National Institute Of Materials Physics - Romania

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5781 articles found

2881. Multifrequency ESR Characterization of Paramagnetic Point Defects in Semiconducting Cubic BN Crystals

Authors: Nistor, SV; Stefan, M; Ghica, D; Goovaerts, E

Published: OCT 2010, APPLIED MAGNETIC RESONANCE, 39, 101, DOI: 10.1007/s00723-010-0136-x

Low-frequency (X-band) electron spin resonance (ESR) investigations on commercially available large-grained cubic boron nitride (cBN) superabrasive powders of various coloration, combined with high-frequency (W-band) ESR measurements on oriented submillimeter-size single crystallites selected from the same powder samples, resulted in a clear identification of several types of paramagnetic point defects. The resulting spin Hamiltonian parameters describing the ESR spectra observed in the 3-293 K temperature range and the photosensitivity of the paramagnetic defects observed in amber-colored cBN samples are reported. It is shown that the nature of the paramagnetic centers depends on the color of the investigated samples and that, in many cases, uncontrolled impurities seem to be involved in their structure.

2882. Preparation and patterning of nanoscale hybrid materials for micro-optics

Authors: Obreja, P; Cristea, D; Teodorescu, VS; Dinescu, A; Obreja, AC; Comanescu, F; Rebigan, R

Published: OCT 2010, JOURNAL OF OPTOELECTRONICS AND ADVANCED MATERIALS, 12, 2013, DOI:

The refractive index of organic polymers can be modified by incorporation of inorganic nanoparticles with higher or lower refractive index. The paper presents our experimental results regarding the preparation of hybrid nanocomposite films with controlled refractive index for micro-optics and integrated optics. These materials were obtained by chemical routes and sol-gel processes from commercial polymers (PMMA, PVA), metal (Ag), metal oxide nanoparticles (TiO2, ZrO2) or organic materials (Rhodamine, Alq3). The properties of these materials depend on the composition, metal/oxide concentration, particle size and dispersion homogeneity. The hybrid nanocomposite films were characterized using spectroellipsometry, UV-VIS spectroscopy, AFM, TEM and the optical properties were correlated with processing parameters. Micro/nano pattering techniques based on combination of electron beam lithography and replication techniques have been developed to obtain structures with applications in micro-optics. Components like micro-lenses, diffractive optical elements and optical waveguides have been obtained and characterized using SEM and AFM.

2883. Synthesis of Eu-doped bismuth germanate nano-ceramic powder

Authors: Polosan, S; Matei, E; Logofatu, C

Published: OCT 2010, OPTOELECTRONICS AND ADVANCED MATERIALS-RAPID COMMUNICATIONS, 4, 1508, DOI:

Europium doped Bi(4)Ge(3)O(12) nanocrystals were successfully obtained by solid state reaction between precursor oxides at 800 degrees C. The XRD and XPS measurements shows an increasing of the lattice constant in the case of Eu:BGO nanocrystals, due to the electropositive character of Eu(III) ions compared with the host cations, which induces an ionic character for Eu-O bound and a more covalent one for the Ge-O bounds. The photoluminescence of Eu:BGO due to (5)D(0) -> (7)F(2) transition have a lower splitting compared with Eu:BGO monocrystals suggesting a lower crystal field in the sintered nanocrystals, due to a mixing between the crystalline and amorphous phases.

2884. New analytical approximation of diffraction size broadened peak profile for spherical crystallites with a lognormal distribution

Authors: Popa, NC; Teodorescu, CM; Frunza, S

Published: OCT 2010, JOURNAL OF APPLIED CRYSTALLOGRAPHY, 43, 1030, DOI: 10.1107/S0021889810024829

A new analytical approximation of the diffraction peak size profile due to the lognormal distribution of spherical crystallites is derived. In comparison with the existing approximation constructed with the condition of conserving the volume-averaged column length of the exact profile [Popa & Balzar (2002). J. Appl. Cryst. 35, 338-346], the new approximation conserves both volume- and area-averaged column lengths. Moreover, it is a better approximation for large distribution dispersions. Like the old approximation, the present one is appropriate for implementation in the whole powder pattern fitting codes, Rietveld included.

