National Institute Of Materials Physics - Romania

The electrical properties of two types of imidic polymers, i.e. copolymaleimides containing azobenzene groups and epiclon-based polyimides with their corresponding poly( amic acid)s, have been investigated. The dielectric constant and electrical conductance were experimentally measured over the 1-100 KHz frequency range. Also, the dielectric constants for all samples were determined from the experimentally and theoretically evaluated refractive indices at optical frequencies, when Maxwell's relationship can be applied. From the current-voltage characteristic I( U), electrical resistance can be determined, providing information on the relationship between the nature of the material and its electric properties. The obtained values show that epiclon-based polyimides and copolymaleimides present higher electrical resistivity than the poly( amic acid) s. The dissipation factor was evaluated, revealing low energy loss under the form of heat in the studied dielectrics. These results show that these polymeric materials are good insulators, which recommends them for microelectronic applications.

The rotational molecular dynamics of water confined to layered oxide materials with brucite structure was studied by dielectric spectroscopy in the frequency range from 10(-2) to 10(7) Hz and in a broad temperature interval. The layered double hydroxide samples show one relaxation process, which was assigned to fluctuations of water molecules forming a layer, strongly adsorbed to the oxide surface. The temperature dependence of the relaxation rates has an unusual saddlelike shape characterized by a maximum. The model of Ryabov et al. (J. Phys. Chem. B 2001, 105, 1845) recently applied to describe the dynamics of water molecules in porous glasses is employed also for the layered materials. This model assumes two competing effects: rotational fluctuations of water molecules that take place simultaneously with defect formation, allowing the creation of free volume necessary for reorientation. The activation energy of rotational fluctuations, the energy of defect formation, a pre-exponential factor, and the defect concentration are obtained as main parameters from a fit of this model to the data. The values of these parameters were compared with those found for water confined to nanoporous molecular sieves, porous glasses, or bulk ice. Several correlations were discussed in detail, such as the lower the value of the energy of defect formation, the higher the number of defects. The pre-exponential factor increases with increasing activation energy, as an expression of the compensation law, and indicates the cooperative nature of the motional process. The involvement of the surface OH groups and of the oxygen atoms of the interlayer anions in the formation of hydrogen bonds was further discussed. For the birnessite sample, the relaxation processes are probably overlaid by a dominating conductivity contribution, which is analyzed in its frequency and temperature dependence. It is found that the conductivity of birnessite obeys the characteristics of semiconducting disordered materials. Especially the Barton/Nakajima/Namikawa relationship is fulfilled. Analyzing the temperature dependence of the direct current (dc) conductivity sigma(0) in detail gives some hint that sigma(0)(T) has also an unusual saddlelike form.

Amorphous SnSe, films were prepared by pulsed laser deposition (PLD) from solid polycrystalline targets. The atomic scale structure has been revealed by X-ray diffraction. Hardness and electrical properties were measured. Strong gas sensing properties have been evidenced for the first time in heat treated films. (c) 2007 Elsevier B.V. All rights reserved.

xTiO(2)-(1- x)alpha-Fe2O3 (x = 0.0-1.0) mixed oxide nanoparticle, system ha, been synthesized by a hydothermal route, X-ray diffraction, Mossbauer spectroscopy and electron microscopy have been used to study the structure, morphology and substitution in the nanosized samples. Experimental evidence for the solubility limits of Ti4+, in alpha-Fe2O3 lattice and of Fe3+ in TiO2 structure is given. (c) 2007 Elsevier B.V. All rights reserved.

The microstructural investigation of a suction-cast Al93Fe3Cr2Ti2 bulk alloy reveals that an icosahedral phase forms together with monoclinic Al-13(Fe,Cr)(4) and tetragonal Al3Ti in an alpha-Al matrix. The icosahedral phase is often surrounded by Al3Ti particles indicating that the formation of the icosahedral phase strongly competes with the Al3Ti phase. Chemical analysis reveals that Ti has a strong influence to stabilize the Al3Ti phase. In contrast, both Cr and Fe are effective to stabilize the icosahedral phase. (C) 2006 Elsevier B.V. All rights reserved.

