Effect of sulfur passivation on 6H-SiC surface stability
Based on first-principles calculation, the surface stability of S-passivated 6H-SiC in different terminals are researched through investigating surface energy, charge distribution and density of states with the change of sulfur coverage from1/4 monolayer (ML) to 1 monolayer (ML). Our calculations show that S-passivated Si-terminal surface performs better than Si-terminal surface. Surface-state of 6H-SiC is reduced by about 1-4 electrons/eV due to S passivation. Meanwhile as the sulfur coverage increasing, surface energy is decreased from about -5.2eV/nm2 to about -40 eV/nm2 which leads to a more stable structure. According to theoretical calculation and mechanism analysis, the most stable structure is 3/4 ML sulfur atoms absorb in bridge site on Si-terminal surface.
First-principles study of the H2S passivation on 6H-SiC (0001) surface
Passivation is an effective way to reduce density of states (DOS) on the surface. In order to verify the differences and stabilities of H2S passivation on Si- and C-face of 6H-SiC, the adsorption of H2S on surface is studied by the first principles method, which is based on the density functional theory. Adsorption configuration and stability are researched with the coverage changing from 1/9monolayer (ML) to 1/3ML on Si-face while 1/9ML to 5/9ML on C-face. Adsorption energy, DOS and charge population of the system are calculated separately. On Si-face, energy calculations show that H2S which is adsorbed at the bridge (BR) site is more stable than that adsorbed at the on-top (OT) site. After H2S is decomposed, S atoms are combined at the BR site, while H atoms are adsorbed at the OT site. The surface states are noticeably reduced after passivation. Surface states are reduced to the lowest when the coverage is 2/9ML. On C-face, H2S which is adsorbed at the OT site is more stable than that adsorbed at the BR site. After H2S is decomposed, HS bonds and H atoms are all combined at the OT site. The surface states are noticeably reduced to the lowest when the coverage is 4/9ML.
Thermodynamics on the Carbochlorination of Titanium-bearing Ores
The equilibrium compositions of the multi-component, multi-phase and multi-reaction system of carbochlorination of titanium-bearing ores are calculated with HSC, a computational chemistry software. The thermodynamic equilibrium calculation of the reaction system shows that a complete conversion of titanium-bearing ores is feasible at a temperature near 200oC. At temperatures ranging from 800 to 1600oC, the products of the reaction system are all in gaseous phase. Thermodynamic analysis accords with the industrial practice. The simulated results provide reasonable scientific basis for improving suitable technological conditions of the carbochlorination reduction.