Showing papers by "Paul Munroe published in 2014"

Porous Graphene Nanoarchitectures: An Efficient Catalyst for Low Charge-Overpotential, Long Life, and High Capacity Lithium–Oxygen Batteries

Abstract: The electrochemical performance of lithium-oxygen (Li-O2) batteries awaits dramatic improvement in the design of porous cathode electrodes with sufficient spaces to accommodate the discharge products and discovery of effective cathode catalysts to promote both oxygen reduction reactions and oxygen evolution reactions. Herein, we report the synthesis of porous graphene with different pore size architectures as cathode catalysts for Li-O2 batteries. Porous graphene materials exhibited significantly higher discharge capacities than that of nonporous graphene. Furthermore, porous graphene with pore diameter around 250 nm showed the highest discharge capacity among the porous graphene with the small pores (about 60 nm) and large pores (about 400 nm). Moreover, we discovered that addition of ruthenium (Ru) nanocrystals to porous graphene promotes the oxygen evolution reaction. The Ru nanocrystal-decorated porous graphene exhibited an excellent catalytic activity as cathodes in Li-O2 batteries with a high reversible capacity of 17,700 mA h g(-1), a low charge/discharge overpotential (about 0.355 V), and a long cycle life up to 200 cycles (under the curtaining capacity of 1000 mAh g(-1)). The novel porous graphene architecture inspires the development of high-performance Li-O2 batteries.

Hierarchical macroporous/mesoporous NiCo2O4 nanosheets as cathode catalysts for rechargeable Li–O2 batteries

Abstract: The key factor to improve the electrochemical performance of Li–O2 batteries is to find bi-functional cathode catalysts to promote the oxygen reduction and evolution reactions. Despite tremendous effects, developing cathode catalysts with high activity remains a great challenge. Herein, we report the synthesis of hierarchical macroporous/mesoporous NiCo2O4 nanosheets as an effective cathode catalyst for Li–O2 batteries. The hierarchical porous catalyst was synthesized by a hydrothermal method, followed by low temperature calcination. SEM and TEM observations clearly present that the as-prepared NiCo2O4 nanosheets showed a hierarchical porous structure with mesopores distributed through the surface of NiCo2O4 nanosheets and macropores formed between the crumpled nanosheets. When investigating as the cathode catalyst in Li–O2 batteries, the as-prepared NiCo2O4 nanosheets exhibited higher reversible capacity, lower charge/discharge overpotential, and better cycling stability than those of pristine carbon black. The enhanced electrochemical performance of NiCo2O4 nanosheets should be attributed not only to the high catalytic activity of NiCo2O4 towards oxygen reduction reaction and oxygen evolution reaction, but also to the novel hierarchical porous structure of NiCo2O4.

Enhancing toughness of CrN coatings by Ni addition for safety-critical applications

Abstract: CrN is one of the most important transition metal nitrides, used as protective and anti-wear coating in modern engineering applications. However, CrN coatings are typically brittle and susceptible to catastrophic failure. In this paper, CrNiN coatings with differing Ni contents were deposited on tool steel substrates using a closed field unbalanced magnetron sputtering system. The effects of Ni addition on the microstructure and mechanical characteristics of CrN thin films were studied by combining nanoindentation tests with cross-sectional transmission electron microscopy. A columnar structure was observed in the CrN coating. With an increase in Ni content, the resultant columnar grains exhibited a high aspect ratio. Ni additions to CrN were found to enhance its damage resistance. Notably, the CrN thin film deformed mainly by intercolumnar shear sliding, whereas plastic deformation was favored in the CrNiN thin films exhibiting high aspect ratio columnar grains. More significantly, this change of microstructure with enhanced Ni content led to improved damage tolerance, manifested by a higher load required for crack formation upon a sharp contact (expressed as 1/HE2), and the plastic energy absorbed during nanoindentation.

