National Institute Of Materials Physics - Romania

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5781 articles found

3361. Optical properties of CaF2 : Eu3+ nanoctystals embedded in transparent oxyfluoride glass ceramic

Authors: Polosan, S; Secu, CE

Published: AUG 2008, JOURNAL OF OPTOELECTRONICS AND ADVANCED MATERIALS, 10, 2137, DOI:

Transparent oxyfluoride glass ceramic in the system SiO2-Al2O3-CaF2-EuF2 containing CaF2:Eu3+ nanocrystals were produced by annealing of the initial glasses slightly above the CaF2 phase crystallisation peak. The relative good transparency of these materials was due to the small 65 nm, nucleated CaF2 nanocrystallites. X-ray diffraction data have shown an increase of the cell parameters for CaF2:Eu3+ compared with CaF2 which was attributed to the substitution of Ca2+ ions with Eu3+ ions in the CaF2 nanoparticles. Optical investigations using UV-VIS-NIR absorption and luminescence spectroscopy have shown an increase of the splitting of the Eu3+-ion associated luminescences and small shifts of the Eu3+ ion characteristic IR absorption peaks. These effects have been attributed to the Eu3+ environment evolving from the glass to the crystalline-like one.

3362. Synthesis and characterization of mesoporous ZnS with narrow size distribution of small pores

Authors: Nistor, LC; Mateescu, CD; Birjega, R; Nistor, SV

Published: AUG 2008, APPLIED PHYSICS A-MATERIALS SCIENCE & PROCESSING, 92, 301, DOI: 10.1007/s00339-008-4545-8

Pure, nanocrystalline cubic ZnS forming a stable mesoporous structure was synthesized at room temperature by a non-toxic surfactant-assisted liquid-liquid reaction, in the 9.5-10.5 pH range of values. The appearance of an X-ray diffraction (XRD) peak in the region of very small angles (similar to 2 degrees) reveals the presence of a porous material with a narrow pore size distribution, but with an irregular arrangement of the pores, a so-called worm hole or sponge-like material. The analysis of the wide angle XRD diffractograms shows the building blocks to be ZnS nanocrystals with cubic structure and average diameter of 2 nm. Transmission electron microscopy (TEM) investigations confirm the XRD results; ZnS crystallites of 2.5 nm with cubic (blende) structure are the building blocks of the pore walls with pore sizes from 1.9 to 2.5 nm, and a broader size distribution for samples with smaller pores. Textural measurements (N-2 adsorption-desorption isotherms) confirm the presence of mesoporous ZnS with a narrow range of small pore sizes. The relatively lower surface area of around 100 m(2)/g is attributed to some remaining organic molecules, which are filling the smallest pores. Their presence, confirmed by IR spectroscopy, seems to be responsible for the high stability of the resulting mesoporous ZnS as well.

3363. Relaxation processes of water confined to AlMCM-41 molecular sieves. Influence of the hydroxyl groups of the pore surface

Authors: Frunza, L; Schonhals, A; Kosslick, H; Frunza, S

Published: AUG 2008, EUROPEAN PHYSICAL JOURNAL E, 26, 386, DOI: 10.1140/epje/i2007-10340-y

A series of AlMCM-41 molecular sieves was prepared with constant composition (Si/Al = 14.7) and presumably same pore structure but different pore diameters (from 2.3 to 4.6 nm). The pore size distribution is narrow for each sample. The rotational fluctuations of water molecules confined inside the pores were investigated applying broadband dielectric spectroscopy (10(-2)-10(7) Hz) over a large temperature interval (213-333 K). A relaxation process, slower than that expected for bulk water, was observed which is assigned to water molecules forming a surface layer on the pore walls. The estimated relaxation time has an unusual non-monotonic temperature dependence, which is rationalized and modeled assuming two competing processes: rotational fluctuations of constrained water molecules and defect formation (Ryabov model). This paper focuses on the defects and notably the influence of the hydroxyl groups of the pore walls. The Ryabov model is fitted to the data and characteristic parameters are obtained. Their dependence on pore diameter is considered for the first time. The found results are compared with those obtained for other types of molecular sieves and related materials.

