Graphite Fragments Research Articles

K+‐Mediated vs Pd‐Catalyzed Cyclotrimerization of 9,10‐Didehydrotribenzo[8]annulene (TribenzoCOTyne): Stereodivergent Access to (α,α,α)‐ and (α,α,β)‐Fragments of Cubic Graphite

AbstractA remarkable stereodivergent cyclotrimerization of 9,10‐didehydrotribenzo[8]annulene (tribenzoCOTyne) to the corresponding (α,α,α)‐ and (α,α,β)‐benzofused derivatives has been developed by controlling the reaction conditions. While the K+‐mediated cyclotrimerization afforded the (α,α,α) stereoisomer, using Pd as a catalyst resulted in the (α,α,β)‐derivative. Both stereoisomers were evidenced by spectroscopic data and crystal X‐Ray analysis. The (α,α,α) stereoisomer is a fragment of cubic graphite (CG), an elusive 3D carbon allotrope that contains carbon cages, since all of its sixty carbons are part of the structure of CG, and 36 constitute a part of the C48 molecular cage of CG. Experimental and computational mechanistic studies revealed that the potassium ion would play a key role as a template to favor the formation of the (α,α,α) stereoisomer.

Fluorine containing diamond-like carbon coatings

Fluorine containing diamond-like carbon (DLC) coatings were obtained by CVD deposition using the destruction of gaseous hydrocarbons by a high-energy ion source with an anode layer. The chemical composition, structure, surface energy, mechano-tribological characteristics of the synthesized thin-film material were studied. It has been shown that the fluorine content in the deposited coatings reaches 31 at.%. It has been established that the presence of fluorine in the DLC coating promotes the formation of nanodispersed graphite-like structures. Fluorine containing coatings with a high content of sp2 hybridized carbon atoms and graphite fragments show a decrease in hardness. Such coatings have a very low coefficient of friction at the level of 0.03–0.07 due to the low surface energy, which can be reduced by 1.5 times compared to pure DLC. It has been established that different fluorine contents in thin-film diamond-like carbon material have different effects on the interaction of the DLC coating with polar/non-polar dielectrics. The research results can be used for the technology of producing solid lubricants in friction units of machines and mechanisms, and are also of interest for medicine, the food industry, the production of plastics, and chemical fibers as chemically inert and corrosion-resistant coatings.

K+-Mediated vs Pd-Catalyzed Cyclotrimerization of 9,10-Didehydrotribenzo[8]annulene (TribenzoCOTyne): Stereodivergent Access to (α,α,α)- and (α,α,β)-Fragments of Cubic Graphite.

A remarkable stereodivergent cyclotrimerization of 9,10-didehydrotribenzo[8]annulene (tribenzoCOTyne) to the corresponding (α,α,α)- and (α,α,β)-benzofused derivatives has been developed by controlling the reaction conditions. While the K+-mediated cyclotrimerization afforded the (α,α,α) stereoisomer, using Pd as a catalyst resulted in the (α,α,β)-derivative. Both stereoisomers were evidenced by spectroscopic data and crystal X-Ray analysis. The (α,α,α) stereoisomer is a fragment of cubic graphite (CG), an elusive 3D carbon allotrope that contains carbon cages, since all of its sixty carbons are part of the structure of CG, and 36 constitute a part of the C48 molecular cage of CG. Experimental and computational mechanistic studies revealed that the potassium ion would play a key role as a template to favor the formation of the (α,α,α) stereoisomer.

Monoisotopic targets production from atomic and molecular positive ion implantation in a tandem accelerator at low energies

We produced monoisotopic targets of 13C and 28,29Si with 1015−16 atoms/cm2 areal density by ion implantation in different matrices at the Accelerator Mass Spectrometry (AMS) facility in Mexico. The tandem accelerator terminal voltage was set at the lowest achievable value (200 kV) that yield beams with energy of 435 keV. It is possible to do ion implantation at lower energies in our facility through the use of molecular beams, so in this work we looked for, and successfully found, bound positively charged molecular beams, although with very low intensity. We inject 235 keV molecules into the charge of exchange canal (argon) of our tandem accelerator and detected the survival fraction of C2,3,4 molecules in 1+ and 2+ charge states or above. Our results showed that some molecules remain bound, with one or more electrons missing, after being forced through a low density gas stripper at relatively high energy. In the production process of negative molecular fluxes in the Source of Negative Ions by Cesium Sputtering (SNICS) we found an odd-even effect in their intensity that we understand as the coupling between the fragment size distribution of a dynamic fragmentation of graphite by cesium impact and the electron affinity of the molecules with n-carbon atoms. The availability of these kind of high energy molecular beams opens the door for discovery both in the study of the physics of negative and positive molecular ions.

