Showing papers on "Carbide published in 2012"

Two-dimensional transition metal carbides.

Abstract: Herein we report on the synthesis of two-dimensional transition metal carbides and carbonitrides by immersing select MAX phase powders in hydrofluoric acid, HF. The MAX phases represent a large (>60 members) family of ternary, layered, machinable transition metal carbides, nitrides, and carbonitrides. Herein we present evidence for the exfoliation of the following MAX phases: Ti2AlC, Ta4AlC3, (Ti0.5,Nb0.5)2AlC, (V0.5,Cr0.5)3AlC2, and Ti3AlCN by the simple immersion of their powders, at room temperature, in HF of varying concentrations for times varying between 10 and 72 h followed by sonication. The removal of the “A” group layer from the MAX phases results in 2-D layers that we are labeling MXenes to denote the loss of the A element and emphasize their structural similarities with graphene. The sheet resistances of the MXenes were found to be comparable to multilayer graphene. Contact angle measurements with water on pressed MXene surfaces showed hydrophilic behavior.

Molybdenum Boride and Carbide Catalyze Hydrogen Evolution in both Acidic and Basic Solutions

Abstract: Molybdenum boride (MoB) and carbide (Mo2C) are excellent catalysts for electrochemical hydrogen evolution at both pH 0 and pH 14.

Fe5C2 nanoparticles: a facile bromide-induced synthesis and as an active phase for Fischer-Tropsch synthesis.

Abstract: Iron carbide nanoparticles have long been considered to have great potential in new energy conversion, nanomagnets, and nanomedicines. However, the conventional relatively harsh synthetic conditions of iron carbide hindered its wide applications. In this article, we present a facile wet-chemical route for the synthesis of Hagg iron carbide (Fe5C2) nanoparticles, in which bromide was found to be the key inducing agent for the conversion of Fe(CO)5 to Fe5C2 in the synthetic process. Furthermore, the as-synthesized Fe5C2 nanoparticles were applied in the Fischer–Tropsch synthesis (FTS) and exhibited intrinsic catalytic activity in FTS, demonstrating that Fe5C2 is an active phase for FTS. Compared with a conventional reduced-hematite catalyst, the Fe5C2 nanoparticles showed enhanced catalytic performance in terms of CO conversion and product selectivity.

A new class of electrocatalysts for hydrogen production from water electrolysis: metal monolayers supported on low-cost transition metal carbides.

Abstract: This work explores the opportunity to substantially reduce the cost of hydrogen evolution reaction (HER) catalysts by supporting monolayer (ML) amounts of precious metals on transition metal carbide substrates. The metal component includes platinum (Pt), palladium (Pd), and gold (Au); the low-cost carbide substrate includes tungsten carbides (WC and W2C) and molybdenum carbide (Mo2C). As a platform for these studies, single-phase carbide thin films with well-characterized surfaces have been synthesized, allowing for a direct comparison of the intrinsic HER activity of bare and Pt-modified carbide surfaces. It is found that WC and W2C are both excellent cathode support materials for ML Pt, exhibiting HER activities that are comparable to bulk Pt while displaying stable HER activity during chronopotentiometric HER measurements. The findings of excellent stability and HER activity of the ML Pt–WC and Pt–W2C surfaces may be explained by the similar bulk electronic properties of tungsten carbides to Pt, as is ...

Mechanical Milling: a Top Down Approach for the Synthesis of Nanomaterials and Nanocomposites

