Showing papers on "Carbide published in 2011"

A review on non-precious metal electrocatalysts for PEM fuel cells

Abstract: With the approaching commercialization of PEM fuel cell technology, developing active, inexpensive non-precious metal ORR catalyst materials to replace currently used Pt-based catalysts is a necessary and essential requirement in order to reduce the overall system cost. This review paper highlights the progress made over the past 40 years with a detailed discussion of recent works in the area of non-precious metal electrocatalysts for oxygen reduction reaction, a necessary reaction at the PEM fuel cell cathode. Several important kinds of unsupported or carbon supported non-precious metal electrocatalysts for ORR are reviewed, including non-pyrolyzed and pyrolyzed transition metal nitrogen-containing complexes, conductive polymer-based catalysts, transition metal chalcogenides, metal oxides/carbides/nitrides/oxynitrides/carbonitrides, and enzymatic compounds. Among these candidates, pyrolyzed transition metal nitrogen-containing complexes supported on carbon materials (M–Nx/C) are considered the most promising ORR catalysts because they have demonstrated some ORR activity and stability close to that of commercially available Pt/C catalysts. Although great progress has been achieved in this area of research and development, there are still some challenges in both their ORR activity and stability. Regarding the ORR activity, the actual volumetric activity of the most active non-precious metal catalyst is still well below the DOE 2015 target. Regarding the ORR stability, stability tests are generally run at low current densities or low power levels, and the lifetime is far shorter than targets set by DOE. Therefore, improving both the ORR activity and stability are the major short and long term focuses of non-precious metal catalyst research and development. Based on the results achieved in this area, several future research directions are also proposed and discussed in this paper.

Carbide‐Derived Carbons – From Porous Networks to Nanotubes and Graphene

Abstract: Carbide-derived carbons (CDCs) are a large family of carbon materials derived from carbide precursors that are transformed into pure carbon via physical (e.g., thermal decomposition) or chemical (e.g., halogenation) processes. Structurally, CDC ranges from amorphous carbon to graphite, carbon nanotubes or graphene. For halogenated carbides, a high level of control over the resulting amorphous porous carbon structure is possible by changing the synthesis conditions and carbide precursor. The large number of resulting carbon structures and their tunability enables a wide range of applications, from tribological coatings for ceramics, or selective sorbents, to gas and electrical energy storage. In particular, the application of CDC in supercapacitors has recently attracted much attention. This review paper summarizes key aspects of CDC synthesis, properties, and applications. It is shown that the CDC structure and properties are sensitive to changes of the synthesis parameters. Understanding of processing–structure–properties relationships facilitates tuning of the carbon material to the requirements of a certain application.

Low-Cost Molybdenum Carbide and Tungsten Carbide Counter Electrodes for Dye-Sensitized Solar Cells

Abstract: Carbide-based catalysts, MoC and WC embedded in ordered nanomesoporous carbon were developed for the redn. of triiodide in DSSCs. CV, EIS, Tafel polarization, and photocurrent/voltage tests confirm the excellent catalytic activity of the synthesized carbide-based composites - comparable to that of expensive Pt catalyst prepd. through pyrolysis. Com. Mo2C and WC particles also effectively catalyze the redn. of triiodide to iodide despite their large particle size. The results show that the addn. of P25 and CD (carbon dye) improves the adhesion, the catalytic activity and the cond. of Mo2C and WC electrodes. The optimum amts. of added P25 and CD added were also detd. Results demonstrate that molybdenum and tungsten carbides are potential alternatives to the expensive and scarce Pt in low-cost DSSCs.

Graphene–aluminum nanocomposites

Abstract: Composites of graphene platelets and powdered aluminum were made using ball milling, hot isostatic pressing and extrusion. The mechanical properties and microstructure were studied using hardness and tensile tests, as well as electron microscopy, X-ray diffraction and differential scanning calorimetry. Compared to the pure aluminum and multi-walled carbon nanotube composites, the graphene–aluminum composite showed decreased strength and hardness. This is explained in the context of enhanced aluminum carbide formation with the graphene filler.

High Activity Carbide Supported Catalysts for Water Gas Shift

Abstract: Nanostructured carbides are refractory materials with high surface areas that could be used as alternatives to the oxide materials that are widely used as support materials for heterogeneous cataly...

