Publications

The films containing Ge nanoparticles embedded in SiO2 matrix were prepared by RF magnetron sputtering and subsequently by thermal annealing. Their structure was investigated by conventional transmission electron microscopy and high resolution transmission electron microscopy together with energy-dispersive X-ray spectroscopy. The electrical behavior of films was studied by measuring current-temperature and current-voltage characteristics. The structure investigation reveals two kinds of features: a low density of big Ge nanoparticles with sizes from 20 to 50 nm and a network of small amorphous Ge nanoregions/nanoparticles (5 nm size or less) with high density, both being embedded in amorphous SiO2 matrix. The electrical transport was shown to take place through the network of amorphous Ge nanoregions. At low temperature, the T-1/4 dependence of the current was evidenced, while at high temperature, the T-1 Arrhenius dependence was found. At both low and high temperatures, the conductivity is nearly constant. The behavior at low temperature was explained by the hopping mechanism on localized states located in a band near the Fermi energy, while at high temperature by the charge excitation to the extended states.

A methodology is proposed to investigate in detail shrinkage kinetics under isothermal spark plasma sintering (SPS) conditions applied to ceramic nano powders such as Y2O3 stabilized ZrO2. To do so, mild SPS conditions were used (low temperatures and pressure, long dwell times). The extracted experimental activation energy has the value of 246 +/- 37 kJ mol(-1) and the slope of the curves on the intense densification stage is around 0.33. Results are in agreement with densification by a grain-boundary diffusion mechanism as for conventional sintering and the contribution from the specific pressure-assisted mechanisms as for hot pressing is insignificant. This result suggests that exploration of mild SPS might prove rewarding in separation and control of the sintering mechanisms leading to production of specific ceramic with new or improved functionality. (C) 2011 Elsevier Ltd and Techna Group S.r.l. All rights reserved.

(Bi0.5Na0.5)TiO3 was doped in situ with 5, 8, and 11 mol% BaTiO3 (BNT-BTx; x = 0.05, 0.08 and 0.11) using a sol-gel technique. The resulting powders from gel precursors showed microstructures consisting of nano-sized grains and crystalline perovskite structure. Spark plasma sintering (SPS) technique was used to prepare high densified (98-99%rho(theor)) BNT-BTx ceramics from these nanopowders. The results confirm the spark plasma sintering method applied to nano-scale powders, obtained by sol-gel, as a viable route in producing nanostructured ceramics. The evolution of the structure and electrical properties of the ceramics with BaTiO3 concentration (x) was investigated. The permittivity of BNT-BT0.08 ceramic is higher (epsilon(r) = 2090, at 100 kHz) than that for x = 0.05 (epsilon(r) = 1350) and x = 0.11 (epsilon(r) = 1800). BNT-BT0.08 ceramic shows maximum values for the frequency constants (N-p, N-t), piezoelectric charge coefficient (d(31)) and piezoelectric voltage coefficient (g(31)), and minimum values for the electromechanical coupling factor (k(p)) and piezoelectric charge coefficient (d(33)). The electrical properties of these ceramics are influenced by grains size, oxygen deficiency and non-uniform internal stresses due to these oxygen deficiencies. BNT-BTx ceramics sintered by SPS seem to be good ceramic resonators with high mechanical quality factor (Q(m)). Crown Copyright (C) 2012 Published by Elsevier B. V. All rights reserved.

Superconducting bulks of MgB2 were obtained by an ex-situ two-temperature route applied to spark plasma sintering (SPS). Processing of samples was performed at lower temperatures than previously reported. Samples produced by the two-temperature route show a higher morphological uniformity, a higher density (above 98%), a higher Vickers hardness, and undesirable stronger microscale flux jumps, as indicated by magnetic relaxation measurements when compared to a sample obtained by the one-temperature route (95.3% relative density). At the same time, all sintered samples show approximately constant crystallite size, critical current density, irreversibility field, critical temperature, weight fraction of impurity phases (MgB4 and MgO), and the amount of carbon accidentally introduced during SPS processing. (C) 2012 Elsevier B.V. All rights reserved.

Ag-doped nanocrystalline hydroxyapatite nanoparticles (Ag:HAp-NPs) (Ca10-x Ag (x) (PO4)(6)(OH)(2), x (Ag) = 0.05, 0.2, and 0.3) with antibacterial properties are of great interest in the development of new products. Coprecipitation method is a promising route for obtaining nanocrystalline Ag:HAp with antibacterial properties. X-ray diffraction identified HAp as an unique crystalline phase in each sample. The calculated lattice constants of a = b = 9.435 , c = 6.876 for x (Ag) = 0.05, a = b = 9.443 , c = 6.875 for x (Ag) = 0.2, and a = b = 9.445 , c = 6.877 for x (Ag) = 0.3 are in good agreement with the standard of a = b = 9.418 , c = 6.884 (space group P6(3)/m). The Fourier transform infrared and Raman spectra of the sintered HAp show the absorption bands characteristic to hydroxyapatite. The Ag:HAp nanoparticles are evaluated for their antibacterial activity against Staphylococcus aureus, Klebsiella pneumoniae, Providencia stuartii, Citrobacter freundii and Serratia marcescens. The results showed that the antibacterial activity of these materials, regardless of the sample types, was greatest against S. aureus, K. pneumoniae, P. stuartii, and C. freundii. The results of qualitative antibacterial tests revealed that the tested Ag:HAp-NPs had an important inhibitory activity on P. stuartii and C. freundii. The absorbance values measured at 490 nm of the P. stuartii and C. freundii in the presence of Ag:HAp-NPs decreased compared with those of organic solvent used (DMSO) for all the samples (x (Ag) = 0.05, 0.2, and 0.3). Antibacterial activity increased with the increase of x (Ag) in the samples. The Ag:HAp-NP concentration had little influence on the bacterial growth (P. stuartii).

