First-principles study on the electronic properties and Schottky barrier of WC/ W S 2 and WC/ WS e 2 heterostructures
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Materials Science↗
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Fundamental Interactions of Bimetallic Cu x Pd y ( x + y = 4) Clusters Supported on the α-WC(0001) Surface and Their Performance for CO 2 Adsorption and Dissociation
The tungsten carbide α-WC(0001) surface, an active system for the activation of H 2 and important hydrogenation processes involving unsaturated hydrocarbons, can serve as a support of bimetallic clusters to produce materials with unique catalytic properties, opening routes for a wide range of technical applications. In particular, Cu x Pd y clusters are of particular interest because they combine metals with different properties. A stochastic method was applied to obtain the geometry of Cu x Pd y (x + y = 4) bare clusters, evaluating thousands of possibilities to obtain stable structures, yielding one isomer for Cu 4 , Cu 2 Pd 2 , Cu 1 Pd 3 , and Pd 4 and two isomers for Cu 3 Pd 1 . These clusters were supported on C and W terminations of the tungsten carbide (0001) surface, exploring all of the binding possibilities. The adsorption energies on the C and W terminations are in the ranges from −2.51 to −3.02 eV and from −2.26 to −3.30 eV, respectively. The strongest and weakest binding was seen for monometallic Cu 4 and Pd 4 clusters on both C and W terminations, while the Cu-Pd bimetallics have intermediate adsorption energies but lack a clear trend in terms of composition. The location of Cu x Pd y clusters over the (0001) surface induces a decrease in the work function relative to the pristine surface, while the cluster-surface Bader charge transfer and variations in the partial density of states point to changes in the electronic structure of the carbide atoms upon binding of the metallic clusters. The d-band center of the Cu x Pd y deposited on WC(0001) indicates an intermediate reactivity among Cu(111) and Pd(111) surfaces, modulating the reactivity with small numbers of Cu and Pd atoms, i.e., atom economy in catalyst design. The likelihood of existence of the most stable Cu x Pd y (x + y = 4) clusters in the temperature range of 298-400 K is 100%. The composite Cu x Pd y /α-WC(0001) (x + y = 4), is a nontrivial system since 22 isomers are needed to completely describe its structural properties. Among the isomers, seven structures are necessary to represent Cu 3 Pd 1 /α-WC(0001), five for Pd 4 /α-WC(0001), two for Cu 4 /α-WC(0001), and four for Cu 2 Pd 2 /α-WC(0001) and Cu 1 Pd 3 /α-WC(0001). The large number of cluster isomers supported on the tungsten carbide surface opens the door for several applications in the heterogeneous catalysis of the Cu x Pd y /α-WC(0001) composite, with the possibility of modulating the geometric, electronic, and chemical properties according to a desired application. Test studies for the adsorption of CO 2 indicate that the Cu x Pd y /α-WC(0001) composites are highly active for the adsorption and decomposition of the molecule, with bimetallic and admetal-carbide interactions playing a key role in the binding performance. In conclusion, this high activity indicates that these systems should be useful as catalysts for the conversion of CO 2 to oxygenates or light alkanes.
37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
The α-WC(0001) Surface as a Hydrogen Sponge: A First Principle Study of H 2 Dissociation and Formation of Low and High Coverages
Tungsten carbide (WC) displays a Pt-like behavior in catalysis, applied in hydrogenation processes. Numerous theoretical studies have modeled the behavior and use of adsorbed hydrogen without obtaining a general picture, missing basic links between H 2 dissociation and generation of high surface coverage (Θ H >0.5 ML). Here, in this study, the capability of C- and W-terminations of the α-WC(0001) surface is analyzed to dissociate several H 2 molecules to produce coverages, Θ H , ranging from low to very high values (0.13<Θ H <2.00 ML). Density functional theory and an ab initio atomistic thermodynamic were used to achieve the conditions for H 2 dissociation. The WC–C surface has higher capacity to dissociate H 2 molecules than WC–W. However, both surfaces can reach full surface coverage, Θ H =1 ML, at mild ambient conditions, T=300 K and P=1 atm, and even up to 500 K at low and high pressures. The H-adatoms on WC–W are more labile than on WC–C. The binding of adsorbates is hindered at high Θ H , implying a need to modulate Θ H according to the application. The results give the basis to understand the capabilities of WC-based catalysts in hydrogenation-related reactions, with the advantage of WC being a hydrogen reservoir at mild practical catalytic conditions.
