National Institute Of Materials Physics - Romania

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5781 articles found

4281. EPR investigation of nanosized La0.67Ca0.33MnO3-delta manganites

Authors: Giurgiu, LM; Grecu, MN; Darabont, A; Raita, O; Filip, X; Pana, O; Toloman, D

Published: 2004, APPLIED MAGNETIC RESONANCE, 27, 150, DOI: 10.1007/BF03166309

We report an electron paramagnetic resonance (EPR) investigation of the spin dynamics in the paramagnetic regime of nanosized La0.67Ca0.33MnO3-delta manganites. The temperature dependences of the EPR line width and integral intensity have been analyzed in terms of the bottlenecked spin relaxation and small polaron hopping scenarios. The exchange coupling integral between Mn3+ and Mn4+ ions and the polaron activation energy decrease with the reduction of grain size. A discussion is given concerning the factors which could explain the observed changes.

4282. Photoluminescence studies on [C60] fullerene solutions in N-methyl-2-pyrrolidinone and pyrrolidine

Authors: Mihut, L; Preda, N; Baibarac, M; Baltog, I; Lefrant, S; Wery, J

Published: 2004, MOLECULAR CRYSTALS AND LIQUID CRYSTALS, 415, 21, DOI: 10.1080/15421400490478209

Photoluminescence (PL) spectra of C-60 solutions in N-methyl-2-pyrrolidinone (NMP) show a drastic increase of the PL intensity if the solutions are stored longer periods of time. Two distinct spectral components, one in the red (1.92-2.00 eV) and another in the green (2.18-2.3 eV) were identified in the whole PL emission. Their weights depend on C-60 concentration and on storage time. The two emissions come from two successive processes running with different kinetics. Charge transfer, leading to (C-60, NMP) loose molecular complexes and a slow aggregation process of these entities seems to originate the red and green emission, respectively. Similar PL properties are found when pyrrolidine (P) is used as solvent.

4283. Electrochemical deposition of PBSe1-xTex nanorod arrays using ion track etched membranes as template

Authors: Sima, M; Enculescu, I; Visan, T; Spohr, R; Trautmann, C

Published: 2004, MOLECULAR CRYSTALS AND LIQUID CRYSTALS, 418, 755, DOI: 10.1080/15421400490478885

Lead chalcogenides (PbS, PbSe, PbTe) are narrow band gap semiconductors which have been studied in the field of IR detection and thermoelectric devices. The template method is a general approach for synthesizing nanomaterials within the pores of membranes. The membranes employed contain cylindrical pores with monodisperse diameters, and corresponding cylindrical nanostructures are obtained. The aim of the present study was to prepare PbSe1-xTex nanorod arrays using electrodeposition. The process was investigated by cyclic voltammetry and electrochemical impedance spectroscopy. The resulted PbSe1-xTex nanostructures were characterized using scanning electron microscopy (SEM) as well as energy dispersive X-ray analysis (EDAX).

4284. Synthesis and piezoelectric properties of nanocrystalline PZT-based ceramics prepared by high energy ball milling process

Authors: Miclea, C; Tanasoiu, C; Gheorghiu, A; Miclea, CF; Tanasoiu, V

Published: AUG-SEP 2004, JOURNAL OF MATERIALS SCIENCE, 39, 5434, DOI: 10.1023/B:JMSC.0000039260.82430.f9

Nanocrystalline powders of a soft FeNbLi-doped PZT material have been prepared by a novel mechanochemical process consisting of mixing the stoichiometric oxides in a planetary ball mill for prolonged times up to 80 h. The constituent oxides were reacted in a tungsten carbide vial with balls of 5, 10 and 20 mm in diameter and a ball/powder ratio of 15/1. The chemical reaction between the component oxides was triggered after 20 h of energetic milling and was completed after 80 h. The XRD of the reacted nanopowder showed the well known perovskite structure. Compacted samples of this powder were sintered between 800-1300degreesC for 3 h and the main piezoelectric properties were determined. The density of the sintered samples reached nearly 99% of the theoretical density at 1100degreesC and showed good piezoelectric characteristics: planar coupling factor of 0.66, dielectric displacement constant d(33) of 550 pm/V, mechanical quality factor of 85, and relative dielectric constant of 3800. The possible mechanisms for solid state reaction of mechanically activated nanopowders such as local heating and pressure at collision as well as defects diffusion are discussed. (C) 2004 Kluwer Academic Publishers.

