National Institute Of Materials Physics - Romania

Publications

5714 articles found

3061. Structural investigations of sol-gel derived silicate gels using Eu3+ ion-probe luminescence

Authors: Secu, CE; Predoi, D; Secu, M; Cernea, M; Aldica, G

Published: SEP 2009, OPTICAL MATERIALS, 31, 1748, DOI: 10.1016/j.optmat.2009.05.009

Undoped and Eu3+-doped CaF2-SiO2 gels were prepared by the sol-gel method and their optical properties have been studied. The UV-VIS-NIR absorption and photoluminescence spectra have shown the bands typical for the Eu3+ ions transitions. When the Eu-doped gel is annealed at temperatures up to 800 degrees C (i.e. above the CaF2 crystallisation peak at similar to 460 degrees C) the photoluminescence spectra intensity increase, the 590 nm (D-5(0) -> F-7(1)) and 620 nm (D-5(0) -> F-7(2)) luminescence bands become comparable and a structuring of the 620 nm band is observed. The phonon sidebands peaks associated with the F-5(0) -> D-7(2) transition of the Eu3+ ion were observed at around 1000 and 620 cm(-1) and have been assigned to the Si-O and Ca-O bonds, respectively. A phonon sideband signal in the range of 3007400 cm(-1) was attributed to Ca-F bonds in the precipitated CaF2 phase. From the optical absorption, photoluminescence and phonon sidebands spectra we have concluded that in the gels annealed at 800 degrees C, the Eu3+ ions are incorporated into the silica network and in the precipitated CaF2 phase. (c) 2009 Elsevier B.V. All rights reserved.

3062. Chalcogenide photonic structures

Authors: Popescu, M; Savastru, D; Popescu, A; Miclos, S; Lorinczi, A; Sava, F; Velea, A; Baschir, L; Ciobanu, M; Matei, E; Socol, G; Mihailescu, IN; Niciu, H

Published: SEP 2009, OPTOELECTRONICS AND ADVANCED MATERIALS-RAPID COMMUNICATIONS, 3, 859, DOI:

Chalcogenide glasses can be tailored for getting special configurations with photonic crystal properties A review on the advances in chalcogenide glass photonics is given Several methods to prepare two-dimensional photonic As-S glass and three-dimensional packing have been developed, and the photonic structures have been characterized Numerical simulations have also been performed The chalcogenide photonic structures fabricated by micro-technological procedures are described and tuned to work in the far infrared region of the electromagnetic spectrum

3063. Effect of annealing upon the structure and adhesion properties of sputtered bio-glass/titanium coatings

Authors: Stan, GE; Morosanu, CO; Marcov, DA; Pasuk, I; Miculescu, F; Reumont, G

Published: AUG 30 2009, APPLIED SURFACE SCIENCE, 255, 9138, DOI: 10.1016/j.apsusc.2009.06.117

Bio-glass films were deposited by radio-frequency magnetron sputtering technique onto medical grade Ti6Al7Nb alloy substrates from prepared silica based bio-glass target. A low deposition temperature was used (150 degrees C) and three different working pressures, followed by annealing in air at 550 and 750 degrees C. A quasi-stoichiometric target to substrate atomic transfer was found for Si, Ca and P, along with strong enrichment in Na and depletion in K and Mg, as evidenced by the energy dispersive microanalysis. The best results, taking into account stoichiometry and surface roughness, were obtained for the BG layers deposited at 0.3 Pa argon working pressure. The infrared spectroscopy of the as-sputtered and of the annealed films evidenced the characteristic molecular vibrations of silicate, phosphate and carbonate functional groups. The as-deposited films are amorphous and became partly crystalline after annealing at 750 degrees C, as evidenced by X-ray diffraction. The pull-out measurements, performed with a certified pull-test machine, gave very strong film-substrate adhesion strength values. For the non-crystalline layers, the pull-out strength is higher than 85 MPa, and decreases after annealing at 750 degrees C to 72.9 +/- 7.1 MPa. The main objective of this work was to establish the influence of the working pressure upon the composition and morphology of the as-deposited films, and of the annealing temperature upon structure and film substrate adhesion. (C) 2009 Elsevier B. V. All rights reserved.

