Carbide Mechanism Research Articles

First-principles study of structure sensitivity of chain growth and selectivity in Fischer–Tropsch synthesis using HCP cobalt catalysts

Co (0001) prefers the CO insertion mechanism with high methane selectivity, but Co (101̄1) prefers the carbide mechanism with high C2-hydrocarbon selectivity.

Atomic-scale strengthening mechanism of dislocation-obstacle interaction in silicon carbide particle-reinforced copper matrix nanocomposites

Using molecular dynamics (MD) simulations, the strengthening mechanism of silicon carbide (SiC) particle acted as barrier for the motion of edge dislocation in copper (Cu) matrix nanocomposites under shear loading is investigated. The dislocation glide behavior and the dislocation-particle interaction accounting for the effect of the temperature and the particle size are discussed in terms of the stress-strain relationship, the crystal energy change, the stress distribution and the dislocation evaluation. The results show that the critical depinning stress and the critical depinning strain are found to vary significantly depending upon the temperature and the particle size, consistent with previous studies. Higher temperature or smaller particle results in lower critical depinning stress to allow dislocation to bypass particle. Moreover, the critical depinning stress from the current atomic study is lower than the theoretical value due to the presence of cross-slip and thermal activation. At low temperature the mechanical activation controls the depinning stress of the dislocation bypassing particle, while at high temperature the thermal activation dominates that. In addition, the trapping sites of dislocations for the dislocation multiplication are observed at high temperature or large particle, due to the lattice mismatch of particle taken as the dislocation source. Compared to the dislocation interacting with void or metal-particle, the interaction of dislocation and SiC-particle in Cu + SiC nanocomposite reveals much complex factors, such as the multiple glide planes, the nanosize effect, as well as the coupling effect of mechanical and thermal activation, on the strengthening mechanism. It maybe apply these results to a high throughput experimental design which provides the dense and targeted material data, to accelerate material discovery with the large-scale and low-cost fabrication strategies.

Formation mechanism of spheroidal carbide in ultra-low carbon ductile cast iron

The formation mechanism of the spheroidal carbide in the ultra-low carbon ductile cast iron fabricated by the metal mold casting technique was systematically investigated. The results demonstrated that the spheroidal carbide belonged to eutectic carbide and crystallized in the isolated eutectic liquid phase area. The formation process of the spheroidal carbide was related to the contact and the intersection between the primary dendrite and the secondary dendrite of austenite. The oxides of magnesium, rare earths and other elements can act as heterogeneous nucleation sites for the spheroidal carbide. It was also found that the amount of the spheroidal carbide would increase with an increase in carbon content. The cooling rate has an important influence on the spheroidal carbide under the same chemical composition condition.

Revisiting alternative pathways in the Fischer–Tropsch process: Accurate density functional theory calculations on “magic” Ru12 clusters

AbstractModels of the Fischer–Tropsch reaction typically focus on two proposed mechanisms for the initial carbon monoxide dissociation: unassisted dissociation (carbide mechanism), and hydrogen‐assisted dissociation via an adsorbed oxymethylidene (HCO*) intermediate. Much evidence for hydrogen‐assisted dissociation comes from density functional theory calculations modeling ruthenium nanoparticle catalysts as infinite, periodic metal slabs. However, the generalized gradient approximations (GGAs) used in these calculations can make significant errors in reaction barrier heights. How these errors affect the predicted selectivity to unassisted vs. hydrogen‐assisted dissociation is not well understood. We address the problem by considering a different regime, applying GGA and beyond‐GGA approximations to CO dissociation on a “magic” nonmagnetic Ru12 cluster modeling supported nanoparticle catalysts. Both approximations concur that hydrogen‐assisted dissociation is facile on this cluster, providing additional support for its potential role in real catalysts.

