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Revisiting the alluaudite NaMnFe2(PO4)(3) sodium insertion material: Structural, diffusional and electrochemical insights
No full textAmong the gamut of sodium battery insertion materials, NaMnFe2(PO4)(3) was reported as the first alluaudite framework compound albeit with poor electrochemical activity Chem. Mater. 22 (2010) 5554], We hereby report auto-combustion synthesis of carbon coated NaMnFe2(PO4)(3) alluaudite and its Na+ diffusion, ionic conductivity and electrochemical activity synergizing experiments with bond valence site energy (BVSE) modeling. It registered a 2.8 V redox activity with a reversible capacity of similar to 60 mAh g(-1) with good cycling stability. BVSE calculations revealed an exceptionally low one-dimensional migration barrier of 0.31 eV for Na-ion diffusion. It was in sync with the low activation energy barrier of 0.162 eV derived from ac impedance spectroscopy. NaMnFe2(PO4)(3) alluaudite cathode was found to have conductivity value of 0.5 x 10(-6)S cm(-1) at room temperature. Among the PO4-based sodium battery insertion materials, NaMnFe2(PO4)(3) alluaudite shows excellent ionic conductivity with very low Na+ migration barrier. It can lead to the realization of superior reversible capacity in this alluaudite cathode comparable to LiFePO4. (C) 2018 Elsevier Ltd. All rights reserved
Experimental Observation of Dirac Nodal Links in Centrosymmetric Semimetal TiB2
No full textThe topological nodal-line semimetal state, serving as a fertile ground for various topological quantum phases, where a topological insulator, Dirac semimetal, or Weyl semimetal can be realized when the certain protecting symmetry is broken, has only been experimentally studied in very few materials. In contrast to discrete nodes, nodal lines with rich topological configurations can lead to more unusual transport phenomena. Utilizing angle-resolved photoemission spectroscopy and first-principles calculations, here, we provide compelling evidence of nodal-line fermions in centrosymmetric semimetal TiB2 with a negligible spin-orbit coupling effect. With the band crossings just below the Fermi energy, two groups of Dirac nodal rings are clearly observed without any interference from other bands, one surrounding the Brillouin zone (BZ) corner in the horizontal mirror plane sigma(h) and the other surrounding the BZ center in the vertical mirror plane sigma(v). The linear dispersions forming Dirac nodal rings are as wide as 2 eV. We further observe that the two groups of nodal rings link together along the G-K direction, composing a nodal-link configuration. The simple electronic structure with Dirac nodal links mainly constituting the Fermi surfaces suggests TiB2 as a remarkable platform for studying and applying the novel physical properties related to nodal-line fermions
Single-particle mass spectrometry with arrays of frequency-addressed nanomechanical resonators
No full textOne of the main challenges to overcome to perform nanomechanical mass spectrometry analysis in a practical time frame stems from the size mismatch between the analyte beam and the small nanomechanical detector area. We report here the demonstration of mass spectrometry with arrays of 20 multiplexed nanomechanical resonators; each resonator is designed with a distinct resonance frequency which becomes its individual address. Mass spectra of metallic aggregates in the MDa range are acquired with more than one order of magnitude improvement in analysis time compared to individual resonators. A 20 NEMS array is probed in 150 ms with the same mass limit of detection as a single resonator. Spectra acquired with a conventional time-of-flight mass spectrometer in the same system show excellent agreement. We also demonstrate how mass spectrometry imaging at the single-particle level becomes possible by mapping a 4-cm-particle beam in the MDa range and above
Cool-core Clusters: The Role of BCG, Star Formation, and AGN-driven Turbulence
No full textRecent observations of cool cluster cores that include the BCG gravity claim that the observed threshold in min(t(cool)/t(ff)) (cooling time to free-fall time ratio) lies at a somewhat higher value, close to 10-30, compared with the threshold seen in numerical simulations. There are only a few clusters in which this ratio falls much below 10. In this paper, we compare 3D hydrodynamic simulations of feedback active galactic nuclei (AGNs) jets interacting with the intracluster medium, with and without a BCG potential. We find that, for a fixed feedback efficiency, the presence of a BCG does not significantly affect the temperature, but increases (decreases) the core density (entropy) on average. Most importantly, min(t(cool)/t(ff)) is only affected slightly by the inclusion of the BCG gravity. Also notable is that the lowest value of min(t(cool)/t(ff)) in the NFW+BCG runs is about twice as large as in the NFW runs. We also look at the role of depletion of cold gas due to star formation, and show that it only affects the rotationally dominant component, while the radially dominant component remains largely unaffected. Stellar gas depletion also increases the repetition rate of AGN jets. The distribution of metals due to AGN jets in our simulations is predominantly along the jet direction, and the equatorial spread of metals is less compared with the observations. We also show that the turbulence in cool-core clusters is weak, which is consistent with recent Hitomi results on the Perseus cluster
