Microcrystalline Graphite Research Articles

Ultrahigh-rate and high-density lithium-ion capacitors through hybriding nitrogen-enriched hierarchical porous carbon cathode with prelithiated microcrystalline graphite anode

Lithium-ion capacitors (LICs) are novel advanced electrochemical energy storage (EES) systems integrating both battery and capacitor functions. Most efforts for developing high-power LICs are currently dedicated to nanostructure design of battery-type anodes, which in general results in low packing densities and cannot fundamentally improve the slow Faradaic reaction. Up to now, little attention has been focused on the effects of porous carbon cathodes and the reasonable matching of cathode/anode on the power performance of LICs. Herein, a novel nitrogen-enriched mesoporous carbon nanospheres/graphene (N-GMCS) nanocomposite is demonstrated, which shows simultaneously hierarchical porous structure, 3D conductive network, as well as very high mass density. When such N-GMCS cathode is coupled with prelithiated microcrystalline graphite (PLMG) anode, the integrated device shows quite high packing density which is highly desirable in EES systems. In particular, the PLMG anode in N-GMCS//PLMG system breaks the limitation of slow Faradaic reaction and lithium-ion bulk diffusion, providing an ultrafast capacitor-like electrochemical response. Quite attractive maximum energy density (80Whkg−1, 68.6WhL−1) and state-of-the-art maximum power density (352kWkg−1, 292kWL−1) can be achieved in N-GMCS//PLMG, which are 5 and 2.8 times as large as those of the supercapacitor counterpart, respectively.

Advantages of natural microcrystalline graphite filler over petroleum coke in isotropic graphite preparation

Graphite materials are used in high-temperature gas-cooled reactor (HTR) as moderator and reflector. Nuclear graphite is currently manufactured using coke as the filler; however, improvements in the performance of nuclear graphite are anticipated in order to enhance the safety and lifetime of HTRs. Natural microcrystalline graphite (MG), which is a metamorphic product of coal, is an emerging candidate as filler material. This paper proposes the approach of preparing isotropic graphite using MG fillers. The characterization results of MG ores and purified powder indicate a polycrystalline and near-isotropic structure of MG particles. Thus, near-isotropic graphite with an isotropy ratio of 1.10–1.15 can be easily obtained via cold isostatic pressing. The thermal diffusivity of MG-based green body is much higher than that of coke-based one, therefore facilitates the baking step. The advantages of MG-based graphite also include a high degree of graphitization, and a low coefficient of thermal expansion, both of which are highly beneficial to nuclear applications. As a result, MG exhibits a large potential for the application in isotropic nuclear graphite.

RECOVERY MECHANISMS OF SERICITE IN MICROCRYSTALLINE GRAPHITE FLOTATION

Sericite is the main contaminant of concentrate in commercial microcrystalline graphite ore flotation. It was necessary to identify its recovery mechanisms so that the appropriate solution can be selected. In this study, the influence of sericite on flotation selectivity of microcrystalline graphite ore and its recovery mechanisms were investigated. Artificial mixtures flotation test suggested that sericite seriously reported into concentrate leading to poor flotation selectivity of microcrystalline graphite ore. However, the aggregation/dispersion behavior of artificial mixtures indicated that a large repulsive energy existed between sericite and microcrystalline graphite particles at pH 7.4, and sericite was not likely to report into graphite concentrate by slime coating. The results obtained from contact angle measurements and a technique of Warren showed that the floated sericite reached the froth via a combination of both entrainment and entrapment mechanisms, not via true flotation.

Preparation of graphene nanosheets from microcrystalline graphite by low-temperature exfoliated method and their supercapacitive behavior

Microcrystalline graphite, a kind of cheap and infrequently used graphite material, used for the rapid synthesis of graphene nanosheets (MGS) by thermal reduction–exfoliation of graphite oxide at low temperature (as low as 100 °C) is reported. The results indicate that the microcrystalline graphite could be oxidized completely. X-ray diffraction patterns show that the obtained MGS exhibit an amorphous structure. Scanning electron microscopy images show that the MGS samples possess nanoporous structure. The results of N2 adsorption–desorption analysis indicate that the MGS samples possess high Brunauer Emmett Teller surface areas (the highest is 374 m2 g−1). The results of electrochemical tests show that the MGS exfoliated at 400 °C (MGS-400) has better supercapacitive performance than that of MGS exfoliated at 100 °C. The specific capacitance value of MGS-400 is about 212 F g−1 at 5 mV s−1. And also the MGS samples have good rate performance and cycle stability.

High‐Performance Electrocatalysts for Oxygen Reduction Based on Nitrogen‐Doped Porous Carbon from Hydrothermal Treatment of Glucose and Dicyandiamide

AbstractA synthetic method was developed for the preparation of N‐doped porous carbon through hydrothermal treatment at controlled temperatures by using glucose and dicyandiamide as precursors and ZnCl2 as an activation reagent. Nitrogen doping was quantitatively determined by using XPS measurements and identified in the forms of pyridinic, pyrrolic, graphitic, and pyridinic N+O−nitrogen atoms. Further structural analysis by using SEM, TEM, XRD, Raman, FTIR, and BET measurements showed that the N‐doped porous carbons exhibited a microcrystalline graphite structure and a high specific surface area up to 1000 m2 g−1. Electrochemical studies showed that the samples all exhibited a remarkable ORR catalytic activity in alkaline media, which was comparable to that of state‐of‐the‐art Pt/C catalysts, and the one prepared at 800 °C was found to be the best among the series with an onset potential of +0.96 V, almost complete reduction of oxygen to OH−, and superior methanol tolerance and cycling stability.

Acoustic emission detection for mass fractions of materials based on wavelet packet technology

Materials are often damaged during the process of detecting mass fractions by traditional methods. Acoustic emission (AE) technology combined with wavelet packet analysis is used to evaluate the mass fractions of microcrystalline graphite/polyvinyl alcohol (PVA) composites in this study. Attenuation characteristics of AE signals across the composites with different mass fractions are investigated. The AE signals are decomposed by wavelet packet technology to obtain the relationships between the energy and amplitude attenuation coefficients of feature wavelet packets and mass fractions as well. Furthermore, the relationship is validated by a sample. The larger proportion of microcrystalline graphite will correspond to the higher attenuation of energy and amplitude. The attenuation characteristics of feature wavelet packets with the frequency range from 125kHz to 171.85kHz are more suitable for the detection of mass fractions than those of the original AE signals. The error of the mass fraction of microcrystalline graphite calculated by the feature wavelet packet (1.8%) is lower than that of the original signal (3.9%). Therefore, AE detection base on wavelet packet analysis is an ideal NDT method for evaluate mass fractions of composite materials.

Graphene microsheets from natural microcrystalline graphite minerals: scalable synthesis and unusual energy storage

Graphene microsheets, directly and efficiently produced from natural microcrystalline graphite mineral, reached 390 mA h g−1 after 220 cycles as a battery anode.

Hydrogen-free synthesis of graphene–graphitic films directly on Si substrate by plasma enhanced chemical vapor deposition

Carbon films with the thicknesses varying from 10 to 600 nm in the architecture range of carbonaceous graphene to graphitic bonding were synthesized on Si wafer. The films were prepared by radio frequency plasma enhanced chemical vapor deposition under 300 °C without any catalyst and hydrogen-assistant ambient as well. The films found to be composed of microcrystalline graphene and graphite inclusions with a maximum graphene film size of 2 µm. Raman spectral characterization verified the bonding form of the graphene–graphitic films to be mostly sp2. In this work, the complicated transfer printing process for carbon/silicon (C/Si) devices and the quality degradation of the hybrid films were avoided by the direct deposition of the carbonaceous films on Si substrate. The current–voltage feature of the devices manifested that the devices possess an excellent rectifying behavior in dark and the characteristic of photovoltaic in the illumination (known as solar cells).

Preparation, characterization and supercapacitive performance of graphene nanosheets from microcrystalline graphite

In this study, graphene nanosheets (GS) were prepared from a kind of economic and resourceful graphite material, natural microcrystalline graphite, by thermal exfoliation method. The scanning electron microscopy and transmission electron microscopy images show that the obtained GS have porous structure and are well separated and composed of approximately 6–8 wrinkled individual monoatomic graphene layers. The results of nitrogen adsorption–desorption analysis show the GS possess high specific surface area. The results of electrochemistry measurements show that the prepared GS have good supercapacitive performance, the capacitance increase with the oxidation degree of microcrystalline graphite oxide. The highest capacitive performance can reach 175 F g−1 at 0.5 A g−1, and also the GS have good rate performance and cycle stability.

