Metal Carbide Phase Research Articles

Implications for the Hydrogenation of Propyne and Propene with Parahydrogen due to the in situ Transformation of Rh2C to Rh0/C.

NMR spectroscopy studies using parahydrogen-induced polarization have previously established the existence of the pairwise hydrogen addition route in the hydrogenation of unsaturated hydrocarbons over heterogeneous catalysts, including those based on rhodium (Rh0). This pathway requires the incorporation of both hydrogen atoms from one hydrogen molecule to the same product molecule. However, the underlying mechanism for such pairwise hydrogen addition must be better understood. The involvement of carbon, either in the form of carbonaceous deposits on the surface of a catalyst or as a metal carbide phase, is known to modify catalytic properties significantly and thus could also affect the pairwise hydrogen addition route. Here, we explored carbon's role by studying the hydrogenation of propene and propyne with parahydrogen on a Rh2C catalyst and comparing the results with those for a Rh0/C catalyst obtained from Rh2C via H2 pretreatment. While the catalysts Rh2C and Rh0/C differ notably in the rate of conversion of parahydrogen to normal hydrogen as well as in terms of hydrogenation activity, our findings suggest that the carbide phase does not play a significant role in the pairwise H2 addition route on rhodium catalysts.

Effect of synergistic variation in Ti and Zr elements on the microstructure and properties of laser cladding AlCoCrFeNi high-entropy alloy coatings

AlCoCrFeNiTi1-xZrx (x = 0, 0.25, 0.5, 0.75, and 1.0) high-entropy alloy coatings were prepared on the surface of 45# steel by laser cladding, and the results showed that the coatings gradually changed from a double body-centered cubic (BCC) + metal carbide (MC) phase to a BCC + face-centered cubic (FCC) + MC phase with the increase in x. The grain morphology transitioned from equiaxed crystals to dendrites and was gradually refined. The solid solution strengthening, fine grain strengthening, and diffusion strengthening made the coatings' microhardness increase substantially to approximately 2.6 to 3.2 times of that of 45# steel. The wear resistance of the coatings was approximately 5.9 to 16.0 times that of the 45# steel, with the highest microhardness and the greatest wear resistance when x = 0.5. The wear mechanism of coatings was mainly abrasive wear and oxidation wear. The corrosion resistance of the coatings was better than that of 45# steel, but the corrosion resistance of the coatings gradually decreased with the increase in x. The increase in grain boundary density makes the galvanic corrosion intensify, and the corrosion resistance of the coating when x = 0, which was approximately 4.2 to 74.0 times that of other coatings. Grain refinement allows for more rapid formation of protective oxide layers with the increase in x, in this study, the resistance of the coating to high-temperature oxidation (800 °C) increased and then decreased with the increase in x, and the coating showed the greatest high-temperature oxidation resistance (800 °C) when x = 0.75, which was 1.4 to 3.5 times that of other coatings. However, the generation of microcracks reduced the high-temperature oxidation resistance of the coating when x = 1.0.

X-RAY DIFFRACTION STUDIES OF THE EFFECT OF PHASE COMPOSITION AND STRUCTURAL STATE CHARACTERISTICS ON THE TRIBOLOGICAL BEHAVIOR OF THE MOLYBDENUM- AND TUNGSTEN-BASED STRENGTHENING COATINGS

A comprehensive study of phase composition, structural state, parameters of fine atomic structure, mechanical and tribological properties of molybdenum-carbon and tungsten-carbon-based coatings deposited by reactive magnetron sputtering in an acetylene-argon gas mixture has been carried out. It has been shown that the resulting coatings have a nanocomposite diamond-like carbon (DLC) structure based on the metal and the metal carbide phases with close sizes of coherently diffracting domains (CDD), approximately 3–7 nm, and on hydrogenated amorphous carbon. The coating nanohardness values were 13–15 and 20–23 GPa for the Mo-DLC and W-DLC coatings, respectively. The tribological tests have demonstrated that the Mo- and W-DLC coatings can reduce friction and effectively protect the steel surfaces hardened by them both under dry friction and under boundary lubrication conditions.