Defect thermodynamics and kinetics in thin strained ferroelectric films: The interplay of possible mechanisms

Abstract: We present a theoretical description of the influence of misfit strain on mobile defects dynamics in thin strained ferroelectric films. Self-consistent solutions obtained by coupling the Poissons equation for electric potential with continuity equations for mobile donor and electron concentrations and time-dependent Landau-Ginzburg-Devonshire equations reveal that the Vegard mechanism (chemical pressure) leads to the redistribution of both charged and electro-neutral defects in order to decrease the effective stress in the film. Internal electric fields, both built-in and depolarization ones, lead to a strong accumulation of screening space charges (charged defects and electrons) near the film interfaces. Importantly, the corresponding screening length is governed by the misfit strain and Vegard coefficient. Mobile defects dynamics, kinetics of polarization and electric current reversal are defined by the complex interplay between the donor, electron and phonon relaxation times, misfit strain, finite size effect and Vegard stresses.

Misfit strain driven cation inter-diffusion across an epitaxial multiferroic thin film interface

Abstract: Cation intermixing at functional oxide interfaces remains a highly controversial area directly relevant to interface-driven nanoelectronic device properties. Here, we systematically explore the cation intermixing in epitaxial (001) oriented multiferroic bismuth ferrite (BFO) grown on a (001) lanthanum aluminate (LAO) substrate. Aberration corrected dedicated scanning transmission electron microscopy and electron energy loss spectroscopy reveal that the interface is not chemically sharp, but with an intermixing of ∼2 nm. The driving force for this process is identified as misfit-driven elastic strain. Landau-Ginzburg-Devonshire-based phenomenological theory was combined with the Sheldon and Shenoy formula in order to understand the influence of boundary conditions and depolarizing fields arising from misfit strain between the LAO substrate and BFO film. The theory predicts the presence of a strong potential gradient at the interface, which decays on moving into the bulk of the film. This potential gradient...

Approaches to greenhouse gas accounting methods for biomass carbon

Abstract: This investigation examines different approaches for the GHG flux accounting of activities within a tight boundary of biomass C cycling, with scope limited to exclude all other aspects of the lifecycle. Alternative approaches are examined that a) account for all emissions including biogenic CO2 cycling – the biogenic method; b) account for the quantity of C that is moved to and maintained in the non-atmospheric pool – the stock method; and c) assume that the net balance of C taken up by biomass is neutral over the short-term and hence there is no requirement to include this C in the calculation – the simplified method. This investigation demonstrates the inaccuracies in both emissions forecasting and abatement calculations that result from the use of the simplified method, which is commonly accepted for use. It has been found that the stock method is the most accurate and appropriate approach for use in calculating GHG inventories, however short-comings of this approach emerge when applied to abatement projects, as it does not account for the increase in biogenic CO2 emissions that are generated when non-CO2 GHG emissions in the business-as-usual case are offset. Therefore the biogenic method or a modified version of the stock method should be used to accurately estimate GHG emissions abatement achieved by a project. This investigation uses both the derivation of methodology equations from first principles and worked examples to explore the fundamental differences in the alternative approaches. Examples are developed for three project scenarios including; landfill, combustion and slow-pyrolysis (biochar) of biomass.

Phase transformation pathways in amorphous germanium under indentation pressure

Abstract: Nanoindentation-induced phase transformations have been studied in amorphous Ge thin films. These films initially tend to deform via plastic flow of the amorphous phase under load but at a critical pressure a sudden phase transformation occurs. This transformation, to a soft metallic (β-Sn-like)-Ge phase confined under the indenter, is signified by a “pop-in” event on loading. Following “pop-in,” the indentation tests fall into two distinct types of behavior. In one case, the rate of deformation with increasing load after “pop-in” increases, and the observed end-phase following complete unloading is observed to be predominately diamond-cubic Ge. In the other case, the deformation rate (slope of the loading curve) remains the same as that before “pop-in,” and the end phases following unloading are found to contain predominantly unstable r8 and more stable hexagonal Ge phases. The different transformation pathways for these two cases are shown to be related to the probability that the soft (β-Sn-like)-Ge phase volume, which suddenly forms at the transformation pressure, is either unconstrained by the indenter tip (the first case) or totally constrained under the indenter tip (in the latter case).

