National Institute Of Materials Physics - Romania

Experimental studies on patterning hexagonal Ge nanostructures have been conducted on Si substrates through deposition of Ge with polystyrene spheres as a mask. The size distribution of the patterned Ge nanostructures is narrow with the full width at half maximum being less than 10% of the dot size. The two-dimensional patterned Ge nanostructures were further introduced in a Si matrix. Cross-section transmission electron microscopy reveals periodic dark stripes representing the deposited Ge dots in an a-Si matrix.

Barium titanate (BaTiO3) has been doped in situ with 5 mol% thulium by a sol-gel method. The as-prepared gel powder consists of nanosized grains (20-30 nm) and crystallizes on the cubic BaTiO3 lattice, at 700 degrees C. Ba0.95Tm0.05TiO3 ceramics derived from this powder have tetragonal perovskite structure and contain a small amount of Tm2Ti2O7 pyrochlore phase. These ceramics exhibit dielectric constants of 4282-3240 and dielectric loss (tan delta) of 0.1077-0.0161 at Curie temperature T-c=132 degrees C and at 10 Hz-100 kHz, respectively. For a drive voltage of 400 V, the hysteresis loop recorded at the frequency of 100 Hz shows a remnant polarization (P-r) value of 76 mu C/cm2 and a coercive field (E-c) of 124 V and for 1 kHz a remnant polarization (P-r) value of 58 mu C/cm2 and a coercive field (E-c) of 116 V.

The article reports the preparation and complex characterization of iron-containing phosphate glasses considered to be ecological materials, as they contain nontoxic compounds related to environment. The oxide system Li2O-MgO-(CaO)-Al2O3-P2O5-(FeO/Fe2O3) was investigated in respect to its structural changes caused by MgO replacement with CaO and by the iron addition. UV-vis-NIR (ultraviolet-visible-near infrared) spectroscopy as well as thermo-gravimetric (TG) measurements, differential thermo-analysis (DTA), X-ray diffraction (XRD) analysis, electronic paramagnetic resonance (EPR), and Mossbauer (nuclear gamma resonance) spectroscopy have been used to investigate redox states and coordination symmetry of iron, together with vitreous network changes during the heat treatment up to 1000 degrees C. UV-vis-NIR transmission spectroscopy revealed no structural modifications when MgO was substituted by CaO, but noteworthy absorption bands attributed to Fe2+/Fe3+ species. TG analysis made in the 20-1000 degrees C range shows low weight loss accompanied by several thermal effects, as evidenced by DTA. XRD patterns for the glass samples heat treated at about 700 degrees C revealed the presence of different phosphate crystalline phases containing Mg, Al, and Fe ions. EPR spectroscopy revealed the presence of paramagnetic Fe3+ ions and the change of the first coordination symmetry, when the samples are heated below the vitreous transition temperature. Mossbauer spectroscopy has evidenced two paramagnetic species, Fe2+ and Fe3+, both in octahedral coordination symmetry and a clustering process supported by only Fe3+ ions.

Polydiphenylamine/single walled carbon nanotube (PDPA/SWNT) composites were synthesized electrochemically aiming at their application as active electrode materials for rechargeable lithium batteries. The electrochemical polymerization of diphenylamine (DPA) on a SWNT film immersed in a 1 M HCl solution was studied by cyclic voltammetry. Comparing cyclic voltammograms recorded on a blank Pt electrode with those obtained for a SWNT film deposited on Pt electrode one observes in the latter case a decrease of the DPA reduction potential. To elucidate electrochemical polymerization mechanism, photoluminescence studies on DPA/SWNT and PDPA/SWNT systems were carried out. Additional information concerning the functionalization process of SWNT with PDPA was obtained by Raman and IR spectroscopy. Using the PDPA/SWNT composite as active material for the positive electrode of a rechargeable lithium cell (LiPF(6) electrolyte), the charge-discharge tests show a specific discharge capacity of ca. 245 mA h g(-1), much higher than the 35 mA h g(-1) for pure PDPA. (C) 2010 Elsevier B.V. All rights reserved.

