Migration Of Carbon Atoms Research Articles

Molecular dynamics simulation of graphene deposition and growth on WC-Co cemented carbides

In this paper, molecular dynamics is used to simulate the self-assembly of carbon atoms and the growth of graphene on the surface of cemented carbide, which has some advantages, such as reducing manufacturing costs, shortening the experimental cycle, and optimizing the preparation parameters of graphene. A proper potential function is designed to simulate the formation of graphene on the WC (<inline-formula><tex-math id="M1">\begin{document}$10 \bar{1} 0$\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="6-20221332_M1.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="6-20221332_M1.png"/></alternatives></inline-formula>) surface and Co (0001) surface by a single carbon atom. The growth process of graphene, such as the deposition of carbon atoms, the formation of carbon chains with different lengths, the transformation about carbon chains into polygons, and the basic units and natural defects of graphene are investigated in detail. Three processes of self-repair of graphene defects, including carbon chain rotation, splitting and embedding, are described respectively. The effects of temperature and carbon deposition rate on the growth of high-quality graphene are also studied. The simulation results show that at low temperature, the mobility of carbon atoms is low and grown graphene contains a lot of defects, and the coverage of the substrate is low, which leads to low quality graphene to be prepared. High temperature promotes the migration of carbon atoms and helps to grow high quality graphene. However, high temperature can cause damage to the substrate and reduce the flatness of the growing graphene. At a higher deposition rate, the nucleation rate of graphene is higher and the distribution is more uniform. However, due to the different ability of each graphene nucleus to absorb carbon atoms, there are many macrocyclic defects in the graphene. The low deposition rate has a longer annealing time, which excessively stimulates the migration of carbon atoms. It leads to the aggregation of carbon atoms and reduce the quality of graphene. The proper deposition rate can ensure the nucleation density of graphene, and at the same time, it has enough time to form six membered rings and repair defects, which is conducive to the formation of the high quality graphene. Therefore, it is significantly important to design the appropriate deposition temperature and deposition rate for the growth of high-quality graphene. After optimizing the simulation parameters, high-quality graphene is successfully grown at 1300 K deposition temperature and 10 ps/C deposition rate.

Migration of carbon atoms in irradiated and non-irradiated alpha-iron studied by magnetic after-effect

Magnetic After-Effect (MAE) and magnetic AC susceptibility studies have been performed on high-purity non-irradiated and Fe-ion irradiated, α-Fe foil samples in the high-temperature range, which became experimentally accessible recently. The pronounced difference in the temperature profile of MAE between non-irradiated and irradiated samples has been identified and ascribed to the trapping of carbon in the irradiated sample into structures that are reluctant to decompose by standard temperature cycling. The accurate background of MAE relaxations at 430 and 610 K in non-irradiated α-Fe samples has been scrutinized by the annealing-type studies in temperature and time domains to conclude that it relies on the formation, decomposition, and mutual transformations of carbide nanoprecipitates nucleated in dislocations, grain boundaries, and in the bcc matrix. Long-term trapping of migrating carbon into carbides and in the 100 nm thick surface layer has also been shown to take place.

Sintering and mechanical properties of carbon bulks from ordered mesoporous carbon and nano diamond

Powder metallurgy is important in material preparation. Due to the inertness of carbon materials, however, sintering powdered carbon into physically coherent bulks has been a great challenge even at a high temperature (2000 °C). Improving the sintering activity of carbon powders is the key to the success of the consolidation of the carbon powders. Here ordered mesoporous carbon (OMC) is used as the starting material to produce highly homogeneous novel carbon bulks. During sintering at 1800 °C, the huge specific surface area of the OMC greatly promotes the migration of carbon atoms and thus the sintering of the OMC by surface diffusion mechanism. When nanodiamond (ND) is added, the volume expansion associated with the phase transformation of diamond to graphite facilitates the densification of the powder compacts. The strong connection between the OMC and the graphite onions derived from the ND bestows the as-prepared carbon bulks with excellent mechanical properties. The current research pioneers a novel way to prepare high-strength carbon materials at relatively low temperatures.

