Formation Of Amorphous Carbon Research Articles

Characterization of Ni-doped pyrolyzed phenolic resin and its addition to the Al2O3–C refractories

The catalytic graphitization of the phenolic resin was investigated in the presence of various amounts of nickel nitrate at 600–1200 °C in a reducing atmosphere for 3 h. For comparison, the effect of catalyzed phenolic resin versus none-catalyzed one was explored on the microstructure and mechanical properties of the Al2O3–C refractory specimens within a range of 800–1400 °C. The phase composition and microstructure evolution were investigated using X-ray diffraction and SEM, respectively. RAMAN spectroscopy was used to measure the graphitization level of phenolic resin. Results indicated that crystalline carbon (graphite) was achieved only from Ni-catalyzed phenolic resin at a high temperature (~1200 °C), while non-catalyzed resin led to the formation of amorphous carbon. Mechanical properties of the Al2O3–C specimens were improved due to the presence of catalyzed phenolic resin. SEM results illustrated that the enhancement was associated with the in-situ formation of the SiC phase in whisker shape in the microstructure of specimens containing Ni-catalyzed phenolic resin.

Identifying the effects of cobalt addition in copper-graphene nanoplatelet composites towards improved tribological performance

Abstract Obtaining good affinity between graphene and copper in copper-graphene composites is the key to make full use of the good anti-wear ability of graphene. Copper-graphene nanoplatelets (Cu-GNPs) composites were prepared by ball-milling and hot pressed sintering with the addition of a small amount of cobalt to form intact graphene/cobalt/copper interfaces. The microstructural characterization shows that the GNPs are uniformly distributed on the grain boundaries of copper grains. The cobalt here acts as “binder”, bringing graphene and copper together efficiently. Benefiting from the unique structural design, Cu-GNPs composite doped with 0.25 wt % cobalt exhibits improved anti-wear performance with a stable friction coefficient of 0.246 and a small volume wear rate of 8.3 × 10−5 mm3/(N·m) (20% of that of pure copper composite). Excessive cobalt causes a significant increase of the structural defects in graphene and the formation of amorphous carbon, resulting in degradation of friction properties.

Simple, Reproducible Synthesis of Pure Monohydrocalcite with Low Mg Content

Monohydrocalcite (MHC) is a metastable hydrous calcium carbonate that requires Mg in the mother solution during formation in the laboratory. MHC prepared by previously reported methods always contains a large amount of Mg (Mg/Ca ratio up to 0.4) because of the simultaneous formation of amorphous Mg carbonate during synthesis, which has hindered detailed elucidation of the mineralogical characteristics of MHC. Here, we synthesized MHC at low temperature (5 °C) and found that it contained little Mg (Mg/Ca ratio < 0.01). X-ray absorption near-edge structure analysis of synthesized MHC revealed that the Mg present was structurally incorporated within the MHC, and that the chemical speciation of this Mg was similar to that of Mg in aragonite. Thus, low-temperature synthesis is an effective means of producing MHC without also producing amorphous Mg carbonate.

Mechanical alloying of iron-fullerite system. Correlation between fullerite content, its structural state and end products

X-ray analysis, Mössbauer and Raman spectroscopies have been used to study mechanosynthesized iron-fullerite powders with the initial fullerite С60/70 content of 10, 25 and 75 at%. The structural changes of С60/70 in mechanocomposites have been studied. It has been shown that in conditions of high-energy mechanosynthesis of iron-fullerite powders the disordering of fullerite С60/70 crystalline structure takes place followed by the total fullerene molecules destruction with the formation of amorphous carbon. At content 75–90 at% iron serves as a catalyst of С60/70 destruction. It has been stated that the phase composition of mechanocomposites depends on the amount of fullerite and its structural state. Carbides formation takes place only after the destruction of fullerene molecules as a result of C–C and CC bonds disrupture. Amorphous carbon, Сam, amorphous phase, Am(Fe–C), carbides, Fe3C, Fe7C3 and a paramagnetic P-phase can be the end products of solid-phase reactions. At the initial stage of Fe–75C60/70 mechanosynthesis, while fullerene structure is preserved, it is possible to obtain iron-fullerene composite. The kinetics of carbide formation in the samples has been analyzed. It has been shown that the speed of solid-phase reactions at С60/70 content 25 at% is higher than at 75 at%.

