i-C₃H₈OH, n-C₃H₈OH, C₂H₅OH, and CH₃OH. The size and flakiness of the ground products and the electrical conductivity of the films made from the ground products were experimentally examined. Grinding the particles under alcoholic vapors slowly reduced the particle size and was similar to grinding in dry air, but grinding in alcoholic vapors produced flakier products. The graphite films, which were composed of flakier particles and were ground in alcoholic vapor, displayed higher electrical conductivities than the feed graphite particles. The products ground in C₂H₅OH, i-C₃H₇O and n-C₃H₇OH vapors had 50% or less of the specific resistance of the feed particles.     

Characterization of packing structure of tape cast with non-spherical natural graphite particles

A three-dimensional microstructure of green and pressed tapes cast with graphite particles of non-spherical shape were examined quantitatively on the basis of the distribution of void sizes among the packed particles. The distributions measured over the cross-sections of the tape in three directions were expressed by the theoretical ones deduced for non-spherical particles. Particle shape was characterized by shape indices defined by Fourier analysis of particle outlines measured over the corresponding cross-sections of the green tape. The relationships were totally established between the limiting packing density, characterizing the void size distribution at the same section voidage, and the shape index of particle over the section. As a result, the normalized median void diameter as well as the limiting packing density of the pressed tape was found to increase with the particle shape index, corresponding to wider void size distribution. Therefore, based on developed correlations, the optimization of packing microstructure of the cast tape can be expected to result in high performance battery by using shape-modified graphite particles.     

Properties and microstructure of expandable graphite particles pulverized with an ultra-high-speed mixer

The expandable graphite (EG) particles were pulverized to achieve different and smaller sizes using an ultra-high-speed mixer. The microstructure of particles was observed by a scanning electron microscope (SEM). The as-received EG particles showed an irregular flake shape. With the increase of the mixing time, the EG particles tended to be circular and the collapses and cracks in the EG surfaces appeared, and their average diameter and average area rapidly reduced. At the same time, their expansion volume after thermal treatment greatly decreased, resulting from the reduction of particle sizes and the direct release of the oxidant inside the EG particles instead of exfoliating the particles. The expanded EG particles revealed the typical wormlike structure.     

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.     

Image analysis method for determining 3-D shape of coarse aggregate

A 3-D method for particle shape determination of coarse aggregates using image analysis, IA, is presented. It is based on the measures the axial length of all three axis of every particle in a coarse aggregate sample. Two images of the entire aggregate sample are taken, in lying and standing positions. Since the particle's intermediate axes are measured in both images they can be used to couple the shortest and longest axial dimensions for each particle. The method allows an interpretation of length/thickness, length/width and width/thickness ratios of all the particles and is thus comparable to the flakiness and shape index tests.     

A comparison of electrochemical performance of natural graphite sulfurized by ball-milling and heat-treating as an anode for lithium ion batteries

Natural graphite (NG) was sulfurized by heat-treating or by high energy ball-milling the blend of NG with sulfur powder. The effect of the surface functional groups, containing sulfur, on the performance of the NG anode for a lithium ion battery was then investigated. X-ray photoelectron spectroscopy revealed that the sulfur was introduced onto the surface of NG in both of these methods. The results of scanning electron microscopy and Raman spectroscopy showed that the surface disorder of NG increased after sulfurization. Charge/discharge tests showed that the reversible capacity of the first cycle was increased after surface sulfurization and that the coulombic efficiency of the first cycle increased for the heat-treated sample but decreased for the ball-milled one. The change in the electrochemical performance was due to a number of factors including an increase in new active sites for lithium storage and an increase in surface area and increased disorder of the sulfurized NG samples.     

Production of finely ground natural graphite particles with high electrical conductivity by controlling the grinding atmosphere

Natural graphite particles with high crystallinity sieved to obtain a particle size range of under 63 μm were ground with a ball mill, under various well-controlled grinding atmospheres such as N₂, O₂, He, H₂, and vacuum. The ratio, X_dif50/X_st50, i.e. between the 50 wt.% Stokes diameter and the 50 wt.% laser diffraction diameter, of the ground particles, was used as an index of the flakiness of the particles. The specific resistance of films composed of the ground graphite particles was systematically measured. The rate of reduction in the size of the particles by grinding was slow under an O₂-rich atmosphere such as 100% O₂ and dry air. On the other hand, it was relatively fast in vacuum, or under an N₂ or He atmosphere, and a gas mixture of 99% N₂ and 1% O₂. The rate of size reduction by grinding under a H₂ atmosphere was intermediate. In our experimental conditions, the flakiness of the ground particles increased with the decrease in the particles’ sizes. The electrical conductivity of the ground particles, however, tended to decrease with the decrease in their sizes. Under the condition that the Stokes diameter of the ground particles remains constant, the electrical conductivity of films made from the ground particles increases with the increase in the flakiness of the particles. It was finally determined from our systematic grinding experiments that small flaky particles, which had a size, X_st of ∼1 μm, with a high electrical conductivity can be produced by grinding in a gas mixture of 99% N₂ and 1% O₂. In this case, the flaky shape of the ground particles was visually confirmed by scanning electron microscopy.     

