National Institute Of Materials Physics - Romania

The (Zr0.8Sn0.2)TiO4 material (ZST), has been prepared by solid state reaction and characterized. The samples were sintered in the temperature range of 1260-1320 degrees C for 2 h. The effects of sintering parameters like sintering temperature (T-s) and MgO addition (0.2 wt.%) on structural and dielectric properties were investigated. Bulk density increases from 4900 to 5050 kg/m(3) with the increase of sintering temperature. The effect of MgO addition is to lower the sintering temperature in order to obtain well sintered samples with high value of bulk density. The material exhibits a dielectric constant epsilon(r) similar to 37 and high values of the Q x f product, greater than 45,000, at microwave frequencies. The dielectric properties make the ZST material very attractive for microwave applications such as dielectric resonators, filters, dielectric antennas, substrates for hybrid microwave integrated circuits, etc. (c) 2006 Elsevier B.V. All rights reserved.

Solid solutions Ba-1 (-) xSrxTiO3 (BST) are of high technological importance, particularly in microwave domain. Barium titanate has "naturally" three transitions, between four stable ferroelectric phases: (C) cubic, (T) tetragonal, (0) orthorhombic, (R) rhombohedral. Jaffe et al. [B. Jaffe, W.R. Cook, H. Jaffe, Piezoelectric Ceramics, Academic Press, 1971] has given the dependence of the transition temperatures up to 30% of Sr content. We have extrapolated these temperatures and we have found that some phases might disappear at higher Sr concentrations. A family of solid solutions with x = 25, 50, 75, 90% was prepared by standard solid-state reaction and sintered at 1230 and 1260 degrees C, respectively. The permitivities and the dielectric losses were measured with a self-acting bridge (1 kHz), on a large temperature range (+/- 200 degrees C). The composition x = 25% shows three peak values of permittivity as expected, while the composition x = 50%, only two peak values, corresponding to phase transitions cubic-tetragonal-rhombohedral, phase orthorhombic being excluded. Compositions with x >= 63%, Sr shows only one peak value corresponding to a genuine transition cubic-rhombohedral. The cubic transition to several lower phases shows almost a linear decrease of the Curie point with the increase of Sr fraction. For Sr concentration x >= 80%, the Curie point appears to fall more rapidly than linear. To our best knowledge, there is for the first time, this effect is reported. (c) 2006 Elsevier B.V. All rights reserved.

To eliminate precipitates-segregates that can easily occur on the thin film surfaces of the multicomponent materials for electronics, a new approach is proposed, consisting of the following aspects: first, on the substrates, artificial steps of predefined height and width are produced, and second, films are grown on such substrates. The width of the step is taken equal to the 'double of the migration length' of the atomic species depositing on the substrate. In these conditions, precipitates migrate and gather at the step edges where the free energy is lowest and the resulting totally precipitate-free surface of the film on the step is suitable for device applications or integration purposes. The method has several other important advantages and they are discussed in the text. Using this new approach we present successful fabrication of a mesa structure showing intrinsic Josephson effect. We have used thin films of Bi-2212/Bi-2223 superstructure grown by MOCVD on (001) SrTiO3 single crystal substrates with artificial steps of about 20 mu m width. (c) 2006 Elsevier B.V. All rights reserved.

Through correlated studies using scanning electron microscopy (SEM), Raman spectroscopy and low-temperature photoluminescence (PL) we have demonstrated that the intercalated PbI2 with ammonia, poly (vinyl alcohol) and polyacrylamide are characterized by a distinctive signature in the Raman and photoluminescence spectra. After intercalation, the Raman spectrum of PbI2 reveals an orthorhombic structure, identical with that observed on the micrometric scale, of KPbI3 rod-like particles resulting from the reaction between Pb(NO3)(2) and KI, carried out in liquid media such as ethanol and acetonitrile. The rods and the intercalated PbI2 are characterized by a new and strong emission band at 2.23 eV (about 550 nm) that appears at 77 K under an excitation wavelength of about 340 nm. Modification of the Raman and PL spectra results from a compressing effect produced by the penetration between the PbI2 layers of different molecular species. The compression acting primarily on the iodine layers has the result of lowering the contribution of the 5p states of I- ions to the constitution of the electronic level situated at the top of valence band of PbI2, so that the photoluminescence of intercalated PbI2 acquires the characteristics of Pb2+ emission when it is dissolved in an alkali halide crystal.

