Catalytic activity of nanoalloys from gold and palladium

We present a quantitative study of the catalytic activity of well defined faceted gold palladium nanoalloys which are immobilized on cationic spherical polyelectrolyte brushes. The spherical polyelectrolyte brush particles used as carriers for the nanoalloys consist of a solid polystyrene core onto which cationic polyelectrolyte chains of 2 aminoethylmethacrylate are attached. Au Pd nanoalloy particles with sizes in the range from 1 to 3 nm have been generated which are homogeneously distributed on the surface of the spherical polyelectrolyte brushes. The reduction of 4 nitrophenol has been chosen as a well controlled model reaction allowing us to determine the catalytic activity of the nanoalloys as a function of the Au Pd composition. The absorption behavior was studied by Langmuir Hinshelwood kinetics. We find a pronounced maximum of the catalytic activity at 75 molar Au. A comparison of gold, platinum, palladium and gold palladium alloy nanoparticles is made in terms of Langmuir Hinshelwood kinetics. Density functional calculations for Au Pd clusters with up to 38 atoms show that the density of states at the Fermi level increases with increasing Pd content, and that the highest occupied orbitals are associated with Pd atoms. The calculations confirm that small changes in the atomic arrangement can lead to pronounced changes in the particles electronic properties, indicating that the known importance of surface effects is further enhanced in nanoalloy

Spinodally decomposed patterns in rapidly quenched Co-Cu melts

The Co-Cu system is analyzed in the region of metastable miscibility gap with separation of the undercooled melt into Co-rich and Cu-rich liquids [1]. Phase separation of undercooled and quenched samples of Co50Cu50 melt are investigated experimentally using electromagnetic levitation (EML) technique, quenching the undercooled melt onto a Pb-solder coated copper chill substrate and by splat quenching methods. It is found that quenching of the liquid samples with cooling rates of 106-107 K/s leads to a frozen in microstructure of spinodally decomposed liquids. The composition of the Co-rich phase measured by TEM-EDS is Co71.7Cu28.3 and that of the Cu-rich phase is Co26.8Cu73.2. These compositions are inside the spinodal region and close to the calculated spinodal boundary in the phase diagram of the Co-Cu system. The spinodally separated samples have periodicity of mean distance between patterns of about 0.12-0.4 μm. Using the model of fast spinodal decomposition (see Ref. [2] and references therein), computational modelling is carried out using semi-implicit numeric scheme as described in Ref. [3]. The results of modelling confirm the ability to quantitatively reproduce experimentally frozen spinodal patterns by their periodicity. The calculated time for the complete phase separation into the Co-rich and Cu-rich phases (in evolution spinodally decomposing patterns) is greater than the time for samples solidifying at cooling rates of 106-107 K/s. References [1] M. Kolbe, C.D. Cao, X.Y. Lu, P.K. Galenko, B. Wei, D.M. Herlach, Materials Science and Engineering A 375-377, 520 (2004). [2] P. Galenko, D. Jou, Physica A 388, 3113 (2009). [3] N. Lecoq, H. Zapolsky, P. Galenko, The European Physical Journal ST 177 165 (2009)

Tensile Behavior and Evolution of the Phases in the Al10Co25Cr8Fe15Ni36Ti6 Compositionally Complex/High Entropy Alloy

Compositionally complex alloys, or high entropy alloys, are good candidates for applications at higher temperatures in gas turbines. After their introduction, the equiatomic Al17Co17Cr17Cu17Fe17Ni17 (at.%) served as a starting material and a long optimization road finally led to the recently optimized Al10Co25Cr8Fe15Ni36Ti6 (at.%) alloy, which shows promising mechanical properties. Investigations of the as-cast state and after different heat treatments focus on the evolution of the microstructure and provide an overview of some mechanical properties. The dendritic solidification provides two phases in the dendritic cores and two different ones in the interdendritic regions. Three of the four phases remain after heat treatments. Homogenization and subsequent annealing produce a γ-γ’ based microstructure, similar to Ni-based superalloys. The γ phase is Co-Cr-Fe rich and the γ’ phase is Al-Ni-Ti rich. The understanding of the mechanical behavior of the investigated alloy is supported and enhanced by the study of the different phases and their nanohardness measurements. The observations are compared with mechanical and microstructural data from commercial Ni-based superalloys, Co-based alloys, and Co-Ni-based alloys at the desired application temperature of ~800 °C

Research Update: Inhomogeneous aluminium dopant distribution in magnetron sputtered ZnO:Al thin films and its influence on their electrical properties

The spatial distribution of Al in magnetron sputtered ZnO:Al films has been investigated in depth. Two different kinds of inhomogeneities were observed: an enrichment in the bulk of the film and an enrichment at the interface to the substrate. This has been correlated to the electrical properties of the films: the former inhomogeneities can lead to trap states at the grain boundaries limiting the free carrier mobility. The latter can promote the formation of secondary phases, which leads to an electrical inactivation of the dopant. Furthermore, this effect can contribute to the thickness dependence of the electrical properties of ZnO:Al films

On the Path to Optimizing the Al-Co-Cr-Cu-Fe-Ni-Ti High Entropy Alloy Family for High Temperature Applications

The most commonly investigated high entropy alloy, AlCoCrCuFeNi, has been chosen for optimization of its microstructural and mechanical properties by means of compositional changes and heat treatments. Among the different available optimization paths, the decrease of segregating element Cu, the increase of oxidation protective elements Al and Cr and the approach towards a γ-γ′ microstructure like in Ni-based superalloys have been probed and compared. Microscopical observations have been made for every optimization step. Vickers microhardness measurements and/or tensile/compression test have been carried out when the alloy was appropriate. Five derived alloys AlCoCrFeNi, Al23Co15Cr23Cu8Fe15Ni16, Al8Co17Cr17Cu8Fe17Ni33, Al8Co17Cr14Cu8Fe17Ni34.8Mo0.1Ti1W0.1 and Al10Co25Cr8Fe15Ni36Ti6 (all at.%) have been compared to the original AlCoCrCuFeNi and the most promising one has been selected for further investigation

Microstructural investigations of bulk metallic glass using small-angle neutron scattering techniques

Bulk metallic glasses (BMG) are very attractive materials exhibiting high specific strength, decent corrosion resistance and other benefiting features due to their amorphous microstructure. However, the mechanisms of mechanical properties as an issue of structure-properties relation in BMGs are not understood as well as those in polycrystalline materials. For example, the driving force for fatigue in crystalline materials is connected to grain boundary slip and the formation of dislocations i.e. to those structural elements whose existence in BMGs is still debatable. In order to find a link between the mechanical properties and the microstructure in BMG, researchers investigate structural heterogeneities i.e. clusters. The size order of the clusters and intercluster boundaries are within the resolution of small-angle neutron scattering (SANS) techniques. Here we present the results of SANS and very-small-angle neutron scattering (VSANS) studies of Zr46.75Ti8.25Cu7.5Ni10Be27.5 bulk metallic glass after deformation with and without ultrasonic vibrations. VSANS measurements revealed the creation and growth of large micropores induced by ultrasonic vibration

Selective etching of InP in InAs/InP nanowires resulting in 11 nm nanogaps

We demonstrate a wet chemical method to selectively etch InP segments within InAs/InP heterostructure nanowires based on a photo-assisted HAc/HBr solution etching process. We successfully etched InP segments ranging from 60 nm down to about 10 nm in size