A new model for the synthesis of graphite encapsulated nickel nanoparticles when using organic compounds in an arc-discharge system

Herein we propose a new “three-step” model of synthesizing graphite encapsulated nickel nanoparticles (Ni-GEM), including the pyrolysis reaction of organic compounds which the conventional two-step model ignores. According to the results of XRD, Raman, and TEM, we found that the Ni-GEM synthesized by using both PF resin and benzene vapor as the carbon sources has two favorable characteristics: thicker shells (~5–10 nm) and smaller particle sizes (~30 nm), which are much better than those using only PF resin or PF resin mixed with cyclohexane vapor (thinner shell less than 5 nm and larger particle sizes ~50 nm). Benzene decomposes into large aromatic molecules and tiny graphitic flakes at 1200–3500K, while cyclohexane prefers to decompose and form small hydrocarbon molecules at 1000K. As a result, the two compounds go through two different reaction paths. Benzene will decompose and directly attach onto the surface of Ni nanoparticles, forming smaller sized but thicker shell structured Ni-GEM, while cyclohexane will lead to the formation of amorphous carbon coating on the Ni-GEM. By including the above two distinct hydrocarbon pyrolysis reactions, this study modifies the conventional model and successfully explains the formation processes of Ni-GEM with very different morphologies. Furthermore, the new model may help in controlling the morphologies of other GEM nanoparticles with a number of core-metals.

Bottom-up synthesis of graphene via hydrothermal cathodic reduction

To date, considerable effort had been devoted to developing a bottom-up synthesis method of graphene, but the methods have been mostly limited to chemical vapor deposition and SiC surface decomposition. Herein, we demonstrated the bottom-up synthesis of graphene via hydrothermal cathodic reduction. In the developed process, the hydrothermal conditions were tuned to enhance the electrochemical reaction and prevent the formation of undesirable amorphous carbon. The nature of the deposited carbonaceous material depends strongly on the temperature, and changes from amorphous carbon into graphitic carbon and graphene with increasing temperature. This should be because of the enhanced oxidative etching effect of amorphous carbon under hydrothermal condition. Finally, the hydrothermal electrochemical reaction enabled graphene growth on a cathode surface at 300 °C.

Effect of electric field on oxide layer structure at zirconium oxidation in H2O, CO2 and H2O/CO2 supercritical fluids

Oxidation of zirconium plates in supercritical fluids H2O, CO2 and H2O/CO2 was studied at 823 K and 24–39 MPa. An experimental cell was a flat capacitor, where the zirconium plate acted as the anode and the copper plate of the same size was the cathode. The interelectrode gap was 2 mm. The experiments were carried out in the absence of electric field and in the electric field of 300 kV m−1. It has been found that in all cases the oxidized layer consists of two parts: the outer loose layer and the inner dense layer forming an interface with Zr. At oxidation in a medium containing H2O, there is non-uniformity in distribution of oxygen near the boundary region of the loose and dense layers. When Zr is oxidized in the H2O/CO2 mixture, zirconium carbonate is found on the surface. In the electric field, an average specific rate of an increase in the sample mass decreases linearly with an increase in the fraction of CO2 in the H2O/CO2 mixture, and without the electric field it increases. Apparently, the reason for this is formation and subsequent decomposition of zirconium carbonate, leading to additional destruction of the oxidized layer and accelerated oxidation. When Zr is oxidized in CO2 in the presence of the electric field, a 12-fold increase in the rate of zirconium oxidation is registered, and this is associated with an increase in the surface leakage current due to formation of amorphous carbon.