Evolution and mechanism of dissolutive wetting between hot metal and carbon brick

The dissolutive wetting between hot metal and carbon brick is a complex problem from the viewpoint of carbon brick erosion. The current work aims to clarify the mechanism of dissolutive wetting at the interface between hot metal and carbon brick. The interfacial morphology is analyzed using SEM and EDS, and the change in wetting angle is studied using CCD camera and ImageJ software. Also, the interface shape model is established, the coupling control mechanism of dissolution and wetting is explored. The results reveal that the concave-like interface is formed after dissolutive wetting and the concave depth decreases with the increase of carbon content in hot metal. Moreover, a large amount of graphite flakes appear at the interface between hot metal and carbon brick after dissolutive wetting. The upper wetting angle initially exhibits a rapid decrease with time, followed by a gradual decrease. The lower wetting angle increases with time. The overall wetting angle initially decreases with time, followed by a stable trend. Hence, the dissolutive wetting can be divided into two stages.Furthermore, the interface shape model is established based on the scaling law and Noyes-Whitney equation to describe the dissolutive wetting. The variation of interface shape can be obtained by the interface shape model. The governing equation is obtained through the Onsager principle to clarify the coupling control mechanism of dissolutive wetting. Finally, wetting is determined by the relative situation of interfacial energy and dissolution free energy. The liquid structure of hot metal is changed due to the migration of carbon atoms from carbon brick to hot metal during dissolutive wetting, altering the free energy and facilitating the transition from the first stage to the second stage.

Effects of masking titanium with a one-atom-thick carbon layer on the adsorption of nitrogen monoxide, nitrogen dioxide, ozone, and formaldehyde

We investigated the adsorption of each of four pollutant molecules, namely nitrogen monoxide, nitrogen dioxide, ozone, and formaldehyde, on the surface of a [0001] titanium slab, when it has adsorbed a one-atom-thick carbon layer. The tile is made of four titanium layers. Density functional theory and molecular dynamics were utilized. Atmospheric pressure and 300 K were considered. We found chemisorption of the four molecules on the [0001] titanium surface. Then, we optimized the interaction of the metal surface and a one-atom-thick carbon layer. The carbon atoms end up bound to titanium atoms on the surface. Carbon atoms' quasilinear chains are formed. The adsorption energy per carbon atom is − 2.054 eV. There is no migration of carbon atoms to the bulk. We found significant changes in the adsorption properties. The new system repels formaldehyde and nitrogen oxide molecules. In the ozone case, chemisorption of one oxygen atom on the surface is found, and the remaining oxygen molecule is repelled. In the nitrogen dioxide case, chemisorption of one oxygen atom on the surface is found too, and the leftover fraction is also repelled. These calculations were performed using GGA, and afterward, we utilized Vdw-DF2, which includes long-range correlation effects. The same results are obtained.

On monitoring the incubation stage of stress corrosion cracking in steel by the eddy current method

The eddy current method is traditionally used for monitoring the latest stages of metal deterioration (appearance of multiple surface cracks that cause reduction of electric conductivity). However, the preceding incubation stage may take up to 80–90% of the total lifetime, and its monitoring is of obvious importance. Experiments reported in the present work suggest a new monitoring methodology is suggested – of extending the eddy current monitoring to the incubation stage, based on changes of the electromagnetic properties at this stage. The said changes are caused by the migration of carbon atoms from grains interior to grain boundaries and the formation of cementite, Fe3C, along the boundaries.

Influences of Initial Microstructures on Portevin‐Le Chatelier Effect and Mechanical Properties of a Ni–Fe–Cr–Base Superalloy