03 NATURAL GAS↗
W-ZrC composites prepared by reactive melt infiltration of Zr 2 Cu alloy into binder jet 3D printed WC preforms
W-ZrC composites were successfully prepared by reactive melt infiltration (RMI) of stoichiometric and excess amounts of Zr 2 Cu into sintered and un-sintered WC preforms made from binder jet 3D printing. The focus of this work was to study the conversion of reactant powders and liquid infiltrant with varying preform density and infiltrant amount by controlling the processing time to reach high conversion yield while understanding the phase composition, microstructure, and hardness. To investigate the effect of time, the reactive melt infiltration was conducted at 1400 °C for 2, 4 and 8 h in a furnace with 96% Ar - 4% H2 gas atmosphere. The increase in reaction time from 2 to 8 h increased the W and W 2 C phase contents and decreased the ZrC phase content when using sintered WC preforms. Samples prepared from un-sintered WC preforms exhibited improved reactive melt infiltration compared to sintered samples, and there was no detectable W 2 C phase and nearly full consumption of WC. Similar to sintered WC samples, the content of W and ZrC phases increased with the increase in time from 2 to 8 h. The interfaces and phases at reaction interfaces were investigated using electron diffraction analysis and S/TEM-EDS to understand material stability; the phases were identified and consistent with XRD analysis. Additionally, there was no Cu present at the interfaces. Increasing the amount of infiltrant led to better reactive melt infiltration. In general, the hardness increased with reaction time and the highest Vickers hardness was found in the W-ZrC sample formed from sintered WC reacted with excess Zr 2 Cu. Finally, this research addresses the critical comparison of sintering and RMI time and shows that by using un-sintered samples for 8 h we are able to achieve W-ZrC composites with fewer undesired phases.
36 MATERIALS SCIENCE↗
Pushing Cu uphill of the volcano curve: Impact of a WC support on the catalytic activity of copper toward the hydrogen evolution reaction
Here, the adsorption of atomic H and H 2 on copper mono- and submonolayers supported on hexagonal WC(0001) surfaces has been investigated using density functional theory with the Perdew–Burke–Ernzerhof exchange correlation functional and D2 van der Waals corrections. Results evidence the impact of the termination of the carbide substrate on fundamental properties of Cu adatoms, and, hence, on the stability of molecular and atomic hydrogen, defining copper's catalytic activity for hydrogen evolution reaction. Using H adsorption energy as a descriptor, catalytic activity of Cu adlayers for hydrogen evolution reaction was estimated using traditional volcano curves and a curve, obtained at low hydrogen coverage. Obtained results evidence that copper adlayers supported on the WC may present a viable low-cost alternative to noble metal-based catalysts, with improved catalytic activity compared to that of copper. This, potentially, can be a useful basis for designing and developing novel functional materials with predetermined catalytic properties.
08 HYDROGEN↗
Residual elastic strain evolution due to thermal cycling of a ceramic-metal composite (WC-Cu) via high energy X-ray diffraction and analytical modeling
Residual stress, when superimposed with in-service loading, can significantly reduce the lifetime and performance of a component. Ceramic-metal composites are susceptible to residual stresses due to the thermal expansion mismatch of the ceramic and metallic phases. The WC-Cu composite explored in the present study provides a promising combination of thermal conductivity and strength properties, while exhibiting counterintuitive improvements in strength and ductility after thermal cycling. Further, this work quantifies the evolution of the residual elastic strains as a result of processing and cyclic thermal loading in a co-continuous WC-Cu composite through experimental high energy X-ray diffraction and kinetics-based modeling. Both analyses indicate that processing-induced residual tensile stress in the copper phase is relieved upon subsequent thermal cycling, with kinetics modeling revealing the cyclic-dependent nature of the active power-law creep mechanisms. The results indicate that, through stress relaxation, this material system maintains structural stability during thermal cycling. The illustrated kinetics of relaxation can inform general material processors and designers of ceramic-metal composites to minimize detrimental residual stress and improve performance of these material systems.