2885. Bioactive glass thin films deposited by magnetron sputtering technique: The role of working pressure

Authors: Stan, GE; Marcov, DA; Pasuk, I; Miculescu, F; Pina, S; Tulyaganov, DU; Ferreira, JMF

Published: SEP 15 2010, APPLIED SURFACE SCIENCE, 256, 7110, DOI: 10.1016/j.apsusc.2010.05.035

Bioglass coatings were prepared by radio frequency magnetron sputtering deposition at low temperature (150 degrees C) onto silicon substrates. The influence of argon pressure values used during deposition (0.2 Pa, 0.3 Pa and 0.4 Pa) on the short-range structure and biomineralization potential of the bioglass coatings was studied. The biomineralization capability was evaluated after 30 days of immersion in simulated body fluid. SEM-EDS, XRD and FTIR measurements were performed. The tests clearly showed strong biomineralization features for the bioglass films. The thickness of the chemically grown hydroxyapatite layers was more than twice greater for the BG films deposited at the highest working pressure, in comparison to those grown on the films obtained at lower working pressures. The paper attempts to explain this experimental fact based on structural and compositional considerations. (C) 2010 Elsevier B.V. All rights reserved.

2886. Structural and photoluminescence characterization of mesoporous silicon-phosphates

Authors: Tiseanu, C; Parvulescu, V; Parvulescu, V; Cotoi, E; Gessner, A; Kumke, M; Simon, S; Vasiliu, F

Published: SEP 5 2010, JOURNAL OF PHOTOCHEMISTRY AND PHOTOBIOLOGY A-CHEMISTRY, 215, 24, DOI: 10.1016/j.jphotochem.2010.07.015

Two different types of mesoporous silicon-phosphate supports using different surfactants (a mixture of (CH3)(3)C13H27NBr with an organophosphorus coupling molecule (HO-PO(i-C3H7)(2)) and with a co-surfactant ((C2H5)(3)(C6H5)PCl), respectively) were synthesized. Trivalent europium (Eu) ions were immobilized via ion-exchange on these supports. The resulting materials were characterized using nitrogen adsorption isotherms at -196 degrees C, thermogravimetric analysis, SEM, TEM, FT-IR, PXRD, CP/MAS. (HSi)-H-1-Si-29 and P-31 NMR, DR-UV-vis as well as steady-state and time-resolved photoluminescence spectroscopy. The results evidenced that the co-polymerization of silicon and phosphorous yielded a unique morphology in these materials. Following calcination at 450 and 900 degrees C europium-exchanged silicon-phosphates with great surface area (BET=600-705 m(2) g(-1)) and 3.4 nm sized mesopores were obtained. The differences among the optical properties of the non-calcined europium materials such as the emission lifetimes, local environment at the europium sites or the relative contribution of the upper excited levels to the total photoluminescence were assigned to the surfactants used in the synthesis. Calcination of the silicon-phosphates at higher temperatures than 450 degrees C did not induce major changes in the structural properties: in contrast, photoluminescence properties of europium were markedly improved in terms of intensity and average lifetime. (C) 2010 Elsevier B.V. All rights reserved.

2887. Optical and structural studies on Ba(Mg1/3Ta2/3)O-3 thin films obtained by radiofrequency assisted pulsed plasma deposition

Authors: Scarisoreanu, ND; Galca, AC; Nedelcu, L; Ioachim, A; Toacsan, MI; Morintale, E; Stoica, SD; Dinescu, M

Published: SEP 1 2010, APPLIED SURFACE SCIENCE, 256, 6530, DOI: 10.1016/j.apsusc.2010.04.041

Single-phase Ba(Mg1/3Ta2/3)O-3 thin films were prepared by radiofrequency plasma beam assisted pulsed laser deposition (RF-PLD) starting from a bulk ceramic target synthesized by solid state reaction. Atomic force microscopy, X-ray diffraction and spectroscopic ellipsometry were used for morphological, structural and optical characterization of the BMT thin films. The X-ray diffraction spectra show that the films exhibit a polycrystalline cubic structure. From spectroscopic ellipsometry analysis, the refractive index varies with the thin films deposition parameters. By using the transmission spectra and assuming a direct band to band transition a band gap value of approximate to 4.72 eV has been obtained. (C) 2010 Elsevier B.V. All rights reserved.

