The formation of self assembled CdSe quantum dots under Stranski Krastanow (S K) mode by low pressure metalorganic chemical vapor deposition (LP MOCVD) was reported for the first time. The samples were grown directly ...The formation of self assembled CdSe quantum dots under Stranski Krastanow (S K) mode by low pressure metalorganic chemical vapor deposition (LP MOCVD) was reported for the first time. The samples were grown directly on GaAs (100) surfaces by LP MOCVD. DimethylSelenide (DMSe) and DimethylCadmium (DMCd) were used as precursors. The growth pressure was kept at 2 93×10 4Pa and the growth temperature was 500℃. CdSe with the thickness of about 2 monolayers was grown directly on GaAs (100) surfaces. For the purpose of AFM observation, this uncapped sample was cooled down immediately to room temperature and was monitored under a Digital Instruments Nanoscope Ⅲa system at the same day of growth. The AFM images show that the average diameter, height and density of those self assembled CdSe quantum dots are 50±15nm, 13±4nm and 5μm -2 , respectively. And those dots’ diameter height ratio is about 4～5, just the same as those results observed in other Ⅱ Ⅵ and Ⅲ Ⅵ compounds which were grown under S K mode by MBE.展开更多
The performance of a reversible pump turbine with S-shaped characteristics is of great importance to the transition processes such as start-up and load rejection. In order to predict the S-shaped curve accurately and ...The performance of a reversible pump turbine with S-shaped characteristics is of great importance to the transition processes such as start-up and load rejection. In order to predict the S-shaped curve accurately and develop a reliable tool for design improvement, a shear stress transport model (SST) with various numerical schemes for pressure term in the governing equation was investigated in a whole pump turbine including spiral casing, stay vanes, guide vanes, runner and draft tube. Through the computation, it was shown that different zones in the curve should employ different schemes to get the solution converged. Comparison of discharge-speed performance showed that good correspondence is got between experimental data and CFD results. Based on this, internal flow analysis was carried out at three typical operating points representing turbine mode, shut-off mode and reversible pump mode, respectively. According to the flow field concerned, the mechanism for the speed-no-load instability was explained, which provides good guidelines to take countermeasures in future design.展开更多
文摘The formation of self assembled CdSe quantum dots under Stranski Krastanow (S K) mode by low pressure metalorganic chemical vapor deposition (LP MOCVD) was reported for the first time. The samples were grown directly on GaAs (100) surfaces by LP MOCVD. DimethylSelenide (DMSe) and DimethylCadmium (DMCd) were used as precursors. The growth pressure was kept at 2 93×10 4Pa and the growth temperature was 500℃. CdSe with the thickness of about 2 monolayers was grown directly on GaAs (100) surfaces. For the purpose of AFM observation, this uncapped sample was cooled down immediately to room temperature and was monitored under a Digital Instruments Nanoscope Ⅲa system at the same day of growth. The AFM images show that the average diameter, height and density of those self assembled CdSe quantum dots are 50±15nm, 13±4nm and 5μm -2 , respectively. And those dots’ diameter height ratio is about 4～5, just the same as those results observed in other Ⅱ Ⅵ and Ⅲ Ⅵ compounds which were grown under S K mode by MBE.
基金supported by the National Natural Science Foundation of China (Grant No. 50979095)
文摘The performance of a reversible pump turbine with S-shaped characteristics is of great importance to the transition processes such as start-up and load rejection. In order to predict the S-shaped curve accurately and develop a reliable tool for design improvement, a shear stress transport model (SST) with various numerical schemes for pressure term in the governing equation was investigated in a whole pump turbine including spiral casing, stay vanes, guide vanes, runner and draft tube. Through the computation, it was shown that different zones in the curve should employ different schemes to get the solution converged. Comparison of discharge-speed performance showed that good correspondence is got between experimental data and CFD results. Based on this, internal flow analysis was carried out at three typical operating points representing turbine mode, shut-off mode and reversible pump mode, respectively. According to the flow field concerned, the mechanism for the speed-no-load instability was explained, which provides good guidelines to take countermeasures in future design.
