HfCsub.xNsub.1−x nanoparticles were synthesized using the urea-glass route, employing hafnium chloride, urea, and methanol as raw materials. The synthesis process, polymer-to-ceramic conversion, ...microstructure, and phase evolution of HfCsub.xNsub.1−x/C nanoparticles were thoroughly investigated across a wide range of molar ratios between the nitrogen source and the hafnium source. Upon annealing at 1600 °C, all precursors demonstrated remarkable translatability to HfCsub.xNsub.1−x ceramics. Under high nitrogen source ratios, the precursor exhibited complete transformation into HfCsub.xNsub.1−x nanoparticles at 1200 °C, with no observed presence of oxidation phases. In comparison to HfOsub.2, the carbothermal reaction of HfN with C significantly reduced the preparation temperature required for HfC. By increasing the urea content in the precursor, the carbon content of the pyrolyzed products increased, leading to a substantial decrease in the electrical conductivity of HfCsub.xNsub.1−x/C nanoparticle powders. Notably, as the urea content in the precursor increased, a significant decrease in average electrical conductivity values was observed for the R4-1600, R8-1600, R12-1600, and R16-1600 nanoparticles measured at a pressure of 18 MPa, yielding values of 225.5, 59.1, 44.8, and 46.0 S·cmsup.−1, respectively.
Boron carbide plays a crucial role in various extreme environment applications, including thermal barrier coatings, aerospace applications, and neutron absorbers, because of its high thermal and ...chemical stability. In this study, the temperature-dependent elastic stiffness constants, thermal expansion coefficient, Helmholtz free energy, entropy, and heat capacity at a constant volume (Csub.v) of rhombohedral Bsub.4C have been predicted using a quasi-harmonic approach. A combination of volume-dependent first-principles calculations (density functional theory) and first-principles phonon calculations in the supercell framework has been performed. Good agreement between the elastic constants and structural parameters from static calculations is observed. The calculated thermodynamic properties from phonon calculations show trends that align with the literature. As the temperature rises, the predicted free energy follows a decreasing trend, while entropy and Csub.v follow increasing trends with temperature. Comparisons between the predicted room temperature thermal expansion coefficient (TEC) (7.54×10sup.−6 Ksup.−1) and bulk modulus (228 GPa) from the quasi-harmonic approach and literature results from experiments and models are performed, revealing that the calculated TEC and bulk modulus fall within the established range from the limited set of data from the literature (TEC = 5.73–9.50 ×10sup.−6 Ksup.−1, B = 221–246 GPa). Temperature-dependent Csub.ijs are predicted, enabling stress analysis at elevated temperatures. Overall, the outcomes of this study can be used when performing mechanical and thermal stress analysis (e.g., space shielding applications) and optimizing the design of boron carbide materials for elevated temperature applications.
Due to the complex products and irradiation-induced defects, it is hard to understand and even predict the thermal conductivity variation of materials under fast neutron irradiation, such as the ...abrupt degradation of thermal conductivity of boron carbide (B.sub.4C) at the very beginning of the irradiation process. In this work, the contributions of various irradiation-induced defects in B.sub.4C primarily consisting of the substitutional defects, Frenkel defect pairs, and helium bubbles were re-evaluated separately and quantitatively in terms of the phonon scattering theory. A theoretical model with an overall consideration of the contributions of all these irradiation-induced defects was proposed without any adjustable parameters, and validated to predict the thermal conductivity variation under irradiation based on the experimental data of the unirradiated, irradiated, and annealed B.sub.4C samples. The predicted thermal conductivities by this model show a good agreement with the experimental data after irradiation. The calculation results and theoretical analysis in light of the experimental data demonstrate that the substitutional defects of boron atoms by lithium atoms, and the Frenkel defect pairs due to the collisions with the fast neutrons, rather than the helium bubbles with strain fields surrounding them, play determining roles in the abrupt degradation of thermal conductivity with burnup. Keywords: boron carbide (B.sub.4C); thermal conductivity; fast neutron irradiation
Using first-principles calculations within the density functional theory we studied the electronic structure, elastic properties, and stability of M sub(23)C sub(6) carbides, where M are the ...platinum-metals: Ru, Rh, Pd, Os, Ir, or Pt. The lattice constants, elastic parameters, formation energies, and densities of states of M sub(23)C sub(6) were compared with those for mono-carbides MC. We demonstrated that these carbides have the positive formation energies and predicted the mechanically stable phases. We found that M sub(23)C sub(6) carbides are energetically more favorable than the corresponding MC carbides due to the stronger M-M interactions in M sub(23)C sub(6). Imageomitted Mechanically stable and unstable M sub(23)C sub(6) and MC carbides.
