Plane Of Graphite Research Articles

Anchoring silicon on the basal plane of graphite via a three-phase heterostructure for highly reversible lithium storage

Interfacial stability is crucial to electrochemical performance, especially the cycle life and Coulombic efficiency (CE), of anodes for lithium-ion batteries (LIBs). However, stabilization of silicon (Si) nanoparticles on the basal plane of graphite nanosheets (GNs) and their effect on both cycling performance and CE have been lacking. Herein, deposition of Si nanoparticles on the basal plane of GNs is achieved by magnesiothermic reduction of mixture of silica (SiO2) nanoparticles and GNs. When the molar ratio of SiO2 to GNs in the reactant is above 0.33, Si nanoparticle and GN form an interfacial 3C-SiC layer. Both Si and interfacial 3C-SiC are oriented in their [111] crystallographic direction in parallel with [002] direction of GNs. The energetics of this three-phase heterostructure, (111) [11¯0]Si//(111) [11¯0]3C-SiC//(002) [110]GNs, is calculated using density functional theory (DFT), which indicates strong bonding at two interfaces of Si(111)/3C-SiC(111) and 3C-SiC(111)/GNs(001). The composite anode with a three-phase heterostructure demonstrates a high initial CE of 97.8% and a rapid increase of the CE to 99.4% after only five cycles with a capacity retention of 82.9% after 100 cycles, owing to the strong bonding between Si and GNs via the interfacial 3C-SiC layer, which maintains electrical connection despite the volume changes of Si during cycling.

Flash Graphene Morphologies.

Flash Joule heating (FJH) can convert almost any carbon-based precursor into bulk quantities of graphene. This work explores the morphologies and properties of flash graphene (FG) generated from carbon black. It is shown that FG is partially comprised of sheets of turbostratic FG (tFG) that have a rotational mismatch between neighboring layers. The remainder of the FG is wrinkled graphene sheets that resemble nongraphitizing carbon. To generate high quality tFG sheets, a FJH duration of 30-100 ms is employed. Beyond 100 ms, the turbostratic sheets have time to AB-stack and form bulk graphite. Atomistic simulations reveal that generic thermal annealing yields predominantly wrinkled graphene which displays minimal to no alignment of graphitic planes, as opposed to the high-quality tFG that might be formed under the direct influence of current conducted through the material. The tFG was easily exfoliated via shear, hence the FJH process has the potential for bulk production of tFG without the need for pre-exfoliation using chemicals or high energy mechanical shear.

Synthesis and Characterization of Aero-Eutectic Graphite Obtained by Solidification and Its Application in Energy Storage: Cathodes for Lithium Oxygen Batteries

Aero-eutectic graphite can be defined as a new light material with hierarchically structured porosity. It is obtained from the solidification of gray cast irons, followed by the dissolution of the ferrous matrix by an acidic sequence. The result is a continuous and interconnected network of graphite sheets with varied dimensions randomly oriented. X-ray diffraction characterization has revealed graphite crystallographic planes (002), (100), (101), (102) and (004), while the surface area measured by BET and Langmuir methods has been determined in the order of 90 m2 g−1 and 336 m2 g−1, respectively. The process of obtaining eutectic aero-graphite also allows the deposit of Cu nanofilms and TiC particles. Aero-eutectic graphite has been tested as cathode in Li–O2 batteries as it has been prepared, without the addition of binders or conductive carbons, showing an appropriate contact with the electrolyte, so that the oxygen reduction and evolution reactions may develop satisfactorily. In the discharge-charge galvanostatic tests, the battery accomplishes 20 complete cycles with area capacity limited to 1.2 mAh cm−2.