Effects of initial microstructures on the Portevin‐Le Chatelier effect (PLC) and mechanical properties of a Ni–Fe–Cr‐base superalloy are investigated by hot tensile deformation tests. The inverse PLC effect only occurs in the ST (solution treated) and SAT (solution plus δ phases aging precipitation) superalloys when the deformation temperature is above 873 K. Effects of initial microstructures on the temperature range of PLC effect are obvious, that is, the narrowest temperature range for the HS (solution plus γ′/γ″ phases aging precipitation) superalloy, while the widest temperature range for the ST superalloy. The average activation energy for type A/A+B/B serrations can be determined as 65–81 kJ mol−1, which contributes to the migration of carbon atoms through pipe diffusion. Meanwhile, the average activation energy for type C serration is about 132–175 kJ mol−1, which is induced by the pipe diffusion of substitutional atoms. An obvious increase in the yield strength with increasing the deformation temperature from 900 to 973 K is observed for the ST and SAT superalloys. Also, the negative strain rate sensitivity is found in the PLC effect regime. The ST and HS superalloys exhibit the ductile fracture. However, a mixture of ductile transgranular and cleavage fracture is observed in the SAT superalloy.

Catalyst-Free In Situ Carbon Nanotube Growth in Confined Space via High Temperature Gradient.

Carbonaceous materials, such as graphite, carbon nanotubes (CNTs), and graphene, are in high demand for a broad range of applications, including batteries, capacitors, and composite materials. Studies on the transformation between different types of carbon, especially from abundant and low-cost carbon to high-end carbon allotropes, have received surging interest. Here, we report that, without a catalyst or an external carbon source, biomass-derived amorphous carbon and defective reduced graphene oxide (RGO) can be quickly transformed into CNTs in highly confined spaces by high temperature Joule heating. Combined with experimental measurements and molecular dynamics simulations, we propose that Joule heating induces a high local temperature at defect sites due to the corresponding high local resistance. The resultant temperature gradient in amorphous carbon or RGO drives the migration of carbon atoms and promotes the growth of CNTs without using a catalyst or external carbon source. Our findings on the growth of CNTs in confined spaces by fast high temperature Joule heating shed light on the controlled transition between different carbon allotropes, which can be extended to the growth of other high aspect ratio nanomaterials.

Enhanced carbon tolerance on Ni-based reforming catalyst with Ir alloying: A DFT study

Carbon deposition is a major cause of performance degradation for the Ni-based catalyst used in steam reforming of hydrocarbons. In this work, we perform first principle calculations to show that carbon tolerance behavior can be significantly enhanced by alloying Ni with Ir. The most stable atomic structure predicted by the surface phased diagram shows that Ir atoms prefer to stay on the surface of the alloy ensuring their exposure to the incoming gas. We find that the presence of Ir atoms suppress the surface migration of carbon atoms and weaken the stability of the adsorbed carbon agglomerates. Finally, we elucidate that the local reactivity change caused by the shift in the d-band structure is responsible for such good carbon tolerance behavior.

Phase transition and in situ construction of lateral heterostructure of 2D superconducting α/β Mo2C with sharp interface by electron beam irradiation.

Lateral heterostructures of 2D materials have several interesting properties and potential applications, and they are usually fabricated by chemical vapor deposition. However, it still remains a great challenge to fabricate 2D lateral heterostructures with well-controlled patterns and sharp interfaces. Herein, we found that the 2D α-Mo2C crystal, a recently emerging 2D superconductor, experiences a phase transition from the α phase to β phase on electron beam irradiation in a transmission electron microscope because of the migration of carbon atoms among the molybdenum octahedrons. Combined with first-principles calculations, the carbon atom migration paths and the corresponding energy barriers were discussed. Utilizing this unique phase transition property of 2D α-Mo2C crystal, we demonstrated the precise in situ construction of the lateral heterostructure of 2D superconducting α/β Mo2C with a well-controlled pattern and sharp interface using advanced aberration-corrected scanning transmission electron microscopy.

Dry Reforming of Methane on Rh-Doped Pyrochlore Catalysts: A Steady-State Isotopic Transient Kinetic Study

Increases in worldwide methane production from biological and fossil sources have led to an increased level of interest in the dry reforming of methane (DRM) to produce syngas. Experimental efforts have shown that select pyrochlore materials, such as the Rh-substituted lanthanum zirconate pyrochlore (LRhZ), are catalytically active for DRM, exhibit long-term thermal stability, and resist deactivation. This work seeks to allow further catalyst improvements by elucidating surface reaction kinetics via steady-state isotopic transient kinetic analysis (SSITKA) of dry reforming on the LRhZ pyrochlore. Isotopically labeled CH4 and CO2 were used in multiple SSITKA experiments to elucidate the migration of carbon atoms to product species. Short surface residence times at 650 and 800 °C (<0.6 s) were observed for DRM intermediates involved in reversible reactions, and the participation of all surface metal atoms as active sites for DRM, not only Rh, is suggested. Isotopic responses and kinetic isotope effects are ...

Влияние внешних напряжений на тетрагональность мартенсита углеродистых сталей

The theory of carbon atoms’ ordering in iron martensite based on the idea of non-uniform filling of octahedral interstices is considered. The results show that external pressure decreases the long-range order parameter and the degree of tetragonality abruptly. It is shown that an equilibrium condition needs to be considered together with free energy minimum requirement. Joint solution of these equations gives the critical stress value that turns the order parameter to zero, as a function of temperature and carbon content. Zero value of the order parameter means that the distribution of carbon atoms among sublattices is uniform, and, thus, the lattice is cubic. Molecular dynamics method with EAM interatomic potential was applied to model redistribution of carbon atoms over sublattices under external stress. It was found that migration of carbon atoms from z octahedral interstices occurs under compression along Oz axis, so that the tetragonality direction changes from Oz axis to Ox or Oy, with exact direction being determined randomly. The dependences of critical stress on temperature and carbon content were obtained. The computer simulation results are consistent with theoretical data.

Molecular dynamics simulation on the initial stage of 1 eV carbon deposition on silicon

The deposition process of 1 eV carbon on silicon has been investigated by molecular dynamics (MD) simulations up to a fluence of 5.3 × 1014 atoms cm−2 which corresponds more or less to monolayer coverage. At such low impact energies, atoms are expected to stay on the sample surface, which is also observed up to a fluence of 2 × 1014 atoms cm−2. For higher fluence, carbon atoms start mixing into the silicon substrate. This process seems to get initiated by the increasing strain caused by the carbon atoms deposited on the silicon surface, and which leads to some gradual distortions. The latter are important for the migration of carbon atoms into the silicon lattice. During the whole process the top part of the silicon sample gets amorphized and the coordination of the carbon atoms increases from 1 or 2 to mostly 4-fold coordinated carbon atoms. The process can be considered as the starting point of silicon carbide formation and allows to explain how nm thick films can be formed from 1 eV deposition energies. The low carbon concentration of about 7% in the modified layer is, however, too low to observe a transition towards the latter.

Experimental Study of Damage Mechanism of Carbon Nanotube as Nanocomponent of Electronic Devices Under High Current Density

The damage mechanisms of carbon nanotubes are considered to be the oxidation by Joule heating and migration of carbon atoms by high-density electron flows. In this study, a high current density testing system was designed and applied to multiwalled carbon nanotubes (MWCNTs) collected at the gap between thin-film electrodes. Local evaporation of carbon atoms occurred on the cathode side of the MWCNTs under relatively low current density conditions, and the center area of the MWCNTs under high current density conditions. The damaged morphology could be explained by considering both Joule heating and electromigration behavior of MWCNTs.

Computer simulation of carbon diffusion near b/2[010](001) dislocation in cementite

Partial contributions U i to the activation energy for the diffusion of carbon atoms in the Fe3C lattice have been calculated. The U i values have been compared upon the migration of carbon atoms in the ideal lattice, near the stacking-fault plane, and near the core of a partial edge dislocation with a Burgers vector b/2[010]. The most preferable ways of the migration of carbon atoms near the studied structural defects in the (001) cementite plane have been revealed. The values of the stacking-fault energy in this plane have been calculated. The possibility of splitting the dislocation with a Burgers vector b/2[010] into two partial dislocations has been shown.

