National Institute Of Materials Physics - Romania

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5840 articles found

5291. EXACT DC CONDUCTIVITY FOR A CLASS OF ONE-DIMENSIONAL INTERACTING HOPPING MODELS

Authors: PITIS, R

Published: AUG 1 1993, PHYSICAL REVIEW B, 48, 4199, DOI: 10.1103/PhysRevB.48.4196

A class of periodic one-dimensional models with nearest-neighbor interactions is presented in which particles can hop between adjacent sites with symmetric jump rates. The dc conductivity is calculated exactly for a wide choice of the factor describing the influence of the nearest-neighbor interaction on the jump rates. By allowing the cell size to become infinite, the result is extended to several nonperiodic chains including the random ones.

5292. TIME-RESOLVED NEUTRON-DIFFRACTION STUDY OF THE SUPERCONDUCTING PHASE-FORMATION PROCESS IN THE BI(PB)-SR-CA-CU-O SYSTEM

Authors: ALDICA, G; MIRONOVA, GM; POPA, NC; STOICA, AD; STOICA, MG

Published: AUG 1993, JOURNAL OF SUPERCONDUCTIVITY, 6, 278, DOI: 10.1007/BF00626269

The results of real-time neutron diffraction measurements during the superconducting phase formation process in the Bi(Pb)-Sr Ca-Cu-O system are reported. A Sr-Ca-Cu-O type precursor, with the same stoichiometry as the 2223 phase, was used as starting material, and the temperature range favorable to the formation of the 2223 phase was investigated. The diffraction patterns were processed by a multiphase Rietveld refinement. The formation and decomposition of the 2201 and 2212 phases were directly observed. Experimental evidence on the existence of a partially melted phase in the range 855-860-degrees-C, involved in the formation of the 2223 phase, is discussed.

5293. DISSIPATION IN CURRENT-CARRYING HIGH-T(C) SUPERCONDUCTING CERAMICS IN APPLIED MAGNETIC-FIELD

Authors: MIU, L; POPA, S; CRISAN, A; ALDICA, G; JAKLOVSZKY, J

Published: AUG 1993, JOURNAL OF SUPERCONDUCTIVITY, 6, 284, DOI: 10.1007/BF00626270

The dissipation mechanism in high-T(c) superconducting ceramics was studied by analyzing the magnetic field dependence of the transport critical current and the form of the current-voltage characteristics. It was found that the actual magnetic field dependence of the transport critical current is significantly slower than that predicted by the weak-link-quenching model. The low-voltage-level current-voltage characteristics were described in terms of thermally activated flux creep at grain boundaries, taking into account collective pinning of intergranular Josephson vortices. For the investigated samples, a low-field, high-temperature mean pinning energy barrier of a few tens of meV was determined.

5294. THEORETICAL AND EXPERIMENTAL-STUDY OF CYLINDRICAL GAS-FLOW CEMS DETECTORS

Authors: KUNCSER, V; FILOTI, G

Published: JUL 1993, NUCLEAR INSTRUMENTS & METHODS IN PHYSICS RESEARCH SECTION B-BEAM INTERACTIONS WITH MATERIALS AND ATOMS, 82, 191, DOI: 10.1016/0168-583X(93)95100-J

The operational range of the gas counters used in conversion electron Mossbauer spectroscopy (CEMS) is at the limit between the proportional counter and Geiger Muller domains. Starting from the theory of phenomena in gas detectors under space charge conditions, a general expression for the multiplication factor was obtained. The theoretical results, presented graphically, are discussed in connection with experimental data.

5295. CHARGE CORRELATION FACTOR OF THE RANDOM BINARY CHAIN DETERMINED BY MONTE-CARLO SIMULATION

Authors: PITIS, R

Published: JUN 1 1993, PHYSICAL REVIEW B, 47, 15293, DOI: 10.1103/PhysRevB.47.15290

We present stationary-state Monte Carlo experiments evaluating the dc conductivity of the one-dimensional lattice gas with binary energetic disorder. The resulting correlation factor is compared to previous analytic calculations.

