Graphitic Clusters Research Articles

Tetrahedral Amorphous Carbon Coatings with Al Incorporation Deposited by a Hybrid Technique of Sputtering and Arc Evaporation

In this paper, tetrahedral amorphous carbon (ta-C) coatings containing Al were deposited by a hybrid technique of sputtering and arc evaporation. The influence of Al incorporation in the structure and properties of the ta-C coatings were studied as a function of the Al concentration. It is found that Al tends to form a Al-O-C bond when the Al concentration is small. An Al-C bond was detected when the Al concentration is high. Al can facilitate the graphitization of the ta-C coatings and the graphite cluster size as well as the sp2/sp3 ratio of the coatings increase as the Al concentration increases. The decline of the sp3 fraction causes the drop in the hardness of the coatings. The incorporation of Al can effectively decrease the residual stress of the ta-C coatings. During friction tests, Al can facilitate the formation of the sp2-rich graphitic tribo-layer and decrease the friction coefficient. Nevertheless, the decline of the hardness due to the Al incorporation will result in the increase in the wear rate of the coating. It is believed that the ta-C coating with a proper concentration of Al appears to achieve a good comprehensive performance with high hardness, low residual stress, and a low friction coefficient and wear rate.

Electrochemical sensor based on PVP coated cobalt ferrite/graphite/PANI nanocomposite for the detection of hydrazine

In this study, we developed a multi-layered electrode as an efficient nanocomposite electrochemical sensor for detecting carcinogenic hydrazine in water. Nano-cobalt ferrite (CoFe2O4) was prepared using poly (vinylpyrrolidone) (PVP), mixed with an optimum quantity of graphite and pasted on a stainless-steel current collector. The nanocomposite was further hybridised by electrodepositing with polyaniline (PANI). The obtained composite was characterized using XRD, FTIR, SEM, and electrochemical techniques. XRD analysis shows the successful formation of composite from individual precursors. According to SEM, wrinkled and layered morphology for graphite and aggregate clusters for cobalt with spike or tubular structure for polyaniline were observed. When subjected to amperometry current, the prepared electrode showed different peaks for different concentrations of hydrazine, such as 1 μM to 100 μM. Cyclic voltammetry studies showed an increase in oxidation and reduction peaks. These studies will lead to a new platform for their potential applications in detecting toxic materials in real samples such as water, plastic bottle, water etc.,

Labile graphitic monolayers as a partition phase for containment of organic chemicals by graphene-based nanomaterials

Graphene-based nanomaterials (GBNMs) (e.g., graphene oxides and carbon nanotubes) display superior electronic and thermal conductivities and varying abilities to contain organic substances. This study sheds light to the idea that GBNMs behave as a dual sorbent, rather than a sole adsorbent, to extract nonionic organic solutes from water by both (competitive) adsorption and (noncompetitive) partition because of the solute interactions with various GBNM nanostructures formed by atomically-thin graphitic monolayers. Essential solute-sorption data with three model GBNMs from this research and similar data from the literature lead to a coherent view that labile graphitic monolayers in GBNMs undergo a liquid-like motion at room temperature to retain nonionic organic solutes by partition while structurally rigid graphitic clusters behave as adsorbents. Because the partition is noncompetitive, the GBNMs possessing high levels of labile graphene layers, as reflected by high BET surface areas, are capable of sequestering vastly higher levels of multiple organic solutes (especially, those of liquids) than conventional adsorbents, e.g., activated carbon (AC). Moreover, the postulated dual functionality of GBNMs makes sense of many otherwise puzzling phenomena, such as the highly concentration-dependent solute competitive effect with certain GBNMs and highly variable “adsorbed capacities” per unit surface area for different organic solutes with a GBNM versus those by a conventional adsorbent (e.g., graphite or AC).Graphical

Fe7C3 nanoparticles anchored on N-doped biochar as electrocatalyst for highly efficient oxygen evolution reaction