Atomic behavior of carbon atoms on a Si removed 3C-SiC (111) surface during the early stage of epitaxial graphene growth

The understanding of the formation of graphene at the atomic scale on Si-terminated 3C-SiC for obtaining high-quality graphene sheets remains elusive, although epitaxial graphene growth has been shown to be a well-known method for economical mass production of graphene/SiC heterojunctions. In this paper, the atomic behavior of carbon atoms on a Si removed 3C-SiC (111) surface for the formation of graphene buffer layer during the early stage of epitaxial graphene growth was investigated using a molecular dynamics simulation. Observation of the behavior of the remaining carbon atoms on the Si-terminated 3C-SiC (111) surface after removal of the silicon atoms revealed that graphene clusters, which were formed by sp2-bonded carbon atoms, start to appear at annealing temperatures higher than 1300 K. Our simulations indicated that the structural stability of the whole system increased as the number of sp2-bonded carbon atoms on the Si-terminated 3C-SiC (111) surface increased. It was also found that the diffusion energy barrier for the migration of carbon atoms from the on-top site to the bridge site on the Si-terminated 3C-SiC (111) surface mainly determines the critical temperature of graphene cluster formation.

10.1007/s11508-008-2012-6

Snoek relaxation parameters in a quenched ternary Fe-Al-Si alloy have been studied during subsequent aging of samples. Substitutional atoms contribute to a broadening of the Snoek peak on the high-temperature side due to an additional relaxation induced by migration of carbon atoms, which interact elastically with the substitutional atoms. Upon subsequent heating (aging) to ∼350°C, there occurs a decomposition of the oversaturated solid solution, which is accompanied by a redistribution of carbon atoms over different positions, which is seen from an analysis of the parameters of the Snoek peak. At temperatures about 400°C, there are observed anomalies related to the re-pinning of dislocations by interstitial atoms and to the formation of new internal-friction peaks.

Long fine single-wall carbon nanotube growth by Nano-spin-threading: model schematics and simulations

We have investigated model schematics for a long fine single-wall carbon nanotube growth inside a larger diameter nanotube. Our proposed schematics are as follows: fullerenes are encapsulated into the nano-channel connected with fullerene storage tank; and then a inner nanotube grows via fullerene coalescence under ∼1200°C in the nano-channel. Then the grown carbon nanotube is extracted from the nano-channel by mechanical control. We have investigated fullerene mergence inside single-wall carbon nanotube using classical molecular dynamics simulations based on the Tersoff–Brenner potential and the Lennard–Jones potential. During fullerene-encapsulating, since the fullerenes naturally have the kinetic energies due to the suction force and can be also accelerated by external force fields to improve the fullerene encapsulation rate, they can be migrated toward the other side of the nano-channel with kinetic energies. Our molecular dynamics simulations showed that the structural relaxation of dynamically free atoms affected on the growth of inner carbon nanotube rather than the Stone-Wales transformations. Since the broken bonds make the structural relaxation during merging to be easily achieved from the migration of carbon atoms or carbon chains, the inner nanotube grows via the re-bonding-reactions of dynamically free carbon atoms or chains as well as the Stone-Wales transformations. We could conclude that the growth rate of the inner CNT could be increased when bond-breakings between carbon atoms of fullerenes were easily achieved.

Successive implantation of aluminium by carbon and nitrogen ions

The results of Rutherford backscattering analysis and transmission electron microscopy of the surface layer of aluminium after monoelemental and successive implantation by carbon and nitrogen ions are presented in this work. The energy of implanted ions was 40 keV, the implantation dose varied in the range (3.3–6.5)×10 17 ions cm −2. The findings showed that monoelemental carbon and nitrogen implantation led to the formation of Al 4C 3 or AlN compound, respectively. Successive implantation resulted in AlN formation and considerable migration of carbon atoms into the depth.

Thermal Stability of Stacking Faults in β-SiC Specimens

Stacking faults in SiC are one of the important factors in treat SiC, since they affect physical and chemical properties. Ultra-pure β-SiC specimens with stacking faults were annealed at various temperatures for various duration in an Ar atmosphere under a condition without grain growth. Stacking faults decreased in two steps by heat treatment characterized by fast decrease and by slow decrease, respectively. The apparent activation energy for the former was estimated to be 182kJ/mol and that for the latter was estimated to be 260kJ/mol. The value of the latter corresponded to the activation energy for migration of carbon atoms in β-SiC. It was also observed that lattice strain decreased with decreasing stacking faults.