Dependence of electronic structures of multi-walled boron nitride nanotubes on layer numbers

By using first-principle calculations based on density functional theory, the electronic structures of multi-walled armchair and zigzag boron nitride nanotubes (BNNTs) are investigated. Band shifts between the two layers of the double-walled nanotubes narrow their band gaps and form significant coupling. With the increase of the layer number of the multi-walled BNNTs, the similarity of the electronic structures for the two outer layers is enhanced obviously and the influence of band shift is weakened. Electronic structures of the BNNTs formed with more than three layers are not sensitive to their layer numbers. These results are meaningful for the application researches of BNNTs

Coordinated Frequency Regulation of Smart Grid by Demand Side Response and Variable Speed Wind Turbines

Frequency stability of the power system is impacted by the increasing penetration of wind power because the wind power is intermittent. Meanwhile, sometimes the demand side loads increase quickly to require more power than total power produced. So balancing the active power in the power system to maintain the frequency is the main challenge of the high penetration of wind power to the smart grid. This paper proposes coordination rotor speed control (RSC), pitch angle control (PAC) and inertial control (IC) to control wind turbines, together with demand side response (DSR) participating in frequency regulation to balance active power in the power system. Firstly, the model of a single area load frequency control (LFC) system is obtained, which includes variable-speed wind turbines (VSWT) and DSR containing aggregated air conditioners and plug-in electric vehicles (PEVs). Then the RSC, PAC and IC, which controls wind turbines participating in frequency regulation in the power system, are introduced, respectively. Finally, the coordination of these three methods for wind turbines in different wind speeds is proposed. Case studies are carried out for the single area LFC system with a wind farm and DSR supported grid frequency. Coordination RSC and PAC combined IC are used to control wind turbines with DSR to balance active power in the power system. The proposed method used in the power system with high penetration of wind power and fluctuation of demand load is tested, respectively. Coordinated RSC or PAC with DSR can increase penetration of wind power and reduce peak load

Coordinated Frequency Regulation of Smart Grid by Demand Side Response and Variable Speed Wind Turbines

Granovetter's "strength of weak ties" hypothesizes that isolated social ties offer limited access to external prospects, while heterogeneous social ties diversify one's opportunities. We analyze the most complete record of college student interactions to date (approximately 80,000 interactions by 290 students -- 16 times more interactions with almost 3 times more students than previous studies on educational networks) and compare the social interaction data with the academic scores of the students. Our first finding is that social diversity is negatively correlated with performance. This is explained by our second finding: highly performing students interact in groups of similarly performing peers. This effect is stronger the higher the student performance is. Indeed, low performance students tend to initiate many transient interactions independently of the performance of their target. In other words, low performing students act disassortatively with respect to their social network, whereas high scoring students act assortatively. Our data also reveals that highly performing students establish persistent interactions before mid and low performing ones and that they use more structured and longer cascades of information from which low performing students are excluded.Comment: 12 pages, 5 figure

Monte Carlo analysis of transient electron transport in wurtzite Zn1−xMgxO combined with first principles calculations

Transient characteristics of wurtzite Zn1−xMgxO are investigated using a three-valley Ensemble Monte Carlo model verified by the agreement between the simulated low-field mobility and the experiment result reported. The electronic structures are obtained by first principles calculations with density functional theory. The results show that the peak electron drift velocities of Zn1−xMgxO (x = 11.1%, 16.7%, 19.4%, 25%) at 3000 kV/cm are 3.735 × 107, 2.133 × 107, 1.889 × 107, 1.295 × 107 cm/s, respectively. With the increase of Mg concentration, a higher electric field is required for the onset of velocity overshoot. When the applied field exceeds 2000 kV/cm and 2500 kV/cm, a phenomena of velocity undershoot is observed in Zn0.889Mg0.111O and Zn0.833Mg0.167O respectively, while it is not observed for Zn0.806Mg0.194O and Zn0.75Mg0.25O even at 3000 kV/cm which is especially important for high frequency devices