Plastic deformation behaviors and hardening mechanism of M7C3 carbide

This paper reports an investigation of inelastic deformation behaviors of M7C3 carbide in high chromium cast iron by compression testing with a strain of 20% at 450°C. The carbide exhibits plasticity and a work-hardening capacity upon compressive loading. Dislocations, twins and torsional deformation are introduced into the larger compressed block-shaped M7C3 carbide. Multiple gliding and twist grain boundaries are firstly observed inside the deformed M7C3 carbide. The corresponding microhardness of the compressed carbide measured at room temperature is found to increase by approximately 100HV compared to the undeformed one. The plastic deformation behaviors of M7C3 carbide provide a new and valuable understanding of the strain-hardening modes of carbide contained steel and iron materials.

Material removal mechanism of 6H-SiC studied by nano-scratching with Berkovich indenter

The aim of this paper was to analyze the deformation characteristics and material removal mechanism of mono-crystal silicon carbide. The nano-scratching tests were conducted on the surface of 6H-SiC (0001) by using Berkovich nano-indenter. In the presence of various characterization techniques, a variety of new achievements have been reached. Phase transformation behavior in the nano-scratching process of mono-crystal silicon carbide (6H-SiC) is revealed by using the transmission electron microscope in this work. Amorphous phase and dislocation activities are found near the surface area under the bottom of the scratch which stands as the major cause triggering the plastic removal of this material, with no other forms of crystalline structure found, by which the plastic removal mechanism of mono-crystal silicon carbide was well identified. With a crack-free surface that had been processed, the chip broke away from the bulk in the form of plastic mode, but the subsurface was covered by cracks with their lengths many times longer than the machined depth. The results of laser Raman indicated that residual amorphous phase exits in the chips under the condition of the plastic removal of this material.

Study on wear mechanism of solid carbide cutting tool in milling CFRP

Abstract

Study on the machined depth when nanoscratching on 6H-SiC using Berkovich indenter: Modelling and experimental study

In order to investigate the deformation characteristics and material removing mechanism of the single crystal silicon carbide at the nanoscale, the nanoscratching tests were conducted on the surface of 6H-SiC (0001) by using Berkovich indenter. In this paper, a theoretical model for nanoscratching with Berkovich indenter is proposed to reveal the relationship between the applied normal load and the machined depth. The influences of the elastic recovery and the stress distribution of the material are considered in the developed theoretical model. Experimental and theoretical machined depths are compared when scratching in different directions. Results show that the effects of the elastic recovery of the material, the geometry of the tip and the stress distribution of the interface between the tip and sample have large influences on the machined depth which should be considered for this kind of hard brittle material of 6H-SiC.

铸造镍基高温合金中初生 M C碳化物的退化过程和机理

Primary MC carbide is one of the most important phases in cast Ni-based superalloys. During longterm thermal exposure, the primary MC carbide is not stable and tends to degenerate, exhibiting various degeneration reactions, such as MC+g →M6C+g′, MC+g→M6C + M23C6+ g′ and MC+g→M6C + M23C6+h. It is widely known that the degeneration of primary MC carbide has obvious influence on the microstructural evolutions of superalloys, including coarsening of g′ phase, coarsening of grain boundaries and precipitation of topologically closepacked (TCP) phase, and consequently the mechanical properties of alloys. Much research work has focused on the degeneration mechanism of primary MC carbide during long-term thermal exposure, however, it is not very clear so far. In this work, a cast Ni-based superalloy is fabricated and thermally exposed at 850 °C for 500~10000 h in order to study the degeneration mechanism of primary MC carbide. The degeneration of primary MC carbide is observed by OM, SEM and TEM. High-angle annular dark field (HAADF) mode of TEM is used to clearly observe the degeneration of primary MC carbide and the element distribution in the degeneration areas. The results show that the primary MC degeneration is an inter-diffusion process which occurs between the primary carbide and the g matrix. During the degeneration, C is released from the primary carbide, Ni, Al and Cr are provided by the g matrix, while Ti, W and Mo come from both primary MC and g matrix. The precipitation of h phase is determined by *国家自然科学基金项目51001101和国家能源局项目NY20150102资助 收到初稿日期: 2015-07-17,收到修改稿日期: 2015-11-24 作者简介:孙文,男, 1986年生,博士生 DOI: 10.11900/0412.1961.2015.00399 第455-462页 pp.455-462

