Showing papers on "Carbide published in 2010"

High-rate electrochemical capacitors based on ordered mesoporous silicon carbide-derived carbon.

Abstract: Microporous carbons, produced by selective etching of metal carbides in a chlorine-containing environment, offer narrow distribution of micropores and one of the highest specific capacitances reported when used in electrical double layer capacitors (EDLC) with organic electrolytes. Previously, the small micropores in these carbons served as an impediment to ion transport and limited the power storage characteristics of EDLC. Here we demonstrate, for the first time, how the preparation and application of templated carbide-derived carbon (CDC) can overcome the present limitations and show the route for dramatic performance enhancement. The ordered mesoporous channels in the produced CDC serve as ion-highways and allow for very fast ionic transport into the bulk of the CDC particles. The enhanced transport led to 85% capacitance retention at current densities up to ∼20 A/g. The ordered mesopores in silicon carbide precursor also allow the produced CDC to exhibit a specific surface area up to 2430 m2/g and a ...

Stability and Reactivity of ϵ−χ−θ Iron Carbide Catalyst Phases in Fischer−Tropsch Synthesis: Controlling μC

Abstract: The stability and reactivity of ϵ, χ, and θ iron carbide phases in Fischer−Tropsch synthesis (FTS) catalysts as a function of relevant reaction conditions was investigated by a synergistic combination of experimental and theoretical methods. Combined in situ X-ray Absorption Fine Structure Spectroscopy/X-ray Diffraction/Raman Spectroscopy was applied to study Fe-based catalysts during pretreatment and, for the first time, at relevant high pressure Fischer−Tropsch synthesis conditions, while Density Functional Theory calculations formed a fundamental basis for understanding the influence of pretreatment and FTS conditions on the formation of bulk iron carbide phases. By combining theory and experiment, it was found that the formation of θ-Fe3C, χ-Fe5C2, and ϵ-carbides can be explained by their relative thermodynamic stability as imposed by gas phase composition and temperature. Furthermore, it was shown that a significant part of the Fe phases was present as amorphous carbide phases during high pressure FT...

Ionic Liquid Monomers and Polymers as Precursors of Highly Conductive, Mesoporous, Graphitic Carbon Nanostructures

Abstract: In this contribution a template-free preparation of mesoporous graphitic carbon nanostructures with high electric conductivity is presented, using ionic liquid monomers or poly(ionic liquid) polymers as carbon precursors. The carbonization was performed in the presence of FeCl2 at temperatures between 900 and 1000 °C. It was found that FeCl2 plays a key role in controlling both the chemical structure and the texture morphology of the graphitization process. A detailed investigation on the carbonization process demonstrated that 900 °C is a threshold temperature where a synergistic formation process enables the development of the superior physical properties, such as large surface area and low resistance. The as-synthesized carbon products are graphitic, mesoporous, and highly conductive, as proven by XRD and TEM characterizations and conductivity measurements. Via an acid etching process, iron and iron carbide nanoparticles, the remainder of the primary catalyst, can be removed, leaving pure mesoporous ca...

Effect of machining parameters on surface roughness and tool wear for 7075 Al alloy SiC composite

Abstract: In the present study, an attempt has been made to investigate the influence of cutting speed, depth of cut, and feed rate on surface roughness during machining of 7075 Al alloy and 10 wt.% SiC particulate metal-matrix composites. The experiments were conducted on a CNC Turning Machine using tungsten carbide and polycrystalline diamond (PCD) inserts. Surface roughness of 7075Al alloy with 10 wt.% SiC composite during machining by tungsten carbide tool was found to be lower in the feed range of 0.1 to 0.3 mm/rev and depth of cut (DOC) range of 0.5 to 1.5 mm as compared to surface roughness at other process parameters considered. Above cutting speed of 220 m/min surface roughness of SiC composite during machining by PCD tool was less as compared to surface roughness at other values of cutting speed considered. Wear of tungsten carbide and PCD inserts was analyzed using a metallurgical microscope and scanning electron microscope. Flanks wear of carbide tool increased by a factor of 2.4 with the increase of cutting speed from 180 to 240 m/min at a feed of 0.1 mm/rev and a DOC of 0.5 mm. On the other hand, flanks wear of PCD insert increased by only a factor of 1.3 with the increase of cutting speed from 180 to 240 m/min at feed of 0.1 mm/rev and DOC 0.5 mm.

