Publications

TlBa2Ca2Cu3Oy (Tl-1223) superconducting films were prepared under identical conditions, on MgO substrates with different morphologies resulting from heat treatments at temperatures between 600 and 1350 degreesC. The superconducting films have almost the same morphology, in-plane alignment and composition, but different critical current densities J(c). Critical current density J(c) determined at 77.3 K differs for films with various annealed MgO substrates, by a factor of 5 in zero field and 10 in 1 T. The behavior of J(c) is discussed in relation with the flatness of the MgO surface, and with pinning effects induced by the Ca-segregates. For the present work, the best films were obtained for the un-annealed (as-received) substrates and for substrates treated at 1350 degreesC. In these samples, the substrate's flatness and morphology with regular steps are essential for high quality of the superconducting films. In the films grown on MgO annealed between 800 and 1200 degreesC and showing rough surface and Ca-segregates, J(c) was lower. However, J(c) was increasing with heat treatment temperature of the substrate, possibly due to the Ca-segregates inducing or acting as pinning centers. The effects of Ca-segregates are of interest for maximization of the quality of the high-temperature superconducting films. In the literature, up to now, these effects have not been considered and therefore Ca-segregates removal has been recommended. (C) 2002 Elsevier Science B.V. All rights reserved.

Thin films of the infinite-layer compound Sr0.6Ca0.4CuO2-delta have been prepared by off-axis rf magnetron sputtering on SrTiO3 (001). For an opt mum level of oxygen vacancy superconductivity was observed (T-c onset = 42 K and T-c(rho=0) = 11 K). Structural and transport data suggesting n-type superconductivity are presented and discussed. Infinite-layer thin films of Sr0.5Ca0.4CuO2-delta, without trivalent cation doping and showing n-type superconductivity, are reported for the first time.

We report an electron paramagnetic resonance (EPR) investigation of the spin dynamics in the paramagnetic regime of the colossal magnetoresistive manganites La2/3Ca1/3Mn1-xMexO3 (Me = Al, In; x less than or equal to 0.05). The temperature dependences of the EPR linewidth and integral intensity have been analyzed in terms of the bottleneck spin relaxation and small-polaron hopping models. The exchange coupling integral between Mn3+ and Mn4+ ions and the polaron activation energy decrease with increasing doping level. A discussion is given concerning the factors which could explain the observed changes.

The simulation of interstitial void evolution in a model of crystalline silicon subjected to topological disordering revealed that the void size distribution broadens and shifts towards smaller mean diameter size, when the lattice disordering increases. The size distribution is not gaussian and the mean diameter of the voids is lower than the crystalline value. The calculation of the structure factor corresponding to void configuration evidences the absence of the short and long range order and point out to the presence of the medium range order.

The paper refers to the study of patch antennas with a high permittivity dielectric substrate ((Zr-0.8 Sn-0.2)TiO4) obtained at National Institute of Materials Physics Bucharest. Because of the high value of the permittivity of the substrate material it was necessary to perform a comparative investigation of different prototypes and arrays. It was considered two substrate thickness h(1)=1mm and h(2)=2mm and two frequencies f(1)=2.4 GHz and f(2)=1.8 GHz. For each case we presented the radiation pattern in the two principal planes (E and H) and input impedance. Also, some radiation pattern for rectangular array with n=4, 16, 36 patches are presented. In the Conclusion section a discussion on the suitability of patch antennas with high permittivity substrate is done.

Effects of high temperature, up to 1700 degreesC, annealing in vacuum of CVD diamond on its structure, optical and mechanical properties are investigated. Translucent polycrystalline diamond films of thickness 0.06-1.0 mm were grown by microwave plasma CVD method, and examined with transmission electron microscopy, optical absorption spectroscopy, and three-point bending technique to measure the fracture strength. A progressive darkening of the samples, with appearance of absorption features specific for graphite-like material, was observed upon annealing at temperatures above 1300 degreesC. The formation along grain boundaries of amorphous carbon and/or well crystallized graphite layers, 5-20 nm thick, as well as intra-granular graphite islands, was directly observed with TEM. This internal diamond-graphite transformation process can be described by two activation energies, both values being much less than those known for the surface graphitization of diamond. The fracture strength of the diamond films increases up to 50% with annealing temperature (1460-1640 degreesC), this being ascribed to a build up of compressive stress as a result of local diamond-graphite conversion. (C) 2003 Elsevier Science B.V. All rights reserved.

The paper describes the investigations of Nd doped Barium titanate (BNT) ferroelectric ceramic materials prepared by solid state reaction techniques. The decrease of the Curie temperature with the increase of the Nd2O3 concentration from 0 to 28 wt % was observed. High density values of BNT ceramics were obtained by adding 0.5 mol % PbO substitutional for BaO. The samples were sintered at temperatures T-s=1200divided by12600degreesC. At room temperature, the materials are in paraclectric phase, suitable for high frequency applications.

This paper presents the results of an analysis using the finite plate technique of Y-cut langasite plan-parallel resonators with Al and Ag electrodes. We have extracted values for the piezoelectrically stiffened elastic stiffness c(66)(D), the elastic stiffness c(66)(E), the piezoelectric stress constant e(11), and the dielectric permittivity epsilon(11)(S). The measured material constants are in very good agreement with other reported values. This paper will present our new results on the langasite samples, and will include a comparison between an analysis of the data using the measurement method recommended by the current IEEE Standard on Piezoelectricity and the finite plate technique.

The structural characterization by means of transmission electron microscopy (high resolution, electron diffraction, energy filtered, in-situ heating) is discussed for some advanced silicide processes. Both blanket layers and narrow line structures are investigated. Special attention is given to the influence of the initial layer stack on the epitaxial growth modes in the layer.

The chemical interaction of chalcogenide vitreous semiconductors (ChVS) with absorbed impurities induced by high-energetic (E>1 MeV) gamma-irradiation has been investigated at the example of vitreous v-As2S3. Radiation-induced processes of oxidation, hydrogenization, hydratation, carbonization and hydrocarbonization have been observed in this material after prolonged gamma-irradiation and studied using IR spectroscopy (4000-400 cm(-1)), laser mass-spectroscopy methods and electron microprobe analysis.