Abstract: Synthesis of nanomaterials by a simple, low cost and in high yield has been a great challenge since the very early development of nanoscience. Various bottom and top down approaches have been developed so far, for the commercial production of nanomaterials. Among all top down approaches, high energy ball milling, has been widely exploited for the synthesis of various nanomaterials, nanograins, nanoalloy, nanocomposites and nano -quasicrystalline materials. Mechanical alloying techniques have been utilized to produce amorphous and nanocrystalline alloys as well as metal/non-metal nano- composite materials by milling and post annealing, of elemental or compound powders in an inert atmosphere. Mechanical alloying is a non-equilibrium processing technique in which different elemental powders are milled in an inert atmosphere to create one mixed powder with the same composition as the constituents. In high-energy ball milling, plastic deformation, cold-welding and fracture are predominant factors, in which the deformation leads to a change in particle shape, cold-welding leads to an increase in particle size and fracture leads to decrease in particle size resulting in the formation of fine dispersed alloying particles in the grain-refined soft matrix. By utilizing mechanical milling various kind of aluminium/ nickel/ mag- nesium/ copper based nanoalloys, wear resistant spray coatings, oxide and carbide strengthened aluminium alloys, and many other nanocomposites have been synthesized in very high yield. The mechanical milling has been utilized for the synthesis of nanomaterials either by milling and post annealing or by mechanical activation and then applying some other process on these activated materials. This review is a systematic view of the basic concept of mechanical milling, historical view and appli- cations of mechanical milling in the synthesis of various nanomaterials, nanosomposites, nnaocarbons and nano quasicrys- talline materials.

Iron Particle Size Effects for Direct Production of Lower Olefins from Synthesis Gas

Abstract: The Fischer–Tropsch synthesis of lower olefins (FTO) is an alternative process for the production of key chemical building blocks from non-petroleum-based sources such as natural gas, coal, or biomass. The influence of the iron carbide particle size of promoted and unpromoted carbon nanofiber supported catalysts on the conversion of synthesis gas has been investigated at 340–350 °C, H2/CO = 1, and pressures of 1 and 20 bar. The surface-specific activity (apparent TOF) based on the initial activity of unpromoted catalysts at 1 bar increased 6–8-fold when the average iron carbide size decreased from 7 to 2 nm, while methane and lower olefins selectivity were not affected. The same decrease in particle size for catalysts promoted by Na plus S resulted at 20 bar in a 2-fold increase of the apparent TOF based on initial activity which was mainly caused by a higher yield of methane for the smallest particles. Presumably, methane formation takes place at highly active low coordination sites residing at corners a...

The Phase of Iron Catalyst Nanoparticles during Carbon Nanotube Growth

Abstract: We study the Fe-catalyzed chemical vapor deposition of carbon nanotubes by complementary in situ grazing-incidence X-ray diffraction, in situ X-ray reflectivity, and environmental transmission electron microscopy. We find that typical oxide supported Fe catalyst films form widely varying mixtures of bcc and fcc phased Fe nanoparticles upon reduction, which we ascribe to variations in minor commonly present carbon contamination levels. Depending on the as-formed phase composition, different growth modes occur upon hydrocarbon exposure: For γ-rich Fe nanoparticle distributions, metallic Fe is the active catalyst phase, implying that carbide formation is not a prerequisite for nanotube growth. For α-rich catalyst mixtures, Fe3C formation more readily occurs and constitutes part of the nanotube growth process. We propose that this behavior can be rationalized in terms of kinetically accessible pathways, which we discuss in the context of the bulk iron–carbon phase diagram with the inclusion of phase equilibri...

Monolayer graphene growth on Ni(111) by low temperature chemical vapor deposition

Abstract: In contrast to the commonly employed high temperature chemical vapor deposition growth that leads to multilayer graphene formation by carbon segregation from the bulk, we demonstrate that below 600 °C graphene can be grown in a self-limiting monolayer growth process. Optimum growth is achieved at ∼550 °C. Above this temperature, carbon diffusion into the bulk is limiting the surface growth rate, while at temperatures below ∼500 °C a competing surface carbide phase impedes graphene formation.

A simple chemical route toward monodisperse iron carbide nanoparticles displaying tunable magnetic and unprecedented hyperthermia properties.

Abstract: We report a tunable organometallic synthesis of monodisperse iron carbide and core/shell iron/iron carbide nanoparticles displaying a high magnetization and good air-stability. This process based on the decomposition of Fe(CO)(5) on Fe(0) seeds allows the control of the amount of carbon diffused and therefore the tuning of nanoparticles magnetic anisotropy. This results in unprecedented hyperthermia properties at moderate magnetic fields, in the range of medical treatments.