Erasure coding for distributed storage: an overview
No full textIn a distributed storage system, code symbols are dispersed across space in nodes or storage units as opposed to time. In settings such as that of a large data center, an important consideration is the efficient repair of a failed node. Efficient repair calls for erasure codes that in the face of node failure, are efficient in terms of minimizing the amount of repair data transferred over the network, the amount of data accessed at a helper node as well as the number of helper nodes contacted. Coding theory has evolved to handle these challenges by introducing two new classes of erasure codes, namely regenerating codes and locally recoverable codes as well as by coming up with novel ways to repair the ubiquitous Reed-Solomon code. This survey provides an overview of the efforts in this direction that have taken place over the past decade
Surface Severe Plastic Deformation of an Orthopedic Ti-Nb-Sn Alloy Induces Unusual Precipitate Remodeling and Supports Stem Cell Osteogenesis through Akt Signaling
No full textThis work presents a strategy to augment the bioactivity of a new-generation metastable beta-Ti-Nb-Sn alloy through surface severe plastic deformation. Foremost, the alloy was strengthened by precipitation of alpha phase using a well-designed thermo-mechanical processing route. Subsequently, the surface of the aged alloy was subjected to severe plastic deformation via surface mechanical attrition treatment (SMAT). Upon SMAT, a unique phenomenon of strain induced precipitate coarsening was observed. A possible mechanism is proposed wherein the precipitates first dissolve due to significant slip transfer across the alpha/beta-interface followed by reprecipitation along the other precipitates thereby leading to coarsening. Coarsening of the precipitates abrogated the strengthening caused by plastic deformation as a result of which the hardness did not increase significantly after SMAT in sharp contrast to other alloys. SMAT led to a decrease in the attachment of human mesenchymal stem cells because of an increase in the roughness-mediated surface hydrophobicity. On the other hand, an increase in the roughness led to the formation of more number of focal adhesions. This in turn enhanced the proliferation rate and more importantly, osteogenic differentiation of stem cells. Detailed investigation into the underlying mechanism revealed that an increase in focal adhesions activated the Akt-mediated mechano-transduction signaling pathway that enhanced the osteogenic differentiation. In summary, the potential of surface severe plastic deformation to impart bioactivity to the next-generation of orthopedic beta-Ti alloys is underscored in this work
Nitridation of Sapphire as a Precursor to GaN Growth: Structure and Chemistry
No full textNitridation of sapphire substrates is used as a precursor to the growth of GaN films to provide a wetting layer which is closer in terms of structure and chemistry to the overlayer. Nitridation has been carried out by metal-organic chemical vapor deposition at 530, 800, and 1100 degrees C in an environment of NH3 and H-2. The structure and chemistry of the nitrided layer grown at these different temperatures have been studied by X-ray photoelectron spectroscopy, electron diffraction, high resolution electron microscopy, and electron energy loss spectroscopy. The low temperature nitridation process results in a nitrided layer in which oxygen has been partially replaced by nitrogen to form a cubic spinel-AL(x)O(y)N(z) structure. Nitridation at 800 degrees and 1100 degrees C results in complete substitution of oxygen atoms by nitrogen to form a cubic rock salt AIN structure. These structures are stable on thermal annealing at 1000 degrees C prior to epitaxial GaN growth
Temperature-dependent elongation of the H-H bond in dihydrogen complexes of Ru(II) bearing an NHC ligand: Effect of the NHC and trans ligands
No full textRuthenium hydride complexes bearing an N-heterocyclic carbene ligand RuHCl(CO)(IMes)(PPh3)(L/L')] (L = py, 2; 4Mepy, 3; L' = MeCN, 4; Me3CCN, 5) have been synthesized in high-yields via reaction of RuHCl(CO)(IMes)(PPh3)] (1) with pyridyl ligands L (L = py and 4Mepy) or nitrile ligands L' (L' = MeCN and Me3CCN). The ligands L/L' are labile in all the ruthenium hydride complexes; they can be easily replaced by Lewis bases. The X-ray structures of complexes 2 and 3 show intramolecular pi-pi interactions between the aromatic ring of PPh3, IMes, and pyridyl ligands. The protonation reaction of 2-5 gives the corresponding dihydrogen complexes of the type RuCl(eta(2)-H-2)(CO)(IMes)(PPh3)(L/L')]OTf] complexes (L = py, 6; 4Mepy, 7; L' = MeCN, 8; Me3CCN, 9). In all the dihydrogen complexes, H-H bond distances of eta(2)-H-2 ligand is temperature-dependent 0.98 angstrom to 0.93 angstrom in the temperature range of 183-233 K. Attempts to synthesize analogous ruthenium hydride complexes bearing phosphine ligands resulted in a mixture of cis and trans- RuHCl(CO)(PPh3)(2)(L)] L = py, 10/11 (trans(HCl)/cis(HCl)); 4Mepy, 12/13 (trans(HCl)/cis(HCl))] complexes. A comparative study has been done to get an insight into the temperature-dependent H-H bond distances in complexes 6-8 by synthesizing analogous ruthenium dihydrogen complexes, RuCl(eta(2)-H-2)(CO)(PPh3)(2) (L)](OTf) (L = py, 15; 4Mepy, 17). All the complexes have been characterized using NMR spectroscopy. The X-ray crystal structures of complexes 2, 3, and 12 have also been determined