The purpose of this work was to obtain precipitated calcium carbonate (PCC) particles in polymorphic form of vaterite via gas-liquid route in controlled pH conditions. The effect of CO2 concentration (12.5-100%), feed gas (CO2-air) flow rate, pH, and conductivity of solution upon the PCC particles properties was studied. On the basis of the experimental data, the main factors leading to vaterite formation as major product were established. It was found that the buffer solution has a decisive role in determining polymorphic phase of PCC while CO2 concentration and feed gas flow rate have no significant influence. It was demonstrated that spherical vaterite particles of high purity can be produced under controlled reaction conditions. Also, some considerations on the mechanism of carbonation process were formulated.

The rectifying properties of Nb:SrTiO3-Bi1-xGdxFeO3-Pt structures (x = 0, 0.05, 0.1) displaying diode-like behavior were investigated via current-voltage characteristics at different temperatures. The potential barrier was estimated for negative polarity assuming a Schottky-like thermionic emission with injection controlled by the interface and the drift controlled by the bulk. The height of the potential barrier at the Nb:SrTiO3-Bi1-xGdxFeO3 interface increases with Gd doping. The results are explained by the partial compensation of the p-type conduction due to Bi vacancies with Gd doping in addition to the shift of the Fermi level towards the middle of the bandgap with increasing dopant concentration. (C) 2012 American Institute of Physics. [http://dx.doi.org/10.1063/1.4729816]

We studied the anisotropic properties of mechanically stretched bulk carbon nanotube sheets using magnetoresistance (MR) measurements in magnetic fields applied under different orientations with respect to the stretching direction. The stretching direction was either parallel or perpendicular to the direction of the electric current. The magnetic field was rotated either in-the-plane or out-of-the-plane of the sheets. We found that the angular dependence of the MR is a superposition of two terms, one with twofold symmetry and the other one with fourfold symmetry. We also found that the field-dependence of the MR is composed of two terms, one positive and one negative, whose magnitudes are largest when the field is parallel with the stretching direction. If the sheets are treated with nitric acid, the positive term is removed and the MR is smallest when the field is aligned with the magnetic field. We attribute these anisotropic features to magnetoelastic effects induced by the coupling between the magnetic catalyst nanoparticles, the magnetic field, and the network of nanotubes. (C) 2012 American Institute of Physics. [http://dx.doi.org/10.1063/1.4729538]

ZnO films were electrodeposited from an aqueous nitrate bath on ITO/glass substrates. The process was performed not by the usual potentiostatic approach but by using potential ramps with different sweep rates. We tested these ramps in both directions i.e. either towards electronegative (direct) or electropositive (inverse) potential. As expected, the samples prepared in different deposition conditions show different morphology, different quality of crystalline structure and different optical properties. By employing inverse ramps we prepared films with high quality structural and optical properties. We assume that in these conditions the growth is followed by an etching process which preferentially removes the areas with high defect concentration and leads to the formation of hollow hexagonal prisms. (c) 2012 Elsevier B.V. All rights reserved.

We use a generalized master equation (GME) formalism to describe the nonequilibrium time-dependent transport of Coulomb interacting electrons through a short quantum wire connected to semi-infinite biased leads. The contact strength between the leads and the wire is modulated by out-of-phase time-dependent potentials that simulate a turnstile device. We explore this setup by keeping the contact with one lead at a fixed location at one end of the wire, whereas the contact with the other lead is placed on various sites along the length of the wire. We study the propagation of sinusoidal and rectangular pulses. We find that the current profiles in both leads depend not only on the shape of the pulses, but also on the position of the second contact. The current reflects standing waves created by the contact potentials, like in a wind musical instrument (for example, a flute), but occurring on the background of the equilibrium charge distribution. The number of electrons in our quantum "flute" device varies between two and three. We find that for rectangular pulses the currents in the leads may flow against the bias for short time intervals, due to the higher harmonics of the charge response. The GME is solved numerically in small time steps without resorting to the traditional Markov and rotating wave approximations. The Coulomb interaction between the electrons in the sample is included via the exact diagonalization method. The system (leads plus sample wire) is described by a lattice model.