Enhanced thermal stability of W-25Re/Ti/carbon-carbon composites via gradient diffusion-bonding

A carbon-carbon (C-C) composite was joined with a W-25Re alloy through a Ti interlayer using the diffusion-bonding technique. The stacked components were hot-pressed with an applied pressure of 25 MPa at 1600 °C for two hours. The cross-sections of the joints were investigated to examine the intermediate phase and elemental distribution near the Ti interlayer. It was confirmed that tungsten, rhenium, and carbon diffused into the Ti interlayer to form various intermediate phases; as a result, the joining interface remained stable through the gradient distribution of the intermeditate phases. The Ti interlayer was divided into two regions according to the diffusion range, and the Ti reacted with the carbon that had rapidly diffused from the C-C composite to generate precipitates. To evaluate the high-temperature durability of the joint interface, a thermal stability test was conducted at 2000 °C in an Ar atmosphere. The diffusion of the components accelerated at high temperatures, forming various solid solution phases (W(Re)-Ti and W-25Re-Ti) and metal carbide phases ((W,Ti)C1−x and TiC) in the expanded interfacial region.

Study of (Ti,Zr)C:H/a-C:H nanocomposite thin film formation by low temperature reactive high power impulse magnetron sputtering

Single-phase nanocrystalline hydrogen-containing titanium zirconium carbide thin films and hydrogen-containing nanocomposite thin films consisting of a nanocrystalline hydrogen-containing titanium zirconium carbide phase in combination with an amorphous hydrogen-containing carbon network, (Ti1-xZrx)C1-δ:H/a-C:H, were deposited by low temperature reactive high power impulse magnetron sputtering (HiPIMS) in a methane-argon atmosphere. High power impulse magnetron sputtering offers a high degree of ionization of sputtered particles and therefore a precise control of the thin film growth. The thin film deposition in the Ti-Zr-C-H system was carried out by a combinatorial approach via the variation of both the titanium-to-zirconium ratio as well as the carbon and hydrogen content in the coatings. A single, metallic, segmented target consisting of two triangles of pure titanium and zirconium enabled the deposition of thin films with different titanium-to‑zirconium-ratio dependent of the substrate position in front of the target within one deposition process. By variation of the CH4 reactive gas flow at a constant Argon gas flow the carbon and hydrogen content of individual thin films was precisely adjusted. During the HiPIMS process a short pulse time of 50 μs was combined with a comparably low duty cycle of 3.3% in order to ensure a high ionization. In order to investigate the chemical composition of the thin films including hydrogen content electron probe microanalysis was combined in a self-consistent way with elastic recoil detection analysis. Structural analysis was done by a combination of different techniques: Raman spectroscopy suggests metal‑carbon bonds attributable to a nanocrystalline metal carbide phase as well as the formation of an amorphous hydrogen-containing carbon network. X-ray diffraction shows fcc (Ti1-xZrx)C1-δ:H grains with different sizes in all coatings. Transmission electron microscopy confirms the structure model of single phase nanocrystalline (Ti1-xZrx)C1-δ:H grains embedded in a hydrogen-containing amorphous carbon matrix. Scope of this work is the low temperature deposition of the (Ti,Zr)C1-δ:H thin films and the (Ti,Zr)C1-δ:H/a-C:H nanocomposites by HiPIMS with precise control of the resulting chemical composition and microstructure.

Determination of atomic positions from time resolved high resolution transmission electron microscopy images

For many reaction processes, such as catalysis, phase transformations, nanomaterial synthesis etc., nanoscale observations at high spatial (sub-nanometer) and temporal (millisecond) resolution are required to characterize and comprehend the underlying factors that favor one reaction over another. The combination of such spatial and temporal resolution (up to 600 µs), while rich in information, produces a large number of snapshots, each of which must be analyzed to obtain the structural (and thereby chemical) information. Here we present a methodology for automated quantitative measurement of real-time atomic position fluctuations in a nanoparticle. We leverage a combination of several image processing algorithms to precisely identify the positions of the atomic columns in each image. A geometric model is then used to measure the time-evolution of distances and angles between neighboring atomic columns to identify different phases and quantify local structural fluctuations. We apply this technique to determine the atomic-level fluctuations in the relative fractions of metal and metal-carbide phases in a cobalt catalyst nanoparticle during single-walled carbon nanotube (SWCNT) growth. These measurements provided a means to obtain the number of carbon atoms incorporated into and released from the catalyst particle, thereby helping resolve carbon reaction pathways during SWCNT growth. Further we demonstrate the use of this technique to measure the reaction kinetics of iron oxide reduction. Apart from reducing the data analysis time, the statistical approach allows us to measure atomic distances with sub-pixel resolution. We show that this method can be applied universally to measure atomic positions with a precision of 0.01 nm from any set of atomic-resolution video images. With the advent of high time-resolution direct detection cameras, we anticipate such methods will be essential in addressing the metrology problem of quantifying large datasets of time-resolved images in future.

