Graphitization Of Amorphous Carbon Research Articles

Electron Energy Loss Spectroscopy (EELS) of Iron Fischer–Tropsch Catalysts

Electron energy loss spectroscopy (EELS), X-ray photoelectron spectroscopy (XPS), and transmission electron microscopy have been used to study iron catalysts for Fischer-Tropsch synthesis. When silica-containing iron oxide precursors are activated in flowing CO, the iron phase segregates into iron carbide crystallites, leaving behind some unreduced iron oxide in an amorphous state coexisting with the silica binder. The iron carbide crystallites are found covered by characteristic amorphous carbonaceous surface layers. These amorphous species are difficult to analyze by traditional catalyst characterization techniques, which lack spatial resolution. Even a surface-sensitive technique such as XPS shows only broad carbon or iron peaks in these catalysts. As we show in this work, EELS allows us to distinguish three different carbonaceous species: reactive amorphous carbon, graphitic carbon, and carbidic carbon in the bulk of the iron carbide particles. The carbidic carbon K edge shows an intense "pi*" peak with an edge shift of about 1 eV to higher energy loss compared to that of the pi* of amorphous carbon film or graphitic carbon. EELS analysis of the oxygen K edge allows us to distinguish the amorphous unreduced iron phase from the silica binder, indicating these are two separate phases. These results shed light onto the complex phase transformations that accompany the activation of iron catalysts for Fischer-Tropsch synthesis.

Characterization of the Carbon-Coating of LiFePO4 by Transmission Electron Microscopy and Raman Spectroscopy

AbstractWe present the properties of the carbon layer deposited at the surface of the LiFePO4 particles. Characterizations include scanning electron microscopy, high-resolution transmission electron microscopy, and Raman scattering spectroscopy. Analuysis of Raman spectra reveals that the carbon deposit is hydrogenated with very small hydrogen/carbon ratio, so that it belongs to the family of the amorphous graphitic carbon. It is expected to have the same properties (small hardness, high electronic conductivity) that favor both the Li diffusion from the LiFePO4 bulk and the charge-discharge rate of the cell.

Template synthesis of microporous carbon for direct methanol fuel cell application

Ordered microporous carbon with a structure of amorphous carbon core and graphitic carbon shell was prepared using hydrogen-form zeolite Y as the template. Impregnation and chemical vapor deposition methods were employed to infiltrate carbon in the pores of the template. Physical adsorption of nitrogen, X-ray diffraction, thermogravimetric analysis, field-emission scanning electron microscope, and field-emission transmission electron microscope techniques were employed to study the structural and morphological properties of the samples. The electrochemical properties of Pt supported on the carbon samples were examined and compared with a commercial catalyst. It was observed that Pt catalyst supported on a carbon with a core/shell structure has a higher specific activity for room-temperature methanol oxidation than the commercial catalyst.

Role of the catalyst particle size in the synthesis of single-wall carbon nanotubes

It is argued that the growth mechanism of single-wall carbon nanotubes (SWNTs) is a kind of solid–liquid–solid (SLS) catalytic graphitization of non-graphitic forms of carbon (predominantly amorphous carbon, condensed at early stages of the carbon vapor relaxation) catalyzed by molten supersaturated carbon–metal nanoparticles. The liquefaction of nanoparticles of the catalytic metals in contact with the amorphous carbon below the equilibrium eutectic temperature of the corresponding metal–carbon alloys is considered as a decisive condition for the SWNT formation both at the nucleation and at the growth stages. It leads to a pronounced increment of carbon solubility (up to 50 at.% C), which alters the character of the interaction of the precipitated graphitic form of carbon with the catalyst surface and presumably can result in the SWNT nucleation. Furthermore, it lowers the energy barrier between the initial amorphous and final graphitic carbon phases. The experimental dependencies of the SWNT abundance in the soot on the synthesis temperature, pressure, carrier gas flow velocity, and the initial catalyst content in the target support the idea of the a “catalyst particle size window” in which the supersaturated metal–carbon alloy may remain liquid below the equilibrium eutectic temperature.

Photochemical Effects in Carbide Synthesis of d-group Transition Metals (Ti, Zr; V, Nb, Ta; Cr, Mo, W) in a Solar Furnace at PSA (Plataforma Solar de Almerı´a)

During the course of our recent series of solar carbide and carbonitride synthesis work for d-group transition elements (Ti,Zr;V,Nb,Ta;Cr,Mo,W), we detected several intriguing evidences apparently indicative of involvement of photochemical effects in some reactions under solar radiation. 1) Acceleration of graphitization of amorphous carbon during carburization of W under solar radiation. 2) Promoted formation of high temperature mono-carbide phase of Mo under solar radiation at a temperature lower than the one indicated in equilibrium phase diagram. 3) Formation of sub-carbide Ta2C besides mono-carbide TaC in the carburization of Ta with amorphous carbon under solar radiation. Only mono-carbide TaC formed by the carburization of Ta with graphite under solar radiation in accordance with the equilibrium phase diagram. These features were already published individually elsewhere. This article is the brief summary of the reported facts in our earlier publications. Although the results so-far-obtained are rather primitive and still lacking quantitative characterization, there seems to be no doubt that solar materials processing would open new routes for unique products which cannot be prepared through traditional industrial manufacturing processes.

