National Institute Of Materials Physics - Romania

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5714 articles found

2671. Hybrid organic-inorganic materials based on poly(o-phenylenediamine) and polyoxometallate functionalized carbon nanotubes

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

Published: JAN 31 2011, JOURNAL OF MOLECULAR STRUCTURE, 985, 218, DOI: 10.1016/j.molstruc.2010.10.044

The chemical polymerization of o-phenylenediamine (OPD) on single-walled carbon nanotubes (SWCNTs) in the presence of phosphomolybdic acid (H3PMo12O40 xH(2)O) has been studied by surface enhanced resonant Raman scattering (SERRS) spectroscopy. One demonstrates that an organic-inorganic hybrid composite of the type poly(o-phenylenediamine)/polyoxometallate-functionalized SWCNTs is produced by the chemical interaction between polyoxometallate-functionalized SWCNTs and poly(o-phenylenediamine) (POPD) doped with [H2PMo12O40](-) ions. According to TEM investigations, a result of the chemical interaction of SWCNT with H3PMo12O40 xH(2)O is the formation into the composite mass of tube fragments of shorter length, which behave like closed shell fullerenes since Raman fingerprint is given by lines situated at 240-275 and 1450-1472 cm(-1). The chemical polymerization of OPD on SWCNTs achieved in the absence of H3PMo12O40 xH(2)O leads to a covalent functionalization of the wall side of the tubes, which is revealed in Raman spectra, recorded at the excitation wavelength of 514 nm, by an enhancement of the lines associated with the tangential vibrational modes of SWCNTs. Using FTIR spectroscopy, significant hindrance steric effects are evidenced in the POPD/polyoxometallate-functionalized SWCNT composite. (c) 2010 Elsevier B.V. All rights reserved.

2672. Substitutional and surface Mn2+ centers in cubic ZnS:Mn nanocrystals. A correlated EPR and photoluminescence study

Authors: Stefan, M; Nistor, SV; Ghica, D; Mateescu, CD; Nikl, M; Kucerkova, R

Published: JAN 20 2011, PHYSICAL REVIEW B, 83, DOI: 10.1103/PhysRevB.83.045301

The EPR, radioluminescence, and photoluminescence of cubic ZnS (cZnS) nanocrystals (NCs) with a narrow size distribution centered at 2 nm, doped with 0.1, 0.2, and 0.5 at.% Mn2+ ions were investigated. Besides the main lines from substitutional Mn2+ ions localized in the core of the NCs next to a stacking defect, the EPR spectra exhibited two broader hyperfine sextets, attributed to the so-called Mn(II)and Mn(III) surface centers, which could be separated by adequate thermal treatments. The contribution to the photoluminescence from the Mn2+ ions at various sites was further determined from the analysis of the steady-state and time-resolved photoluminescence data from cZnS: Mn NCs subjected to thermal treatments and from cZnS: Mn single crystals. Thus, the main emission consisting of two intense overlapping bands peaking at 596 and 630 nm was attributed to the T-4(1)-(6)A(1) transition of the substitutional Mn2+ ions in the core of the cZnS nanocrystals and to residual aggregated Mn2+ ions, respectively, the last ones being responsible for a broad EPR line observed in the X-band spectrum. The Mn(II) and Mn(III) centers, consisting of Mn2+ ions in the oxidized and hydrolyzed surface layer of the NCs, respectively, are only indirectly involved in the energy transfer to the substitutional Mn2+ centers, very likely through pairs interaction.

2673. Microwave properties of Ba(Zn1/3Ta2/3)O-3 dielectric resonators

Authors: Nedelcu, L; Toacsan, MI; Banciu, MG; Ioachim, A

Published: JAN 12 2011, JOURNAL OF ALLOYS AND COMPOUNDS, 509, 481, DOI: 10.1016/j.jallcom.2010.09.069

Ba(Zn1/3Ta2/3)O-3 (BZT) dielectric resonators were prepared by solid-state reaction. The starting materials were BaCO3, ZnO, and Ta2O5 powders with high purity. The double calcined BZT pellets were sintered in air at temperatures of 1575, 1600, 1625, and 1650 degrees C for 4 h. The X-ray diffraction data allowed the study of the unit cell distortion degree and the presence of the secondary phases. A long-range order with a 2:1 ratio of Ta and Zn cations on the octahedral positions of the perovskite structure was observed with the increase of the sintering temperature. The dielectric constant of BZT resonators measured around 6 GHz was between 26 and 28. High values of Q x f product (120 THz) were obtained for BZT resonators sintered at 1650 degrees C/4 h. The temperature coefficient of the resonance frequency exhibits positive values less than 6 ppm/degrees C. The achieved dielectric parameters recommend BZT dielectric resonators for microwave and millimeter wave applications. (C) 2010 Elsevier B.V. All rights reserved.

2674. Green light radiation effects on free radicals inhibition in cellular and chemical systems

Authors: Comorosan, S; Polosan, S; Jipa, S; Popescu, I; Marton, G; Ionescu, E; Cristache, L; Badila, D; Mitrica, R

Published: JAN 10 2011, JOURNAL OF PHOTOCHEMISTRY AND PHOTOBIOLOGY B-BIOLOGY, 102, 44, DOI: 10.1016/j.jphotobiol.2010.09.003

Free radicals generation is inhibited through green light (GL) irradiation in cellular systems and in chemical reactions. Standard melanocyte cultures were UV-irradiated and the induced cellular reactive oxygen species (ROS) were quantified by the fluorescence technique. The same cell cultures, previously protected by a 24 h GL exposure, displayed a significantly lower ROS production. A simple chemical reaction is subsequently chosen, in which the production of free radicals is well defined. Paraffin wax and mineral oil were GL irradiated during thermal degradation and the oxidation products checked by chemiluminescence [CL] and Fourier transform infrared spectra [FT-IR]. The same clear inhibition of the radical oxidation of alkanes is recorded. A quantum chemistry modeling of these results is performed and a mechanism involving a new type of Rydberg macromolecular systems with implications for biology and medicine is suggested. (C) 2010 Elsevier B.V. All rights reserved.

2675. The effects of thermal treatment on the physical properties of Sr2FeMo1-xMxO6 perovskite with M = W, Ta and x <= 0.3

Authors: Burzo, E; Balasz, I; Valeanu, M; Pop, IG

Published: JAN 5 2011, JOURNAL OF ALLOYS AND COMPOUNDS, 509, 113, DOI: 10.1016/j.jallcom.2010.08.126

The Sr2FeMo1-xMxO6 double perovskites with M = W and Ta and x <= 0.3, were obtained by sintering at 1300 degrees C. during 4 and 8 h, respectively. The perovskites crystallize in a tetragonal structure having 14/mmm space group. The grains are more homogeneous when sintering time is increased. The samples sintered longer time show higher values of saturation magnetizations, resistivities and magnetoresistivities than the samples sintered during 4 h. The intergrain tunnelling magnetoresistance as well as the intragrain contributions, respectively were analysed as function of temperature and external field. The changes in the physical properties, when the sintering time is increased, have been correlated with the number of antisite defects as well as the nature of grain boundaries. (C) 2010 Elsevier B.V. All rights reserved.

2676. Stress-induced traps in multilayered structures

Authors: Ciurea, ML; Lazanu, S; Stavarache, I; Lepadatu, AM; Iancu, V; Mitroi, MR; Nigmatullin, RR; Baleanu, CM

Published: JAN 1 2011, JOURNAL OF APPLIED PHYSICS, 109, DOI: 10.1063/1.3525582

The trap parameters of defects in Si/CaF2 multilayered structures were determined from the analysis of optical charging spectroscopy measurements. Two kinds of maxima were observed. Some of them were rather broad, corresponding to "normal" traps, while the others, very sharp, were attributed to stress-induced traps. A procedure of optimal linear smoothing the noisy experimental data has been developed and applied. This procedure is based on finding the minimal value of the relative error with respect to the value of the smoothing window. In order to obtain a better accuracy for the description of the trapping-detrapping process, a Gaussian temperature dependence of the capture cross-sections characterizing the stress-induced traps was introduced. Both the normal and the stress-induced traps have been characterized, including some previously considered as only noise features. (C) 2011 American Institute of Physics. [doi: 10.1063/1.3525582]

2677. Growth and type-I non critical phase matching second harmonic generation of Y1-xRxCa4O(BO3)(3) (R = Sc or Lu) crystals

Authors: Gheorghe, L; Achim, A; Voicu, F; Aka, G

Published: JAN 2011, OPTOELECTRONICS AND ADVANCED MATERIALS-RAPID COMMUNICATIONS, 5, 105, DOI:

Single crystals of Y1-xRxCa4O(BO3)(3) (R = Sc or Lu) of good quality have been grown from the melt by the Czochralski pulling method. The chemical compositions of the grown crystals were determined. Typed non critical phase matching (NCPM) wavelengths for second harmonic generation (SHG) in Y1-xRxCa4O(BO3)(3) grown crystals were also determined.