36 MATERIALS SCIENCE↗
Materials Data on Ni10(WC)3 by Materials Project
Ni10(WC)3 crystallizes in the hexagonal P6_3/mmc space group. The structure is three-dimensional. W is bonded in a distorted rectangular see-saw-like geometry to two equivalent Ni and two equivalent C atoms. Both W–Ni bond lengths are 2.49 Å. Both W–C bond lengths are 2.04 Å. There are three inequivalent Ni sites. In the first Ni site, Ni is bonded in a 12-coordinate geometry to two equivalent W and two equivalent Ni atoms. Both Ni–Ni bond lengths are 2.41 Å. In the second Ni site, Ni is bonded in a distorted bent 150 degrees geometry to one Ni and two equivalent C atoms. The Ni–Ni bond length is 2.61 Å. Both Ni–C bond lengths are 1.99 Å. In the third Ni site, Ni is bonded to twelve Ni atoms to form NiNi12 cuboctahedra that share edges with six equivalent CNi4W2 octahedra and faces with two equivalent NiNi12 cuboctahedra. C is bonded to two equivalent W and four equivalent Ni atoms to form CNi4W2 octahedra that share corners with six equivalent CNi4W2 octahedra and edges with two equivalent NiNi12 cuboctahedra. The corner-sharing octahedra tilt angles range from 33–43°.
36 MATERIALS SCIENCE↗
Materials Data on WC by Materials Project
WC1 is Halite, Rock Salt structured and crystallizes in the cubic Fm-3m space group. The structure is three-dimensional. W4+ is bonded to six equivalent C4- atoms to form a mixture of edge and corner-sharing WC6 octahedra. The corner-sharing octahedral tilt angles are 0°. All W–C bond lengths are 2.19 Å. C4- is bonded to six equivalent W4+ atoms to form a mixture of edge and corner-sharing CW6 octahedra. The corner-sharing octahedral tilt angles are 0°.
36 MATERIALS SCIENCE↗
Materials Data on WC by Materials Project
WC1 is Tungsten Carbide structured and crystallizes in the hexagonal P-6m2 space group. The structure is three-dimensional. W4+ is bonded to six equivalent C4- atoms to form a mixture of distorted corner, edge, and face-sharing WC6 pentagonal pyramids. All W–C bond lengths are 2.21 Å. C4- is bonded to six equivalent W4+ atoms to form a mixture of distorted corner, edge, and face-sharing CW6 pentagonal pyramids.
36 MATERIALS SCIENCE↗
Materials Data on U(WC)4 by Materials Project
UW4C4 crystallizes in the tetragonal P4/m space group. The structure is three-dimensional. there are two inequivalent U4+ sites. In the first U4+ site, U4+ is bonded in a body-centered cubic geometry to eight equivalent C4- atoms. All U–C bond lengths are 2.57 Å. In the second U4+ site, U4+ is bonded in a body-centered cubic geometry to eight equivalent C4- atoms. All U–C bond lengths are 2.50 Å. There are two inequivalent W3+ sites. In the first W3+ site, W3+ is bonded in a rectangular see-saw-like geometry to four C4- atoms. There are a spread of W–C bond distances ranging from 2.15–2.19 Å. In the second W3+ site, W3+ is bonded to four C4- atoms to form corner-sharing WC4 trigonal pyramids. There are a spread of W–C bond distances ranging from 2.08–2.11 Å. There are two inequivalent C4- sites. In the first C4- site, C4- is bonded to two equivalent U4+ and four W3+ atoms to form a mixture of corner, edge, and face-sharing CU2W4 octahedra. The corner-sharing octahedra tilt angles range from 0–68°. In the second C4- site, C4- is bonded to two equivalent U4+ and four W3+ atoms to form a mixture of corner, edge, and face-sharing CU2W4 octahedra. The corner-sharing octahedra tilt angles range from 0–67°.