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.     

Image analysis of particles by modified Ferret method — best-fit rectangle

For the purpose of choosing and designing measurement algorithms to characterize particle size and shape rotation-invariantly, reproducibly, simply, and reasonably, this paper presents an image analysis measurement algorithm-best-fit rectangle for particle size and shape. The best-fit rectangle approach is a combination of the Ferret method and the least 2nd moments minimization, only requiring calculation of three moments about the center of gravity, and maximum and minimum co-ordinates in a co-ordinate system oriented in the direction of the axis of the least 2nd moments, and a simple area ratio. It is a simple rotation-invariance method, reflecting shape (elongation and angularity). In this paper, the method that has been tested in a large number of aggregate particle samples in a laboratory is introduced theoretically in detail, analyzed and compared to other widely used methods. The test results show that by using this method, the results are very close to manual measurements (for size, elongation and angularity). The width accumulative curve is parallel to the curves of sieving and thickness, implying that sieving analysis and thickness measurement can be easily estimated by the width. The method combining other image processing algorithms in an online system does the processing in real time. It is more reasonable and useful than other traditionally used measurement methods in image analysis.     

Electrochemical intercalation of lithium into a natural graphite anode in quaternary ammonium-based ionic liquid electrolytes

Electrochemical intercalation of lithium into a natural graphite anode was investigated in electrolytes based on a room temperature ionic liquid consisting of trimethyl-n-hexylammonium (TMHA) cation and bis(trifluoromethanesulfone) imide (TFSI) anion. Graphite electrode was less prone to forming effective passivation film in 1 M LiTFSI/TMHA-TFSI ionic electrolyte. Reversible intercalation/de-intercalation of TMHA cations into/from the graphene interlayer was confirmed by using cyclic voltammetry, galvanostatic measurements, and ex situ X-ray diffraction technique. Addition of 20 vol% chloroethylenene carbonate (Cl-EC), ethylene carbonate (EC), vinyl carbonate (VC), or ethylene sulfite (ES) into the ionic electrolyte resulted in the formation of solid electrolyte interface (SEI) film prior to TMHA intercalation and allowed the formation of Li-C₆ graphite interlayer compound. In the ionic electrolyte containing 20 vol% Cl-EC, the natural graphite anode exhibited excellent electrochemical behavior with 352.9 mAh/g discharge capacity and 87.1% coulombic efficiency at the first cycle. A stable reversible capacity of around 360 mAh/g was obtained in the initial 20 cycles without any noticeable capacity loss. Mechanisms concerning the significant electrochemical improvement of the graphite anode were discussed. Ac impedance and SEM studies demonstrated the formation of a thin, homogenous, compact and more conductive SEI layer on the graphite electrode surface.     

An improved estimation of size distribution from particle profile measurements

Several methods are available to measure particle size. The majority of them, such as sieving, are off-stream techniques where samples must first be separated from the main stream for analysis.Therefore, the search for on-line particle size analysis systems has provided the impetus for the introduction of image-based particle size analysers to the mineral industry in the past three decades. Generally, the estimation of particle size distribution on the basis of image analysis depends on measuring a single parameter of particle profile. For example the equivalent area diameter (d_A) or mean Feret's diameter (d_F) distributions, then transforming this data to the equivalent size distribution. However, due to the irregularity of particles being analysed, it is believed that this kind of analysis may increase the error in estimation of particle size distribution since profiles of irregular particles carry more information than can be represented by a single parameter.In this paper, a proposed technique which measures two parameters, equivalent area diameter (d_A) and mean Feret's diameter (d_F), for each particle profile has been developed. The accuracy of the technique has then been investigated in the laboratory by successfully estimating (unfolding) the size distribution, where size refers to sieve size, of three samples of different particle shapes with known size distribution.     