Mechanisms of fault mirror formation and fault healing in carbonate rocks

The development of smooth, mirror-like surfaces provides insight into the mechanical behaviour of crustal faults during the seismic cycle. To determine the thermo-chemical mechanisms of fault mirror formation, we investigated carbonate fault systems in seismically active areas of central Greece. Using multi-scale electron microscopy combined with Raman and electron energy loss spectroscopy, we show that fault mirror surfaces do not always develop from nanogranular volumes. The microstructural observations indicate that decarbonation is the transformation process that leads to the formation of smooth surface coatings in the faults studied here. Piercement structures on top of the fault surfaces indicate calcite decarbonation, producing CO2 and lime (CaO). Lime subsequently reacts to portlandite (Ca(OH)2) under hydrous conditions. Nanoscale imaging and electron diffraction reveal a thin coating of a non-crystalline material sporadically mixed with nano-clay, forming a complex-composite material that smooths the slip surface. Spectroscopic analyses reveal that the thin coating is non-crystalline carbon. We suggest that ordering (hybridisation) of amorphous carbon led to the formation of partly-hybridised amorphous carbon but did not reach full graphitisation. Calcite nanograins, <50 nm in diameter, are spatially associated with the carbon and indicate that the decomposition products acted as a crystallisation medium. Within this medium, portlandite back-reacted with CO2 to form nanocrystalline calcite. Consequently, two types of calcite nanograins are present: nanograins formed by grain-size reduction (primary nanograins, >100 nm) and new nanograins formed by back-reaction (secondary nanograins, <50 nm). Hence, we suggest that the new, secondary nanograins are not the result of comminution during slip but originate from pseudomorphic replacement of calcite after portlandite. The continuous coverage of partly-hybridised amorphous carbon on all samples suggests that calcite decarbonation products may develop across the entire fault surface, controlling the formation of carbonate fault mirrors, and may facilitate slip on a decarbonation-product glide film.

A single step process to synthesize ordered porous carbon from coconut shells-eggshells biowaste

A novel method is used to synthesize ordered porous carbon from natural bio waste. Eggshells and coconut shells powder particles were used for synthesis of bio carbon. Eggshell particles were used as template as well as activating agent for synthesis of ordered porous carbon, while coconut shells act as a precursor. The synthesized activated carbon particles were characterized with the aid of X-ray diffraction analysis (XRD), Raman spectroscopy, Thermogravimetric analysis (TGA), Scanning Electron Microscopy (SEM), Transmission Electron Microscopy (TEM), and Surface area analysis. Adsorption capacity of synthesized ordered porous carbon was investigated with UV–vis spectroscopy. XRD results revealed that the optimum temperature for pyrolysis of eggshell-coconut shell mixture is 800 °C. XRD and Raman analysis confirmed the formation of amorphous carbon. It was observed that 1:1 ratio at 800 °C gave optimum results when compared with 1:2, 1:3 and 1:4 eggshells and coconut shells mixture. Brunauer–Emmett–Teller (BET) surface area and pore volume of 1:1 ratio at 800 °C were found to be 375.4 m2 g−1 and 0.2478 cm3 g−1 respectively. The average pore size was found to be 2.64 nm for 1:1 ratio of eggshells-coconut shells mixture pyrolyzed at 800 °C. Morphology analysis confirms the formation of micropores and mesopores on the surface of carbon particles. The adsorption isotherms were well fitted with Langmuir isotherms. Maximum adsorption capacity was found to be 305.76 mg g−1 for methylene blue adsorption on the surface of ordered porous carbon.

Exploring the Conversion Mechanisms of Toluene as a Biomass Tar Model Compound on NiFe2O4 Oxygen Carrier

