National Institute Of Materials Physics - Romania

The contrast variation method, applied in a neutron small angle scattering experiment, was used to simultaneously obtain information about molecular and magnetic structure of ferrofluids. Limitations caused by size polydispersity of particles were considered. (C) 1999 Elsevier Science B.V. All rights reserved.

We have considered the stationary state of a one-dimensional Gaussian polymer chain of length Nl (N is number of segments, l is the Kuhn length) subjected to a one-parameter class of exactly solvable, symmetric, repulsive potentials with multiple (two or three) minima. We have calculated analytically and numerically the exact Green's function G(R\R-';L) and the mean-square end-to-end distance [(R-R-')(2)], respectively, of the polymer chain. The instanton approximation is translated in polymer physics language and used to analyze the conformation of the polymer chain in its ground state, by evaluating the average number of folds that connect the potential minima. All quantities are functions of the barrier height and of the separation between wells. Our results show that for a given length N, the polymer expands with increasing barrier width until a maximum value of [(R-R-')(2)] is reached. Afterwards, the polymer apparently collapses in one of the wells. This behavior defines a critical length N* and may offer the possibility of applications in separation and pattern recognition processes. (C) 1999 American Institute of Physics. [S0021-9606(99)50122-0].

Soft type piezoceramics with stoichiometric composition Pb(Nb0.02Li0.007Zr0.51Ti0.463)O-3 were prepared by the usual ceramic technique. Several samples with lead oxide in excess of up to 4% wt, were prepared. Careful measurements of the density of the sintered disc-shaped materials and the piezoelectric properties (piezoelectric voltage constant, g(31), piezoelectric strain constant, d(31), dielectric permittivity,epsilon(r), electromechanical planar coupling coefficient, k(p), mechanical quality factor, Q(m)) were carried out. All piezoelectric properties were spectacularly enhanced by addition of small amount of lead oxide. The maximum values were reached for samples containing 2-3 wt.% PbO, regardless the sintering temperature. The experimental results were discussed in terms of the two competitive processes: structural rearrangement and dissolution-reprecipitation, which are active in the sintering process in the presence on the excess PbO liquid phase. The last process seems to be dominant and explains the improvement on the piezoelectric properties of PZT materials.

Thin films prepared by pulsed laser deposition and single crystal implanted LiNbO3 wafers used in optoelectronic technology were studied by transmission electron microscopy. The use of high electron beam density for high resolution imaging induced a decomposition effect in the implanted LiNbO3 lattice. It is shown that the most probable decomposition leads to the formation of NbO2 nuclei by a topotactic solid state reaction along the pseudo-cubic directions of the lithium niobate matrix. It is also shown that powdered LiNbO3 specimens or samples obtained by preparation of cross section of LiNbO3 thin films are more stable in the electron beam, compared with the samples obtained from implanted LiNbO3 wafers.

The effect of low amounts of dysprosium, samarium and manganese (0.1 - 0.5 at.%) on the properties of the chalcogenide glassy semiconductors As2S3 and As2Se3 is investigated The metals have significant influence on the structural, optical, electrical and photoelectric characteristics. The fundamental absorption edge shifts towards longer wavelengths with doping. The largest shift was observed for As2S3 glasses doped with Dy and Mn. The conductivity is significantly increased by addition of Mn and Dy. All the dopants hamper the photoconductivity with the exception of 0.5 at. % Dy in As2Se3 which generates a broad secondary band in the photoconductivity spectrum near 1.05 eV, ascribed to small amounts of crystalline phase. Close correlation was revealed between the medium range order in the network, the metal electronegativity and the activation energy of conductivity.

Structural and magnetic properties of melt-spun Cu-Co-(Fe-Si) ribbons, as-quenched and after appropriate annealing, are investigated and correlated with their magnetoresistive properties. The effect of structural refinement of the magnetic phases and of the coexistence of multiple magnetic phases on the magnetoresistance is discussed. The existence of an optimum in the annealing conditions for obtaining a significant magnetoresistive effect, is inferred. (C) 1999 Elsevier Science B.V. All rights reserved.

A reactive DC magnetron discharge produced in a TIN deposition system was investigated using optical emission spectroscopy. The influence of He, Ne and Kr addition into an Ar + N-2 working atmosphere was investigated. Addition of small percentages of He or Ne in the working atmosphere of Ar + N-2, caused a decrease of the N-2(+) (B)/N-2(C) ratio, whle with Kr addition the ratio increased. The known decrease of the relative intensity of Ti lines for N-2 addition was also observed for He addition, while it increases when Ne and Kr are added. XRD analysis of TiN thin films revealed that He addition determines the same effect as N-2. This behavior is explained by N and N+ density increase for He addition. (C) 1999 Elsevier Science Ltd. All rights reserved.

We consider the class of Hamiltonians composed by a quadratic form in the canonical operators and a linear interaction term with time periodic external fields of an arbitrary strength. The Hamiltonian can be made time independent by a unitary transformation in the extended Hilbert space, which has the meaning of a time-dependent translation in the classical phase space. We solve the eigenvalue problem for the Floquet Hamiltonian and we find general expressions for the time-dependent wave functions, of interest for the optical and transport properties of quantum nanostructures.

Cyclic voltammetry and impedance spectroscopy were used for study of polyaniline in acid solutions. Impedance spectra were presented both as Bode diagrams and Nyquist representations. The capacitive charging behaviour and anion (ClO4-, HSO4-) insertion/elimination reaction of the polymer film were discussed on the basis of a simplified Randles circuit model. The examined scanning electron micrographs have also shown that the nature of dopant anion changes the film morphology.

Metal structures of diffusion limited aggregation type were obtained by controlled electrodifussion in quartz crystal without macroscopic damages, Conditions of temperature, applied voltage and the form, shape and dimension of electrodes were found, in order to obtain fractal branches. Similarity with fractal DLA structures are described and discussed.