National Institute Of Materials Physics - Romania

The structure and magnetic properties of films of iron-modified amorphous carbon (a-C:Fe) prepared by magnetron cosputtering of iron and graphite targets are studied. X-ray diffraction measurements show that iron enters the samples in the form of Fe nanocrystals that are typically about 20 nm in size and also forms nanocrystals of hexagonal iron carbide. The temperature dependences of the magnetization, measured under cooling in zero and nonzero magnetic fields, are studied. At temperatures T less than or similar to 8 K, a magnetic transition, which provides evidence for the onset of magnetic ordering in the material, is observed to occur. The magnetization isotherms obtained in the 8- to 20-K temperature range are in agreement with this observation. It is shown that a modified version of Langevin's formalism adequately describes the observed features of a-C:Fe film magnetization. (c) 2005 Pleiades Publishing, Inc.

Cubic boron nitride (c-BN) is a synthetic material which exhibits exceptional physicochemical properties such as: hardness, thermal conductivity, thermo-chemical stability, semiconducting properties and radiations resistance. Such outstanding properties make it a promising multifunctional material for applications in extreme conditions, as those found in the outer space environment. Its further use for such applications requires, however, a much better understanding of the lattice defects and radiation damage properties. Here we present the results of multifrequency ESR studies concerning the native and radiation induced point defects in crystalline c-BN under irradiation with high intensity 1MeV electron beams.

Lead zirconate-titanate (PZT) epitaxial thin films are considered wide-gap p-type semiconductors with standard Schottky contacts. Capacitance-voltage, current-voltage, as well as the dielectric hysteresis are analyzed using a model based on the conventional metal-semiconductor Schottky contact in which the effect of the ferroelectric polarization on band-bending was considered. In addition, presence of the deep traps and its effect on the measured quantities were considered and discussed. For a single-crystal like PZT films with a remnant polarization of 40 mu C/cm(2) the free carrier concentration is estimated to about 3 x 10(18) cm(-3), whereas the effective density of the fixed charges in the depletion region is of about 1.8 x 10(19) cm(-3) .

The goal of this work is to prepare doped PZT-type materials with improved radial piezoelectric properties. Ceramic materials were obtained by solid-state reaction at different sintering temperatures. SAED, bright field TEM and HRTEM methods were used for the microstructure studies. The influence of the composition and nanostructure on the dielectric and piezoelectric properties of the materials is discussed. One of the materials with high coupling constant was used in the construction of a miniature flexural ventilator. Designated to function at an emf of 220 V/50 Hz, this device is characterized by high efficiency, reliability and low energy consumption. (c) 2005 Elsevier Ltd. All rights reserved.

We review our experimental data obtained by Raman spectroscopy on single-walled carbon nanotubes (SWNTs) using surface-enhanced Raman scattering (SERS). We focused on the study of the dependence of the SERS spectra of SWNTs on both the rough metallic substrate type and the film thickness. In particular, we show that degradation of the nanotubes, mainly metallic, occurs for very thin films with the formation of graphitic, fullerene-like or amorphous carbon particles. We show also that SWNTs reacting with H2SO4 form a salt of the hydrogensulfate type similar to that resulting from graphite-H2SO4 interaction. SWNTs exhibit a complicated spectroelectrochemical behavior in aqueous and non-aqueous H2SO4 solutions that depends also on the sweep potential range. We demonstrate that the oxidation-reduction processes have a quasi-reversible or irreversible character. Finally, SERS studies on compressed SWNTs in different host matrices, inorganic and organic, reveal nanotube breaking and that tube fragments may react or not with the host matrix used. Copyright (c) 2005 John Wiley & Sons, Ltd.

The paper presents the photoluminescence of nanocrystalline porous silicon. Two maxima were observed for rather fresh samples, one located at the infrared edge of the visible range and the other one in red. After ageing, the first maximum vanishes, suggesting its relation with the surface states, while the red one undergoes a significant blue shift. A simple quantum confinement model allows to correlate the photon energy of the red maximum with a transition between two confinement levels and to interpret the blue shift in terms of size reduction by oxidation. These results are in good agreement with previous microstructure measurements.

Powders of barium titanate (BT) and lead zirconate-titanate (PZT) were mixed together to form solid solutions in the ceramic system (1 - x)BT (.) xPZT, where 0 <= x <= 1. The mixed compounds were synthesized by sintering pressed samples of the mixed powders at high temperatures between 1100-1400 degrees C, for 3 hours. Dielectric and piezoelectric properties of the sintered samples were investigated at room temperature. X-ray diffractograms showed that monophase ceramics with perovskite structure were formed along the whole compositional range. The dielectric permittivity epsilon(r) increased with increasing amount of PZT, reaching a maximum value of about 6900 for composition with x = 0.6, and then suddenly decreases up to lower values of less than 2000. The piezoelectric properties (electromechanical coupling coefficient k(p), voltage and charge constants g and d respectively) behave as follows: at first there is a slight decrease of their values for all compositions with x <= 0.6 and then suddenly, and rather constantly, their values increase by more than 100 % for compositions with x >= 0.6. Such a behavior could possible be explained in terms of the structural changes brought about by the incorporation of PZT phase into BT one. The mechanical mixture of BT and PZT particles of similar dimensions gives rise to both new compositions as well as to distinct particles of BT or PZT of different dimensions under the influence of temperature, whose properties may differ from the initial ones so that their properties can be either the result of a size effect or the compositional one, but most probably the combination of both.

Superconductivity at about 8 K was observed in the metal-rich Li-Pd-B ternary system. Structural, microstructural, electrical, and magnetic investigations for various compositions proved that the Li2Pd3B compound, which has an antiperovskite cubic structure composed of distorted Pd6B octahedrons, is responsible for the superconductivity. This is the first observation of superconductivity in metal-rich ternary borides containing alkaline metal and Pd as a late transition metal. The compound prepared by arc melting has a high density and is relatively stable in the air. The upper critical fields H-c2(0) estimated by linear extrapolation and the Werthamer-Helfand-Hohenberg theory are 6.2 and 4.8 T, respectively.

UV-VIS transmission and fluorescence spectroscopy have been used to study some intrinsic properties of crystalline films of pure metadinitrobenzene (m-DNB) and, pure and doped benzil. We have evaluated the band gap energy, E-g = 2.92 eV, emphasising the near classical semiconductor behaviour of m-DNB. The two bands split of the (n, pi*) level is assumed to be the origin the two edges absorption in benzil films with the energetic thresholds at E-g1 = 2.84 eV and E-g2 = 3.55 eV. The red shift of the emission peak in m-DNB films has been correlated with the process of self-absorption of the emitted radiation in the thicker film. The effect of the impurities on the shape and position of the emission peaks in benzil has been investigated and we have observed no significant shift of the absorption peak situated at 3.25 eV. We also have identified a three steps process for the relaxation of the excited crystalline lattice of benzil involving the geometrical change of the (S-1) excited state, the intersystem crossing and the radiative decay by phosphorescence with a peak at 2.30 eV. The small blue shift of this emission peak in benzil highly doped with sodium has been attributed to the modification of the molecular geometry of benzil as a result of the change in the trans-planar molecular configuration induced by the chemical reaction between the atoms of alkali metal and the carbonyl groups. (C) 2004 Elsevier B.V All rights reserved.