Tensile behaviour of a nanocrystalline bainitic steel containing 3 wt% silicon

Abstract: Much recent work has been devoted to characterize the microstructure and mechanical properties bainitic nanostructured steels. The microstructure is developed by isothermal heat treatment at temperatures as low as 125–350 °C and adapted steel grades typically contain high carbon contents to achieve sufficient depletion of the B S – M S temperature range, and above 1.5 Si wt.% to suppress carbide formation during isothermal holding. On the latter, most of the published literature agrees on a limit of around 1.2–1.5 wt.% to suppress cementite in high carbon steels. For this reason perhaps, additions of Si significantly above this limit have not been investigated systematically in the context of nanostructured bainitic steels. The present work is concerned with the effect of up to ∼3 Si wt.% in a steel grade adapted to low temperature bainitizing. Tensile properties as compared to similar grades, though with lower Si contents, exhibited unrivalled combinations of strength and ductility, with above 21% total elongation for a UTS above 2 GPa. An attempt is made to explain the mechanical properties of this microstructure in terms of some of its most relevant and unique morphological and microstructural features.

Bimetallic carbide nanocomposite enhanced Pt catalyst with high activity and stability for the oxygen reduction reaction.

Abstract: Nanocomposites consisting of the bimetallic carbide Co6Mo6C2 supported on graphitic carbon (gC) were synthesized in situ by an anion-exchange method for the first time. The Co6Mo6C2/gC nanocomposites were not only chemically stable but also electrochemically stable. The catalyst prepared by loading Pt nanoparticles onto Co6Mo6C2/gC was evaluated for the oxygen reduction reaction in acidic solution and showed superior activity and stability in comparison with commercial Pt/C. The higher mass activity of the Pt–Co6Mo6C2/gC catalyst indicated that less Pt would be required for the same performance, which in turn would reduce the cost of the fuel cell electrocatalyst. The method reported here will promote broader interest in the further development of other nanostructured materials for real-world applications.

Carbide-derived carbon monoliths with hierarchical pore architectures.

Abstract: Porous carbon materials are crucial components in catalysis, gas storage, electronics, and biochemistry. A hierarchical pore architecture in these materials is essential to achieve high surface areas combined with advanced mass transport kinetics. Widely used approaches for the generation of microor mesopores are activation and nanocasting. In contrast, macroporous carbon materials are primarily obtained by carbonization of polymeric precursor gels or replication of larger templates. A relatively new class of microand mesoporous carbon material with tunable porosity are carbide-derived carbon materials (CDCs). High-temperature chlorination of carbides leads to selective removal of metalor semi-metal atoms and allows control over the pore size of the resulting CDCs in a subngstrcm range by changing synthesis conditions or the carbide precursor. These materials have been studied for applications in gas storage and as electrode materials in supercapacitors because of their high specific surface areas. Recently, metal etching from pyrolyzed pre-ceramic components (polysilsesquioxanes or polysilazanes) was found to be a useful route towards carbide-derived carbon materials with enhanced porosity and gas-storage properties. A significant step towards ultrahigh specific surface area combined with a hierarchical mesoporous–microporous system was achieved using nanocasting of silica templates (SBA-15 or KIT-6) with polycarbosilane precursors and subsequent chlorine treatment of the resulting ordered mesoporous silicon carbides. These ordered mesoporous CDCs offer specific surface areas as high as 2800 mg 1 and total pore volumes of up to 2 cmg . Their mesostructure can be easily controlled by changing the silica hard template, resulting in excellent performance in protein adsorption, gas storage, and as electrodes for supercapacitors. However, such carbon materials are available only as nonstructured micrometer-sized powders and cannot be shaped into films without the addition of binders or the use of high mechanical stress, leading to structural deformation. Chlorine treatment of mechanically mixed Si/SiC precursors was found to be a useful route towards monolithic CDC with a hierarchical pore system. The presence of a free metal phase in the precursor system provides the opportunity to introduce a secondary macroporosity of 3 mm sized channels with a volume of 0.23 cmg 1 along with the microporous carbide-derived carbon material system. The introduction of large transport pores in polymerbased CDCs might be an alternative way to form materials that combine high surface areas with efficient fluid transport. The current literature describes a variety of routes for the production of highly macroporous ceramics from precursor polymers with controllable cell and window sizes. In particular, direct blowing of polycarbosilanes was found to be a useful approach for the generation of silicon carbide foams that might be suitable materials for the production of hierarchical CDCs. In the following, we describe a novel synthesis route for monolithic carbide-derived carbon materials, including micro-, meso-, and macroporous structures with extremely high specific surface area. They can be obtained by hightemperature chlorination of macroporous polymer-derived silicon carbide (SiC-PolyHIPE). A soft-templating approach starting from a high internal phase emulsion (HIPE) was used with an external oil phase consisting of liquid polycarbosilane SMP-10 and the cross-linker paradivinylbenzene. Using Span-80 as surfactant to stabilize the internal water phase, the application of oxidic or carbon hard templates and the corresponding template removal under harsh conditions is no longer necessary. After cross-linking the polymer chains, the resulting PolyHIPEs were pyrolyzed to silicon carbides at maximum temperatures of 700, 800, and 1000 8C and subsequently converted into CDCs by chlorine treatment at the maximum pyrolysis temperature (Supporting [*] M. Oschatz, L. Borchardt, Dr. I. Senkovska, N. Klein, Dr. R. Frind, Prof. Dr. S. Kaskel Department of Inorganic Chemistry Dresden University of Technology Bergstrasse 66, 01062 Dresden (Germany) E-mail: stefan.kaskel@chemie.tu-dresden.de

