Effect of temperature on the optical properties of GaAsSbN/GaAs single quantum wells grown by molecular-beam epitaxy

GaAsSbN/GaAs strained-layer single quantum wells grown on a GaAs substrate by molecular-beam epitaxy with different N concentrations were studied using the photoluminescence (PL) technique in the temperature range from 9 to 296 K. A strong redshift in optical transition energies induced by a small increase in N concentration has been observed in the PL spectra. This effect can be explained by the interaction between a narrow resonant band formed by the N-localized states and the conduction band of the host semiconductor. Excitonic transitions in the quantum wells show a successive red/blue/redshift with increasing temperature in the 2-100 K range. The activation energies of nonradiative channels responsible for a strong thermal quenching are deduced from an Arrhenius plot of the integrated PL intensity. (C) 2003 American Institute of Physics.

H-bonding in entrapped water in poly(o-methoxyaniline): Results from a differential scanning calorimetry study

Differential scanning calorimetry (DSC) was used to investigate entrapped water in poly(o-methoxyaniline) (POMA) in powder form. Two endothermic peaks were attributed to removal of water molecules that were adsorbed with distinct energies. By obtaining thermograms at various heating rates, we succeeded in applying Kissinger's approaches to estimate activation energies for the water adsorbed. The values obtained were ca. 25 and 53 kJ/mol, which correspond to H-bonding interactions, probably at the amine and imine centers of POMA, respectively. (C) 2005 Elsevier B.V. All rights reserved.

Electrical relaxation in proton conductor composites based on (NH4)H2PO4/TiO2

We report on electrical relaxation measurements of (1-x)NH4H2PO4-xTiO(2) (x = 0.1) composites by admittance spectroscopy, in the 40-Hz-5-MHz frequency range and at temperatures between 303 and 563 K. Simultaneous thermal and electrical measurements on the composites identify a stable crystalline phase between 373 and 463 K. The real part of the conductivity, sigma', shows a power-law frequency dependence below 523 K, which is well described by Jonscher's expression sigma' = sigma(0)(1 + (omega/omega(p))(n)), where sigma(0) is the dc conductivity, omega(p)/2 pi = f(p) is a characteristic relaxation frequency, and n is a fractional exponent between 0 and 1. Both sigma(0) and f(p) are thermally activated with nearly the same activation energy in the II region, indicating that the dispersive conductivity originates from the migration of protons. However, activation energies decrease from 0.55 to 0.35 eV and n increases toward 1.0, as the concentration of TiO2 nanoparticles increases, thus, enhancing cooperative correlation among moving ions. The highest dc conductivity is obtained for the composite x = 0.05 concentration, with values above room temperature about three orders of magnitude higher than that of crystalline NH4H2PO4 (ADP), reaching values on the order of 0.1 (Omega cm)(-1) above 543 K.

Impedance spectroscopy study of dehydrated chitosan and chitosan containing LiClO4

Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES)

Photodesorption and electron trapping in n-type SnO2 thin films grown by dip-coating technique

Thin films of undoped and Sb-doped (2 atg%) SnO2 have been prepared by sol-gel dip-coating technique on borosilicate glasses. Variation of photoconductivity excitation with wavelength and optical absorption indicate indirect bandgap transition with energy of ≅ 3.5 eV. Conductance as function of temperature indicates two levels of capture with 39 and 81 meV as activation energies, which may be related to an Sb donor and oxygen vacancy respectively. Electron trapping by these levels are practically destroyed by UV photoexcitation (305 nm) and heating in vacuum to 200°C. Gas analysis using a mass spectrometer indicates an oxygen related level, which may not be desorbed in the simpler O2 form.

Non-isothermal study of the devitrification kinetics of fluoroindate glasses

The understanding of the kinetics of devitrification of a glass is important for anticipating its stability in a particular purpose, such as fiber-drawing processes. The crystallization kinetics of (BaF2)16(ZnF2)20(SrF 2)20(NaF)2 (GaF3)5(InF3)36(GdF 3)1 glass prepared by quenching were studied by differential scanning calorimetry (DSC). Avrami's exponent (n) obtained by a non-isothermal method was 4.3 for a solid and 2.4 for a powdered sample. According to the classical interpretation of n, these magnitudes correspond to an interface-controlled crystal growth and a diffusion-controlled crystal growth, respectively. The activation energies for crystallization (E) was 62 ± 1 kJ/mol for solid glass and 245 ± 2 kJ/mol for powdered glass. These results are discussed in terms of glass particle size. © 2000 Elsevier Science B.V. All rights reserved.

Glass-ceramic material from the SiO 2-Al 2O 3-CaO system using sugar-cane bagasse ash (SCBA)

Brazil is the world's largest producer of alcohol and sugar from sugarcane. Currently, sugarcane bagasse is burned in boilers to produce steam and electrical energy, producing a huge volume of ash. The major component of the ash is SiO 2, and among the minor components there are some mineralizing agents or fluxing. Published works have shown the potential of transforming silicate-based residues into glass-ceramic products of great utility. This work reports the research results of SCBA use to produce glass-ceramics with wollastonite, rankinite and gehlenite as the major phases. These silicates have important applications as building industry materials, principally wollastonite, due to their special properties: high resistance to weathering, zero water absorption, and hardness among others. The glasses (frits) were prepared mixing ash, calcium carbonate and sodium or potassium carbonates as flux agents, in different concentrations. X-ray fluorescence was used to determine the chemical composition of the glasses and their crystallization was assessed by using thermal analysis (DTA/DSC/TGA) and X-ray diffraction. The crystallization kinetics was evaluated using the Kissinger method, giving activation energies ranging from 200 to 600 kJ/mol. © 2011 Ceramic Society of Japan.

Isotermas de sorção e cinética de secagem de sementes de pomelo

The drying of grapefruit seeds, by-products from grapefruit processing, was studied at 40, 50, 60 and 70°C and at three air velocities, 0.6, 1.0 and 1.4 m s-1. Sorption isotherms of grapefruit seeds were obtained at each temperature by the static method using saturated salt solutions. The Henderson model adequately described the sorption isotherms, over the entire temperature range. Drying rates indicated that the drying of grapefruit seeds took place under the falling rate period. Effective moisture diffusivity in grapefruit seeds ranged from 4.36 × 10-10 to 6.82 × 10-10 m2 s-1. The temperature dependence of the effective diffusivity followed an Arrhenius relationship, and the activation energies were 12.23, 11.29 and 11.79 kJ mol-1 for dried grapefruit seeds under air velocities of 0.6, 1.0 and 1.4 m s-1, respectively. Three thin-layer models were used to predict the drying curves, Page, Lewis, and Henderson-Pabis model. The Page model presented the best fit for all drying air temperatures and velocities studied.

Aplicação do conceito de maturidade em concreto com adição de cinza de casca de arroz

Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq)

Espectroscopia de impedância em vidros 22Na2O·8CaO·65SiO2·5MO2 (M = Si, Ti, Ge, Zr, Sn, Ce) sem e com troca iônica Ag+ Na+

Pós-graduação em Física - IGCE

Cinética de cristalização em vidros teluritos do sistema TLWN

Pós-graduação em Ciência dos Materiais - FEIS

Influência do óxido de cromo nas propriedades varistoras do óxido de estanho

Pós-graduação em Química - IQ

Imobilização da inulinase de Klyveromyces marxianus var. bulgaricus ATCC 16045: caracterização e produção de açúcar invertido em biorreator

Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq)

Investigação óptica de pontos quânticos de InAs confinados em poços quânticos de In0,15Ga0,85As

Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES)

Propriedades térmicas e estruturais de vidros teluretos

Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES)