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.
Properties of a monopivot centrifugal blood pump manufactured by 3D printing
An impeller the same geometry as the impeller of a commercial monopivot cardiopulmonary bypass pump was manufactured using 3D printing. The 3D-printed impeller was integrated into the pump casing of the commercially available pump to form a 3D-printed pump model. The surface roughness of the impeller, the hydraulic performance, the axial displacement of the rotating impeller, and the hemolytic properties of the 3D-printed model were measured and compared with those of the commercially available model. Although the surface roughness of the 3D-printed model was significantly larger than that of the commercially available model, the hydraulic performance of the two models almost coincided. The hemolysis level of the 3D-printed model roughly coincided with that of the commercially available model under low-pressure head conditions, but increased greatly under high-pressure head conditions, as a result of the narrow gap between the rotating impeller and the pump casing. The gap became narrow under high-pressure head conditions, because the axial thrust applied to the impeller increased with increasing impeller rotational speed. Moreover, the axial displacement of the rotating impeller was twice that of the commercially available model, confirming that the elastic deformation of the 3D-printed impeller was larger than that of the commercially available impeller. These results suggest that trial models manufactured by 3D printing can reproduce the hydraulic performance of the commercial product. However, both the surface roughness and the deformation of the trial models must be considered to precisely evaluate the hemolytic properties of the model.