National Institute Of Materials Physics - Romania

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5781 articles found

3691. Synthesis and characterization of spinelic ferrites obtained from coordination compounds as precursors

Authors: Culita, DC; Patron, L; Teodorescu, VS; Balint, L

Published: APR 25 2007, JOURNAL OF ALLOYS AND COMPOUNDS, 432, 216, DOI: 10.1016/j.jallcom.2006.05.104

Cobalt ferrite and magnetite nanoparticles were obtained by "the complexation method" belonging to soft chemistry, in which cholic acid sodium salt acts as complexation agent. The precursors were characterized by IR, UV-vis spectra and magnetic measurements. X-ray diffraction spectra and transmission electron inicroscopy proved the nanocrystalline structure of the spinelic ferrites. (c) 2006 Elsevier B.V. All rights reserved.

3692. First measurement of the nonlinear coefficient for Gd1-xLuxCa4O(BO3)(3) and Gd1-xScxCa4O(BO3)(3) crystals

Authors: Andersen, MT; Mortensen, JL; Germershausen, S; Tidemand-Lichtenberg, P; Buchhave, P; Gheorghe, L; Lupei, V; Loiseau, P; Aka, G

Published: APR 16 2007, OPTICS EXPRESS, 15, 4901, DOI: 10.1364/OE.15.004893

The effective nonlinear coefficient and temperature acceptance bandwidth of three Lu and Sc co-doped GdCa4O(BO3) 3 type nonlinear crystals were measured. NCPM for SHG in to the blue-UV spectral region can be obtained by controlling the co-dopant concentration. Measurements were based on intra-cavity SHG of a CW Ti: Sapphire laser, and the effective nonlinear coefficients were found to be in the range of 0.5 to 0.6 pm/V for the three crystals used. The FWHM temperature acceptance bandwidth was measured to be more than 35 degrees C using a 6 mm long Gd0.871Lu0.129Ca4O(BO3)(3) crystal. A maximum of 115 mW at 407.3 nm in a single direction was measured using a 6.5 mm long Gd0.96Sc0.04Ca4O(BO3)(3) crystal. (c) 2007 Optical Society of America.

3693. Spectral response of Au-Ti Schottky barrier on semi-insulating GaAs

Authors: Ghita, RV; Logofatu, C; Negrila, C; Cotirlan, C; Ghita, P; Manea, AS; Lazarescu, MF

Published: APR 2007, PHYSICA STATUS SOLIDI A-APPLICATIONS AND MATERIALS SCIENCE, 204, 1029, DOI: 10.1002/pssa.200674107

For the fabrication of the rectifying contact, respectively the Schottky diode structure on semi-insulating GaAs (100) oriented, Cr-doped, with a resistivity of rho similar to 10(6) - 10(7) Omega cm we have used Au-Ti contact. The Ti contact was deposited on a plasma etched GaAs surface in high vacuum (10(-8) torr), Au was deposited in medium vacuum (10(-6) torr) and the Au-Ti/GaAs interface was formed by a rapid thermal annealing procedure at T = 320 - 360 degrees C in low vacuum (10(-1) torr). The representative I-V characteristics in dark and under illumination at different power levels are presented. It is worth to mention that we estimate a barrier height of Phi(B) similar to 0.84 V (in the frame of thermionic emission model). The spectral response at two operating points and at different temperatures is presented. The signal has a maximum in the Delta lambda range (850-950) nm, which corresponds to the generation of photo-carriers due to a band-to-band direct transition. (c) 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

3694. Synthesis by oxalic (citric) route and electrical and magnetic characterization of Sr2FeMoO6 perovskite

Authors: Aldica, G; Plapdanu, C; Badica, P; Valsangiacom, C; Stoica, L

Published: APR 2007, JOURNAL OF MAGNETISM AND MAGNETIC MATERIALS, 311, 670, DOI: 10.1016/j.jmmm.2006.08.031