Metal overlayer on metal carbide substrate: unique bimetallic properties for catalysis and electrocatalysis

Abstract: Transition metal carbides often display electronic and catalytic properties that are similar to Pt-group metals. In this tutorial review, we describe the feasibility of replacing the Pt-group metal component in bimetallic systems with metal carbides. By supporting a metal monolayer on a carbide substrate, these bimetallic surfaces exhibit similar catalytic and electrocatalytic activity to the corresponding Pt-based bimetallic systems while demonstrating the advantages of lower cost and higher thermal stability. Another promising aspect is that the carbide substrates often promote the formation of small, flat metal particles with novel catalytic properties. We review the synthesis, characterization, and utilization of carbide-supported metal surfaces in heterogeneous catalysis and electrocatalysis. An overview is given for both theoretical and experimental investigations, and trends are drawn from the literature. We also discuss opportunities for future research on carbide-supported metal surfaces.

Effect of Austenitizing Heat Treatment on the Microstructure and Hardness of Martensitic Stainless Steel AISI 420

Abstract: The effect of austenitizing on the microstructure and hardness of two martensitic stainless steels was examined with the aim of supplying heat-treatment guidelines to the user that will ensure a martensitic structure with minimal retained austenite, evenly dispersed carbides and a hardness of between 610 and 740 HV (Vickers hardness) after quenching and tempering. The steels examined during the course of this examination conform in composition to medium-carbon AISI 420 martensitic stainless steel, except for the addition of 0.13% vanadium and 0.62% molybdenum to one of the alloys. Steel samples were austenitized at temperatures between 1000 and 1200 °C, followed by oil quenching. The as-quenched microstructures were found to range from almost fully martensitic structures to martensite with up to 35% retained austenite after quenching, with varying amounts of carbides. Optical and scanning electron microscopy was used to characterize the microstructures, and X-ray diffraction was employed to identify the carbide present in the as-quenched structures and to quantify the retained austenite contents. Hardness tests were performed to determine the effect of heat treatment on mechanical properties. As-quenched hardness values ranged from 700 to 270 HV, depending on the amount of retained austenite. Thermodynamic predictions (using the CALPHAD™ model) were employed to explain these microstructures based on the solubility of the carbide particles at various austenitizing temperatures.