36 MATERIALS SCIENCE↗
Materials Data on WC by Materials Project
WC1 is Tetraauricupride structured and crystallizes in the cubic Pm-3m space group. The structure is three-dimensional. W4+ is bonded in a body-centered cubic geometry to eight equivalent C4- atoms. All W–C bond lengths are 2.37 Å. C4- is bonded in a body-centered cubic geometry to eight equivalent W4+ atoms.
36 MATERIALS SCIENCE↗
Materials Data on WC by Materials Project
WC1 is Zincblende, Sphalerite structured and crystallizes in the cubic F-43m space group. The structure is three-dimensional. W4+ is bonded to four equivalent C4- atoms to form corner-sharing WC4 tetrahedra. All W–C bond lengths are 2.04 Å. C4- is bonded to four equivalent W4+ atoms to form corner-sharing CW4 tetrahedra.
36 MATERIALS SCIENCE↗
Materials Data on U(WC)4 by Materials Project
UW4C4 crystallizes in the tetragonal P4/m space group. The structure is three-dimensional. there are two inequivalent U4+ sites. In the first U4+ site, U4+ is bonded in a body-centered cubic geometry to eight equivalent C4- atoms. All U–C bond lengths are 2.49 Å. In the second U4+ site, U4+ is bonded in a body-centered cubic geometry to eight equivalent C4- atoms. All U–C bond lengths are 2.53 Å. There are two inequivalent W3+ sites. In the first W3+ site, W3+ is bonded to four C4- atoms to form corner-sharing WC4 trigonal pyramids. There are one shorter (2.09 Å) and three longer (2.11 Å) W–C bond lengths. In the second W3+ site, W3+ is bonded in a rectangular see-saw-like geometry to four C4- atoms. There are a spread of W–C bond distances ranging from 2.13–2.16 Å. There are two inequivalent C4- sites. In the first C4- site, C4- is bonded to two equivalent U4+ and four W3+ atoms to form a mixture of edge, face, and corner-sharing CU2W4 octahedra. The corner-sharing octahedra tilt angles range from 0–67°. In the second C4- site, C4- is bonded to two equivalent U4+ and four W3+ atoms to form a mixture of edge, face, and corner-sharing CU2W4 octahedra. The corner-sharing octahedra tilt angles range from 0–68°.
36 MATERIALS SCIENCE↗
Mechanical evaluation of WC-Co materials with varying microstructures
Tungsten carbide–cobalt materials are useful in a variety of extreme applications due to a desirable blend of properties, yet the technology has not significantly changed since their initial development in the 1920s. The mechanical properties of this class of materials is highly dependent on two variables, the size of the tungsten carbide grains, and the amount of binder phase present in the final body. In this study, the amount of binder phase is isolated across three commercial materials from the same manufacturer with three different grain sizes to investigate the effect on mechanical properties. The mechanical properties investigated are indentation hardness, flexure and tensile strength, as well as fracture toughness. In general, an increase in hardness and tensile strength with decreasing grain size was observed, while the fracture toughness showed the opposite trend with toughness increasing with increasing grain size. The flexure strength results did not show a correlation to grain size. Fractographic analysis identified the dominant strength-limiting flaw for each sample, which largely were in the form of porosity. Other flaws types, such as inclusions from the milling process, clusters of large grains, and machining cracks from the surface finishing process, were also identified. Finally, Weibull analysis was performed and deemed appropriate for analysis of these materials, but strength-size scaling was not conducted due to the variability in the strength-limiting feature between the different specimen geometries.
36 MATERIALS SCIENCE↗
Microstructure and mechanical properties of WC and Ti(C0.7N0.3) modified NbC solid solution cermets
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Huang, S. G.↗