CHARACTERIZATION OF ta-C AND GRANULAR Co-C FILMS PREPARED BY PULSED FILTERED VACUUM ARC DEPOSITION

A pulsed filtered vacuum are deposition system was used to prepare ta-C thin films and granular Co-C films. The ta-C films prepared at various substrate bias voltages were characterized using Raman spectroscopy and spectroscopic ellipsometry of which the results confirmed that these ta-C films exhibit high sp 3 fraction of over 80%. The composition of the granular Co-C films prepared by the same system was determined by non-Rutherford backscattering spectrometry. The properties of these Co-C Films, as deposited and after vacuum annealing at various temperatures, were studied using Raman spectroscopy, electrical measurements, magnetic measurements by a SQUID magnetometer, atomic force microscopy and magnetic force microscopy. It was found that the dependence of the Raman spectra of these films on annealing temperature was associated with the formation and dissociation of a cobalt carbide phase and the graphitization of amorphous carbon. The magnetic properties showed complicated composition and annealing temperature dependence. The optimum annealing temperature for the maximum coercivity was found to depend on the composition of the film. For a film of Co 65 C 35 after annealing at 623K in vacuum for one hour, the coercivity was measured to be 460 Oe at 300K and 1380 Oe at 3K. Clear MFM images of the domain structures were observed for films after annealing at sufficiently high temperature, showing that there was perpendicular magnetic anisotropy in these films. A nearly-temperature-independent electrical resistance in the range from 20K to 300K was also observed. A more detailed analysis indicated that the low temperature electrical transport is consistent with a theory for granular metal films.

Characterization of pore size distributions on carbonaceous adsorbents by DFT

Carbonaceous adsorbents are amorphous graphitic carbon, consisting of SP 2 hexagonal carbon layers with different sizes of pores ranging from micropores to macropores. Nitrogen adsorption is a standard tool for determination of pore size distribution (PSD). In the present work, we carried out extensive adsorption studies of a series of carbonaceous adsorbents including PAN-based activated carbon fibers, coal activated carbon and pitch carbon bead. The PSDs of samples studied were calculated by employing the regularization method according to Density Functional Theory (DFT). It was shown that adsorption measurement provided profound insight into the structural heterogeneity of carbonaceous adsorbents.

Evaluation of Boundary-Enhancement Additives for Perfluoropolyethers

Six additives were synthesized and evaluated as boundary lubrication enhancers for perfluoropolyalkylethers. These additives included a phosphate, a thiophosphate, a β-diketone, a benzothiazole, an amide/thiol and a sulfite. These additives were evaluated in a vacuum four-ball apparatus, at a one weight percent concentration in a perfluoropolyalkylether based on hexafluoropropene oxide. Tests were performed in vacuum (<6.7 × 10−4 Pa), at room temperature (∼23°C), at an initial Hertzian stress of 3.5 GPa (200N load), and a sliding velocity of 28.8 mm/sec (100 rpm). Infrared (IR) and Raman spectroscopies were used to analyze the steel (AISI 52100) specimens after testing. Wear rates for each formulation were determined from the slope of wear volume as a function of sliding distance. All additives yielded reductions in mean wear rates of at least 50 percent, with the exception of the benzothiazole which had no effect. Two of the additives, an amide/thiol and a sulfite, reduced the mean wear rate by at least 80 percent. IR and Raman analysis indicated the severity of wear can be correlated to the amount of surface fluorinated polymeric acid species (RfCOOH) and graphitic amorphous carbon, in and around the wear scar. Presented as a Sociey of Tribologists and Lubrication Engineers paper at the World Tribology Congress in London, United Kingdom, September 8–12, 1997

Attempted preparation of diamond-like carbon nitride by explosive shock compression of poly(methineimine)

Explosive shock compression of poly(methineimine) produced a large portion of amorphous graphitic carbon nitride of composition CN 0.2 and small amounts of diamond-like carbon containing a slight amount of nitrogen. Codeposition of the two materials suggests the possibility of cubic phase transformation of heterocyclic C-N compounds under well-designed shock-compression conditions.

Chemical vapour deposition of diamond at low substrate temperatures using fluorinated precursors

Diamond deposition from the fluorinated hydrocarbons CF 4 and CHF 3 was investigated in a hot filament chemical vapour deposition (HFCVD) system. The systems H 2CF 4, H 2CHF 3 and H 2CH 4 were compared under equivalent conditions. Our results demonstrate that below 650 °C the diamond growth rate for H 2CHF 3 gas mixtures is higher than that for the H 2CH 4 system. Even at 400 °C it was possible using H 2CHF 3 to measure considerable growth rates with in situ laser interferometry. Mass spectrometry proved the formation of HF at a significant concentration for the system H 2CHF 3. This is due to the abstraction of F from CHF 3 by atomic hydrogen. CF x radical species are postulated from CHEMKIN calculations. They are supposed to play an important role in the diamond growth process using CHF 3. With H 2CF 4 mixtures neither diamond nor amorphous graphitic carbon could be deposited. Mass spectrometry showed the inertness of CF 4 under the conditions of our investigations with the absence of the F abstraction reaction.

X-ray diffraction and thermoanalytical investigations of amorphous carbons derived from C60

Abstract We have synthesized two kinds of amorphous carbon products starting from C 60 , one by low-temperature oxidation and the other by vacuum-annealing in a sealed tube at 900°C. Our characterization studies indicate that the former has a nominal composition C 5 O and consists of two amorphous phases one of which has an amorphous C 60 structure, and may perhaps be a C 60 polymer, whereas the other has an amorphous graphitic carbon structure. The product obtained by vacuum-annealing of C 60 has a negligible oxygen content and is single phase with an amorphous graphitic carbon structure. The TG-DTA thermograms of both these materials in oxygen exhibit close similarity with respect to their ignition temperatures and the two-step burning process. However, their heats of combustion differ. The TG-DTA studies in a helium atmosphere followed by X-ray characterization of the residue indicate that although vacuumamorphized C 60 is stable and remains amorphous up to 1000°C the amorphous product C 5 O exhibits appreciable volatility perhaps due to oxygen-induced instability. The volatilization proceeds in two steps and the gas phase may contain carbon clusters. Together with volatilization, crystallization of amorphous C 5 O also occurs.

Mechanism of bias-enhanced nucleation and heteroepitaxy of diamond on Si

The nucleation of diamond deposited on Si is enhanced by a negative substrate bias. It is proposed that incident CHx+ ions with 70–80 eV energy stabilise a graphitic C overlayer, with some layers oriented normal to the surface. Diamond nucleates on the layer edges. The graphitic amorphous carbon acts as a buffer layer to accommodate the 2:3 lattice mismatch between diamond and Si, without requiring an excessive density of dislocations or dangling bonds.

Vibrational dynamics and structure of graphitic amorphous carbon modeled using an embedded-ring approach.

The Raman spectrum of graphitic amorphous carbon (g-C) is modeled with simple classical methods, using rings as medium-range structural units. g-C provides a simple [nearly two dimensional (2D) with a single type of atom and one dominant bond type] prototypic example of a large class of continuous random network (CRN) solids where ring or cluster vibrations cannot be decoupled from the network in which they are embedded. We determined the in-plane vibrational modes of n=4, 5, 6, 7, and 8-membered symmetric, planar carbon rings using bond-stretching and bond-angle bending force constants; an additional force constant couples each ring node to the surrounding network. Our results support current models of g-C as small rafts of nearly 2D CRN's comprised of \ensuremath{\approxeq}20% n=5, 60% n=6, and 20% n=7 rings, with very few n=4 and 8 rings. Extensions of the embedded-ring approach to more complex polyatomic and 3D CRN systems and to other probes of vibrational dynamics are discussed.

Electronic and transport properties of perfect sp2-bonded amorphous graphitic carbon.

The electronic and transport properties of a perfect amorphous graphitic carbon model constructed by Townsend et al. [Phys. Rev. Lett. 69, 921 (1992)] are studied by first-principles calculations within the local-density approximation. It is shown that such a random system with two-dimensional interaction in a three-dimensional structure is a high-resistivity metal. A zero-temperature resistivity of about 1160 \ensuremath{\mu}\ensuremath{\Omega} cm and a positive thermopower at low temperature are obtained. Localized states near the Fermi level are induced by subtle difference in the \ensuremath{\pi} bonding of different ring structures. Relaxation time due to elastic electron-electron scattering and the Fermi velocity are estimated to be 2.4\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}15}$ s and 2.61\ifmmode\times\else\texttimes\fi{}${10}^{4}$ m/s, respectively.

Complex dielectric function of amorphous diamond films deposited by pulsed-excimer-laser ablation of graphite.