As a novel biomass gasification technology, chemical looping gasification (CLG) could in situ catalytically crack biomass tar by oxygen carrier (OC). This study compares the reactivity of different OCs (Al₂O₃, Fe₂O₃, NiO, NiO+Fe₂O₃, and NiFe₂O₄) to crack a biomass tar model compound (toluene) and preliminarily explores the reaction mechanisms of toluene cracked by NiFe₂O₄ OC. Among five OCs, NiFe₂O₄ with a homogeneous Fe/Ni dispersibility shows the best reactivity for toluene cracking with a toluene conversion and H₂ yield of 96.83% and 0.91 L/g, respectively. Additionally, on the basis of the dispersibility of active sites on the OC sample surface, the reactivity of the other four OCs to crack toluene shows the sequence NiO+Fe₂O₃ > Fe₂O₃ > NiO ≫ Al₂O₃. A large amount of carbon deposition, including amorphous carbon and graphitized carbon, is generated during toluene catalytic cracking, while the addition of steam significantly eliminates the carbon deposition. The NiFe₂O₄ OC shows a dual-function of in situ oxidation–catalysis during toluene cracking. A mechanism of four steps is proposed for the toluene cracking on OCs: (I) toluene cracking at high temperature, (II) reduction of OC, (III) generation of amorphous carbon deposition and (III′) formation of graphitized carbon, and (IV) elimination of carbon deposition.

Investigation of the ignition and combustion of compressed aluminum/polytetrafluoroethylene bulk composites

The thermal pyrolysis process of mold-compressed Al/PTFE composites has been investigated. TG–DSC analysis of the Al/PTFE composites revealed a slight exothermic pre-reaction in the range 500–600 °C. Based on XRD and XPS analyses of Al/PTFE composites subjected to heat treatment at 500–600 °C, this pre-reaction is believed to be fluoridation of the alumina passivating layer on the surface of Al particles. This fluoridation has been confirmed to be a crucial factor in triggering the ignition of aluminum particles in an O2 flow at a temperature much lower than the melting point of aluminum (660 °C). The energetic properties of the Al/PTFE composites have been studied, and the compositions of their combustion residues have been analyzed by XRD. The results indicated that the energy release from the composites and the compositions of their solid combustion residues were determined by their PTFE contents. Composition analysis of these combustion residues has revealed the formation of AlF3, Al2O3, Al4C3, and amorphous carbon.

Plasma-enhanced chemical vapor deposition of graphene optimized by pressure

Fast growth of graphene on non-catalytic substrates by plasma-enhanced chemical vapor deposition (PECVD) usually results in poor crystalline quality and sometimes is accompanied by the formation of amorphous carbon. In this study, we slow down the graphene growth on quartz substrates by elevating pressure in a PECVD setup. The lower growth rate is caused by slower diffusion of gaseous species in the plasma to the substrate surface at higher pressure. The slower growth at high-temperature is crucial for larger critical nucleus size, higher nucleation barrier, and better electrical properties. Better graphene quality under higher pressure growth is verified by Raman spectroscopy. A hole mobility up to 89.6 cm2·V−1·s−1 is acquired with a growth rate of 1.34 nm h−1 at 950 °C, 1.2 Torr.

Deactivation reactions on a commercial lean nox-trap - Effect of hydrocarbon nature, concentration and operation temperature

Engine-out emissions during rich phase operation of a lean NOX trap (LNT) catalyst contain a mixture of different hydrocarbons (HCs) depending on engine operation and fuel type. The impact of temperature, as well as type and concentration of HC on reactions and deactivation potential on a commercial LNT catalyst was analyzed by lean-rich cycle measurements and subsequent temperature programmed oxidation. Deactivation during the rich phase is omitted initially by O species adsorbed or stored by CeO2, but formation of amorphous carbon and significant deactivation of noble metal sites occur in the presence of C2H2. Carbon formation and deactivation of Pt sites correlate with the reforming reaction of C2H2, thus emphasizing the significance of this reaction in the rapid build-up of carbonaceous deposits. Coking and deactivation decreases with decreasing C2H2 concentration and reaction temperature that determine the extent of reforming reaction. DFT calculations of adsorption energies on sites of Pt (111) or Pt10/CeO2 verify that abundance of HCs on the Pt surface, relative to other feed components, determines which reactions occur and thus the coking degree. This aligns with the finding that HCs with lower adsorption energy, such as C2H4 or C3H6, are less reactive and significant catalyst deactivation is absent during their presence.

Amorphous Carbon Generation as a Photocatalytic Reaction on DNA-Assembled Gold and Silver Nanostructures.

