Effect Of Carbon Vacancies Research Articles

Carbon vacancies enhanced oxidation resistance of high-entropy carbides (Ti0.2V0.2Nb0.2Mo0.2W0.2)Cx

High-entropy carbides have attracted considerable attention owing to their high melting point and excellent mechanical properties at high temperatures. However, exposures to high temperature environments are often accompanied by oxidation problems. Previous studies generally focused on improving oxidation properties by adding antioxidant elements. However, improving the oxidation properties intrinsically by modifying the crystal structure is still lacking. In this work, we innovative introduced tunable carbon vacancies into high-entropy carbides successfully.Consequently, the oxidation property of high-entropy carbides was improved, with the effect of carbon vacancies on the oxidation property was systematically investigated in this study. The (Ti0.2V0.2Nb0.2Mo0.2W0.2)Cx exhibits the best oxidation resistance property when x = 0.7, with the highest initial oxidation temperature and peak weight gain temperature (950.2 °C) and the lowest mass gain rate (25.2 %). This is mainly attributed to the formation of a higher number of carbonaceous oxides, which hinder the internal oxidation process.

Effects of carbon vacancies on the CO oxidation on Cu double atom catalyst supported by graphene

The activity of double metal atoms is closely related to their chemical coordination environments. In this work, CO oxidation on Cu double atom catalyst loaded on defective graphene with triple carbon vacancy (Cu2-TG) and quadruple carbon vacancy (Cu2-QG) has been comprehensively studied through DFT calculations. Empty Cu-3d orbital is created through charge transfer from Cu atom to carbon atom of graphene, which is parallel to the basal plane of graphene for one Cu atom and nearly perpendicular to the basal plane for the other Cu atom in Cu2-TG. While the empty Cu-3d orbital is parallel to the basal plane for both Cu atoms in Cu2-QG. The delocalized orientation of empty Cu-3d orbital in Cu2-TG makes it more convenient to accept electrons and more active. This is verified through the larger adsorption energies of -1.74 eV and -1.68 eV for O2 and CO molecules on Cu2-TG, as well as the smaller values of -0.20 eV and -0.40 eV on Cu2-QG, respectively. Because of the weak interactions between Cu2-QG and gas molecules, only Cu2-TG is further considered to catalyze the CO oxidation. The maximum barrier of CO oxidation along ER path is 1.75 eV on Cu2-TG, while that is only 0.39 eV along LH path, which indicates a smooth reaction progress on Cu2-TG. The electronic structures show that the Cu atom can accept electrons through its empty 3d orbital and lose electrons through its filled 3d orbital with an “acceptance and feedback” mechanism. Cu2-TG can move the 5σCO orbital downshift significantly, which causes the 5σCO/1πCO orbitals very close to 5σO2/1πO2 orbitals and facilitates the formation of OC-OO bond along LH path. These results indicate that the synergistic effects between the two Cu atoms and the triple carbon vacancy make Cu2-TG to be an efficient CO oxidation catalyst.

Effects of carbon vacancies on the structures, mechanical properties, and chemical bonding of zirconium carbides: a first-principles study.

Interstitial carbides are able to maintain structural stability even with a high concentration of carbon vacancies. This feature provides them with tunable properties through the design of carbon vacancies, and thus making it important to reveal how carbon vacancies affect their properties. In the present study, using first-principles, we have calculated the properties of a number of stable and metastable zirconium carbides ZrC1-x (x = 0 and 1/n, n = 2-8) which were predicted by the evolutionary algorithm USPEX. Effects of carbon vacancies on the structures, mechanical properties, and chemical bonding of these zirconium carbides were systematically investigated. The distribution of carbon vacancies has significant influence on mechanical properties, especially Pugh's ratio. Nonadjacent carbon vacancies enhance Pugh's ratio, while grouped carbon vacancies decrease Pugh's ratio. This is explained by the changes in strength of Zr-C and Zr-Zr bonding around differently distributed carbon vacancies. We further explored the mechanical properties of zirconium carbides with impurities (N and O) by inserting N and O atoms into the sites of carbon vacancies. The enhanced mechanical properties of zirconium carbides were found.

Lattice dynamics of binary and ternary phases in Ti–Si–C system: A combined Raman spectroscopy and density functional theory study

Results of the x-ray diffraction and the Raman spectroscopy experiments on the multiphase Ti–Si–C system containing Ti3SiC2 as the major phase and TiSi2, TiCx, and Ti5Si3/Ti5Si3Cx impurity phases are reported. Experimental studies are supported by the density functional theory calculations of the Raman spectra performed for the major and concomitant phases. The effect of carbon vacancies and impurities on the TiCx and Ti5Si3Cx Raman spectra is investigated. It is shown that identification and refinement of the phase composition of the multicomponent Ti–Si–C system based on the theoretical Raman spectroscopy can be achieved when both frequencies and intensities of the simulated Raman-active modes are simultaneously considered.

Effect of carbon vacancies on thermodynamic properties of TiC–ZrC mixed carbides

Thermodynamic properties of a TiZrC mixed carbide system are investigated by first-principles methods within density functional theory. Carbon vacancies are found to have a significant contribution to the thermodynamics of TiZrC mixed carbides. The temperature effect on the thermodynamic properties of the system is calculated taking into consideration the corresponding electronic and vibrational thermal excitations.

Effect of carbon vacancies on the electric resistivity of nonstoichiometric VC y vanadium carbide

The influence of the temperature, concentration, and distribution of structure vacancies of the carbon sublattice on the electric resistivity ρ of nonstoichiometric VCy vanadium carbide (0.66 ≤ y ≤ 0.875) has been studied in the temperature range of 300–1200 K. The symmetry and structure characteristics of the ordered V6C5 and V8C7 phases formed owing to low-temperature annealing on various sections of the homogeneity region of the VCy carbide. The dependence of the residual electric resistivity on the content of the disordered vanadium carbide is explained by the atom-vacancy interaction and the change in the carrier concentration in the homogeneity region of VCy.

Optical conductivity in substoichiometric titanium carbides

A theoretical study of the optical absorption of perfectly ordered titanium carbides Ti${\mathrm{C}}_{x}$ is reported for different carbon concentrations $(x$=1, 0.75, 0.50, and 0.25). The optical conductivity is calculated within the framework of the linear muffin-tin orbital method in the atomic-sphere approximation. The effect of carbon vacancies on the optical conductivity is discussed and the origin of the different absorption peaks is traced back to particular interband transitions.