Natural Microcrystalline Graphite Research Articles

Fabrication of matrix graphite with a high degree of graphitization for spherical fuel elements by using natural microcrystalline graphite fillers

Matrix graphite is used as a structural material, thermal conductor, moderator, and secondary fission product barrier for fuel elements in high-temperature gas-cooled reactors (HTRs). Due to its high graphitization degree and compressibility, natural flake graphite (NFG) is used as the main filler in traditional A3-3 matrix graphite, whereas artificial graphite (AG), with a lower graphitization degree than NFG, serves as an additive for toughness and gas permeability. Matrix graphite could be improved in terms of thermal conductivity, oxidation resistance, and irradiation performance by increasing the degree of graphitization. However, reports on the development of new matrix graphite formulations are scarce. In this study, MG-20 matrix graphite was prepared by mixing 60 wt% NFG, 20 wt% natural microcrystalline graphite (MG), and 20 wt% phenolic resin. Due to the high graphitization degree (higher than AG) and low coefficient of thermal expansion (CTE) of MG, MG-20 exhibited higher thermal conductivity (∼6%) and lower CTE (∼2.4%) than A3-3. Thus, MG-20 with higher graphitization degree and better thermal properties than A3-3 could improve the performance of HTR fuel elements in the future.

Improving Natural Microcrystalline Graphite Performances by a Dual Modification Strategy toward Practical Application of Lithium Ion Batteries.

Microcrystalline graphite (MG), as a kind of natural graphite (NG), holds great potential for use as an anode material for lithium-ion batteries (LIBs) due to low raw material cost, good electrolyte compatibility, and relatively long cycle life. Nevertheless, the relatively low reversible capacity and poor initial Coulombic efficiency (ICE) of the MG anode largely limit its practical application in LIBs. In order to improve the lithium storage capacity of MG, three kinds of oxidant intercalators are applied to treat the original MG, and the as-obtained MG is further modified by a thin carbon layer. The results indicate that using H2SO4-C2H2O4 as oxidant intercalators and subsequent carbon coating layer modification are the optimum techniques, and they can increase the interlayer distance, introduce defects to decrease the volume expansion, and generate channels for fast Li+ diffusion. Meanwhile, the carbon coating layer can reduce the specific surface area of graphite and greatly improve the ICE and cycling performance. Especially, the OEMGC-2 anodes prepared by the dual modification strategies represent a high reversible capacity of 349.4 mA h g-1 at 0.2C with a satisfactory ICE of 90.2%, indicating that the MG can also be considered as a high performance and low-cost anode material of LIBs.

N-doped microcrystalline graphite for boosting peroxymonosulfate activation with highly efficient degradation of bisphenol A

With the excellent electrical conductivity and unique structure, carbonaceous materials are expected to prepare green catalysts used in sulfate radical based advanced oxidation processes. Nevertheless, most of researches are focusing on artificial synthetic material to date, thus the studies on natural carbon materials call for attention and development. Herein, impregnation-pyrolysis method was employed by adopting the urea as an extrinsic N-dopant under 850 °C to favor C–N configuration for natural microcrystalline graphite (MG). And N element presented as pyridinic N (27.42 %), pyrrolic N (40.08 %), graphitic N (25.86 %) and nitric oxide (6.65 %) in N-doped microcrystalline graphite (NMG). In addition, the catalysis performance of MG, NMG and calcined microcrystalline graphite (CMG) was examined by the degradation of bisphenol A (BPA). The NMG system exhibited high activity in a broad pH range, and acidic conditions promote the reaction rate greatly. Based on quenching tests and electron paramagnetic resonance, the degradation mechanisms comprised radical and nonradical pathway, in which 1O2 was the dominant reactive oxidative species (ROS). This study suggested that pyridinic N, pyrrolic N, graphitic N, sp (2)-hybridized carbon structure as well as defects in NMG promoted the generation of ROS, and provides a new insight in natural carbon materials for the fabrication of new eco-friendly and highly efficient PMS activation catalysts.

One-Step Binding and Wrapping Fragmented Natural Microcrystalline Graphite via Phenolic Resin into Secondary Particles for High-Performance Lithium-Ion Battery Anode

One-Step Binding and Wrapping Fragmented Natural Microcrystalline Graphite via Phenolic Resin into Secondary Particles for High-Performance Lithium-Ion Battery Anode

Biochemical fulvic acid derived amorphous carbon modified microcrystalline graphite as low-cost anode for potassium-ion storage

