National Institute Of Materials Physics - Romania

Mixed oxides nanocomposites of the type xCr(2)O(3)-(1-x)alpha-Fe2O3 (x = 0.0-1.0) were synthesized using hydrothermal technique and characterized by X-ray diffraction (XRD) and Mossbauer spectroscopy. The same characterization methods were applied to a set of samples annealed at 600 degrees C for 2 h. High chromium content correlated with occurrence of amorphization effects and onset of superparamagnetism in the as-obtained samples. The structure and properties of the thermally annealed samples, however, are dominated by the equilibrium of two phases, Cr:Fe2O3 and Fe:Cr2O3. The final (x = 1.0) particle size of the nanocomposite is close to 19 nm.

Au3Zr intermetallic phase was obtained as pure phase by melt spinning technique. The sample with the same composition obtained by powder metallurgy exhibits some Au4Zr impurity. Rietveld refinements of the X-Ray data were done to obtain the unit cell parameters and atom positions inside the unit cell. Geometry optimizations for the atom positions inside the unit cell were performed by using ab-initio band structure calculations. The calculated values agree well with the ones obtained by Rietveld refinements of the diffraction data. The electronic mechanisms are explained on the basis of the projected densities of states to atom sites.

One of the most important trend in electronic devices manufacturing is miniaturisation. Among other techniques used to decrease the physical dimensions of microwave devices one is to employ materials with high permitivity [1], providing that the dimension of the device is proportional to the wavelength in the material, which is square effective permittivity times less than the wavelengths in free space. The paper shortly presents the manufacturing process to obtain a high permittivity ZST ((Zr(0.8), Sn(0.2))TiO(4)) material used to build a dielectric resonator oscillator, which is used as a proximity Doppler sensor. Computed and experimental results as well as the procedure to measure the parameters of the Doppler sensor are presented. The sensor described in the paper may be considered as a short range radar device.

In the aim to improve the adherence of the nanostructured gold layers onto glass substrate, gold deposition has been first time performed onto a polystyrene (PS) layer. The PS layer of ca. 50nm was spin-coated onto glass plates from a toluene solution. Gold layers of 10-20nm were vacuum deposited at small incidence angle. We found that at variance from the plates with gold deposited directly on the glass, the plates with an intermediate PS layer do not peel under overnight treatment in ethanol solution. The layers were characterized by several methods. X-ray diffraction (XRD) measurements showed that gold peaks have the position corresponding to the face-centered cubic structure: However, the crystallites on the sample with PS layer seem to be a little bit smaller than those with gold deposited directly onto the glass. XR reflectometry measurements have given the thickness of the gold layer in good agreement with the value estimated from quartz monitor readings and with the ellipsometric data as well, Liquid crystal cells were obtained to observe the molecular alignment imposed by the nanostructured gold layer deposited onto PS film. A rather strong interaction of gold atoms with the substrate molecules can be considered on the basis of the XRD and ellipsometry measurements.

We performed magnetization relaxation experiments for optimally doped YBa(2)Cu(3)O(7-delta) (YBCO) films with the external magnetic field H = 20-30 kOe oriented along the c axis and at relatively low temperatures T = 10-32 K, where a crossover elastic (collective) vortex creep-plastic creep is expected. The creep crossover is generated in this case by the T dependent macroscopic currents induced in the specimen, and manifests itself through the occurrence of a maximum in the current density J (or T) variation of the normalized vortex-creep activation energy U*, at which the creep exponent changes sign. With the same magnetization relaxation data the widely used Maley technique to construct the J dependence of the vortex-creep activation energy U supplies an U(J) variation close to the logarithmic one. It appears that the Maley technique may lead to unreliable results, due to the creep crossover characteristic to standard zero-field-cooling magnetization measurements and the reduced overall relaxation (in a convenient relaxation time window) for highly disordered specimens. It is suggested that the results obtained with this scheme should be checked by determining the U*(T, J) variation.

The influence of the shape of silicon quantum dots embedded in an amorphous silica matrix on the quantum confinement energy levels, as well as that of the Si/SiO2 potential barrier, are studied. The energy levels are computed using both the infinite and finite rectangular quantum well models for spherical quantum dots and the infinite rectangular quantum well for prolate spheroidal quantum dots. The results are compared with each other and also with the experimental activation energies obtained from the temperature dependence of the dark current. These activation energies are identified with the differences between the quantum confinement energies, subject to the selection rules. The finite rectangular quantum well model takes into account the experimental value of the finite potential barrier and the matrix-to-dot electron mass ratio. The energy levels are smaller than those for the infinite rectangular quantum well case; they decrease when the potential barrier decreases and the mass ratio increases. Different aspects of the models are discussed. All the errors are less than about 4%. The spheroidal shape lifts the degeneracy on the magnetic quantum number. The energy levels can decrease or increase with eccentricity as a consequence of the different quantum confinement effects along the major and minor axes. The supplementary information on the magnetic quantum number is beneficial for optical applications.

