National Institute Of Materials Physics - Romania

Recently, MgB2 was discovered to be a superconductor with a surprisingly high critical temperature of about 39 K and with significant potential for applications. This material is usually prepared from a mixture of Mg and B in sealed tubes with excess Mg. As-prepared samples usually are very porous and mechanically weak. High density superconducting samples of MgB2 undoped and doped with SiC and B4C were produced by Field Assisted Sintering Technique (FAST) (T-c = 38.5 K). Samples are relatively large of 1.9 cm in diameter, apparently uniform, and can be easily extracted from the die. Processing times are short for easily attainable and relatively low temperatures. The effects of FAST consolidation on crystalline morphology revealed by microtomography and Scanning Electron Microscopy (SEM) and bulk density were investigated. Most samples have bulk density above 90% of the theoretical one. Despite such high bulk density, microtomography in combination with SEM revealed that in the samples there are some regions of low and high density. High density regions are not the consequence of FAST processing and they were observed in the raw material.

Metallo- phtalocyanines were obtained in good yields by the solvothermal method, a new and original synthesis method, optimized in real laboratory conditions. The structures were characterized by elemental analysis, UV-VIS and IR spectroscopy. Some of the compounds (iron phthalocyanine) were assigned by Mossbauer spectroscopy.

As recently shown, HgBa2Can-1CunOy (n >= 6) cuprate superconductors have a critical temperature of about 100 K independent of n. This remarkable property can be explained by the very imbalanced distribution of carriers among the inequivalent CuO2 planes in the unit cell. We discovered that these materials also have a common vortex melting line that resembles the theoretical melting lines of magnetically coupled pancake vortices. We suggest that there are two types of pancake pairs situated in the superconducting CuO2 outer planes: those separated by the thin charge reservoir layer are strongly (Josephson) coupled, while those separated by the thick block of (n-2) CuO2 nonsuperconducting inner planes are weakly (magnetically) coupled, forming magnetically coupled pancake vortex molecules.

The N,N-dimethylbiguanide (HDMBG) complexes [Cu-2(HDMBG)(2)Cl-4] (1) and respectively [Cu(HDMBG)(2)]Cl-2 center dot 2H(2)O (2) exhibit in vitro antimicrobial activity. The complexes were characterised by IR, electronic as well as EPR spectra. The IR spectra of complexes show the pattern of N,N-dimethylbiguanide coordinated as chelate. The electronic and EPR data are in agreement with a square pyramidal stereochemistry for (1) and a square planar one for (2). The in vitro qualitative and quantitative antimicrobial activity assays showed that the complexes exhibited variable antimicrobial activity against Gram-negative strains (Escherichia coli, Klebsiella spp. and Enterobacter sp.) isolated from the hospital environment. The thermal analysis has evidenced the thermal intervals of stability and also the thermodynamic effects that accompany them. The thermal behaviour in nitrogen is complex according to TG and DTA curves including melting., dehydration as well as compounds decomposition.

Double perovskite Sr2FeMoO6 type compounds show special properties, such as magnetoresistance response at relatively small-applied fields and at rather high temperatures (ca. 400 K). Perovskite precursors were obtained unconventionally, in liquid phase, using oxalic acid as complexing agent, in the presence of ethylene glycol as polymerization agent. Ordering control concerning the magnetic properties is predictable to be strongly influenced by the synthesis route. The thermal stability of the precursors and the phase transitions of the oxide samples were analyzed by thermal analysis method, IR spectroscopy, Mossbauer spectroscopy, X-ray diffraction method and magnetic measurements. The experimental data for the perovskite compounds prepared by using oxalic acid as complexing agent revealed a double perovskite structure of analyzed samples, convenient magnetorezistive response (3.51 mu(B)/f.u), in good agreement with the Mossbauer spectra. Moreover, Mossbauer spectreoscopy allows the distinction between the ordered and the disordered phases of the perovskite structure and put in evidence the strong dependence of the structural and magnetic characteristics of the double perovskites on the synthesis route.