As the crucial step of electrolytic water, oxygen evolution reaction (OER) is urgent to be enhanced by developing the most efficient electrocatalysts to accelerate the sluggish OER kinetics. Herein, nitrogen-doped carbon-based Fe7C3 clusters with hierarchical pore structure (NHAC@Fe7C3) were synthesized via a two-step method of FeCl3 and seaweed liquid-phase impregnation combined with physical activation. The optimum NHAC@Fe7C3 = 3:1 catalyst (with a mass ratio of Sargassum Horneri to FeCl3 of 3:1) demonstrated much better catalytic activity and durability towards OER in alkaline electrolyte than the most noble-metal-free catalysts, of which the overpotential reached 181 mV at 10 mA cm−2. Long-term discharging stability of over 33 h at 10 mA cm−2 current density. The excellent performance is attributed to the synergistic effect of graphitic carbon and Fe7C3 clusters, hierarchical pore structure and good electrical conductivity. Density functional theory calculations confirm that the free energy for the potential-limiting step is reduced by the synergistic effect of N-doped carbon and Fe7C3 substrance. This study employs a simple and effective method to synthesize iron carbide embedded in algae-derived biochar, providing an opportunity for the rational design of robust performance electrocatalysts for oxygen evolution reaction.

Complexity of graphite formation in response to metamorphic methane generation and transformation in an orogenic ultramafic body

The fate of carbon in subduction is explored by considering fluid-rock interactions in a body of metamorphosed ultramafic rocks from the Appalachian belt as potential proxies for metasomatism and carbon recycling (e.g., infiltration, solution, precipitation) in the mantle wedge at convergent margins. Ultramafic rocks may record intense redox variations affecting the exchanges between solid and fluid carbon-bearing phases. The Belvidere Mountain Complex (BMC) in Vermont, USA is an ultramafic body that underwent Ordovician Taconic subduction metamorphism up to 510–520 °C and 0.9–1.3 GPa. Previous investigations indicate that the BMC experienced partial serpentinization and carbon recycling in the subduction zone. Here bulk δ11B, 87Sr/86Sr and δ13C, data, in-situ microRaman spectroscopy and bulk δ13CCH4 and δ2HCH4 on fluid inclusions, and numerical modeling results aim to constrain the origin and formation mechanisms of carbon-bearing solid and fluid phases in the BMC. Bulk 87Sr/86Sr ratios and δ11B of the BMC rocks suggest infiltration of metasediment-derived fluids inside the mafic/ultramafic body. Carbonates present in the studied ultramafic and mafic rocks have δ13C values ranging from −7.26 to −1.27 ‰. This range was found to record progressive reduction of an initial isotopically light carbonate to methane under reducing conditions. MicroRaman spectroscopy on fluid inclusions confirms CH4-rich gaseous compositions, along with N2, NH3, and S-H compounds, which support a metasedimentary origin for the fluid infiltrating the BMC. Methane-rich fluid inclusions were observed in partially serpentinized peridotites, in carbonate veins, and in amphibolite bodies associated with the ultramafic body, and as well in the surrounding metasedimentary rock units. The δ13C and δ2H of methane in fluid inclusions show a wide range of values in ultramafic rocks from −45.2 ‰ to −12.6 ‰ for carbon and −226 ‰ to −140 ‰ for hydrogen. The methane stable isotope data suggest the presence of two types of methane preserved in studied samples, formed by high-temperature thermogenic processes of metamorphosed graphitic carbon and by abiotic conversion of more oxidized carbon bearing species in the presence of H2-rich fluids produced during metamorphic serpentinization. The BMC also hosts a remarkable example of graphite deposit, with up to 3.90 wt% graphite, along a ∼ 6 m-thick zone within the serpentinized ultramafic body and along lithologic contacts. The δ13C composition of the graphite clusters at −15 ‰ (VPBD), which is consistent with the composition of graphite precipitating from the mixing of the two types of methane-bearing fluids identified in fluid inclusions. The mixing between metasediment-derived and serpentinization-related methane-rich fluids is proposed as the mechanism leading to graphite precipitation. Further complexity is suggested by carbonate reduction evidenced in the samples, which may have locally contributed to the formation of graphite on the precursor carbonate domains. The BMC rocks highlight the variability of carbon recycling processes, including carbon mobilization from multiple solid sources, and mixing of multiple carbon-bearing fluids in a dynamic fluid-rock system, with carbon mobilization and sink adapting to evolving redox conditions.