Sr2FeMoO6 was synthesizes by soft chemistry route using citric and/or oxalic acid as complexing agents. This method provides good compositional control with promising results as a base for further improvements. The material shows complex temperature- and field(magnitude and orientation) dependent electro-magnetic behavior as probed by magnetic and transport measurements. The zero-field cooling curves of magnetization versus temperature taken at different magnetic fields, show two temperature regions separated by a transition temperature T-cups(ZFC). When H increases, T-cups(ZFC) values decrease. At 5 K and 5 T the saturation moment is 3.55 JIB per formula unit, and the coercitive force is 0.021 T. The relative value of the magnetoresistive effect (i.e. 100 (rho(H)-rho(0))/rho(H), %, with rho being resistivity) at 5 K is 68% and 36% when applied field H is parallel and perpendicular the measuring current I, respectively. The results are discussed in relation to magnetic sublattices and the interaction that produces anti-site defects and spin-dependent electron anisotropy. (C) 2006 Elsevier B.V. All rights reserved.

3695. Novel microwave bandpass filters using defected-ground structures

Authors: Mititaru, N; Banciu, MG; Lojewski, G

Published: APR 2007, JOURNAL OF OPTOELECTRONICS AND ADVANCED MATERIALS, 9, 1064, DOI:

In this paper a study of some microwave microstrip bandpass filters (BPF) using defected ground structures (DGS) is presented. This technique allows designs of tight couplings without the necessity of using very narrow coupling gaps. Based on the results of the study, four-pole cross-coupled planar microwave bandpass filters with a single and with two ground slots are designed, fabricated and measured. The performances of these novel structures indicate some technological advantages, compared to classical microstrip filters, without defected ground.

3696. Thermal analysis of some polynuclear coordination compounds

Authors: Patron, L; Budrugeac, P; Balu, A; Carp, O; Diamandescu, L; Feder, M

Published: APR 2007, JOURNAL OF THERMAL ANALYSIS AND CALORIMETRY, 88, 277, DOI: 10.1007/s10973-006-8080-6

The thermal behaviour of five polynuclear coordination compounds containing tartaric anion as ligand, namely (NH(4))(3)[LnFe(C(4)O(6)H(4))(3)(OH)(3)] (Ln=La and Eu), (NH(4))(2)[PrFe(C(4)O(6)H(4))(3)(OH)(2)] and (NH(4))[LnFe(C(4)O(6)H(4))(3)(OH)]center dot 3H(2)O (Ln=Nd and Gd) was investigated. The reaction progress was studied by TG/DTA and FTIR measurements. Oxalates and oxocarbonates were identified as intermediates. In the case of Ln=La, Nd, Pr, Eu and Gd, pure LnFeO(3) was obtained as final decomposition product. The thermal decomposition of Eu-Fe compound, leads to a mixture of mixed (ortho-ferrite (EuFeO(3)) and garnet (Eu(3)Fe(5)O(12))) and simple oxides (Eu(2)O(3) and alpha-Fe(2)O(3)).

3697. T-c behaviour of 2212 and 2223 phases in (Bi,Pb)Sr(Ca2-xYx)CuO/2223

Authors: Mihalache, V; Aldica, G

Published: APR 2007, JOURNAL OF OPTOELECTRONICS AND ADVANCED MATERIALS, 9, 922, DOI:

The critical temperature, T-c, for (Bi,Pb)Sr(Ca2-x, Y-x)CuO/2223, with x between 0 and 0.45, polycrystalline samples have been investigated. T-c of (Bi,Pb)(2)Sr2Ca2-xYxCu3O10+delta phase, 2223, found in samples with x < 0.05, decreases whereas Tc of (Bi,Pb)(2)Sr2Ca1-xYxCu2O8+delta phase, 2212, found in samples with x > 0.05, displays a parabolic dependence with the increase of Y content. The change of T-c for 2223 originates mainly from the increase of x. The variation of both x and oxygen content, delta, influences the T-c of 2212. The doping state of 2212 and 2223 in Y substituted samples suggests that, generally, in the samples consisting of both superconducting phases, 2212 is in over-doped state whereas 2223 is in under-doped state. The last affirmation suggests that 2212 could bring a significant contribution in the transport parameters of (Bi,Pb)SrCaCuO/2223 samples.