Effects of recrystallization annealing temperature on carbide precipitation, microstructure, and mechanical properties in Fe-18Mn-0.6C-1.5Al TWIP steel

Abstract: The microstructure, dimple structure, and mechanical properties of a cold-rolled Fe–18Mn–0.6C–1.5Al TWIP steel were investigated as a function of annealing temperature. The recrystallization started at 600 °C and finished at 700 °C for the holding time of 10 min. The coarsening rate of recrystallized grains was increased over about 840 °C and Rockwell hardness was greatly decreased between 800 and 900 °C, which shows a good agreement with the equilibrium dissolution temperature of M 3 C carbides. The reversion of the tensile strength occurred between 700 and 800 °C because of the carbide precipitation hardening. The precipitation-time-temperature diagram was generated by dilatometric tests, showing a nose temperature of 800 °C. The dimple size was decreased to 700 °C and then increased again with higher annealing temperature, having a strong proportional relationship with austenite grain size.

Methane dry reforming with CO2: A study on surface carbon species

Abstract: Methane dry reforming with a mixture of 29% CO 2 and 71% CH 4 over 8 wt% NiMgAl 2 O 4 in a plug flow reactor with temperature programmed mode has been investigated. It was established that in sequential reactions the catalyst was deactivated due to graphite like carbon deposit measured by means of X-ray diffraction (XRD), transmission electron microscopy (TEM) while by X-ray photoelectron spectroscopy (XPS) only Ni–carbide was observed. The discrepancy found by XPS and TEM can be explained by the different types of carbon species, i.e. it is assumed that after the first reaction mostly carbide is deposited, whereas after several subsequent reactions the carbon on the surface is graphitized forming carbon nanotubes (TEM). Addition of 0.5 wt% gold to NiMgAl 2 O 4 improves the catalytic activity and on gold containing bimetallic catalyst the formation of nanotubes is vanished. The results are interpreted by the formation of non-crystalline NiC x which – depending on the conditions – is transferred to graphitic carbon species.

Density Functional Theory Study of Iron and Cobalt Carbides for Fischer-Tropsch Synthesis

Abstract: Carbides are important phases in heterogeneous catalysis. However, the understanding of carbide phases is inadequate: Fe and Co are the two commercial catalysts for Fischer−Tropsch (FT) synthesis, and experimental work showed that Fe carbide is the active phase in FT synthesis, whereas the appearance of Co carbide is considered as a possible deactivation cause. To understand very different catalytic roles of carbides, all the key elementary steps in FT synthesis, that is, CO dissociation, C1 hydrogenation, and C1+C1 coupling, are extensively investigated on both carbide surfaces using first principles calculations. In particular, the most important issues in FT synthesis, the activity and methane selectivity, on the carbide surfaces are quantitatively determined and analyzed. They are also discussed together with metallic Fe and Co surfaces. It is found that (i) Fe carbide is more active than metallic Fe and has similar methane selectivity to Fe, being consistent with the experiments; and (ii) Co carbide ...

Ceramic composite materials containing carbon nanotube-infused fiber materials and methods for production thereof

Abstract: In various embodiments, composite materials containing a ceramic matrix and a carbon nanotube-infused fiber material are described herein. Illustrative ceramic matrices include, for example, binary, ternary and quaternary metal or non-metal borides, oxides, nitrides and carbides. The ceramic matrix can also be a cement. The fiber materials can be continuous or chopped fibers and include, for example, glass fibers, carbon fibers, metal fibers, ceramic fibers, organic fibers, silicon carbide fibers, boron carbide fibers, silicon nitride fibers and aluminum oxide fibers. The composite materials can further include a passivation layer overcoating at least the carbon nanotube-infused fiber material and, optionally, the plurality of carbon nanotubes. The fiber material can be distributed uniformly, non-uniformly or in a gradient manner in the ceramic matrix. Non-uniform distributions may be used to form impart different mechanical, electrical or thermal properties to different regions of the ceramic matrix.

