National Institute Of Materials Physics - Romania

(Na1/2Bi1/2)TiO3 doped in situ with 11 inol% BaTiO3 (NBT-BT0.11) powders were synthesized by a sol-gel method, and the electrical properties of the resulting ceramics were investigated. The powders consisting of uniform and fine preliminary particles of about 50 nm were prepared by calcining the gel precursor at 700 degrees C. (Na1/2Bi1/2)(0.89)Ba0.11TiO3 ceramics, sintered at temperatures up to 1150 degrees C have a rhombohedral symmetry while the ceramic sintered at 1200 degrees C exhibits a tetragonal crystalline structure. The ceramics show high dielectric constant (epsilon(r) similar to 5456), dielectric loss of 0.02, depolarization temperature T-d similar to 110 degrees C and temperature corresponding to the maximum value of dielectric constant T-m similar to 262 degrees C. The dielectric constant (epsilon(33)) and the piezoelectric constant (d(33)) attain the maximum values of 924 and 13 pC/N, respectively, while the electromechanical coupling factor (k(p)) value is 0.035. The NBT-BT0.11 ceramics derived from sol-gel present high mechanical quality factor (Q(m) similar to 860). The dielectric and piezoelectric properties values of NBT-BT0.11 ceramics derived from sol-gel are smaller than those of samples produced by the conventional solid state reaction method, due to the grains size and oxygen vacancies that generate dipolar defects. (C) 2012 Elsevier Ltd. 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.

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]

High quality pulsed laser deposited lead zirconate titanate (PZT) films are obtained by pulsed laser deposition on SrRuO3(111) single crystal layers and characterized by X-ray photoelectron spectroscopy (XPS), to determine the surface composition. It is found that a minor amount of Pb forms PbO2 at the surface and also some Pb is included into the contamination layer, in form of a Pb(CO3)2 layer of about 1?nm thickness, occupying about one quarter of the PZT surface. The stoichiometry of the outermost 45?nm layers yielded as PbZr0.25Ti0.80O2.5, which suggest the formation of an oxygen depleted, Brownmillerite-like layer at the surface, of at least 5?nm thickness.

Oxyfluoride glass-ceramics in the system SiO2-Al2O3-CaF2-EuF3 containing Eu3+-doped CaF2 nanocrystals were produced by using controlled crystallization of melt-quenched glass. X-ray diffraction and scanning electron microscopy data have revealed the formation of CaF2 nanocrystals of about 50 nm size. The Eu3+-dopant ions act as the nucleating agent necessary to initiate the crystallization process. The refractive index is higher in the glass ceramics than in the initial glass and varies as the annealing time increases. Two competitive processes are responsible for this behavior, the crystallization of the CaF2 phase and the decrease of the glass ceramic mass density. (C) 2012 Elsevier B.V. All rights reserved.

We present the preparation, structural and magnetic properties of nanosized magnetite obtained by the crystallization of a series of Fe-containing borosilicate glasses. Several compositions with the ratio Fe2O3/SiO2 spanning from 0.37 to 0.67 were investigated as a function of two nucleators Cr2O3 and P2O5, respectively, and modifiers and intermediates (Al2O3 and MgO). Mossbauer spectroscopy revealed the degree, the type and the location of disorder induced by a specific composition and nucleators. In addition to magnetite, it was also revealed the presence of large amounts of Fe-rich paramagnetic phases. The magnetic response is analysed in relation with the amount of Fe ions which remain dispersed in the glassy matrix as noninteracting (paramagnetic) ions. We discuss the role of the nucleators on the disorder in both tetrahedral and octahedral sites of the magnetite.

Epitaxial growth of Bi1.5-xZn0.92-yNb1.5O6.92-1.5x-y (BZN) thin films was achieved on (100)(pc) LaAlO3 substrate by pulsed laser deposition (PLD) and by chemical solution deposition based on Pechini process. Effect of bismuth and zinc deficiency on the BZN thin films obtained by PLD was discussed, in relation with the starting target composition. Dielectric permittivity and bandgap values were determined from electrical and spectroscopic ellipsometry measurements performed on randomly oriented films grown on Pt/Si substrate. BZN thin films obtained by PLD exhibit, at 100 kHz, a dielectric constant of epsilon(r) = 203 and quite low dielectric losses of tan delta = 5 x 10(-2). Epitaxial ferroelectric - dielectric KTa0.65Nb0.35O3 (KTN) -Bi1.5-xZn0.92-yNb1.5O6.92-1.5x-y (KTN on BZN and BZN on KIN) bilayers were obtained by PLD on (100)(pc) LaAlO3 with the insertion of a suitable buffer layer of KNbO3 in the case of KTN on BZN. Such multilayer heterostructures with an epitaxial growth control of each layer are promising candidates for potential integration in microwave devices. (C) 2011 Elsevier B.V. All rights reserved.

Using a pulsed laser deposition system in a plane reflector configuration, we have grown single crystal ZnO nanowires using different gold catalyst quantities and different local particle fluxes. The number of ZnO nanowire over the square micrometer substrate surface proved to depend on both gold quantity and particle flux, in specific conditions. If for a "low particle flux" the nanowire density seems to strongly depend on the gold quantity, for a "high particle flux", the dependence is no longer observable. The results were interpreted in terms of the catalyst absorbed particle concentration dependence on the catalyst droplet size and incident particle flux. (C) 2011 Elsevier B.V. All rights reserved.

The paper presents the optical properties of amorphous-like indium zinc oxide and indium gallium zinc oxide thin films with various In/(In + Zn) ratios obtained by Pulsed Laser Deposition. Thickness results obtained from simulations of X-ray Reflectivity and Spectroscopic Ellipsometry data were very similar. The dependence of density on stoichiometry resembles the corresponding dependence of the refractive index in the transparency range. A free carrier absorption was noted in the visible spectral range, leading to a weak absorbing thin transparent conductive oxide. On the other hand, the refractive index is smaller than those of based oxides (ZnO and In2O3), and counterbalance therefore the weak light absorption. (C) 2011 Elsevier B.V. All rights reserved.

A new noniterative method, for determining the dielectric, piezoelectric and elastic constants, in complex form, for piezoceramic materials, in the radial mode, was proposed. This method uses the standard procedure to determine the elastic compliance and Poisson factor and the measurement of admittance at two frequencies to calculate the dielectric and piezoelectric constants, by solving a system of two equations. The accuracy of the new method was determined for materials with different planar coupling coefficients (k(p) = 2.5-57%) and mechanical quality factors (Q(m) = 20-3000). This method proved to be very accurate for all materials especially for those with large coupling factors. The accuracy of standard method was also evaluated for the same materials. (C) 2011 Elsevier Ltd. All rights reserved.