Carbon Carbide Research Articles

Foam-based optical absorber for high-power laser radiometry

We report damage threshold measurements of novel absorbers comprised of either liquid-cooled silicon carbide or vitreous carbon foams. The measurements demonstrate damage thresholds up to 1.6x10(4) W/cm(2) at an incident circular spot size of 2 mm with an absorbance of 96% at 1.064 microm. We present a summary of the damage threshold as a function of the water flow velocity and the absorbance measurements. We also present a qualitative description of a damage mechanism based on a two-phase heat transfer between the foam and the flowing water.

Temperature programed desorption of C2H4 from pure and graphite-covered Pt(111)

Ethylene adsorption on Pt(111) at 95K was studied by temperature programed desorption (TPD), low energy electron diffraction (LEED), and x-ray photoelectron spectroscopy. Ethylene desorbs reversibly at 112K and irreversibly at 255 and 280K. It is generally accepted that annealing of ethylene adsorbed on Pt(111) to 300K results in a dehydrogenation to ethylidyne through an ethylidene intermediate. This was observed by a hydrogen desorption peak at 300K. Also, hydrogenation of the adsorbed ethylene was observed by a small ethane desorption peak at 300K. Upon heating to 700K, the ethylidyne species will further dehydrogenate to carbidic carbon species with hydrogen desorption peaks at 460 and 640K. If the carbidic species is heated to higher temperatures (up to 1000K), it will further dehydrogenate and form graphitic islands which will accumulate by Ostwald ripening in larger islands at the step edges of the surface. After annealing the sample to 1000K, a statistically distributed 8×8 superstructure of these graphite islands is achieved, as interpreted from a ring pattern in the LEED data. The TPD results indicate that ethylene adsorption on Pt(111) results in the formation of graphitic islands upon heating to 1000K, contrary to previous conjectures of formation of a full graphite monolayer.

Chemistry of Methyl Formate with TiC(100): Comparison of Experiment with Density Functional Calculations

Titanium carbide is an important tribological material used as an antiwear coating on mechanical components. The TiC(100) surface has demonstrated interesting low-temperature chemistry with a variety of adsorbates, including methyl formate, the simplest ester and a model for this chemical functionality found in many aerospace lubricants. In an effort to identify and characterize surface adsorption and reaction products, we have undertaken a comparison of theoretical results employing density functional methods with experimental results using high-resolution electron energy loss spectroscopy (HREELS) obtained on clean, single-crystal TiC surfaces. The DFT results presented in this work have enabled the identification of an elusive surface reaction product with a characteristic vibration of 1120 cm-1 as a formyl (CHO) species bridging between a surface carbon and a titanium site. This result and others have demonstrated the importance of the lattice carbidic carbon atom in the adsorption of the breakdown products of methyl formate. Specifically, any atomic adsorbate and every reaction product with an unsatisfied valence on a carbon atom showed a theoretical preference for bonding with a lattice carbon rather than the titanium. Alternatively, reaction fragments containing oxygen preferred bonding to surface titanium sites through the oxygen atom, with the exception of carbon monoxide. The calculations show that the breakdown of methyl formate to methoxy and formyl groups, or to surface formate and methyl groups, is exothermic, while further decomposition reactions are endothermic. These results provide a rationale for the experimental observation of these surface reaction products at temperatures ranging from 150 to 300 K.

