National Institute Of Materials Physics - Romania

Impedance spectroscopy studies on polyaniline doped by various amounts of HCl are reported. The measurements were performed at room temperature, over a wide range of frequencies [1000 Hz to 45 MHz]. The experimental data shows that the resistance dominates the AC behavior of polyaniline confirming that the charge transport occurs through an one-dimensional hopping process.

Dopant yttrium at 150 p.p.m. level (Y to Al atomic ratio) in ultra -high purity polycrystalline alumina has been found to be, together with the coefficient of porosity, the reason of grain growth : with abnormal grain growth (AGG) or with uniform grains. The powder, green bodies and sintering samples were studied by transmission electron microscopy (TEM), scanning electron microscopy (SEM), and packing models. In this paper we studied by mathematical modeling the correlation between structure of powder and green bodies in correlation with the porosity in sintering compacts. A formula, which describes the packing of the spherical-cylindrical homogeneous particles, has been obtained. The porosity of the low Y doped alpha-Al2O3 was investigated.

Modelling of several large molecular clusters possible to be formed in non-crystalline arsenic chalcogenides (As2S3, As2Se3 or mixed compositions) during cooling down of the melts has been performed. The structure of minimum free energy was calculated in the frame of valence force field theory using a Monte-Carlo-Metropolis method. The structural characteristics of the clusters were computed after energy relaxation. There was demonstrated that specific cluster configurations of the type nano-hat, nano-pie and nanotube are crystallo-chemically compatible with the non-crystalline structure of the arsenic chalcogenides.

The possibility to enhance the self-nonlinear emission characteristics in Nd-activated nonlinear crystals by direct pumping into the emitting level F-4(3/2) is discussed. It is inferred that the reduction of threshold and the increase of the slope efficiency for the intracavity emission at the fundamental frequency caused by the reduction of quantum defect between the pump and the emitted quanta in the case of direct pumping are strongly amplified in the self-nonlinear processes. Moreover, the diminished heat generation could contribute to a more stable emission and to the scaling to a higher power. The possibility to use the direct pumping is determined by the spectroscopic properties of the laser material. This is illustrated for Nd3+ in Ca4GdO(BO3)(3), where, for direct pumping, the thermally activated transition I-4(9/2)(Z(2)) --> F-4(3/2)(R-1) at 887 nm is suitable. Pumping in this band induces an enhancement of the self-doubled emission power at 530 nm by similar to2.5 times for an absorbed power four to five times above threshold as compared with the 811-nm pumping into the level F-4(1/2) of Nd3+. (C) 2004 Optical Society of America.

The atomic structure of dislocations in cubic boron nitride has been investigated by high resolution transmission electron microscopy. Most of the perfect dislocations, screw and 60degrees edge, are dissociated. A 60degrees dislocation which was undissociated has been analysed. Computer simulation is performed in an attempt to characterise the core structure. Twinning dislocations and dislocations resulting from the intersection of stacking faults are also revealed. (C) 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

New magnetic composite materials, ribbon samples, with nominal compositions FexCu1-x (x=0.2; 0.5; 0.8) were obtained by the melt spinning technique. Various information was provided via X-ray diffraction, Mossbauer and magnetic measurements. The structural aspects: phase composition, size and texture effects, have been corroborated with magnetic properties. Finally, the possibility to choose the adequate composition and the suitable processing in order to obtain desired anisotropic magnetic materials were presented.

Selenium-sulphur layers prepared by warm pressing or deposited by pulsed laser deposition (PLD) on silicon wafers were subjected to laser pulses in order to test the possibility of recording the optical information in these glassy materials. There was demonstrated that the contrast of reflectivity between the irradiated places and the amorphous matrix could be used for storage of information at a reasonable density.

The crystalline structure, the interlayer and the epitaxial growth stage of PZT/IrO2 deposited by sol-gel method, were investigated using: X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM) (in bright field (BFTEM) and in dark field (DFTEM)), in correlation with selected area electron diffraction (SAED), high resolution electron microscopy (HRTEM) and X-ray energy dispersive spectroscopy (EDS). On the IrO2 electrodes, preferential (110) orientation was obtained. The crystalline growth of PZT film depends on the initial state of surface at the initial moment and on the relationship between PZT and bottom electrode IrO2. In particular, the interlayer Ir3Ti, formed between PZT and IrO2 plays an important role in the epitaxial growth of PZT film.

We have investigated the effect of Li2CO3 addition on the transport and magnetic properties of Bi1.7Pb0.4Sr1.5Ca2.5Cu3.6Ox, samples in connection with the structural changes observed by scanning electronic microscopy (SEM) and X-ray diffraction. In addition to the results reported in literature, we have extracted new information from the magnetic hysteresis loops and AC susceptibility measurements. One such information is the splitting of the AC loss peak corresponding to the dissipation inside the Bi-2223 grains. The results reported here have been compared with those obtained for Bi1.7Pb0.4Sr1.5Ca2.5Cu3.6Ox/(LiF)(y) and Bi1.7Pb0.4Sr1.5Ca2.5Cu3.6Ox/(LiCl)(y) specimens, and the effect of halogen is pointed out. (C) 2004 Elsevier B.V. All rights reserved.

Cubic boron nitride (c-BN) crystalline superabrasive powder (Borazon** CBN 400), consisting of 200-300 microns sized amber colored crystallites prepared by HP/HT synthesis, has been examined from 2.1 K to 293 K by X-band ESR spectroscopy. The observed spectrum consists of a component line A1, visible in the whole temperature range, and two component lines A2 and A3, visible at high and low temperatures, respectively. The A1 and A3 lines originate from transitions inside S = 1/2 ground states of distinct paramagnetic species and A2 from transitions inside an excited state of another paramagnetic center. The intensity of the A1 and A3 lines changes differently during in situ low temperature illumination in the UV-VIS range. (C) 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.