National Institute Of Materials Physics - Romania

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847 articles found

741. EPR characterization of Mn2+ impurity ions in PbWO4 single crystals

Authors: Nistor, SV; Stefan, M; Goovaerts, E; Nikl, M; Bohacek, P

Published: AUG-DEC 2004, RADIATION MEASUREMENTS, 38, 658, DOI: 10.1016/j.radmeas.2003.12.024

The electron paramagnetic resonance (EPR) properties of the Mn2+ ions in PbWO4 single crystals grown by the Czochralski method have been investigated in the X-band microwave frequency, at T = 20 K. The angular dependence of the EPR line positions obtained by rotating the magnetic field in the main crystallographic planes shows that the local symmetry at the Mn2+ impurity ions is tetragonal, strongly suggesting that the Mn2+ ions substitute for the Pb2+ lattice cations, without charge compensation. The resulting spin Hamiltonian parameters compare well with the corresponding values for the Mn2+ ions in other isomorphous tungstates. The observed strong angular variation of the EPR linewidth has been quantitatively described considering a random distribution of lattice strains. (C) 2003 Elsevier Ltd. All rights reserved.

742. Piezoelectric and optical properties of Sr-doped PT-PZ-Pb(Mg1/3Nb2/3)O-3 ceramics

Authors: Dimitriu, E; Ramer, R; Ghica, C; Iuga, A; Ghiordanescu, V

Published: 2004, JOURNAL OF THE EUROPEAN CERAMIC SOCIETY, 24, 1708, DOI: 10.1016/S0955-2219(03)00478-3

Ceramics in the ternary system (Pb,Sr)TiO3-PbZrO3-Pb(Mg1/3Nb2/3)O-3, in the ratio 85.976/5.817/8.207 mol%, was prepared by mixed oxide route. With about 13.98 at % Sr+2, as isovalent substitution in A-position of the lead titanate, high-density samples were obtained for the sintering temperatures of about 1220 and 1230 degreesC. Tetragonal phases were identified both by XRD method and selected area electron diffraction pattern (SAED). The lattice parameter ratio, c/a, of about 1.017 indicated a decrease of tetragonal distortion. The typical TEM micrographs showed domains of about 50 nm wide. The reflection spectra were recorded to prove the presence of the transition Ti+4 to Ti+3. The principal mechanical, electrical and electromechanical properties were discussed as a function of the sintering temperature. The variation of the relative constant with the temperature and frequency evidenced a relaxation-like behavior of this ternary system. By combining the ferroelectric and relaxor end member we tried to improve the piezoelectric properties of a ceramic with a high content of lead titanate, and to achieve a material with both piezoelectric and relaxor-like properties. (C) 2003 Elsevier Ltd. All rights reserved.

743. Microstructure of Si/Sio(2) nanocomposite films

Authors: Teodorescu, VS; Ciurea, ML; Iancu, V; Blanchin, MG

Published: 2004, 2004 INTERNATIONAL SEMICONDUCTOR CONFERENCE, VOLS 1AND 2, PROCEEDINGS, 62, DOI: 10.1109/SMICND.2004.1402802

The paper presents the inicrostructure of the Si/SiO2 nanocomposite films prepared by co-sputtering from Si and SiO2 targets. High resolution transmission electron microscopy and electron diffraction were used to reveal and compare the structures with different silicon/oxygen (SiOx, 0 < x < 2) coinposition. Si nanocrystallites have been evidenced clearly in the SiO1.6 areas. Percolation limits between the Si crystallites was observed in the x=1 areas. The morphology of the Si nanocrystallites and the amorphous SiO2 zones were compared in regions with different composition.

744. Influence of crystal growth technology on the tolerance to radiation of silicon for detectors at future accelerators

Authors: Lazanu, S; Lazanu, I; Ciurea, ML

Published: 2004, 2004 International Semiconductor Conference, Vols 1and 2, Proceedings, 422, DOI:

Silicon detectors represent an important option for the Large Hadron Collider and for its upgrades in luminosity and energy, especially for the tracking system. The main limitation in their utilisation comes from the degradation in the hostile radiation environment where they, will work for long time, without the possibility to be changed. The main goal of this paper is to study, the role of growth technology on stable defect concentrations produced by long-time continuous irradiation in the radiation field estimated for LHC and SLHC.

745. EPR study of the low temperature ferroelectic phase transition in Cu2+ doped Rb2ZnCl4 single crystals

Authors: Stefan, M; Nistor, SV; Schoemaker, D; Ursu, L

Published: SEP 2003, SOLID STATE COMMUNICATIONS, 127, 698, DOI: 10.1016/S0038-1098(03)00649-5

Temperature dependent EPR measurements on copper doped Rb2ZnCl4 single crystals allowed us to evidence and study the P2(1)cn <----> ClcI structural phase transition that takes place in this compound at 74.6 K. From the two types Of Cu2+ centers localized at different anionic sites, called Cu2+(I) and Cu2+(II), which are formed in this compound, only the Cu2+(II) centers exhibit observable changes in their EPR spectra, attributable to the symmetry lowering. The observed changes have been related to the soft-mode responsible for the structural phase transition. (C) 2003 Elsevier Ltd. All rights reserved.