Effect of surface reactions of low-energy carbon ions on the secondary electron emission of TiN:O thin films

Low-energy secondary electron emission coatings are required for antimultipactor applications in several important technologies and their study and development is also a matter of scientific interest. For this purpose, titanium nitride was deposited on Si(100) substrates by reactive sputtering and the influence of low-energy carbon ion bombardment on the secondary electron emission yield of TiN:O coatings was studied. The composition and chemical bonds formed in the films after carbon ion implantation were analyzed by x-ray photoelectron spectroscopy (XPS). XPS revealed the formation of both carbidic and graphitic bonds. N was displaced while Ti-C bonds were formed up to a limiting value of 0.103 for the carbidic carbon atomic fraction, beyond which a graphitic surface layer was deposited. The secondary electron emission yields for TiN:O were measured before and after low-energy CHn+ ion bombardment and air exposure, and after heating. Initially, the carbon ion implantation reduced the secondary electron emission yield. Then an increase in secondary electron emission was obtained when the excess graphitic carbon was deposited on the sample. On the other hand, subsequent thermal treatment at 700 °C of the carburized samples produced a further reduction of the secondary electron emission yield. The maximum yields are about 53% lower for thermal annealed films than for similarly treated previously measured as-deposited layers. A narrowing of XPS peak line shapes is observed as a consequence of the annealing away of structural and chemical defects in the near-surface region. In addition, secondary electron emission (SEE) yield curves were used in a simulation of multipactor discharge. Both experimental multipactor threshold tests and computer multipactor simulations indicate that SEE yield values for low primary-electron energies are the most influencing parameters on multipactor threshold.

CO2 addition on the non-oxidative dehydro-aromatization of methane over MoMCM-22

This work aims at studying the effect of CO2 addition in the non-oxidative conversion of methane over a Mo-containing MCM-22 zeolite. Catalyst characterization made use of X-ray fluorescence, textural analysis, X-ray diffraction and diffuse reflectance spectroscopy. Molybdenum was present as highly condensed species with octahedral coordination, such as (Mo7O24)6 and MoO3 species. Catalytic tests were carried out at atmospheric pressure and 700 °C, with WHSV of 90gCH4 · gMo−1 h−1. Coke species were studied by thermogravimetric analysis and thermoprogrammed oxidation. Besides the carbidic carbon, two other types of coke were observed: one associated to molybdenum and the other associated to Bronsted acid sites located both on the catalyst surface and inside zeolite pores. The species associated to acid sites, known to be responsible for polyaromatics formation and catalyst deactivation, were reduced in a more significant way by the use of CO2, increasing the catalyst stability.

Microstructure of C<sub>f</sub>/SiC Composite with Precursor of Hexamethyldisilazane by Chemical Vapor Infiltration

A method of waved-thermal field chemical vapor infiltration was introduced. And interphases of silicon carbide layer and carbon layer were processed via the route. The preforms with the interfacial coatings were densified by method of forced-flow thermal-gradient chemical vapor infiltration (FCVI) employing hexamethyldisilazane (HMDS) as precursor material of the matrix. The matrix of the composites annealed at 1400°C consists of nano-polycrystalline silicon carbide. The configuration of fracture surface was observed by scanning electronic microscopy (SEM). The interphases behaved successfully as mechanical fuse for the reinforcing fibers.

Synthetic resin-bonded transition-metal carbide–carbon hetero-modulus ceramics

Hetero-modulus ceramics (HMC) present an opportunity to combine a ceramic matrix having high Young's modulus with inclusions of a material having a considerably lower value of Young's modulus. The manufacturing method for the powder transition-metal carbide–graphite HMC with synthetic binder was studied. A statistical model for the cured-pressed carbide–carbon powder composition with resin binder was developed employing the statistical fractional factorial experimental design with the 2 5–2 matrix type for several processing parameters. During the subsequent carbonisation of these compacts, the mutual influence between the carbide phase and the phenol–formaldehyde binder leads to the specific transformations in resin-bonded HMC. The pyrolysis reactions in the resin begin at lower temperatures and the products oxidise carbides with the formation of metal oxycarbides and oxides, so the behaviour of HMC depends strongly on the carbide content. The mass gain of carbonised compacts is compensated for by their swelling, so the bulk density in the wide range of contents does not vary after carbonisation. Possessing lower strength, the resin-bonded HMC have better thermal shock/fatigue resistance than materials hot-pressed at high temperatures.

Comparison of nanoindentation results obtained with Berkovich and cube-corner indenters

There is increasing interest in using sharp cube-corner indenters in nanoindentation experiments to study plastic properties. In combination with finite element methods, it is, for example, possible to extract stress–strain curves from load–displacement curves measured with differently shaped pyramidal indenters. Another example is the fracture toughness of coatings, which can be studied using cracks produced during indentation with cube-corner tips. We have carried out indentation experiments with Berkovich and cube-corner indenters on eight different materials with different mechanical properties. To gain information about the formation of pile-up and cracks, indentation experiments with cube-corner indenter were performed inside a scanning electron microscope (SEM) using a custom-built SEM-microindenter. The results show that reliable hardness and modulus values can be measured using cube-corner indenters. However, the fit range of the unloading curve has a much bigger influence on the results for the cube-corner than for the Berkovich tip. The unloading curves of a cube-corner measurement should, therefore, be carefully inspected to determine the region of smooth curvature, and the unloading fit range chosen warily. Comparison of the modulus results shows that there is no significant difference between cube-corner and Berkovich measurements. Also for hardness, no fundamental difference is observed for most of the investigated materials. Exceptions are materials, such as silicon nitride, cemented carbide or glassy carbon, where a clear difference to the hardness reference value has been observed although the modulus difference is not pronounced.

Investigation of the corrosion resistance of fuel micropellet cladding made of silicon carbide and pyrolytic carbon for the operating conditions of light-water reactors in nuclear power plants

The results of experimental and theoretical investigations of the corrosion of outer protective coatings, made of silicon carbide and pyrolytic carbon, of fuel microspherules in high-pressure water and water vapor in the range 350–950°C are presented. The tests lasted for times ranging from 300 to 11800 h were conducted for the operating conditions of light-water reactors.

Elasticity, strength, and toughness of single crystal silicon carbide, ultrananocrystalline diamond, and hydrogen-free tetrahedral amorphous carbon

In this work, the authors report the mechanical properties of three emerging materials in thin film form: single crystal silicon carbide (3C-SiC), ultrananocrystalline diamond, and hydrogen-free tetrahedral amorphous carbon. The materials are being employed in micro- and nanoelectromechanical systems. Several reports addressed some of the mechanical properties of these materials but they are based in different experimental approaches. Here, they use a single testing method, the membrane deflection experiment, to compare these materials’ Young’s moduli, characteristic strengths, fracture toughnesses, and theoretical strengths. Furthermore, they analyze the applicability of Weibull theory [Proc. Royal Swedish Inst. Eng. Res. 153, 1 (1939); ASME J. Appl. Mech. 18, 293 (1951)] in the prediction of these materials’ failure and document the volume- or surface-initiated failure modes by fractographic analysis. The findings are of particular relevance to the selection of micro- and nanoelectromechanical systems materials for various applications of interest.

Kinetic study of a ceria-promoted [formula omitted] catalyst in dry-methane reforming

The kinetics of dry-methane reforming has been studied over a ceria-promoted Mo 2 C / γ - Al 2 O 3 (3 wt% Ce, 30 wt% Mo) catalyst at temperatures between 800 and 900 ∘ C , and at CH 4 and CO 2 partial pressures up to 0.31 bar. Oxidation of the Mo 2 C catalyst, which has plagued earlier kinetic studies, was avoided by co-feeding CO to produce a CO : CO 2 ratio of 3. The activation energy obtained from the data fit to a power law rate expression of 45.5 kcal/mol was found to be similar to that of bulk Mo 2 C . However, the activity of the ceria-promoted catalyst, on a per unit mass of Mo 2 C , was significantly higher than that of the bulk and is attributed to the higher metal dispersion of the former. A reaction mechanism, involving CH 4 activation on Mo 2 C particles and CO 2 activation on both ceria and Mo 2 C particles, with carbidic carbon extraction by oxygen on ceria as the rate-determining step, is consistent with the derived kinetic model and the high activation energy.