Thermal stability of the nanostructure of mechanically milled Cu–5 vol% Al2O3 nanocomposite powder particles

Abstract: Isothermal annealing in the temperature range of 300–600 °C, microstructural characterization, and analysis of the grain growth kinetics during annealing were carried out for Cu–5 vol% Al2O3 nanocomposite powder particles produced by high energy mechanical milling. When the annealing temperature was 400 °C or lower, only reduction in dislocation density occurred during annealing. When the annealing temperature was 500 °C or higher, reduction in dislocation density, abnormal grain growth of the nanocrystalline Cu matrix, and coarsening of the Al2O3 nanoparticles occurred. It has been found that the microstructure of the nanocrystalline Cu matrix of the nanocomposite exhibits a far higher thermal stability than that of monolithic nanocrystalline Cu, even though the apparent activation energy of the grain growth of the former is similar to that of the latter over the temperature range of 400–600 °C, showing the dramatic drag effects of finely distributed Al2O3 nanoparticles and Al3+/O2− clusters on the grain boundary motion.

Oxidation resistance of Mo(Si1−xAlx)2 nanocrystalline films and characterization of their oxide scales by electrochemical impedance spectroscopy

Abstract: To explore the influence of Al alloying on the oxidation resistance of MoSi2, four Mo(Si1−xAlx)2 nanocrystalline films, with differing Al contents, were fabricated on Ti–6A1–4V substrates by a double-cathode glow discharge technique and their cyclic oxidation behavior was characterized at 500 °C in air. The oxidation kinetics of the four Mo(Si1−xAlx)2 films was found to obey a subparabolic behavior with respect to the overall exposure, and their oxidation resistance was improved by Al additions. On the other hand, the electrochemical behavior of the oxide scales developed on the four Mo(Si1−xAlx)2 nanocrystalline films in a 3.5 wt% NaCl solution was studied using electrochemical-impedance spectroscopy (EIS). The impedance data showed that with increasing oxidation time, the oxide scales transformed from a homogeneous and dense structure to a duplex structure consisting of a porous outer layer and a denser inner layer. The resistance of the oxide scales increased with increasing Al addition, implying an enhanced protective ability of the oxide scales by Al alloying in media containing chlorine ions. The findings represent a step forward in improving the surface integrity of alloy components used in the hot zones of jet engines.

Through-thickness variations in recrystallization behavior in an Al-based ARB composite sheet

Abstract: A composite sheet of commercially pure aluminum and an Al-03 wt% Sc alloy (in the supersaturated solid solution condition) was produced by accumulative roll bonding at 200°C The material was then subjected to isothermal annealing at 300°C for 1-30 minutes and cold water quenched The transverse section was investigated by electron back-scatter diffraction (EBSD) to investigate the variations in microstructure and texture within Al layers through the sheet thickness A faster spheroidization of the highly elongated lamellar band deformation structures was observed in the surface aluminum layer as compared to the mid- and quarter-thickness layers In the quarter thickness aluminum layer so-called continuous recrystallization occurred and, thus, the p-fiber rolling texture was retained Further growth in this layer led to secondary recrystallization of cube orientations In contrast, in the surface aluminum layers the recrystallization and grain growth texture were relatively random Intermediate behavior was observed in the mid-thickness aluminum layer

The Benefit and Impact of On-Line Tools for Microscopy and Microanalysis Training and Education in Core Facilities.

Abstract: The Australian Microscopy and Microanalysis Research Facility (AMMRF) is a national grid of equipment, instrumentation and expertise in microscopy and microanalysis that provides nanostructural characterisation capability and services, from widely used optical, electron, X-ray and ion-beam techniques to world-leading flagship platforms. This collaborative facility, comprising a distributed network of microscopy and microanalysis core facilities spread over fourteen institutions, supports more than 3,000 researchers each year of which approximately one-third are “new users” and with indications that this is growing by 10% per annum.