Modification of Co/Al2 O3 Fischer-Tropsch Nanocatalysts by Adding Ni: A Kinetic Approach

In this article, the effects of Ni (Co:Ni ratio = 4:1) on the performances and kinetic parameters of Co/Al2O3 nanocatalysts are explored in Fischer–Tropsch synthesis. As a result, the probability of chain growth increases and the deactivation rate decreases with addition of Ni. The properties of catalysts are characterized at different stages using Brunauer–Emmett–Teller analysis, Fourier transform infrared spectroscopy, scanning electron microscopy, and the vibrating sample magnetometer technique. The Co/Ni/Al2O3 catalyst with higher surface area and pore size exhibits higher activity and C5+ selectivity. The different results obtained for room-temperature ferromagnetism have been attributed to the different metal oxides in the catalysts. The Fischer Tropsch kinetic study is done on the basis of the Langmuir–Hinshelwood–Hougen–Watson adsorption theory. Seven kinetic expressions based on the carbide and enolic mechanisms are selected. All experimental data are theoretically examined with these kinetic expressions to derive the best kinetic according to the linear and nonlinear approaches. The kinetic parameters of the fitted model, including the rate constant (k) and activation energy (Ea), are calculated with the Levenberg–Marquardt and genetic algorithm. The results show that by adding Ni the activation energy (Ea) decreases, the reaction rate (–RCO) increases, and the CO adsorption on the Co/Ni/Al2O3 catalyst is faster as well.

Advanced fundamental modeling of the kinetics of Fischer–Tropsch synthesis

The kinetic modeling of Fischer–Tropsch Synthesis on an iron catalyst starting from experimental data collected in a spinning basket reactor and proceeding through the Single Event approach to limit the number of independent parameters is reported. The elementary steps are based on the carbide mechanism with hydrogen assisted CO insertion and CH2 as the growth monomer. The evolution with the chain length of alkane‐, alkene‐, and alcohol‐selectivities in the synthesis product and their exponential‐like behavior is fundamentally generated by accounting for the effect of the structure of reactants and transition state intermediates on the rate coefficients. The olefin readsorption is included in the model but its influence on the product distribution is found to be negligible. © 2016 American Institute of Chemical Engineers AIChE J, 62: 1668–1682, 2016

Model System Studies of Wear Mechanisms of Hard Metal Tools when Cutting CFRP

Within the publicly-funded ZAFH research project SPANTEC light, the Competence Center for Tribology in Mannheim, Germany, examines the wear mechanisms of hard metal tools when cutting carbon-fiber-reinforced plastic in drilling and milling processes. The research project includes a test series with a tribological model test.The development of a new tribological simulation model system, a modified pin-on-disc system respectively called as edge-on-disc system, for cutting CFRP with hard metal composites is systematically analyzed and presented in this work. With this model system, a new method for tool wear analysis of the tool materials against CFRP is described.As result, there are further findings on the wear mechanism of the hard metal tungsten carbide in cobalt matrix (WC-Co) compared with carbon-fiber-reinforced plastic (CFRP). Research parameters for the tribological system in this paper are the WC-Co materials, with their WC-grain size and cobalt content, and the carbon fiber orientation.This study introduces a first approach to determine WC-Co wear compared with a CFRP-specimen, unlike most hard metal wear tests. Furthermore it is shown, that properties of the drilling and milling processes can be adapted by altering the model systems rotating speed and normal force. The results originality is also provided by using a variety of WC grain size classes (sub-μm to approximately 7-9μm grain size) and cobalt contents far above usual CFRP-machining properties to show the wear process between carbon fibers and WC-Co in detail.