Metal and silicon containing capping layers for interconnects

Abstract: Disclosed methods cap exposed surfaces of copper lines with a layer of metal or metal-containing compound combined with silicon. In some cases, the metal or metal-containing compound forms an atomic layer. In certain embodiments, the methods involve exposing the copper surface first to a metal containing precursor to form an atomic layer of adsorbed precursor or metal atoms, which may optionally be converted to an oxide, nitride, carbide, or the like by, e.g., a pinning treatment. Subsequent exposure to a silicon-containing precursor may proceed with or without metallic atoms being converted.

A quantitative assessment of nanometric machinability of major polytypes of single crystal silicon carbide

Abstract: The influence of polymorphism on nanometric machinability of single crystal silicon carbide (SiC) has been investigated through molecular dynamics (MD) simulation. The simulation results are compared with silicon as a reference material. Cutting hardness was adopted as a quantifier of the machinability of the polytypes of single crystal SiC. 3C-SiC offered highest cutting resistance (∼2.9 times that of silicon) followed by the 4H-SiC (∼2.8 times that of silicon) whereas 6H-SiC (∼2.1 times that of silicon) showed the least. Despite its high cutting resistance, 4H-SiC showed the minimum sub-surface crystal lattice deformed layer depth, in contrast to 6H-SiC. Further analysis of temperatures in the cutting zone and the percentage tool wear indicated that single point diamond turning (SPDT) of single crystal SiC could be limited to either 6H-SiC or 4H-SiC depending upon quality and cost considerations as these were found to be more responsive and amenable to SPDT compared to single crystal 3C-SiC.

Precipitation behavior of grain boundary M23C6 and its effect on tensile properties of Ni–Cr–W based superalloy

Abstract: Precipitation behavior of grain boundary (GB) M23C6 and its effect on tensile properties at elevated temperature were investigated systematically in a Ni–Cr–W based superalloy. The results show that the M23C6 precipitation behavior is influenced obviously by grain boundary character (GBC) and interfacial energy. The Σ≤9 GBs and low angle GBs have low interfacial energy, and no M23C6 carbide precipitates at these GBs. Plenty of M23C6 carbide particles precipitate at the large angle GBs with high interfacial energy. The coherent orientation relationship between M23C6 and the matrix plays an important role on the precipitation morphology of M23C6. M23C6 carbides with four typical morphologies distribute at the large angle GBs, including lamellar carbide which grows into the matrix near one side or both sides of the GBs, rod-like carbide and small lamellar carbide both of which grow along GBs. Moreover, the decrease of both tensile and yield strength of the aged alloy is mainly caused by the lamellar M23C6 carbide breaking. The tensile properties vary irregularly with increasing aging time.

Study of surface quality in high speed turning of Inconel 718 with uncoated and coated CBN tools

Abstract: Inconel 718 is known to be among the most difficult-to-machine materials due to its special properties which cause the short tool life and severe surface damages. The properties, which are responsible for poor machinability, include rapid work hardening during machining; tendency to weld with the tool material at high temperature generated during machining; the tendency to form a built-up edge during machining; and the presence of hard carbides, such as titanium carbide and niobium carbide, in their microstructure. Conventional method of machining Inconel 718 with cemented carbide tool restricts the cutting speed to a maximum 30 m/min due to the lower hot hardness of carbide tool, high temperature strength and low thermal conductivity of Inconel 718. The introduction of new coated carbide tools has increased cutting speed to 100 m/min; nevertheless, the time required to machine this alloy is still considerably high. High speed machining using advanced tool material, such as CBN, is one possible alternative for improving the productivity of this material due to its higher hot hardness in comparison with carbide tool. This paper specifically deals with surface quality generated under high speed finishing turning conditions on age-hardened Inconel 718 with focus on surface roughness, metallographic analysis of surface layer and surface damages produced by machining. Both coated and uncoated CBN tools were used in the tests, and a comparison between surfaces generated by both tools was also discussed.

Complete and Incomplete Wetting of Ferrite Grain Boundaries by Austenite in the Low-Alloyed Ferritic Steel

Abstract: Low-carbon low-alloyed ferritic steels are the main material for the production of high-strength pipes for the transportation of oil and gas. The formation of brittle carbide network during the lifetime of a pipeline could be a reason for a catastrophic failure. Among other reasons, it can be controlled by the morphology of grain boundary (GB) carbides. The microstructure of a low-alloyed ferritic steel containing 0.09 at.% C and small amounts of Si, Mn, Nb, Cu, Al, Ni, and Cr was studied between 300 and 900 °C. The samples were annealed very long time (700 to 4000 h) in order to produce the equilibrium morphology of phases. The (α-Fe)/(α-Fe) GBs can be either completely or incompletely wetted (covered) by the γ-Fe (austenite) above the temperature of eutectoid transition. The portion of (α-Fe)/(α-Fe) GBs completely wetted by γ-Fe is around 90% and does not change much between 750 and 900 °C. The (α-Fe)/(α-Fe) GBs can be either completely or incompletely wetted (covered) by the Fe3C (cementite) below the temperature of eutectoid transition. The portion of (α-Fe)/(α-Fe) GBs completely wetted by Fe3C changes below 680 °C between 67 and 77%. The formation of the network of brittle cementite layers between ductile ferrite grains can explain the catastrophic failure of gas- and oil-pipelines after a certain lifetime.

New insights into hard phases of CoCrMo metal-on-metal hip replacements

Abstract: The microstructural and mechanical properties of the hard phases in CoCrMo prosthetic alloys in both cast and wrought conditions were examined using transmission electron microscopy and nanoindentation. Besides the known carbides of M(23)C(6)-type (M=Cr, Mo, Co) and M(6)C-type which are formed by either eutectic solidification or precipitation, a new mixed-phase hard constituent has been found in the cast alloys, which is composed of ∼100 nm fine grains. The nanosized grains were identified to be mostly of M(23)C(6) type using nano-beam precession electron diffraction, and the chemical composition varied from grain to grain being either Cr- or Co-rich. In contrast, the carbides within the wrought alloy having the same M(23)C(6) structure were homogeneous, which can be attributed to the repeated heating and deformation steps. Nanoindentation measurements showed that the hardness of the hard phase mixture in the cast specimen was ∼15.7 GPa, while the M(23)C(6) carbides in the wrought alloy were twice as hard (∼30.7 GPa). The origin of the nanostructured hard phase mixture was found to be related to slow cooling during casting. Mixed hard phases were produced at a cooling rate of 0.2 °C/s, whereas single phase carbides were formed at a cooling rate of 50 °C/s. This is consistent with sluggish kinetics and rationalizes different and partly conflicting microstructural results in the literature, and could be a source of variations in the performance of prosthetic devices in-vivo.

Influence and effectivity of VC and Cr3C2 grain growth inhibitors on sintering of binderless tungsten carbide

Abstract: For the production of hard, high temperature and abrasion resistant parts, like water-jet nozzles or pressing tools for forming glass lenses, binderless cemented carbide is used. In this work, the consolidation of tungsten carbide with additions of VC and Cr3C2 grain growth inhibitors is studied. Tungsten carbide powder dry or wet milled was consolidated by dry pressing, debindering and gas pressurized sintering and, alternatively, by spark plasma sintering. The effect of adding VC and Cr3C2 to binderless tungsten carbide on the grain growth was studied with contents being 0; 0.1; 0.3; 0.5; 0.7 and 1.0 wt.%. Samples with an ultrafine microstructure free of abnormal grain growth, a hardness of 25.5 GPa and a fracture toughness of 7.2 MPa·m1/2 were archived by conventional sintering. Both carbides reduce grain growth, but with Cr3C2 a finer microstructure can be achieved at lower amounts. Compared to the same amount of Cr3C2, the addition of VC results in smaller grains but lower hardness and fracture toughness.