National Institute Of Materials Physics - Romania

X- and Q-band electron paramagnetic resonance (EPR) investigation of different crystalline Alq(3) (tris(8-hydroxyquinoline)aluminium (III)) fractions formed by a train sublimation method are reported. Several paramagnetic defect centres corresponding to 1/2, 1, and 3/2 spin are observed at room temperature. Their intensity is dependent on the temperature, nature of the crystalline phase, and preparation conditions. Spectra simulation and analysis based on the spin Hamiltonian appropriate to a high spin system (S >=, 1) suggest the existence of randomly oriented triplets and quartets in annealed Alq(3) fractions. The crystalline Alq(3) phases responsible for the EPR powder spectra have been identified by transmission electron microscopy measurements performed on these sample fractions.

Addition of diamond like carbon (DLC) phase to Ag-base overlay prepared by ECR-DC hybrid sputtering was found to reduce its coefficient of friction compared to that of the substrate material in dry sliding. The coefficient of friction, tested in dry conditions, of the Ag matrix overlay decreased by sp(3)-rich C formation. It decreased from 0.72 +/- 0.05 to 0.25 +/- 0.05 by increasing the graphite/Ag target area ratio from 1/9 to 515 at low sliding speed (0.01 ms(-1)), 5 N load in dry sliding. Its drastic decrease was found when the grain size of the prepared films was lower than 10 mn as measured by transmission electron microscopy (TEM). The prepared coatings were characterized also by Xray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), Raman spectroscopy, and atomic force microscopy (AFM). (c) 2005 Elsevier B.V. All rights reserved.

Fe-sheathed tapes of MgB2 with addition of SiC have been prepared by the powder-in-tube (PIT) method using a powder mixture mechanically milled in an argon or hydrogen atmosphere. The reactivity of the component powders in the mixtures, Mg, B and SiC, was significantly enhanced by mechanical milling, so that MgB, formed faster than in tapes fabricated from a hand-milled mixture. At the same time the number of impurity phases was higher, and the impurity phases of Mg2Si and Fe2B were observed only in these tapes. Mechanical milling also induced a lower quality of MgB2, especially for tapes produced from powders milled in a hydrogen atmosphere, with a significant influence on the values of the critical temperature T-C and critical field, J(C). (c) 2005 Published by Elsevier B.V.

Simultaneous discharges of Fe and Cu metal sources by thermoionic vacuum arc (TVA) were used as a new procedure for preparation of granular Fe-Cu thin films with sensibly enhanced room temperature giant magnetoresistive effects. The possibilities to produce thin films with a wide range of structural and magneto-transport characteristics are emphasized. The structural aspects of granular Fe-Cu thin films deposited onto Kapton are investigated via X-ray diffractometry, atomic force microscopy and Mossbauer spectroscopy. The magneto-transport properties are strongly related to composition, structure and magnetic interactions inside the films. (c) 2005 Elsevier B.V. All rights reserved.

The modification of the polyester track membranes by treatment in radiofrequency plasma generated by capacitive coupled parallel plate discharge in air and ammonia was studied. The investigation of the surface properties (roughness, pore size distribution, contact angle) before and after the plasma treatments was performed. The treatment resulted in increase of the surface roughness and of the pores effective diameter and change of wettability. Membranes presenting lower or higher contact angles, as compared with the initial one can be obtained by selecting adequate values for the radiofrequency power and treatment time values. (c) 2005 Elsevier B.V. All rights reserved.

The effect of light-induced photodarkening in amorphous As100-xSex (x=40 divided by 98) thin films for different thickness and film composition was investigated. It was established a strong dependence of the red shift of the absorption edge and refractive index under light irradiation on the film composition and thermal treatment. It was established that the more sensitive films to photodarkening and holographic recording correspond to non-stoichiometric compositions of As55Se45 and As60Se40 glasses. The experimental results are interpreted in terms of structural optical polymerization process, which includes the transformation of As4Se4 and Se-2 structural units in homogenius AsSe3/2 network.

Microbiological load of green coffee is a real problem considering that it is extremely sensitive to contamination. Irradiation is a decontamination method for a lot of foodstuffs, being a feasible, very effective and environment friendly one. Beans and ground green coffee were irradiated with electron beams up to 40 kGy. Microbial load, rheological behavior, electron paramagnetic resonance (EPR) and visible spectroscopy were carried out. The results show that electron beam irradiation of green coffee could decontaminate it without severe changes in its properties. (c) 2005 Elsevier B.V. All rights reserved.

X-ray Photoelectron Spectroscopy (XPS) was used to investigate the chemical state of ZrC coatings deposited by cathodic arc method. A chemistry model of the ZrC films has been proposed. It was pointed out that on the films surface a highly oxidized passive layer, containing also carbon and oxygen as contaminants, was formed. The film bulk was composed by a mixture of ZrC (the dominant phase), ZrO2 and free carbon.

The simultaneous discharge of Fe (or Co) and, respectively, CU metal sources via thermionic vacuum arc (TVA) was used as a new processing method to obtain granular magneto-resistive films. The nano-structured films with thicknesses of 120 +/- 1 nm and 250 +/- 1 nm were obtained in definite conditions and subsequently characterized via electrical measurements at room temperature, in applied magnetic field. The paper reveals the importance of different specific factors (from size and dispersion effects to annealing influence) on the magnetoresistance behavior. A special contribution of the Mossbauer Spectroscopy, concerning the local structure and interactions as well as the magnetic phase characterization, was emphasized.

Quantum confinement effects in different kinds of nanocrystalline silicon systems are experimentally and theoretically investigated. Porous silicon structured as a nanowire network and silicon nanodots embedded in amorphous silicon dioxide are studied. The main quantum confinement effect in both cases is represented by the appearance of new energy levels in the silicon band gap. The corresponding energies can be experimentally determined from the current - temperature characteristics, which show an Arrhenius-like behavior. The curves present several activation energies between liquid nitrogen temperature and room temperature. The energy levels can be evaluated from a quantum well model. The fundamental level is located at the top of the valence band. The change of the activation energy is then related with the filling of the levels. The ratios of the consecutive activation energies in the current - temperature characteristics prove that the excitation undergoes the angular momentum conservation law imposed by the applied electric field. The estimation of the mean size of the nanocrystals from the values of the activation energies is in good agreement with the microstructure investigations performed on the samples. The confinement levels are also in good agreement with the photoluminescence measurements..