John C. Walmsley

Composition of β″ precipitates in Al–Mg–Si alloys by atom probe tomography and first principles calculations

The composition of β″ precipitates in an Al–Mg–Si alloy has been investigated by atom probe tomography, ab initio density functional calculations, and quantitative electron diffraction. Atom probe analysis of an Al-0.72% Si-0.58% Mg (at. %) alloy heat treated at 175 °C for 36 h shows that the β″ phase contains ∼20 at. % Al and has a Mg/Si-ratio of 1.1, after correcting for a local magnification effect and for the influence of uneven evaporation rates. The composition difference is explained by an exchange of some Si with Al relative to the published β″-Mg5Si6 structure. Ab initio calculations show that replacing the Si3-site by aluminum leads to energetically favorable compositions consistent with the other phases in the precipitation sequence. Quantitative electron nanodiffraction is relatively insensitive to this substitution of Al by Si in the β″-phase.

Formation of a zirconium-titanium based conversion layer on AA 6060 aluminium

Abstract Structure and chemistry of zirconium-titanium base conversion layers were characterised as a function of immersion time in the aqueous conversion bath to understand the mechanism of film formation. Characterisation was performed by glow discharge optical emission spectroscopy, scanning electron microscopy and transmission electron microscopy. Preferential nucleation of the zirconium-titanium oxide film and its growth occurred on and around intermetallic particles, resulting in reduced cathodic activity of the particles. Passivation of the cathodes in this manner constituted a limitation in the formation of a good quality conversion layer.

The influence of composition and natural aging on clustering during preaging in Al–Mg–Si alloys

This work provides a detailed atom probe tomography study of clustering in the Al–Mg–Si system. Focus is on separating and understanding the influence of natural aging, preaging, and alloy composition on the clustering behavior of solute atoms. Two dilute alloys with the same total solute content have been studied, one Mg-rich and one Si-rich. The detrimental effect of natural aging for these alloys is investigated by comparing directly preaged samples to samples stored at room temperature before the preaging treatment. Clusters were identified in the atom probe datasets by the maximum separation method employing heuristically determined input parameters. It was found that seven days of intermediate natural aging gave a five times lower number density of clusters as compared to direct preaging for both alloy types. The clusters were of comparable size but their compositions depended on heat treatment history. Preaging promoted the formation of clusters with an Mg:Si ratio close to 1 in both alloys, while ...

Toward Three-Dimensional Nanoengineering of Heterogeneous Catalysts

Cobalt-based Fischer-Tropsch systems are widely used to convert synthesis gas to clean hydrocarbon fuel. However, surprisingly little is known about the morphology of the catalysts on the nanoscale. Here we show that scanning transmission electron tomography reveals their true 3-D morphology and provides direct evidence that the support controls the final morphology of the catalyst. Such direct local three-dimensional measurements provide unprecedented insight into catalysis, and can henceforth transform our understanding of these complex materials.

Anodising as pre-treatment for structural bonding

Degradation of adhesive bonded aluminium profiles in various environments has been investigated in terms of the tensile strength of lap shear joints and wedge testing. Prior to adhesive bonding, the extrusions were pre-treated using various anodising processes. The results showed that anodising conditions, and thus structure and thickness of the anodised films, affect adhesion and durability of the adhesive joints. Tensile lap-shear testing of samples exposed in corrosive environments showed that hot AC anodising, using either sulphuric or phosphoric acid electrolytes, performed as well as standard PAA in spite of much shorter anodising times and no etching step prior to anodising. This was confirmed by wedge testing at high relative humidity. Wedge results, however, showed adhesive fracture of samples hot AC anodised in sulphuric acid, while cohesive fracture was found for samples anodised in phosphoric acid.

Intergranular Corrosion of Copper-Containing AA6xxx AlMgSi Aluminum Alloys

AlMgSi (AA6xxx-series) aluminum alloys are generally resistant to intergranular corrosion (IGC). However, copper may introduce susceptibility to IGC; its role was investigated by using model alloys with 0.02, 0.18, and 0.7 wt % Cu. The lowest copper-containing alloy was resistant to IGC in accelerated corrosion testing. The 0.18 wt % copper alloy showed superficial etching in the naturally aged condition and was highly susceptible to IGC in the underaged temper, but was only slightly susceptible in the peak aged or overaged condition. High-resolution field emission scanning electron microscopy imaging showed no visible grain boundary precipitation in the T4 and underaged tempers, whereas the T6 and overaged tempers had grain boundaries decorated with Cu-containing precipitates. Field emission transmission electron microscopy investigation of the underaged material showed a copper-enriched grain boundary layer and an adjacent copper-depleted zone. The reduced susceptibility to IGC upon extended artificial aging was attributed to the consumption of the copper-rich grain boundary film by the growth of grain boundary precipitates.

