Sudden stoppage of rotor in a thermally driven rotary motor made from double-walled carbon nanotubes

In a thermally driven rotary motor made from double-walled carbon nanotubes, the rotor (inner tube) can be actuated to rotate within the stator (outer tube) when the environmental temperature is high enough. A sudden stoppage of the rotor can occur when the inner tube has been actuated to rotate at a stable high speed. To find the mechanisms of such sudden stoppages, eight motor models with the same rotor but different stators are built and simulated in the canonical NVT ensembles. Numerical results demonstrate that the sudden stoppage of the rotor occurs when the difference between radii is near 0.34 nm at a high environmental temperature. A smaller difference between radii does not imply easier activation of the sudden rotor stoppage. During rotation, the positions and electron density distribution of atoms at the ends of the motor show that a sp(1) bonded atom on the rotor is attracted by the sp(1) atom with the biggest deviation of radial position on the stator, after which they become two sp(2) atoms. The strong bond interaction between the two atoms leads to the loss of rotational speed of the rotor within 1 ps. Hence, the sudden stoppage is attributed to two factors: the deviation of radial position of atoms at the stator's ends and the drastic thermal vibration of atoms on the rotor in rotation. For a stable motor, sudden stoppage could be avoided by reducing deviation of the radial position of atoms at the stator's ends. A nanobrake can be, thus, achieved by adjusting a sp(1) atom at the ends of stator to stop the rotation of rotor quickly.The authors are grateful for financial support from the National Natural-Science-Foundation of China (Grant Nos. 50908190, 11372100)

A test of the power law relationship between gamma-ray burst pulse width ratio and energy expected in fireballs or uniform jets

Recently, under the assumption that the Doppler effect of the relativistically expanding fireball surface is important, Qin et al. showed that in most cases the power law relationship between the pulse width and energy of gamma-ray bursts (GRBs)would exist in a certain energy range. We check this prediction with two GRB samples which contain well identified pulses. A power law anti-correlation between the full pulse width and energy and a power law correlation between the pulse width ratio and energy are seen in the light curves of the majority (around 65%) of bursts of the two samples within the energy range of BATSE, suggesting that these bursts are likely to arise from the emission associated with the shocks occurred on a relativistically expanding fireball surface. For the rest of the bursts, the relationships between these quantities were not predicted previously. We propose to consider other spectral evolutionary patterns or other radiation mechanisms such as a varying synchrotron or Comptonized spectrum to check if the observed relationships for these rest bursts can also be accounted for by the Doppler model. In addition, we find that the upper limits of the width ratio for the two samples do not exceed 0.9, in agrement with what predicted previously by the Doppler model. The plateau/power law/plateau and the peaked features predicted and detected previously by Qin et al. are generally observed, with the exceptions being noticed only in a few cases. According to the distinct values of two power law indices of FWHM and ratio and energy, we divide the bursts into three subsets which are located in different areas of the two indices plane. We suspect that different locations of the two indices might correspond to different mechanisms.Comment: 16 pages, 7 figures, MNRAS accepte

Constructing Restoration Strategies with Availability Risk Constraints

System restoration strategies are established and implemented based on the availabilities of components of a system. However, due to lack of accurate information, availabilities of some components may not be known during establishment of restoration strategies after a blackout. In this paper, a novel risk-based methodology is proposed for constructing restoration strategies with stochastic availability constraints for both individual components and restoration paths. Based on a stochastic model of the availability, a multistage stochastic optimization model is constructed. A bi-level method is used to solve the proposed model. The established restoration strategy is to achieve a reasonable restoration duration subjects to operating constraints and acceptable risk levels. The proposed risk constraints are introduced into the EPRI's System Restoration Navigator (SRN) with a little modification. Case studies demonstrate the proposed model and methods.published_or_final_versio

Minimum throughput maximization in LoRa networks powered by ambient energy harvesting

In this paper, we investigate the uplink transmissions in low-power wide-area networks (LPWAN) where the users are self-powered by the energy harvested from the ambient environment. Demonstrating their potential in supporting diverse Internet-of-Things (IoT) applications, we focus on long range (LoRa) networks where the LoRa users are using the harvested energy to transmit data to a gateway via different spreading codes. Precisely, we study the throughput fairness optimization problem for LoRa users by jointly optimizing the spreading factor (SF) assignment, energy harvesting (EH) time duration, and the transmit power of LoRa users. First, through examination of the various permutations of collisions among users, we derive a general expression of the packet collision time between LoRa users, which depends on the SFs and EH duration requirements. Then, after reviewing prior SF allocation work, we develop two types of algorithms that either assure fair SF assignment indeed purposefully `unfair' allocation schemes for the LoRa users. Our results unearth three new findings. Firstly, we demonstrate that, to maximize the minimum rate, the unfair SF allocation algorithm outperforms the other approaches. Secondly, considering the derived expression of packet collision between simultaneous users, we are now able to improve the performance of the minimum rate of LoRa users and show that it is protected from inter-SF interference which occurs between users with different SFs. That is, imperfect SF orthogonality has no impact on minimum rate performance. Finally, we have observed that co-SF interference is the main limitation in the throughput performance, and not the energy scarcity

Effect of disorder with long-range correlation on transport in graphene nanoribbon

Transport in disordered armchair graphene nanoribbons (AGR) with long-range correlation between quantum wire contact is investigated by transfer matrix combined with Landauer's formula. Metal-insulator transition is induced by disorder in neutral AGR. Thereinto, the conductance is one conductance quantum for metallic phase and exponentially decays otherwise when the length of AGR is infinity and far longer than its width. Similar to the case of long-range disorder, the conductance of neutral AGR first increases and then decreases while the conductance of doped AGR monotonically decreases, as the disorder strength increases. In the presence of strong disorder, the conductivity depends monotonically and non-monotonically on the aspect ratio for heavily doped and slightly doped AGR respectively.Comment: 6 pages, 8 figures; J. Phys: Condensed Matter (May 2012