196,221 research outputs found
Layered LaSrGa3O7-based oxide-ion conductors: cooperative transport mechanisms and flexible structures
No full textNovel melilite-type gallium-oxides are attracting attention as promising new
oxide-ion conductors with potential use in clean energy devices such as solid
oxide fuel cells. Here, an atomic-scale investigation of the LaSrGa3O7-based
system using advanced simulation techniques provides valuable insights into
the defect chemistry and oxide ion conduction mechanisms, and includes comparison
with the available experimental data. The simulation model reproduces
the observed complex structure composed of layers of corner-sharing GaO4
tetrahedra. A major finding is the first indication that oxide-ion conduction in
La1.54Sr0.46Ga3O7.27 occurs through an interstitialcy or cooperative-type mechanism
involving the concerted knock-on motion of interstitial and lattice oxide
ions. A key feature for the transport mechanism and high ionic conductivity
is the intrinsic flexibility of the structure, which allows considerable local
relaxation and changes in Ga coordination
On the area of the graph of a piecewise smooth map from the plane to the plane with a curve discontinuity
Get PDFIn this paper we provide an estimate from above for the value of the relaxed area functional for a map defined on a bounded domain Ω of the plane taking values in the real plane and discontinuous on a simple curve, with two endpoints. We show that, under certain assumptions, the relaxed area does not exceed the area of the regular part of the function, with the addition of a singular term measuring the area of a disk-type solution of the Plateau's problem spanning the two traces of the function across the jump set. The result is valid also when the minimal surface has self-intersections. A key element in our argument is to show the existence of what we call a semicartesian parametrization of the minimal surface, namely a conformal parametrization defined on a suitable parameter space, which is the identity in the first component. To prove our result, various tools of parametric minimal surface theory are used, as well as some results from Morse theory
Semicartesian surfaces and the relaxed area of maps from the plane to the plane with a line discontinuity
No full textIn this paper, we estimate the area of the graph of a map u: Ω⊂ R2→ R2 discontinuous on a segment Ju, with Ju either compactly contained in the bounded open set Ω , or starting and ending on ∂Ω. We characterize A ̄ ∞(u, Ω) , the relaxed area functional in a sort of uniform convergence, in terms of the infimum of the area of those surfaces in R3 spanning the graphs of the traces of u on the two sides of Ju and having what we have called a semicartesian structure. We exhibit examples showing that A ̄ (u, Ω) , the relaxed area in L1(Ω; R2) , may depend on the values of u far from Ju and also on the relative position of Ju with respect to ∂Ω. These examples confirm the highly non-local behavior of A ̄ (u, ·) and justify the interest in the study of A ̄ ∞. Finally we prove that A ̄ (u, ·) is not subadditive for a rather large class of discontinuous maps u
Feeling the strain:enhancing ionic transport in olivine phosphate cathodes for Li- and Na-ion batteries through strain effects
Get PDFOlivine-type phosphates LiFePO4 and NaFePO4 are among the most widely studied cathode materials for rechargeable batteries. To improve their rate behaviour for future electronic and vehicle applications, it is vital that the Li+ and Na+ conductivities be enhanced. In this study, atomistic simulation methods (including molecular dynamics) are used to investigate the effect of lattice strain on ion transport and defect formation in olivine-type LiFePO4 and NaFePO4, as these properties are directly related to their intercalation behaviour. The results suggest that lattice strain can have a remarkable effect on the rate performance of cathode materials, with a major increase in the ionic conductivity and decrease in blocking defects at room temperature. Such understanding is important for the future optimization of high-rate cathodes for rechargeable batteries, and is relevant to the growing interest in developing thin film solid-state batteries
Design of Perovskite-Type Fluorides Cathodes for Na-ion Batteries: Correlation between Structure and Transport