Reaction Kinetics Investigation of Ni Ohmic Contacts on N-Type 4H-SiC

Investigation of the reaction kinetics between Ni film and 4H-SiC substrate at temperatures which are usually used for ohmic contacts formation provides valuable insights into the studies on fundamental properties of ohmic contacts to 4H-SiC, which are limiting the switching speed, energy efficiency and high-temperature thermal stability of SiC MOSFETs. High Resolution Scanning Electron Microscope (HRSEM) and Raman spectroscopy were used to elaborately characterize the interfacial reaction products under various annealing conditions and to assess the thicknesses of reaction diffusion layers. The square of reaction layer thicknesses versus time followed parabolic law and the apparent active energy of interfacial reaction was derived as 1.5 eV (145 kJ/mol). For Raman spectra, the intensity ratio of two Raman peaks for each nickel silicide detected varied monotonically with temperature in the same trend, indicating that crystal quality of nickel silicide film was improved with annealing temperature rising at micron scale. The red-shift of Ni2Si peak locations at about 140 cm-1 with temperature was suggestive of the polycrystalline Ni2Si film with weaker stress/strain status. Moreover, the in-plane size of graphite cluster aggregating at top surface increased with annealing temperature rising until about 1000°C, which is detrimental to the ohmic contacts from the perspective of device applications.

Features of wear of DLC-Si coating under microcontact conditions during the formation of secondary structures

Diamond-Like Carbon (DLC) coatings have high wear resistance, hardness, biocompatibility and chemical inertness. Secondary structures, which formed at the track in dry friction conditions, provide friction reduction. Doping DLC with both metallic and non-metallic elements leads to a rearrangement of the structure of graphite clusters, a change in their properties, and facilitates the formation of secondary structures. In the present, the silicon atoms were used to create the compositional structure of DLC surface layers at the level of the crystal lattice. DLC-Si coatings with the thickness of 2 µm were deposited on a (Cr, Al, Si)N layer with the thickness of 3 µm formed on a steel substrate. DLC-Si layer was deposited by plasma-enhanced chemical vapor deposition using acetylene and tetramethylsilane. Nanoindentation in the wear mode was used to study the nanomechanical properties of the surface. Atomic force microscopy was used to visualize the surface morphology and wear traces. In this study, secondary structures were created in a microfriction process using a NanoScratch mode. The surface structures after microfriction was observed by atomic force microscopy, and the specific surface energy was measured. The specific volumetric wear rate of the coatings after microfriction was estimated. During microtribological testing with double squares the specific volumetric wear rate of DLC coatings was 5.39·10-14 m3/N·m for 0.8 %Si and 12.39·10-14 m3/N·m for 10 % Si. When tested in the form of scratches, the specific volumetric wear rate for the coating was for the DLC-0.8 % Si (1.22 – 10.71)·10-14 m3/N·m and for the DLC-10 % (6.17 – 49.55) ·10-14 m3/N·m.

Conductive black carbon promoted biotransformation of undissolved 2, 2′-dinitrobiphenyl by mediating electron transfer

With low bioaccessbility, persistence of the undissolved organic pollutants in soil and sediments poses threat to health of the resident. Although ubiquitous black carbon catalyzes a wide range of biogeochemical reactions in nature, its role in biotransformation of the compounds in non-aqueous phase like 2, 2′-nitrobiphenyl remains unclear. Reduction rate constants of 2, 2′-dinitrobiphenyl by Shewanella oneidensis MR-1 increased from 0.0044 h−1 by 7-fold to 0.035 h−1 in the presence of black carbons produced at pyrolysis temperature of 250–900 °C. Accordingly, electrical conductivity of black carbon was enhanced from 0 to 5.56 S∙cm−1. The reactivity of black carbon for catalyzing the biotransformation positively correlated with its electrical conductivity (R2 > 0.89), which was strongly associated with conductive graphitic clusters in it. The surface oxygenated groups in black carbon were likely not involved in the bioreduction. This work attaches importance to role of the ubiquitous black carbon in natural biotransformation of the undissolved pollutants, and elucidates new mechanism for the biotransformation.

