Streamer evolution arrest governed amplified AC breakdown strength of graphene and CNT colloids

The present article experimentally explores the concept of large improving the AC dielectric breakdown strength of insulating mineral oils by the addition of trace amounts of graphene or CNTs to form stable dispersions. The nano-oils infused with these nanostructures of high electronic conductance indicate superior AC dielectric behaviour in terms of augmented breakdown strength compared to the base oils. Experimental observations of two grades of synthesized graphene and CNT nano-oils show that the nanomaterials not only improve the average breakdown voltage but also significantly improve the reliability and survival probabilities of the oils under AC high voltage stressing. Improvement of the tune of ~ 70-80 % in the AC breakdown voltage of the oils has been obtained via the present concept. The present study examines the reliability of such nano-colloids with the help of two parameter Weibull distribution and the oils show greatly augmented electric field bearing capacity at both standard survival probability values of 5 % and 63.3 %. The fundamental mechanism responsible for such observed outcomes is reasoned to be delayed streamer development and reduced streamer growth rates due to effective electron scavenging by the nanostructures from the ionized liquid insulator. A mathematical model based on the principles of electron scavenging is proposed to quantify the amount of electrons scavenged by the nanostructures. The same is then employed to predict the enhanced AC breakdown voltage and the experimental values are found to match well with the model predictions. The present study can have strong implications in efficient, reliable and safer operation of real life AC power systems

Highly Permeable Perfluorinated Sulfonic Acid Ionomers for Improved Electrochemical Devices: Insights into Structure-Property Relationships.

Rapid improvements in polymer-electrolyte fuel-cell (PEFC) performance have been driven by the development of commercially available ion-conducting polymers (ionomers) that are employed as membranes and catalyst binders in membrane-electrode assemblies. Commercially available ionomers are based on a perfluorinated chemistry comprised of a polytetrafluoroethylene (PTFE) matrix that imparts low gas permeability and high mechanical strength but introduces significant mass-transport losses in the electrodes. These transport losses currently limit PEFC performance, especially for low Pt loadings. In this study, we present a novel ionomer incorporating a glassy amorphous matrix based on a perfluoro(2-methylene-4-methyl-1,3-dioxolane) (PFMMD) backbone. The novel backbone chemistry induces structural changes in the ionomer, restricting ionomer domain swelling under hydration while disrupting matrix crystallinity. These structural changes slightly reduce proton conductivity while significantly improving gas permeability. The performance implications of this trade-off are assessed, which reveal the potential for substantial performance improvement by incorporation of highly permeable ionomers as the functional catalyst binder. These results underscore the significance of tailoring material chemistry to specific device requirements, where ionomer chemistry should be rationally designed to match the local transport requirements of the device architecture

Annealing Relaxation of Ultrasmall Gold Nanostructures

Except serving as an excellent gift on proper occasions, gold finds applications in life sciences, particularly in diagnostics and therapeutics. These applications were made possible by gold nanoparticles, which differ drastically from macroscopic gold. Versatile surface chemistry of gold nanoparticles allows coating with small molecules, polymers, biological recognition molecules. Theoretical investigation of nanoscale gold is not trivial, because of numerous metastable states in these systems. Unlike elsewhere, this work obtains equilibrium structures using annealing simulations within the recently introduced PM7-MD method. Geometries of the ultrasmall gold nanostructures with chalcogen coverage are described at finite temperature, for the first time

Multiscale understanding of tricalcium silicate hydration reactions

Tricalcium silicate, the main constituent of Portland cement, hydrates to produce crystalline calcium hydroxide and calcium-silicate-hydrates (C-S-H) nanocrystalline gel. This hydration reaction is poorly understood at the nanoscale. The understanding of atomic arrangement in nanocrystalline phases is intrinsically complicated and this challenge is exacerbated by the presence of additional crystalline phase(s). Here, we use calorimetry and synchrotron X-ray powder diffraction to quantitatively follow tricalcium silicate hydration process: i) its dissolution, ii) portlandite crystallization and iii) C-S-H gel precipitation. Chiefly, synchrotron pair distribution function (PDF) allows to identify a defective clinotobermorite, Ca11Si9O28(OH)2.8.5H2O, as the nanocrystalline component of C-S-H. Furthermore, PDF analysis also indicates that C-S-H gel contains monolayer calcium hydroxide which is stretched as recently predicted by first principles calculations. These outcomes, plus additional laboratory characterization, yielded a multiscale picture for C-S-H nanocomposite gel which explains the observed densities and Ca/Si atomic ratios at the nano- and meso- scales.This work has been supported by Spanish MINECO through BIA2014-57658-C2-2-R, which is co-funded by FEDER, BIA2014-57658-C2-1-R and I3 (IEDI-2016-0079) grants. We also thank CELLS-ALBA (Barcelona, Spain) for providing synchrotron beam time at BL04-MSPD beamline

Tunable mechanical and thermal properties of ZnS/CdS core/shell nanowires

Using all atom molecular dynamics (MD) simulations, we have studied the mechanical properties of ZnS/CdS core/shell nanowires. Our results show that the coating of a few atomic layer CdS shell on the ZnS nanowire leads to a significant change in the stiffness of the core/shell nanowires compared to the stiffness of pure ZnS nanowires. The binding energy between the core and shell region decreases due to the lattice mismatch at the core-shell interface. This reduction in binding energy plays an important role in determining the stiffness of a core/shell nanowire. We have also investigated the effects of the shell on the thermal conductivity and melting behavior of the nanowires

Ab initio vibrations in nonequilibrium nanowires

We review recent results on electronic and thermal transport in two different quasi one-dimensional systems: Silicon nanowires (SiNW) and atomic gold chains. For SiNW's we compute the ballistic electronic and thermal transport properties on equal footing, allowing us to make quantitative predictions for the thermoelectric properties, while for the atomic gold chains we evaluate microscopically the damping of the vibrations, due to the coupling of the chain atoms to the modes in the bulk contacts. Both approaches are based on a combination of density-functional theory, and nonequilibrium Green's functions.Comment: 16 pages, to appear in Progress in Nonequilibrium Green's Functions IV (PNGF4), Eds. M. Bonitz and K. Baltzer, Glasgow, August 200