2888. Morphological and optical properties of doped potassium hydrogen phthalate crystals

Authors: Enculescu, M

Published: SEP 1 2010, PHYSICA B-CONDENSED MATTER, 405, 3727, DOI: 10.1016/j.physb.2010.05.074

Potassium hydrogen phthalate (KAP) crystals doped with rhodamine 6G (Rh 6G) and polyvinylpyrrolidone (PVP) were grown by solution evaporation technique. Nucleation occurred without the use of seeds and optically transparent crystals were obtained. The grown crystals were characterized by XRD measurements performed on crystals and their powders, optical transmission and photoluminescence measurements. The influence of the dopants on the structural, morphological and optical properties of the KAP crystals was analysed. Dopants do not change the structure of the single crystals while the addition of PVP changes the morphology of crystals from pseudo-hexagonal to rhomb. Three new XRD reflections are observed in all single-crystal and powder XRD spectra and are probably (0 3 0), (0 4 0) and (0 5 0) lines. The UV cut-off and transparency of the crystals are not changed by doping. Dye-doped KAP crystals exhibit a strong emission band centred at 550 nm excited with 480 nm wavelength. For the dye-doped crystals the up-conversion was investigated and its second harmonic origins are proved using photoluminescence measurements. (C) 2010 Elsevier B.V. All rights reserved.

2889. A new ZnO photoanode for dye-sensitized solar cell

Authors: Sima, M; Secu, M; Sima, M; Vasile, E

Published: SEP 2010, OPTOELECTRONICS AND ADVANCED MATERIALS-RAPID COMMUNICATIONS, 4, 1328, DOI:

The present work reports the preparation of nanostructured ZnO nanorods in a hydrothermal-electrochemical process and preliminary attempt to fabricate a dye-sensitized solar cell using this material as photoanode. Synthesis of ZnO nanorods in solutions with and without eosin Y dye, used as nanostructuring agent, was studied. ZnO nanorod arrays were characterized by photoluminescence spectroscopy, scanning electron microscopy and X-ray diffraction analysis. Fabricated dye-sensitized solar cell comprised a photoanode prepared with nanostructured ZnO nanorods and eosin Y dye, a platinum counter electrode and an electrolyte consisting of 0.5M KI + 0.03M I-2 in acetonitrile/ethylene carbonate (v/v=1/4). Performance parameters of dye-sensitized solar cell were extracted from standard current-voltage characteristic.

2890. Flux jumps anomalous behaviour in FAST-processed MgB2 composites

Authors: Aldica, G; Badica, P; Plapcianu, C; Groza, JR; Popa, S

Published: SEP 2010, JOURNAL OF OPTOELECTRONICS AND ADVANCED MATERIALS, 12, 2003, DOI:

Superconductor MgB2 has a significant potential for practical applications, but undesirable flux jumps in magnetization curves at low temperature and at moderate or intermediate magnetic fields prevent its widespread use. In the present work, we analyzed the flux-jumps tendency in MgB2 bulk doped with SiC, B4C and metallic Mg. Samples were prepared by field-assisted-sintering (FAST) as composites with up to three intercalated Fe-foils. Samples produced by FAST showed quite different phase structures clearly influencing pinning and thermo-magnetic instability features due to inhomogeneous materials structure or due to dopants. The largest flux jumps are observed in samples with high critical current density, as usually observed in conventional MgB2 bulks. However, jumps patterns are different and do not perfectly follow the jumps critical current dependence, especially for the samples with Mg-addition.