Highly graphitic Fe-doped carbon xerogels as dual-functional electro-Fenton catalysts for the degradation of tetracycline in wastewater

Fe-doped carbon xerogels with a highly developed graphitic structure were synthesized by a one-step sol-gel polymerization. These highly graphitic Fe-doped carbons are presented as promising dual-functional electro-Fenton catalysts to perform both the electro-reduction of O2 to H2O2 and H2O2 catalytic decomposition (Fenton) for wastewater decontamination. The amount of Fe is key to the development of this electrode material, since affects the textural properties; catalyzes the development of graphitic clusters improving the electrode conductivity; and influences the O2-catalyst interaction controlling the H2O2 selectivity but, at the same time is the catalyst for the decomposition of the electrogenerated H2O2 to OH• radicals for the organic pollutants oxidation. All materials achieve the development of ORR via the 2-electron route. The presence of Fe considerably improves the electro-catalytic activity. However, a mechanism change seems to occur at around −0.5 V in highly Fe-doped samples. At potential lower than −0.5 eV, the present of Feδ+ species or even Fe–O–C active sites favour the selectivity to 2e-pathway, however at higher potentials, Feδ+ species are reduced favoring a O–O strong interaction enhancing the 4e-pathway. The Electro-Fenton degradation of tetracycline was analyzed. The TTC degradation is almost complete (95.13%) after 7 h of reaction without using any external Fenton-catalysts.

Tuning of solid-to-solid structural transitions in amorphous carbon films by optical pumping and chemical modification

Amorphous carbon (a-C) attracts great attention in tribology research and thin film technologies due to its versatile properties. However, high temperatures and mechanical stresses may cause significant changes in the structural ordering of the a-C network. We present an optical method to initiate structural ordering and to probe solid-to-solid structural transitions of element modified a-C films. A pulsed pump laser introduces heat into the film in a controlled manner, while a second laser probes confocally the first- and second-order Raman scattering signatures of the a-C network. For low pump power, the number of defects and non-sixfold aromatic rings is reduced. A further increase in the laser power leads to sharply evolved changes in the Raman scattering features, indicating a transition from a-C to defected graphite and an effusion of hydrogen. Moreover, graphite-dominant defect relaxation and an enhancement in hexagonal lattice areas occur and, in turn, activate second-order Raman scattering lines. A rising laser power subsequently results in progressive graphitization. Chemical modification of the films with Si or Cu enhances their thermal stability and even shifts the upper thermal limit of the film ablation, while the a-C:W film demonstrates a more efficient enrichment of nanocrystalline graphitic clusters.

Impact of blending with polystyrene on the microstructural and electrochemical properties of SiOC ceramic

AbstractIn this work, we present the electrochemical behavior and microstructural analysis of silicon oxycarbide (SiOC) ceramics influenced by an addition of polystyrene (PS). Polymer‐derived ceramics were obtained by pyrolysis (1000°C, Ar atmosphere) of different polysiloxanes prepared by sol–gel synthesis. This method is very effective to obtain desired composition of final ceramic. Two alkoxysilanes phenylthriethoxysilane and diphenyldimethoxysilane were used as precursors. Before pyrolysis polysiloxanes were mixed with PS using toluene as a solvent. Blending with PS affects the microstructure and free carbon content in the final ceramic material. Free carbon phase has been confirmed to be a major lithium storage host. Nevertheless, we demonstrate here that capacity does not increase linearly with increasing carbon content. We show that the amount of SiO4 units in the SiOC microstructure increases the initial capacity but decreases the cycling stability and rate capability of the material. Furthermore, the microstructure of the free carbon influences the electrochemical performance of the ceramic: More ordered graphitic clusters favor better rate capability performance.

Effect of diamond surface structure on the interfacial reaction and properties of diamond/SiC composites

The changes in the surface microstructure of diamond annealing under different environment and the influence of this change on the interfacial reaction of diamond/SiC composites were reported. The resulting changes in the properties of composites were analyzed. The newly {111} crystal family was formed on (100) plane by surface atom reconstruction, and the steps were formed on (111) plane, during annealing in vacuum. The diamond undergoes a similar transformation expect that the changes were more drastic, and graphitic clusters appeared on surface when annealing in air. The inhomogeneous nucleation of SiC was found on both (100) and (111) plane of the diamond. Growth of SiC particles on interface are controlled by diffusion-reaction mechanism. The modified diamond enhances the interfacial reaction and facilitates the improvement of mechanical and thermal properties, but the graphitic clusters generated on diamond surface after annealing in air has a negative effect on the thermal conductivity of composites.