3064. RPA APPROACH TO NON-LINEAR TRANSPORT IN QUANTUM DOTS

Authors: Tanatar, B; Moldoveanu, V

Published: AUG 20 2009, INTERNATIONAL JOURNAL OF MODERN PHYSICS B, 23, 4421, DOI: 10.1142/S0217979209063560

An accurate theoretical treatment of electron-electron interactions in mesoscopic systems is available in very few cases and approximation schemes are developed in most of the applications, especially for many-level quantum dots. Here we present transport calculations within the random-phase approximation for the Coulomb interaction using the Keldysh Green's functions formalism. We describe the quantum dot systems by a tight-binding Hamiltonian. Our method is similar to the one used by Faleev and Stockman [Phys. Rev. B 66 085318 (2002)] in their study of the equilibrium properties of a homogeneous 2D electron gas. The important extension at the formal level is that we combine the RPA and the Keldysh formalism for studying non-linear transport properties of open quantum dots. Within the Keldysh formalism the polarization operator becomes a contour-ordered quantity that should be computed either from the non-interacting Green functions of the coupled quantum dot (the so-called G(0)W approximation) either self-consistently (GW approximation). We performed both non-selfconsistent and self-consistent calculations and compare the results. In particular we recover the Coulomb diamonds for interacting quantum dots and we discuss the charge sensing effects in parallel quantum dots.

3065. Synthesis and sensing properties of zirconium-doped hematite nanoparticles

Authors: Sorescu, M; Diamandescu, L; Tomescu, A; Krupa, S

Published: AUG 1 2009, PHYSICA B-CONDENSED MATTER, 404, 2165, DOI: 10.1016/j.physb.2009.04.006

Zirconium-doped hematite particles of the type xZrO(2)-(1-x)alpha-Fe(2)O(3) (x = 0.1, 0.5) were synthesized using mechanochemical activation and characterized by X-ray diffraction (XRD) and Mossbauer spectroscopy. For x = 0.1 all zirconia was dissolved in the hematite lattice after 12 h of ball milling and a particle size of 9 nm was obtained. We obtained the recoilless fraction as function of the ball milling time for each value of the molar concentration x. The appearance of nanoparticles in the system was demonstrated based on these plots. We further correlated the structural properties of the zirconium-doped hematite system with the sensing properties of the best candidate in the series. These were measured as function of temperature, gas concentration (carbon monoxide and methane) and variable humidity of air. The material system was found to be sensitive over the entire range of CO concentrations and the linearity of the sensor signal was not affected by the relative humidity of air, qualities which make it the ideal system for gas sensing. (C) 2009 Elsevier B.V. All rights reserved.

3066. Adherent functional graded hydroxylapatite coatings produced by sputtering deposition techniques

Authors: Stan, GE

Published: AUG 2009, JOURNAL OF OPTOELECTRONICS AND ADVANCED MATERIALS, 11, 1138, DOI:

The aim of this paper was to study the influence of crystallization heat treatments up to 750 degrees C, upon the integrity and structure of hydroxylapatite thin films. Two different types of hydroxylapatite coatings were investigated: abrupt structures prepared by magnetron sputtering and respectively functional graded structures deposited by a co-sputtering technique. A clear dependence was noticed between adherence values and annealing temperature and was correlated with the SEM, XRD and FTIR results. X-ray diffraction and FTIR measurements showed that the hydroxylapatite coatings annealed at 400 degrees C were less crystalline, similar to biological apatites. Over 550 degrees C, the apatite coatings obtained were well crystallized and preferentially oriented on (002) HA crystal plane. The adherence of HA coating increased with annealing the temperature up to 550 degrees C due to diffusion phenomena. At 550 degrees C the adhesion test results indicated better adhesion values in case of functional graded structures (> 85 MPa) in comparison with abrupt ones (similar to 70 MPa). At 750 degrees C an adherence decreasing occurred for both types of structures. This is mainly due to the effect of a significant thermal stress owed to the mismatch of the thermal expansion coefficients between the crystallized HA and Ti substrate. The presence of rutile lines in XRD spectrum suggests an increasing of the diffusion phenomena and a strong substrate oxidation at the HAM interface with annealing temperature.