Microstructure and Mechanical Properties of an Ultrahigh-Strength 40SiMnNiCr Steel during the One-Step Quenching and Partitioning Process

Abstract: The quenching and partitioning (Q&P) process is a novel heat treatment for the enhancement of the strength level of steels without a significant deterioration of ductility. In this work, a study of 40SiMnNiCr steel subjected to the one-step Q&P process is presented. The study results suggest that the strength level of the steel subject to one-step Q&P increases at first and subsequently decreases with the partitioning time because of the synergistic effect of the increase in the retained austenite fraction, the decrease in carbon supersaturation in martensite, the change in the dislocation density in martensite, and the formation of transition carbide. The presence of the transition carbide markedly increases the strength level of the one-step quenched and partitioned steel, with the ultimate tensile strength (UTS) over 2400 MPa and the ductility more than 10 pct during partitioning at 453 K (180 °C) for 180 seconds. Isothermal martensite transformation possibly occurred in this medium-carbon ferrous alloy during the one-step Q&P processing. Meanwhile, in the early stages of the low-temperature partitioning process, carbon partitioning from martensite to austenite plays a dominant role in the carbon redistribution competitions. In addition, the relationship between the microstructure and mechanical properties of the one-step quenched and partitioned steel is discussed.

Low-temperature martensitic transformation and deep cryogenic treatment of a tool steel

Abstract: The tool steel X220CrVMo 13-4 (DIN 1.2380) containing (mass%) 2.2C, 13Cr, 4V, 1Mo and the binary alloy Fe–2.03 mass% C were studied using transmission electron microscopy, Mossbauer spectroscopy, X-ray diffraction and internal friction with the aim of shedding light on processes occurring during deep cryogenic treatment. It is shown that the carbon atoms are essentially immobile at temperatures below −50 °C, whereas carbon clustering in the virgin martensite occurs during heating above this temperature. An increase in the density of dislocations, the capture of immobile carbon atoms by moving dislocations, the strain-induced partial dissolution of the carbide phase, and the abnormally low tetragonality of the virgin martensite are found and interpreted in terms of plastic deformation that occurs during martensitic transformation at low temperatures where the virgin martensite is sufficiently ductile.

Comparison of Microstructures and Mechanical Properties for Solid and Mesh Cobalt-Base Alloy Prototypes Fabricated by Electron Beam Melting

Abstract: The microstructures and mechanical behavior of simple, as-fabricated, solid geometries (with a density of 8.4 g/cm3), as-fabricated and fabricated and annealed femoral (knee) prototypes, and reticulated mesh components (with a density of 1.5 g/cm3) all produced by additive manufacturing (AM) using electron beam melting (EBM) of Co-26Cr-6Mo-0.2C powder are examined and compared in this study. Microstructures and microstructural issues are examined by optical metallography (OM), scanning electron microscopy (SEM), transmission electron microscopy (TEM), energy-dispersive X-ray spectrometry (EDS), and X-ray diffraction (XRD), while mechanical properties included selective specimen tensile testing and Vickers microindentation hardness (HV) and Rockwell C-scale hardness (HRC) measurements. Orthogonal (X-Y) melt scanning of the electron beam during AM produced unique, orthogonal and related Cr23C6 carbide (precipitate) arrays (a controlled microstructural architecture) with dimensions of ~2 μm in the build plane perpendicular to the build direction, while connected carbide columns were formed in the vertical plane, parallel to the build direction, with microindentation hardnesses ranging from 4.4 to 5.9 GPa, corresponding to a yield stress and ultimate tensile strength (UTS) of 0.51 and 1.45 GPa with elongations ranging from 1.9 to 5.3 pct. Annealing produced an equiaxed fcc grain structure with some grain boundary carbides, frequent annealing twins, and often a high density of intrinsic {111} stacking faults within the grains. The reticulated mesh strut microstructure consisted of dense carbide arrays producing an average microindentation hardness of 6.2 GPa or roughly 25 pct higher than the fully dense components.

