National Institute Of Materials Physics - Romania

Bulks and tapes of MgB2 with 5 wt.% SiC were prepared from Mg, B and SiC. Mixing of the raw powders was performed in Ar atmosphere by high energy milling using a planetary mill. Number of balls (10 or 20), size (10 mm or 7 mm, respectively), rotation speed (200 rpm or 400 rpm) and milling time (up to 3 h) were varied. Powders were compared through XRD and SEM. From the as-prepared powders, bulk and tape samples were prepared and investigated by magnetic (SQUID) and transport (four-probe method) measurements. Results suggest that through variation of the milling conditions such as rotation speed, size and number of balls it is possible to control the value of the critical current density, J(c) and the slope of the J(c)(B) curve, For different heat treatment conditions milling with 400 rpm and using 20 balls of 7 mm always resulted in lower J(c)(B) than for the tapes or bulks produced from hand-milled mixture. Use of 10 balls of 10 mm keeping all the other conditions constant changed the slope of the J(c)(B) curve possibly improving J(c) above a crossover at 4 T. Mild milling with a rotation speed of 200 rpm (20 balls x 7 mm) resulted in enhancement of J(c) in the entire investigated region B <= 14 T, but the slope was steeper than for the best sample produced from the hand-milled mixture and a crossover is likely to occur for B > 14 T leading to lower J(c)-values. The best samples had at 4.2 K J(c) = 3.6 x 10(-2) A center dot cm(-2) at 14 T and J(c) = 5 x 10(3) A center dot cm(-2) at 10 T.

The experimental studies on III-V semiconductor compounds surface passivation phenomena are mainly dedicated to solve some technological problems as those regarding the ways to keep the chemical stability of native oxides on surfaces. Self-assembled monolayers (SAMs) provide a simple way to produce relatively ordered structures at a molecular scale, which seems to be capable to protect the clean surface against the evolution of oxidation process. In this respect, thin films of SAMs of aliphatic thiol (dodencanthiol-CH3(CH2)(11)SH) and aromatic thiol (4, 4' tiobisbenzenthiol-S (C6H4SH)(2) have been deposited on the surface of Gap (111) samples. The electrical properties measurements of some structures based on GaP compound was performed. There were recorded current-voltage (I-V) characteristics for complex structures AuGeNi/R-SH/GaP and AuGeNi/Ar-SH/GaP in darkness and also exposed to a Xe lamp. In dark and in "reverse bias" way, the I-V characteristics present the feature of a Zenner diode for GaP/Ar-SH and a gradual increase of current for GaP/R-SH. In dark and "in forward bias" way, the current increases as for a normal diode for both GaP/Ar-SH and GaP/R-SH structures. The complex structures (e.g.: In/AuGeNi/R-SH/GaP/R-SH/AuGeNi/In) are less sensitive to light. The SEM analysis performed on a GaP/R-SH surface shows a continuous packed up layer while GaP/Ar-SH looks like an inhomogeneous deposition of layers with different thickness regions. The diodes' ideality factors determined from I-V characteristics are unusually high (n >> 2) as a possible result of inhomogeneous Schottky contacts or due to ageing effects, in the field of degradation. (C) 2009 Elsevier Ltd. All rights reserved.

Super - fine calcium carbonate was prepared by double jet precipitation method. The values of the estimated kinetics parameters show that nucleation is the dominant mechanism in all experimental runs.

The possibility to determine the kinetic parameters for temperature programmed reduction of Cu/Fe3O4 using only one TPR profile is analyzed. The same data are analyzed both by Friedman's iso-conversional method and another one previously derived and published by the authors. One shows that taking into account the experimental restrictions of Monti and Baiker, the Friedman's method, although gives values of the activation energy smaller than the real values, indicates a very similar dependence of these on the reduction degree. On the basis of some synthetic data one shows that the errors are very large when these recommendations are neglected, being possible to determine a false dependence on the degree of reduction.

N-doping is often used to improve the photocatalytic properties of TiO2, films in order to achieve visible light response. In this work, we study the effect of annealing treatment (temperature and atmosphere) on the structural and optical properties of undoped and N-doped TiO2, films by means of X-ray diffraction (XRD), Atomic Force Microscopy (AFM), Rutherford backscattering (RBS), X-ray photoelectron spectroscopy (XPS), spectroscopic ellipsometry (SE) and UV-VIS optical transmission spectroscopy. Porous five-layer TiO2, films were deposited by sol-gel method on quartz substrate and thermally treated in oxygen or NH3 flow at 500 and 600 degrees C. Significant doping effect was achieved after annealing at 600 degrees C in NH3 and a shift in optical bandgap value down to visible range (2.69 eV) was observed. (C) 2008 Elsevier B.V. All rights reserved.

Hydroxyapatite (HA) biocoatings were obtained using two different deposition methods: sol-gel and sputtering. A direct comparison of films obtained using different techniques is necessary because of possible formation of multi-phases and components in HA films. HA films were investigated using spectroscopic ellipsometry (SE) as a powerful method to evaluate composition and structural properties of the films. For both types of films, sputtering and sol-gel, a weak dispersion of the refractive index within the wavelength range 0.4-0.7 pm was found. This corresponds to a high energy bandgap and a high optical transparency in the UV-VIS range. The internal porosity varying in a wide range in the sol-gel films was evaluated using a modeling of SE data. Additionally to SE Studies, structural, chemical and optical characterizations of the Films were performed using Fourier transform infrared spectroscopy (FTIR), X-ray photoelectron spectroscopy (XPS) and X-ray diffraction (XRD). Two absorption bands observed in the wavelength ranges 550-600 cm(-1) and 1000-1200 cm(-1) can be attributed to the major absorption modes associated with HA in agreement with published data. Prom XPS analysis, the atom ratio Ca/P was found for sol-gel films close to the stoichiometric value 1.67. For sputtering films, a higher Ca/P ratio of about 2.1 was estimated. However, similar FTIR spectra have been recorded for both sputtering and sol-gel films. XRD Studies show also similar crystallization conditions by annealing at temperatures higher than 500 degrees C. (C) 2008 Elsevier B.V. All rights reserved.

Ceramics (Ba1-xSrx)TiO3, (BST) with x=0.25; 0.35; 0.40; 0.50; 0.60; 0.75; 0.90 were prepared by sintering method from BaCO3, SrCO3 and TiO2 of high purity (99.98%). The solid-state reaction was employed to obtain pure and doped BST ceramic samples. In order to improve the sintering process the following additives were added: 1.0 wt.% MgO and 1 wt.% MnO2. The sintering treatment was performed in die temperature range 1200 to 1260 degrees C, for 2 h. Archimedean method was used for ceramic density measurements, which was found to depend linearly on strontium content: rho(x)=5.4(2)-0.8(8).x, (g/cm(3)). SEM, EDX and XRD methods were used for sample characterization. Morphology, pores and grain size distribution of both pure and doped ceramics were investigated by SEM analysis. A bimodal grain distribution could be observed in pure BST for x <= 0.25, whereas a uniform one was noticed for doped samples of x = 0.25. Well faceted grains of similar to 5 pm size, Could be seen for x=0.75 samples. Pores size has grown with the increase of Sr content. X-ray diffraction patterns have shown a dominant perovskite structure and some other minor phases. Dielectric permittivity and loss were investigated at 1kHz, in the temperature range - 150 to + 150 degrees C, at a pace of similar to 2 degrees C /min. Sharp transitions, with higher peaks values of permittivity were noticed with the increase of the sintering temperature. A shift Of the Curie points to lower values was noticed with the increase of Sr content, according to the equation: Tc = 127.4-331.x. Measured at room temperature, in the frequency range of similar to 1 GHz, permittivity was found to increase and losses to decrease with the sintering temperature. For x <= 0.35 permittivity of the samples, being in the ferroelectric phase could not be measured in microwave domain at room temperature. Our results point to important application of these BST compositions for microwave devices. (C) 2008 Elsevier B.V. All rights reserved.

High spectral (1.5 cm(-1)) and spatial (similar to 1000 nm lateral and vertical) resolution Raman spectra taken at room temperature in the depth of a thin (350 nm) YBCO film with good superconducting properties are presented and discussed. The differences between Raman spectra corresponding to different depths in the film are interpreted as depth-dependant changes in the film's structure, degree of oxygenation, and superconducting properties. These changes are in agreement to what we expect from a film obtained by pulsed laser deposition technique. The resonance of the most significant Raman shift (500 cm(-1)), monitoring the oxygen content of YBCO films, for 488 nm excitation light, was used to overcome the relatively small penetration depth of visible light in YBCO (similar to 200 nm). We found the range of parameters for the depth profiling Raman system such as to obtain the maximum signal intensity and a reasonable number of distinctive Raman spectra corresponding to different depths of the YBCO film. (C) 2008 Elsevier B.V. All rights reserved.

We introduce a theoretical formalism describing a wide class of 'Which Path' experiments in mesoscopic/nanoscopic transport. The physical system involves a mesoscopic interferometer (e.g. an Aharonov-Bohm ring with embedded dots or a side-coupled quantum dot) which is electrostatically coupled to a nearby quantum point constriction. Due to the charge sensing effect the latter acts as a charge detector. Therefore the interference pattern can be monitored indirectly by looking at the current characteristics of the detector as shown in the experimental work of Buks et al. [E. Buks, R. Schuster, M. Heiblum, D. Mahalu, V. Umansky, Nature (London) 391 (1998) 871]. We use the non-equilibrium Green-Keldysh formalism and a second order perturbative treatment of the Coulomb interaction in order to compute the relevant transport properties. it is shown that in the presence of the Coulomb interaction the current through the detector exhibits oscillations as a function of the magnetic field applied on a single-dot AB interferometer. We also discuss the dependence of the visibility of the Aharonov-Bohm oscillations on the gate potential applied to the dot. (C) 2008 Elsevier B.V. All rights reserved.