Transition metal carbides (TMCs) feature high catalytic activity and superior stability for the hydrogen evolution reaction (HER). However, their platinum‐like HER catalytic performance is heavily ...hindered, due to their strong interaction with hydrogen. Herein, Ni activation of TMCs (M = V, Fe, Cr, and Mo) is proposed through introducing adsorbed nickel atoms on the TMC surface (Ni/TMC). In both acidic and alkaline solutions, a sharp decrease of both overpotentials and Tafel slopes of the Ni/TMC catalysts for HER is achieved. At 10 mA cm−2, the overpotentials of the Ni/vanadium carbide (VC) and Ni/Fe3C catalysts are 128 and 93 mV in 1 m KOH, 111 and 112 mV in 0.5 m H2SO4, respectively. Even at 150 mA cm−2, they exhibit the overpotentials of as low as 270 and 291 mV, respectively. In the alkaline solutions, the performance of these Ni/TMC catalysts is even superior to a Pt/C catalyst. As confirmed from density functional theory calculations and X‐ray absorption fine structure analysis, such adsorbed Ni atoms effectively optimize the d‐electron structure and improve HER performance. As a versatile strategy, this work provides a universal route to activate TMCs for highly efficient HER in different media.
Nickel activation or the introduction of adsorbed Ni atoms onto transition metal carbides optimizes the d‐electron structure and enhances hydrogen evolution reaction (HER) performance. Compared to a Pt/C catalyst, Ni‐activated vanadium carbide and Fe3C exhibit lower overpotentials in alkaline solutions. This versatile strategy provides a new pathway to promote HER catalytic activity of transition metal carbides in both acidic and alkaline media.
The ablation behavior of (Hf–Ta–Zr–Nb–Ti)C high‐entropy carbide (HEC‐0) was investigated using a plasma flame in air for different times (60, 90, and 120 s) at about 2100°C. The effect of SiC content ...on the ablation resistance of HEC–xSiC composites (x = 10 and 20 vol%) was also studied. The linear ablation rate of HEC‐0 decreases with increasing ablation time, showing the positive role of the oxide layer with a complex composition. The linear ablation rate of HEC–10 vol% SiC (0.3 µm s−1) is only a 10th of that of HEC‐0, showing a significant improvement in ablation resistance, probably due to the formation of a protective oxide layer containing melted SiO2 and refractory Hf–Zr–Si–O oxides.