Geometrically confined favourable ion packing for high gravimetric capacitance in carbon–ionic liquid supercapacitors

A supercapacitor, a safe and durable electrical energy storage device with fast charge–discharge capability, will achieve more widespread use if the specific energy can be improved. However, current understanding of pore characteristic effects on gravimetric capacitance has limited the development of electrode materials. We derive a model of ion packing in cylindrical nanopores, and it quantitatively reveals the significant impact of pore geometric characteristics on the gravimetric capacitance in neat ionic liquid, which is confirmed experimentally using a series of sponge-like carbons (carbon nanosponge). With the favourable ion packing proposed by the model, the electrode using the carbon nanosponge as an active material delivered double-layer capacitances of 290 F g−1 (20 °C) and 387 F g−1 (60 °C) with an operating cell voltage of 4 V. This study also provides systematical strategies for rational design of various carbon materials and ionic liquids by optimized ion packing for ultrahigh gravimetric capacitance.

Effect of Excess Silicon and Small Copper Content on Intergranular Corrosion of 6000-Series Aluminum Alloys

The required strength and ductility of heat-treatable AlMgSi (6000-series) alloys are often obtained by alloying with either a small amount of Cu or a large excess of Si compared to the stoichiometric Mg/Si ratio corresponding to the Mg 2 Si phase. Both approaches may cause susceptibility to intergranular corrosion (IGC) as a result of unfavorable heat-treatment. Whether alloying with Cu or excess Si gives the optimal combination of mechanical properties and IGC resistance is a controversial subject. The corrosion behavior of a model alloy containing 0.2% Cu is compared with an essentially Cu-free alloy with an excess Si/Mg composition ratio (i.e., unbalanced) by using accelerated corrosion tests and electron optical characterization. In general, the Cu-containing alloy showed a higher susceptibility to IGC than the Cu-free, excess Si alloy. The Cu-containing alloy was especially susceptible in the underaged condition. The Cu-free, excess Si alloy became completely resistant to IGC by removing the cathodic intermetallic particles from the surface by selective etching or by purging the dissolved oxygen from the solution, whereas the Cu-containing alloy was still susceptible to IGC after the same treatment. The difference in IGC susceptibility between the two alloys was attributed to the presence of a cathodic Cu-rich film and discrete Cu-containing particles along the grain boundaries of the Cu-containing alloy, while the cathodic sites on the unbalanced variant were restricted to the material surface. In addition, the IGC susceptibility of both alloys depended on the presence of solute (Si and Cu)-depleted zones adjacent to the grain boundaries.

Coaxial carbon/metal oxide/aligned carbon nanotube arrays as high-performance anodes for lithium ion batteries.

Coaxial carbon/metal oxide/aligned carbon nanotube (ACNT) arrays over stainless-steel foil are reported as high-performance binder-free anodes for lithium ion batteries. The coaxial arrays were prepared by growth of ACNTs over stainless-steel foil followed by coating with metal oxide and carbon. The carbon/manganese oxide/ACNT arrays can deliver an initial capacity of 738 mAh g(-1) with 99.9 % capacity retention up to 100 cycles and a capacity of 374 mAh g(-1) at a high current density of 6000 mA g(-1). The external carbon layer was recognized as a key component for high performance, and the mechanism of performance enhancement was investigated by electrochemical impedance spectroscopy, electron microscopy, and X-ray diffraction analysis. The layer increases rate capability by enhancing electrical conductivity and maintaining a low mass-transfer resistance and also improves cyclic stability by avoiding aggregation of metal-oxide particles and stabilizing the solid electrolyte interface. The resultant principle of rational electrode design was applied to an iron oxide-based system, and similar improvements were found. These coaxial nanotube arrays present a promising strategy for the rational design of high-performance binder-free anodes for lithium ion batteries.

One-step electrochemical synthesis of tunable nitrogen-doped graphene

A one-step electrochemical approach for the production of tunable nitrogen-doped graphene has been developed in this work. The simultaneous production and nitrogen incorporation of graphene are realized by electrochemical exfoliation of graphite in an aqueous electrolyte containing (NH4)2SO4 and NH3·H2O, which serve as an exfoliating agent and a nitrogen source, respectively. Both nitrogen contents and nitrogen bonding configurations can be manipulated by tuning the exfoliation conditions. The mechanism of nitrogen doping is proposed, based on analysis of the released gas from the graphite electrode, partially exfoliated graphite and graphene obtained. This green, efficient, low cost and scalable method provides a possible way for the large scale production of high-quality nitrogen-doped graphene. The nitrogen-doped graphene obtained is evaluated as a catalyst for the oxygen reduction reaction. It is demonstrated to be among the best nitrogen-doped graphene-based catalysts for the oxygen reduction reaction, even though the preparation process is extremely facile. Significantly, an Al–air battery is assembled, for the first time, by utilizing nitrogen-doped graphene as the cathode catalyst in this work. It can deliver a high specific capacity of 619 mA h gAl−1, corresponding to a specific energy of 817 W h kgAl−1, which is on a par with or better than that of Al–air batteries based on metal catalysts.