Synthesis of transition metal carbides and high-entropy carbide TiZrNbHfTaC5 in self-shielding DC arc discharge plasma

The paper shows the feasibility of synthesizing micro- and nano-sized particles of binary metal carbides (Me–C) and high-entropy carbide (HEC) TiZrNbHfTaC₅ by vacuum-free electric arc method. The method is based on the effect of self-shielding of the reaction volume from atmospheric oxygen by carbon monoxide CO, which is generated during arcing in air. The synthesis results in a solid solution with a NaCl-type carbide with a cubic lattice, which simultaneously contains atoms of titanium, zirconium, niobium, hafnium, tantalum, and carbon. The lattice parameter of the HEC TiZrNbHfTaC₅ phase is ～4.532 Å that is in line with the known data on this compound. The synthesis product contains micro-sized particle agglomerates of transition metal carbides. The synthesis products also contain nano-sized particles with a shell-core structure, in which the core can consist of metal carbide (TiC, ZrC, NbC, HfC, TaC) or HEC TiZrNbHfTaC₅, and the shell is a graphite phase.     

Carbon nanohorns hybridized with a metal-included nanocapsule

A novel nanocarbon structure, carbon nanohorns (CNHs) particle which includes one Ni-contained carbon nanocapsule (CNC) in its center, is produced by submerged arc in liquid nitrogen using Ni-contained (0.7 mol%) graphite anode. Transmission electron microscopy revealed that each CNHs particle in nearly spherical shape of diameter around 50-100 nm has one Ni nanoparticle of diameter 5-20 nm encapsulated by graphitic shells as CNC.     

Composite graphitic nanolayers prepared by self-assembly between finely dispersed graphite oxide and a cationic polymer

Chemical flaking of graphite has been performed by reacting natural graphite with a strong oxidizing agent, NaClO₃/HNO₃. The formed hydrophilic, negatively charged graphite oxide (GO) colloids can be dispersed in water which allows the deposition of thin GO/cationic polymer (poly(diallyldimethylammoniumchloride, PDDA) multilayer films on a glass substrate by wet-chemical self-assembly. The feasibility of the charge-regulated layer-by-layer deposition is demonstrated by mutual charge titrations of the film-forming species. Visible-light spectroscopy revealed progressive growth of the film thickness with the number of deposition of steps, while XRD and AFM showed that partially exfoliated, highly anisometric (aspect ratio >50) graphite oxide platelet aggregates were deposited with an average thickness of the stacked graphite oxide platelets of 10 carbon layers (7.4 nm). Reduction of multilayer assemblies of GO and PDDA on glass yielded a non-conductive turbostratic carbon nanofilm. The original, conductive graphite-like structure was restored by reduction with N₂H₄ and annealing at 400 °C which, by gradual ordering of the carbon crystallites, caused a significant decrease in the resistivity.     

Synthesis and characterization of double-walled carbon nanotubes from multi-walled carbon nanotubes by hydrogen-arc discharge

Double-walled carbon nanotubes (DWNTs) were synthesized in a large scale by a hydrogen arc discharge method using graphite powders or multi-walled carbon nanotubes/carbon nanofibers (MWNTs/CNFs) as carbon feedstock. The yield of DWNTs reached about 4 g/h. We found that the DWNT product synthesized from MWNTs/CNFs has higher purity than that from graphite powders. The results from high-resolution transmission electron microscopy observations revealed that more than 80% of the carbon nanotubes were DWNTs and the rest were single-walled carbon nanotubes (SWNTs), and their outer and inner diameters ranged from 1.75 to 4.87 nm and 1.06 to 3.93 nm, respectively. It was observed that the ends of the isolated DWNTs were uncapped and it was also found that cobalt as the dominant composition of the catalyst played a vital role in the growth of DWNTs by this method. In addition, the pore structures of the DWNTs obtained were investigated by cryogenic nitrogen adsorption measurements.     

Multi-walled carbon nanotubes on amorphous carbon films

Nanotubular structures composed of layered graphite sheets or other layered materials have been studied intensely by both scanning and transmission electron microscopy. In this paper, we will show how graphite structures, that are inherent to the production process of the amorphous carbon support films, used for both SEM and TEM studies can be easily mistaken for the actual sample structures. We will further report that these artifacts appear in both commercial as well as homemade holey carbon support films on copper grids, and suggest that to successfully study the “real” nanotubular structures only support films made from materials other than carbon should be used.     

Growth process of carbyne crystals by synchrotron irradiation

Growth of small carbyne crystals in a thin amorphous carbon film has been observed by high-resolution transmission electron microscopy (HRTEM). The as-deposited film was composed of diamond and graphite crystallites of size 1 nm. Circular α-phase carbyne crystals predominantly grew to 20 nm in size and transformed into (α+β)-phase crystals with an elongated shape of 100 nm in length. The typical correlation during the transformation is (220)_α//(301)_(α+β). The c-axes of both grown crystals were oblique to the direction of the electron beam. The growth process of carbyne crystals will be discussed in terms of selective excitation of graphite crystallites by an SR beam.     

Co-synthesis, purification and characterization of single- and multi-walled carbon nanotubes using the electric arc method

We report the simultaneous synthesis of single-walled (SWNT) and multi-walled (MWNT) carbon nanotubes using the electric arc method. Two distinct deposits are formed when an electric arc is struck between a Co/Ni impregnated carbon electrode (anode) and a graphite electrode (cathode). The cobweb-like deposit harvested from the walls of the growth chamber (chamber deposit) contained SWNT bundles while the MWNTs were present in the deposit formed at the tip of the cathode (cathode deposit). Energy dispersive spectroscopy (EDS), thermogravimetric analysis (TGA), scanning electron microscopy (SEM) and Raman spectroscopy of each deposit confirmed the presence of respective carbon nanotubes along with carbonaceous impurities. A systematic purification procedure for separating SWNT bundles from the rest of the impurities is discussed. The optimal yield of purified SWNT bundles is ∼25 wt.% of the as-prepared chamber deposit.     

Decomposition of methane over Ni catalysts supported on carbon fibers formed from different hydrocarbons

In order to get pure hydrogen without CO and CO₂, the decomposition of methane into hydrogen and carbon fibers (CF) over Ni/carbon catalysts has been investigated. The reason for the use of carbon materials as supports is to avoid a costly elimination treatment of the catalyst from the formed CF. The Ni/carbon catalysts prepared by the impregnation of various carbon materials with Ni(NO₃)₂ dissolved in acetone, followed by reduction in hydrogen at 573 K, showed better catalytic performance in the decomposition of methane than those prepared by the impregnation with aqueous Ni(NO₃)₂. The Ni(40 wt%)/CF(from 1-C₄H₈) showed the highest catalytic performance giving a C/Ni value (moles of deposited carbon per mole of Ni on the catalyst) of 1920 until complete deactivation of the catalyst. SEM and TEM images of the CF formed from methane indicated their thickness to be ≈10-150 nm with the same size of Ni particles at their tips at the early stage of the decomposition of methane, but the thickness changed to ≈40-100 nm at the final stage of decomposition. An estimate of the average size of Ni crystallites from XRD measurement suggested that the carbons deposited from methane on various Ni/CF would modify the size of Ni crystallites during the reaction. It is suggested that ≈20 nm Ni crystallites are most active for the growth of carbon nanofibers.     

High-purity fibrous carbon deposit on the anode surface in hydrogen DC arc-discharge

A brittle porous deposit consisting of high-purity fibrous carbon products with a diameter of 25-100 nm was obtained on a heated anode surface in hydrogen DC arc-discharge. Hydrogen arc plasma was generated between a graphite cathode and a carbon/metal composite anode consisting of 1.0 at.% Fe, 0.6 at.% Co, 2.4 at.% Ni, and 0.4 at.% FeS. X-ray diffraction analysis revealed that the fibrous products had a turbostratic carbon structure, with an interlayer spacing of 0.346 nm. Elemental analysis showed that the fibrous products were composed of 98.4 mass% of carbon. Three types of nano-structured fibrous products were observed using transmission electron microscopy, (1) with a bamboo structure, (2) with a hollow core, and (3) without a hollow core. The formation of fibrous products was initiated by arc-generated metal particles with a diameter of 5-75 nm, then carbon for further growth was supplied by the decomposition of polycyclic aromatic hydrocarbons that were created by interactions between arc-generated carbon clusters and atomic hydrogen. The nano-structure of the fibrous products is thought to depend on the size and morphology of the catalytic metal particles. The synthesis conditions, microstructural characterization and the growth mechanism of the fibrous carbon products are reported.     

Synthesis of polyynes in a submerged electric arc in organic solvents

The products formed by the electric arc between graphite electrodes submerged in organic solvents like acetonitrile, n-hexane and methanol consist of a series of polyynes having the general chemical structure: H(CC)_mH (with m an integer 1,2,3,…m). The polyynes were separated through liquid chromatography (HPLC) and identified through their characteristic electronic absorption spectra recorded by a diode-array detector. When acetonitrile was arced at −40 °C, the entire polyynes series to m=9 (chain with 18 carbon atoms) were produced. Instead, at room temperature the entire polyynes series to m=8 (chain with 16 carbon atoms) were obtained. Similar results were obtained by arcing graphite electrodes in n-hexane and in methanol. The purest polyynes mixture is obtained in methanol while in n-hexane and in acetonitrile by-products are formed together with the polyynes series. In particular, in acetonitrile, a series of monocyanopolyynes was detected together with the normal polyynes series.The polyynes are formed by the vaporization of the elemental carbon from the graphite electrodes and dissolved in the solvent where the electrodes are submerged. The demonstration that the polyynes series H(CC)_mH is hydrogen-capped is based on several experimental data from electronic and FT-IR spectroscopy as well as their reactivity with a specific reagent for terminal acetylenes. Some chemical properties of the polyynes solutions (hydrogenation, oxidation) are also treated and discussed.     

Effect of arc current on characteristics of nanocarbons prepared by cryogenic arc discharge method

A variety of nanocarbons (nanohorns, nanoflowers and nanoclusters) could be prepared by arc discharge in cryogenic nitrogen with either graphite–graphite or iron–graphite electrodes manipulated by a strategy of automatic electrode delivering. Based on local thermal equilibrium assumption, magneto-hydrodynamic equations were taken into account for estimating the arc power efficiency of 60–84%, depending on the electrode combination. The effects of arc current on the morphology and yield of nanocarbons were investigated within a range of 75–150 A. Transmission electron microscopic analyses revealed that the synthesized product consisted of single-walled carbon nanohorns and multi-walled carbon nanoflowers with nominal diameters of 100–200 nm when graphite–graphite electrodes were employed but nanoclusters containing Fe nanoparticles inside carbon nanoshells with smaller size of 70–120 nm were mainly synthesized by iron–graphite electrodes subject to arc discharge in cryogenic nitrogen.     

Reduction of cathode carbon deposit by buffer gas outflow

The problem of the cathode carbon deposit arising from the carbon vapor developed by conventional arc discharge plasma during fullerene synthesis is addressed. This carbon deposit significantly reduces fullerene productivity, wastes valuable energy and graphite material, and presents a major obstacle in achieving desirable productivity and scalability. A study of the deposit microstructure and its mechanical and physical properties revealed that the deposit is of a three-dimensional fractal structure having a rigid and microporous structure.Analysis reveals that the graphite evaporation rate determines carbon deposit formation, soot content, fullerene yield, and vapor quality, as well as defines the deposit formation. The buffer gas outflow approach utilizes the injection of a buffer gas into the hot plasma zone, provides quick evacuation of the produced carbon vapor away from the plasma zone, allows usage of large currents and bigger electrodes without a significant reduction of fullerene yield, and reduces the carbon deposit by more then 99%. Based on this work, the elimination of the cathode deposit is a major step in the development of efficient, productive and scalable fullerene and nanotube technology.     