The development of a low‐cost, energy‐efficient, and environmentally friendly alternative to the currently utilized Haber‐Bosch process to produce ammonia is of great importance. Ammonia is an ...essential chemical used in fertilizers and a promising high‐density fuel source. The nitrogen reduction reaction (NRR) has been explored intensively as a potential avenue for ammonia production using water as proton source, but to this day a catalyst capable of producing this chemical at high Faradaic efficiency (FE) and commercial yield and rates has not been reported. Here, we investigate the activity of transition metal carbide (TMC) surfaces in the (100) facets of the rocksalt (RS) structure as potential catalysts for the NRR. In this study, we use density functional theory (DFT) to model reaction pathways, estimate stability, assess kinetic barriers, and compare adsorbate energies to determine the overall performance of each TMC surface. For pristine TMC surfaces (with no defects) we find that none of the studied TMCs possess both exergonic adsorption of nitrogen and the capability to selectively protonate nitrogen to form ammonia in the desired aqueous solution. ZrC, however, is shown to be a potential catalyst if used in a non‐aqueous electrolyte. To circumvent the endergonic adsorption of nitrogen onto the surface, a carbon vacancy was introduced. This provides a well‐defined high coordination active site on the surface. In the presence of a vacancy VC, NbC, and WC showed efficient nitrogen adsorption, selectivity towards ammonia, and a low overpotential (OP). NbC did, however, display an unfeasible kinetic barrier to nitrogen dissociation for ambient‐condition purposes, and thus it is suggested for high tempearture/pressure ammonia synthesis. Both WC and VC in their RS (100) structure are promising materials for experimental investigations in aqueous electrolytes, and ZrC could potentially be interesting for non‐aqueous electrolytic systems.
Herein we report a potential catalyst capable of electrochemical formation of ammonia at standard temperature and pressure. The activity of (100) rocksalt transition metal carbide (TMC) surfaces was tested via density functional theory computational calculations following the unrestricted mechanism. Results indicate that some of the TMC catalysts could be capable of efficient electrochemical ammonia formation and that activity is greatly enhanced in the presence of a surface carbon vacancy.
Transition metal carbides are of great potential for electrochemical applications. The phase and facet of molybdenum carbides greatly affect the electrochemical performance. Carburization of MoO3 ...inside a transmission electron microscope to monitor the growth process of molybdenum carbides is performed. Carbon sources with different activities are used and the controllable growth of molybdenum carbides is investigated. The results show that the relatively inert amorphous carbon film produces Mo2C, where the interstitial sites formed by hexagonal closed packing molybdenum atoms are partially occupied by carbon atoms. In contrast, the carbon decomposed from the sucrose has a high portion of sp3 hybridized and crosslinked carbon atoms with high reactivity, leading to the formation of MoC with full occupation of interstitial sites by carbon atoms. In addition, the MoC growth experiences a (111) to (100) facets change with the increase of temperature. The (111) facet formed at low temperature has Mo‐terminated or C‐terminated surface with higher surface energy and higher reactivity, while the (100) facet with 1:1 C/Mo ratio on the surface exhibits enhanced stability. The phase and facet control by carbon source and temperature allow us to tune the crystal structures and surface atoms as well as their electrochemical properties.
The carburization of molybdenum trioxide is conducted inside a transmission electron microscope to monitor the growth process of molybdenum carbides. It is revealed that the phase and facet of molybdenum carbides are controlled by the carbon source and temperature, which allows the structures and surface atoms to be tuned, as well as the electrochemical properties of molybdenum carbides.
Case-hardened steels, widely used in high-performance ball and roller bearings, have high surface hardness and a gradient in material properties (hardness, yield strength, etc.) as a function of ...depth; therefore, they behave as functionally graded materials. Understanding the mechanical properties due to gradients in the subsurface microstructure of case-hardened steels is important for modeling the effects of cyclic damage induced by rolling contact fatigue. In the current study, two different commercially available case-carburized steels (P675, M-50 NiL) and two through-hardened steels (M-50, case P675) were characterized to obtain relationships among the volume fraction of subsurface carbides, indentation hardness, elastic modulus, and yield strength as a function of depth. A variety of methods including microindentation, nanoindentation, ultrasonic measurements, compression testing, rule of mixtures, and upper and lower bound models were used to determine the above relationships and compare the experimental results with model predictions. In addition, the morphology, composition, and properties of the carbide particles are also discussed. It was found that the subsurface hardness and volume fraction of carbides are linearly related. Finally, it was found that the estimation of composite modulus from a well-established model compares with measurements from the ultrasonic method and compression tests. The results presented are of immediate engineering relevance to the bearing industry, with importance to modeling of microstructure and its effects on rolling contact fatigue life.
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