National Institute Of Materials Physics - Romania

We have studied the growth of ZnSe crystals by chemical transport in a closed system in nonstoichiometric conditions, and we have deduced that the interface kinetics is the phenomenon limiting the growth process. The effect on the growth process of the deviation from stoichiometry of the II-VI compound was investigated using a mathematical model that involves indirect data computed from directly obtained experimental values. The experimental crystallization rate was compared with the maximum value of the transport flux calculated using the Arizumi-Nishinaga model. The influence of the stoichiometry of the source material and of the variations in the growth parameters (supercooling, geometrical dimension, specific loading of the ampoule and iodine concentration) on the ZnSe crystal growth process has also been studied. (C) 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

The synthesis of a new hybrid composite based on PUS nanoparticles and poly( methyl methacrylate-2acrylamido-2-methylpropane sulfonic acid) [P(MMA-AMPSA)] copolymer is reported. The chemical synthesis consists in two steps: (i) a surfactant-free emulsion copolymerization between methyl methacrylate and 2-acrylamido-2-methylpropane sulfonic acid and (ii) the generation of PbS particles in the presence of the P( MMA-AMPSA) latex, from the reaction between lead nitrate and thiourea. The composite was studied by scanning electron microscopy (SEM), X-ray diffraction, FTIR spectroscopy, thermogravimetric analysis and differential scanning calorimetry. The microstructure observed using SEM proves that the PbS nanoparticles are well dispersed in the copolymer matrix. The X-ray diffraction measurements demonstrate that the PbS nanoparticles have a cubic rock salt structure. It was also found that the inorganic semiconductor nanoparticles improve the thermal stability of the copolymer matrix. (C) 2010 Elsevier Ltd. All rights reserved.

Oxyfluoride glass samples in the system SiO2-Al2O3-CaF2-SmF3 were produced by using the melt-quenched technique and the kinetics of non-isothermal crystallization of CaF2 nano-crystals has been investigated. Differential thermal analysis (DTA) has shown an exothermic peak at about 700 degrees C which was assigned to the CaF2 nano-crystals precipitation within the glass matrix as was identified by X-ray diffraction (XRD) analysis of the annealed glass samples. It was shown the Ozawa model is most suitable for describing the behaviour of non-isothermal crystallization of CaF2 nano-crystals within the glass matrix. The mean value of the Avrami exponent was found 1.16 which indicates the diffusion-controlled growth process of nano-particles in the glass system with zero nucleation rate as the single operative mechanism; the activation energy of the crystallization is E-d = 379 +/- 25 kJ/mol (3.93 +/- 0.26 eV). The activation energy of the nano-crystals growth was computed within the same model and agrees very well with the activation energy of the crystallization. (C) 2010 Elsevier B.V. All rights reserved.

A special mechanism of thermoelectric transport is described, consisting of pulses of charge carriers which "fly" periodically through the external circuit from the hot end of the sample to the cold end, with a determined duration of the "on" and "off" times of the electric contacts, while maintaining continuously the thermal contacts. It is shown that such a "resonant" ideal thermogenerator may work cyclically, with the same efficiency quotient as the ideal efficiency quotient of the thermoelectric devices operated in the usual stationary transport regime but the electric flow and power are increased, as a consequence of the concentration of the charge carriers on pulses of small spatial extent. The process is reversible, in the sense that it can be operated either as a thermoelectric generator or as an electrothermal cooler. (C) 2010 American Institute of Physics. [doi: 10.1063/1.3456037]