3698. TiO2-based nanopowders obtained from different Ti-alkoxides

Authors: Crisan, M; Braileanu, A; Raileanu, M; Crisan, D; Teodorescu, VS; Birjega, R; Marinescu, VE; Madarasz, J; Pokol, G

Published: APR 2007, JOURNAL OF THERMAL ANALYSIS AND CALORIMETRY, 88, 176, DOI: 10.1007/s10973-006-8125-x

Pure TiO2 and S-doped TiO2 sol-gel nanopowders were prepared by controlled hydrolysis-condensation of titanium alkoxides. The influence of different Ti-alkoxides (tetraethyl-, tetraisopropyl- and tetrabutyl-orthotitanate) used in obtaining TiO2 porous materials in similar conditions (water/alkoxide ratio, solvent/alkoxide ratio, pH and temperature of reaction) has been investigated. The relationship between the synthesis conditions and the properties of titania nanosized powders, such as thermal stability, phase composition, crystallinity, morphology and size of particles, BET surface area and the influence of dopant was investigated. The nature of the alkyl group strongly influences the main characteristics of the obtained oxide powders, fact which is pointed out by thermal analysis, X-ray diffraction, TEM and BET surface area measurements.

3699. Preparation through the vapor transport and growth mechanism of the first-order hierarchical structures of MoO3 belts on sillimanite fibers

Authors: Badica, P

Published: APR 2007, CRYSTAL GROWTH & DESIGN, 7, 801, DOI: 10.1021/cg060893s

First-order branched tree structures of MoO3 belts on sillimanite fibers (refractor wool) were grown by a vapor transport method. At longer growth times, MoO3 micro-belts on micro-belts form a flower-like structure. Most belts are of alpha-MoO3, and they grow in the two opposite directions along the c-axis. They show a high level of crystal quality, clean surfaces, sharp edges, and a triangular-shape tip with certain angles. The most frequent are angles of 47 degrees and 94 degrees. The growth mechanism is of vapor-solid type. In the initial stages of growth, elements specific for spontaneous spread and island formation on the surface of the sillimanite fibers described in the literature are observed. Significant enhancement of the thickness suggests that a layer-by-layer 2D growth is also probable. It is thought that responsible for such growth is direct vapor deposition of the Mo-O vapors on the already formed belts, this being similar to thin film growth. Curved surfaces of the substrate (in our case of the sillimanite fibers), on which the MoO3 belts are growing, are important and can be used for the directional growth control of the belts and formation of hierarchical structures. The vapor transport method allows the formation of beta-MoO3 belts that grow in the b-axis direction.

3700. Thermal behaviour of CuS (covellite) obtained from copper-thiosulfate system

Authors: Simonescu, CM; Teodorescu, VS; Carp, O; Patron, L; Capatina, C

Published: APR 2007, JOURNAL OF THERMAL ANALYSIS AND CALORIMETRY, 88, 76, DOI: 10.1007/s10973-006-8079-z

Thermal behaviour of CuS (covellite) obtained from the Cu(CH3COO)(2)center dot H2O and Na2S2O3 center dot 5H(2)O system, working at different molar ratio (1:6 and 1:4) in presence/absence of NH4VO3, was studied. It was established that the presence of vanadium in the system induces a densification of CuS nodules, but do not change the hexagonal CuS structure. It has an important influence in thermal behaviour of copper sulfide CuS obtained also. The morphological characteristics of CuS play an important role in the thermal stability and the stoichiometry of the thermal decompositions. Also, the possibility to obtain copper sulfides with greater cooper content was investigated.