Sérgio Michielon de Souza

X-ray diffraction, Raman, and photoacoustic studies of ZnTe nanocrystals

Nanocrystalline ZnTe was prepared by mechanical alloying. X-ray diffraction (XRD), energy dispersive spectroscopy, Raman spectroscopy, and photoacoustic absorption spectroscopy techniques were used to study the structural, chemical, optical, and thermal properties of the as-milled powder. An annealing of the mechanical alloyed sample at 590 °C for 6 h was done to investigate the optical properties in a defect-free sample (close to bulk form). The main crystalline phase formed was the zinc-blende ZnTe, but residual trigonal tellurium and hexagonal ZnO phases were also observed for both as-milled and annealed samples. The structural parameters, phase fractions, average crystallite sizes, and microstrains of all crystalline phases were obtained from Rietveld analyses of the X-ray patterns. Raman results corroborate the XRD results, showing the longitudinal optical phonons of ZnTe (even at third order) and those modes of trigonal Te. Nonradiative surface recombination and thermal bending heat transfer mechani...

Nanocomposite based on polyaniline emeraldine-base and α-Al2O3: A structural characterization

Abstract A ceramic-matrix nanocomposite based on polyaniline emeraldine base and aluminum oxide (PANI-EB/α-Al2O3) was obtained by in-situ polymerization. X-ray diffraction pattern presented peaks related to both materials. The level of crystallinity was estimated at about 53%. The average crystallite sizes of PANI-EB and α-Al2O3 were found to be ∼40 Å and 570 Å, respectively. Scanning electron microscopy showed polymerization over ceramic particles. Fourier-transform infrared spectroscopy suggested physical deposition. The electrical conductivity of the PANI-EB/α-Al2O3 nanocomposite was decreased by a factor of 80 when compared with that of pure PANI-EB. Therefore, the polymeric reinforcement and the ceramic matrix maintained their original structural features, but the electrical conductivity in the nanocomposite was reduced.

Synthesis and properties of nanostructured GeSb 4 Te 7 prepared by mechanical alloying

A nanostructured GeSb4Te7 (GST147) phase was produced by mechanical alloying (MA), starting from a mixture with nominal GeSb2Te4 composition of high-purity elemental Ge, Sb and Te powders. The effect of milling on z-coordinates of Wyckoff sites 2c and 2d occupied by the Sb and Te atoms was studied using an approach combining the Rietveld method and pair distribution function analysis. The thermal stability of GST147 and the Raman active modes of GST147 were investigated. The results of photoacoustic absorption spectroscopy show that the thermal diffusivity of nanostructured GST147 produced by MA is similar that of its bulk counterpart.

Structural, thermal and optical studies of nanocomposite powder NiSb + Sb produced by mechanical alloying

Abstract A nanostructured composite powder formed from 75 wt.% NiSb and 25 wt.% Sb was obtained by mechanical alloying binary mixtures of elemental Ni and Sb powders in the composition NiSb2 for 39 hours. The structural evolution was followed by X-ray diffraction measurements and Rietveld structural refinement. At the end of milling, the crystallites of Sb and NiSb reached a mean size of 17 and 15 nm, respectively. Differential scanning calorimetry measurement showed two exothermic processes at 263 °C and 309 °C. Variable temperature powder X-ray diffraction measurements at 25, 100, 200 and 300 °C showed that the sample is structurally stable until 100 °C. The beginning of the transition NiSb + Sb → NiSb2 was observed at 200 °C and the nucleation of Sb2O3 was observed at 300 °C in accordance with differential scanning calorimetry measurement. Raman spectroscopy measurements showed neither NiSb nor NiSb2 compounds produce significant Raman signals when compared with Sb and Sb2O3.