Complex interactions underpin social behaviour in Dictyostelium giganteum
No full textIn the wild, social groups of the cellular slime mould amoeba Dictyostelium giganteum are genetically heterogeneous more often than not. When studied as 1:1 binary mixes, amoebae of one strain almost always form more spores than the other, an observation that leads one to wonder what might be responsible for the long-term persistence of different strains in nature. We have monitored a number of individual and collective traits bearing on reproductive fitness in chimaeras of Dictyostelium giganteum obtained by mixing pairs of starved amoebae belonging to distinct wild-type strains in proportions ranging from 1:9 to 9:1. The main findings are that intercellular interactions take place at more than one stage of the life cycle and that generalisations drawn after mixing cells only in a 1:1 ratio can be misleading. A strain that does better than another in respect of some component of fitness (for example, spore formation) may do worse in respect of a different component (for example, growth rate). Also, a strain that is a more efficient sporulator than a second strain in one context may be less efficient in another context. In addition to such trade-offs, spore formation in chimaeras can exhibit negative frequency dependence, which too can lead to stable co-existence.Significance statementHumans live in social groups whose members perform different but equally important tasks. When they do so freely, the long-term stability of the group depends on give and take (trade-offs') between individuals who are generally unrelated. In contrast, studies of cooperation in non-human animals have focussed on relatedness through common descent as responsible for the maintenance of group cohesion. But animal groups too contain unrelated individuals. Using the example of a social amoeba, we show that this need not rule out a high level of cooperation including so-called altruism, as long as inter-individual interactions permit it. In both genetically homogeneous and heterogeneous groups, interactions can result in trade-offs between different traits. In genetically heterogeneous groups, they can also lead to a negative correlation between the efficiency with which cells of a genotype reproduce and the proportion of cells belonging to that genotype
Insulin dimer dissociation in aqueous solution: A computational study of free energy landscape and evolving microscopic structure along the reaction pathway
No full textThe dissociation of an insulin dimer to two monomers is an important life process. Although the monomer is the biologically active form of the hormone, it is stored in the beta-cells of the pancreas in the hexameric form. The latter, when the need comes, dissociates to dimers and the dimers in turn to monomers to maintain the endogenous delivery of the hormone. In order to understand insulin dimer dissociation at a molecular level, we perform biased molecular dynamics simulations (parallel tempering metadynamics in the well-tempered ensemble) of the dissociation of the insulin dimer in water using two order parameters and an all-atom model of the protein in explicit water. The two order parameters selected (after appropriate studies) are the distance (R-MM ) between the center of mass of two monomers and the number of contacts (N-MM) among the backbone-C alpha atoms of the two monomers. We calculated the free energy landscape as a function of these two order parameters and determined the minimum free energy pathway of dissociation. We find that the pathway involves multiple minima and multiple barriers. In the initial stage of dissociation, the distance between the monomers does not change significantly but the N-MM decreases rapidly. In the latter stage of separation, the opposite occurs, that is, the distance R-MM increases at nearly a constant low value of N-MM. The configurations of the two monomeric proteins so formed are found to be a bit different due to the entropic reasons. Water is seen to play a key role in the dissociation process stabilizing the intermediates along the reaction path. Our study reveals interesting molecular details during the dissociation, such as the variation in the structural and relative orientational arrangement of the amino acid residues along the minimum energy path. The conformational changes of monomeric insulin in the stable dimer and in the intermediate states during dimer dissociation have been studied in detail. Published by AIP Publishing