Spatial and temporal evolution of floc size distribution in a stirred square tank investigated using PIV and image analysis

In this study, non-intrusive measurements were performed in order to determine the spatial and temporal evolution of the floc size distribution in a square 7.3 L tank stirred with an A310 hydro foil impeller. The data was collected in situ using particle image velocimetry. The analysis of the data was done using a connected component labelling technique. It was found that the reproducibility of the system was adequate. The results show that there are large spatial differences in the mean size and the shape of the floc size distribution within the tank. It was found that steady-state was reached when stirrer speed was increased. It was also found that effects of the interaction between flocs and the water surface had substantial influence on the local size distribution. It was determined that the surface was more important for breakage of flocs than the impeller region. Results also showed clear number gradients in the tank. It is clear from this study that any population balance model developed in order to predict floc size distributions should incorporate spatial dependence.     

Surface modification of sepiolite particles with polyurethane and polyvinyl alcohol

Sepiolite is a clay mineral that has many industrial applications due to its advantageous properties such as white color, low specific gravity, high absorption capacity, chemical composition and low thermal conductivity. In this study, the effect of the addition of polymers, polyvinyl-alcohol and polyurethane, on the rheological properties of the sepiolite dispersions have been investigated. The rheological parameters of clay suspensions can be used to examine particle–particle interactions. The polymers that have been added to the clay suspensions interact with clay particles, depending on their ionic or non-ionic character. Firstly, the sepiolite dispersions were characterized by the rheological properties, mineral structure and content. Then the effect of the polymers on the flow, structural, and surface properties of sepiolite dispersions was investigated by rheological, electrokinetical, and scanning electron microscopy (SEM) measurements. The measurements showed that polymer molecules bind on the surface of sepiolite particles and changed the flow properties of the dispersions as stable dispersions at some certain concentrations. It is also determined that PU polymer covered the surface of the sepiolite faster than PVA, but the coverage of the PVA was much more smoothly. The thermal properties of the sepiolite improved with PVA more than PU, as a result of the homogenous surface coverage.     

Growth of highly oriented graphite films at room temperature by pulsed laser deposition using carbon-sulfur targets

Carbon-sulfur films were grown by pulsed laser deposition at room temperature using different graphite-sulfur mixtures as targets. The structure of the films was characterized by X-ray diffraction and transmission electron microscopy. The composition and the chemical bonds were analyzed by Rutherford-backscattering spectroscopy, X-ray photoelectron spectroscopy and energy dispersive X-ray analysis. The films were composed of amorphous carbon with sp²-, sp³- and S-C-C-S bonds and textured graphite on the top of the film. The thin graphite layer on top of the carbon-sulfur films is highly oriented, comparable to highly oriented pyrolytic graphite, and free of sulfur in the graphite lattice. The lateral size of the oriented graphite grains in the films was up to 8 μm. Magnetic measurements reveal that the films prepared under the conditions of our study show neither magnetic ordering nor superconductivity in the studied temperature range T > 2 K.     

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.     

Study on graphite surface modification by non-electrode plasma electrolysis

A novel non-electrode plasma electrolysis technique was developed in the paper in order to rapidly prepare a coating on graphite. The coating was mainly composed of SiO₂ with the thickness of ~ 7?µm. A small amount of SiC was also detected in the coating. The coating exhibited superior oxidation resistance at 1000?°C in air, evidenced by the visible polishing marks, smooth surface and the much decreased roughness from ~ 120?µm to ~ 50?µm. The simulation of thermal effects was conducted in order to establish the formation mechanism of the coating on graphite. We believe that the novel non-electrode plasma electrolysis technique will find a wide range of applications in preparing high-performance graphite and its composites.     

Enhancement of adsorption on graphite (HOPG) by modification of surface chemical functionality and morphology

The effects of chemical functional groups and surface morphology on the adsorption/desorption behavior of a model non-polar organic adsorbate (propane) on model carbonaceous surfaces [air-cleaved highly oriented pyrolytic graphite (HOPG) and plasma-oxidized HOPG], were investigated using temperature-programmed desorption (TPD). Oxygen- and hydrogen-containing functional groups exist on both air-cleaved HOPG and plasma-oxidized HOPG. The presence of these groups almost completely suppresses propane adsorption at 90 K. However, these groups can be removed from both air-cleaved and plasma-oxidized HOPG by thermal treatment (>500 K), leading to more than an order of magnitude increase in adsorption capacity. It is essential for both air-cleaved HOPG and plasma-oxidized HOPG to be outgassed at over 1273 K for all the adsorption sites to be chemically accessible for propane molecules. The effect of morphological heterogeneity is evident for plasma-oxidized HOPG as this substrate provides greater surface area available for adsorption, as well as higher energy binding sites.