National Institute Of Materials Physics - Romania

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.

We investigated that in HgBa2Can-1CunO2n+2+delta, [Hg-1 2(n-1)n], for n >= 6, T-c of about 105 K, vortex melting lines remains constant and independent of number of inner plane CuO2 layers. However the vortex dynamics of the supermultilayer system could be crucially different from that of single or double layer cuprates. Therefore we measured the fundamental and third harmonic AC susceptibility responses to see the vortex dynamics and vortex matter phase diagram of Hg-12(n-1)n (n= 14), the higher member of super-multilayered cuprate superconductors. The Lindemann type vortex melting line was estimated using onset of third harmonic susceptibility (chi 3), at very low h(AC) (50 mOe), and represented by 3D Ginzburg Landau rescaling. Frequency dependence of out of phase susceptibility chi"(T) measured at lower fields of 500 and 1000 Oe. for 10 Hz to 10 kHz, showed resonance behaviour in the temperature region 55-68 K, suggesting rotational and translational creep dynamics of vortex molecule, even below usual melting line. We present schematic vortex phase diagram, considering the response of vortex dynamics to applied AC frequencies, taking into account the crossover temperatures obtained from critical slowing down of spin glass model and extended thermal activated model. (C) 2008 Elsevier B.V. All rights reserved.

Analyses of standard current-voltage (I-V) characteristics of disordered high-temperature superconductors (HTS) indicate that the vortex phase at high magnetic fields H should be an elastic vortex glass, where the vortex pinning barriers diverge at low current densities, whereas do magnetization relaxation measurements reveal the presence of nondiverging (plastic) pinning barriers in a wide H-T domain. We show that the different conclusions concerning the nature of the vortex phase at high H in disordered HTS seem to be due to the ordering effect of the driving force existing in various experiments. (C) 2008 Elsevier B.V. All rights reserved.

A recent report on multilayered superconductors establishes unique constant T-c although the number of inner CuO2 plane (IP) increases. Many hypotheses proposed that the irreversibility field (B-irr) and critical current density (J(c)) are depends on the crystal structures. We have measured the J(c) and B-irr of the HgBa2Can-1CunO2n+2+delta sample containing n = 6-15 phases to investigate the effect of the number of IP's on the B-irr and J(c) and its pinning properties. The rate of fall of J(c) increases and irreversible lines (IL's) shift to lower temperatures with increasing the number of IP's, which suggests that anisotropy increases with n. The IL settled in between those for Bi2Sr2CaCu2Oy with high anisotropy value and YBa2Cu3O7 with less anisotropy value. The double logarithmic plot of irreversibility field versus [1 - (T/T-c)] analysis suggests that the flux line melting model is adopted. The flux pinning force density F-p (approximate to J(c)B) exhibits scaling behavior when the magnetic field B is normalized by the irreversibility field B;- Analysis of the normalized pinning force reveals that a surface pinning mechanism is dominant and reduced magnetic field b(max) = 0.2 agree with surface pinning mechanism with closely spaced pins. (C) 2008 Elsevier B.V. All rights reserved.

The effects of heating-induced crystallization on the structural and mechanical properties of Mg-Ni-Fe amorphous ribbons were studied by anelastic spectroscopy, differential scanning calorimetry (DSC) and X-ray diffraction. DSC results show that the crystallization occurs through several non-reversible steps, which correspond to significant changes in the Young's modulus and concomitant irreversible elastic energy loss peaks. Moreover, an anelastic peak is found at 215 K, which for the first time indicates the presence of some dynamical process related to the simultaneous presence of different phases. The formation of a metastable Mg(6)Ni phase is detected, which transforms into Mg and Mg(2)Ni stable phases. A quantitative analysis of the different phases present at the different steps was also carried out. (C) 2007 Elsevier B.V. All rights reserved.