Background signals from in situ-formed amorphous carbon, despite not being fully understood, are known to be a common issue in few-molecule surface-enhanced Raman scattering (SERS). Here, discrete gold and silver nanoparticle aggregates assembled by DNA origami were used to study the conditions for the formation of amorphous carbon during SERS measurements. Gold and silver dimers were exposed to laser light of varied power densities and wavelengths. Amorphous carbon prevalently formed on silver aggregates and at high power densities. Time-resolved measurements enabled us to follow the formation of amorphous carbon. Silver nanolenses consisting of three differently-sized silver nanoparticles were used to follow the generation of amorphous carbon at the single-nanostructure level. This allowed observation of the many sharp peaks that constitute the broad amorphous carbon signal found in ensemble measurements. In conclusion, we highlight strategies to prevent amorphous carbon formation, especially for DNA-assembled SERS substrates.

V2O3/C nanocomposites with interface defects for enhanced intercalation pseudocapacitance

V2O3 nanoparticles highly dispersed in amorphous carbon composites (VO-C) were successfully synthesized by the calcination of (NH4)2V3O8/C precursor, which was fabricated through the hydrothermal reaction using commercial NH4VO3 and glucose as raw materials. The release of gaseous byproduct during the formation of V2O3 and amorphous carbon resulted in the formation of highly dispersed oxide nanoparticles dispersed in porous carbon matrix. The composites exhibited a high specific intercalation pseudocapacitance of 458.6 F g−1 at 0.5 A g−1 with a retention of 86% after 1000 cycles in aqueous electrolyte. Such good electrochemical properties is attributed to the high diffusion coefficient and promoted electron transfer of the composites possibly as a result of surface modification of V2O3 nanoparticles. In addition, highly porous carbon network with homogeneously dispersed V2O3 permits efficient intercalation reaction with much enhanced stability of V2O3. Flexible asymmetric supercapacitors using synthesized VO-C and active carbon electrodes were assembled and demonstrated excellent electrical energy storage performance.

Phenyl Disulfide Additive for Solution‐Mediated Carbon Dioxide Utilization in Li–CO2 Batteries

AbstractPhenyl disulfide (PDS) is employed as an electrolyte additive in lithium–carbon dioxide (Li–CO2) batteries to allow for a solution‐mediated carbon dioxide reduction pathway. Thiophenolate anions, generated via electrochemical reduction of PDS, act as CO2 capture agents by forming the adduct S‐phenyl carbonothioate (SPC−) in solution. A mechanism of SPC−‐mediated CO2 capture and utilization is proposed and supported via carbon‐13 nuclear magnetic resonance spectroscopy and Fourier‐transform infrared spectroscopy. Reversible formation and decomposition of lithium carbonate and amorphous carbon during cycling, facilitated by the solution‐mediated pathway, are demonstrated with an array of characterization techniques. Li–CO2 batteries employing the PDS additive show vastly improved capacity, energy efficiency, and cycle life. The enhanced Li–CO2 battery performance offered by the proposed solution‐mediated reaction pathway offers a compelling step forward in the pursuit of reversible CO2 utilization.

Formation of low resistivity layers on singlecrystalline diamond by excimer laser irradiation

A singlecrystalline diamond (100)(Ib) plate immersed in 2% boric acid was irradiated by 193-nm ArF excimer laser beams for the formation of conductive layers on the surface of an insulating diamond substrate. From current-voltage measurements of the irradiated areas, it was confirmed that semiconducting layers with high conductivities are formed on the diamond surface. It was possible to form ohmic contacts by directly touching tungsten probes with the layer surface. Since Raman spectra exhibited only peaks due to diamond and no peaks due to amorphous carbon, the drastically enhanced conductivity is not attributed to amorphous carbon formation but due to the incorporation of boron atoms into the diamond subsurface from the dopant acid. Secondary ion mass spectrometric depth profile showed the incorporation of boron atoms up to 40 nm depths from the surface. From cathodoluminescence measurements at low temperatures, it was difficult to detect clear peaks for the substitutional incorporation of boron atoms into diamond lattices, which could be attributed to the small thickness of the doped layer for detection. The proposed technique is a new potential method for shallow doping and formation of conductive layers on singlecrystalline diamond surfaces.