Influence of Two Types of Few-Layer Graphite Fragments on Viscoelastic Properties of Plastic Lubricants

Influence of Two Types of Few-Layer Graphite Fragments on Viscoelastic Properties of Plastic Lubricants

Direct Synthesis of Fluorinated Carbon Materials via a Solid‐State Mechanochemical Reaction Between Graphite and PTFE

AbstractFluorinated carbon materials (FCMs) have received significant attention, because of their exceptional stability, which is associated with the strong C‐F bonding, the strongest among carbon single bonds. However, the fluorination of carbon materials requires extremely toxic and moisture‐sensitive reagents, which makes it inapplicable for practical uses. Here, a straightforward and relatively safe method are reported for the scalable synthesis of FCMs, by mechanochemical depolymerization of polytetrafluoroethylene (PTFE) and fragmentation of graphite. The resultant FCMs are evaluated as anode materials for lithium‐ion batteries (LIBs). An optimized FCM delivered capacities as high as 951.6 and 329.3 mAh g −1 at 0.05 and 10 A g −1, respectively. It also demonstrated capacity retention as high as 76.6% even after 1000 cycles at 2.0 A g −1.

A Fullerene Seeded Strategy for Facile Construction of Nitrogen‐Doped Carbon Nano‐Onions as Robust Electrocatalysts

AbstractCarbon nano‐onions (CNOs) as a novel form of carbon materials hold peculiar structural features but their electrocatalytic applications are largely discouraged by the demanding synthesis conditions (e.g., ≥1500 °C and vacuum). Using C60 fullerene molecules as the sacrificial seeds and melamine as the main feedstock, herein, a novel strategy for the facile construction of CNOs nanoparticles is presented with ultrafine sizes (≈5 nm) at relatively low temperatures (≤900 °C) and atmospheric pressure. During the calcination, in‐depth characterizations reveal that C60 can retain the melamine‐derived graphitic carbon nitride from complete sublimation at high temperatures (≥700 °C). Owing to the N removal and subsequent pentagon generation, severely deformed graphitic fragments together with the disintegrated C60 molecules merge into larger sized nanosheets with high curvature, eventually leading to the formation of N‐doped defect‐rich CNOs. Owing to the integration of multiple favorable structural features of pentagons, edges, and N dopants, the CNOs obtained at 900 °C present superior oxygen reduction half‐wave potential (0.853 VRHE) and zinc–air cathode performance to the commercial Pt/C (0.838 VRHE). Density functional theory calculation further uncovers that the carbon atoms adjacent to the N‐doped edged pentagons are turned into the ORR‐active sites with O2 protonation as the rate‐determining step.

Mesomorphism, dielectric permittivity, and ionic conductivity of cholesterol tridecylate doped with few-layer graphite fragments