The influence of carbon nanotube (CNT) morphology and diameter on the processing and properties of CNT-reinforced aluminium composites

Abstract: Ball milling was used to disperse MWCNTs of two different morphologies (stiff and straight vs. bent and entangled) and diameters (very large diameter and 3.5 times smaller diameter) in aluminium powders, which were subsequently hot consolidated by hot extrusion. Characterization of the produced composites revealed that the CNT morphology plays an important role in affecting dispersion. It was found that the smaller diameter bent and entangled CNTs were more difficult to disperse with increase in CNT content compared to the larger diameter stiff and straight ones; which in turn affected the tensile properties and hardness of the composites. Furthermore, cold welding of the milled powders as well as carbide formation in the final composite was found to depend on the CNT diameter. The smaller diameter CNTs – having a larger effective interfacial contact area with the aluminium matrix compared to the larger ones for a given CNT wt.% – were found to reduce particle welding during milling and to be more affected by carbide formation. Nano-sized particles of aluminium oxide as well as nano-rods of aluminium carbide, in addition to CNT damage were observed upon TEM analysis of the smaller diameter CNTs.

Graphene growth on Ni(111) by transformation of a surface carbide.

Abstract: A novel growth mechanism of graphene on Ni(111) has been discovered that occurs at temperatures below 460 °C. At these conditions, a surface-confined nickel−carbide phase coexists with single layer graphene. The graphene grows by in-plane transformation of the carbide along a one-dimensional phase-boundary, which is distinctively different from known growth processes on other transition metals and on Ni above 460 °C, where carbon atoms attach to “free” edges of graphene islands.

Magnetic Nanoparticles of Iron Carbide, Iron Oxide, Iron@Iron Oxide, and Metal Iron Synthesized by Laser Ablation in Organic Solvents

Abstract: Iron-based nanoparticles can have useful magnetic and catalytic properties. We investigated the synthesis of iron-based nanostructures by laser ablation of bulk iron with 1064 nm nanosecond pulses in the following organic solvents: tetrahydrofuran, acetonitrile, dimethylformamide, dimethylsulfoxide, toluene, and ethanol. Structural analysis carried out by transmission electron microscopy and X-ray diffraction revealed that the solvent has a dramatic influence on both the composition and the nanostructure of nanoparticles. Various magnetic nanoparticles like iron carbide (Fe3C), magnetic iron oxide (magnetite/maghemite), metal iron (α-Fe), and iron@iron oxide are obtained by varying the solvent and keeping unchanged all the other experimental conditions. These results are the consequences of the different reactivity of solvent molecules exposed to the plasma plume generated during the ablation process.

Graphene growth and stability at nickel surfaces

Abstract: The formation of single-layer graphene by exposure of a Ni(111) surface to ethylene at low pressure has been investigated. Two different growth regimes were found. At temperatures between 480 and 650 °C, graphene grows on a pure Ni(111) surface in the absence of a carbide. Below 480 °C, graphene growth competes with the formation of a surface Ni2C carbide. This Ni2C phase suppresses the nucleation of graphene. Destabilization of the surface carbide by the addition of Cu to the surface layer facilitates the nucleation and growth of graphene at temperatures below 480 °C. In addition to the growth of graphene on Ni substrates, the interaction between graphene and Ni was also studied. This was done both experimentally by Ni deposition on Ni-supported graphene and by density functional theory calculation of the work of adhesion between graphene and Ni. For graphene sandwiched between two Ni-layers, the work of adhesion between graphene and the Ni substrate was found to be four times as large as that for the Ni-supported graphene without a top layer. This stronger interaction may cause the destruction of graphene that is shown experimentally to occur at ~200 °C when Ni is deposited on top of Ni-supported graphene. The destruction of graphene allows the Ni deposits to merge with the substrate Ni. After the completion of this process, the graphene sheet is re-formed on top of the Ni substrate, leaving no Ni at the surface.

Study on the nano-powder-mixed sinking and milling micro-EDM of WC-Co

Abstract: Present study investigates the feasibility of improving surface characteristics in the micro-electric discharge machining (EDM) of cemented tungsten carbide (WC–Co), a widely used die and mould material, using graphite nano-powder-mixed dielectric. In this context, a comparative analysis has been carried out on the performance of powder-mixed sinking and milling micro-EDM with view of obtaining smooth and defect-free surfaces. The surface characteristics of the machined carbide were studied in terms of surface topography, crater characteristics, average surface roughness (Ra) and peak-to-valley roughness (Rmax). The effect of graphite powder concentration on the spark gap, material removal rate (MRR) and electrode wear ratio (EWR) were also discussed for both die-sinking and milling micro-EDM of WC–Co. It has been observed that the presence of semi-conductive graphite nano-powders in the dielectric can significantly improve the surface finish, enhance the MRR and reduce the EWR. Both the surface topography and crater distribution were improved due to the increased spark gap and uniform discharging in powder-mixed micro-EDM. The added nano-powder can lower the breakdown strength and facilitate the ignition process thus improving the MRR. However, for a fixed powder material and particle size, all the performance parameters were found to vary significantly with powder concentration. Among the two processes, powder-mixed milling micro-EDM was found to provide smoother and defect-free surface compared to sinking micro-EDM. The lowest value of Ra (38 nm) and Rmax (0.17 μm) was achieved in powder-mixed milling micro-EDM at optimum concentration of 0.2 g/L and electrical setting of 60 V and stray capacitance.

Development and microstructural characterization of microwave cladding on austenitic stainless steel

Abstract: In the present work microwave cladding was explored as a new processing method for enhancement of surface properties of austenitic stainless steel (SS-316). Cladding of nickel based powder (EWAC) was developed using microwave radiation as the heating source. This paper explains the possible mechanism of clad formation using microwave hybrid heating with the help of a schematic model. The developed clads were characterized using field emission scanning electron microscope (FE-SEM), energy dispersive X-ray spectroscope (EDS), X-ray diffraction (XRD) and measurement of Vicker's microhardness. Typical X-ray diffraction (XRD) pattern of the clad showed the presence of chromium carbide, nickel silicide and nickel iron phases that eventually contribute to enhancement in microhardness of the clads. Clads of approximately 1 mm thickness were developed without any visible interfacial cracking and had significantly less porosity (1.09%). Microstructure of clad transverse section revealed good metallurgical bond with SS-316 substrate by partial mutual diffusion of constituent elements. The microstructure of the clad was found dominantly cellular in nature. Chromium was observed segregated around the cell boundaries while iron and nickel were identified inside the cells. Chromium carbides (Cr 23 C 6 , Cr 3 C 2 ) were formed during the processing and appeared at the cell boundaries. Vicker's microhardness study revealed that the hardness profile varies within the clad zone and the average microhardness of the developed clad was observed to be 304 ± 48 H v .

Dissolved Carbon Controls the Initial Stages of Nanocarbon Growth

Abstract: Carbon is a versatile material that, depending on its hybrid-ization and assembly in one-, two-, or three-dimensionalnetworks, exhibits important electronic and chemical proper-ties with countless practical applications. For example, it isfound in printer inks, pencils, water purification systems,thermal isolation, and antistatic materials.

Innovative porous SiC-based materials: From nanoscopic understandings to tunable carriers serving catalytic needs

Abstract: Progress in developing a new class of catalyst carrier materials based on porous self-bonded silicon carbide (SiC) is reviewed. Since the demonstration of scalable economically viable β-SiC production process, innovative β-SiC-based materials with tunable physical and chemical properties were successfully synthesized. Silicon carbide has superior mechanical and thermal properties which, coupled to chemical inertness, avoids several of the problems inherent in the use of commercial oxide and carbon based supports and catalysts. High surface area SiC (35 m 2 /g) can now be easily synthesized, with unmatched mechanical properties, tailored pore size distribution (meso- and macroporous network and total pore volume up to 1 cm 3 /g) and at reasonable cost. It can be shaped directly into extrudates, (μ-) spheres, monoliths, open cell foams, 3D forms depending to the downstream applications. Furthermore, it can also be chemically modified for specific catalytic applications through the addition of promoters (oxides like Al 2 O 3 , TiO 2 , ZrO 2 , carbides and metals) rendering the fabrication simple and cost effective. In many respects, it combines the best properties of oxide and carbon based supports without suffering many of their disadvantages. New structured TiO 2 /SiC composites have been prepared and are expected to be the next photocatalytic media. The ability of the SiC material to be used as catalyst support will be illustrated in the present work by two exothermic reactions, namely the selective oxidation of trace amount of H 2 S and the Fischer–Tropsch synthesis. For this later, a direct comparison was also made with a traditional support, i.e. alumina.

Recent advances in understanding metal dusting: A review

Abstract: Recent experimental investigations have widened the understanding of metal dusting significantly. Microscopic observations have been used to dissect dusting mechanisms. Iron dusts by growing a cementite surface scale, which catalyses graphite nucleation and growth. The resulting volume expansion leads to cementite disintegration. Cementite formation on iron can be suppressed by alloying with germanium. Nonetheless, dusting occurs via the direct growth of graphite into the metal, producing nanoparticles offerrite.This process is faster, because carbon diffusion is more rapid in α-Fe than in Fe 3 C. Austenitic materials cannot form cementite, and dust via formation of graphite at external surfaces and interior grain boundaries. The coke deposit consists of carbon nanotubes with austenite particles at their tips, or graphite particles encapsulating austenite. TEM studies demonstrate the inward growth of graphite within the metal interior. It is therefore concluded that the dusting mechanism of austenitic materials like high alloy Cr-Ni steels and Ni base materials is one of graphite nucleation and growth within the near surface metal. In all alloys examined, both ferritic and austenitic, the principal mass transfer process is inward diffusion of carbon. Alloying iron with nickel leads to a transformation from one mechanism with carbide formation to the other without. Copper alloying in nickel and high nickel content stainless steels strongly suppresses graphite nucleation, as does also an intermetallic Ni-Sn phase, thereby reducing greatly the overall dusting rate. A surface layer of intermetallic Ni-Sn Fe-base materials facilitates the formation of a Fe 3 SnC surface scale which also prevents coking and metal dusting. Current understanding of the roles of temperature, gas composition and surface oxides on dusting rates are summarised. Finally, protection against metal dusting by coatings is discussed in terms of their effects on catalysis of carbon deposition, and on protective oxide formation.

Iron carbide inclusions in lower-mantle diamond from juina, brazil

Abstract: Iron carbides in association with native iron, graphite, and magnetite were identified in a crystal of diamond from the Juina area, Brazil, that contains a series of other, deep-mantle mineral inclusions Among the iron carbides, Fe 3 C, Fe 2 C (“chalypite”), and Fe 23 C 6 (haxonite) are present; the two latter phases are identified in the terrestrial environment for the first time Some of the analyzed iron carbide grains contain 73–91 at% N and are, in fact, nitrocarbide We suggest, on the basis of the high-pressure mineral parageneses previously observed in the diamond and experimental data on the system Fe–C, that “chalypite” crystallized within a pressure interval of 50–130 GPa from an iron–carbon melt rich in nitrogen Following crystallization, iron carbides and native iron were partially oxidized to magnetite, and encapsulated in diamond along with other high-pressure minerals The finds of various iron carbides, some of which are rich in nitrogen, in lower-mantle diamond confirm a significant role of carbides and nitrogen in the Earth’s interior

Thermodynamic assessment of the Fe–Mn–C system

Abstract: The Fe–Mn–C ternary system has been thermodynamically evaluated using the CALPHAD method. Published data on thermochemical properties and phase equilibria have been critically examined and used to optimise the model parameters. The Fe–Mn–C system has been evaluated previously by Huang [W. Huang, Metall. Trans. A 21A (1990) 2115–2123]. In comparison to that evaluation, experimental liquidus data are now considerably better described and carbide equilibria at low temperatures are also more reasonable now. The differences in equilibria involving the γ (austenite), α (ferrite), e (hcp) and M 3 C (cementite) phases are very small. The improvement of carbide equilibria was possible by doing ab initio calculations of the enthalpies of formation of the metastable Fe 3 C, Fe 23 C 6 , Fe 5 C 2 and Fe 7 C 3 carbides. Calculations using the presently developed thermodynamic description are extensively compared with experimental data.

Recent developments on WC-based bulk composites

Abstract: In order to achieve improved properties and performance with WC-based cemented carbides, research efforts have been directed towards the development of nanostructured cemented carbides. With the recent development of ‘spray conversion process’ for synthesizing nanosized powders and the advent of spark plasma sintering technique, it has been possible to successfully develop bulk nanostructured cemented carbides, possessing improved hardness and wear resistance. On a different note, realisation of the fact that the presence of metallic binder phase is deleterious towards certain applications of WC-based cermets has led to a recent surge of interest towards the development of novel ‘binderless’ WC-based ceramics by replacing the metallic binder phase with ceramic sinter-additives. More recently, it has been possible to develop dense WC-based ceramic composites without considerable deterioration of fracture toughness in the absence of the metallic binder phase. In the above perspective, the present review focuses mainly on the recent research results concerned with the processing and characterisation of nanostructured WC-based cermets and binderless WC-based ceramic composites.