Multiferroic composite materials, consisting of piezoelectric and piezomagnetic phases, show electric polarization in the presence of a magnetic field and magnetization on applying an electric field, i.e. piezomagnetic phase causes polarization while piezoelectric phase causes changes in magnetization. The figure of merit of such multiferroic particulate composites is the magnetoelectric voltage coefficient representing the amount of polarization induced by a given magnetic field. In the present investigation we prepared and studied the properties of bulk particulate composites made from cobalt ferrite (COF) and a soft piezoelectric material (PZT). Composite samples, according to the formula xCOF center dot(1-x)PZT with 0 <= x <= 1 were prepared by mixing the COF and PZT powders, pressing them as discs and then sintered at 1200 degrees C for 3 hours. Structural, magnetic, piezoelectric and magnetoelectric characteristics of the sintered composites were determined as a function of the mass fraction of COF into PZT and magnetic field. The ME coefficient shows a maximum value of about 70 mV/cm.Oe for composites containing 40 % COF and 60 % PZT, in a DC magnetic field of 3 kOe at a frequency of 1 KHz of the ac magnetic field. Some of the dielectric and piezoelectric properties of the composite samples, as a function of the mass fraction x, were also determined.

Due to its wide band-gap (ca. 3.4 eV), ZnO is a possible candidate material to be used as transparent electrode for a new class of photovoltaic (PV) cells. Also, an increased interest for the photovoltaic properties of several organic monomers and polymers (merocyanines, phthalocyanines and porphyrins) was noticed, because of their high optical absorption in the visible region of the spectrum allowing them to be used as potential inexpensive materials for solar cells. Preparation and properties of CuPc (copper phthalocyanine) based photovoltaic cells using ZnO thin films as transparent conductor electrodes are presented in this paper. ZnO layers are grown by pulsed laser deposition, while the organic layers are obtained by thermal evaporation. Structural characterization is performed by electron microscopy. Optical and transport properties of the mutilayered structures are obtained by electrical and spectro-photometric measurements. The influence of the ZnO-polymer interface on the external quantum efficiency (EQE) of the photovoltaic cell is clearly evidenced by our measurements.

The paper describes a four-band, small-dimensions printed antenna intended to be integrated into modern wireless systems. The printed antenna has resonant slots, at slightly different frequencies. in order to provide a simultaneous response at four frequency bands in the operating spectral region. This type of antennas is proposed for the emerging multi-mode multi-band transceivers, which operate over a wide range of bands required by modern telecommunication systems. Measured data including return loss, antenna gain and radiation patterns are in good agreement with the simulated data.

Nano-sized iron oxide-based particles have been directly synthesized by the laser induced pyrolysis of a mixture containing iron pentacarbonyl/air (as oxidizer)/ethylene (as sensitizer). In this paper we further demonstrate the possibility to vary the chemical composition and the nanoparticle dimensions of the iron oxide-based materials by handling the oxidation procedure in the frame of the laser pyrolysis process. Thus, nanoparticles with major maghemite/magnetite content may change composition into mixtures with variable amounts of three components: major gamma-Fe2O3/Fe3O4 iron oxide, metallic Fe and cementite Fe3C. By X-ray diffraction (XRD) it is found that the relative proportion of these phases differs in function of the reaction temperature (laser power). As revealed by transmission electron microscopy (TEM), mean particle sizes between about 4 nm and 6 nm and between about 9 and 11 nm may be prepared by varying the oxidation procedure and the laser power, respectively. By the controlled heating of samples (maximum temperature 185 degrees C), increased crystallinity for the gamma-Fe2O3/Fe3O4 oxide phase was found as well as an increase of the mean particle diameters. The examination of the magnetization curves for samples obtained for different laser powers indicates notable differences in the magnetic behavior and parameters. The temperature dependent Mossbauer measurements confirm the formation of larger particles at higher laser power densities as well as the presence of inter-particle magnetic interactions. On this basis, the estimation of phase composition for the different representative samples is given.