3364. Investigations of conduction mechanism in Cr2O3 gas sensing thick films by ac impedance spectroscopy and work function changes measurements

Authors: Pokhrel, S; Simion, CE; Quemener, V; Barsan, N; Weimar, U

Published: JUL 28 2008, SENSORS AND ACTUATORS B-CHEMICAL, 133, 83, DOI: 10.1016/j.snb.2008.01.054

This paper investigates the conduction mechanism in Cr2O3 gas sensing thick films. A citrate combustion method was used for the preparation of the metal oxide and ethanol vapour as a test gas. The manner in which surface reactions induced electrical changes are affecting the sensor signals inputs was explored by simultaneous dc and work function changes (Kelvin probe method). The identification of the contributions to conduction of the different sensing layer elements was made possible by ac impedance spectroscopy measurements. A conduction model, which qualitatively explains the experimental findings, was elaborated on the basis of the acquired experimental data and the information provided in literature. The model validity should apply, besides Cr2O3, to all p-type metal oxides used as gas sensitive materials. (C) 2008 Elsevier B.V. All rights reserved.

3365. Analytical expression for the quantum dot contribution to the quasistatic capacitance for conduction band characterization

Authors: Buda, M; Iordache, G; Mokkapati, S; Fu, L; Jolley, G; Tan, HH; Jagadish, C; Buda, M

Published: JUL 15 2008, JOURNAL OF APPLIED PHYSICS, 104, DOI: 10.1063/1.2959681

This paper demonstrates an analytical expression for the quasistatic capacitance of a quantum dot layer embedded in a junction, where the reverse bias is used to discharge the initially occupied energy levels. This analysis can be used to determine the position and the Gaussian homogeneous broadening of the energy levels in the conduction band, and is applied for an InGaAs/GaAs quantum dot structure grown by metal organic chemical vapor deposition. It is shown that the Gaussian broadening of the conduction band levels is significantly larger than the broadening of the interband photoluminescence (PL) transitions involving both conduction and hole states. The analysis also reveals a contribution from the wetting layer both in PL and modeled C-V profiles which is much stronger than in typical molecular beam epitaxy grown dots. The presence of a built-in local field oriented from the apex of the dot toward its base, contrary to the direction expected for a strained dot with uniform composition (negative dipole), is also derived from fitting of the C- V experimental data. (c) 2008 American Institute of Physics.

3366. A study of the LiNH2-MgH2 system for solid state hydrogen storage

Authors: Barison, S; Agresti, F; Lo Russo, S; Maddalena, A; Palade, P; Principi, G; Torzo, G

Published: JUL 14 2008, JOURNAL OF ALLOYS AND COMPOUNDS, 459, 347, DOI: 10.1016/j.jallcom.2007.04.278

The influence of different high energy milling times and of the addition of catalysts such as Nb2O5, TiCl3 and graphite on the hydrogen absorption/desorption (AID) kinetics of a mixture of 2LiNH(2) + 1.1MgH(2) has been studied in the temperature range 220-240 degrees C. It is found that a prolonged milling time is effective in improving the A/D kinetics, irrespective of the presence or not of any kind of tested additive. The enthalpy of decomposition reaction results to be about 40.4 kJ/mol, as derived from van't Hoff plot using the values of the plateau pressures measured in desorption mode. This thermodynamic parameter fits well with the current literature data. (c) 2007 Elsevier B.V. All rights reserved.