In situ tensile and fracture behavior of monolithic ultra-thin amorphous carbon in TEM

Even while being important components in day-to-day life and in advanced technology, the wider application of amorphous solids is limited by their brittle behavior. Although amorphous solids have been reported to show plasticity at the nanoscale, studies have so far been limited to metallic and oxide glasses. Here, we report on the tensile and fracture behavior of monolithic ultra-thin amorphous carbon (a-C) films during in situ nanomechanical testing inside a transmission electron microscope (TEM). Our results show that ultra-thin a-C films exhibit large plastic strain under uniaxial tension while retaining high strength. Beam-off tests confirm that the plasticity is not induced by electron-beam effects during testing. Consecutive cyclic tests and Raman spectra reveal that the plasticity results from an increased nanoporosity, and graphitic cluster size increases and bond/cluster alignments along the tensile direction occur and likely contributes to stiffening of the a-C film. Despite the large plastic strain, catastrophic failure still occurred accompanied by the formation of multiple shear bands, which has never been reported for amorphous carbon. This study serves as a basis for our better understanding of the mechanical behavior of amorphous solids such as ultra-thin a-C, and provides new opportunities in design of flexible electronics, mechanical nanocomponents, and nanocomposites.

Adhesion and corrosion of Al–N doped diamond-like carbon films synthesized by filtered cathodic vacuum arc deposition

Diamond-like carbon (DLC) films (thickness: ∼118 nm) doped with Al and N on AISI 4140 steel were synthesized using a filtered cathodic vacuum arc technique. The adhesion and corrosion of the ta-C, ta-C:N, ta-C:Al, and ta-C:Al:N films were examined in terms of structure and bonding. X-ray reflectivity, X-ray photoelectron spectroscopy (XPS), and Raman spectroscopy revealed that the Al-doped and Al and N co-doped films had lower densities and internal stresses than the other films, as indicated by their higher ID/IG ratios and graphite cluster size (La) and lower sp3 relative fractions. All DLC films exhibited improved corrosion resistance as compared to AISI 4140, as indicated by their decreased corrosion rate and increased corrosion potential and polarization resistance. The corrosion resistances of ta-C:Al:N and ta-C:Al films were high owing to the combined effect of sp3 C–N and Al2O3, as confirmed by near-edge X-ray absorption fine structure spectroscopy and XPS. All doped DLC films exhibited improved adhesion performance compared to the ta-C film owing to their improved lubrication performance (increased sp3-hybridized C–N bonds) and improved toughness (formation of Al2O3).

The high-cycle fatigue fracture mechanism and fatigue strength prediction of compacted graphite iron

The tensile properties, high-cycle fatigue (HCF) properties and corresponding fatigue fracture mechanism of compacted graphite iron (CGI) were investigated. It is found that the tensile strength, yield strength and fatigue strength of CGI decrease as the temperature increases. At 25 °C, the fatigue cracks of CGI are mainly caused by local cleavage fracture at ferrite around the tip of vermicular graphite. At 400 °C and 500 °C, the gradual occurrence of grain boundary softening and oxidation may reduce the difficulty of fatigue crack propagation. Similar to temperature, the change of microstructure content also has a significant effect on the fatigue properties of CGI. By analyzing the damage characteristics of fatigue fracture morphology, it is shown that the fatigue crack initiation of CGI is mainly dependent on the damage localization caused by the ferrite and vermicular graphite inside the graphite cluster and the pearlite and spheroidal graphite outside the graphite cluster under cyclic loading. Based on those results, a fatigue strength prediction model associated with tensile strength, yield strength and microstructure area percentage was proposed. This model can provide a reasonable prediction of HCF strength of CGI at different temperatures.

Using DEM method to study graphite migration of copper-based graphite composites during friction

The self-lubrication of copper-based graphite composites depends on the formation of a graphite lubricating film on the frictional interface, which results from the graphite migration from the matrix. In fact, the graphite migration process is difficult to observe and record during friction by experimental methods. In our work, a discrete element method (DEM) software “the Particle Flow Code in 2 Dimensions (PFC2D)” was employed to simulate and analyze the graphite migration dynamic process of copper-based graphite composites sliding against 45 steel. The simulation results show that a migrated graphite particle layer was formed on the composite surface during friction, and maintained dynamic equilibrium relied on graphite migration from the matrix. The graphite migration was affected by applied load: the larger the applied load was, the greater the migration depth and number of migration particles were. And the sliding speed has little effect on graphite migration. With increase of graphite content, the migration depth first decreased and then increased, and the number of migration particles increased. With increase of graphite cluster size, the migration depth first decreased and then increased, and the number of migration particles decreased. The simulation results were verified by test analysis on a self-made in-situ observation tribometer, the test results was consistent with simulation results.