4285. Raman studies of carbon nanotubes and polymer nanotube composites

Authors: Lefrant, S; Buisson, JP; Schreiber, J; Chauvet, O; Baibarac, M; Baltog, I

Published: 2004, MOLECULAR CRYSTALS AND LIQUID CRYSTALS, 415, 132, DOI: 10.1080/15421400490482844

In this paper, we present Raman studies of different carbon nanotubes systems like single-walled nanotubes (SWNTs), multi-walled nanotubes (MWNTs) or composites prepared with different conjugated or non-conjugated polymers. We have focused our studies on the interactions that can take place in these systems and in general modify the characteristics of individual tubes. By introducing Van der Waals interactions between tubes, we are able to interpret experimental Raman spectra. In the case of SWNTs, we have put in evidence the so-called "bundle'' effect that consists of an upshift of the frequency of the RBM of 10 to 16 cm(-1), depending on the size of the bundles. In MWNTs, one can calculate the low frequency Raman modes observed experimentally. We found that they originate from the radial breathing modes of isolated tubes with an upshift that depends on the number of shells constituting the tube and we were then able to calculate the intensity of such modes. Finally, in polymer/nanotubes composites, we demonstrate that interactions take place between the two constituents in two different cases: PPV/SWNTs and PANI/SWNTs. The role of the method of preparation of composites is particularly put in evidence.

4286. Emission properties of T-a-centres in KCl

Authors: Topa, V; Tsuboi, T; Polosan, S

Published: AUG-DEC 2004, RADIATION MEASUREMENTS, 38, 734, DOI: 10.1016/j.radmeas.2004.01.021

Electrolytic colouration at about 600degreesC and 350 V/cm, for KCl crystal containing Pb2+ and for KCl:Pb2+ crystals co-doped with Li+, Na+ and Rb+ has been undertaken. Several absorption bands were observed in both doped and co-doped crystals in the visible-UV region. Excitation into these bands gives rise to the same 0.86 eV emission band except for the Li+-co-doped crystal which gives rise to a 0.80 eV emission band. These absorption bands are due to the same T-a-centre related to Pb-. The observed infrared emission intensity of the crystal with F-centres is higher than of without F-centres. (C) 2004 Elsevier Ltd. All rights reserved.

4287. Carbon nanostructures produced by CCVD with induction heating

Authors: Lupu, D; Biris, AR; Jianu, A; Bunescu, C; Burkel, E; Indrea, E; Mihailescu, G; Pruneanu, S; Olenic, L; Misan, I

Published: 2004, CARBON, 42, 507, DOI: 10.1016/j.carbon.2003.12.034

The catalytic chemical vapor deposition (CCVD) method in which the outer furnace is replaced by the high frequency induction heating (IH) has been used for synthesis of carbon nanostructures. Using different catalysts, various types of carbon nanofibers were obtained, with the absence of the catalyst particles at their tip as a common characteristic. The IH mode combined with the CCVD method seems attractive for the synthesis of carbon nanostructures, allowing a significant decrease of energy consumption and of the overall reaction time as compared with the heating mode with outer furnace. (C) 2003 Elsevier Ltd. All rights reserved.

4288. Surface study of the corrosion of carbon steel in solutions of ammonium salts using Mossbauer spectrometry

Authors: Samide, A; Bibicu, I; Rogalski, MS; Preda, M

Published: 2004, JOURNAL OF RADIOANALYTICAL AND NUCLEAR CHEMISTRY, 261, 596, DOI: 10.1023/B:JRNC.0000037100.27713.bf

The effect of the NO3- anion on the corrosion of carbon steel in a solution of 0.1M NH4Cl (pH 5.5) was studied by galvanostatic polarization and Mossbauer spectrometry. The anion has an inhibiting effect by decreasing the expansion rate of generalized corrosion of carbon steel in a solution of 0.1M NH4Cl-Mossbauer spectroscopy shows that a superficial compound is formed on the electrode surface as a result of corrosion, presenting no magnetic ordering. Its parameters show the initial stage of corrosion. We assume that at this stage the main corrosion product is a mixture of ferrihydrite Fe(III) and FeOOH (alpha and/or gamma).

4289. 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.

4290. Structural, electrical, and photoelectrical properties of CdxPb1-xS thin films prepared by chemical bath deposition

Authors: Pentia, E; Draghici, V; Sarau, G; Mereu, B; Pintilie, L; Sava, F; Popescu, M

Published: 2004, JOURNAL OF THE ELECTROCHEMICAL SOCIETY, 151, G733, DOI: 10.1149/1.1800673

CdxPb1-xS (0 less than or equal to x less than or equal to 1) thin films were prepared on glass substrates using the chemical bath deposition method. The structural, electrical, and photoelectrical properties of the films were investigated, and all were dependent on the x value. For x > 0.5 (high Cd content), the films show a poor crystallinity and weak photoconductivity. For x < 0.5 (high Pb content), the films exhibit a good crystallinity, with an average size of the crystallites of 100 nm, and good photoconductivity. The threshold voltage of photoconductivity is shifted to lower wavelengths with increasing Cd content. The experimental results point toward possible ternary deposition on some composition ranges (near pure PbS or CdS); whereas, for the rest, the deposited film is rather a mixture of phases [not CdxPb1-xS but (CdS)(x)(PbS)(1-x)]. (C) 2004 The Electrochemical Society.