Electron states at the SiO2/4H-SiC interface have been investigated using capacitor structures and especially, the influence of excess nitrogen, introduced by ion implantation, at the interface is studied in detail. Implanted and nonimplanted n-type samples with an interfacial concentration of nitrogen of similar to 10(19) cm(-3) and 10(16) cm(-3), respectively, were analyzed by capacitance-voltage (C-V) measurements, performed at different temperatures and probe frequencies, and thermal dielectric relaxation current (TDRC) measurements performed in the temperature range of 35-295 K. Three main categories of electron states are disclosed, true interface states (D-it), fast near interface states (NIToxfast) and slow near interface states (NIToxslow). The density versus energy distributions of D-it and NIToxfast have been deduced from the TDRC data and they are shown to give a close quantitative agreement with the shape and frequency dependence of the C-V curves. Further, the amount of NIToxslow extracted from TDRC is demonstrated to be responsible for the parallel shifts and hysterezis effects occurring in the C-V characteristics. All three categories of electron states are reduced in concentration in the implanted samples. This holds particularly for NIToxfast with a peak at similar to 0.1 eV below the conduction band edge of 4H-SiC that is suppressed by at least two orders of magnitude relative to the nonimplanted samples. The decrease for D-it is also substantial (a factor of similar to 10) while the loss for NIToxslow is considerably smaller (only similar to 30%). The results provide firm evidence that NIToxfast and NIToxslow do not originate from the same kind of defect center. (C) 2010 American Institute of Physics. [doi:10.1063/1.3457906]

The thermal spike model developed for the electronic stopping power regime is extended to consider both ionization and nuclear energy loss processes of the projectile as electronic and atomic heat distinct sources. The time and space dependencies of the lattice and electron temperatures near the projectile trajectory are calculated and discussed for different ions in silicon, at room and cryogenic temperatures, taking into account the peculiarities of electron-phonon interaction in both domains. The model developed contributes to the understanding of transient microscopic processes immediately after the projectile interaction in the target. (c) 2010 Elsevier B.V. All rights reserved.

The preparation of nanoporous materials with enhanced stability using an improved synthesis route using reactive inorganic silica and alumina species is reported. This way improved mesoporous molecular sieves were obtained. The synthesized aluminum substituted mesoporous molecular materials (Al-MMS) contain very large pores of 50-200 angstrom size combined with an improved pore wall thickness. Increased wall thickness and Al substitution lead to an improved chemical stability against alkaline solution. The textural, structural and acid properties are investigated by physico-chemical methods. The catalytic performance acidic materials was tested in the benzoylation reaction; amino functionalized materials were studied in the base catalyzed Michael addition. (C) 2010 Elsevier B.V. All rights reserved.

A new method is introduced for estimating the effects of the inhomogeneities on the propagation of the elastic waves in isotropic bodies. The method is based on the Kirchhoff electromagnetic potentials. It is applied here for estimating the effect of a static density inhomogeneity, either extended or localized, on the elastic waves propagating in an infinite, or a semi-infinite (half-space) body. For a semi-infinite body the method leads to coupled integral equations, which are solved. It is shown that such a density inhomogeneity may renormalize the waves velocity, or may even produce dispersive waves, depending on the geometry of the body and the spatial extension of the inhomogeneity. The method can be extended to other types of geometries or inhomogeneities, as, for instance, those occurring in the elastic constants. (C) 2010 Elsevier Ltd. All rights reserved.

Nitrogen doped TiO2 aerogels were obtained by sol gel process followed by supercritical drying with liquid CO2. Urea and NH3 (25% aqueous solution) were used as nitrogen sources. The as-prepared aerogels were submitted to thermal treatments performed under different experimental conditions (temperature and heating time). Increasing the temperature from 450 degrees C to 550 degrees C a decrease about 32% and 15.4% of the particle size and surface OH groups' concentration, respectively and an increase about 21.4% of the incorporated nitrogen in TiO2 lattice were observed. The immersion of the TiO2 aerogels in NH3 solution induced a decreasing by 48.86% and by 80% of the S-BET and the aerogel porosity, respectively. The photocatalytic activity of the obtained N-doped TiO2 was found to depend mainly on the nitrogen incorporation in the TiO2 lattice. (C) 2009 Elsevier Inc. All rights reserved.