Graphite anode has great potential toward potassium ion storage for abundant reserves, yet it suffers from the large volume expansion and slow diffusion rate. Herein, the low-cost biochemical fulvic acid-derived amorphous carbon (BFAC) is employed to modify the natural microcrystalline graphite (BFAC@MG) by a simple mixed carbonization strategy. The BFAC smooths the split layer and folds on the surface of microcrystalline graphite and builds the heteroatom-doped composite structure, which effectively alleviates the volume expansion caused by K+ electrochemical de-intercalation processes, together with improving electrochemical reaction kinetics. As expected, the optimized BFAC@MG-0.5 exhibits superior potassium-ion storage performance, which delivers a high reversible capacity (623.8 mAh g−1), excellent rate performance (147.8 mAh g−1 at 2 A g−1), and remarkable cycling stability (100.8 mAh g−1 after 1200 cycles). As a practical device application, the potassium-ion capacitors are assembled using the BFAC@MG-0.5 anode and commercial activated carbon cathode, which exhibits a maximum energy density of 126.48 Wh kg−1 and superior cycle stability. Significantly, this work demonstrates the potential of microcrystalline graphite as the host anode material for potassium-ion storage.

Purification mechanism of microcrystalline graphite and lithium storage properties of purified graphite

In order to improve the application value of natural microcrystalline graphite with carbon content of 49.5%, high-purity microcrystalline graphite was prepared by emulsifying kerosene flotation firstly, and then purifying hydrofluoric acid and hexafluorosilicic acid. Then the purified microcrystalline graphite was prepared for the lithium-ion battery anode material, its microstruture and electrochemical properties were analyzed, the purification mechanism and lithium storage mechanism were discussed. The research results show that carbon content of microcrystalline graphite after emulsified kerosene flotation and mixed acid purification are 93.5% and 99.0% respectively. After pickling, high-purity microcrystalline graphite shows the largest layer spacing, which is 0.351 5 nm and is 0.001 4 nm higher than that of natural microcrystalline graphite. Size disparity of acid washing sample is larger, layered structure is more obvious, cycle performance and magnification performance are better than those of floation sample. The pickled sample has the highest initial reversible specific capacity of 477.4 mAh/g, and the first Coulomb efficiency is 61.3%. Charge transfer impedance, interface impedance and SEI membrane impedance, and lithium ion diffusion impedance in electrode material are significantly lower than those of microcrystalline graphite after flotation.

Utilization of impurities and carbon defects in natural microcrystalline graphite to prepare silicon-graphite composite anode for high-performance lithium-ion batteries

Utilization of impurities and carbon defects in natural microcrystalline graphite to prepare silicon-graphite composite anode for high-performance lithium-ion batteries

Silicon/carbon composites based on natural microcrystalline graphite as anode for lithium-ion batteries

Silicon/carbon composite is considered as one of the most potential anode materials to be commercialized because of silicon’s high specific capacity and carbon’s stability. Natural microcrystalline graphite features abundant resources, low price, and high stability. In consideration of these, it is an ingenious strategy to combine natural microcrystalline graphite with silicon as the anode material of lithium-ion batteries. In this paper, silicon/natural microcrystalline graphite@ carbon layer composites were fabricated by a two-step chemical vapor deposition (CVD) method. In this structure, natural microcrystalline graphite serves as the carbon matrix; silicon grows evenly on the surface and then pyrolytic carbon provides a tight coating. Therefore, the prepared composites deliver a high specific capacity of 560 mA h g−1 after 200 cycles at a current density of 100 mA g−1. This paper provides a potential method for large-scale production and is expected to solve the high-level application problem of natural microcrystalline graphite.

Hydrothermal synthesized magnesium silicate hydroxide/graphene nanocomposites in a MgO-SiO2-graphite-H2O alkaline system and its application in anti-wear additive toward infinitesimal wear

The paper describes a hydrothermal synthesis of one new-style inorganic modified graphene nanomaterial called magnesium silicate hydroxide/graphene (MSH/graphene) in a MgO-SiO2-graphite-H2O system, and reveals its tribological behaviors as additives in poly-alpha-olefin (PAO). These MSH/graphene nanocomposites consist of MSH and various forms of graphene evolved from natural microcrystalline graphite. Four-ball experiments indicate that MSH/graphene nanocomposites as additives in PAO display outstanding tribological performances and nearly eliminate wear despite a severe contact pressure of 3.91 GPa. This can be attributed to the attachment of graphene to MSH, enabling it to successfully enter the asperity clearances so as to bring into full play to graphene's extreme mechanical strength and/or forming a tribofilm consisting of nanocrystalline graphite and amorphous carbon (a-C) on the sliding surfaces.

Effect of purity and proportion of microcrystalline graphite ore on the electrical, mechanical and tribological performance of copper-carbon composites

The copper-carbon composites were prepared by using powder metallurgy method with copper powder and natural microcrystalline graphite or flake graphite as raw materials. The microstructure and properties of copper-carbon composites were investigated by using scanning electron microscopy, conductivity tests, hardness tests, three-points flexural tests, friction and wear tests. The effects of the purity and proportion of the natural graphite on the microstructure, conductivity, flexural strength, hardness and tribological properties of the composites were analyzed. The results showed that the main performance of this copper-carbon composite was enhanced with increasing purity of microcrystalline graphite. When the content of the natural graphite in raw materials is 7.5 wt%, the conductivity, friction and wear properties of copper-carbon composites reached the optimal value. Furthermore, compared with flake graphite, microcrystalline graphite is more closely bound to copper, which leads to lower wear rate.

Shape Modification and Size Classification of Microcrystalline Graphite Powder as Anode Material for Lithium-Ion Batteries

Natural microcrystalline graphite (MCG) composed of many crystallites is a promising new anode material for lithium-ion batteries (LiBs) and has received considerable attention from researchers. MCG with narrow particle size distribution and high sphericity exhibits excellent electrochemical performance. A nonaddition process to prepare natural MCG as a high-performance LiB anode material is described. First, raw MCG was broken into smaller particles using a pulverization system. Then, the particles were modified into near-spherical shape using a particle shape modification system. Finally, the particle size distribution was narrowed using a centrifugal rotor classification system. The products with uniform hemispherical shape and narrow size distribution had mean particle size of approximately 9 μm, 10 μm, 15 μm, and 20 μm. Additionally, the innovative pilot experimental process increased the product yield of the raw material. Finally, the electrochemical performance of the prepared MCG was tested, revealing high reversible capacity and good cyclability.

Cost-effective fabrication of graphene-like nanosheets from natural microcrystalline graphite minerals by liquid oxidation–reduction method

Graphene-like nanosheets were fabricated using natural microcrystalline graphite minerals (NMGM) instead of flake graphite (FG) using a liquid oxidation–reduction method.

Fixed carbon content and reaction mechanism of natural microcrystalline graphite purified by hydrochloric acid and sodium fluoride

In order to improve the fixed carbon content of microcrystalline graphite (MG) and broaden its application, this article discussed the fixed carbon content and purification reaction mechanism of natural MG by using hydrochloric acid and sodium fluoride. The influence of sodium fluoride amount, liquid-solid ratio, reaction temperature and reaction time were respectively investigated using MG as raw materials. Results indicate that the fixed carbon content of MG increased with the increasing amount of fluoride; with the increment of the liquid-solid ratio, reaction temperature, and reaction time. In all cases, the fixed carbon content significantly increased at first then slowed down its increasing rate. The fixed carbon content of MG can be increased from 83.08% to 98.37% when 13.5g sodium fluoride amount and 30g microcrystalline graphite reacted with 120.5ml hydrochloric acid at 70°C for 2.5h. Hydrochloric acid can be used to convert the impurities such as metal compounds into soluble chloride. Hydrofluoric acid generated from sodium fluoride removes most of the complex silicate impurities, and then to be washed and filtered to achieve the separation of graphite. The crystal structure and morphology of MG will not be changed according to the SEM images and the XRD patterns. Results indicate that the hydrochloric acid and sodium fluoride method can enhance the fixed carbon content of MG to a higher level.

Wettability of natural microcrystalline graphite filler with pitch in isotropic graphite preparation

Natural microcrystalline graphite (MG), which shows highly graphitized structure and near-isotropic material properties, can be used as filler material in isotropic graphite preparation. The final properties of graphite artifacts are largely determined by the interaction between pitch and filler particles during kneading. To achieve a better understanding of the wetting mechanism of MG filler, wetting behavior of pitch on substrates with different crystal structures was studied by measuring their contact angles. The results showed that both crystalline orientation and degree of graphitization of substrates could significantly influence the pitch wettability. Edge planes of graphite surface showed better wetting capacity than basal planes. And cokes obtained at very high temperature with highly graphitized structure tended to prevent pitch from wetting. Further wetting test showed that MG substrate was totally wetted by pitch drop, and its wetting time was shorter than coke. The wetting behavior of MG was related to its unique crystalline structure and surface morphology, and beneficial to the mechanical strength of MG-based isotropic graphite.

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.

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.

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.

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.

Preparation of natural microcrystalline graphite with high sphericity and narrow size distribution

Natural microcrystalline graphite with high sphericity and narrow size distribution was prepared by powder engineering methods. The shape of microcrystalline graphite (MCG) particles was modified using a wet agitating method and then the particles were separated into groups with narrow size distribution by means of gravity settling and hydrocyclone combined. Several methods such as scanning electron microscope (SEM), Raman spectroscopy and X-ray diffraction (XRD) were employed to investigate the structure of the sample. The result showed that the present method was effective in classifying the MCG particles into a narrow size range. Galvanostatic cycling was used to investigate the electrochemical performance of the as-prepared MCG as the anode material of lithium ion cells. Compared with raw MCG, the modified MCG shows a large improvement in reversible capacity, though the cycleability is slightly reduced.