National Institute Of Materials Physics - Romania

The molecular structures for N-phenyl-N'-acetylurea, N-phenyl-N'-capronylthiourea, and N - phenyl - N' - benzoyl thiourea have been characterized by X-ray diffraction and NMR spectroscopy. The relative positions of atoms in the parent molecular fragment, common to all three analyzed compounds, are discussed. (C) 1998 Elsevier Science B.V.

Co, Ni, Mn, Cr, and Cu substituted magnetites were prepared by the hydrothermal method at 300 degrees C, with concentrations ranging from 8.2% to 12.5%. Transmission electron microscopy determined the average particle diameter (Phi) to be in the hundred of nm range and the morphological modifications induced by the various substitutions employed, Hysteresis loop measurements were performed to determine the coercive field H-c, and saturation magnetic moment m(s). While H-c decreased with increasing (Phi), the particle shape was found to play an important role in explaining the dependence of m(s) on (Phi). Transmission Mossbauer spectroscopy was used to determine the site preference of the substitutions and their effect on the hyperfine magnetic fields. The room-temperature Mossbauer spectra were analyzed assuming a random distribution of substitutents using the binomial distribution from the ionic crystal point of view. Superparamagnetic particles were observed at room temperature in the case of Cu and Cr substituted magnetites. (C) 1998 American Institute of Physics.

The practical importance of these studies comes from the fact that, the local degradation of dielectric layers due to the treeing discharge leads to the increment of energy losses and finally to the electric breakdown of the films. The thickness of the deposition layers was 1000 Angstrom. For the application of high voltage, we used the needle-plane system. The Mirror Electron Microscopy (MEM) investigations in polyethylene films without the application of high voltage have shown electrical inhomogeneities. After the application of high voltage (4 to 12 kV), thin polyethylene films have no electrical inhomogeneities because the sites corresponding to local conduction filaments are the starting points for pre-breakdown phenomenon. (C) 1998 Elsevier Science S.A.

The non-ionising energy loss and the concentration of primary radiation defects induced by charged pions, in the energy range 50-500 MeV, in bulk diamond have been calculated in order to characterise its radiation resistance. The mechanisms by which the pion imparts energy to the diamond lattice have been analysed and their contribution has been evaluated starting from the nuclear data on pion-carbon interaction. The energy partition of the primary recoil nuclei between ionisation and displacements has been calculated in the frame of the Lindhard theory. It has been found that diamond, from the point of view of concentration of primary defects, is approximately one order of magnitude harder than silicon in pion fields, in the energy range of the Delta(33) resonance, and about two orders of magnitude harder than GaAs in the same energy region. (C) 1998 Elsevier Science B.V. All rights reserved.

Models are presented which are compatible with the Rietveld method for the anisotropic broadening of the diffracted peaks produced by crystallite size and microstrain. The models, derived from general crystallographic considerations, are different for different Laue groups.

Polyethylene thin films of different thickness have been obtained by evaporation method, We have produced the treeing discharge by electron beam irradiation. The films have been examined by transmission electron microscopy (TEM) and electron diffraction (ED), before and after irradiation. It is observed that: (1) the polyethylene thin films have a structure of the fine spherulites before irradiation; (2) structural damages (channels, treeing figures and zones in which the material was melted and resolidified) accompanying the space charge produced in thin polyethylene films. (C) 1998 Elsevier Science S.A.

In this paper we have considered a Gaussian polymer chain of length L as an intrinsic object enclosed on a surface embedded in the Euclidean space. When the surfaces are the sphere SD-1 in D dimensions and the cylinder, the cone and the curved torus in R-3. We have calculated analytically and numerically (using the diffusion equation and the path-integral approach) the probability distribution function G(R\R';L) of the end-to-end vector R-R' and the mean-square end-to-end distance ((R-R')(2)) of the polymer chain. Our findings are that the curvature of the surfaces induces a geometrical localization area; at short scales (L-->0) the polymer is locally flat and the mean-square end-to-end distance is just the Gaussian value Ll (l = Kuhn length), independent of the metric properties of the surface; at large scales (L-->infinity), ((R-R')(2)) tends to a constant value in the sphere case, and it is linear in L for the cylinder and reaches different constant values las a function of the geometry of the surface) for the curved torus. In the case of the cone, contraction of the chain is induced at all length scales by the presence of the vertex, as a function of the opening angle 2 alpha and the end position R' of the chain. Explicit crossover formulas are derived for G(R\R';L) and ((R-R')(2)).

The effect of thermal and epithermal neutron irradiation on the superconducting critical temperature and critical current density of some Li-doped YBa2Cu3O7-x, samples was studied. The critical temperature exhibits a peak and the critical current density a valley in their dependence on neutron fluence, for moderate dose. A simple model, based on the Van Hove scenario and the kinetics of the defect production, is used to describe both phenomena.

The systems Li1-xMn2-xO4-delta acid Li1-xNi1+xO2 were studied as oxidic cathode materials for 4 V rechargeable Li-ion cells. From a comparison of the results it follows that the spinel system can be subdivided into three regions: in region I (0 less than or equal to x < 0.05) the electrochemical performance is unsatisfying due to a large volume reduction of about 7.5%, inducing stress, multiphase materials and capacity fading. Region II (0.05 less than or similar to x less than or similar to 0.2) contains the best cathode material with a composition near x = 0.1. The spinel framework is stable against Li extraction. In region III (0.2 less than or equal to x less than or equal to 1/3) the capacity is too low for an application in the 4 V region. In the system Li1-xNi1+xO2 the highest capacity is observed for LiNiO2 with x = 0. Experimental difficulties during material synthesis were overcome by suppression of the decomposition of LiNiO2 into Li1-xNi1+xO2, Li2O and O-2. (C) 1998 Elsevier Science S.A. All rights reserved.

Iron-containing pyrochlores, having ions with various valencies on the B-sites, were investigated by magnetic measurements and Mossbauer spectroscopy. The influence of the ionic radius and of the valence state of the different cations accommodated on A or B sites on the super-exchange interactions and local distortions was emphasized. Three local configurations of iron were given as evidence in agreement with a model based on a random distribution of ions over the B-sites. (C) 1998 Elsevier Science S.A.