Progress of the NbAl3 Combustion Synthesis Reaction

Parallelopipedal (60 × 10 × 10-mm) bars obtained by uniaxial compaction of Nb-Al powder mixtures with a stoichiometric NbAl₃ composition (75 a/o Al) were submitted to reaction synthesis using the SHS mode (self-propagating high-temperature synthesis). The experimental procedures used were such as to allow for the reaction ignition, propagation, and extinction at a certain plane along the bar length. The microstructure of the reacted bars was characterized by optical microscopy, scanning electron microscopy, and EDS. The various phases present along the bar length evidenced that the local composition was changed due to the presence of a molten aluminum flux during the reaction. All the observed phases are predicted by the Nb-Al equilibrium phase diagram. A possible explanation for the reaction sequence is advanced based on the experimental observations.     

Analysis and optimization of wavelet-coded communication systems

Since its advent, wavelet coding has been recognized as a promising technique for wireless transmission systems, especially due to its low decoding complexity and good performance over multipath fading. However, this technique requires unusual modulation schemes in order to be fully exploited. A BER analysis of wavelet-coded transmission systems over Rayleigh fading channels is presented in this work. This analysis is used in the design of novel signal constellations for wavelet-coded systems by means of numerical performance optimization. Numerical results obtained from analysis and simulations show that the wavelet-coded systems derived herein have very good performance characteristics over fading channels.     

A high-power-factor half-bridge doubler boost converter without commutation losses

This paper proposes a high-power-factor half-bridge doubler boost converter without commutation losses, which provides high output voltages, i.e., from 600 to 900 V. The voltages across the semiconductor devices are low and approximately equal to the output voltage, as doubled output voltages and reduced high-frequency ripple can be achieved. A detailed mathematical analysis concerning its operation is presented, and simulation and experimental results describe the converter performance.     

Purification and characterization of pectin methylesterase from acerola (Malpighia glabra L.)

The enzyme pectinmethylesterase (PME) from acerola was extracted and purified by gel anion‐exchange chromatography (Q Sepharose) and filtration on Sephadex G‐100. The results showed two different PME isoforms (PME1 and PME2), with molecular masses of 25.10 and 5.20 kDa, respectively. PME1 specific activity increased by 9.63% after 60 min incubation at 98 °C, while PME2 retained 66% of its specific activity under the same conditions. The K_m values of PME1, PME2 and concentrated PME were 0.94, 0.08 and 0.08 mg mL^?1, respectively. The V_max value of PME1, PME2 and concentrated were 204.08, 2, 158.73 and 2.92 µmol min^?1 mg^?1 protein, respectively. Copyright © 2007 Society of Chemical Industry     

Planar circularly symmetric EBG structures for reducing surface waves in printed antennas

This paper discusses the design and analysis of planar circularly symmetric (PCS) electromagnetic band gap (EBG) structures for reducing the surface waves excited by printed antennas on dense dielectric substrates. The key advantage of the circularly symmetric geometries is that a surface wave generated by a source located at its center experiences the same band gap effect in all radial directions. To obtain simple design rules of the PCS-EBGs for the optimization of the bandwidth, an equivalence is established between 2-D-EBGs and PCS-EBGs. Integrated planar printed antennas with bandwidths up to 20% are designed, manufactured and tested.     

External Equivalent for Electric Power Distribution Networks With Radial Topology

This paper presents a method for evaluating external equivalent in electric distribution networks (EDN). The proposed method has as its main objectives the reduction of the computational costs in distribution network reconfiguration, investigation of the optimal allocation of banks of capacitors, investigation of the allocation of distributed generation, etc. In these sorts of problems, a large number of alternative projects must be assessed in order to identify the optimal solution. The optimal solution comes up with the voltage level in the load points within specified limits. Consequently, the EDN must retain the external network load points but without major increasing in the dimension of the equivalent circuit. The proposed method has been tested and validated in a substation of the Electricity Utility of MaranhÃo-CEMAR, in Brazil.      

Using space-time TCM with rotated constellations for transmissionover mobile communications channels

Space-time trellis coded modulation is combined with a rotated signal set for transmission in mobile communication channels. Two systems are proposed and their performances simulated over slow selective fading channels     

Evaluation of W-Rh permanent modifier for lead determination in sugar by graphite furnace atomic absorption spectrometry

An alternative procedure that improves the performance of graphite atomizers for the determination of Pb in sugar by electrothermal atomic absorption spectrometry is described. The procedure is based on the injection of 10 μl of an acidified aqueous solution containing 8% w/v sugar and 0.2% v/v HNO₃ into integrated graphite platforms. Either transversely (THGA) as well as longitudinally heated graphite atomizers (LHGA) were evaluated by using conventional co-injection of 0.03% Pd + 0.05% Mg(NO₃)₂ or thermally treated platforms with 250 μg W + 200 μg Rh and co-injection of 5 μg l⁻¹ Rh solution. With W-Rh under the same analytical conditions, the lifetimes of THGA and LHGA reached up to 1110 and 900 firings, respectively. With Pd + Mg the LHGA tube lifetime was limited to approximately 500 firings, but for THGA up to 1020 firings were made with a single tube. Characteristic masses were 11 and 29 pg Pb for LHGA and THGA, respectively. Detection limits (3 s) based on sugar blank solution and on integrated absorbance were 5.0 mg kg⁻¹ with LHGA and 9.3 mg kg⁻¹ Pb for THGA. In general, the coefficients of variation of 20 consecutive measurements of a solution containing 50 μg l⁻¹ Pb were lower than 5%. The obtained detection limits are in consonance with the Codex Alimentarius recommendation for the maximum Pb content in the sugar.