Study of pulsed-current loading of direct methanol fuel cells using a new time-domain model based on bi-functional methanol oxidation kinetics

This work describes a non-linear time-domain model of a direct methanol fuel cell (DMFC) and uses that model to show that pulsed-current loading of a direct methanol fuel cell does not improve average efficiency. Unlike previous system level models, the one presented here is capable of predicting the step response of the fuel cell over its entire voltage range. This improved model is based on bi-functional methanol oxidation reaction kinetics and is derived from a lumped, four-step reaction mechanism. In total, six states are incorporated into the model: three states for intermediate surface adsorbates on the anode electrode, two states for the anode and cathode potentials, and one state for the liquid methanol concentration in the anode compartment. Model parameters were identified using experimental data from a real DMFC. The model was applied to study the steady-state and transient performance of a DMFC with the objective to understand the possibility of improving the efficiency of the DMFC by using periodic current pulses to drive adsorbed CO from the anode catalyst. Our results indicate that the pulsed-current method does indeed boost the average potential of the DMFC by 40 mV; but on the other hand, executing that strategy reduces the overall operating efficiency and does not yield any net benefit.     

Analysis and correction techniques of doppler shift for non-geosynchronous communication satellites

A theoretical and experimental study of Doppler correction strategies for elliptical orbit satellite communications is presented. Exact analytical expressions for Doppler shift and elevation angle as a function of perigee time of the ground-station are also presented. It is shown that for a 63° inclined Molnya orbit and a U.K. observer, the Doppler shift remains below about ± 10 ppm and the Doppler rate below about 6 × 10^?4 ppm/s. Various strategies for continuous Doppler correction of the on-board oscillators are reviewed or proposed. These include a closed loop ground–satellite system and an on-board programmable correction approach. Two detailed theoretical and experimental studies of on-board encoded Doppler correction distribution systems are presented in the context of the UK CERS/TSAT mobile communications experiment. A frequency lock system which requires the distribution of only one signal and a phase-lock system requiring a master reference and the time-variant code word are described. Numerical values for a 1·5 GHz carrier are presented throughout but can be scaled to other frequencies.     

Neural network implementation of a fade countermeasure controller for a VSAT link

In this paper, an algorithm and its practical implementation to activate adaptive modulation as a fade countermeasure (FCM) is presented. Characteristics of the algorithm are derived from the propagation studies and the algorithm uses a simple four parameter model. A neural network architecture was used to implement the decision making block of the controller. The algorithm has been implemented on a fixed point DSP. An experimental set‐up with an emulated K_a‐band satellite link and a terrestrial return path connection has been used with previously recorded propagation data for experimental verification and performance analysis. Performance of the implemented FCM system is compared with that of fixed, non‐adaptive systems. Over concatenated rain events, the adaptive system yields better throughput at or below a given BER than any fixed mode system and only marginally worse BER availability than the most robust scheme in the system. The FCM is thus essential if throughput is of importance as well as availability. The added complexity of the FCM system is not great by contemporary technology standards and is, in authors' opinion, well worth the investment. Copyright © 2002 John Wiley & Sons, Ltd.     

Transient structural ordering of the RNA-binding domain of carnation mottle virus p7 movement protein modulates nucleic acid binding

Plant viral movement proteins bind to RNA and participate in the intra- and intercellular movement of the RNAs from plant viruses. However, the role and magnitude of the conformational changes associated with the formation of RNA-protein complexes are not yet defined. Here we describe studies on the relevance of a preexisting nascent alpha-helix at the C terminus of the RNA-binding domain of p7, a movement protein from carnation mottle virus, to RNA binding. Synthetic peptide analogues and single amino acid mutation at the RNA-binding domain of recombinant p7 protein were used to correlate the transient structural order in aqueous solution with RNA-binding potential.     

Indentation size effect in nickel and cobalt laser clad coatings

In this work it is reported a comparative study of the indentation response at different length scales in nickel and cobalt samples produced by laser cladding. The indentation tests were carried out in the load range 57 μN to 2 N. Both in Ni and Co, the indentation hardness increases as the indentation dimension decreases from the micrometer to the nanometer range. It is shown that the experimental results can be accurately described by an analytical function, whose terms account for the contributions of dislocation density and surface free energy to the material's hardness. Although the good relation between the model and the experimental points is not a fully evidence, it strongly suggests that, as the hardness scale decreases, the increase of dislocation density and the increase of the surface-to-volume ratio of the plastically deformed region are the most important causes for the observed indentation size effect.     