Sonochemically Generated Air-Stable Bimetallic Nanopowders of Group 4 Transition Metals with Aluminum

Air-stable reactive metal nanopowders (RMNPs) of group 4 transition metals and aluminum were produced via a sonochemically agitated reaction of MCl4 (M = Ti, Zr, Hf) with 4 equiv of lithium aluminum hydride (LiAlH4, LAH) under an atmosphere of N2 in Et2O, generating in situ the tetrahydroaluminate of the respective group 4 metal. The subsequent decomposition of the tetrahydroaluminate complex via elimination of H2 produced black powders that were further dried and subsequently annealed in vacuo to temperatures ranging from 620 to 860 °C, simultaneously subliming LiCl and Al byproducts for ease of purification, and producing air-stable RMNPs, which were then analyzed by elemental analysis, X-ray diffraction (XRD), scanning electron microscopy/energy-dispersive spectroscopy (SEM/EDS), magic-angle spinning (MAS) NMR, thermogravimetric analysis/differential scanning calorimetry (TGA/DSC), and oxygen bomb calorimetry. Elemental analysis of the materials showed a significant presence of H, C, and N in the product materials. For each of the metal combinations the XRD analysis showed the presence of two main phases that have the same lattice parameters as the cubic metal carbide phase and the tetragonal Al3M phase of each of the group 4 transition metals. Each of the phases exhibited nanocrystallites, with the more refractory carbide phase producing approximately 5 nm crystallites, and the aluminide phase producing 20 nm crystallites based on Scherrer analysis. SEM confirmed that the materials consist of small individual crystallites that are agglomerated into larger structures. The observed 27Al MAS NMR resonances can be assigned to Al0, Al3M alloys, and Aln+ ionic species. The TGA data showed the superior stability of these materials exhibiting reactivity with O2 only at temperatures approaching 400 °C. The combustion energy of these RMNPs was then measured using oxygen bomb calorimetry, which confirmed that the air-exposed RMNPs are capable of producing at least 26 kJ/g when combusted in O2 (as in the case of the titanium aluminum material), and therefore may have potential to be useful fuels.

Bulk diffusion induced structural modifications of carbon-transition metal nanocomposite films

The influence of transition metal (TM = V,Co,Cu) type on the bulk diffusion induced structural changes in carbon:TM nanocomposite films is investigated. The TMs have been incorporated into the carbon matrix via ion beam co-sputtering, and subsequently the films have been vacuum annealed in the temperature range of 300 – 700 °C. The structure of both the dispersed metal rich and the carbon matrix phases has been determined by a combination of elastic recoil detection analysis, x-ray diffraction, transmission electron microscopy, and Raman spectroscopy. The as-grown films consist of carbidic (V and Co) and metallic (Cu) nanoparticles dispersed in the carbon matrix. Thermal annealing induces surface segregation of Co and Cu starting at ≥ 500 °C, preceded by the carbide-metal transformation of Co-carbide nanoparticles at ∼ 300 °C. No considerable morphological changes occur in C:V films. In contrast to the surface diffusion dominated regime where all the metals enhance the six-fold ring clustering of C, in the bulk diffusion controlled regime only Co acts as a catalyst for the carbon graphitization. These results are consistent with the metal-induced crystallization mechanism in the C:Co films. The results are discussed on the basis of the metal-carbide phase stability, carbon solubility in metals or their carbides, and interface species.

Preparation and Nanomechanical Characterisation of Metal Containing Amorphous Hydrogenated Carbon Nanocomposite Films

The relationship between metal-induced chemical bonding and the mechanical properties of Me/a:C-H (Me- Ti and Ni) films are discussed. Nanocomposite films were deposited onto the Si substrates via biased target ion beam sputtering of metal combined with reactive ion beam deposition of a:C-H using CH4/Ar gas mixture. The chemical composition, microstructure and mechanical properties were characterized using X-ray photoelectron spectroscopy (XPS), Raman spectroscopy and nanoindentation. XPS analysis revealed that both Ti and Ni atoms were preferentially bonded to carbon to form its metal carbide phase. Irrespective of its kind, both the metal carbide clusters induces more graphite like bondings in a:C-H matrix. From the nanoindentation analysis it was found that there is no crack propagation and damage around the indent region even at very high indentation loads. Additionally, it was found that a:C-H film shows higher hardness compared to that of Ti/a:C-H and Ni/a:C-H films.

