Plastic Deformation of a Nano‐Precipitate Strengthened Ni‐Base Alloy Investigated by Complementary In Situ Neutron Diffraction Measurements and Molecular‐Dynamics Simulations

In situ neutron‐diffraction experiments at the spallation neutron source, simultaneously illuminating the diffraction of the matrix and the strengthening nano precipitates, allow the determination of their plastic deformation. An irreversible neutron‐diffraction‐profile evolution of the nano precipitates is observed. However, there is no conclusive trend of the nano‐precipitate peak‐width evolution subjected to the greater stress levels. Hence, in the present work, molecular‐dynamics simulations are applied to reveal the deformation mechanisms of the nano precipitate and its interaction with the surrounding matrix. The microstructure size, dislocation content, and structural parameters of the nano precipitates, quantified by X‐ray, transmission electron microscopy, and small‐angle neutron scattering, are used as the simulation input and reference. The simulation results show that there are two competing deformation mechanisms, which lead to the fluctuation of the nano‐precipitate‐diffraction widths, occurring during the higher plastic deformation stages.     

Homology modelling and molecular dynamics studies of human placental tissue protein 13 (galectin-13)

The primary structure of the newly sequence analysed placentaltissue protein 13 (PP13) was highly homologous to several membersof the ß-galactoside-binding S-type lectin (galectin)family. By homology modelling, the three-dimensional structureof PP13 was built based on high-resolution crystal structuresof homologues and also their characteristic `jellyroll' foldwas found in the case of PP13. Our model has been depositedin the Brookhaven Protein Data Bank. By multiple sequence alignmentand structure-based secondary structure prediction, we underlinedthe structural similarity of PP13 with its homologues. The secondarystructure of PP13 was identical with `proto-type' galectinsconsisting of a five- and a six-stranded ß-sheet,joined by two     

Reaction pathway studies of C6 hydrocarbons on palladium model catalysts: effects of hydrogen/hydrocarbon ratio, molecular structure, surface structure and temperature

Reaction pathways of C₆ hydrocarbons, such as hydrogenolysis, dehydrogenation, aromatization and dehydrocyclization, have been studied on small surface area palladium model catalysts, including Pd(111) single crystal and polycrystalline palladium foil. These reactions were performed in a batch reactor, at atmospheric pressure and at two different temperatures (573 and 673 K). This revised version was published online in July 2006 with corrections to the Cover Date.     

Structural peculiarities of 0.67 Pb(Mg1/3Nb2/3)O3–0.33 PbTiO3 thin films grown directly on SrTiO3 substrates

Growth of Pb(Mg_1/3Nb_2/3)O₃–33PbTiO₃ thin films by pulsed-laser deposition directly on non-conductive SrTiO₃ substrates for d₃₃-mode energy harvesters (EHs) was studied, as they offer a higher figure-of-merit than d₃₁-mode EHs. It was found that a high defect density, present in the film grown at 0.13?mbar, is manifested in the form of splitting of the (00l) peaks in X-ray diffraction, which was avoided by raising the process pressure to 0.27?mbar. Nevertheless, both films grow in a combined 2D and 3D manner, and form out-of-phase boundaries (OPBs) with a PbO rock-salt structure between the as-grown islands. It was found that the overall composition of the sample with optimized structural and functional properties was Pb_1.07Mg_0.19Nb_0.44Ti_0.32O₃, which is close to stoichiometric. The surplus of Pb is compensated by the formation of OPBs and Mg deficit maintains macroscopic electroneutrality. In-plane and out-of-plane relative permittivities of 1900 and 980, respectively, imply macroscopic out-of-plane polarization.     

Ultrasonic inspection system for powder metallurgy parts

The demand for powder metallurgy (P/M) parts in its traditional automotive market is predicted to grow, but future sector expansion depends directly upon its capability to manufacture zero-defect parts for industries such as aerospace and medicine. The lack of adequate inspection systems has important implications from the point of view of quality assurance, since it increases the costs, time and wasted material. In recent years the applicability of several techniques for the inspection of P/M parts has been investigated, such as Eddy current testing, computer tomography or X-ray imaging, but studies have revealed that all of them have deficiencies that make them unavailable for a complete and reliable flaw detection and density defect recognition. A new inspection tool has been developed based on pulse echo ultrasonic technology combined with robotics, which makes it possible to provide a global density map of sintered.     