We show that, due to band mixing, the eigenstate localization within the disordered Landau bands gets an asymmetric structure: the degree of localization increases in the lower part of the band and decreases in the upper one. The calculation is performed for a two-dimensional lattice with the Anderson disorder potential and we prove that this effect is related to the upper shift of the extended states within the band and is enhanced by the disorder strength. The asymmetric localization and the energy shift disappear when the interband coupling is switched off.

FexOy-SiO2 nanocomposites were prepared by sol-gel method by using two SiO2 sources and Fe(SO4)(2)center dot 7H(2)O as raw materials. The amorphous gels were thermally treated up to 1000 degrees C. The initial gel and the thermally treated samples were characterized by thermo-gravimetric analysis (TGA) and differential thermal analysis (DTA) and infrared spectroscopy. The presence of hematite was confirmed by the obtained Mossbauer spectra which showed the characteristic sextet. The total amount and the size distributions of the hematite nanoparticles can be controlled via the initial precursors and subsequent by annealing conditions. (C) 2006 Elsevier B.V. All rights reserved.

Ceramic based barium strontium titanate (BST) solid solutions with the formula Ba1-xSrxTiO3 are very important candidates for a wide range device applications. Several doped (Mn and Mg) and undoped samples were prepared by standard solid-state reaction. Special emphasis was put on compositions with x=0.35 and 0.60, with high potential for applications. The samples were sintered at temperatures in the 1200 divided by 1260 degrees C range. Structural X-ray diffraction analysis preformed confirms the perovskite structure. The dielectric parameters were investigated in a wide temperature range between - 150 and 150 degrees C. The temperature was cyclically changed in both directions, up and down, at a rate of less than 2 degrees C/ min. Both permittivity and dielectric loss were measured at low frequencies, 1 kHz. The peak values of the permittivity are increasing from 2000 to 4000 with the sintering temperature increase. Moreover the dielectric parameters were measured at room temperature in microwave domain (1 divided by 2 GHz). The Curie temperature of BST samples with x=0.35 and x=0.6 is in agreement with the Curie point dependence on Sr content, as we have previously reported. (C) 2007 Published by Elsevier B.V.

A comparative study of the sol-gel films prepared in the SiO2-P2O5 system starting with triethylphosphate, triethylphosphite and phosphoric acid as P precursors was performed. The work addresses basic aspects of physics, chemistry, and engineering of oxide films for applications in microelectronics, sensing, nano-photonics, and optoelectronics by establishing the influence of different precursors on the composition, structure and optical properties of the obtained films. The influence of the type of substrate (glass and indium tin oxide coated glass) and of the thermal treatment (200 and 500 degrees C on their structure and properties was studied. By spectroscopic ellipsometry, X-Ray photoemission spectroscopy and atomic force microscopy measurements the high vaporization of the phosphorous during the densification of the films by thermal treatment was noticed when P-alkoxides were used. The phosphoric acid that forms chemical bond with silica network during the sol-gel process leads to better incorporation of P in the silica network as compared to the P-alkoxides. (C) 2006 Elsevier B.V. All rights reserved.

The sol-gel deposition method has been successfully applied to obtain Pb(Zr0.2Ti0.8)03 thin films on platinized silicon wafers. Addition of different amounts (7-15 wt.%) of organic macromolecular polyvinylpyrrolidone in the precursor solution prior to spin coating proves to be an excellent method for obtaining porous films. The crystal structure of as deposited films was analyzed by X-ray diffraction. The porous films show perovskite phase after annealing at 650 degrees C. The surface morphology has been studied by Atomic Force Microscopy and Scanning Electron Microscopy. The surface profile indicates a roughness of the film of 5 nm and no microcracks on the surface. The ferroelectric behavior was proved for each film, by hysteresis loops and by the "butterfly" shape of the capacitance-voltage characteristics. The remnant polarization and the coercive field decrease while the amount of added PVP increases. (C) 2006 Elsevier B.V. All rights reserved.