Characteristics of a high brightness gaseous field ion source employing tungsten-carbon doped NiAl needles

We report on the characterization of a high brightness gaseous field ion source using an emitter made of a NiAl needle containing tiny spherical tungsten-carbon precipitates. By field evaporation of such a multiphase alloy, a surface protrusion is formed out of a precipitate, which can act as a small source size field ion emitter. The emission current-voltage characteristics of this emitter were recorded for a variety of parameters. The results obtained suggest that its application as a stable ion source is possible even on long term operation.     

Energy-efficient power/rate control and scheduling in hybrid TDMA/CDMA wireless sensor networks

We consider a hybrid TDMA/CDMA wireless sensor network (WSN) and quantitatively investigate the energy efficiency obtained by combining adaptive power/rate control with time-domain scheduling. The energy efficiency improvement is carried out with respect to interfering-cluster scheduling, intra-cluster node scheduling, and transmission powers and times (durations) control (PTC) for individual nodes. The interfering-cluster scheduling is formulated as a vertex-coloring problem, which can be solved efficiently using existing numerical algorithms in graph theory. For the node scheduling problem, we present a heuristic algorithm, which iteratively searches for the best schedule in such a way that the energy consumption keeps decreasing after every iteration. Compared with the exponentially complicated exhaustive search algorithm, this heuristic algorithm has polynomial computing complexity and can provide optimal solutions in the most simulated cases. For the transmission power/time control, two simplified PTC schemes, namely, PTC-UT and PTC-USG, are proposed and studied based on our previous optimization work PTC-IPT. We show that PTC-UT and PTC-USG provide comparable energy efficiency to PTC-IPT at only half of its complexity. Numerical examples are used to validate our findings.     

Parallel Fiber Coding in the Cerebellum for Life-Long Learning

Continuous and real-time learning is a difficult problem in robotics. To learn efficiently, it is important to recognize the current situation and learn appropriately for that context. To be effective, this requires the integration of a large number of sensorimotor and cognitive signals. So far, few principles on how to perform this integration have been proposed. Another limitation is the difficulty to include the complete contextual information to avoid destructive interference while learning different tasks.We suggest that a vertebrate brain structure important for sensorimotor coordination, the cerebellum, may provide answers to these difficult problems. We investigate how learning in the input layer of the cerebellum may successfully encode contextual knowledge in a representation useful for coordination and life-long learning. We propose that a sparsely-distributed and statistically-independent representation provides a valid criterion for the self-organizing classification and integration of context signals. A biologically motivated unsupervised learning algorithm that approximate such a representation is derived from maximum likelihood. This representation is beneficial for learning in the cerebellum by simplifying the credit assignment problem between what must be learned and the relevant signals in the current context for learning it. Due to its statistical independence, this representation is also beneficial for life-long learning by reducing the destructive interference across tasks, while retaining the ability to generalize. The benefits of the learning algorithm are investigated in a spiking model that learns to generate predictive smooth pursuit eye movements to follow target trajectories.     

Determination of the surface coverage of Re/Al2O3 catalysts by ion scattering spectroscopy and low‐temperature CO adsorption

The surface coverage of a series of rhenia–alumina catalysts prepared by equilibrium adsorption was determined by low‐temperature CO adsorption. The results show a linear increase in the coverage of the Re oxide on the alumina support up to 24% with increasing Re loading from 0 to ca. 5.6 wt%. This is consistent with a uniform build‐up of the surface species. The coverage values measured by this method agreed very well with those obtained by ion scattering spectroscopy. This revised version was published online in July 2006 with corrections to the Cover Date.     

A fully implicit conjugate heat transfer method

Abstract

This article deals with the development of an implicit and conservative method for conjugate heat transfer at solid-fluid interfaces. The technique is applicable for both conformal and non-conformal meshes. The method, which is implemented within a fully coupled in-house code, is symmetric in its treatment of the solid and fluid regions and is shown to be very robust for highly complex configurations. To demonstrate the performance of the method, two compressible turbulent conjugate heat transfer test cases, the Mark II and C3X with film cooling, which are benchmarks for simulating the hydrodynamic and thermal fields around and inside turbine blades, are used. Numerical results generated are in good agreement with available experimental measurements.     