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.

SiC–CNT nanocomposites: high pressure reaction synthesis and characterization

Silicon carbide–carbon nanotube composite was fabricated using the high pressure reactivesintering technique. Samples were synthesized at high pressures, 2 and 8 GPa,and temperatures, 1770 and 1970 K. Their structures were studied using x-raydiffraction, x-ray photoelectron spectroscopy, transmission electron microscopy, andRaman scattering techniques. The composites produced at high pressure havepronounced nanocrystalline structure (the mean crystallite size of the SiC matrixwas 32–37 nm) and very promising mechanical properties: fracture toughness of6.8–7.1 MPa m0.5 and Vickers hardness of 20–21 GPa.

Sintering shrinkage of WC–Co materials with different compositions

The composition of cemented carbides affects not only the properties of the finished product, but also changes the sintering behaviour. In this work the sintering of medium–coarse WC–Co materials has been investigated. Descriptions of the constitutive behaviour, uniaxial viscosity, viscous Poisson’s ratio and sintering stress, obtained from dilatometry experiments under uniaxial load were extended to cover different material compositions. The proposed models were used to express the linear shrinkage rate, and the model parameters fitted to shrinkage data from materials with different carbide grain size, cobalt content and carbon content. The influence of grain size distribution was also investigated. Finally, the shrinkage rate was used to numerically integrate for sintering shrinkage under different thermal cycles.

Growth of SiC–C nanocables on SiO 2 films derived by gaseous composition control using Ti

Hexagonal silicon carbide (α-SiC) and amorphous carbon coaxial nanocables were synthesized on silica films that were cobated with nickel (Ni) by flowing a methane (CH 4) and hydrogen (H 2) mixture in presence of titanium (Ti). Ti powder plays an important role in the growth of the SiC–C nanocables by controlling the gas composition in the growth system. The thickness of the Ni also affects the growth characteristics of SiC–C nanocables. The density of the nanowires was found to be dependent on the relative location of the Ti powder and the substrates. It is possible that Ti decomposes CH 4 to form titanium carbide (TiC) and create a high H 2 concentration in the system.

Mechanistic Study of Methane Reforming with Carbon Dioxide on a Supported Nickel Catalyst

The nature of activation of methane on supported nickel catalyst has been investigated by means of temperature programmed surface reaction (TPSR), temperature programmed desorption (TPD), X-ray photoelectron energy spectroscopy (XPS) and pulse reaction and a synergetic mechanism of carbon dioxide reforming with methane is tentatively postulated. TPSR, TPD and XPS results indicate that carbidic Cα, carbonaceous Cβ and carbidic clusters Cγ surface carbon species formed by decomposition of methane show different surface mobility, thermal stability and reactivity. Cα and Cβ species on the nickel surface are thermally unstable and can be rapidly converted into Cγ species upon increasing temperature. The carbidic carbon is a very active and important intermediate in carbon dioxide reforming with methane and the carbidic clusters Cγ species might be the precursor of the surface carbon deposition.

Mechanistic study on the methane activation over various supported molybdenum carbide catalysts with isotopic tracer methods

The mechanism of methane activation in CH 4 CO 2 reaction over various unsupported and supported Mo 2C catalysts was studied by means of an isotopic tracer technique employing 13CH 4 and CD 4 as tracers. Incorporation of carbide carbons into CO took place in the case of CH 4 CO 2 reaction over unsupported Mo 2C, which was retarded significantly by dispersing Mo 2C onto ZrO 2 support. Methane dissociation step was also retarded significantly over supported Mo 2C catalysts, and the durability of the catalysts was improved by preventing inactive carbon accumulation. By dispersing Mo 2C on Al 2O 3, novel active acidic sites were produced, where CH 4 CD 4 exchange reaction took place even below room temperature through an associative mechanism.