Docking-based virtual screening for ligands of G protein-coupled receptors: not only crystal structures but also in silico models

G protein-coupled receptors (GPCRs) regulate a wide range of physiological functions and hold great pharmaceutical interest. Using the β(2)-adrenergic receptor as a case study, this article explores the applicability of docking-based virtual screening to the discovery of GPCR ligands and defines methods intended to improve the screening performance. Our controlled computational experiments were performed on a compound dataset containing known agonists and blockers of the receptor as well as a large number of decoys. The screening based on the structure of the receptor crystallized in complex with its inverse agonist carazolol yielded excellent results, with a clearly delineated prioritization of ligands over decoys. Blockers generally were preferred over agonists; however, agonists were also well distinguished from decoys. A method was devised to increase the screening yields by generating an ensemble of alternative conformations of the receptor that accounts for its flexibility. Moreover, a method was devised to improve the retrieval of agonists, based on the optimization of the receptor around a known agonist. Finally, the applicability of docking-based virtual screening also to homology models endowed with different levels of accuracy was proved. This last point is of uttermost importance, since crystal structures are available only for a limited number of GPCRs, and extends our conclusions to the entire superfamily. The outcome of this analysis definitely supports the application of computer-aided techniques to the discovery of novel GPCR ligands, especially in light of the fact that, in the near future, experimental structures are expected to be solved and become available for an ever increasing number of GPCRs.     

Neural Network Based Recognition of Smoke Signatures from Lidar Signals

The automatic recognition of smoke signatures in lidar signals collected during very small-scale experimental forest fires using neural-network algorithms was studied. An algorithm for pre-processing of raw lidar signals is proposed, which selects suspicious backscattering peaks and makes them unbiased and scale independent. The resulting patterns can be successfully classified as corresponding to alarm or no-alarm conditions by a neural-network algorithm based on a simple one-neuron structure (perceptron). In the case of an alarm, the pre-processing algorithm provides the location of the smoke plume. Five algorithms selected from the literature, and one that was specially developed, all with learning rate adaptation, were used for training the perceptron. Their efficiencies and statistical properties were compared. The best perceptron classifier presented an efficiency of 97% in the classification of smoke-signature patterns and a false alarm rate of 0.9%.     

Micro-to-Nano Indentation and Scratch Hardness in the Ni–Co System: Depth Dependence and Implications for Tribological Behavior

Although size effects in hardness have been extensively reported and analyzed for the static (indentation) case, much less attention has been given to these effects in non-static (scratch) hardness measurements. In the present work, size effects in the indentation and scratch hardness response of the Ni–Co system are evaluated by performing tests at several penetration depths, from the micro to the nanometer range. It is shown that, for all the range of compositions, the hardness response of these materials is strongly affected by the depth of penetration of the indenter: when the depth decreases, both the indentation and scratch hardness increase several times. This result denotes that, when studying the wear behavior of materials, special care must be taken concerning the scale one is dealing with, since the tribo-mechanical response of the material may change significantly from the micrometric to the nanometric contact scale.     

Energy loss in electrochemical diaphragm process of chlorine and alkali industry – A collateral effect of the undesirable generation of chlorate

Contamination of NaOH with chlorate constitutes a major problem for the chlorine–alkali industry, particularly when electrolytic cells based on the diaphragm process are employed. In this paper, pilot and laboratory cell experiments revealed that chlorate contamination in diaphragm cells also inhibits hydrogen evolution and gives rise to a significant increase in electrical energy consumption. Electrolysis carried out under conditions that simulated the industrial process (current density 240 mA cm⁻²; temperature 90 °C; brine flux 23 L cm⁻² h⁻¹) revealed that chlorate formation depends on brine flux and NaOH production. The inhibitory effect of chlorate on the main cathodic reaction was demonstrated in bench cell experiments, with cathodic displacement of the hydrogen evolution reaction by more than 100 mV in the presence of 0.4% chlorate compared with ideal conditions in which chlorate formation was absent. This hydrogen generation overpotential can charge the total electric energy balance in more than 5% of the total value, consisting of a critical loss for this process.     

Détermination électrochimique du nombre de sherwood minimum pour un milieu macro‐poreux (fritté — lit fixe de grains)

In this work, an electrochemical method used for the determination of solid‐liquid mass transfer coefficients has been applied to the measurement of the Sherwood number for the stationary diffusional mass transfer between a porous material and a stationary liquid in the pores. Such a minimum Sherwood number was determined for sintered metals and for fixed beds of spherical grains. The theoretical aspects are based on the analogy of the problem with competition between molecular diffusion and heterogeneous chemical reaction in a porous catalyst.