A silicon basis for synaptic plasticity

This paper describes analogue VLSI synaptic circuitry incorporating two forms of plasticity: a nonassociative mechanism analogous to paired-pulse facilitation (PPF), and an associative form of adaptation analogous to the short-term potentiation (STP) variety of plasticity. Simulation results demonstrate expected temporal characteristics of the plasticity mechanisms. The time-course of decay of associative potentiation is established by utilising diode leakage current.     

Numerical modelling and mechanical behaviour analysis of ancient tunnel masonry structures

As with all older construction, ancient tunnels feature particular characteristics, especially as regards past construction methods, geometrical design considerations and the set of construction materials used. Old tunnels usually display a unique vaulted section shape built with masonry. The present paper proposes two numerical models of an old tunnel supported by masonry; these models were developed by the well-known Universal Distinct Element Code (UDEC). A masonry mechanical behaviour analysis and numerical simulation of the masonry ageing phenomenon will also be addressed by means of an experimental design to study the influence of masonry block physical properties on the mechanical behaviour of masonry structures in old tunnels.     

Epoxy–lignin polyblends. III. Thermal properties and infrared analysis

Differential scanning calorimetry (DSC), dynamic mechanical analysis (DMA) and IR analysis were performed on a nonviscous epoxy polymer system (EP) with Kraft lignin (L) up to 20%. Mixtures of EP with similar amounts of silica (S) as in EP–L polyblends were used as a reference system for the analyses performed. EP–L polyblends cured at room temperature exhibit a single T_g, a fact characteristic for the monophasic systems. One-step tensile storage modulus vs. temperature curves, and related one-peak tensile loss modulus vs. temperature curves were found for all the EP–L polyblends. At about 30°C the tensile storage modulus of EP does not change in the presence of lignin in amounts up to 20% by weight. All these showed that L is miscible with EP and it does not affect the crosslinking at room temperature. IR spectra led to similar conclusions. The gradual decrease of the peak values of tan δ with the amount of L in polyblends is due to an increase in the tensile storage modulus and a decrease of the tensile loss modulus at temperatures close to T_g. This fact is explained by a stronger bond between EP and L, which could be formed at higher temperatures. The DSC and DMA data are in agreement with the mechanical properties of EP–L polyblends, which were reported previously.     

Failure and Dilatancy Properties of Sand at Relatively Low Stresses

Analysis of geotechnical problems concerned by low confinement such as design of shallow foundations and analysis of slope stability and soil liquefaction requires modeling of the soil behavior at low stresses. This note includes a laboratory study of the behavior of Hostun RF sand at low cell pressure (20–50?kPa). Isotropic and triaxial compression drained tests were performed. Drained tests show that both failure and dilatancy angles at low stresses are stress dependent. The contractive/dilative phase transition is observed for loose sand, which may result from the overconsolidated nature of this sand for low values of cell pressure.     

False-Path Removal Using Delay Fault Simulation

Some false paths are caused by redundant stuck-at faults. Removal of those stuck-at faults automatically eliminates such false paths from the circuit. However, there are other false paths that are not associated with any redundant stuck-at fault. All segments of such a false path are shared with other testable paths. We focus on the elimination of this type of false paths. We use a non-enumerative path delay fault simulator based on the path status graph (PSG) data-structure, which duplicates selected gates to separate the detected and undetected path delay faults. The expanded circuit may contain new redundant stuck-at faults, corresponding to those undetected paths that are false. This happens because the expanded circuit has some new interconnects with only false paths passing through them. Such links become the sites for redundant stuck-at faults. Removal of these redundant faults eliminates false paths. The reported results show that the quality of the result may depend on the coverage of testable paths by the vectors that are simulated. When non-enumerative path delay simulation and implication-based redundancy removal techniques are used, the present procedure of false-path elimination can be applied to very large circuits.