Amanda Carvalho

Incorporation of nitrogen in carbon nanotubes

Abstract Nitrogen-doped carbon nanotubes were obtained by the arc-discharge technique in a helium–nitrogen atmosphere and using iron–nickel–cobalt catalysts, The samples were analyzed using spectroscopic techniques (Raman, EELS, X-ray photoelectron spectroscopy) and transmission electron microscopy (TEM). Pure helium atmosphere conditions led to bundles of single-wall nanotubes with diameters of ∼1.5 nm. The presence of nitrogen during tube formation produced irregular and thickly textured tubes. TEM micrographs showed that N suppresses the formation of bundles of single-wall nanotubes, giving rise to nested nanofibers. Quantum-chemical calculations were carried out to study the influence of substitutional N on the tube conformation. The calculations show that the combination of hexagons and pentagons at low N concentration produces kinks that account for the irregular shaped nanotubes.

Cloud‐to‐ground lightning in southeastern Brazil in 1993: 1. Geographical distribution

About I. I million cloud-to-ground lightning flashes were recorded by a lightning positioning and tracking system in southeastern Brazil in 1993. The 1-year continuous lightning data set is the first obtained in Brazil. It has been analyzed for geographical distribution of total flash density, percentage of positive flashes, negative and positive flash densities, and negative and positive flash peak currents. The dependence of the flash density and peak current on latitude, altitude, and soil resistivity was investigated. Negative flash peak current was found to be inversely correlated with latitude, but no other significant correlation was found for flash density and peak current with these parameters. Positive flashes were found to be contaminated by intracloud flashes. The maximum total, negative, and positive flash densities were 15.5, 9.1, and 7.7 flashes/km 2 per year, respectively. The average percentage of positive flashes was 36.5%. The gcomctric means of negative and positive peak current were 30.9 kA and 17.8 kA. The high density, high percentage and low average peak current of positive flashes found in this study are probably a result of such a contamination. Neglecting positive flashes below 15 kA, assuming that they correspond to intracloud flashes erroneously identified by the system, the maximum positive and total flash densities would be 3.9 flashes/km per year and 11. 7 flashes/km 2 per year. The percentage and geometric mean peak current of positive flash would be 23% and 38. 7 kA, respectively. The results are discussed in the context of other similar measurements made at different parts of the world.

Cloud‐to‐ground lightning in southeastern Brazil in 1993: 2. Time variations and flash characteristics

About 1.1 million cloud-to-ground lightning flashes were recorded by a lightning positioning and tracking system in southcastern Brazil in 1993. The data have been analyzed in terms of their monthly, scasonal (summer/winter), and diurnal (local time) variations. The monthly variation shows a double peak characteristic of tropical lightning activity. The seasonal variation indicates that most flashes occur in the spring and summer seasons, with less than 25% occurring in the autumn and winter. The lightning flash polarity and multiplicity were found to be very similar in the summer and winter seasons. Radiation field and direct current lightning data were obtained in towers located in the same region of the network to verify the multiplicity data obtained by the network. The results indicate that the multiplicity obtained by the system is much lower than that obtained by radiation field measurements of close lightning in the same region of Brazil. The lightning flash peak current were found to be larger in the summer than in the winter, in contrast with results obtained in other parts of the world. The diurnal variation of the negative flashes shows in the summer and winter seasons the same behavior, with a peak around 1500-1800 LT, associated with the maximum convective activity in the afternoon. The diurnal variation of positive flashes, in turn, shows this behavior only in the winter. In the summer, it shows a maximum around 1400-1500 LT, with a secondary peak at 1900 LT. However, considering only positive flashes with peak currents higher than 15 kA, the diurnal distribution in the summer is similar to that for negative flashes. This fact indicates that the positive flashes with a peak current less than 15 kA are probably intracloud flashes erroneously identified by the network. The results are discussed in association with the findings presented in paper 1 [Pinto et al., this issue] and compared with results obtained in other parts of the world.

Stabilizing Y‐junctions and ring structures through nitrogen substitution

In this work we theoretically investigate the conformational structure of nanotube junctions and bent tubules. Geometry optimizations were performed through the semi‐empirical quantum chemical Parametric Method 3 technique. The defective regions of junctions and bends were built including five‐, seven‐ and eight‐membered rings in the otherwise hexagonal network of carbon bonds. In order to reduce the stress caused by the curvature, a chemical doping through nitrogen substitution is proposed. The energy associated to nitrogen incorporation was obtained. Results are consistent with the shortening of bonds within the junctions and bends and an increased chemical stability of the defects.