Microwave atmospheric pressure plasma for surface treatment and reactive coating on steel surfaces

The microwave atmospheric plasma technology developed at DANA corporation has previously been used for various heat treatment applications. In this paper, we demonstrate the use of atmospheric plasma assisted chemical vapor deposition (APACVD) process to coat steel surfaces. The method described in this paper involves two steps. The first step is to coat a Si based interlayer and then to coat C either by CH 4 or C 2H 2 containing plasma. At the end of the coating process, a thick, colorful, reactive coating is formed on the steel surface. The XRD and compositional analyses have established the presence of metal carbide phases such as Cr 7C 3, Fe 15.1C in these coatings. These phases are well bonded to the steel surface with no separate interlayer formation. Hardness tests show that carbide phases formed in a nitrogen containing plasma are 2.5 times harder compared to the bulk. The faster deposition rates at atmospheric pressure clearly indicate that this process is not only rapid (∼ 0.2 μm/min) but also cost-effective.

CALPHAD modelling of the unmixing of transition metal carbide phases

Phase separation in a pseudo-binary solid solution of the form (M 1,M 2)C where M 1 and M 2 are chosen among minor addition in 9% Cr, martensitic-ferritic steels such as Nb, V, Ta and other transition elements such as Hf and Zr has been studied using the Calphad-Thermocalc approach. Ternary excess parameters accounting for phase equilibria in pseudo-binary sections have been optimised using suitable combinations of already published excess parameters which have been calculated on a physical basis. The ternary parameters obtained by optimisation were checked, when possible, with experimental information on isothermal ternary sections.

Low-friction carbon-rich carbide coatings deposited by co-sputtering

Low-friction coatings are used more and more frequently, particularly in situations and applications with insufficient or no lubrication. A good example of such coatings is amorphous carbon, which is produced both in pure form (a-C:H) and doped with metal (Me-C:H). The knowledge of what actually occurs when one metal in a Me-C:H coating is exchanged with another has so far been rather limited. Also, when producing these films hydrogen is incorporated in the substrate as well as in the film, which can be detrimental to the overall properties. Here, a newly adopted co-sputtering technique, utilizing a carbon target partly covered by metal-foil strips, was used to deposit non-hydrogenated carbon coatings alloyed with Ta, W and Zr on ball-bearing steel (BBS) substrates. The metal content varied between 0 and 41 at.%, and the resulting films were analyzed with respect to phase composition and textures, chemical composition, microstructural morphology, as well as mechanical and tribological properties. All alloyed coatings displayed a nanocomposite microstructure, with 3–6 nm metal-carbide crystallites embedded in a matrix of amorphous carbon. The amount of metal-carbide phase increased with increasing amounts of metal which led to a large increase in hardness and elastic modulus. An increased metal content did however not affect the carbide size to any notable extent. Ball-on-disk tests show that metal additions cause a sharp drop in friction coefficient from 0.21 to about 0.05, depending on the metal used. This is however accompanied by an increase in wear rate. The coating best combining low friction and low wear rate was alloyed with 20 at.% Ta. Best possible protection of the counter surface was offered by coatings containing 30 at.% Ta or more.

X-ray photoelectron spectroscopy study of TiC films grown by annealing thin Ti films on graphite

Thin Ti films with thicknesses in the range between 200 and 300 Å were evaporated onto graphite substrates under UHV conditions. The samples were annealed up to temperatures of 1470 K. After reaching steady state, subsequent x-ray photoelectron spectroscopy analysis showed formation of different TiCx phases dependent on the annealing temperature. For temperatures lower than 600 K, little carbide formation occurs in the metal film; a mixed metal–carbide phase forms only at the metal–graphite interface. At higher temperatures, the concentration ratio x of carbidic C to Ti ranges from about 0.6 at 750 K to about 0.95 at 1200 K and higher annealing temperatures. It was observed that the binding energies of the Ti 2p electrons rise continuously with increasing concentration ratio of carbidic C to Ti. Depth profiles were made to investigate chemical phases in the Ti film, as well as at the interface. It is illustrated from the profiles that formation of carbide proceeds from the metal–graphite interface, as well as from surface segregated carbon and advances into the Ti layer from both sides.

New ferromagnetic materials containing free metal and metal carbide phases from aminotriazole complexes with transition metals

New ferromagnetic materials (M, M′)C have been synthesized by thermal decomposition in absence of oxygen of animotriazole complexes with transition metals (MAX) combined with metal oxalates (M′ox). Free metal and metal carbide phases were found in the (M, M′)C materials by X-ray diffraction analysis.

Synthesis and characterization of nanophase Group 6 metal (M) and metal carbide (M2C) powders by chemical reduction methods

The group 6 metal chlorides CrCl[sub 3], MoCl[sub 3], MoCl[sub 4], and WCl[sub 4] were reduced in toluene solution with NaBEt[sub 3]H at room temperature to form the corresponding metal colloids in high yield. When the same metal chlorides were reduced in THF solution with LiBEt[sub 3]H and NaBEt[sub 3]H metal carbides (M[sub 2]C) were formed in approximately 95% yield. The metal and metal carbide colloids were shown to be comprised of 1-5-nm-sized particles by TEM. The metal and metal carbide powders derived from the colloids by centrifuging and washing with various solvents were characterized with SEM, TEM, EDS, and ED which showed that the powders were comprised of agglomerates of the primary 1-5-nm-sized crystallite. X-ray powder diffraction studies of these black powders exhibited broad peaks for the s-prepared powders at room temperature where the crystallite size estimated from the broadening analysis generally corresponded to the primary particle size as determined by TEM. Variable-temperature X-ray powder diffraction allowed unambiguous identification of the crystalline metal and metal carbide phases present. Carbon and hydrogen combustion elemental analysis was also used to identify the metal and metal carbide materials. Thermogravimetric analysis in air provided information on the oxidation behavior of these materials andmore » also enabled distinction between the formation of metals versus metal carbides, M[sub 2]C. Experiments to determine the exact origin of the carbide carbon were inconclusive. Treatment of nanocrystalline W with THF and THF/LiBet[sub 3]H solutions did not result in further reaction to form the metal carbide. 38 refs., 10 figs.« less

Correlation of 3d transition metal carbide phase formation with 3d electrons by carbon ion implantation

A systematic study of carbide phase formation by carbon ion implantation showed NaCl type f.c.c. carbide formation in Ti, V and Cr, and hexagonal carbide formation in Fe, Co and Ni. No carbide formation was observed in the system Cu, Ag, Au and Zn. This indicates that 3d transition metal carbide phase formation by carbon ion implantation correlates with incomplete 3d shell electrons. The s shell electrons do not contribute. The electronic factor was further demonstrated in the structural alternation from f.c.c. in systems of Ti, V and Cr to hexagon in Fe, Co and Ni with increasing 3d electron concentration, which follows the Hume-Rothery rule.

Reaction sintering of diamond using a binary solvent-catalyst of the Fe-Ti system

Reaction sintering of diamond was investigated using a starting mixed powder of purified natural graphite and a binary solvent-catalyst of the Fe-Ti system under high pressure (7G Pa) and temperature (1700°C) conditions for e treatment time of 1 to 15 min. Diamond sintered compact of about 100% conversion ratio from graphite todiamond was obtained with the binary solvent-catalyst content: 11.4 to 17.0 vol% (30 to 40wt%) Fe and 6.6 to 7.7vol% (10wt%) Ti. The sintered compact having the bulk density of 4.1 to 5.5g cm−3, consisted of diamond phase and metal carbide (Fe3C and TiCx) phase. The Vickers microhardnesm (under 1000 g load) mfthesintered diamond phase was >8OO0, while that ofthe metal carbide phase was 1000 to 2000. The transformation from graphite to diamond proceeded in a short time (< 1 min), which was followed by a particle joining between the formed diamond grains, when the densification would be attained at the reaction time of 15 min by pooling out the melt of carbon and solvent-catalyst.

Atomic and microstructural characterization of metal impurities in synthetic diamonds

The phase and microstructure of Ni, Co, and Fe impurities found in synthetic diamonds have been characterized in some detail using a combination of extended x-ray absorption fine structure (EXAFS) utilizing intense synchrotron radiation as a light source, and conventional transmission electron microscopy (TEM), x-ray diffraction, and fluorescence analyses. In all three cases, the metal impurity exists as an fcc metallic phase dispersed in the diamond matrix. The particles are submicron in size and not facetted. There was no evidence of a metal carbide phase in these systems. Quantitative simulations of the first-shell EXAFS signal showed that the Co and Ni particles contain, respectively, 2.3 and 1.5 at. % of carbon in solution.