A new model for the thermal spike produced by the nuclear energy loss, as source of transient processes, is derived analytically, for power law dependences of the diffusivity on temperature, as solution of the heat equation. The contribution of the ionizing energy loss to the spike is not included. The range of validity of the model is analysed, and the results are compared with numerical solutions obtained in the frame of the previous model of the authors, which takes into account both nuclear and ionization energy losses, as well as the coupling between the two subsystems in crystalline semiconductors. Particular solutions are discussed and the errors induced by these approximations are analysed. (C) 2011 Elsevier B.V. All rights reserved.

The temperature distribution and solid-liquid interface shape during benzil growth have been studied both experimentally and numerically. The heat transfer equation with appropriate boundary conditions has been solved by modelling a vertical Bridgman-Stockbarger growth configuration. Two models have been developed, namely a global numerical model and a pseudo-transient approximation in an ideal configuration. (C) 2010 Elsevier B.V. All rights reserved.

Porous nanocomposites based on TiO2 aerogels and Au and, respectively, Ag particles were synthesized and their dual functionality to decontaminate water, via photocatalysis, and to detect low concentrations of pollutants from water, via SERS technique, was evaluated using model pollutants. The apparent photodegradation rate constants of a standard pollutant molecule revealed a considerable improvement of the composites photocatalytic performances (even better than one order of magnitude) in comparison with the commercial product Degussa P25. The essential role of the contact between TiO2 and Au nanoparticles was demonstrated and the potential of all synthesized porous composites for photocatalytic experiments with visible light was pointed out. The lowest concentrations of contaminants adsorbed on the noble metal particles detectable by SEAS were found to depend on the composite and pollutant species type and were determined to be between 10(-3) and 5 x 10(-6) M, and 10(-8) and 10(-10) M for off and under resonant excitation, respectively. The morphological (BET, pore size distribution, TEM) and structural (Raman, XRD, diffuse reflectance) particularities of the obtained porous composites were also discussed. (C) 2010 Elsevier B.V. All rights reserved.

It is proposed a processing route to obtain dense (>= 90%) nanometric La0.9Sr0.1Ga0.8Mg0.2O3-x (LSGM) electrolyte ceramic by reacting under spark-plasma-sintering (SPS) conditions a relatively poor mixture of intermediate powdered La/Sr- and Ga/Mg-based products synthesized separately by precipitation and hydrothermal methods, respectively. Samples with particle size of 9-20, 37-93, and 120-240 nm were measured from the electrical conductivity viewpoint. A fast transport path along grain boundaries was not observed and, a higher nano particle size improves total conductivity in our samples. Results suggest that it is necessary to make a clear distinction between particle size from microscopy observation and crystallite size from structural (e.g. XRD) measurements when looking at the general conductivity - grain size dependence. This dependence is complex and the use of an unconventional, far from equilibrium technique such as SPS might influence it. Currently, a good understanding is missing, but pending on materials, technology specifics and the complex relationship between them, some of the usually promoted ideas from literature such as the necessity of high mixing levels in the precursor powders, of high pressure application during sintering, of low particle size (nano) in the sintered ceramic for a higher conductivity, of high purity phase and so on require careful consideration. (C) 2010 Elsevier B.V. All rights reserved.

A dimer chromium(III)-salen complex immobilized on modified silica is an effective catalyst for the epoxidation/epoxide ring-opening reaction of trans-methylcinnamate ester and gave significantly high ee in methyl (2R,3S)-2,3-dihydroxy-3-phenylpropionate. For the first time, it is shown that a donor ligand (e.g., Et3N) can be involved into the enantioselective mechanism of the product configuration. It is suggested that the additive is interacting with the reaction intermediate in a manner that prolongs its lifetime, thus affording a free rotation of the C-C single bond in this species and a selective collapse to the cis-epoxide product. In the presence of water, this epoxide led to methyl (2S,3R)-2,3-dihydroxy-3-phenylpropionate. Alternatively, the donor additives may give rise to a new chromium-based oxidant that effects epoxidation and its consecutive ring opening to diols via an unusual pathway.