Synthesis of boron/nitrogen substituted carbons for aqueous asymmetric capacitors

Boron/nitrogen substituted carbons were synthesized by co-pyrolysis of polyborazylene/coal tar pitch blends to yield a carbon with a boron and nitrogen content of 14 at% and 10 at%, respectively. The presence of heteroatoms in these carbons shifted the hydrogen evolution overpotential to −1.4 V vs Ag/AgCl in aqueous electrolytes, providing a large electrochemical potential window (∼2.4 V) as well as a specific capacitance of 0.6 F/m². An asymmetric capacitor was fabricated using the as-prepared low surface area carbon as the negative electrode along with a redox active manganese dioxide as the positive electrode. The energy density of the capacitor exceeded 10 Wh/kg at a power density of 1 kW/kg and had a cycle life greater than 1000 cycles.     

Mechanical behaviour and microstructural evolution of constrained groove pressed nickel sheets

Pure nickel sheets are subjected to severe plastic deformation by constrained groove pressing technique at room temperature thereby imparting an effective plastic strain of 3.48. The evolution of mechanical behaviour with increasing number of passes revealed intensive increase in strength properties after first pass; however marginal increase is observed subsequently. Gain in ductility which is attributed to dislocation recovery is observed after third pass along with marginal drop in strength. Microstructural evolution during groove pressing of sheets is characterized by X-ray diffraction profile analysis using Williamson–Hall method. Besides the observation of strong shear texture in constrained groove pressed sheets, improvement in strain isotropy with increased straining is revealed from Williamson–Hall plots. The sub-grain/cell size estimated by analysing the diffraction profile of deformed sheets is found to be ～1390 nm after processing up to three passes. Low grain refinement efficiency observed in this process compared to other severe plastic deformation techniques at similar strain conditions is explained by the deformation characteristics and loading behaviour experienced by the sheets during constrained groove pressing.     

Roll bonding behaviour of Ti-6Al-4V sheets during pack rolling

Low density, excellent corrosion resistance, moderate strengths at high temperatures make titanium based alloys candidate materials for advanced aerospace structures. Advanced joining techniques are emulating for fabrication of complex aerospace structures. Roll bonding is one such solid state joining technique where bonding between two rough and clean surfaces is obtained under high temperature and pressure. The work horse dual phase titanium alloy Ti-6Al-4V finds extensive structural application in aerospace and space industries. In the present study, two Ti-6Al-4V sheets are stacked together and encapsulated in a mild steel can followed by evacuation and hot crimping. The pack is extensively deformed to ∼84% reduction in thickness employing conventional hot rolling mill. Partially roll bonded 0.74mm thick sheets were obtained and microstructural evolution was studied using light microscopy and scanning electron microscopy. The incompletely bonded interface was analyzed through elemental mappings by Electron Probe Micro Analyzer and compared with completely bonded region. The partially bonded region revealed the presence of aluminium oxide at the interface which possibly could have hindered complete bonding at that region. The room temperature tensile properties of as- roll bonded Ti-6Al-4V nearly approaches the base material properties.     

Strengthening of Short Shear Span Reinforced Concrete T Joists with Fiber-Reinforced Plasic Composites

In this work, the results of an experimental study conducted in a 1964-vintage building are presented. Twelve reinforced concrete (RC) T-joists strengthened with fiber-reinforced plastic (FRP) composites were loaded until failure in a short shear span configuration. Different strengthening schemes, including different FRP materials and a new FRP anchorage system, were adopted in order to compare the performance of the different installations. Carbon FRP and aramid FRP sheets in an epoxy matrix were bonded to the RC joists using the wet layup technique. All of the joists were loaded close to one end support and showed similar cracking patterns at failure. The design calculations were based on experimental results. All of the unanchored FRP strengthened beams showed failure due to peeling, while the anchored FRP strengthened members showed failure due to anchor pullout at higher load values. It was found that an increase in the amount of FRP did not result in a proportional increase in the shear capacity, as expected by design equations, but all of the beams showed a considerable increase in stiffness. The experimental results are compared with the results expected by analytical models in order to discuss the structural behavior of FRP strengthened beams tested in a real building with a short shear span. It was found that theoretical calculations resulted in nonconservative results for the tested specimens.     