Electrochemical performance of pyrolytic carbon-coated natural graphite spheres

Natural graphite (NG) spheres were coated by pyrolytic carbon from the thermal decomposition of C₂H₂/Ar at 950 °C in a fluidized bed reactor. Scanning electron microscopy and secondary electron focused ion beam (FIB) images clearly showed that a pyrolytic carbon layer with a thickness of ∼250 nm was uniformly deposited on the surface of the NG spheres. Electrochemical performance measurements for the original and coated NG spheres as anode materials of a lithium-ion battery indicated that the first coulombic efficiency and cyclability were significantly improved in the coated sample. The reasons for this were investigated by analyzing structural characteristics, specific surface area, pore size distribution, and solid electrolyte interphase (SEI) film. Using a FIB workstation, we demonstrated, by cross-section imaging of a coated NG sphere that had experienced five electrochemical cycles, that the SEI film formed on the non-graphitic pyrolytic carbon surface became thinner (60-150 nm) and more uniform in composition compared with that on the surface of uncoated NG spheres; and the formation of an “internal SEI film” inside the NG spheres was also remarkably suppressed due to the uniform coating of pyrolytic carbon.     

Morphology of carbon/TiC composite films prepared by carbonization of polyimide/titania composites

Polyimide/titania (PI/TiO₂) composite was synthesized by in situ sol–gel polymerization, and carbon/titanium carbide (TiC) composite films were prepared by carbothermal reduction of the PI/TiO₂ composite at 1,600 °C under flowing argon. The structure and properties of the composites were studied by wide angle X-ray diffraction (WAXD), Fourier transform infrared spectrometer (FTIR), scanning electron microscopy, and energy dispersive X-ray spectroscopy. The carbon/TiC composite films exhibited metallic luster on the surface and compact structure in cross section with well dispersed TiC particles. WAXD intensity distribution revealed that TiC particles formed by tightly bonding between elemental carbon and titanium formed crystallites which as a filler provided tough films. The results indicated that heat treatment of PI/TiO₂ under argon is a promising method for preparing tough carbon composite films.     

Role of the cathode deposit in the carbon arc for the synthesis of nanomaterials

The atmospheric pressure carbon arc in helium is an important method for the production of nanomaterials. Typical arcs operate in a dc mode between a graphite anode, which is consumed, and a cathode which may be a lower melting point material. During arc operation, a carbon deposit is formed on the cathode surface. This deposit may contain different forms of the synthesised fullerenes. It is shown that this deposit plays a crucial role in conducting the arc current. Temperature measurements demonstrate that a sufficiently large area of the cathode deposit is hot enough for thermionic emission to be the source of most of the arc current. Due to the deposit’s low thermal conductivity, the cathode behind the deposit does not reach its melting point. The role of the deposit in emitting electrons can probably be generalized for other arc synthesis methods with consumed anodes.     

The synthesis and microstructure of morph-genetic TiC/C ceramics

Wood is a natural material with a novel and ordered hierarchical structure. In the present work, it is used as a bio-template to produce morph-genetic TiC/C ceramics. This is obtained by infiltrating the carbon preform pyrolyzed from wood with tetrabutyl titanate. It was subsequently sintered at 1400 °C to produce the final ceramic structure.By observing the microstructure under the scanning electron microscope and the transmission electron microscope, the morph-genetic TiC/C ceramics are shown retaining the complex morphology of the original template structure. The crystalline TiC was formed through the reaction of tetrabutyl titanate with carbon preform, and it was distributed mainly at the surface layer of the cellular wall. During the conversion of wood into carbon preform, the specific surface area of samples increased from 28.2 to 35.7 m² g⁻¹, and its porosity also increased from 64.4% to 80.3%. However, during the conversion of carbon preform into morph-genetic ceramics, the specific surface area of samples decreased from 35.7 to 33.8 m² g⁻¹, and its porosity also decreased from 80.3% to 76.5%. At the synthesis process, the variation of pore-size distribution is mainly in the range from 0.1 to 1 μm.     