Amorphous diamond films have been synthesized by pulsed excimer- (ArF) -laser ablation of graphite at room temperature. Detailed studies of the energy-dependent complex dielectric function in the energy range up to 40 eV have been carried out by Kramers-Kronig dispersion analysis of transmission electron-energy-loss spectroscopy and spectroscopic ellipsometry. Distinct from those of graphitic amorphous carbon and diamondlike carbon, the optical constants of amorphous diamond films are closely related to those of crystalline diamond, but with a smooth structure, a decrease in strength, and band shift towards lower energies. The variation of the dielectric function from crystalline diamond to amorphous diamond is analogous to that of crystalline and amorphous semiconductors such as Si and Ge. The extracted optical energy band gap reaches 2.6 eV, the highest value among the reported results for amorphous carbon films. The study of the dielectric function and its relation to the band structure of amorphous diamond films has also confirmed their diamond character. All of this information provides further evidence of the existence of amorphous diamond.

Infrared analysis of the irradiation effects in aromatic polyimide films

A Fourier transform infrared (FTIR) spectroscopic investigation of the ion-beam-induced chemical and structural modification in polyimide (PI) films is presented. Our experimental considerations provide a foundation for a quantitative evaluation of the infrared (IR) results for this thermally stable polymer on the deterioration of chemical bonding with irradiation fluence. It is found that chemical structure in PI is subjected to a severe alteration within the dose range from 1 × 10 14 to 1 × 10 15 B +/cm 2. The radical structure modification is completed only as irradiation reaches 1 × 10 16 B +/cm 2, at which point one may find that there exists a resemblance in profile shape between the IR spectrum of the high-dose irradiated PI film and the one-phonon states density (DOS) of amorphous graphite. This suggests that a structure similar to amorphous graphitic carbon films will be formed in the ion-beam-modified PI film when dose comes to a certain threshold. This “graphitization” process under ion bombardment is also supported by the experimental results from ultraviolet (UV)-visible spectroscopic study.

Anomalous electrical conduction in graphite-vaporized films

Abstract A transition to a higher electrical-conduction state is observed at temperatures between 30 K and 115K in graphite-vaporized films, depending on preparation conditions. This transition may be related to carbon clusters [C n , with n = 30–100] being contained in graphitic amorphous carbon.

Direct Pyrolysis of Raw Rice Husks for Maximization of Silicon Carbide Whisker Formation

A new alternate route for the production of SiC from rice husks has been described. The proposed single‐stage process has the advantage of reduced time of production and a much higher yield of SiC whiskers than the conventional two‐stage process. Direct pyrolysis of raw rice husks was carried out in a graphite furnace under vacuum and the reaction products were analyzed through macroscopic examination, XRD, SEM, and chemical analysis. Effects of time and temperature on the relative yields of the various reaction products have been determined over a wide range. The pyrolysis of raw rice husks has been found to be accompanied by four different competitive processes, viz., formation of SiC whiskers, formation of SiC polycrystals, crystallization of amorphous silica, and graphitization of amorphous carbon. Crystallization of silica and formation of SiC(w) are dominant processes at 1150° to 1310°C. At higher temperatures formation of SiC polycrystals and graphitization of carbon are more favored. A multistep pyrolysis to 1310°C was found to yield a still higher amount of SiC(w) than direct pyrolysis.

Modeling the graphitization of amorphous carbon

Three-dimensional ordering in carbon materials during graphitization has been studied by numerical simulation. At small values of the diameter of the graphite layers (∼1 nm), there is a region of values of the relative displacement of neighboring layers in which the interaction energy reaches a minimum at each point. There are no potential barriers between the points in this region. If the diameter of the neighboring graphite layers increases to infinity, only two fixed displacements with a minimum binding energy of the layers are possible. These particular displacements are characteristic of the graphite structure. The relative positions of neighboring graphite layers are thus determined by the size of these layers. The process which limits the formation of the threedimensional graphite structure is the increase in the diameter of the graphite layers.

Graphitization of amorphous carbon at high pressures to 15 GPa

Graphitization of amorphous carbon has been studied at high static pressures up to 15 GPa and high temperatures up to 1300 °C, primarily under diamond-stable conditions. Upon heating at 2 GPa, amorphous carbon turns into graphite at ∼900 °C. The temperature for the graphitization decreases with increasing pressure and approaches 600 °C at 10 GPa or higher. The graphitization process at 15 GPa comprises two stages over a temperature range 600–1300 °C. In the first stage, below 900 °C, graphite layers develop and the interlayer spacing decreases markedly. The second stage, above 1000 °C, corresponds to in-layer arrangement and partial development of three-dimensional ordering. At temperatures approaching diamond formation under this pressure, the graphitization does not acquire completion. Along 1300 °C isotherm, the structure and texture of graphite exhibit appreciable changes across the graphite-diamond equilibrium line and also across 11 GPa.