Correlation between structural properties and electrical conductivity of porous carbon derived from hemp bast fiber

A comprehensive study of the correlation between the structural and morphological parameters and the frequency-temperature dependences of the conductivity of porous carbon materials obtained by carbonization of hemp fibers has been carried out. The effect of treatment with nitric acid and additional annealing on the transport of charge carriers in carbon materials carbonized in the temperature range of 500–1000 °C has been established. Porous carbon materials have been described as a heterophase system, in which highly conductive graphitized particles are separated by low conductivity amorphous carbon, from the standpoint of the effect of carbonization temperature and changes in the morphology of materials due to acid treatment and annealing on the formation of conduction channels with the simultaneous implementation of fluctuation-assisted tunneling between graphitic clusters separated by low conductive barriers and the formation of conduction channels as a result of contact of individual graphitized carbon clusters. A model has been constructed that relates the results of measuring the specific surface area and pore size distribution, changes in the size of graphite crystallites, and changes in conductivity with temperature. The possibility of applying the theory of percolation conductivity to describe the charge transport has been analyzed and the percolation threshold has been established.

Quantum-Chemical Modeling of Elementary Processes That Accompany the Generation of Diamond and Graphite Clusters in the Gas Phase Without a Substrate

Quantum-Chemical Modeling of Elementary Processes That Accompany the Generation of Diamond and Graphite Clusters in the Gas Phase Without a Substrate

Phase transformation and subsurface damage formation in the ultrafine machining process of a diamond substrate through atomistic simulation

This report explores the effects of machining depth, velocity, temperature, multi-machining, and grain size on the tribological properties of a diamond substrate. The results show that the appearance of graphite atoms can assist the machining process as it reduces the force. Moreover, the number of graphite atoms relies on the machining speed and substrate temperature improvement caused by the friction force. Besides, machining in a machined surface for multi-time is affected by its rough, amorphous, and deformed surface. Therefore, machining in the vertical direction for multi-time leads to a higher rate of deformation but a reduction in the rate of graphite atoms generation. Increasing the grain size could produce a larger graphite cluster, a higher elastic recovery rate, and a higher temperature but a lower force and pile-up height. Because the existence of the grain boundaries hinders the force transformation process, and the reduction in the grain size can soften the diamond substrate material.

Few-layer graphene prepared via microwave irradiation of black sesame for supercapacitor applications

Traditional heating often generates amorphous carbon or very small graphite clusters, leading to poor electron transport capabilities and limiting the application of material in supercapacitors. Nevertheless, the superiority of microwave heating, a molecular-level heating that is different from traditional heating, has been seldom exploited to date. Herein, under microwave irradiation, we built biomass-based (black sesame) few-layer porous graphene using molysite (MPG). The MPG, with an IG/ID value of 2.37, an IG/I2D of about 1.05, possessed a thickness of 1.192 nm verified the few-layer structure. The proportion of sp2 hybridized carbon obtained 41.99% corresponding to graphitic carbon. Compared with traditionally heated sample (TGPC), it can be found that hyperthermic point provided by microwave can supply energy for the rearrangement from amorphous to ordered. The molysite created catalysis-wave absorption dual sites simultaneously, realizing high-density “hot spots”, allowing targeted, precise heating, and promoting the exfoliation of graphite layers was demonstrated through tracking the microwave reaction. The MPG formed by microwave technology not only possessed excellent specific surface area (2092.8 m2 g−1) but also formed natural nitrogen/oxygen self-doping. The electrochemical test demonstrated that MPG had an outstanding specific capacitance of 333.3F g−1 and a small charge transfer resistance (Rct) of 0.047 Ω. The energy density of 3.32 W h kg−1, relaxation time constant of 1.645 s and cyclability of 97.6% could be maintained at 20 mg cm−2. Our work demonstrated the potential of using microwave for preparing graphene and provides a new idea for high-energy and power supply in lightweight supercapacitors.