Electrochemical polymerization of N-vinyl carbazole (VK) on carbon nanotube (CN) films was studied by cyclic voltammetry in LiClO4/acetonitrile solutions. Cyclic voltammograms recorded on a blank Pt electrode were compared with those obtained when single or multi-walled CN films were deposited on the Pt electrode; in the latter case, a down-shift of the VK reduction peak potential was observed. Functionalization of CNs with poly(N-vinyl carbazole) (PVK) was invoked by Raman scattering and UV-VIS-NIR spectroscopic studies. The influence of sweep rate on the electrochemical properties of the PVK/CN nanocomposite and the performance of supercapacitors constructed using PVK-functionalized single-walled carbon nanotube electrodes were also evaluated. (c) 2006 Elsevier Ltd. All rights reserved.

NdFeB-based bulk nanocomposite permanent magnets (with Dy and Co partially substituting Nd and Fe, respectively) with additions of Early Transition Metals (ETM = Mo, Ti, Zr, Cu, Nb, V, W), in form of rods with diameters ranging from 0.5 to 0.8 mm, have been prepared by devitrification annealing of amorphous and partly-amorphous precursors produced by injection die casting. A fully amorphous structure was obtained for rods with the diameter as large as 0.6 mm. The as-cast rod samples are magnetically soft and the magnetic hardness develops as the samples are devitrified to the optimum nanostructure. The best-achieved hard magnetic properties have been obtained for the optimum devitrified (15 min at 630 degrees C) Nd(3)Dy(1)F(3)e(66)Co(10)B(20) samples of 0.6 mm in diameter: H-i(c) = 296 kA/m, mu M-0(r) = 0.86 T, Mr/M-max = 0.65 and (BH)(max), = 74 kJ/m. The Mossbauer spectra indicate the coexistence of 4 magnetic phases: 2:14:1, alpha-Fe, Fe3B and a short-range ordered intermetallic Fe-B-type phase.

The Fe-57 Mossbauer spectroscopy on alpha-iron(II) phthalocyanine (FePc) as a function of temperature (1.3 < T < 295 K) and applied field (0 < B < 10 T) has been used to study the peculiar magnetic properties of this ferromagnetic quasilinear chain type compound. One sextet with an internal hyperfine field B-int=66.2 T was observed at 1.3 K, a very large value for a bivalent iron with S=1 pointing to the existence of large positive orbital and dipolar contributions in the investigated FePc. Under an applied field, the experimental spectra exhibited two nonequivalent Fe positions, due to spin canting, with the values for the hyperfine fields of the split sextets increasing with increasing field, an indication that unlike most cases, B-int in alpha-FePc is positive, i.e., parallel to the magnetic moment of iron. Therefore, the origin of the large hyperfine field is the orbital moment rather than the Fermi's contact interaction. This fact is ascribed to the orbital degeneracy of the ground state of Fe(II) in the present configuration, where an unpaired hole occupies the orbital doublet (d(xz), d(yz)). This feature supports and explains the magnetization and susceptibility data as well as the anomalously high hyperfine field observed at Fe-57 nucleus. The relaxational behavior in the ac susceptibility and Mossbauer spectra found in the region 5-20 K was ascribed to solitonlike motion of domain walls within the magnetic chains, with a single-kink activation energy of 72 K.

Polycarbonate templates of ( 30 +/- 1) mu m thickness containing cylindrical etched-track nanochannels of ( 500 +/- 50) nm diameter were used for electrodeposition of Ni nanowires. Using 10(4) channels per cm(2), the most favourable deposition potential of -1.0 V was determined in a potentiostatic mode by varying the deposition potential with respect to an Ag/AgCl reference electrode over a range between -0.1 V and -1.5 V. The deposition efficiency at -1.0 V was estimated around 10%. The resulting single wires had a resistance around 200 W and showed an anisotropic magnetoresistance (AMR) effect of 1%, applicable to directionally sensitive magnetic field sensors.

We review the current status of the growth, characterization and applications of Bi-Sr-Ca-Cu-O (BSCCO) whiskers from the materials science point of view. Our analysis leads to the conclusion that reconsideration of the importance of the whisker studies is necessary; whiskers can play a major role in studies of the growth mechanism and defects, properties control and their understanding, and new devices and applications. Arguments favourable to the idea that BSCCO whiskers can possibly have a significant impact on future synthesis of nano-objects in this system or other high-T-c superconducting systems, as well as in investigation and understanding of the growth processes under elevated magnetic fields, are discussed. Emphasis is made on superconducting whiskers, but non-superconducting ones are also briefly addressed.

The structure of bulk glassy As2Se3 has been investigated by X-ray diffraction. The structure was compared with that of bulk glass As2S3 measured in the same conditions. Similarities and differences have been pointed out.