1,721,053 research outputs found
Factors affecting cyclic durability of all-solid-state lithium batteries using poly(ethylene oxide)-based polymer electrolytes and recommendations to achieve improved performance
Get PDFA detailed experimental analysis of the factors affecting cyclic durability of all-solid-state lithium batteries using poly(ethylene oxide)-based polymer electrolytes was published in EES by Nakayama et al. We use quantum mechanics to interpret these results, identifying processes involved in the degradation of rechargeable lithium batteries based on polyethylene oxide (PEO) polymer electrolyte with LiTFSI. We consider that ionization of the electrolyte near the cathode at the end of the recharge step is probably responsible for this degradation. We find that an electron is likely removed from PEO next to a TFSI anion, triggering a sequence of steps leading to neutralization of a TFSI anion and anchoring of another TFSI to the PEO. This decreases the polymer conductivity near the cathode, making it easier to ionize additional PEO and leading to complete degradation of the battery. We refer to this as the Cathode Overpotential Driven Ionization of the Solvent (CODIS) model. We suggest possible ways to confirm experimentally our interpretation and propose modifications to suppress or reduce electrolyte degradation
Effects of filling in CoSb3: Local structure, band gap, and phonons from first principles
No full textWe use ab initio computations to investigate the effect of filler ions on the properties of CoSb3 skutterudites. We analyze global and local structural effects of filling, using the Ba-filled system as an example. We show that the deformation of Sb network induced by the filler affects primarily nearest neighboring Sb sites around the filler site as the soft Sb rings accommodate the distortion. Rearrangement of Sb atoms affects the electronic band structure and we clarify the effect of this local strain on the band gap. We compute the phonon dispersions and identify the filler-dominated modes from the lowest-frequency optical modes at Gamma. Their weak dispersion across the Brillouin zone indicates that they are localized and a force-constant analysis shows that the filler vibration is strongly coupled with nearby Sb atoms.Massachusetts Institute of Technology. Energy InitiativeRobert Bosch Gmb
Electronic, vibrational, and transport properties of pnictogen-substituted ternary skutterudites
No full textFirst principles calculations are used to investigate electronic band structure and vibrational spectra of pnictogen-substituted ternary skutterudites. We compare the results with the prototypical binary composition CoSb3 to identify the effects of substitutions on the Sb site, and evaluate the potential of ternary skutterudites for thermoelectric applications. Electronic transport coefficients are computed within the Boltzmann transport formalism assuming a constant relaxation time, using a methodology based on maximally localized Wannier function interpolation. Our results point to a large sensitivity of the electronic transport coefficients to carrier concentration and to scattering mechanisms associated with the enhanced polarity. The ionic character of the bonds is used to explain the detrimental effect on the thermoelectric properties.National Science Foundation (U.S.), United States. Dept. of Energy Partnership in Thermoelectrics (CBET-0853350
Going Beyond Counting First Authors in Author Co-citation Analysis
Get PDFThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Get PDF“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
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Nanocalorimetry Study of Phase Transformations in Thin-Film Shape Memory Alloys
No full textThe nanocalorimetry technique enables measurement of novel thin film materials at non-equilibrium conditions. This study utilizes the synergy between nanocalorimetry, transmission electron microscopy, density functional theory simulations, and theoretical models to explore the Cu-Zr and Ni-Ti shape memory alloy systems, revealing interesting phenomena and their underlying mechanisms.
We have investigated the phase evolution of sputter-deposited equiatomic Cu-Zr thin films. The nanocalorimetry heat treatment to form the shape memory phases are determined. We found that in CuZr and some of its ternary alloys, the austenitic phase can be supercooled below the normal temperature range of martensitic transformation, resulting in an explosive-type transformation that converts the entire sample in microseconds. This phenomenon is due to a lack of nucleation sites and can be controlled by annealing of defects.
We carried out a computational-experimental study on the ternary alloying effect of CuZr-based shape memory alloy. The computed energy difference between martensite and austenite suggests that both Co and Ni increases the transformation temperature. However, experiments show that the effect of Co goes in the opposite direction. We attribute this discrepancy to the microstructure-related twin boundary energy and strain energy terms.
