Suppression of Phase Separation in LiFePO4 Nanoparticles During Battery Discharge

Using a novel electrochemical phase-field model, we question the common belief that LixFePO4 nanoparticles separate into Li-rich and Li-poor phases during battery discharge. For small currents, spinodal decomposition or nucleation leads to moving phase boundaries. Above a critical current density (in the Tafel regime), the spinodal disappears, and particles fill homogeneously, which may explain the superior rate capability and long cycle life of nano-LiFePO4 cathodes.Comment: 27 pages, 8 figure

Atomic-scale structure and properties of highly stable antiphase boundary defects in Fe3O4

The complex and intriguing properties of the ferrimagnetic half metal magnetite (Fe3O4) are of continuing fundamental interest as well as being important for practical applications in spintronics, magnetism, catalysis and medicine. There is considerable speculation concerning the role of the ubiquitous antiphase boundary (APB) defects in magnetite, however, direct information on their structure and properties has remained challenging to obtain. Here we combine predictive first principles modelling with high-resolution transmission electron microscopy to unambiguously determine the three-dimensional structure of APBs in magnetite. We demonstrate that APB defects on the {110} planes are unusually stable and induce antiferromagnetic coupling between adjacent domains providing an explanation for the magnetoresistance and reduced spin polarization often observed. We also demonstrate how the high stability of the {110} APB defects is connected to the existence of a metastable bulk phase of Fe3O4, which could be stabilized by strain in films or nanostructures

Surface roughness imparts tensile ductility to nanoscale metallic glasses

Experiments show an intriguing brittle-to-ductile transition on size reduction on nanoscale metallic glasses (MGs). Here we demonstrate that such phenomena is linked to a fundamental characteristic size effect in the failure mode under tensile loading. Large-scale molecular dynamics simulations reveal that nanoscaled MGs with atomistically smooth surfaces exhibit catastrophic failure via sharp, localized shear band propagation. In contrast, nanosized specimens with surface imperfections exhibit a clear transition from shear banding to necking instability above a critical roughness ratio of  ξ ∼ 1/20, defined as the ratio between the average surface imperfection size and sample diameter. The observed brittle-to-ductile transition that emerges in nanosized MGs deformed at room temperature can be strongly attributed to this roughness argument. In addition, the results suggest that the suppression of brittle failure may be scale-free and be realizable on length scales much beyond those considered here, provided the threshold roughness ratio is exceeded. The fundamental critical roughness ratio demonstrated sheds light on the complex mechanical behavior of amorphous metals and has implications for the application of MGs in nano- and micro-devices

Scalable parallel debugging with statistical assertions

Traditional debuggers are of limited value for modern scientific codes that manipulate large complex data structures. This paper discusses a novel debug-time assertion, called a "Statistical Assertion", that allows a user to reason about large data structures, and the primitives are parallelised to provide an efficient solution. We present the design and implementation of statistical assertions, and illustrate the debugging technique with a molecular dynamics simulation. We evaluate the performance of the tool on a 12,000 cores Cray XE6

Correlation between optical absorption redshift and carrier density in phase change materials

Here, we report an optical absorption redshift map for GeTe-Sb2Te3 pseudo-binary alloys. We found that, with phase change from amorphous to crystalline, the observed redshift increases with Ge concentration along pseudo-line of compositions, which directly reflects the enhanced electron delocalization/resonant bonding and increased carrier concentrations in the respective crystal compounds. The measured valence band maximum shift towards the Fermi energy from amorphous to crystalline phase supports the observed similar trend in redshift and carrier density. We show that the correlation between optical redshift and carrier density, attributed to the resonant bonding, can be rationalized by calculating the valence electron concentration, the ionicity, and hybridization.ASTAR (Agency for Sci., Tech. and Research, S’pore)Published versio