Angle-resolved X-ray photoelectron spectroscopy (ARXPS) analysis has been performed on GaAs (10 0) surfaces in different conditions as naturally oxidized, Ar+ ion sputtering (E = 1-5 keV) and chemical etching in H2SO4/H2O2/H2O(3: 1: 1). The most sensitive angle to the surface compositional changes was the take-off angle (TOA): 25 degrees. Native oxide phases on GaAs consist of a mixture of Ga2O3, As2O3 and As2O5. Ar+ ion sputtering procedure modifies the surface composition, in the altered layer where the concentration ratio C-Ga/C-As tends to 1.5-1.6. Wet chemical etching removes the oxide layer and the As-rich region from the surface. In the experiment combining chemical etching with Ar+ ion sputtering for cleaning purpose, the native oxides are removed from the surface and C-Ga/C-As tends to stoichiometry. The experiment on native oxide reconstruction after storage in high-vacuum conditions (P similar to 10(-8) Torr) provides evidence of the high reactivity of GaAs (10 0) surfaces. We have observed the presence of an As oxide (BE = 43 eV) within a concentration range of 2-3%. (C) 2007 Elsevier B.V. All rights reserved.

In the last years the ternary stabilized zirconia ceramics have attracted a considerable attention as a source of materials for structural ceramics. In present paper, the simultaneous influence of Eu3+ and Y3+ cations on zirconia powders (obtained by co-precipitation of aqueous salts solutions) on stabilization process in conjunction with the structural characteristics was investigated. A conventional sintering method (in electric furnace) at 1350 degrees C temperature range was applied. SEM+EDAX and XPS (X-ray Photoelectron Spectroscopic) analysis were performed. The Eu3+ Mossbauer parameters: isomer shift and quadrupole splitting have been obtained for 21.6 keV radiation of Eu-151 are in good agreement with the XPS and XRD-obtained crystallographic data of the studied compositions.

Nanosized magnetite particles embedded in polypyrrole matrix have been studied by Mossbauer and electron magnetic resonance spectroscopy. Comparison with as grown magnetite is made. Hyperfine fields distribution is determined and line shape of resonance curves are discussed in terms of composite structure, sizes and treatments.

Fe-Pt alloys have gained much interest for the occurrence of permanent magnetic features resulting from the L1(0) ordered face-centred-tetragonal FePt phase with very high magnetic crystalline anisotropy (7x10(6) J/m(3)). An amorphous melt spun ribbons of the composition Fe51Pt27Nb2B20 has been synthesized by the rapid solidification technique and its microstructure and magnetic properties were studied. After appropriate annealing, an ordered face-centred-tetragonal (f.c.t.) L1(0) phase is formed. X-ray analysis revealed a structural phase transformation from the body-centered-cubic Al to f.c.t. L1(0) phase and this produce magnetic hardening of the alloy, upon appropriate annealing conditions. Extremely performant magnetic properties, typical for exchange spring magnets, are obtained.

Fluorescence measurements have been made on pure and doped bulk, mechanically polished wafers of crystalline m-DNB and benzil obtained by cutting ingots grown by the Bridgman-Stockbarger method modified for organic compounds crystallization. By comparison with pure matrices, we have investigated the effect of an inorganic dopant (iodine, silver, sodium) and of an organic dopant (m-DNB, naphthalene) on the emission characteristics (position and shape) of these molecular crystals. A slight shift of the emission peaks through high energy and an intense emission peak situated around 2.35 eV correlated with the local trapping level attributed to structural defects, which are involved in radiative processes, have been evidenced in iodine-doped m-DNB. The emission peak of m-DNB-doped benzil situated in the high-energy range (2.97 eV) is associated with direct emission activity of m-DNB, suggesting that this is an active impurity in benzil molecular matrix. We have not observed in benzil any evidence of indirect action of the impurity molecules (atoms) associated with the traps represented by the structural defects that generate changes in the energy levels of the neighbouring molecules and are correlated with different growth conditions. We have not remarked any involvement of the studied inorganic metallic impurities and of some organic impurities, such as naphthalene, in the radiative recombination processes in benzil matrix. (c) 2007 Elsevier B.V. All rights reserved.