Selective Nanocatalysis of Organic Transformation by Metals: Concepts, Model Systems, and Instruments

Monodispersed transition metal (Pt, Rh, Pd) nanoparticles (NP) in the 0.8–15 nm range have been synthesized and are being used to probe catalytic selectivity in multipath organic transformation reactions. For NP systems, the turnover rates and product distributions depend on their size, shape, oxidation states, and their composition in case of bimetallic NP systems. Dendrimer-supported platinum and rhodium NPs of less than 2 nm diameter usually have high oxidation states and can be utilized for catalytic cyclization and hydroformylation reactions which previously were produced only by homogeneous catalysis. Transition metal nanoparticles in metal core (Pt, Co)––inorganic shell (SiO₂) structure exhibit exceptional thermal stability and are well-suited to perform catalytic reactions at high temperatures (>400 °C). Instruments developed in our laboratory permit the atomic and molecular level study of NPs under reaction conditions (SFG, ambient pressure XPS and high pressure STM). These studies indicate continuous restructuring of the metal substrate and the adsorbate molecules, changes of oxidation states with NP size and surface composition variations of bimetallic NPs with changes of reactant molecules. The facile rearrangement of NP catalysts required for catalytic turnover makes nanoparticle systems (heterogeneous, homogeneous and enzyme) excellent catalysts and provides opportunities to develop hybrid heterogeneous-homogeneous, heterogeneous-enzyme and homogeneous-enzyme catalyst systems.     

Major Successes of Theory-and-Experiment-Combined Studies in Surface Chemistry and Heterogeneous Catalysis

Experimental discoveries followed by theoretical interpretations that pave the way of further advances by experimentalists is a developing pattern in modern surface chemistry and catalysis. The revolution of modern surface science started with the development of surface-sensitive techniques such as LEED, XPS, AES, ISS and SIMS, in which the close collaboration between experimentalists and theorists led to the quantitative determination of surface structure and composition. The experimental discovery of the chemical activity of surface defects and the trends in the reactivity of transitional metals followed by the explanations from the theoretical studies led to the molecular level understanding of active sites in catalysis. The molecular level knowledge, in turn, provided a guide for experiments to search for new generation of catalysts. These and many other examples of successes in experiment-and-theory-combined studies demonstrate the importance of the collaboration between experimentalists and theorists in the development of modern surface science.     

The interaction of short-chain model lubricants with the surfaces of hydrogenated amorphous carbon films

The adsorption of water and small model perfluorinated lubricants on hydrogenated amorphous carbon (aC-H) films of varying hydrogen content was investigated using thermal desorption spectroscopy (TDS). Hydrogen content of the carbon films was measured by Rutherford back scattering (RBS) and elastic recoil spectroscopy (ERS) and correlated to changes in surface free energies measured by contact angle analysis. Hydrogenated carbon films exhibiting the highest surface free energy provided a greater attractive interaction for the model lubricants. All model lubricant species studied - water (D₂O), perfluorodiethyl ether (CF₃CF₂OCF₂CF₃), perfluoropentane (CF₃(CF₂)₃CF₃), perfluorooctane (CF₃(CF₂)₆CF₃), 2,2,2-trifluoroethanol (CF₃CH₂OH), and 1,1,7-H-perfluoroheptanol (CF₂H(CF₂)₅CH₂OH)—reversibly adsorbed to the carbon surface with little chemical reaction. Increases in desorption energies with increasing chain length were observed among the adsorbates and are ascribed to increasing van der Waals interactions. Incorporation of alcoholic end groups provided an avenue of hydrogen bonding to the surface and produced an ~20 kJ/mol increase in desorption energy relative to a perfluorinated alkane of the same chain length. Ether linkages within the model lubricant provide little increase in desorption energy as fluorine substituents effectively screen the oxygen. Together these findings implicate a predominantly physisorbed state for perfluorinated lubricants on hydrogenated carbon surfaces.     

CO Poisoning of Ethylene Hydrogenation over Pt Catalysts: A Comparison of Pt(111) Single Crystal and Pt Nanoparticle Activities

The ethylene hydrogenation reaction was studied on two platinum model catalyst systems in the presence of carbon monoxide to examine poisoning effects. The catalysts were a Pt(111) single crystal and lithographically fabricated platinum nanoparticles deposited on alumina. Gas chromatographic results for Pt(111) show that CO adsorption reduces the turnover rate from 10¹ to 10^-2 molecules/Pt site/s at 413 K, and the activation energy for hydrogenation on the poisoned surface becomes 20.2 ± 0.1 kcal/mol. The activation energy for ethylene hydrogenation over Pt(111) in the absence of CO is 10.8 kcal/mol. The Pt nanoparticle system shows the same rate for the reaction as over Pt(111) in the absence of CO. When CO is adsorbed on the Pt nanoparticle array, the rate of the reaction is reduced from 10² to 10⁰ nmol/s at 413 K. However, the activation energy remains largely unchanged. The Pt nanoparticles show an apparent activation energy for ethylene hydrogenation of 10.2 ± 0.2 kcal/mol in the absence of CO and 11.4 ± 0.6 kcal/mol on the CO-poisoned nanoparticle array. This is the first observation of a significant difference in catalytic behavior between Pt(111) and the Pt nanoparticle arrays. It is proposed that the active sites at the oxide--metal interface are responsible for the difference in activation energies for the hydrogenation reaction over the two model platinum catalysts.