Macro- and Nanotribological Properties of Graphite Tribofilms: Influence of the Sliding Interface

Macrotribological studies of microcrystalline graphite powder reveal a drastic decrease in the friction coefficient when the experiments are carried out in the presence of low-viscosity liquids. The friction reduction is attributed to the simultaneous presence of particles and liquid in the sliding contact, but the mechanisms involved remain unclear. In order to contribute to the understanding of liquid action in friction reduction mechanisms, nanoscale investigations of the tribofilms have been performed using lateral force microscopy. Attention is devoted to the nanostructure of the film surfaces and their nanofriction behavior using an atomic force microscope. The influence of the tip/sample interfaces on friction properties is investigated by using AFM tips constituted of different compounds (silicon, gold/chromium alloy, silicon nitride or carbon-covered AFM tip) and by performing the nanofriction tests in air or liquid environments. The results indicate that the friction reduction observed at macroscale is attributed neither to the lowering of the shear strength of the carbon/carbon interface in the presence of liquid nor to the nanostructure of the film surface. Collective liquid/particles effects inside the contact during sliding are probably involved.

Effective production of nano-sized graphene via straight-forward exfoliation of microcrystalline graphite

We report a facile and effective technique for the large-scale production of nano-sized graphene sheets via subjecting the microcrystalline graphite to ball milling. The products consist of single- or few-layer (≤5 layers) graphene with lateral dimensions concentrated in the range of 100–200 nm. It displays a high intensity of photoluminescence at a wavelength of 339 nm and graphene dispersions at concentrations up to 1.22 mg ml−1.

Microstructures and mechanical properties of Al2O3–C refractories using nickel-loaded ultrafine microcrystalline graphite and silicon additives

Ultrafine microcrystalline graphite (UMCG) and nickel-loaded ultrafine microcrystalline graphite (NMCG) were introduced into Al2O3–C refractories to totally replace graphite flake (GF), respectively. Their interactions with silicon additive were exploited through the observation of microstructures, mechanical and thermo-mechanical properties of Al2O3–C refractories after firing at 800–1400°C in a coke powder bed. It revealed that with NMCG, carbon nanotubes and SiC whiskers are in-situ formed out of nickel-catalytic pyrolysis of phenol resin binder at the temperature low to 1000°C. Moreover, the amount of SiC whiskers increases with increasing fire temperature. The synergistic effect of the ultrafine microcrystalline graphite, carbon nanotubes and SiC whiskers contributes to the substantial improvement of mechanical properties and thermal shock resistance of Al2O3–C refractories using nickel-loaded ultrafine microcrystalline graphite as carbon source.

A hard carbon/microcrystalline graphite/carbon composite with a core-shell structure as novel anode materials for lithium-ion batteries

Hard carbon and microcrystalline graphite (MG) core-shell structured composite materials are prepared, and their electrochemical performances as an anode material for lithium-ion batteries are reported. The composite materials are obtained by coating a mixture of MG and pitch onto hard carbon particles, followed by heating at 1200°C under an argon atmosphere for 1h. The surface of the hard carbon is subsequently covered with a layer of the MG/pitch carbon composite. In the coating layer of the MG/pitch carbon composite, the MG particles are divided into nanoscale graphite sheets, and uniformly dispersed within the pitch of carbon matrix. The composite particles have a rounded shape, especially when the content of MG increases, which can improve their packing density compared to hard carbon having sharp edges. Anodes prepared from these composite materials exhibit enhanced electrochemical performances, including a high reversible capacity, high initial coulombic efficiency, high charging/discharging rate capability, and desirable cycling stability.

Microstructures and mechanical properties of Al2O3–C refractories with addition of microcrystalline graphite

Al2O3–C refractories were prepared in coke bed in the range from 800°C to 1400°C by using tabular alumina, α-Al2O3, Al, Si powder, silica fume, natural graphite flake and microcrystalline graphite as starting materials. Firstly the ultrafine microcrystalline graphite (UMCG) powders were produced by high-energy ball milling natural microcrystalline graphite and micron sized α-Al2O3 powders, and then partially or totally replaced with graphite flake in Al2O3–C refractories. The microstructures and mechanical properties of all the obtained Al2O3–C specimens were investigated by X-ray diffraction (XRD), scanning electron microscopy (SEM) coupled with energy dispersive X-ray spectroscopy (EDS) and three-point bending test. The results showed that UMCG with less than 5μm in size could be obtained by a high-energy ball milling process. The mechanical properties such as cold modulus of rupture (CMOR), modulus of elasticity (E), force and displacement of Al2O3–C refractories with UMCG powders were improved in comparison with those without UMCG. This improvement was attributed to the fact that the addition of UMCG affected the microstructural evolution of Al2O3–C refractories. The UMCG powders could accelerate the in-situ formation of AlN, Al4C3, and SiC ceramic whiskers in specimens because of their higher reactivity than graphite flake.

EFFECT OF GRAPHITE PRECURSOR ON OXIDATION DEGREE, HYDROPHILICITY AND MICROSTRUCTURE OF GRAPHENE OXIDE

Graphene oxide (GO) has attracted much attention as a derivative of graphene. In addition, it appears to have many unique physicochemical properties and has been investigated widely in many areas. Herein, we prepare GOs using flake graphite (FG), expandable graphite (EG) and microcrystalline graphite (MG) as graphite precursors by the modified Hummers method. According to the X-ray diffraction (XRD), Fourier transform infrared spectroscopy, X-ray photoelectron spectroscopy (XPS) and Raman spectroscopy, we characterize the component, the functional group, the chemical state of the element and the structural disorder of the obtained GOs to reveal their oxidation degree. Besides, we evaluate the hydrophilicity of the obtained GOs with the water contact angle, and observe their microstructures by transmission electron microscopy (TEM). We find that the GO prepared with EG has a higher-degree oxidation and better hydrophilicity, and it will be exfoliated easily and forms a monolayer or quasi-monolayer structure. Finally, based on the structural characteristic of graphite precursor, we build the intercalation and oxidation model to illuminate the phenomenon.

The entrainment behaviour of sericite in microcrystalline graphite flotation

Few publications focus on the influence of hydrophobic minerals such as graphite or coal on entrainment behaviour of hydrophilic gangue, although they coexist with one another in flotation pulp. This paper presented an experimental study on entrainment behaviour of sericite in the absence and presence of hydrophobic microcrystalline graphite, and explained the poor flotation selectivity of microcrystalline graphite ore.For sericite and microcrystalline graphite single mineral, batch flotation tests and contact angle measurements were conducted, the results indicated that hydrophilic sericite was easy to entrain into concentrate and this process was strongly affected by its particle size; the particle size and solid concentration of hydrophobic graphite intensely affected water recovery of concentrate. Batch flotation tests of artificial mixtures composed of sericite and microcrystalline graphite were also carried out. The results indicated that the particle size and concentration of hydrophobic graphite significantly affected the recovery of water, the entrainment factor of sericite, and the recovery of sericite in concentrate. The serious entrainment or entrapment of sericite in the presence of hydrophobic microcrystalline graphite leads to poor selectivity in microcrystalline graphite ore flotation.

Synthesis and some aspects of the formation mechanism of carbon structures under hydrothermal conditions

The formation of carbon structures upon the reactions of organic compounds with strong mineral acids under hydrothermal conditions was studied. The reactions were found to give amorphous carbon, microcrystalline graphite, nanodiamonds, diamond-like carbon, diamonds, C60 and C70 fullerenes, and carbolite. Hydrocarbons of various compositions including aromatics and naphthenes were detected among the reaction products. A possible mechanism of the formation of carbon structures with sp2 bond hybridization under hydrothermal conditions was proposed.

High concentration and stable few-layer graphene dispersions prepared by the exfoliation of graphite in different organic solvents

A series of stable graphene dispersions in different organic solvents were prepared by the liquid-phase exfoliation of microcrystalline graphite. The pristine graphite was heat-treated in N-methyl-2-pyrrolidone before it was exfoliated in solvents containing 0.1 mg mL−1 polyvinyl pyrrolidone (PVP). X-ray diffraction and scanning electron microscopy (SEM) results implied that the interlayer spacing of graphite increased after the heat treatment. The increased interlayer spacing makes it favorable for organic molecules to enter the lattice of graphite, and is helpful in the exfoliation of graphite for the production of graphene flakes. High resolution transmission electron microscopy (HRTEM) and Raman spectroscopy indicated that the resulting graphene flakes are high-quality products without any significant structural defects. Atomic force microscopy (AFM) showed that the graphene flakes consist of single-to-few layer graphene, and also demonstrated that the concentration of PVP is an essential factor in the deposition of graphene flakes onto a mica substrate. More importantly, the resulting graphene dispersions can be used to fabricate graphene films and disk arrays on quartz glass. Therefore, it is possible that the graphene dispersions in this study can be used for the preparation of large-area graphene films on different substrates.

Influence of radio-frequency electromagnetic field on optical characteristics of cast-iron surface

Variations in spectral coefficients of specular reflection and Raman scattering spectra were detected during exposure of a cast-iron surface to an electromagnetic field in the radio-wave range. The observed variations were supposed to be caused by both field-induced changes in the surface morphological structure, density, and crystalline state and ordering of the carbon structures accompanied by a reduction in the amount of defects. Spectral reflectometry and Raman scattering methods were shown to be applicable to nondestructive remote monitoring of the morphological state of the cast-iron surface and changes in the structure and size of its microcrystalline graphite inclusions.

Microcrystalline Graphite Oxide as Durable Catalyst Support for PEM Fuel Cell

Microcrystalline Graphite Oxide as Durable Catalyst Support for PEM Fuel Cell