We found that the crystallization of NiTi and NiTiFe splits into two steps at ultrafast heating rates. The first step forms large grains of supersaturated austenite, and the second step forms secondary phases within the large grains. When a small percentage of ternary element Fe is added, the apparent activation energies of both steps drop significantly, and the resulting grain size is much smaller. Further increase of heating rate to above 10,000 K/s results in less time for nucleation and rapid grain growth of NiTiFe
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Nanoscale spin-polarized imaging of magnetic Weyl semimetal CeBi
No full textWeyl fermions are massless excitations with definite chirality. They manifest around band-touching points in the bulk band structure when inversion or time-reversal symmetry is broken. Among Weyl semimetals, those breaking time-reversal symmetry are of special interest due to their potential for the controlled manipulation of Weyl points through magnetic structure engineering. Here, we present a systematic investigation of the Weyl state in different magnetic phases of CeBi and report the discovery of a tunable magnetic Weyl semimetal state within nanoscale fully-polarized domains in the antiferromagnetic (++−−) CeBi.
In the ferrimagnetic (+++−) phase, quasiparticle interference (QPI) measurements show a 100 meV band splitting in the Bi 6p band, which supports the existence of Weyl points in this phase. Similarly, in the fully-polarized (++++) state, we observed at least a 200 meV band splitting in the Bi 6p band along the kx axis, supporting the existence of Weyl nodes in fully-polarized CeBi.
We demonstrate the creation of Weyl nodes by inducing local ferromagnetic (FM) domains in antiferromagnetic CeBi. These FM domains consist of co-aligned Ce moments and can be generated through in-plane magnetic field training or by employing a scanning tunneling microscopy (STM) tip to induce local strain. We image the formation of Weyl fermions around these FM domains by measuring QPI patterns. Our results not only demonstrate CeBi as an excellent magnetic Weyl semimetal for investigating intrinsic Weyl physics but also show the possibility of controllably writing the Weyl phase at the nanoscale.
Finally, we present a novel way to accelerate QPI measurements by acquiring sparsely measured dI/dV maps and full-grid topography maps simultaneously. The acquired full-grid topography maps facilitate the computation of drifting phase maps, essential for correcting lattice distortions within reconstructed dI/dV maps. Our results demonstrate the effectiveness of sparse sampling, thereby broadening the scope of QPI applications
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Large-scale atomistic simulation at near-quantum accuracy with equivariant machine learning
No full textSince the first digital computers were built, scientists have programmed them to make concrete predictions about real-world systems by numerically solving the relevant physical laws that govern the behavior of those systems. Many of these efforts have focused on simulating the behavior of the atoms that make up materials and chemicals.
As computational power has grown, these simulations have shifted from using simple qualitative models of idealized interatomic interactions to sophisticated techniques for solving the quantum physics of realistic systems of electrons and atomic nuclei. These first-principles models offer a remarkable degree of accuracy and predictive power, but their computational cost also imposes severe limitations on the kinds of length- and time-scales that can be simulated.
Machine learning models of interatomic interactions---called machine learning interatomic potentials (MLIPs)---can be used as fast approximations of these accurate but costly quantum calculations. In principle, MLIPs can enable much longer and larger simulations to be run at near-quantum accuracy.
This thesis presents algorithms and implementations that can realize this promise through "equivariant" machine learning techniques that leverage the symmetries of the underlying physics. The first half discusses the equivariant MLIP NequIP, with its significantly improved data efficiency, accuracy, generalization, and robustness. The second half presents the development of the novel Allegro architecture. Allegro makes it possible to scale up equivariant machine learning: this ability is demonstrated through comprehensive performance experiments that apply a powerful Allegro model trained on a large dataset of more than 1M quantum calculations to biomolecular systems of realistic sizes of up to 44M atoms while taking advantage of up to 5120 GPUs
Appropriate Similarity Measures for Author Cocitation Analysis
Get PDFWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
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