National Institute Of Materials Physics - Romania

Publications

5781 articles found

3401. Rapidly solidified ferromagnetic shape memory alloys

Authors: Craciunescu, CM; Ercuta, A; Mitelea, I; Valeanu, M; Teodorescu, VS; Lupu, N; Chiriac, H

Published: MAY 2008, EUROPEAN PHYSICAL JOURNAL-SPECIAL TOPICS, 158, 165, DOI: 10.1140/epjst/e2008-00670-6

Ferromagnetic shape memory alloys have been manufactured by various techniques involving rapid solidification. Bulk alloys have been obtained by extracting the melted alloy in especially designed copper molds; glass coated wires have been obtained by drawing the melt from glass recipients followed by water cooling and ribbons have been fabricated by melt-spinning. Microstructural observations show particular solidification aspects of fractured areas, while ferromagnetic behavior has been detected in glass coated wires obtained by rapid solidification. The martensitic microstructure was observed on Co-Ni-Ga rapid solidified bulk alloys and Fe-Pd ribbons. The memory effect was detected using a Vibran system that allows the detection of the phase transition for the ribbons and by visual observation for other specimens. The conclusions of the observations are related to the comparison between the ferromagnetic behaviors of shape memory alloys solidified using different techniques.

3402. Single-beam pumped coherent anti-Stokes Raman scattering on carbon nanotubes thin films excited through surface plasmons

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

Published: MAY 2008, PHYSICA E-LOW-DIMENSIONAL SYSTEMS & NANOSTRUCTURES, 40, 2385, DOI: 10.1016/j.physe.2007.07.022

In this paper, we propose an interpretation for an abnormal anti-Stokes Raman emission observed on nanometric thin films of different materials and in particular carbon nanotubes. We demonstrate that under a tight-focusing of the excitation light, a coherent anti-stokes Raman scattering (CARS) emission is produced, resulting from a wave mixing process between the incident laser light (omega(1)) and Stokes Raman light (omega(s)) generated by a surface enhanced Raman scattering (SERS) mechanism. Although the Stokes/anti-Stokes intensity ratio has been explained differently, we present here the results which corroborate the CARS emission. They can be summarized as follows: (i) a square relationship between the CARS signal intensity and the film thickness; (ii) a square relationship between the CARS signal intensity and the exciting laser intensity; (iii) a dependence of the CARS intensity on the numerical aperture (NA) of the microscope objective used for the detection of the anti-Stokes emission. Such effects are not specific to carbon nanotubes and have been observed with other materials accommodated in similar conditions on rough metallic surfaces acting as SERS supports. (c) 2007 Elsevier B.V. All rights reserved.

3403. Vibrational and photoluminescence properties of composites based on zinc oxide and single-walled carbon nanotubes

Authors: Baibarac, M; Baltog, I; Lefrant, S; Mevellec, JY; Husanu, M

Published: MAY 2008, PHYSICA E-LOW-DIMENSIONAL SYSTEMS & NANOSTRUCTURES, 40, 2564, DOI: 10.1016/j.physe.2007.09.034

Composites based on zinc oxide (ZnO) nanoparticles and single-walled carbon nanotubes (SWNTs) were investigated by photoluminescence (PL) and Raman scattering. The room temperature PL spectrum of ZnO at the excitation wavelength of 335 nm displays a narrow excitonic band, peaking at 368 nm (3.36eV), and a wide one, disclosing two components with maxima at about 444 nm (2.79eV) and 560 nm (2.21eV) which are associated to the local oxygen vacancies and oxygen excess, respectively. Regardless the preparation method of the ZnO/SWNTs compound (electrochemical, mechanicochemical and hydrothermal synthesis), PL spectra reveal an increased quenching tendency with the concentration of nanotubes in the composite mass. As distinct feature in the PL spectra of ZnO/SWNTs composites are two bands, one at 450 nm (2.75 nm) and another one attributed to the formation of a donor-acceptor complex at 680 nm (1.82eV). The role of the dispersion of carbon nanotubes in the ZnO/SWNTs composite is observed by Raman scattering with the excitation wavelength of 676 nm. (C) 2007 Elsevier B.V. All rights reserved.

3404. Electrical properties of electrodeposited CdS nanowires

Authors: Ghenescu, M; Ion, L; Enculescu, I; Tazlaoanu, C; Antohe, VA; Sima, M; Enculescu, M; Matei, E; Neumann, R; Ghenescu, O; Covlea, V; Antohe, S

Published: MAY 2008, PHYSICA E-LOW-DIMENSIONAL SYSTEMS & NANOSTRUCTURES, 40, 2488, DOI: 10.1016/j.physe.2007.09.188

A template method was used to obtain cadmium sulfide (CdS) nanowires. Polymer ion tracks foils (30 mu m tick) were used as templates, after etching with solutions containing NaOH and methanol. CdS nanowires were electrochemically grown in the resulting pores. The nanowires were contacted by sputtering a gold layer on top of the membrane, and the electrical properties were recorded in the temperature range 40-300K. An activated electrical resistance was observed, with activation energy of 0.27eV at temperatures larger than 180 K. I-V characteristics show a symmetric, non-linear shape, in the voltage range used in this experiment. (C) 2007 Elsevier B.V. All rights reserved.