3367. Optical cooling of single-walled carbon nanotubes as revealed by their anti-Stokes Raman spectra

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

Published: JUL 9 2008, JOURNAL OF PHYSICS-CONDENSED MATTER, 20, DOI: 10.1088/0953-8984/20/27/275215

Semiconducting single-walled carbon nanotubes resonantly excited by the interband E-22(S) electronic transitions (at 1064 nm) display for the two components of the radial Raman band-one associated with the isolated tubes and the other associated with the bundled tubes-an anti-Stokes/Stokes Raman intensity ratio (I-aS/I-S) which deviates oppositely from the predictions of the Maxwell-Boltzmann formula. A cooling and heating vibration process, evidenced by an enhancement and diminishment of (I-aS/I-S), appears in the isolated and bundled nanotubes, respectively. Here we confirm a cooling process, observed only for semiconducting nanotubes, which emerges from the relaxation of the E-22(S) excited state by the electronic relaxation from E-22(S) to E-11(S) that precedes the spontaneous luminescence emission at E-11(S). Metallic nanotubes do not exhibit luminescence and no cooling effect is observed. Both semiconducting and metallic nanotubes show for the bundled component of the radial Raman band an enhancement of (I-aS/I-S) such as is frequently observed in a coherent anti-Stokes Raman scattering process.

3368. Re-Cr-Ni high-temperature resistant coatings on Cu substrates prepared by thermionic vacuum arc (TVA) method

Authors: Surdu-Bob, CC; Lungu, CP; Mustata, I; Frunza, L

Published: JUL 7 2008, JOURNAL OF PHYSICS D-APPLIED PHYSICS, 41, DOI: 10.1088/0022-3727/41/13/132001

Re-Cr-Ni composite metallic films were prepared using an original plasma deposition method developed at INFLPR, Bucharest, called thermionic vacuum arc ( TVA). The method is based on the evaporation of a metal followed by ignition of a plasma in the vapours. These three-component films/alloy films were deposited using three simultaneous TVA plasma sources in the same vacuum chamber. Surface corrosion at temperatures up to 1000 degrees C was found not to take place in these Re-Cr-Ni alloy films as shown by thermogravimetric analysis. The current results demonstrate that the TVA method is a promising candidate tool for the synthesis of multiple compound films. Films of uniform and controlled composition can be simultaneously obtained using this method. Moreover, high melting point metals can be involved in these superalloy films, thus leading to applications in extremely hot conditions such as turbine blades and aircraft parts.

3369. Exchange bias and spin valve systems with Fe-Mn antiferromagnetic pinning layers, obtained by the thermo-ionic vacuum arc method

Authors: Kuncser, V; Valeanu, M; Schinteie, G; Filoti, G; Mustata, I; Lungu, CP; Anghel, A; Chiriac, H; Vladoiu, R; Bartolome, J

Published: JUL 2008, JOURNAL OF MAGNETISM AND MAGNETIC MATERIALS, 320, E230, DOI: 10.1016/j.jmmm.2008.02.054

Exchange bias and spin valve systems with Fe-Mn antiferromagnetic layers of different Fe concentrations were obtained by the thermo-ionic vacuum arc method. The adherence of the multilayer system to the Si substrate depends on the Fe-Mn composition. The overall roughness depends on the growing order of the antiferromagnetic/ferromagnetic layers. Very low blocking temperatures for exchange bias were observed for the considered compositions of the Fe-Mn layer. The coercive forces of both the pinned and the free layers of spin valve structures can be considerably modified along a large set of samples simultaneously prepared. (C) 2008 Elsevier B.V. All rights reserved.

3370. Concentration dependence of luminescence properties in praseodymium and praseodymium/ytterbium-doped lutetium double phosphates

Authors: Legendziewicz, J; Guzik, M; Cybinska, J; Stefan, A; Lupei, V

Published: JUL 2008, OPTICAL MATERIALS, 30, 1671, DOI: 10.1016/j.optmat.2007.11.004

In this paper we present a comparative optical study of praseodymium and praseodymium/ytterbium-doped lutetium double phosphates in the wide spectral range (VUV-IR) and for various concentrations of the activators. The absorption and emission spectra of Na3Lu(PO4)(2):Pr3+ and Na3Lu(PO4)(2):Pr3+, Yb3+ were measured at room and low temperatures. It was shown that spectroscopic properties of investigated systems strongly depend on the concentration ratio of the Pr3+ and Yb3+ ions. (c) 2007 Published by Elsevier B.V.