RESUMO
A new class of two-dimensional scandium carbide nanosheet has been identified by using first-principles density functional theory. It has a primitive cell of Sc3C10, in which there are two pentagonal carbon rings surrounded by one scandium octagon. Being as the precussor of Volleyballene Sc20C60 and ScC nanotubes, the Sc3C10 nanosheet is exceptionally stable. By rolling up this Sc3C10 sheet, a series of stable ScC nanotubes have been obtained. All the nanotubes studied have been found to be metallic. Furthermore, the hydrogen storage capacity of the ScC nanotubes has been explored. The calculated results show that one unit of the (0,3) ScC nanotube can adsorb a maximum of 51 hydrogen molecules, reaching up to a 6.25 wt% hydrogen gravimetric density with an average binding energy of 0.23 eV/H2.
RESUMO
OBJECTIVE: To study the changes and influence factors of an Orbscan II topography system in myopia after excimer laser in situ keratomileusis (LASIK). METHODS: Two hundred and sixteen myopia patients (230 eyes) were treated by LASIK, their corneal thickness and curve of corneal posterior surface was measured with an Orbscan II topography system before and after surgery. The results of measurement in different time points were compared. RESULTS: The thinnest point of cornea, the mean keratometry of the posterior best fit surface (BFS) and the distance from the apex to the BFS were significant difference before and after operation (P < 0.05). There were significant difference among those groups in different degree of myopia in the same time point (P < 0.05) after surgery, but there were no significant difference in the same group in difference time (P > 0.05). A significant difference in the area of the thinnest cornea before surgery and after surgery was detected among those groups (P < 0.01). The regression equation for preoperative Diff value was established as Y = 0.0941 - 0.000,53X(1) - 0.000,471X(2) - 0.0063X(3) + 0.001,22X(4) (P < 0.01), X(1) = corneal thickness of the thinnest point (microm), X(2) = central corneal thickness (microm), X(3) = corneal diameter (mm), X(4) = length of ocular axis (mm). The regression equation for postoperative decrease Diff value between the third month and the first month was produced as Y = - 0.027 - 0.000,78X(1) + 0.002,01X(2) - 0.0055X(3) (P < 0.01). X(1) = decrease in corneal thickness of the thinnest point (microm), X(2) = intraocular pressure pre-surgery (mmHg), X(3) = decrease in curvature of posterior corneal surface (D). CONCLUSIONS: The application of Orbscan II has an important clinical significance before and after LASIK. The Diff value measured before LASIK for myopia is influenced by the corneal thickness, corneal diameter and ocular axis. The postoperative decrease in Diff value between the third and first month is influenced by the postoperative decrease of corneal thinnest point and curvature of posterior corneal surface, and preoperative intraocular pressure.