Synthesis of multiwall carbon nanotubes by electric arc discharge in liquid environments

This work reports the experimental results from the production of multiwall carbon nanotubes (MWCN) synthesized by an electric arc discharge performed in liquid environments between pure graphite electrodes. Both liquid nitrogen and deionised water were suitable for a successful synthesis of this form of carbon aggregation. We report a successful synthesis of MWCN by arc discharge submerged in deionised water. Electron microscopy observations of both the reaction products and the surface of the as-synthesized raw material showed the presence of structural degradation of the MWCN, which probably operates after their growth at the cathode. The degradation is tentatively ascribed to a combination of overheating and high current density experienced by the as-synthesized MWNT, which can be caused by the loose structure of the as-deposited material. The damage appeared to be less severe in water environments, probably owing to the better cooling capacity of water relative to liquid nitrogen.     

Proportion and dispersion of rhombohedral sequences in the hexagonal structure of graphite powders

Using the Phase Matrix Method, we derive a calculation scheme giving the analytical form of the intensity diffracted by graphite crystallites containing rhombohedral stacking faults. The physical model accounts for all possible types of interstratification of the hexagonal and rhombohedral forms in graphite. The relevant statistical parameters are the proportion of the rhombohedral phase, its “dispersion” within the hexagonal structure and the average number of graphene layers in the crystallites. A mean-squares refinement program allows calculation of the (10?) diffraction profiles and the X-ray data reduction into physically meaningful statistical parameters. In the case of powdered materials, the parameters describing the rhombohedral phase are found to depend on the material origin and on its preparation process. Reducing the grain size by milling always increases the proportion of the rhombohedral phase whereas its dispersion in the hexagonal structure can significantly vary, depending upon both raw material type and milling conditions. X-ray diffraction profile fitting also evidenced the fact that compacting a powder of crystallites in which the two phases are segregated (coexistence of large rhombohedral and hexagonal crystallographic domains) leads to the destruction of the large domains constituting both phases and results in strong inter-mixing of the two crystalline forms.     

Evaluation of carbonized medium-density fiberboard for electrical applications

The conversion of wood-based fiberboard materials into crack-free, monolithic, porous hard carbons is of significant interest due to their ability to perform in a multifunctional capacity. Three varieties of carbonized medium-density fiberboard (c-MDF) were studied for electrical, mechanical, and structural properties. X-ray diffraction data suggested that the volume fraction of large turbostratic crystallites increased with carbonization temperature (T_carb). The volume fraction of large turbostratic crystallites had a positive correlation with elastic modulus and electrical conductivity. The c-MDF materials were approximately isotropic with respect to elastic modulus and exhibited increasing stiffness with increasing T_carb (up to 4.5 GPa). Between 600 and 1400 °C, the electrical resistivity of c-MDF varied by seven orders of magnitude. The electrical resistivity of the hard carbon material in c-MDF 1400 °C was found to be within about an order of magnitude of polycrystalline graphite.     

Carbon atmosphere effect on Ti3SiC2 based composites made from TiC/Si powders

The effect of carbon activity and CO pressure in the furnace atmosphere is investigated with respect to the phase reactions during heat treatment of TiC/Si powders. Special attention is given to the production and decomposition of Ti₃SiC₂. Samples were heated in graphite and alumina furnaces, connected to a dilatometer which enabled in situ analysis of the phase reactions. The phase compositions of the heat treated samples were determined by X-ray diffraction. The reducing atmosphere of the graphite furnace enhanced the reactivity of the starting powder and enabled phase reactions to take place at a lower temperature than in the alumina furnace. TiSi₂ and SiC phases formed at temperatures below the melting point of Si and were continuously consumed at higher temperatures. Ti₃SiC₂ formed at the melting point of Si regardless of furnace atmosphere. No decomposition of the Ti₃SiC₂ was observed in either furnace.