Heteroatoms (N, F, O)-Doped CNTs on NiCo-Silica Nanocomposites for Oxygen Evolution Reaction

The more electronegative atoms of element like nitrogen, fluorine and oxygen in CNTs are highly desirable for electrochemical catalytic reactions like oxygen evolution reaction (OER). Herein, we report the synthesis of in situ nitrogen-doped CNTs by CVD method on fcc NiCo alloy-silica nanocomposites in ethylene precursor containing 10% acetonitrile and their catalytic behaviour for oxygen evolution in alkaline media. The XPS analysis revealed that about 2 atomic% nitrogen was successfully doped in CNTs. Although the amount of doped nitrogen was not substantial, it played a significant role in the formation of bamboo-shaped CNTs by facilitating the conical shape of nanocatalyst. The internal core of the CNTs was analysed by TEM studies and it was found that CNTs have irregular cup and cone compartments repeated at a distance of about 50 nm. The oxygenated functional groups in the form of C=O and O–F were also found which owe their presence on the surface of CNTs to the aqueous HF treatment. The unique features like the presence of heteroatoms (N, F, O) at the graphitic planes of CNTs and its bamboo shape have collectively improved the OER performance of our synthesised carbonaceous material. As a result it exhibited OER overpotential of 315 mV at current density of 10 mA/cm2, which was better than many reported carbonaceous materials in alkaline media.

Superlubricity between Graphite Layers in Ultrahigh Vacuum

Graphite has been conventionally believed to exhibit an inferior lubricating performance with significantly larger friction coefficient and wear rate in a vacuum environment than in ambient air. Dangling bonds at the edge planes of graphite, accounting for the high friction in inert atmosphere are saturated by chemisorbed vapor molecules in air, which contributes to low surface adhesion and low friction. However, there is still a lack of direct experimental evidence whether basal planes of graphite excluding the negative effects of edges or dangling bonds shows intrinsic lubricity when sliding under ultrahigh vacuum (UHV) conditions. By the interlayer friction measurement enabled by graphite flake-wrapped atomic force microscope tips in UHV, we show a record-low friction coefficient of 4 × 10-5 (slope of friction vs normal force curve) when sliding between graphite layers, which is much lower than that in ambient air. This discrepancy manifests the intrinsic sliding frictional behavior between the graphite basal planes when the tribo-materials and experimental conditions are well-designed and strictly controlled. In addition, the temperature dependence of the kinetic friction between the graphite layers has been investigated under UHV conditions over the temperature range of 125-448 K, which is consistent with the thermally activated process.

Experimental measurement of thermal conductivity along different crystallographic planes in graphite

In this work, the time-domain thermoreflectance (TDTR) method is used to measure the thermal conductivity of graphite along different crystallographic planes at room temperature for the first time and the thermal conductivities along the non-principal axes of graphite are obtained. A focused ion beam is used to cut graphite samples along different crystallographic planes for the TDTR measurement. Then, a thermal model is developed to extract the thermal conductivity of graphite along different crystallographic planes from the measured signals of the TDTR method. The measured thermal conductivities along different crystallographic planes in graphite agree well with the anisotropy model, revealing that the traditional TDTR method can be used to measure the non-principal axis thermal conductivity of anisotropic layered materials. Moreover, the experimental results demonstrate that once the crystallographic plane deviates from the cross-plane direction, the in-plane phonon modes will dominate the heat transfer in graphite.

Quantum-dot assisted spectroscopy of degeneracy-lifted Landau levels in graphene

Energy spectroscopy of strongly interacting phases requires probes which minimize screening while retaining spectral resolution and local sensitivity. Here, we demonstrate that such probes can be realized using atomic sized quantum dots bound to defects in hexagonal Boron Nitride tunnel barriers, placed at nanometric distance from graphene. With dot energies capacitively tuned by a planar graphite electrode, dot-assisted tunneling becomes highly sensitive to the graphene excitation spectrum. The spectra track the onset of degeneracy lifting with magnetic field at the ground state, and at unoccupied excited states, revealing symmetry-broken gaps which develop steeply with magnetic field - corresponding to Landé g factors as high as 160. Measured up to B = 33 T, spectra exhibit a primary energy split between spin-polarized excited states, and a secondary spin-dependent valley-split. Our results show that defect dots probe the spectra while minimizing local screening, and are thus exceptionally sensitive to interacting states.

Chemistry and Mechanism of One-Step Formation of Graphene from Agrowaste

The one-step synthesis of high-quality graphene derivatives via CVD process has gained considerable importance nowadays for high-performance electronics and sensors. However, the use of harsh chemicals, high temperature, sensitivity, and the problem of separation of graphene from the substrate, motivated the one-step synthesis of graphene from a non-graphitic precursor, bypassing the use of graphite. In this paper, we have reported for the first time, the synthesis of graphene nanosheets from sugarcane bagasse at the normal atmospheric condition in a single step, avoiding the formation of GO. Here, the pyrolysis of sugarcane bagasse was carried out in the temperature range of 250-450o C in the presence of sodium hydroxide. The results suggested that even the low temperature (250–450o C) facilitated the development of graphitic planes via condensation and aromatization of the glucose monomers present in the precursor. The XRD pattern showed 2θ at around 25o in each case, which confirmed the formation of graphene instead of GO. The FESEM, TEM, and EDX analysis proved the formation of few-layer nanosheets of graphene from carbon-rich waste precursors in a single step.

The high electrochemical performance of N-doped active carbon with the shape of nanospheres in 1-ethyl-3-methylimidazolium tetrafluoroborate

In our investigation, N-doped active carbon with the shape of nanospheres with a diameter of 85 nm was synthesized. The TEM image shows that the NAC has graphitic regions and the surface graphitic (002) plane can be observed. The specific surface area of NAC was 2256 m2/g. XPS demonstrated that elemental nitrogen (pyrrolic-N, pyridine-N, quaternary-N) was successfully doped into the active carbon structure with a total N content of 8.1%. The sample was tested in a symmetrical system in 1 M LiClO4/acetonitrile electrolyte and 1-ethyl-3-methylimidazolium tetrafluoroborate. It showed a high specific energy density of 65.3 Wh/kg, due to the high potential window in the 1-ethyl-3-methylimidazolium tetrafluoroborate. It could retain 95% of the initial capacitance even after 8000 cycles.

Introduction of a Methodology to Enhance the Stabilization Process of PAN Fibers by Modeling and Advanced Characterization.

A methodology for designing the oxidative stabilization process of polyacrylonitrile (PAN) fibers is examined. In its core, this methodology is based on a model that describes the characteristic fiber length variation during thermal processing, through the de-convolution of three main contributors (i.e., entropic and chemical shrinkage and creep elongation). The model demonstrated an additional advantage of offering further insight into the physical and chemical phenomena taking place during the treatment. Validation of PAN-model prediction performance for different processing parameters was achieved as demonstrated by Fourier Transform Infrared Spectroscopy (FTIR) and Differential Scanning Calorimetry (DSC). Τensile testing revealed the effect of processing parameters on fiber quality, while model prediction demonstrated that ladder polymer formation is accelerated at temperatures over 200 °C. Additionally, according the DSC and FTIR measurements predictions from the application of the model during stabilization seem to be more precise at high-temperature stabilization stages. It was shown that mechanical properties could be enhanced preferably by including a treatment step below 200 °C, before the initiation of cyclization reactions. Further confirmation was provided via Raman spectroscopy, which demonstrated that graphitic like planes are formed upon stabilization above 200 °C, and thus multistage stabilization is required to optimize synthesis of carbon fibers. Optical Microscopy proved that isothermal stabilization treatment did not severely alter the cross section geometry of PAN fiber monofilaments.

Nano-engineering the material structure of preferentially oriented nano-graphitic carbon for making high-performance electrochemical micro-sensors

Direct synthesis of thin-film carbon nanomaterials on oxide-coated silicon substrates provides a viable pathway for building a dense array of miniaturized (micron-scale) electrochemical sensors with high performance. However, material synthesis generally involves many parameters, making material engineering based on trial and error highly inefficient. Here, we report a two-pronged strategy for producing engineered thin-film carbon nanomaterials that have a nano-graphitic structure. First, we introduce a variant of the metal-induced graphitization technique that generates micron-scale islands of nano-graphitic carbon materials directly on oxide-coated silicon substrates. A novel feature of our material synthesis is that, through substrate engineering, the orientation of graphitic planes within the film aligns preferentially with the silicon substrate. This feature allows us to use the Raman spectroscopy for quantifying structural properties of the sensor surface, where the electrochemical processes occur. Second, we find phenomenological models for predicting the amplitudes of the redox current and the sensor capacitance from the material structure, quantified by Raman. Our results indicate that the key to achieving high-performance micro-sensors from nano-graphitic carbon is to increase both the density of point defects and the size of the graphitic crystallites. Our study offers a viable strategy for building planar electrochemical micro-sensors with high-performance.

Interfacial effect between graphite and iron substrate on basal plane orientation and lubricity of graphite

The role of graphite/substrate interface in low-friction behavior of graphite was investigated. Iron and graphite powder mixtures were plastically deformed in either Ar–H2 or air atmospheres to obtain bare iron or iron oxide surface. Then, graphite particles adhered to and formed interfaces with surfaces. Friction force microscopy revealed that friction on particle surface milled in Ar–H2 atmosphere was obviously higher. X-ray absorption near-edge structure and transmission electron microscopy revealed that although the basal plane of graphite oriented along the oxidized surface, it did not occur along the bare iron surface, which explains the difference in lubricity on each surface.

Molecular dynamics simulation study on the wetting behavior of liquid iron and graphite

Understanding the wetting behavior of the molten iron and graphite is of great significance to guide the gas permeability of the blast furnace. This paper used the molecular dynamics simulation method to analyze the wetting behavior of molten iron on the graphite substrate at the atomic scale. In this paper, it was found that high temperatures can help reduce the time it takes for the wetting process to reach equilibrium. And the graphite basal plane can promote the melting of the iron block at a temperature lower than the melting point. In addition, the graphite basal plane showed good hydrophilicity to the molten iron, and the contact angle can reach 55.65° at 2000 K. The graphite prism plane is hydrophobic to the molten iron. And the results of molecular dynamics simulation showed that high temperature can improve the hydrophilicity of graphite basal plane and reduce the hydrophobicity of graphite prism plane. Therefore, adjusting the temperature and the contact surface between graphite and molten iron can provide theoretical guidance for improving the gas and liquid permeability of blast furnace.

The preparation of graphite/silicon@carbon composites for lithium-ion batteries through molten salts electrolysis

High-specific-capacity materials are crucial for the high-energy-density lithium-ion secondary batteries as the automakers and customers are both eager to extend the cruising range of electric vehicles. The current commercial silicon/carbon composites are based on the mechanical mixture of silicon and graphite, but this weak combination is not suitable for the higher-capacity materials. Here, low-cost raw materials are used for the preparation of a graphite/silicon@carbon composite negative electrode material, which synergizes ball milling, molten salts electrolysis and carbon coating. Silica is in situ electrochemically reduced to silicon on the flaky graphite serving as the conducting substrate during the electrolysis process. It is found that ball milling increases the active sites on the basal plane of graphite, which is beneficial for the nucleation and growth of the silicon, and enhances the bonding of silicon particles and graphite. As for the electrochemical results of coin-type cells, this graphite/silicon@carbon composite material exhibits a better cycle performance than the commercial Si/C 650 silicon-based composite material.

Characterization of Carbon Properties Influencing on Lib Performance at Low Temperature

Introduction : In order to promote development of electric vehicle, it is required to improve the capacity density of a battery, the battery performance at a low temperature, and the high rate property of a battery. Characterization of the size distribution and the average size of carbon in the negative electrode significantly influence on the battery performance. We have studied clarification of graphitic carbon’s properties influencing on performance of LIB especially at low temperature. Firstly, the X-ray diffraction profiles of several graphitic carbons were analyzed by the fundamental parameter (FP) method. Further, the structure changes of lithium-intercalated graphite during the charge and discharge processes were investigated by operando measurement using synchrotron radiation at BL28XU of SPring-8 in Japan. The time resolution of the apparatus is the order of milliseconds for analyzing one diffraction pattern. Experimental : Crystallite size and its distribution of several graphitic carbons were analyzed by the fundamental parameter (FP) method using X-ray diffraction profiles. X-ray wavelength of 0.154 nm was used to obtain X-ray diffraction profiles. In order to analyze crystallite size and its distribution, X-ray diffraction profiles were calculated by using software called PDXL2 (Rigaku Corporation).Coin cells composed of carbon and NMC electrodes were prepared and subjected to the evaluation of the battery performances from the low temperature to the room temperature. Coin cells conditioned by several cycles were evaluated.Further, crystal structures of lithium-intercalated graphite during charge and discharge processes were investigated by operando analysis using synchrotron radiation diffraction at BL28XU beam line of SPring-8. Al-laminated half-cells composed of graphitic carbon and Li electrodes were prepared and subjected to the evaluation of operando measurement. The graphitic carbons of different crystallite sizes and their distributions were analysed. Al-laminated half-cells conditioned by several cycles were used for the measurement. Operando measurements were carried out during charge and discharge processes. X-ray of 25keV (its wavelength of 0.0496 nm) with a beam size of 0.2 mm ×0.5 mm was used to obtain diffraction patterns. The exposure time was set from 0.5 s to 10 s. The diffraction patterns were obtained by two-dimensional detector (Rigaku Corporation). Results and Discussion : The experimental X-ray diffraction profile of graphitic carbon and that calculated profile by the fundamental parameter (FP) method were obtained. The calculated profile showed a good fit with the measured one. The each plane of graphitic carbon was analyzed properly.The analysis of FP method revealed that crystallite sizes and their distributions of graphitic carbons influences on LIB performance especially at low temperature. The results indicate that the low temperature characteristics improve as the crystallite size and distribution of the graphite (102) plane decreases. The same results were also obtained for the relationship between the crystallite size distribution of the graphite (102) plane and the high-rate discharge characteristics of LIB. The crystallite size distribution of the graphite (102) plane is thought to express the Li diffusivity.Figure shows the result of synchrotron radiation diffraction pattern of lithium-intercalated graphite at the discharge process with current density of 0.2C. The stage change was analysed clearly as shown in Figure.The difference of stage change of each carbon was revealed in considerable detail by Operando measurement.The crystallite size and its distribution influencing on the battery performance at low temperature was examined in detail. The result showed that crystallite sizes and their distributions influence on the phase structure changes and give major effect on charge and discharge performances of graphite negative electrode at low temperature. Their battery reaction mechanism influenced by crystallite size distribution was elucidated by the Operando measurement. The detail results will be shown in the conference. Acknowledgment: This work was supported by the Research and Development Innovative for Scientific Innovation of New Generation Battery 2 (RISING2) project of the New Energy and Industrial Technology Development Organization (NEDO). Figure 1

Finetuning hierarchical energy material microstructure via high temperature material synthesis route

Hierarchical energy materials such as graphite are a backbone of various scientifically and commercially important emerging technologies including high-energy density energy storage systems with fast charging capability, multifunctional catalyst systems, selective membrane separation systems, and next-generation nuclear material systems. Consequently, it is extremely crucial to develop an efficient and cost-effective route of bulk hierarchical material synthesis (e.g., carbonaceous materials with a well-controlled fraction of the graphitic content) to cater the extraordinary operational and energy material requirements in a very complex coupled thermophysicochemical environment. Here we present a fabrication of Polyacrylonitrile (PAN) derived carbon films and fibers (~with linear dimension ~100 nm) via electrospinning and spin coating methods ensued by a heat-treatment in the range of 1000–3000 °C under inert atmospheric conditions. Intriguingly, we observed at least a two orders of magnitude enhancement (~134%) in length of graphitic plane accompanied by 36% more graphitization when the carbonization temperature increased from 1000 °C to 3000 °C. Such significant enhancements were attributed to the differences in the fundamental nanomorphology of initial carbonaceous materials and their subsequent kinetic evolution as it was more favorable for underlying graphene layers in films to stack and bond to the adjacent ones without strong rotations as compared to fibers, which were further evident from fewer voids and cracks in the films. The covalent cross-links, substrate effect and physical entanglements of carbon domains in PAN-derived carbon films contributed to a higher graphitic length owing to more shear stress between the graphene layers, compared to fibers and undergoes an enormous transformation from turbostratic structures to ordered state along with nitrogen removing over high temperature heating. This morphology dependent graphitization was also investigated from computational approach and concluded in the similar thoughts. The outcomes from this systematic study can be beneficial to the carbon research community focusing in the morphology dependent applications, for instance catalysis, energy storage, sensors etc.

