Review of diffusion and vaporization of Group 4 and 5 transition metal carbides

New Insights into the Structure of the C-Terminated β-Mo2C (001) Surface from First-Principles Calculations

“…Upon relaxation, all reconstructions lower the total energy of the system with respect to the corresponding value for the polar bulk-cut relaxed C-terminated surface, in agreement with the predictions of the Tasker model for ionic materials. The calculated values of the cleavage energy E cleav in Table suggest that the C-non polar termination is the most stable surface, whose calculated C–C distances are in remarkable agreement with experimental values. ,, Nevertheless, the relative close cleavage energies, and hence stabilities, of the different reconstructions strongly suggest that fractions of the material surface are likely to be covered by Hexagonal and Stripes motifs as these surfaces are normally prepared by annealing at high temperature. It is worth noting that all results reported are obtained considering perfect UHV conditions (i.e., in the absence of any gas in contact with the surface).…”

“…Further experiments have provided evidence that annealing time also modifies the surface structure . The combination of both temperature and annealing time modifies the carbon coverage on the surface (θ C ), due to its segregation and C oxidation toward CO 2 . For instance, it was noted that the honeycomb structure changes to a c(2 × 4) zigzag motif …”

Ab-initio modeling and experimental investigation of properties of ultra-high temperature solid solutions TaxZr1-xC

“…The diffusion coefficients in solid solutions of refractory carbides mainly depend on the atomic radius of the metallic species, which is 145 pm for both Ta and Nb and 155 for Zr[63]. Coefficients of diffusion D0 are also similar for TaC and NbC (1.04 and 1.2 cm 2 s -1 , correspondingly)[64]. Therefore, we investigated the phase stability of TaxZr1-xC solid solutions using a methodology employed for NbxZr1-xC by Gusev [62] and compared the obtained results.…”

Nucleation experiments on a titanium-carbon system imply nonclassical formation of presolar grains

“…where β is a concentration-dependent parameter, B is the carbontitanium ratio, Q is the activation energy, and R is the universal gas constant. If the carbon-titanium ratio is assumed to be unity, and the values of D 0 , Q, and T are 5.75 × 10 −6 m 2 s −1 , 3.0 × 10 5 J mol −1 , and 1079 K, respectively (48), then D = 1.5 × 10 −20 m 2 s −1 . In the case of representative sizes of the core-mantle grains (1 μm) and TiC particles (10 nm) obtained in this experiment, the diffusion times (=r 2 /D) are estimated to be 2.2 years and 1.9 hours, respectively; 2.2 years is a reasonable period for the time scale of grain formation in the gas ejecta of a supernova.…”