3067. Direct synthesis of Y doped BaTiO3 powder by sol-precipitation method

Authors: Cernea, M

Published: AUG 2009, JOURNAL OF OPTOELECTRONICS AND ADVANCED MATERIALS, 11, 1195, DOI:

Ultra fine and homogeneous BaTiO3 doped with 0.5 at% yttrium powder was prepared by sol-precipitation method using barium acetate, titanium (IV) diisopropoxide bis-acetylacetonate and yttrium (III) butoxide as starting precursors. The effect of pH value on the precipitate formation from the sot is pointed out in this paper. The grain size of the precipitate was about 100-200 nm. The structural evolution of the precursor precipitate as function of temperature was investigated. Ba0.995Y0.005TiO3 single phase was obtained at 1300 degrees C. The ceramics manufactured from the as-prepared powder presented good dielectric characteristics (dielectric constant = 2152 and dielectric loss (tan delta) = 4.54% at the Curie temperature of 125 degrees C). These ceramics are expected to be useful in dielectric applications.

3068. Optical Advanced Spectroscopic Techniques for the Study of Nano-Structured Materials Such as Carbon Nanotubes

Authors: Lefrant, S; Baltog, I; Baibarac, M

Published: AUG 2009, JOURNAL OF NANOELECTRONICS AND OPTOELECTRONICS, 4, 206, DOI: 10.1166/jno.2009.1022

Optical techniques allowing observations and spectroscopic investigations of materials at a nanoscale have been recently developed and applied to carbon nanotubes. They are briefly presented, including Scanning near-field optical microscopy, together with the Surface Enhanced or Tip Enhanced Raman Scattering. Even if new physical properties are difficult to extract from observed data, we show that nanostructures can generate non linear phenomena that can be exploited for studying nano-materials and in particular carbon nanotubes. By measuring the anti-Stokes/Stokes ratios of the Raman bands which present strong anomalies with respect to the prediction of the Maxwell-Boltzman law, one can distinguish between semi-conducting and metallic tubes on one hand and between isolated and bundled nanotubes on the other hand.

3069. LITHIUM FERRITES OBTAINED BY THERMAL DECOMPOSITION OF UREATE COMPLEX COMPOUNDS

Authors: Gingasu, D; Mindru, I; Patron, L; Diamandescu, L; Cizmas, CB; Firastrau, I

Published: AUG 2009, REVUE ROUMAINE DE CHIMIE, 54, +, DOI:

Lithium ferrites have been obtained by combustion reaction of ureate precursors. Various physico-chemical techniques: IR spectra, XRD, magnetic determinations, Mossbauer spectra have been employed to characterize the precursors and end products. The single spinel phase alpha-Li(0.5)Fe(.2.5)O(4) with saturation magnetization of similar to 56 Am(2)/kg were achieved from ureate precursor with ratio 2.5Fe(3+) : 0.5Li(+) : 7urea after a thermal treatment at 800 degrees C/1h.

3070. Amorphous research in Romania: Past, present and latest results

Authors: Popescu, M; Croitoru, N

Published: AUG 2009, PHYSICA STATUS SOLIDI B-BASIC SOLID STATE PHYSICS, 246, 1793, DOI: 10.1002/pssb.200982006

In the early days of amorphous research, Romania was deeply involved in the development of the field of amorphous semiconductors within a close collaboration with Boris Kolomiets. The first researches in this field are discussed and further rapid developments up to the present are shown. The latest results in the field of amorphous semiconductor:, with special emphasis on chalcogenide glasses are presented. (C) 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim