National Institute Of Materials Physics - Romania

Publications

5975 articles found

5431. 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.

5432. 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.

5433. 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.

5434. 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.

5435. 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.

5436. SPECTROSCOPY OF MN-2+ IONS IN CDCL2

Authors: GHIORDANESCU, V; VODA, M; PEDRINI, C

Published: APR 16 1993, CHEMICAL PHYSICS LETTERS, 205, 414, DOI: 10.1016/0009-2614(93)87143-Q

An investigation of ultraviolet absorption and excitation spectra of Cd1-xMnCl2 Crystals is reported. At lower temperatures, T < 77 K, it is shown that a number of lines are associated with the manganese ions. No assignments are given for room temperature excitation spectra because the Mn2+ states are in the fundamental absorption edge.

5437. EXISTENCE OF EXPONENTIALLY LOCALIZED WANNIER FUNCTIONS FOR NONPERIODIC SYSTEMS

Authors: NENCIU, A; NENCIU, G

Published: APR 15 1993, PHYSICAL REVIEW B, 47, 10115, DOI: 10.1103/PhysRevB.47.10112

For a large class of nonperiodic systems, exponentially localized Wannier functions, associated with isolated parts of the energy spectrum, are proved to exist.

5438. UNUSUAL SCALING OF THE SPECTRUM IN A DETERMINISTIC APERIODIC TIGHT-BINDING MODEL

Authors: DULEA, M; JOHANSSON, M; RIKLUND, R

Published: APR 1 1993, PHYSICAL REVIEW B, 47, 8551, DOI: 10.1103/PhysRevB.47.8547

Local and global scaling properties of the integrated density of states of the tight-binding Rudin-Shapiro model are numerically derived by investigating the dependence of the bandwidths of its periodic approximants on the size of the unit cells. Scaling relations intermediate between the power and exponential laws are found for various values of the energy and amplitude of the on-site potential V. An analysis of the global properties of the spectrum performed in the case when V is equal to the hopping integral t points out its multifractal structure. Multifractal arguments together with earlier results concerning the nature of the wave functions indicate a pure point spectrum for V greater t, while for smaller values of the amplitude V the spectrum reveals a mixed character.

5439. THE MECHANISM OF THE INTERACTION BETWEEN CO AND AN SNO2 SURFACE - THE ROLE OF WATER-VAPOR

Authors: BARSAN, N; IONESCU, R

Published: MAR 15 1993, SENSORS AND ACTUATORS B-CHEMICAL, 12, 75, DOI: 10.1016/0925-4005(93)85017-5

A model for the interaction between CO and an SnO2 surface, which takes into account the role of water vapour, is presented. An assumption, supported by literature data, about the interaction of water vapour with the SnO2 surface as a generating process of chemisorption sites for oxygen is made. By means of the quasi-chemical reaction formalism and of an originally written rule of conservation of the number of electrons, we find an equation in agreement with experimental data. We conclude that in the conditions in which the model is valid (working temperature of about 600 K, etc.) the reacting chemisorbed species of oxygen is doubly ionized oxygen atoms and that their interaction with reducing gases is enhanced by the presence of water vapour.

5440. COMPLEX CARBIDES IN SYNTHETIC DIAMOND CRYSTALS PRODUCED AT SIMILAR-TO 5.5 GPA

Authors: PAVEL, E; BALUTA, G; BARB, D; LAZAR, DP; MORARIU, M; POPESCU, M; SORESCU, M

Published: MAR 15 1993, JOURNAL OF MATERIALS SCIENCE, 28, 1647, DOI: 10.1007/BF00363361

Characterization of (Fe1-xNix)3C and (Fe1-xCOx)3C identified in metallic inclusions in synthetic diamond crystals grown in Fe-Ni-C and Fe-Co-C systems at a pressure of approximately 5.5 GPa and a temperature of approximately 1800 K was carried out using X-ray microprobe analysis, X-ray diffraction, hardness data, Mossbauer and magnetic measurements. The result suggests that in (Fe1-xCox)3C, metallic atoms have an electronic structure similar to that of Co.