Sintered Silicon Carbide: A New Ceramic Vessel Material for Microwave Chemistry in Single-Mode Reactors

Abstract: Silicon carbide (SiC) is a strongly microwave absorbing chemically inert ceramic material that can be utilized at extremely high temperatures due to its high melting point and very low thermal expansion coefficient. Microwave irradiation induces a flow of electrons in the semiconducting ceramic that heats the material very efficiently through resistance heating mechanisms. The use of SiC carbide reaction vessels in combination with a single-mode microwave reactor provides an almost complete shielding of the contents inside from the electromagnetic field. Therefore, such experiments do not involve electromagnetic field effects on the chemistry, since the semiconducting ceramic vial effectively prevents microwave irradiation from penetrating the reaction mixture. The involvement of electromagnetic field effects (specific/nonthermal microwave effects) on 21 selected chemical transformations was evaluated by comparing the results obtained in microwave-transparent Pyrex vials with experiments performed in SiC vials at the same reaction temperature. For most of the 21 reactions, the outcome in terms of conversion/purity/product yields using the two different vial types was virtually identical, indicating that the electromagnetic field had no direct influence on the reaction pathway. Due to the high chemical resistance of SiC, reactions involving corrosive reagents can be performed without degradation of the vessel material. Examples include high-temperature fluorine-chlorine exchange reactions using triethylamine trihydrofluoride, and the hydrolysis of nitriles with aqueous potassium hydroxide. The unique combination of high microwave absorptivity, thermal conductivity, and effusivity on the one hand, and excellent temperature, pressure and corrosion resistance on the other hand, makes this material ideal for the fabrication of reaction vessels for use in microwave reactors.

Synthesis and Characterization of Crystalline Silicon Carbide Nanoribbons

Abstract: In this paper, a simple method to synthesize silicon carbide (SiC) nanoribbons is presented. Silicon powder and carbon black powder placed in a horizontal tube furnace were exposed to temperatures ranging from 1,250 to 1,500°C for 5–12 h in an argon atmosphere at atmospheric pressure. The resulting SiC nanoribbons were tens to hundreds of microns in length, a few microns in width and tens of nanometers in thickness. The nanoribbons were characterized with electron microscopy, energy-dispersive X-ray spectroscopy, X-ray diffraction, Raman spectroscopy and X-ray photoelectron spectroscopy, and were found to be hexagonal wurtzite–type SiC (2H-SiC) with a growth direction of . The influence of the synthesis conditions such as the reaction temperature, reaction duration and chamber pressure on the growth of the SiC nanomaterial was investigated. A vapor–solid reaction dominated nanoribbon growth mechanism was discussed.

Microstructural evolution during the infiltration of boron carbide with molten silicon

Abstract: The previously reported model that accounts for the formation of the core-rim structure in reaction-bonded boron carbide composites (RBBC) is expanded and validated by additional experimental observations and by a thermodynamic analysis of the ternary B–C–Si system. The microstructure of the RBBC composites consists of boron carbide particles with a core-rim structure, β-SiC and some residual silicon. The SiC carbide particles have a polygonal shape in composites fabricated in the presence of free carbon, in contrast to the plate-like morphology when the initial boron carbide is the sole source of carbon. In the course of the infiltration process, the original B4C particles dissolve partly or fully in molten silicon, and a local equilibrium is established between boron carbide, molten silicon and SiC. Overall equilibrium in the system is achieved as a result of the precipitation of the ternary boron carbide phase B12(B,C,Si)3 at the surface of the original boron carbide particles and leads to the formation of the rim regions. This feature is well accounted for by the “stoichiometric saturation” approach, which takes into account the congruent dissolution of B4C particles. The SiC phase, which precipitates form the silicon melt adopts the β-allotropic structure and grows preferably as single plate-like particles with an {1 1 1}β habit plane. The morphology of the SiC particles is determined by the amount of carbon available for their formation.

High coercivity cobalt carbide nanoparticles processed via polyol reaction: a new permanent magnet material

Abstract: Cobalt carbide nanoparticles were processed using polyol reduction chemistry that offers high product yields in a cost effective single-step process. Particles are shown to be acicular in morphology and typically assembled as clusters with room temperature coercivities greater than 3.4 kOe and maximum energy products greater than 20 kJ m−3. Consisting of Co3C and Co2C phases, the ratio of phase volume, particle size and particle morphology all play important roles in determining permanent magnet properties. Further, the acicular particle shape provides an enhancement to the coercivity via dipolar anisotropy energy as well as offering potential for particle alignment in nanocomposite cores. While Curie temperatures are near 510 K at temperatures approaching 700 K the carbide powders experience an irreversible dissociation to metallic cobalt and carbon thus limiting operational temperatures to near room temperature. These findings warrant more extensive investigation of this and other magnetic carbide systems in which particle size, chemistry and morphology are optimized.

Evaluation of vanadium carbide coatings on AISI H13 obtained by thermo-reactive deposition/diffusion technique

Abstract: Vanadium carbide coatings on AISI H13 steel were prepared by thermo-reactive deposition/diffusion process (TRD) in molten salt bath for 1 to 6 h at 920 °C and 1000 °C, respectively. The obtained coatings were characterized by scanning electron microscopy (SEM), energy dispersive X-ray spectrometry (EDX) and X-ray diffraction analysis (XRD). Equiaxed grains were observed throughout the coatings. The grain size gradually increased from the coating/substrate interface to the top surface. The coatings were composed of ordered state V6C5 phase and disordered state VCx (x = 0.83–0.88) phases and had a preferential orientation of (111) and (200) planes. The values of nano-indentation hardness and elastic modulus of the coating are 28.1 ± 0.7 GPa and 421 ± 14 GPa, respectively. The growth of the vanadium carbide coating by the TRD process followed a parabolic kinetics with an activation energy of 199.3 kJ/mol. The variation of the coating thickness on the AISI H13 steel with treating time and temperature can be determined.

Production of Calcium Carbide from Fine Biochars

Abstract: Carbon is the most abundant source of energy and chemicals on the earth. Biomass produced from photon-activated conversion of atmospheric CO2, and biomass fossils such as coal and petroleum are all carbon-rich sources. In around only one century of heavy industrial use of petroleum, this hydrocarbon source has already depleted to a point of a widespread concern over its scarcity in the decades to follow. Biocarbon, also known as biochar, can be readily produced from a vast sustainable supply of lignocellulosic biomass through pyrolysis. It is often in fine form and characterized by low mechanical strength and high activity in comparison to coal-derived chars. The ability to use biochar for the production of chemicals with high energy efficiency will largely alleviate our dependence on shrinking petroleum feedstock. Herein, we show reaction of fine biochars with fine CaO for the production of CaC2, an important starting material for production of many commodity chemicals. The process offers the potential to redirect the carbon conversion pathway. CaC2 is produced by the reaction 3C + CaO + E ! CaC2 + CO, where E 1 is the energy required for the process, about 445.6 kJmol 1 at above 2000 8C. CaC2 can be readily converted into acetylene by treatment with water: CaC2 + 2H2O!C2H2 + Ca(OH)2. Acetylene is an oxygen-free platform chemical for production of chemicals, for example, polyvinylchloride (PVC), vinyl acetate, and 1,4-butanediol. In this carbon conversion process, the main products CaC2 and then C2H2 are readily separated from other components. The current CaC2 production technology dates back to 1892 and has not changed much since then. 5] It uses an electric arc furnace, which is limited only to small-scale operations, typically less than 40 kt CaC2 per year. This process requires granular char and CaO of 5–30 mm in size and with sufficient mechanical strength, such as coal char, to allow unrestricted release of byproduct CO. Because of the low reaction rate resulting from the low surface area and poor contact between the large feed particles, high temperatures (about 2200 8C) and long reaction times (1–2 h) are usually required. These constraints inevitably result in high energy consumption (4000 kWh tCaC2 ), high production cost, and high CO2 emissions in electricity generation. [7] Autothermal heating by combustion of chars has been studied as an alternative process for CaC2 preparation. [8–10]

