Current-induced transition in atomic-sized contacts of metallic alloys

We have measured conductance histograms of atomic point contacts made from the noble-transition-metal alloys CuNi, AgPd, and AuPt for a concentration ratio of 1:1. For all alloys these histograms at low-bias voltage (below 300 mV) resemble those of the noble metals, whereas at high bias (above 300 mV) they resemble those of the transition metals. We interpret this effect as a change in the composition of the point contact with bias voltage. We discuss possible explanations in terms of electromigration and differential diffusion induced by current heating.

The basis for the formation of stable metal clusters on an electrode surface

Metal nanoclusters can be produced cheaply and precisely in an electrochemical environment. Experimentally this method works in some systems, but not in others, and the unusual stability of the clusters has remained a mystery. We have simulated the deposition of the clusters using classical molecular dynamics and studied their stability by grand-canonical Monte Carlo simulations. We find that electrochemically stable clusters occur only in those cases where the two metals involved form stable alloys.

Chemoenzymatic synthesis of chiral 4,4 '-bipyridyls and their metal-organic frameworks

The first enantiopure 4,4'-bipyridyls, 6, 8, and 9 have been prepared in four or five steps via bacterial dioxygenase-catalysed cis-dihydroxylation of 4-chloroquinoline 1 and C-C coupling; ligands 6 and 9 are found to be effective building blocks for the preparation of chiral metal-organic frameworks as demonstrated with the rational synthesis of two pillared-grid structures [Zn-2(fumarate)(2)(L)], which exhibit interesting structural and dynamic aspects.

A metal complex that imitates a micelle

A metal complex with a micelle-like, core-shell structure adopts higher nuclearity in water than in organic solvents, thereby imitating also the growth of a micelle, but through covalent rather than non-covalent aggregation.

Modelling and characterization of ultrasonic consolidation process of Aluminium alloys

Ultrasonic consolidation process is a rapid manufacturing process used to join thin layers of metal at low temperatures and low energy consumption. In this work, finite element method has been used to simulate the ultrasonic consolidation of Aluminium alloys 6061 (AA-6061) and 3003 (AA-3003). A thermomechanical material model has been developed in the framework of continuum cyclic plasticity theory which takes into account both volume (acoustic softening) and surface (thermal softening due to friction) effects. A friction model based on experimental studies has been developed, which takes into account the dependence of coefficient of friction upon contact pressure, amount of slip, temperature and number of cycles. Using the developed material and friction model ultrasonic consolidation (UC) process has been simulated for various combinations of process parameters involved. Experimental observations are explained on the basis of the results obtained in the present study. The current research provides the opportunity to explain the differences of the behaviour of AA-6061 and AA-3003 during the ultrasonic consolidation process. Finally, trends of the experimentally measured fracture energies of the bonded specimen are compared to the predicted friction work at the weld interface resulted from the simulation at similar process condition. Similarity of the trends indicates the validity of the developed model in its predictive capability of the process. © 2008 Materials Research Society.

Advanced Ceramic Substrate with Ordered and Designed Micro-Structure for Applications in Automotive Catalysts

This study describes an innovative monolith structure designed for applications in automotive catalysis using an advanced manufacturing approach developed at Imperial College London. The production process combines extrusion with phase inversion of a ceramic-polymer-solvent mixture in order to design highly ordered substrate micro-structures that offer improvements in performance, including reduced PGM loading, reduced catalyst ageing and reduced backpressure.

This study compares the performance of the novel substrate for CO oxidation against commercially available 400 cpsi and 900 cpsi catalysts using gas concentrations and a flow rate equivalent to those experienced by a full catalyst brick when attached to a vehicle. Due to the novel micro-structure, no washcoat was required for the initial testing and 13 g/ft3 of Pd was deposited directly throughout the substrate structure in the absence of a washcoat.

Initial results for CO oxidation indicate that the advanced micro-structure leads to enhanced conversion efficiency. Despite an 79% reduction in metal loading and the absence of a washcoat, the novel substrate sample performs well, with a light-off temperature (LOT) only 15 °C higher than the commercial 400 cpsi sample.

To test the effects of catalyst ageing on light-off temperature, each sample was aged statically at a temperature of 1000 °C, based on the Bench Ageing Time (BAT) equation. The novel substrate performed impressively when compared to the commercial samples, with a variation in light-off temperature of only 3% after 80 equivalent hours of ageing, compared to 12% and 25% for the 400 cpsi and 900 cpsi monoliths, respectively.

Advances in single-shot, dry drilling of composite/metal stacks

Carbon fibre reinforced polymers (CFRP) are increasingly being used in the aerospace, automotive and defence industry due to their high specific stiffness and good corrosion resistance. In a modern aircraft, 50-60% of its structure is made up of CFRP material while the remainder is mostly a combination of metallic alloys (typically aluminium or titanium alloys). Mechanical fastening (bolting or riveting) of CFRP and metallic components has thus created a pressing requirement of drilling several thousand holes per aircraft. Drilling of stacks in a single-shot not only saves time, but also ensures proper alignment when fasteners are inserted, achieving tighter geometric tolerances. However, this requirement poses formidable manufacturing challenges due to the fundamental differences in the material properties of CFRP and metals e.g. a drill bit entering into the stack encounters brittle and abrasive CFRP material as well as the plastic behaviour of the metallic alloy, making the drilling process highly non-linear.

Over the past few years substantial efforts have been made in this direction and majority of the research has tried to establish links between how the process parameters (feed, depth of cut, cutting speed), tooling (geometry, material and coating) and the wear of the cutting tool affect the hole quality. Similarly, multitudes of investigations have been conducted to determine the effects of non-traditional drilling methods (orbital, helical and vibration assisted drilling), cutting zone temperatures and efficiency of chip extraction on the hole quality and rate of tool wear during single shot drilling of CFRP/alloy stacks.

In a timely effort, this paper aims at reviewing the manufacturing challenges and barriers faced when drilling CFRP/alloy stacks and to summarise various factors influencing the drilling process while detailing the advances made in this fertile research area of single-shot drilling of stack materials. A survey of the key challenges associated with avoiding workpiece damage and the effect these challenges have on tool design and process optimisation is presented. An in depth critique of suitable hole making methods and their aptness for commercialisation follows. The paper concludes by summarising the future work required to achieve repeatable, high quality single shot drilled holes in CFRP/alloy stacks.