2678. Improved Critical Current Densities in YBa2Cu3O7-delta Multilayer Films Interspaced with Palladium Nano-dots

Authors: Sarkar, A; Mikheenko, P; Dang, VS; Kechik, MMA; Abell, JS; Crisan, A

Published: JAN 2011, JOURNAL OF SUPERCONDUCTIVITY AND NOVEL MAGNETISM, 24, 177, DOI: 10.1007/s10948-010-0914-6

Power applications of superconducting coated conductors in high magnetic fields require thick films with high critical current density J (c) and strong artificial pinning centers. Here, we report on the artificial pinning centers induced in YBCO quasi-multilayer films interspaced with palladium (Pd) nano-dots. Quasi-multilayered (QM) YBa2Cu3O7-delta (YBCO) films composed of YBCO layers interspaced with quasi-layers of palladium nano-dots were grown by pulsed laser deposition on SrTiO3(100) substrates. DC magnetization and frequency-dependent measurements showed high J (c) comparable with best YBCO films in thin quasi-multilayers and significant improvement of J (c) in thick quasi-multilayers. TEM study shows regions of planar defects, stacking faults, and pore formations suitable for immobilizing vortices. These defects significantly contribute to the pinning of magnetic flux and increase critical current in the films.

2679. Structural investigations of Ge nanoparticles embedded in an amorphous SiO2 matrix

Authors: Stavarache, I; Lepadatu, AM; Gheorghe, NG; Costescu, RM; Stan, GE; Marcov, D; Slav, A; Iordache, G; Stoica, TF; Iancu, V; Teodorescu, VS; Teodorescu, CM; Ciurea, ML

Published: JAN 2011, JOURNAL OF NANOPARTICLE RESEARCH, 13, 232, DOI: 10.1007/s11051-010-0021-4

Transmission electron microscopy and X-ray photoelectron spectroscopy analyses are performed to investigate Ge nanoparticles embedded in an amorphous SiO2 matrix. GeSiO thin films are prepared by two methods, sol-gel and radio frequency magnetron sputtering. After the deposition, the sol-gel films are annealed in either N-2 (at 1 atm and 800 A degrees C) or H-2 (at 2 atm and 500 A degrees C), and the sputtered films in H-2 (at 2 atm and 500 A degrees C), to allow Ge segregation. Amorphous Ge-rich nanoparticles (3-7 nm size) are observed in sol-gel films. Crystalline Ge nanoparticles in the high pressure tetragonal phase (10-50 nm size) are identified in the sputtered films. The size of the nanoparticles increases with Ge concentration in the volume of the film. At the film surface, the Ge concentration is much larger that in the volume for both sol-gel and sputtered films. At the same time, at the film surface, only oxidized Ge is observed.

2680. Effect of aqueous comonomer solubility on the surfactant-free emulsion copolymerization of methyl methacrylate

Authors: Preda, N; Matei, E; Enculescu, M; Rusen, E; Mocanu, A; Marculescu, B; Enculescu, I

Published: JAN 2011, JOURNAL OF POLYMER RESEARCH, 18, 30, DOI: 10.1007/s10965-009-9387-3

Surfactant-free emulsion copolymerization was used to prepare methyl methacrylate-hydroxyethyl methacrylate (MMA-HEA) and methyl methacrylate-hydroxypropyl methacrylate (MMA-HPMA) latex particles. Also, glycidyl methacrylate (GMA) was grafted onto the surface of the preformed MMA-HPMA latex particles by seeded surfactant-free emulsion copolymerization. The copolymerization reactions were conducted at 75 A degrees C using a water-soluble initiator, potassium persulfate (KPS). The morphologies of copolymer latex particles were observed using Scanning electron microscopy (SEM). The influence of different reactions parameters (the MMA saturation concentration (Sr), the KPS concentration and the aqueous solubility of the comonomers (HEA or HPMA)) on the particles average diameter and particles size dispersity was investigated. The experimental results showed that the increase of initiator concentration induces in all investigated cases the increase of particles average diameter, while the presence of HEA or HPMA as comonomers in the copolymerization reaction of MMA (1,000% Sr) lead to a decrease of particles average diameter. At small KPS concentration the latex is monodisperse, the increase of the initiator concentration leading to the formation of polydisperse latex. In the case of grafting reaction of GMA onto the monodisperse preformed MMA-HEA latex particles, although the average diameter of the final particles doubles the latex remains quasi-monodisperse.