Comparing Graphene Oxide and Reduced Graphene Oxide as Blending Materials for Polysulfone and Polyvinylidene Difluoride Membranes

Graphene is a single atomic plane of graphite, and it exhibits unique electronic, thermal, and mechanical properties. Exfoliated graphene oxide (GO) contains various hydrophilic functional groups, such as hydroxyl, epoxide, and carboxyl groups, that can modify the hydrophobic characteristics of a membrane surface. Though reduced graphene oxide (rGO) has fewer functional groups than GO, its associated sp2 structures and physical properties can be recovered. A considerable amount of research has focused on the use of GO to obtain a pristine graphene material via reduction processes. In this study, polysulfone (PSf) and polyvinylidene fluoride (PVDF) membranes that were blended with GO and rGO, respectively, were fabricated by using the immersion phase inversion method and an n-methylpyrrolidone (NMP) solvent. Results showed that the graphene nanomaterials, GO and rGO, can change the pore morphology (size and structure) of both PSf and PVDF membranes. The optimum content of both was then investigated, and the highest flux enhancement was observed with the 0.10 wt% GO-blended PSf membrane. The presence of functional groups in GO within prepared PSf and PVDF membranes alters the membrane characteristics to hydrophilic. An antifouling test and rejection efficiency evaluation also showed that the 0.10 wt% membrane provided the best performance.

In situ measurement of elastic and total strains during ambient and high temperature deformation of a polygranular graphite

In situ neutron diffraction and synchrotron X-ray diffraction, combined with image correlation analysis of 2D optical and 3D X-ray tomography datasets, have been used to investigate the relationship between elastic lattice strain and total strain during deformation of Gilsocarbon (IM1-24) polygranular nuclear grade graphite. The specimens were flat-end Brazilian discs under diametral loading, such that a compressive-tensile biaxial stress state was developed in the central region. The X-ray study was at ambient temperature, and the neutron diffraction was conducted at temperatures from ambient to 850 °C. When under compression, there is a temperature-insensitive linear relationship between the total strain and the lattice strain that is measured perpendicular to the graphite basal planes. However, when under tensile stress, the total strain and elastic strain relationship is temperature sensitive: below 600 °C, the lattice tensile strain saturates with increasing total tensile strain; above 600 °C, significantly higher tensile lattice strains are sustained. The saturation in tensile lattice strain is attributed to microcracking in the graphite microstructure. Improved resistance to microcracking and damage tolerance at elevated temperature explains the increase in tensile strength of polygranular graphite.

In situ atomic force microscopy: the case study of graphite immersed in aqueous NaOH electrolyte

In situ atomic force microscopy (AFM) measures, with the scanner head immersed inside a liquid (namely an electrolyte), are—generally speaking—not easy to be acquired in real time, i.e., during electrochemical processes, the immersion of a massive and wide system (with respect to the electrochemical cell mean radius) can significantly perturb the electrolyte and the I/V profile of the electrochemical characterization (CV). Despite this fact, the information that can be obtained from an electrode morphological inspection is very precious, and the coupling between an AFM and an electrochemical cell is in any case successful. In this work, we discuss the surface morphological evolution of a graphite electrode when immersed inside an aqueous NaOH electrolyte. 1.0 M sodium hydroxide added inside electrochemical baths was shown to promote the intercalation of ions between the stratified graphite structure or graphene layers. Possible consequences of diluted NaOH electrolyte on the quality of the graphite basal plane have been never considered in detail yet. By means of an in situ AFM, we show the effects of CVs on the surface morphology of the graphite basal plane. Our result shows that a 1.0 M NaOH solution, generally used for promoting intercalation, changes the electrode surface quality as a consequence of a partial carbon corrosion.

Why is Graphite Spherical in Ductile Iron? A Study of Elemental Distributions at Interfaces in Ductile Iron Using Atom Probe Tomography and Transmission Electron Microscopy

Atom probe tomography (APT) with high sensitivity and spatial resolution was applied to study the elemental distributions in spheroidal graphite in ductile iron. A specimen of quenched ductile iron with some graphite nodules in direct contact with liquid phase was used in this study to understand the role of alloying elements in spheroidal graphite formation. Compositional analysis was performed at the graphite/matrix interface and graphite/nuclei interface using APT and EDX (energy-dispersive X-ray). Distributions of various alloying elements at interfaces were obtained. Compositional gradients were observed at both the graphite/liquid interface and the graphite/nuclei interface. The spheroidal graphite nuclei were differentiated as the complex compound made of oxide, nitride and sulfide. The graphite–nuclei interface was shown to be semi-coherent/incoherent using the transmission electron microscopy, with a high density of crystallographic defects and high curvatures in the graphite basal planes growing off the nuclei. Intercalation of large nodulizing elements within and in between the graphite basal planes may account for the spherical morphology of the graphite in ductile irons.