No full textTransition metal-based sodium fluoro-perovskite of general formula NaMF3 (M = Fe, Mn, and Co) were investigated as cathode materials for rechargeable Na-ion batteries. Preliminary results indicated Na-ion reversible intercalation but highlighted the need to find optimization strategies to improve conductivity and to modulate the operating voltages within experimentally accessible electrolytes’ stability windows, in order to fully exploit their potential as high-voltage cathodes. In this study, we combined experimental and computational techniques to investigate structures, defects, and intercalation properties of the NaFe1-xMnxF3 and NaCo1-xMnxF3 systems. Through the use of a simple solvothermal synthesis, we demonstrated the possibility to modulate the sample’s morphology in order to obtain fine and dispersed powder samples. The structural results indicated the formations of two solid solutions with a perovskite structure over the entire compositional range investigated. Atomistic simulations suggested that Na-ion diffusion in these systems was characterized by relatively high migration barriers and it was likely to follow three-dimensional paths, thus limiting the effect of anti-site defects. The correlation between structural and computational data highlighted the possibility to modulate both ionic and electronic conductivity as a function of the composition
Na+diffusion mechanism and transition metal substitution in tunnel-type manganese-based oxides for Na-ion rechargeable batteries
No full textNa0.44MnO2 (NMO) with a tunnel-type structure is a reference cathode material for rechargeable Na-ion batteries. In this work, structural, electrochemical and computational investigations are combined to study the properties of this material, particularly with reference to Cu substitution in the structure. For the first time, molecular dynamics (MD) is used to obtain insights into the mechanisms of Na+ diffusion in NMO, highlighting the role of structural modifications and Na distribution. The main results allow the investigation of the implication of high temperature treatments and the effect of Cu substitution on the defect and transport properties of the material with a tunnel-type structure. From an experimental point of view, the substitution promotes an increased stability of the material upon cycling and an improved capacity particularly at higher discharging rates, that stems from the synergistic effects of the composition, morphology and multiple polymorphs of the sample
Design of a bioinspired ray robot with flexible fins
Get PDFThis paper presents the design and construction of a biomimetic swimming robot inspired by the locomotion of rays. These fishes move by flapping their pectoral fins and creating a wave that moves in the opposite direction to the direction of motion, pushing the water back and giving the fish a propulsive force due to momentum conservation. The robot's fins are molded from silicone rubber and moved by a servo motor that drives a mechanism inside the leading edge of each fin. The traveling wave, mimicking the movement of the fin, is passively generated by the flexibility of the rubber itself. The robot is also equipped with a tail that acts as a rudder, helpful in performing maneuvers. The rigid central body of the robot is the housing for motors, electronics, and batteries
A Bioinspired Cownose Ray Robot for Seabed Exploration
Get PDFThis article presents the design and the experimental tests of a bioinspired robot mimicking the cownose ray. These fish swim by moving their large and flat pectoral fins, creating a wave that pushes backward the surrounding water so that the fish is propelled forward due to momentum conservation. The robot inspired by these animals has a rigid central body, housing motors, batteries, and electronics, and flexible pectoral fins made of silicone rubber. Each of them is actuated by a servomotor driving a link inside the leading edge, and the traveling wave is reproduced thanks to the flexibility of the fin itself. In addition to the pectoral fins, two small rigid caudal fins are present to improve the robot’s maneuverability. The robot has been designed, built, and tested underwater, and the experiments have shown that the locomotion principle is valid and that the robot is able to swim forward, perform left and right turns, and do floating or diving maneuvers
Lattice strain effects on doping, hydration and proton transport in scheelite-type electrolytes for solid oxide fuel cells
Get PDFLattice strain is considered a promising approach to modulate the structural and functional properties of oxide materials. In this study we investigate the effect of lattice strain on doping, hydration and proton transport for the family of scheelite-type proton conductors using both atomistic and DFT computational methods. The results suggest that tensile strain improves the dopant solubility and proton uptake of the material. The anisotropic proton pathways change from being within the a-b plane to being in the a-c plane. However, the predicted reduction in the migration barrier suggests that improvements in ionic conductivity due to lattice strain effects will be limited, in contrast with the work on oxide ion conduction. Such results are rationalized in terms of structural changes and differences in migration steps between oxide ions and protonic species.</p
- …