3405. Doping dependence of vortex regimes in Y1-xPrxBa2CU3O7-delta single crystals

Authors: Gyawali, P; Sandu, V; Almasan, CC; Taylor, BJ; Maple, MB

Published: MAY 2008, PHYSICAL REVIEW B, 77, DOI: 10.1103/PhysRevB.77.174504

Magnetic relaxation measurements on a series of Y1-xPrxBa2CU3O7-delta (x=0.13, 0.34, and 0.47) single crystals were performed over a large field and temperature range in order to investigate the characteristics of the vortex matter across the second magnetization peak (SMP). The magnitude of the SMP varies nonmonotonically with Pr concentration, i.e., the irreversible magnetization normalized by its value at the onset field H-on displays a maximum for the x=0.34 single crystal. The two characteristic fields, H,,, and H,,, follow different temperature T dependences: H-on proportional to T-nu on and H-sp proportional to [1-(T/T-c)(2)](nu sp). The extracted values of the apparent activation energy U* and the creep exponent mu display a maximum at a field H-on < H* < H-sp. Their field dependences point toward the coexistence of both elastic and plastic creep for H > H-on. The degree of participation of each creep mechanism is determined by the charge carrier density, which controls both the elastic properties of the vortex matter and the pinning potential.

3406. Luminescence and EPR study of ZnO : Mn : Cu nanowire array

Authors: Sima, M; Enculescu, I; Grecu, MN; Secu, M; Sima, M; Matei, E; Vasile, V

Published: MAY 2008, PHYSICA E-LOW-DIMENSIONAL SYSTEMS & NANOSTRUCTURES, 40, 2498, DOI: 10.1016/j.physe.2007.08.075

We report on photoluminescence studies on manganese and copper-doped ZnO nanowire array prepared by template method. The ZnO:Mn:Cu semiconductor grown using the electrochemical technique was characterized by electron paramagnetic resonance (EPR) and optical reflection measurements. Morphology and the structure were observed by SEM and X-ray diffraction. The photoluminescence of this system was correlated with manganese and copper concentration in the ZnO matrix. (C) 2007 Elsevier B.V. All rights reserved.

3407. WO(x)-CeO(2) and WO(x)-Nb(2)O(5) catalysts deactivation during hexane isomerization

Authors: Mamede, AS; Payen, E; Granger, P; Florea, M; Parvulescu, VI

Published: MAY 2008, , DOI: 10.1002/aic.11468

3408. Mo-V-M-O (M = Ni, Cu, Zn, Sb, Ta) MIXED METAL OXIDES PREPARED BY SOLID-SOLID REACTIONS FOR OXIDATIVE DEHYDROGENATION OF ISOBUTANE

Authors: Mitran, G; Marcu, IC; Florea, M; Sandulescu, I

Published: MAY 2008, , DOI:

3409. Charge fluctuations in quantum point contact

Authors: Bulka, BR; Dinu, IV; Tolea, M

Published: MAY 2008, PHYSICA E-LOW-DIMENSIONAL SYSTEMS & NANOSTRUCTURES, 40, 2617, DOI: 10.1016/j.physe.2007.08.001

We present studies of the role of charge fluctuations in transport through a quantum point contact (QPC). Our model of QPC is based on assumption of a specific electronic energy structure with a resonant level below the electronic sub-band. We show that charge fluctuations lead to a dynamical Coulomb blockade effect, which is responsible for reduction of the conductance to the value 2/3 x (2e(2)/h). The evolution of conductance with a magnetic field and as a function of source-drain voltage is presented as well. The conductance plateau at 2/3 x (2e(2)/h) evolves continuously with an increase of the voltage to a 0.8 or 0.4 plateau, depending on the relative position of the resonant state with respect to the Fermi energy at zero bias. Our simple model shows many similarities with experimental characteristics. (C) 2007 Elsevier B.V. All rights reserved.

3410. Kinetics corrosion process of carbon steel in hydrochloric acid in absence and presence of 2-(cyclohexylaminomercapto) benzothiazole

Authors: Samide, AP; Bibicu, I

Published: MAY 2008, SURFACE AND INTERFACE ANALYSIS, 40, 952, DOI: 10.1002/sia.2825

The corrosion and inhibition behavior of carbon steel in 1 m HCl in the presence of 2-(cyclohexylaminomercapto) benzothiazole (CMB) was investigated using weight-loss measurements, galvanostatic polarization studies and UV-VIS spectrophotometry. The morphology of a surface was analyzed by using Mossbauer spectroscopy. Some samples were examined by scanning electron microscopy. The inhibition efficiency increased with increasing CMB concentration and decreased with increasing temperature of the corrosion medium. The activation energy (E(a)) was determined from Arrhenius equation. The activation enthalpy (Delta H(0)) and the activation entropy (Delta S(0)) were calculated from the diagram of the transition state. The experimental results suggest that the presence of CMB in the solution increases the surface coverage (theta) and therefore, indicates the adsorption of CMB. The adsorption of this compound on the metal surface is found to obey Langmuir's adsorption isotherm. The UV-VIS spectrophotometry shows a decrease of the concentration of CMB in HCl solution after corrosion, indicating that an adsorption process occurs between an organic compound from aqueous phase and the electrode surface. From the Mossbauer spectroscopy studied at this stage, it is expected that the main product of corrosion is a nonstoichiometric amorphous Fe(3+) oxyhydroxide, consisting of a mixture of alpha,beta and gamma-FeOOH, where gamma-FeOOH is the main phase. Copyright (C) 2008 John Wiley & Sons, Ltd.