On the chemistry of the carbides in a molybdenum base Mo-Hf-C alloy produced by powder metallurgy

The main focus of the present work is on the investigation of carbides present after sintering in the molybdenum base alloy MHC (Mo-Hf-C) which is produced via a powder route. The alloy has a nominal content of 0.65 at.% hafnium and 0.65 at.% carbon and can be hardened by precipitation of nm-sized hafnium carbides. The assessment of the molybdenum-rich corner of the ternary (quaternary) Mo-Hf-C(-O) phase diagram is required to exploit the full precipitation potential of the alloy. Thus, it is important to gain information about the accurate chemical composition of all individual phases. Therefore, the chemistry and structure of hafnium and molybdenum carbides were analyzed by means of X-ray diffraction, energy dispersive spectroscopy and atom probe tomography. The analysis of hexagonal molybdenum carbides revealed a significant amount of hafnium (∼1 at.%) in addition to molybdenum and carbon. The face centered cubic hafnium carbide phase on the other hand contains high amounts of oxygen (∼4.4 at.%) and molybdenum (∼2 at.%) in addition to small amounts of zirconium and nitrogen. In order to understand the high oxygen content in the hafnium carbide, energies of formation and lattice parameters for hafnium carbides with different contents of oxygen and molybdenum were calculated theoretically using an ab initio approach. Subsequently, the theoretically calculated lattice parameter of hafnium carbide was compared to the experimentally determined one of the carbide present in the MHC alloy. The experimental and modeling results are in good agreement. Finally, the formation mechanism of hafnium carbide during the sintering process is discussed.

How do the preparation methods impact the kinetic parameters of the two Co/Ni/Al2O3 nanocatalysts in Fischer–Tropsch process?

The influence of the preparation methods on the kinetics of Fischer–Tropsch (FT) synthesis in a fixed-bed reactor is investigated by kinetic tests employing two 80 %Co/20 %Ni/Al2O3 catalysts prepared through: (a) co-gelling support and cobalt, as well as nickel (SG), and (b) impregnating cobalt and nickel with sol–gel derived support (IM-SG). The kinetics for both the catalysts is developed on the basis of Langmuir–Hinshelwood–Hougen–Watson (LHHW) type models at the same range: T = 200–250 °C, P = 1–10 bar, gas hourly space velocity (GHSV) = 4200 h−1 and H2/CO mole ratio of 1/1–3/1. According to the enolic and carbide mechanisms, eight kinetic expressions of the CO consumption rate have been tested. The achieved best fitted model is subject to a good correlation between experimental and theoretical data; and the kinetic parameters are estimated by the use of Levenberg–Marquardt algorithm. Kinetic parameters such as rate constant (k) and activation energy (E a) suggest that the preparation method remarkably impact the FT behavior. The IM-SG sample was selected as the better catalyst due to a higher reaction rate and less activation energy comparing with the SG catalyst.

Synthesis and photoluminescence properties of in-situ synthesized core–shell (m-VC@C) nanocomposites

Core–shell structure of mesoporous vanadium carbide nanoparticles encapsulated with carbon layers (m-VC@C) have been successfully synthesized by single-step, non-toxic and economical route. The texture, morphology and optical properties of the obtained product were studied by various characterization techniques. X-ray diffraction analysis shows that the optimization of reaction time facilitates the reduction process of the precursor and hence carburization. High resolution transmission electron microscopy analysis reveals that the synthesized vanadium carbide nanoparticles with average size of 30–40 nm were encapsulated in 20–22 layers of carbon. High thermal stability of the obtained product was found at high temperatures. N2 adsorption/desorption isotherm shows that the sample has a specific surface area of 62.4560 m2/g and pore volume 0.30 cm3/g with pore size in the mesoporous range (3–14 nm). The formation mechanism of carbide and carbon layer has been explained on the basis of experimental results. The as-obtained m-VC@C shows good absorption and luminescence properties. Its application in photocatalytic degradation of the organic pollutant has been studied.