Boron carbide and boron carbonitride thin films as protective coatings in ultra-high density hard disk drives

Boron carbide (B 4C) and boron carbon nitride (B x C y N z ) thin films were synthesized by pulsed DC magnetron sputtering. Effects of target power, target pulse frequency, substrate bias and pulse frequency on surface roughness were studied by AFM over sampling areas of 20 μm × 20 μm. For B 4C, the combination of target power of 75 W/50 kHz and substrate bias of −100 V/2 kHz results in the smallest surface roughness. Compressive stress in these films is around 2.5 GPa. These B 4C films have a hardness of 30 ± 5 GPa and reduced modulus of 250 ± 50 GPa. Nitrogen incorporation into B 4C films, which gives B x C y N z thin films, has a beneficial effect. When deposited under similar conditions with substrate bias pulsed at −100 V / 20 kHz, the root-mean-square surface roughness decreases to 0.19 nm, compared with 0.28 nm for B 4C. The hardness of B x C y N z is 20 ± 3 GPa, and reduced modulus is 210 ± 30 GPa. Auger electron spectroscopy was used to characterize the film composition. High-resolution cross-sectioned TEM images and diffraction show that both films are amorphous. Corrosion studies indicate that B x C y N z is a better protective coating for hard disk substrates than B 4C and CN x films. This is attributed mainly to the smoother morphology of B x C y N z films.

Dry reforming kinetics over a bulk molybdenum carbide catalyst

A bulk molybdenum carbide catalyst was used to determine a rate expression for dry methane reforming at temperatures between 800 and 900 ∘ C , with CH 4 and CO 2 partial pressures from 0.15 to 0.32 bar and from 0.11 to 0.23 bar, respectively. Intrinsic kinetic measurements were made possible by co-feeding CO along with reactants at a constant CO : CO 2 feed ratio of 3.0 in order to prevent oxidation of the Mo 2 C catalyst to MoO 2 . Methane conversion data as a function of inverse methane space velocity were obtained under conditions of negligible gas–solid mass transfer resistance and subjected to differential analysis. Partial pressure and rate data from all experiments were agglomerated and fit to a power law rate expression, which gave reaction orders of 1.2 and 0.7 for CH 4 and CO 2 , respectively. The activation energy obtained in this temperature range was 172 kJ/mol, higher than reported values for supported nickel or noble metals for the same reaction. It is proposed that the rate data are consistent with a mechanism that involves a combination of carbidic carbon exchange and chemisorbed CO 2 , accounting for the high activation energy and the susceptibility of the catalyst to oxidation.

Novel silicon carbide/polypyrrole composites; preparation and physicochemical properties

Novel silicon carbide/polypyrrole (SiC/PPy) conducting composites were prepared using silicon carbide as inorganic substrate. The surface modification of SiC was performed in aqueous solution by oxidative polymerization of pyrrole using ferric chloride as oxidant. Elemental analysis was used to determine the mass loading of polypyrrole in the SiC/PPy composites. Scanning electron microscopy showed the surface modification of SiC by PPy. PPy in composites was confirmed by the presence of PPy bands in the infrared spectra of SiC/PPy containing various amounts of conducting polymer. The conductivity of SiC/PPy composites depends on PPy content on the surface. The composite containing 35 wt.% PPy showed conductivity about 2 S cm −1, which is in the same range as the conductivity of pure polypyrrole powder prepared under the same conditions using the same oxidant. PPy in the composites was clearly detected by X-ray photoelectron spectroscopy (XPS) measurements by its N1s and Cl2p peaks. High resolution scans of the C1s regions distinguished between silicon carbide and polypyrrole carbons. The fraction of polypyrrole at the composite surface was estimated from the silicon and nitrogen levels. The combination of XPS and conductivity measurements suggests that the surface of the SiC/PPy composites is polypyrrole-rich for a conducting polymer mass loading of at least 12.6 wt.%.