Electron spin resonance of Gd3+ in the normal state of RNi2B2C (R=Y,Lu)

Electron spin resonance (ESR) of Gd3+ in the normal state (T>T-c) of R1-xGdxNi2B2C (R=Y,Lu) is reported. The results show that the exchange coupling between the rare-earth localized magnetic moment and the conduction electrons depends on the conduction electrons momentum transfer (\k(F)(in)-k(F)(out)\ = q), i.e., J(fs)(q). The temperature dependence of the ESR linewidth yields a value for one of the exchange parameters, [J(fs)(2)(q)](EF)(1/2), which is in agreement with that estimated from the slope of the initial linear decrease of T-c by the Gd3+ impurities. These results indicate that the R1-xGdxNi2B2C (R=Y,Lu) compounds behave as conventional BCS superconductors, in agreement with previous reports. [S0163-1829(98)02806-9].5763668367

Pressure effects on the structural and superconducting transitions in La₃Co₄Sn₁₃

La3Co4Sn13 is a superconducting material with transition temperature at Tc = 2.70 K, which presents a superlattice structural transition at T ∗ ≃ 150 K, a common feature for this class of compounds. However, for this material, it is not clear that at T ∗ the lattice distortions arise from a charge density wave (CDW) or from a distinct microscopic origin. Interestingly, it has been suggested in isostructural non-magnetic intermetallic compounds that T ∗ can be suppressed to zero temperature, by combining chemical and external pressure, and a quantum critical point is argued to be observed near these critical doping/pressure. Our study shows that application of pressure on single-crystalline La3Co4Sn13 enhances Tc and decreases T ∗ . We observe thermal hysteresis loops for cooling/heating cycles around T ∗ for P & 0.6 GPa, in electrical resistivity measurements, which are not seen in x-ray diffraction data. The hysteresis in electrical measurements may be due to the pinning of the CDW phase to impurities/defects, while the superlattice structural transition maintains its ambient pressure second-order transition nature under pressure. From our experiments we estimate that T ∗ vanishes at around 5.5 GPa, though no quantum critical behavior is observed up to 2.53 GPa

Selective hydrogenation of dimethyl adipate on titania-supported RuSn catalysts

Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES)The hydrogenation of dimethyl adipate over RuSn/TiO(2) catalysts was studied to examine the effect of tin species and titania on selective production of 1,6-hexanediol. The bimetallic catalysts were prepared by co-impregnation and the reaction was carried out in a liquid phase semi-batch reactor at moderate conditions (255 degrees C and 50 bar). The optimal Sn/Ru ratio to reach maximum selectivity changed according to the catalyst reduction temperature. A remarkable selectivity of 70% of 1,6-hexanediol was accomplished over the catalyst with Sn/Ru ratio of 2 and reduced at 400 degrees C. The characterization of the bimetallic catalysts reduced at such a high temperature was performed by Mossbauer spectroscopy and it was found that all systems presented a similar distribution of tin species and no entity revealed to be preferentially formed. The establishment of strong-metal-support interaction and thus a synergistic effect between TiO(x) moieties and Sn(n+) species was Suggested to determine the reaction pathway. (C) 2008 Elsevier B.V. All rights reserved.3531101106Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES)Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES

Role of catalyst preparation on determining selective sites for hydrogenation of dimethyl adipate over RuSn/Al2O3

The hydrogenation of dimethyl adipate over RuSn/Al2O3 catalysts was studied. Attention was given to the effects of the preparation method and pretreatment conditions on the selective production of 1,6-hexanediol. The catalysts were prepared by co-impregnation and sol gel methods. The liquid phase catalytic reaction revealed that hexanediol can only be produced over the catalyst obtained by co-impregnation and calcined at low temperature. This system allowed 49% selectivity at practically total ester conversion. No diol could be formed when the same system was formerly calcined at 600 degrees C or over the sol gel catalyst. Mossbauer spectra revealed the occurrence of the same tin species in all catalysts; the distinct performance was thus attributed to the location of such species on the surface as evidenced by XPS. The results presented in this work indicated that alumina-supported Ru-Sn bimetallic catalysts may indeed be appropriate for 1,6-hexanediol production from DMA. However, the generation of tin oxidic species is not solely decisive for the formation of catalytically active sites. A suitable preparation method must be applied in order to better localize the SnOx moieties and tailor the required Ru-SnOx selective sites. (c) 2006 Elsevier B.V. All rights reserved.25341671626

Anomalous variation of coercivity with annealing in nanocrystalline NiZn ferrite films

The sputter deposited NiZn ferrite thin films were studied as a function of annealing temperature. The magnetization showed a monotonic increase with increasing annealing temperature. The coercivity shows a minimum at annealing temperature of 400 degreesC and shows a value of 14 Oe. Transmission electron microscopy study indicated that the grain size increases from similar to3 nm for the as-deposited case to similar to15 nm for the film annealed at 800 degreesC. The observed coercivity behavior could be attributed to the defects present in the films, the change in cation distribution, and the grain growth. (C) 2002 American Institute of Physics