Globally optimal transmitter placement for indoor wireless communication systems

A global optimization technique is applied to solve the optimal transmitter placement problem for indoor wireless systems. An efficient pattern search algorithm - DIviding RECTangles (DIRECT) of Jones et al.- has been connected to a parallel three-dimensional radio propagation ray tracing modeler running on a 200-node Beowulf cluster of Linux workstations. Surrogate functions for a parallel wideband code-division multiple-access (WCDMA) simulator were used to estimate the system performance for the global optimization algorithm. Power coverage and bit-error rate are considered as two different criteria for optimizing locations of a specified number of transmitters across the feasible region of the design space. This paper briefly describes the underlying radio propagation and WCDMA simulations and focuses on the design issues of the optimization loop.     

ECG coding by wavelet-based linear prediction

Presents a novel coding scheme for the electrocardiogram (ECG). Following beat delineation, the periods of the beats are normalized by multirate processing. After amplitude normalization, a discrete wavelet transform is applied to each beat. Due to the period and amplitude normalization, the wavelet transform coefficients bear a high correlation across beats at identical locations. To increase the compression ratio, the residual sequence obtained after linear prediction of the significant wavelet coefficients is transmitted to the decoder. The difference between the actual period and the mean beat period, and that between the actual scale factor and the average amplitude scale factor are also transmitted for each beat. At the decoder, the inverse wavelet transform is computed from the reconstructed wavelet transform coefficients. The original amplitude and period of each beat are then recovered. The approximation achieved, at an average rate of 180 b/s, is of high quality. The authors have evaluated the normalized maximum amplitude error and its position in each cycle, in addition to the normalized root mean square error. The significant feature of the proposed technique is that, while the error is nearly uniform throughout the cycle, the diagnostically crucial QRS region is kept free of maximal reconstruction error     

Constrained-storage vector quantization with a universal codebook

Many image compression techniques require the quantization of multiple vector sources with significantly different distributions. With vector quantization (VQ), these sources are optimally quantized using separate codebooks, which may collectively require an enormous memory space. Since storage is limited in most applications, a convenient way to gracefully trade between performance and storage is needed. Earlier work addressed this problem by clustering the multiple sources into a small number of source groups, where each group shares a codebook. We propose a new solution based on a size-limited universal codebook that can be viewed as the union of overlapping source codebooks. This framework allows each source codebook to consist of any desired subset of the universal code vectors and provides greater design flexibility which improves the storage-constrained performance. A key feature of this approach is that no two sources need be encoded at the same rate. An additional advantage of the proposed method is its close relation to universal, adaptive, finite-state and classified quantization. Necessary conditions for optimality of the universal codebook and the extracted source codebooks are derived. An iterative design algorithm is introduced to obtain a solution satisfying these conditions. Possible applications of the proposed technique are enumerated, and its effectiveness is illustrated for coding of images using finite-state vector quantization, multistage vector quantization, and tree-structured vector quantization.     

Fullerene photoelectrochemical solar cells

The photoelectrochemistry of single crystal C₆₀ and fullerene photoelectrochemical solar cells is studied. Illuminated and immersed, C₆₀ is shown to drive oxidation of several solution-phase redox couples. Utilization of a photoelectrochemical solid/liquid junction, rather than solid-state photovoltaic junction, improves the observed photocurrent. Utilization of a single crystal, rather than a polycrystalline film, of C₆₀ decreases dark current to the extent that light-driven charge transfer dominates. The spectral response and current-voltage behaviour in several electrolytes is studied. A low-power fullerene photoelectrochemical solar cell, utilizing a regenerative polyiodide and ferri/ferrocyanide redox couple, is demonstrated.