Insights into Hägg Iron-Carbide-Catalyzed Fischer–Tropsch Synthesis: Suppression of CH4 Formation and Enhancement of C–C Coupling on χ-Fe5C2 (510)

Probing the product selectivity of Fischer–Tropsch catalysts is of prime scientific and industrial importance—with the aim to upgrade products and meet various end-use applications. In this work, the mechanisms for CH4 formation and C1–C1 coupling on a thermodynamically stable, terraced-like χ-Fe5C2 (510) surface were studied by DFT calculations. It was found that this surface exhibits high effective barriers of CH4 formation for the three cases (i.e., 3.66, 2.81, and 2.39 eV), indicating the unfavorable occurrence of CH4 formation under FTS conditions. The C + CH and CH + CH are the most likely coupling pathways, which follow the carbide mechanism. Subsequently, the effective barrier difference between CH4 formation and C1–C1 coupling was used as a descriptor to quantify FTS selectivity. A comparison of the selectivity between this surface and the reported FTS catalysts’ surfaces was discussed in detail. More interestingly, this surface shows unexpectedly high C2+ selectivity. This indicates that manipul...

Polishing of optoelectronic components made of monocrystalline silicon carbide

The authors of this paper have substantiated the appropriateness of application of polishing suspensions of diamond micron powders, boron nitride and Ti3AlC2 MAX-phase powders in the study of the mechanism of polishing monocrystalline silicon carbide. They have proved suitability of using the following parameters as criteria for efficiency of the machining process: the specific value of the energy of transfer, the density of the energy of transfer, and the density of vibrational energy spent for the formation of debris particles. Computer modeling of the polished surface micro- and nanoprofile produced the profile charts from which the surface roughness parameters have been determined and the characteristic relations between the parameters and the most probable debris particle size have been defined.

Deformation and removal characteristics in nanoscratching of 6H-SiC with Berkovich indenter

In order to analyze the deformation characteristics and removal mechanism of the single crystal silicon carbide at the nanoscale, the nanoscratching test was conducted on 6HSiC (0001) by using a Berkovich diamond indenter. The deformation and removal characteristics of 6H-SiC during a nanoscratching process by using a sharp indenter are significantly different from those through a single-point diamond turning (SPDT) method. The characteristics in different regimes of the scratching process like plastic flow and the form of fracture behavior were analyzed. The ductile/brittle transition (DBT) of this material was discussed in detail theoretically and experimentally. X-ray diffraction and a laser micro-Raman spectrometer were used for phase analysis after the scratching process. The results indicate that phase transformation has not occurred during the nanoscratching process of this material.

Application of atmospheric CVD for internal surface coating of graphite conduit by silicon carbide

Mechanism of silicon carbide coating on the internal surface of the graphite tube of dimension ϕ 3mm ID×ϕ 10mm OD×ϕ 235mm and surface area ∼24.5cm2 has been investigated. Atmospheric pressure chemical vapor deposition (APCVD) method in mass controlled domain has been applied. Methyltrichlorosilane (MTS) as precursor was used in the present investigation. The coating was characterized by X-ray micro tomography (μCT). The coating thickness and composition were analyzed by scanning electron microscopy energy-dispersive spectroscopy (SEM–EDS). The coating phase analyzed by X-ray diffraction (XRD). It has been concluded that silicon carbide coating is a predominantly boundary layer phenomena and the engineering of the same could open up the prospects to accomplish SiC coating on complex pipe line structures on macro scale.

Microstructure and Properties of Laser Surface Carburized Titanium and Titanium Alloys

Titanium and titanium alloys are widely used in many fields due to their some characteristics such as light density, high strength and excellent corrosion resistance. However, their practical applications are limited by their poor surface mechanical properties. Laser surface carburizing can improve the surface properties of titanium and titanium via the formation of titanium carbide and other reinforcing phases. In this paper, the properties of the alloyed coatings, the growth morphologies and the formation mechanism of titanium carbide are discussed.