National Institute Of Materials Physics - Romania

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5781 articles found

2671. New insight into phase change memories

Authors: Velea, A

Published: JUL 1 2011, JOURNAL OF NON-CRYSTALLINE SOLIDS, 357, 2631, DOI: 10.1016/j.jnoncrysol.2011.02.014

The switching mechanism in phase change memories was described on the basis of minimum switching unit: the commuton. A commuton is a minimum cluster of atoms that supports a reversible phase change from high to low electrical conduction state and back under the influence of an external signal. The switching process in a phase change chalcogenide film was modeled using two dimensional cellular automata approach. A system of 50 x 50 cells, each cell containing a commuton, was simulated. In the particular case of Ge(2)Sb(2)Te(5) (investigated here) this system corresponds to a 30 x 30 nm area. The formation of the percolation path as a function of phase change induced in commutons explains the switching phenomenon. The influence of the percent of defects in the material on the percolation threshold has been studied. (C) 2011 Elsevier B.V. All rights reserved.

2672. Angle-resolved evanescent-wave cavity ring-down spectroscopy for thin film-solid interface characterization

Authors: Cotirlan, C; Logofatu, C; Rizea, A; Lazarescu, MF

Published: JUL 2011, OPTOELECTRONICS AND ADVANCED MATERIALS-RAPID COMMUNICATIONS, 5, 714, DOI:

A new method, that involves Evanescent-Wave Cavity Ring-Down Spectroscopy (EW-CRDS), is presented. This method is useful for the study of thin films on optical solid surfaces. The new design of experimental set-up incorporates a semicylindrical prism with a plane surface where the Total Internal Reflection (TIR) effect appears. The antireflex (AR) coated cylindrical surface of the prism induces the stability of resonant cavity. The evanescent wave generated at the plane surface of the prism probes the absorption by matter in the vicinity of the prism. A general discussion of design criteria is presented to quantify intrinsic losses, and then absorption spectra for Rhodamine chloride R590 from 555 to 560 nm are presented to demonstrate the sensitivity of the system. The layer of R590 on BK7 surfaces is deposited from a solution of R590 in ethanol (92% purity) with 51 mg/l concentration. After vaporization of the ethanol the dye layer on surfaces is supposed to be uniform. This implementation of TIR surface in a resonant cavity provides a powerful new spectroscopic tool especially to angle-resolved diagnostic for interfaces and thin-films. The loss spectrum can be obtained for an angular range from the TIR critical angle to a maximum angle when the resonance of system is lost.

2673. Transient thermal effects in solid noble gases as materials for the detection of Dark Matter

Authors: Lazanu, I; Lazanu, S

Published: JUL 2011, JOURNAL OF COSMOLOGY AND ASTROPARTICLE PHYSICS, DOI: 10.1088/1475-7516/2011/07/013

Noble solid gases are promising detector materials to be used in the search for dark matter. In the present paper a systematic analysis of the transient phenomena associated with the stopping of recoils in noble gases in the solid phase is performed for the first time. The investigated energy range of the recoils corresponds to the elastic scattering of WIMPs from the galactic halo in these materials. A thermal spike model, previously developed by the authors, is extended and applied to solid noble gases. Ionization, scintillation and nuclear. energy loss processes are considered and included in the model, as well as the coupling between the subsystems. The development of the temperature pulse in space and time in solid Ar, Kr and Xe is analysed for different energies of the WIMP, and for different initial temperatures of the material. Phase transitions are possible in particular cases. The results of the model could be used as supplementary information in respect to ionization and scintillation, for detection and particle identification.

2674. BET and XRD studies on the hydroxyapatite and europium doped hydroxyapatite

Authors: Ciobanu, CS; Andronescu, E; Predoi, D

Published: JUL 2011, JOURNAL OF OPTOELECTRONICS AND ADVANCED MATERIALS, 13, 824, DOI:

Europium doped hydroxyapatite (Eu:HAp) nanocristalline powders was synthesized by co-precipitation method by setting the atomic ratio of Eu/[Eu + Ca] at 0% and 2% and [Ca+Eu] /P at 1.67. The structural properties were characterized by X-ray diffraction (XRD). Nitrogen adsorption-desorption isotherms were obtained on different samples using BET method. The X-ray diffraction analysis revealed that hydroxyapatite is the unique crystalline constituent of all the samples, indicating that Eu has been successfully inserted into the HAp lattice. Based on N-2 adsorption-desorption isotherms investigation, the pore size, surface area and pore volume of europium doped hydroxyapatite are 16.57 nm, 115.06 m(2)/g and 0.48 cm(3)/g.

2675. Synthesis and characterization of indium oxide-hematite magnetic ceramic solid solution

Authors: Sorescu, M; Xu, TH; Diamandescu, L; Hileman, D

Published: JUL 2011, HYPERFINE INTERACTIONS, 199, 386, DOI: 10.1007/s10751-011-0267-y

Indium oxide-doped hematite xIn(2)O(3*) (1 - x) alpha-Fe2O3 (molar concentration x = 0.1-0.7) solid solutions were synthesized using mechanochemical activation by ball milling. XRD patterns yield the dependence of lattice parameters and grain size as function of milling time. After 12 h of milling, the completion of In3+ substitution of Fe3+ in hematite lattice occurs for x = 0.1. For x = 0.3, 0.5 and 0.7, the substitutions between In3+ and Fe3+ into hematite and respectively, In2O3 lattices occur simultaneously. The lattice parameters of alpha-Fe2O3 (a and c) and In2O3 (a) vary with milling time. For x = 0.1, Mossbauer spectra were fitted with one, two, or three sextets versus milling time, corresponding to gradual substitution of In3+ for Fe3+ in hematite lattice. For x = 0.3, Mossbauer spectra after milling were fitted with three sextets and two quadrupole-split doublets, representing In3+ substitution of Fe3+ in hematite lattice and Fe3+ substitution of In3+ in two different sites of In2O3 lattice. For x = 0.5 and 0.7, Mossbauer spectra fitting required two sextets and one quadrupole-split doublet, representing coexistence of In3+ substitution of Fe3+ in hematite lattice and Fe3+ substitution of In3+ in indium oxide lattice. The recoilless fraction studied versus milling time for each molar concentration exhibited low values, consistent with the occurrence of nanoparticles in the system. SEM/EDS measurements revealed that the mechanochemical activation by ball milling produced xIn(2)O(3*)(1 - x)alpha-Fe2O3 solid solution system with a wide range of particle size distribution, from nanometer to micrometer, but with a uniform distribution of Fe, In, and O elements.

2676. Structural and electrical properties of yttrium-doped ceria ceramic composites

Authors: Ionascu, AM; Mercioniu, I; Notingher, PV; Popescu-Pogrion, N

Published: JUL 2011, OPTOELECTRONICS AND ADVANCED MATERIALS-RAPID COMMUNICATIONS, 5, 777, DOI:

The attractive properties of Yttria doped ceria, particularly the structural and material properties, have led to vast research efforts to investigate and develop such materials. One of the main applications of this ceramic is in the IT-SOFC. The aim of this paper is the preparation and characterization of Y doped Ceria and low doped alpha-Al2O3, a new composite for IT-SOFC (10 mol %) Y:CeO2 and (150 ppm) Y:alpha-Al2O3 composite were obtained by sol-gel or mechanical route for 10YDC and 150 Y:alpha-Al2O3. Shapes, size distributions and the mean of grains were determined by SEM, EDX and statistical investigations.

2677. The pinning force density in polycrystalline Bi1.8Pb0.4Sr2Ca2-xYxCu3Oy multiphase systems

Authors: Mihalache, V; Deac, IG; Pop, AV; Miu, L

Published: JUL 2011, CURRENT APPLIED PHYSICS, 11, 1014, DOI: 10.1016/j.cap.2011.01.015

ac susceptibility measurements as a function of temperature T and the ac magnetic field amplitude, Hac, have been performed on five superconducting Bi1.8Pb0.4Sr2Ca2-xYxCu3Ox polycrystalline samples (x = 0, 0.005, 0.04, 0.15 and 0.36, and with the volume fraction of the Bi2Sr2Ca2Cu3Oy (2223) phase, f approximate to 100%, 100%, 64%, 0% and 0%, respectively. By using the Muller model for granular superconductors, we found that the pinning force density obtained from the imaginary peak of chi ''(T) is strongly dependent on the doping level. To describe this dependence we adopted the multilayer model (a stack of superconducting, S, and normal, N, layers) of a superconductor. In the framework of this model, we demonstrated that (a) the yttrium addition influences the degree of coupling between the adjacent S layers (and thereby, the pinning strength) through the modification of the carrier concentration and of the effective thickness of the N layers, and (b) the intergrowth of Bi2Sr2CaCu2O delta (2212) layers within the 2223 grains has an important influence on the thickness of N. (C) 2011 Elsevier B.V. All rights reserved.

2678. Grain growth, microstructure and surface modification of textured CeO2 thin films on Ni substrate

Authors: Mihalache, V; Pasuk, I

Published: JUL 2011, ACTA MATERIALIA, 59, 4885, DOI: 10.1016/j.actamat.2011.04.029

CeO2 films were prepared from solutions of different concentrations (0.05-1.0 M) on textured Ni substrates. Homogeneous nucleation and growth of CeO2 nanocrystals <8 nm occurs upon calcination at 350-500 degrees C. At the heating and sintering stage, the homogeneous growth is inhibited in favor of the development of grains with (0 0 1) texture. The grain growth normal to the film surface is well described by a stretched exponential function with a relaxation time of up to 60 min and with Kohlrausch exponent values of less than unity. The increase in grain size is accompanied by the relaxation of the microstrain. During the relaxation time, the grain coarsening is controlled by surface diffusion characterized by an activation energy as low as 0.6 eV. At the relaxation time, the surface morphology is strongly concentration dependent. The surface morphology changes from separated (agglomerated) grains to a continuous grain configuration as the concentration increases from 0.05 M to 0.8 M. After the relaxation time, both the grain size normal to the film surface and the lateral grain size continue to grow, and the grain configuration continues to change. These processes are concentration dependent. In the films with a nominal thickness >170 nm (>0.8 M), the transition to classical curvature-driven grain-growth kinetics is evident (at 1000 degrees C, below the literature value of 1100 degrees C). The decrease in the Kohlrausch exponent for these thick films suggests that the grain coarsening through grain boundary migration is responsible for the stretched regime of grain-size relaxation. (C) 2011 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

2679. Nanoparticles size effects in thermoluminescence of oxyfluoride glass-ceramics containing Sm3+-doped CaF2 nanocrystals

Authors: Secu, M

Published: JUL 2011, JOURNAL OF NANOPARTICLE RESEARCH, 13, 2732, DOI: 10.1007/s11051-010-0181-2

Oxyfluoride glass-ceramic in the system SiO2-Al2O3-CaF2-SmF3 containing Sm3+-doped CaF2 nanocrystals in the range from 15 to 150 nm size were produced by using the controlled ceramization of the precursor glass. The incorporation of the Sm3+-dopant ion in the glass ceramic creates new electron-trapping centers and thermoluminescence (TL) method has been used in order to trace their evolution during glass ceramization. The 370 A degrees C TL peak observed in precursor glass has been assigned to the recombination of the electrons released from the Sm2+-traps in the amorphous glass network. In the glass-ceramic sample containing nanocrystals with about 15 nm size the new weak TL peaks at 270, 290, and 310 A degrees C were attributed to the recombination of the electrons released from the Sm2+-traps located mainly at the surface of the CaF2 nanocrystals. In the glass-ceramic sample containing nanocrystals with about 150 nm size, the new TL peaks at 232, 270, and 302 A degrees C size have been assigned to the recombination of the electrons released from the Sm2+-traps located inside the CaF2 nanocrystals.

2680. Non-linear and resonance effects in carbon nanotube structures

Authors: Lefrant, S; Buisson, JP; Mevellec, JY; Baibarac, M; Baltog, I

Published: JUL 2011, OPTICAL MATERIALS, 33, 1414, DOI: 10.1016/j.optmat.2011.03.014

In this paper, we report on Raman scattering and Surface Enhanced Raman Scattering (SERS) studies of single walled carbon nanotubes (SWNTs) and carbon nanotube/conjugated polymers composites. We demonstrate that under SERS conditions we induce an abnormal anti-Stokes Raman emission, that can be interpreted as being due to a "single-beam pumped" Coherent Anti-Stokes Raman Scattering (CARS) effect. We also investigate in detail the anti-Stokes/Stokes (aS/S) intensity ratios of the radial breathing modes (RBMs) of SWNTs as a function of several parameters. From calculations, we show that resonance phenomena mostly explain the aS/S intensity ratio anomalies, but only at low frequencies. In addition, we describe results obtained with polymers like poly(bithiophene) (PBTh) polymerized on carbon nanotube thin films which exhibit also an amplification of its high frequency Raman modes in the anti-Stokes branch, generated by the plasmon excitation of metallic tubes. This phenomenon occurs in several other materials such as composites based on SWNTs and conjugated polymers such as poly(3,4-ethylenedioxythiophene) (PEDOT) and polyparaphenylene-vinylene (PPV) for modes located around 1500 cm(-1). (C) 2011 Elsevier B.V. All rights reserved.