746. Morphology, structure and optical properties of sol-gel ITO thin films

Authors: Stoica, TF; Teodorescu, VS; Blanchin, MG; Stoica, TA; Gartner, M; Losurdo, M; Zaharescu, M

Published: AUG 15 2003, MATERIALS SCIENCE AND ENGINEERING B-SOLID STATE MATERIALS FOR ADVANCED TECHNOLOGY, 101, 226, DOI: 10.1016/S0921-5107(02)00667-0

The alkoxidic route and the spinning deposition were used to prepare monolayer sol-gel indium tin oxide (ITO) films. The morphology and crystalline structure were investigated by cross-section transmission electron microscopy (XTEM) and atomic force microscopy (AFM). The ITO sol-gel mono-layer contains three regions of different porosities. The basic crystalline structure is that of the In2O3 lattice. The optical properties have been studied by optical transmission and spectroscopic ellipsometry. (C) 2003 Elsevier Science B.V. All rights reserved.

747. Modeling of optical charging spectroscopy investigation of trapping phenomena in nanocrystalline porous silicon

Authors: Iancu, V; Ciurea, ML; Draghici, M

Published: JUL 1 2003, JOURNAL OF APPLIED PHYSICS, 94, 223, DOI: 10.1063/1.1576301

A model for trapping phenomena in nanocrystalline silicon investigated by the optical charging spectroscopy method is proposed. The model takes into account all the possible contributions to the discharge current. The results previously obtained on fresh and passivated porous silicon samples are interpreted within the framework of the model, which provides a good fit to the experimental data. The role played by the different kinds of trapping centers (donors and acceptors, surface and bulk) and the different trap parameters is analyzed. (C) 2003 American Institute of Physics.

748. Coupled confinement effect on the photoluminescence and electrical transport in porous silicon

Authors: Ciurea, ML; Draghici, M; Iancu, V; Reshotko, M; Balberg, I

Published: MAY 2003, JOURNAL OF LUMINESCENCE, 102, 497, DOI: 10.1016/S2313(02)00589-6

In this paper we report correlations between the structure, the photoluminescence and the transport properties of luminescent porous silicon. These correlations combined with the observed temperature dependence of tunneling characteristics yield quite a wholesome (pea-pod-like) model for this system. (C) 2002 Elsevier Science B.V. All rights reserved.

749. Densification and crystallization of SnO2 : Sb sol-gel films using excimer laser annealing

Authors: Sandu, CS; Teodorescu, VS; Ghica, C; Canut, B; Blanchin, MG; Roger, JA; Brioude, A; Bret, T; Hoffmann, P; Garapon, C

Published: MAR 15 2003, APPLIED SURFACE SCIENCE, 208, 387, DOI: 10.1016/S0169-4332(02)01412-5

We have successfully applied laser annealing to sol-gel deposited SnO2:Sb thin films in order to achieve their crystallization. The as-deposited films are quasi-amorphous and electrically non-conductive. After laser annealing they crystallize and become conductive. This paper presents a comparative study of the laser annealed films and shows the influence of the irradiation parameters on the crystallization process and the electrical behavior of the films. Our results are quite promising in view of applying this kind of treatment to films deposited on thermally sensitive substrates (e.g. polymers). (C) 2002 Elsevier Science B.V. All rights reserved.

750. Calcium phosphate thin film processing by pulsed laser deposition and in situ assisted ultraviolet pulsed laser deposition

Authors: Nelea, V; Pelletier, H; Iliescu, M; Werckmann, J; Craciun, V; Mihailescu, IN; Ristoscu, C; Ghica, C

Published: DEC 2002, JOURNAL OF MATERIALS SCIENCE-MATERIALS IN MEDICINE, 13, 1173, DOI: 10.1023/A:1021150207350

Calcium orthophosphates (CaP) and hydroxyapatite (HA) were intensively studied in order to design and develop a new generation of bioactive and osteoconductive bone prostheses. The main drawback now in the CaP and HA thin films processing persists in their poor mechanical characteristics, namely hardness, tensile and cohesive strength, and adherence to the metallic substrate. We report here a critical comparison between the microstructure and mechanical properties of HA and CaP thin films grown by two methods. The films were grown by KrF* pulsed laser deposition (PLD) or KrF* pulsed laser deposition assisted by in situ ultraviolet radiation emitted by a low pressure Hg lamp (UV-assisted PLD). The PLD films were deposited at room temperature, in vacuum on Ti-5Al-2.5Fe alloy substrate previously coated with a TiN buffer layer. After deposition the films were annealed in ambient air at 500-600 C. The UV-assisted PLD films were grown in (10(-2)-10(-1) Pa) oxygen directly on Ti-5Al-2.5Fe substrates heated at 500-600 degreesC. The films grown by classical PLD are crystalline and stoichiometric. The films grown by UV-assisted PLD were crystalline and exhibit the best mechanical characteristics with values of hardness and Young modulus of 6-7 and 150-170 GPa, respectively, which are unusually high for the calcium phosphate ceramics. To the difference of PLD films, in the case of UV-assisted PLD, the GIXRD spectra show the decomposition of HA in Ca2P2O7, Ca2P2O9 and CaO. The UV lamp radiation enhanced the gas reactivity and atoms mobility during processing, increasing the tensile strength of the film, while the HA structure was destroyed. (C) 2002 Kluwer Academic Publishers.