The effect of doping a cholesteric liquid crystal – cholesterol tridecylate (X-20) with few-layer graphite fragments/nanoflakes (GNF) has been studied with the aim to search for carbon nanostructures that able to improve the performance characteristics of helicoidal liquid–crystal materials for electro-optical devices. A nonlinear effect of dopant concentration on the phase transition temperature, thermodynamics, specific ionic conductivity, and the energy of ion transitions between equilibrium positions of X-20/GNF systems has been established. A split SmA → N* transition found upon the addition of GNF, is discussed within the framework of the model of stepwise phase subtransitions (at first, the transition occurs near the nanoparticle surface, and then it takes place in the bulk of the liquid crystal). Comparison of differential scanning calorimetry data with the characteristics of dielectric permittivity and ionic conductivity let us to suggest the generality of the factors that affect both the mesomorphism features during doping of the host liquid crystal matrix with GNF, and the differences in dielectric permittivity, ionic conductivity, and activation energy between the pure X-20 and its doped systems. The observed effects in X-20/GNF systems are explained by the coexistence of two differently directed processes: 1) interaction between GNF particles, leading to their aggregation and formation of percolation networks, 2) dipole–dipole interactions between X-20 molecules and GNF particles, leading to the orientation of X-20 on flat GNFs (anchoring effect). The dominance of one process over another depends on the GNF concentration and temperature. The dipole–dipole interactions of X-20 and GNF increase the order parameter of nematic phase at the macroscopic level, thereby improving the dielectric properties of the host liquid crystal matrix.

Study of the electrophysical characteristics of plastic lubricants

Actively developing electric automobile transport assumes the creation of conductive lubricants. We pres­ent a setup designed to study the electrophysical properties of industrially produced and model plastic lubricants, as well as the samples of a similar consistency. The setup operates within the current frequency range from 0.1 to 1 kHz and includes a temperature-controlled measuring cell that allows changes in the sample thickness and temperature in the range from 20 to 120 °C. A method for determination of cur­rent-voltage characteristics with subsequent calculation of the specific electrical conductivity is proposed. The specific electrical conductivity of model plastic lubricants based on medical vaseline added with car­bon nanostructures (few-layer graphite fragments and their modified analogs) was studied using the de­veloped setup. Vaseline was used as a base model lubricant, since its rheological properties are similar to that of plastic lubricants and it does not contain any additional additives that can affect the measurement result. The electrically conductive properties of the dispersions of carbon nanostructures and their modi­fied analogs in vaseline were analyzed. It is shown that the introduction of carbon nanostructures into dielectric vaseline turns it into an electrically conductive material. The developed setup makes it possible to study the specific electrical conductivity of systems simulating plastic lubricants with a sufficient accuracy. The results obtained can be used to improve the methodology for studying the specific electrical conductivity of model and industrially produced plastic lubricants, including those with conductive additives.

Optoelectric Properties of Doped Carbon Dot Nanostructures

Carbon dots (C-Dots) can be defined as spherical-like carbon particles (graphitic fragments) with sizes less than 10 nm. Typically, C-Dots exhibit ample room temperature photoluminescence (PL) whereby the PL properties of C-Dots with a well-defined crystalline core is related to the size or conjugated domains of C-Dots accompany with contribution from the surface state, conjugated core, molecular configuration, and so on. However, up to date the exact mechanism of the PL origins of C-Dots depends on synthesis path and remains unsettled [1-3]. In this contribution we will discusses the synthesis, characterization and origins of PL properties of doped C-Dots their potential for applications as absorbers in solar cells. We discuss to use nitrogen doped C-Dots as active absorbing layer in a solar cell and how the electrical, optical, and chemical properties can be engineered by doping. We demonstrate that the doping can be efficiently used to modify the band structure of the C-Dots and meet the requirements for application in solar cells. To rationalize the measured experimental data, we also calculated using DFT in the sTDA approximation the absorbance spectra of pristine and nitrogen doped C-Dots. For computationally feasibility the diameter, d = 2.3 nm, of the C-Dots is chosen to be at the lower end of experimentally observed size distribution and the C-Dots consist of 3 carbon layers. Subsequently, these C-Dots have potential to be used as an absorbing material in next-generation solar cells for high energy photons to be absorbed by a lower bandgap transporter.

A study investigating the cutting mechanism of compacted graphite iron based on a novel microstructure of finite element model

Compacted graphite iron is internationally recognized as a material that has the potential to be applied for preparing high performance engine components. However, difficulties during machinability seriously limit the mass production of these. A novel method based on MATLAB image recognition and python programing was used to establish a microstructural model that included graphite and pearlite particles. The thermal conductivity and specific heat capacity at different temperatures was measured and the results were used to validate the model. Using this novel model, the chip morphology and formation mechanism, cutting surface morphology and formation mechanism and cutting temperature distribution were studied. The simulation results show that crack formation on the chip surface and generation of graphite fragments occurs during the cutting process. The crack opening angle during this is approximately 33°. Significant graphite cavities also appeared on the cutting surface. The stress concentration at the boundary of graphite and pearlite during cutting process was analyzed, and demonstrated to be the primary reason for these phenomena. The temperature of both the rake face and flank face are simulated with the highest cutting temperature concentrated in the secondary deformation zone reaching approximately 322 °C. Furthermore, the simulation results are consistent with the experimental results.