5296. SELECTIVE HYDROGENATION OF STYRENE ON PD-HO/GAMMA-AL2O3 AND PD/GAMMA-AL2O3 CATALYSTS

Authors: PARVULESCU, VI; PARVULESCU, V; FRUNZA, L; GRECU, N; ENACHE, C; ANGELESCU, E

Published: JUN 1993, BULLETIN DES SOCIETES CHIMIQUES BELGES, 102, 399, DOI:

Pd-Ho/gammaAl2O3 catalysts exhibit higher activity for styrene hydrogenation than Pd/gammaAl2O3 ones. The addition of holmium to Pd catalysts changes the place of the palladium impregnation front depending on the state of the supported palladium. As a consequence of the interaction between holmium and Pd/gammaAl2O3 support, oxidized forms as holmium aluminate were formed. These forms are catalytically inactive but represent adsorption centers for styrene. On holmium containing catalysts, hydrogenation takes place between the styrene adsorbed on basic centers and the hydrogen spilled over from palladium particles.

5297. INCLUSION-EXCLUSION CALCULATION OF THE DIPOLE-DIPOLE ENERGY OF HEXAGONAL ICE AND OF CUBIC ICE

Authors: HUCKABY, DA; PITIS, R; KINCAID, RH; HAMILTON, C

Published: MAY 15 1993, JOURNAL OF CHEMICAL PHYSICS, 98, 8109, DOI: 10.1063/1.464566

The principle of inclusion and exclusion is adapted to derive an expansion for properties of disordered hydrogen-bonded systems. A dendritic approximation to this expansion is used to calculate the dipole-dipole energy of hexagonal ice and of cubic ice. The calculated difference in dipole-dipole energy of these open forms of ice favors the hexagonal one in stability and accounts for most of the experimental energy difference.

5298. FE-57 HYPERFINE FIELDS AND MAGNETIC-ANISOTROPY OF SM2FE17NX INTERSTITIALS WITH INTERMEDIATE NITROGEN CONCENTRATIONS

Authors: ROSENBERG, M; ZHOU, RJ; KATTER, M; SCHULTZ, L; FILOTI, G

Published: MAY 15 1993, JOURNAL OF APPLIED PHYSICS, 73, 6037, DOI: 10.1063/1.353461

From the compositional dependence of the hyperfine fields at the iron sites of Sm2Fe17Nx alloys with 0 less-than-or-equal-to x less-than-or-equal-to 2.94, evidence for cone anisotropy for x=0.4 and uniaxial anisotropy at least for x greater-than-or-equal-to 0.81 has been found. The analysis of the volume dependence of the hyperfine fields allowed us to separate contributions arising from the anisotropy of the orbital component of the iron moment and from the change of the spin moments owing to magnetovolume and bonding effects. The temperature dependence of the average hyperfine field and of the hyperfine fields at the four different nonequivalent Fe sites suggests weak itinerant ferromagnetism for Sm2Fe17N2.94.

5299. UNIVERSAL STATISTICAL BEHAVIOR OF THE COMPLEX ZEROS OF WIENER TRANSFER-FUNCTIONS

Authors: FOURNIER, JD; MANTICA, G; MEZINCESCU, A; BESSIS, D

Published: MAY 10 1993, EUROPHYSICS LETTERS, 22, 331, DOI: 10.1209/0295-5075/22/5/002

To N real random variables the sample autocorrelation coefficients, which are also the N Fourier coefficients of a measure on the unit circle are associated. The polynomials orthogonal with respect to this measure define the transfer functions of the Wiener-Levinson predictors. We show that the statistics of the zeros of those random polynomials exhibits a universal law of crystallization on a circle of radius [1 - (lnN)/2n], n being the order of the predictor. These results are supported by extensive computer experiments and backed by a theoretical scaling argument in the asymptotic domain In N << n << N. These results are independent of the nature of the noise and robust for signals of finite length N.

5300. XANES AND EXAFS INVESTIGATION OF SD HYBRID BONDS IN ALLOYS OF GOLD WITH GALLIUM, GERMANIUM AND TIN THROUGH THE SOLID-LIQUID TRANSITION

Authors: ORTON, BR; STANTON, CM; GULLEY, NA; VORSATZ, DA; MANAILADEVENYI, R

Published: MAY 1993, JOURNAL OF NON-CRYSTALLINE SOLIDS, 156, 136, DOI: 10.1016/0022-3093(93)90146-O

The XANES for the Au L(III) edge in solid Au and Au-Ga alloys have been measured using synchrotron radiation. The excess white line areas in the alloys have been compared with the same region of pure Au to give the d-state charge transfer from Au to Ga. It is in good agreement with values published for an Au-Al alloy. Backtransfer for s-electrons from Ga to Au is evident from X-ray photoelectron spectroscopy and confirms an sd hybrid bond in this alloy system. On melting, this bond persists in the molten state, but with diminished effect which is evident from the reduced coordination number obtained from EXAFS. A similar bond is found in molten AuSn, but not in an Au-Ge alloy.