Experimental investigation into the mechanical properties of metal anodes in lithium-ion batteries

In the search for high-capacity lithium-ion batteries, metal anodes, such as silicon and aluminum among -others, have emerged possessing a theoretical specific charge capacity significantly greater than their conventional graphite counterpart. In spite of their high capacity, metal anodes based lithium-ion batteries have shown poor structural integrity and cycle life, which has been the bottleneck to the large scale implementation of these kinds of batteries. This issue arises from the large dimensional changes that accompany the phase transformations during lithiation/delithiation (alloying/dealloying with/from lithium) in metal anodes. The evolution of mechanical behavior during phase transformation determines the deformation and fracture behavior and also affects the kinetic processes involved in phase transformation. Thus, understanding the evolution of mechanical properties during electrochemical phase transformation and its dependence on microstructure is necessary in mechanical reliability and electrochemical performance analysis. In this article, we report an experimental investigation into the evolution of mechanical properties in select metal anodes in which phase transformations result in contrasting changes in microstructure and mechanical properties. The metal anodes studied consisted of brittle Si, which transforms to ductile lithiated Li ×Si and ductile Al, which transforms to brittle lithiated Li × Al. Uniaxial tensile tests and nanoindentation were utilized to investigate the mechanical properties, and X-ray diffraction used to examine the structural evolution of Si and Al anodes during lithiation. Fractographic studies were performed in a scanning electron microscope to elucidate the fracture mechanisms. The effect of the transition in mechanics of metal anodes on the kinetic processes during phase transformation in Si and Al is discussed

Multiscale experimental investigation and numerical simulation of deformation and failure in polycrystalline alloys under shear loading

Recently, there has been a revived interest in ductile fracture of metallic alloys, especially under dominant shear loading condition. Conventional ductile fracture models such as the Gurson–Tvergaard–Needleman model has been developed based on the mechanics of void growth due to volumetric strain and subsequent void coalescence leading to fracture. It has now become evident that these models are not capable of capturing fracture in dominant shear deformation scenarios, which arise commonly in the form of shear localizations in polycrystalline metallic materials. Ad-hoc phenomenological modifications by means of an artificial augmentation of void growth in shear have been introduced to extend the applicability of these models to low stress triaxiality regimes. However, the mechanics and physics of deformation and failure in dominant shear loading at the microstructure of polycrystalline alloys are not well understood. In this article, we report in-situ multiscale examination of deformation processes and failure mechanisms in Al 6061-T6 – a polycrystalline alloy with a dispersion of second phase particles in the microstructure – under low stress triaxiality levels using modified Arcan specimens. Strains at the grain and subgrain levels are measured in a scanning electron microscope by in-situ tracking of the changes in grain size and morphology, and at the macroscale level using digital image correlation. Grain level strains in the range of 2–2.5 are sustained in the material without any indication of failure and shown to be significantly higher than estimates of strain measured using a specimen dimension as the gage length. A continuum material failure model based on grain level strain measurement was introduced and used in numerical simulations to assess the suitability of the proposed failure model. The results from the failure model were compared to those from commonly used Johnson–Cook model. It was noted that although the proposed failure model based on grain based deformation was able to reproduce the essential features observed in the experiments, the results from Johnson–Cook model predicted a premature failure in the material. It was concluded that calibration of failure models requires a suitable length scale and the grain size as an intrinsic property of the material can be used as an appropriate length scale to define strain in failure model calibrations

The Sandia Fracture Challenge: blind round robin predictions of ductile tearing

Existing and emerging methods in computational mechanics are rarely validated against problems with an unknown outcome. For this reason, Sandia National Laboratories, in partnership with US National Science Foundation and Naval Surface Warfare Center Carderock Division, launched a computational challenge in mid-summer, 2012. Researchers and engineers were invited to predict crack initiation and propagation in a simple but novel geometry fabricated from a common off-the-shelf commercial engineering alloy. The goal of this international Sandia Fracture Challenge was to benchmark the capabilities for the prediction of deformation and damage evolution associated with ductile tearing in structural metals, including physics models, computational methods, and numerical implementations currently available in the computational fracture community. Thirteen teams participated, reporting blind predictions for the outcome of the Challenge. The simulations and experiments were performed independently and kept confidential. The methods for fracture prediction taken by the thirteen teams ranged from very simple engineering calculations to complicated multiscale simulations. The wide variation in modeling results showed a striking lack of consistency across research groups in addressing problems of ductile fracture. While some methods were more successful than others, it is clear that the problem of ductile fracture prediction continues to be challenging. Specific areas of deficiency have been identified through this effort. Also, the effort has underscored the need for additional blind prediction-based assessments

Electrodeposition of Ni and Te-doped Cobalt Triantimonide in Citrate Solutions

Skutterudite compounds form a new class of potential candidates for thermoelectric applications. Cobalt triantimonide (CoSb3) shows good thermoelectric properties at medium and high temperatures. Doping this system with substitution elements, for either Co or Sb or both, may result in an increase of the thermoelectric figure of merit (ZT). This work focused on the electrochemical doping and characterization of films and nanowires of Co-Sb system in citrate solutions using gold-coated PCTE templates. The electrodeposition was performed on gold surface that was pre-treated electrochemically to ensure reproducible results. The electrochemical treatment acted as an annealing process for the surface, which resulted in an increase in Au(111) as demonstrated by XRD. Detailed electrochemical studies including deposition-stripping experiments was performed in order to develop a better understanding of the co-deposition kinetics and a better control over the composition of doped Co-Sb system. Scanning electron microscopy (SEM/EDS) helped study the morphology and the composition of the doped and undoped Co-Sb system. Co-deposition of Co-Sb showed that the amount of Co is higher in nanowires than in film or mushroom caps due to the slow Sb deposition rate dictated by slow Sb(III) complex diffusion. Doped nanowires have been also obtained. Both Ni and Te electrochemical doping of the Co-Sb system affected the composition of the deposit but there was no effect on nanowire morphology

Overview paper: New insights into aerosol and climate in the Arctic

Motivated by the need to predict how the Arctic atmosphere will change in a warming world, this article summarizes recent advances made by the research consortium NETCARE (Network on Climate and Aerosols: Addressing Key Uncertainties in Remote Canadian Environments) that contribute to our fundamental understanding of Arctic aerosol particles as they relate to climate forcing. The overall goal of NETCARE research has been to use an interdisciplinary approach encompassing extensive field observations and a range of chemical transport, earth system, and biogeochemical models. Several major findings and advances have emerged from NETCARE since its formation in 2013. (1) Unexpectedly high summertime dimethyl sulfide (DMS) levels were identified in ocean water (up to 75&thinsp;nM) and the overlying atmosphere (up to 1&thinsp;ppbv) in the Canadian Arctic Archipelago (CAA). Furthermore, melt ponds, which are widely prevalent, were identified as an important DMS source (with DMS concentrations of up to 6&thinsp;nM and a potential contribution to atmospheric DMS of 20&thinsp;% in the study area). (2) Evidence of widespread particle nucleation and growth in the marine boundary layer was found in the CAA in the summertime, with these events observed on 41&thinsp;% of days in a 2016 cruise. As well, at Alert, Nunavut, particles that are newly formed and grown under conditions of minimal anthropogenic influence during the months of July and August are estimated to contribute 20&thinsp;% to 80&thinsp;% of the 30–50&thinsp;nm particle number density. DMS-oxidation-driven nucleation is facilitated by the presence of atmospheric ammonia arising from seabird-colony emissions, and potentially also from coastal regions, tundra, and biomass burning. Via accumulation of secondary organic aerosol (SOA), a significant fraction of the new particles grow to sizes that are active in cloud droplet formation. Although the gaseous precursors to Arctic marine SOA remain poorly defined, the measured levels of common continental SOA precursors (isoprene and monoterpenes) were low, whereas elevated mixing ratios of oxygenated volatile organic compounds (OVOCs) were inferred to arise via processes involving the sea surface microlayer. (3) The variability in the vertical distribution of black carbon (BC) under both springtime Arctic haze and more pristine summertime aerosol conditions was observed. Measured particle size distributions and mixing states were used to constrain, for the first time, calculations of aerosol–climate interactions under Arctic conditions. Aircraft- and ground-based measurements were used to better establish the BC source regions that supply the Arctic via long-range transport mechanisms, with evidence for a dominant springtime contribution from eastern and southern Asia to the middle troposphere, and a major contribution from northern Asia to the surface. (4) Measurements of ice nucleating particles (INPs) in the Arctic indicate that a major source of these particles is mineral dust, likely derived from local sources in the summer and long-range transport in the spring. In addition, INPs are abundant in the sea surface microlayer in the Arctic, and possibly play a role in ice nucleation in the atmosphere when mineral dust concentrations are low. (5) Amongst multiple aerosol components, BC was observed to have the smallest effective deposition velocities to high Arctic snow (0.03&thinsp;cm&thinsp;s−1).</p