Publications

The Rayleigh scattering of Mossbauer radiation (RSMR) is a well-established and successful method for dynamics and structural investigations of ordered and disordered systems. The recoilless emission and absorption of low-energy gamma rays, combined with the high selectivity of the nuclear resonance absorption phenomenon, permits an energy resolution of the order of 10(-8) eV for the 14.4 keV transition of Fe-57. The results of our studies by RSMR on pyrolytic graphite (C) and rubidium tetrachlorozincate (Rb(2)nCl(4)) are given. The contribution of the coherent inelastic scattering to the total intensity in C(002) is described like a consequence of the coupling motions of C atoms in the hexagonal plan. The normal-incommensurate phase transition in Rb2ZnCl4 is discussed in connection with the photon-phonon interaction.

We report here on pentacene based organic field effect transistors (OFETs) with a high-k HfO2 gate oxide. HfO2 layers were prepared by two different methods: anodic oxidation and sol-gel. A comparison of the two processes on the electrical properties of OFETs is given. Ultra thin nanoporous (20 nm) sol-gel deposited oxide films were obtained following an annealing at 450 degrees C. They lead to high mobility and stable devices (mu = 0.12 cm(2)/V S). On the other hand, devices with anodic HfO2 revealed a little bit more leaky and show some hysteresis. Anodization, however, presents the advantage of being a fully room temperature process, compatible with plastic substrates. Stability and response to a bias stress are also reported. (c) 2006 Elsevier Ltd. All rights reserved.

X-Ray Photoelectron Spectroscopy (XPS) and Auger Electron Spectroscopy (AES) have been used to investigate the elemental composition, chemical and electronic state of elements that exist within the Herastrau Lake (Bucharest, Romania) sediments. XPS spectra were obtained by irradiating the sediment samples with an Al source of X-rays (1486.6 eV). A very broad peak at 165-190 eV corresponds to sulfur oxides, sulfate and sulfite ions and sulfur hexafluoride. The measurements indicate metal oxides: (Al2O3, Fe2O3) at 528-531 eV, SiO2 at 533 eV, phosphates, nitrates at 531-533 eV, silicates at 532-533 eV, CaO, CaCO3 and CaS at 346-347 eV, CaSo(4) at 348 eV, Ca(NO3)(2) at 349 eV and slight traces of NH3. A very intense peak at 784 eV also appears due to CoSO4. The analysis of Electron Spin Resonance (ESR) spectra revealed an additional free radical peak in the center of Mn2+ sextet, at a g = 2.0038, attributed to SO3- species as well a very intense and broad signal due to Fe3+ ions.

The new advanced procedure of mechanochemical synthesis was used to prepare barium titanate from barium and titanium oxides. The X-ray examination of the powders milled for different times showed the phase evolution during synthesis. The perovskite phase was initiated after 5 hours of milling and was completed after 50 hours. Sintered samples from these nanopowders show different dielectric properties depending on their morphostructure. Thus for samples with an average grain size of 2 mu m we obtained the highest dielectric constant of 5 800 at room temperature. A sharp maximum of the dielectric constant with values of about 16 000 was recorded at the Curie temperature.

A microstrip defected ground structure dual mode filter is presented. Several methods for coupling between the dual modes are investigated. Defected ground structure is used to enhance the external coupling and to introduce the necessary perturbation to the initially symmetrical structure. The proposed device is compact, requiring a surface area of about 6 times smaller than the area needed for a conventional patch dual mode resonator.

Accuracies of three methods used to determine the mechanical qualily-factors of-piezoceramic resonators were evaluated and discussed-for the specific case of five materials with different mechanical quality factors (between 20 and 3000) and electromechanical coupling coefficients (between 2.5% and 60%). The mechanical quality factor Q(m) was calculatedfor both radial and thickness modes. The three methods are: the equivalent circuit, the band width and the iterative method. The last two methods proved to be very accurate for all materials (the error of Q(m) is less than 0.1%), but for low Q(m) materials the second method requires to redefine the involved frequencies. The first method is less accurate, especially for materials with low mechanical quality and electromechanical coupling factors or large difference between the mechanical quality factors of the resonance and antiresonance bands (the error of Q(m) is more than 10-20%).

Barium strontium titanate (BST) bulk ceramic was used as target for PLD thin film deposition. In order to avoid stoichiometry modification during the deposition, a more reactive oxidant ambient in the chamber was produced by rising a radiofrequency discharge 13.56 MHz, 150 W. Thin films of stoichiometric BST was deposited on alumina substrate with the thickness between 400 and 500 nm heated at 650 degrees C. An aditional anealing was made at 800 degrees C for 6 hours. XRD and SEM were used for sample characterization. Capacity measurements at 100 kHz were performed versus temperature.

We report a microstructural and compositional study of a 90% Pb(Mg1/3Nb2/3)O-3 - 10% PbTiO3 ceramic sample. High resolution transmission electron microscopy (HRTEM) revealed the presence of compositional ordered nano-domains (1 - 4 mn), observable in two specific orientations, [0 (1) over bar 1] and [1 1 (2) over bar]. However, the visibility and the ordering degree of the nano-domains on the HRTEM images are not at all obvious. Fourier filtering of the images clearly revealed the ordered domains. Antiphase boundaries lying in the (111) planes separate them, while (100) and (111) facets separate the ordered domains from the disordered matrix. Their shape is plate-like, not uniformly spread in the disordered matrix. Their average size varies in different regions of the sample, most probably due to a non-uniform distribution of Ti, as observed by energy dispersive X-ray spectroscopy. (c) 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

In the present investigation we studied the effect of lead content on a hard type ceramics starting from lack to excess PbO. Specimens of Pb-x(Mn0.017Sb0.033Zr0.48Ti0.47)O-3 with 0.96 <= x <= 1.06 were prepared by conventional oxide mixing technique and sintered at temperatures between 1280 and 1350 degrees C. In samples with lower Ph concentration the pyrochlore phase appeared, while in those with higher Pb an excess of PbO phase was detected. These phases proved detrimental for the properties of sintered ceramics. The highest values of piezoelectric properties were observed only for samples with an excess lead of about 2% sintered at 1340 degrees C, indicating that it represents the optimum amount of PbO. For such samples, the density p was 7.77 g/cm (3). the remnant polarization P, was 37 mu C/cm(3) the planar coupling coefficient k(P) was 0.57 and the charge constant d(33) was 340 C-P/N. (c) 2007 Elsevier Ltd. All rights reserved.

2.5 mol% Y2O3-stabilised ZrO2 composites with 30 vol.% TiB2 particles, hot pressed in vacuum at 1450 degrees C, were investigated by means of quantitative electron energy loss spectroscopy (EELS) with a nanobeam analytical field emission gun transmission electron microscope (FEG-TEM). The experimental results revealed that B is diffusing out of the titanium boride grains, forming textured polycrystalline BN agglomerates leaving behind non-stoichiometric TiB2-x single crystal grains. The formation of BN was calculated to be thermodynamically favourable under the actual hot pressing conditions. Boron was also detected in the amorphous intergranular phase, together with Y, Si, O, Al, but not in the ZrO2 grains. Ti was only detected in the amorphous material in-between the TiB2-x grains and the micro-textured BN grains, but not in the zirconia grains nor in the intergranular amorphous phase in between. There are indications that Ti forms facetted carbide precipitates inside the TiB2-x grains. Zr was not detected in the titanium boride grains, nor in the intergranular amorphous phase in-between the zirconia grains. (c) 2006 Elsevier Ltd. All rights reserved.