Evolution of photoresist layer structure and surface morphology under fluorocarbon‐based plasma exposure

AbstractIon bombardment of photoresist materials during plasma etching results in the formation of a surface dense amorphous carbon (DAC) layer that contributes to both etch resistance and the development of surface roughness. Real‐time ellipsometric measurements/analysis reveals that a C4F8‐containing plasma interacts with an Ar‐plasma‐formed DAC layer to produce a modified DAC/fluorocarbon (FC) layer by FC deposition/diffusion of fluorine into the surface. The depletion of the DAC layer via modification and ion bombardment causes the etch rate of the bulk layer to increase. As the modified surface layer is formed, a noticeable decrease in surface roughness decrease is observed. These findings provide an understanding of the mechanisms of atomic layer etching processes in photoresist materials.

Synergistic catalysis in monodispersed transition metal oxide nanoparticles anchored on amorphous carbon for excellent low-temperature dehydrogenation of magnesium hydride

Magnesium hydride (MgH2) has been considered to be one of the most promising solid-state hydrogen storage materials owing to its high hydrogen capacities, excellent reversibility and abundant source. However, the high dehydrogenation energy barrier and poor kinetics embarrass the practical application of MgH2 in fuel cell. Doping nano-catalyst is deemed to be the most effective method to improve kinetics property of hydrogen storage materials, but the nanoparticles generally suffer from agglomeration and inactivation during the cycling hydrogen storage. Here we present a promising strategy to facilely prepare a high-efficiency transition metal oxide nano-catalyst, TiO2 nanoparticles, in which monodispersed single-crystal-like TiO2 nanoparticles are wrapped with amorphous carbon. The in-situ synthesized TiO2 nanoparticles/amorphous carbon catalyst exhibit superior catalytic effect on the dehydrogenation properties of MgH2. A significant reductions of hydrogen desorption temperature (163.5 °C) and activation energy (69.2 kJ mol−1) have been obtained for the TiO2 nanoparticles/amorphous carbon catalyzed MgH2, which can be fully rehydrogenated with a reversible capacity of about 6.5 wt% at 200 °C within 5 min, and then completely dehydrogenated at 275 °C within 10 min. It is demonstrated that such significantly improved hydrogen desorption properties can be attributed to the in-situ formation of TiO2 nanoparticles, amorphous carbon and multi-valance Ti species, which play the synergistically catalytic roles in the nano-catalyst. In particular, the presence of amorphous carbon in the catalyst can not only prevent the aggregation and growth of catalyst nanoparticles, but also dramatically reduce the desorption energy value of H in MgH2, according to the density functional theory calculation. This finding opens a new venue for the synthesis of monodispersed single-crystal-like TiO2 nanoparticles/amorphous carbon catalyst with high-activity, safety, low cost, and its practical application in MgH2 and other hydrogen storage systems.

Structural and Morphological Transformations in Aqueous Graphite-Based Lubricant Subjected to Extreme Tribological Interactions of Low-Speed Intermittent Machining

We study morphological and structural changes in graphite flakes present in aqueous graphite-based lubricants when repeatedly subjected to extreme tribological interactions at the tool–chip interface in low speed, high depth of cut intermittent machining, a process involving combined high normal and shear stress. Orthogonal tube turning experiments are conducted on a specially fabricated mild steel workpiece completely submerged in an aqueous graphite solution. Surface profiler, electron microscopy and Raman spectroscopy studies on initial and sheared graphite flakes indicate crushing followed by layer sliding, rolling, edge curling, tearing and exfoliation of layers. Raman spectroscopy studies highlight formation of low amorphous carbon by structural disorder and edge defects formed while crushing and tearing with no new basal defects in the sheared graphite layers. Some regions of the flake thin down to highly disordered few-layer graphene with a thickness reduction from about 30 micrometres to about 1–2 nanometres with an I2D/IG value of around 0.7. When reused back into the tribological region, these morphological changes can significantly influence the continued lubrication process by the generation of few-layer graphene.