Adsorption and reaction of CO and hydrogen on iron-based Fischer–Tropsch synthesis catalysts

Abstract: H2, CO chemisorptions and the carbon hydrogenation on promoted iron Fischer–Tropsch synthesis (FTS) catalysts (Fe/SiO2 and FeK/SiO2) were investigated using temperature programmed surface reaction with X-ray photoelectron spectroscopy and laser Raman spectroscopy. It is found that potassium, used as promoter, does not lead to a distinct variation in the carbon species but changes the surface H/C ratio of carburized catalysts. Besides bulk iron carbide, several carbon species with different cluster sizes exist in the carburized catalysts, which have different reactivities towards hydrogen. The surface atomic carbon, the oligomerized carbon species and the bulk iron carbide are more reactive to hydrogen, whereas the large-size amorphous carbons are relatively inert to hydrogen. There is a good correlation between the chemisorption of H2 or CO and the corresponding feed gas conversion activity in FTS reaction. Meanwhile, the methane selectivity is correlated with the hydrogenation capability of catalysts, indicating that the surface H concentration has an important effect on the selectivity of hydrocarbons.

Nano-silicon carbide supported catalysts for PEM fuel cells with high electrochemical stability and improved performance by addition of carbon

Abstract: Nano-silicon carbide was applied as a novel catalyst support in proton exchange membrane (PEM) fuel cells to improve catalyst stability, due to its excellent resistance to electrochemical oxidation. Homogeneously dispersed Pt nanoparticles were deposited on β-SiC (Pt/SiC) using an ethylene glycol reduction method. Furthermore, carbon was introduced into this Pt/SiC catalyst, (Pt/SiC/C), to improve its electrocatalytic performance by increasing the electrical conductivity. The electrochemical stability of SiC was investigated and showed almost no changes in the redox region after oxidation for 48 h at 1.20 V. Based on accelerated durability tests (ADT) and high-resolution transmission electron microscopy (HRTEM), the electrochemical stability of Pt/SiC/C was remarkably enhanced compared with the Pt/C catalysts, which could be attributed to the excellent stability of the SiC support and the addition of high electrical conductivity carbon.

QM/MD simulation of SWNT nucleation on transition-metal carbide nanoparticles.

Abstract: The mechanism and kinetics of single-walled carbon nanotube (SWNT) nucleation from Fe- and Ni-carbide nanoparticle precursors have been investigated using quantum chemical molecular dynamics (QM/MD) methods. The dependence of the nucleation mechanism and its kinetics on environmental factors, including temperature and metal-carbide carbon concentration, has also been elucidated. It was observed that SWNT nucleation occurred via three distinct stages, viz. the precipitation of the carbon from the metal-carbide, the formation of a “surface/subsurface” carbide intermediate species, and finally the formation of a nascent sp2-hybidrized carbon structure supported by the metal catalyst. The SWNT cap nucleation mechanism itself was unaffected by carbon concentration and/or temperature. However, the kinetics of SWNT nucleation exhibited distinct dependences on these same factors. In particular, SWNT nucleation from NixCy nanoparticles proceeded more favorably compared to nucleation from FexCy nanoparticles. Altho...