Actinides and fission products in reactor graphite after loss-of-flow accident

Samples of irradiated graphite from a uranium-graphite reactor contaminated with fuel debris from loss-of-flow accidents, are investigated. Peculiarities of spatial distribution, speciation, and kinetics of leaching by aqueous solution of actinides and fission products are studied at scales from the whole reactor stack down to sub-mm graphite fragments. The main fraction of fuel debris is associated with aluminosilicate-based phases decorating cracks and other defects of damaged graphite details. Elemental composition of actinides and fission products depends on both irradiation and leaching conditions. Significant differences are observed between leach kinetics of various radionuclides; integral leached fraction varies between ~5% for 244Сm to ~90% for 134,137Cs. For some radionuclides sudden massive release due to uneven dissolution of the carrier phases is observed. Disposal of irradiated graphite with significant fraction of fuel debris requires dedicated treatment aimed either at decontamination or at conversion of the radionuclides to insoluble well-fixed form.

Structural variety and stability of carbon honeycomb cellular structures

A new synthesized carbon honeycomb allotrope reported previously, built from graphene nanoribbons connected by sp3-bonded carbon junction lines, forms a family of cellular structures with high porosity and sorption capacity. In this work we first propose a complete set of possible honeycomb structures of different wall chiralities both the armchair and zigzag types, including considered earlier only theoretically, for the structural analysis of such structures by means of the high-energy electron diffraction method. The “completeness” of the model set made it possible to obtain nearly perfect coincidence of the experimental and calculated diffraction intensities. The contribution of graphite fragments and random structures, also involved in the analysis, turned out to be zero. Only a limited number of honeycomb structures of different types almost ideally describes the experiment. Thus we conclude that polydomain structures corresponding to a set of basic models formed in this investigation rather than formations dominated by random structures. The samples under study have demonstrated the unique cellular stability since were stored in vacuum ∼4.5 months before the reported measurements. Along with the original results the history of the carbon honeycomb cellular structures is briefly presented.

Few-layer versus mono-layer N-doped graphenes in oxygen reduction reaction

Doped graphenes, nitrogen-doped (N-doped) in particular, are known to possess promising electrocatalytic performance in oxygen reduction reaction (ORR). We study the effect of the structure of N-doped few-layer (N-Gr) and mono-layer (N-gr) graphenes prepared within one low-temperature approach on their functional behavior as ORR electrocatalysts. Spectral studies reveal the nitrogen content in N-Gr being a quarter greater than in N-gr and nearly one third of all nitrogen atoms in N-Gr being within graphitic fragments, while about half of all nitrogen atoms of N-gr are pyrrolic ones; N-Gr particles are characterized by the presence of defects at the edges, while vacancies are also characteristic of N-gr. It is established that the structural differences of N-Gr and N-gr determine higher electrocatalytic performance of N-Gr in ORR and its four-electron mechanism, which are associated with a significant increase in the charge carrier density and shift of the flat-band potential to negative potential range. The significance of the work is directly related to practical application, since N-Gr could be prepared by fundamentally simpler method, its preparation is ecologically friendly, easily scalable and, therefore, cost effective.

Effect of milling time in characteristics of the powder Cu-5wt.%Graphite

Composites of Cu-5wt.%Graphite were prepared by high-energy milling, under argon atmosphere for milling time of up to 50 h, to investigate the influence of the milling time on the size and dispersion of the copper and carbon phases. The formation of a monophasic carbon-copper solid solution was also investigated. The powder samples were collected at different times and characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM and FESEM), transmission electron microscopy (TEM), energy dispersive X-ray spectroscopy (EDS) and Raman spectroscopy. Composite particles were formed by fragments of graphite embedded in the soft Cu matrix. After 50 h of milling, the Cu phase had a crystallite size of 24 nm and micro-strain of 0.26 %. The lattice parameter showed a reduction of 0.001545 nm and reached a value of 0.360152 nm. Furthermore, no carbon diffraction peak was observed in the milled powders, due to the small graphite crystallites. Meanwhile, the Raman spectra showed that the carbon phase remains crystalline, even after 50 h of milling. When the composite was annealed at 600 °C, for 1 h and under argon atmosphere, no carbon precipitate was observed. These results suggest the absence of the formation of a solid solution of carbon in copper.

Numerical Simulations of Laser-Induced Shock Experiments on Graphite.

The development of particle accelerators with ever increasing energies is raising the standards of the structures which could interact with the particle beams. These structures could be subjected to strong shockwaves in accidental scenarios. In order to test materials in such conditions, one of the most promising techniques is the impact with high-power lasers. In view of the setting up of future experimental campaigns within the Petawatt High-Energy Laser for Heavy Ion Experiments (PHELIX), the present work aims at the development of a numerical approach for the simulation of graphite impacted by laser beams. In particular, the focus is on the spallation damage caused by shockwave reflection: a sufficiently intense laser beam could ablate the matter until plasma conditions, hence producing a shockwave which could travel inside the material and reach a free surface. A numerical model to properly describe the spall fragmentation of graphite has been calibrated on the basis of literature-available experimental data. The numerical approach is a ‘two-step’ procedure: the first step is the definition of the laser–matter interaction and the second one concerns the description of the shockwave evolution into matter. The simulations satisfactorily reproduce the dynamic response of graphite impacted by two different laser sources with various intensities, despite the difficulties of characterising a phenomenon which is extremely fast and chaotic.

Room-temperature deposition of ZnO-graphene nanocomposite hybrid photocatalysts for improved visible-light-driven degradation of methylene blue

ZnO-graphene nanocomposites (NCs) of ZnO and graphite microparticles were directly deposited by a one-step kinetic spray process at room temperature on a nickel foam (NF) porous substrate. The ZnO-graphene NCs/NF with various graphite contents (25, 50, and 75 wt%) were utilized as hybrid photocatalysts for methylene blue (MB) degradation. Analysis of Raman spectra revealed the improvement of various disorder-related phonons, confirming the fragmentation of microparticles into the nano-size range in all the ZnO-graphene NCs/NF hybrid photocatalysts. Also, the degree of layer separation and fragmentation of micro-sized graphite in the ZnO-graphene NCs/NF was shown to be strongly dependent on the graphite content. The surface morphology of the nanostructured ZnO/NF thin films consisted of nanorods and nanosheets. The increase of graphite content in the ZnO-graphene NCs/NF hybrid photocatalysts was accompanied by a two-step morphology transformation into graphene nanoflakes and nanoflowers. The interfacial bonding states of the ZnO-graphene NCs/NF were investigated using X-ray photoelectron spectroscopy, which indicated synergy improvement by the evolution of multiple bonded states between the ZnO and graphene species in all the hybrid photocatalysts. UV–visible optical absorbance spectra revealed a decrease in the optical bandgap of the ZnO-graphene NCs/NF with graphene species incorporation, indicating the improvement of visible light harvesting. The NCs hybrid photocatalyst with 50 wt% graphite content exhibited the lowest bandgap of 2.9 eV compared with the nanostructured ZnO/NF with 3.18 eV. The photocatalytic activity of ZnO-graphene NCs/NF was evaluated using the degradation of MB under visible light irradiation. The NCs hybrid photocatalysts with 50 wt% graphite content exhibited the highest degradation efficiency and the highest reaction kinetics rate constant compared with the other nanostructured photocatalysts.

Dual-modal label-free genosensor based on hemoglobin@gold nanocluster stabilized graphene nanosheets for the electrochemical detection of BCR/ABL fusion gene

For the first time, we have successfully synthesized stable graphene nanosheets from graphite powder through sonication in the hemoglobin-capped gold nanoclusters (Hb@AuNCs) solution for biosensing application. This approach, as a simple method for the exfoliation and fragmentation of graphite in a nanocluster solution, enabled us to produce stable aqueous graphene dispersions at low cost and without the need for hazardous chemicals or tedious experimental procedures. In this method, Hb@AuNCs were used not only as stabilizing agent of graphene through non-covalent bonding, but also as dispersing agent of few-layer graphene nanosheets. The Hb@AuNCs stabilized graphene (Hb@AuNCs-G) was characterized by high resolution transmission electron microscopy (HRTEM), zeta-sizer and Raman spectroscopy. Then, the graphene nanosheets were applied as a novel versatile electrochemical platform for ultrasensitive biosensing of short DNA species of chronic myelogenous leukemia (CML) based on the “signal off” and “signal on” strategies. For this purpose, a single strand DNA (ssDNA) was immobilized on the Hb@AuNCs-G/AuNPs modified electrode surface and acted as the biorecognition element. Methylene blue (MB), as the signaling probe, was then intercalated into the ssDNA. The intercalated MB was liberated upon interaction with the synthetic complementary DNA (cDNA, target), thereby resulting in the apparent reduction of MB redox signal. This designed “signal off” sensing system enabled the voltammetric determination of the target cDNA over a dynamic linear range (DLR) of 0.1 fM to 10 pM with a limit of detection (LOD) of 0.037 fM. In the “signal on” strategy, the response to the cDNA was detected by monitoring the change in the electron transfer resistance (Rct) using the ferro/ferricyanide system as a redox probe. The charge transfer resistance of the probe was found to increase linearly with increasing concentration of target cDNA in the range of 0.1 fM–10 pM with a limit of detection of 0.030 fM. Finally, the selectivity and feasibility of genosensor was evaluated by the analysis of derived nucleotides from mismatched sequences and the clinical samples of patients with leukemia as real samples, respectively.

Effect of Graphite Content on Cu–Ni–Graphite Composite for Use as Switch Slide Baseplate Materials Sliding Against U75V Steel

Abstract The rapid development of high-speed railways necessitates the development of new materials for switch slide baseplates. In this study, a Cu–Ni–graphite composite, containing 1 wt% to 6 wt% graphite and prepared by powder metallurgy, was used as a potential material. Pin-on-disk wear tests were conducted to measure the sliding friction of the Cu–Ni–graphite composite against U75 V steel. The results showed that the friction coefficients gradually decreased when the graphite content in the composite ranged from 1 wt% to 4 wt% in the composite. When the graphite content was 4 wt%, the friction coefficient reached the minimum value (0.153). When the graphite content was low (1 wt% to 4 wt%), the primary wear mechanism was microcutting. An increased graphite content facilitated the generation of lubricating films and decreased the wear damage. As the graphite content increased from 4 wt% to 6 wt%, the friction coefficients also increased. The variation in the wear volume rate had the same tendency as the friction coefficient. When the graphite content exceeded 4 wt%, the primary wear mechanism was delamination and fatigue wear. Due to the tendency to form cracks on the subsurface and the plentiful generation of the spalled pits, the graphite fragments could not completely form lubricating films but separated as wear debris. The lubricating films existing on the U75 V steel were in proportion to the graphite content in the composite. The wear weight loss of the U75 V steel exhibited a reduction with increasing graphite content.

Preparation of Graphene Nanolayers through Surfactant-assisted Pure Shear Milling Method

In this study, graphite powder was used to prepare few-layer graphene sheets through shear milling. During the process, graphite was well-dispersed in double distilled water as a lubricant and sodium dodecylsulfate (SDS), followed by shaking and milling under low energy. The exerted sheer force led to continuous delamination of graphene flakes. The microstructural investigation was performed by SEM. Also, the energy dispersive X-ray spectroscopy (EDS) analysis was performed to determine distinct levels of carbon in different fragments of graphite. The ultrathin multilayer structure of graphite was successfully obtained using the surfactant of SDS, which can lead to the production of molecularly thin sheets by mechanical peeling. Moreover, it was found that this synthesis method has advantages, including cost-effectiveness and ease in performance for producing a lot of graphene nanolayers.