Temperature‐Dependent Electrical Transport in Polymer‐Sorted Semiconducting Carbon Nanotube Networks

The temperature dependence of the electrical characteristics of field‐effect transistors (FETs) based on polymer‐sorted, large‐diameter semiconducting carbon nanotube networks is investigated. The temperature dependences of both the carrier mobility and the source‐drain current in the range of 78 K to 293 K indicate thermally activated, but non‐Arrhenius, charge transport. The hysteresis in the transfer characteristics of FETs shows a simultaneous reduction with decreasing temperature. The hysteresis appears to stem from screening of charges that are transferred from the carbon nanotubes to traps at the surface of the gate dielectric. The temperature dependence of sheet resistance of the carbon nanotube networks, extracted from FET characteristics at constant carrier concentration, specifies fluctuation‐induced tunneling as the mechanism responsible for charge transport, with an activation energy that is dependent on film thickness. Our study indicates inter‐tube tunneling to be the bottleneck and implicates the role of the polymer coating in influencing charge transport in polymer‐sorted carbon nanotube networks.     

Vehicle capacity planning system: a case study on vehicle routing problem with time windows

In this paper, we consider a local logistic company that provides transportation service for moving empty and laden containers within Singapore. Due to the limited capacity of its own fleet of vehicles, the company cannot handle all the job orders and have to outsource some orders to other smaller local transportation companies. The current operation of assigning jobs for outsourcing goes through two steps. In the first step, a certain percentage of jobs will be preselected for outsourcing according to some simple rules. Then at the second step, the rest of the jobs will be put into an in-house computer system which assigns jobs to its internal fleet of vehicles according to some greedy rules and the remaining jobs that cannot be served by the internal fleet of vehicles will be outsourced. This paper presents a vehicle capacity planning system (VCPS), which models the problem as a vehicle routing problem with time window constraints (VRPTW) and tabu search (TS) is applied to find a solution for the problem. From the simulation results, some new rules on how to assign jobs for outsourcing are derived, which are shown to be about 8% better than existing rules currently adopted by the company.     

Characterization of the dynamic response of proton exchange membrane fuel cells – A numerical study

The dynamic response of PEM (Proton exchange membrane) fuel cells is a complex phenomenon which is affected by numerous factors related to their designs and operating conditions. Despite that experimental data is available in the literature, a systematic numerical study to explain the dynamic behavior of PEM fuel cells is currently unavailable. In this paper, a one-dimensional, two-phase, dynamic model of PEM fuel cell is developed to achieve this principal objective. Transient profiles of cell voltage, activation and ohmic over-potentials, saturation level of liquid water, oxygen concentration, and membrane water content are predicted under various operating conditions. Under constant fuel and air flow rates, it is found that the cell voltage exhibits undershoot behavior following a step increase in current density due to the inherent time delay experienced by the redistribution of membrane water content with a response time of ∼50 s. The undershoot is followed by an overshoot in the presence of flooding with a significantly longer predicted response time of ∼150–200 s. It is found that the various operating conditions mainly affect the specific details of the undershoot and overshoot profiles without changing their general behavioral forms.     

Overall Effect of Unreliable Machines on Production Capacity

The effect of unreliable machines on the probability distribution of the number of acceptable items produced is discussed. Such a distribution is shown to depend on two other distributions, namely, the distribution of the number of items actually produced and the distribution associated with the probability of an item's satisfying some performance criterion. Relations governing the moments of the distributions are given.     

Evaluation of Conditional Failure Density from Hazard Rate

Further to the recent discussion regarding the concepts of hazard rate and conditional failure density, it is shown that the latter may be expressed in terms of the former. Moreover, six functions, namely, failure time distribution function, failure time density, reliability function, hazard rate, conditional failure distribution, and conditional failure density, are shown to be equivalent to the extent that if one of them is known, the other five are completely determined. The results are summarized in a table.     

Study of deep-level defects and annealing effects in undoped and Sn-doped GaAs solar cells irradiated by one-MeV electrons

DLTS and C-V techniques have been employed to determine the defect energy levels and density, carrier capture cross sections, lifetimes and diffusion lengths in the Sn-doped and the undoped GaAs solar cells irradiated by one-MeV electrons under different electron fluences (10¹⁴ to 10¹⁶ cm^?2), fluxes (2 × 10⁹, 4 × 10¹⁰ e/cm²-s), and annealing conditions (150 ? T ? 230°C). The results show that density of both electron and hole traps will in general increase with incresing electron fluence and flux, and decrease with increasing annealing temperature and annealing time. Some distinct difference in defeat spectrum was observed in the undoped and the Sn-doped GaAs solar cells studied. The low temperature thermal annealing and the recombination enhanced annealing processes are found to be very effective in reducing the density of deep-level defects induced by one-MeV electrons. The results of our findings are discussed in detail in this paper.     

Photonic wideband array antennas

Presents an introduction to the optical control of array antennas by using fiber optic links for remote control and a photonic time shift network for wide instantaneous bandwidth. An overview of the development of a wideband conformal array designed for airborne surveillance radars is given. The paper covers the system design and the performance of an L-band (850-1400 MHz) M-element array controlled by photonics. Packaging techniques of the photonic components and the array aperture are discussed. The wideband performance of the system is highlighted. A nano-second impulse response has been measured to demonstrate a 50% instantaneous bandwidth (550 MHz, 30 cm range resolution) for target ID and imaging. A built-in signal injection technique based on time-domain impulse measurement was used to calibrate the wideband components in the time-shift beamforming network