This paper studies the stability problem of linear systems with two additive time-varying delays. Firstly, a modified Lyapunov-Krasovskii functional (LKF) is constructed involving the delay-dependent ...non-integral items and some augmented single-integral items. Secondly, based on the improved LKF, a stability criterion is derived via the generalized free-matrix-based inequality application, which is less conservative than some recent existing ones. Thirdly, a generalized refinement of allowable delay set removed some impossible delay subsets is proposed to further reduce the conservatism of the stability criterion. Finally, a numerical example and an application of the load frequency control for a single area power system are presented to show the effectiveness of the proposed approach.
Teaching assistants (TAs) that lead reformed recitations and labs must understand and buy into the design of the course and the research-based instructional strategies that the course requires in ...order to create high-fidelity implementations. We present a model that outlines possible influences on TAs' buy-in and their in-class actions coupled with a method, using a Real-time Instructor Observation Tool-based E. A.West et al. "Phys. Rev. ST Phys. Educ. Res." 9, 010109 (2013) exercise, to measure the effect of these influences that is not only quicker than interviews but also allows one to quantify these effects. We use this method to measure the influences on six graduate TAs teaching algebra-based introductory mechanics and electricity and magnetism recitations and labs ("mini studios") at the University of Central Florida. The results from the exercise are confirmed by interview responses from the TAs. We find a relatively high degree of buy-in to the design of the course, yet this is not reflected in the TAs' actions. The TAs' actions appear to be most influenced by student responses and expectations which do not align with the design of the course. Our study examines the effect of three influences shown in our model, and we argue that our method could be easily adapted to examine additional influences.
Rechargeable Lithium ion batteries (LIBs) are commonly used power source in everyday electrical devices. Understanding the real-time atomistic evolution of the electrodes in a LIB is indispensable ...since the performance of a LIB is governed by electrochemical reactions and possible mechanical degradation of the electrodes during the operation of a battery. With advances in high resolution transmission electron microscopy and microfabrication techniques, it has been possible to investigate the evolution of solid electrolyte interphase, electrolyte decomposition, and the response of the electrode upon lithiation-delithiation at the atomic scale. This article provides a comprehensive review of the current understanding of lithiation-delithiation mechanisms in LIBs.
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•In situ TEM observation provides an extensive overview of structural evolution of electrodes during lithiation and delithiation mechanism.•Understanding fracture mechanism helps to develop strategies to control crack formation and mechanical degradation during cycling.•Lithiation studies show that electrochemical reactions are material-specific, size-dependent, and phase-governed processes.
Nanoparticles have enormous potential in diagnostic and therapeutic studies. We have demonstrated that the amyloid beta mixed with and conjugated to dihydrolipoic acid- (DHLA) capped CdSe/ZnS quantum ...dots (QDs) of size approximately 2.5 nm can be used to reduce the fibrillation process. Transmission electron microscopy (TEM) and atomic force microscopy (AFM) were used as tools for analysis of fibrillation. There is a significant change in morphology of fibrils when amyloid β (1–42) (Aβ (1–42)) is mixed or conjugated to the QDs. The length and the width of the fibrils vary under modified conditions. Thioflavin T (ThT) fluorescence supports the decrease in fibril formation in presence of DHLA-capped QDs.
The comprehension of fundamental electrochemical behavior and sodiation mechanism is critical for the design of high-performance electrode materials for sodium-ion (Na-ion) batteries. In this paper, ...the electrochemical sodiation process and microstructure evolution of individual Co9S8-filled carbon nanotubes (CNTs) have been directly visualized and studied using in situ transmission electron microscopy. Upon the first sodiation, a reaction front propagates progressively along the filling nanowire, causing the filled CNT to inflate. The filled CNTs behave differently depending on their structures and the magnitude of the sodiation voltage. For a Co9S8-filled CNT with an open end, the sodiated Co9S8 filler shows a substantial axial elongation of 120.8% and a small radial swelling due to the extrusion of CNT walls. In contrast, the closed CNT shows a major radial expansion of 40.6% and a small axial elongation because of the mechanical confinement of the carbon shells. After sodiation, the spacing between the carbon shells increases from 3.4 to 3.8 Å due to the Na+-ion insertion and the single-crystalline Co9S8 filler converts to numerous Co nanograins dispersed in a Na2S matrix. Compared with the gentle microstructure evolution of the CNT under small charging voltage, a strong electrochemical reaction accompanying drastic swelling and fracturing of CNT shells is observed for the CNT electrode under large charging voltage. Our observations provide direct evidence and important insights for the electrochemical process of CNT-based composite materials in Na-ion batteries.
Carbon nanotube (CNT)-encapsulated metal sulfides/oxides are promising candidates for application as anode materials in lithium ion battery (LIB), while their electrochemical behavior and mechanism ...still remain unclear. A comprehensive understanding of the lithiation mechanism at nanoscale of this type of composites will benefit the design and development of high-performance LIB materials. Here, we use Co9S8/Co nanowire-filled CNTs as a model material to investigate the lithium storage mechanism by in situ transmission electron microscopy. For a Co9S8/Co nanowire-filled closed CNT, the reaction front propagates progressively during lithiation, causing an axial elongation of 4.5% and a radial expansion of 32.4%, while the lithiated nanowire core is still confined inside the CNT. Contrastingly, for an open CNT, the lithiated Co9S8 nanowire shows an axial elongation of 94.2% and is extruded out from the open CNT. In particular, a thin graphite shell is drawn out from the CNT wall by the extruded lithiated Co9S8. The thin graphite shell confines the extruded filler and protects the filler from pulverization in the following lithiation–delithiation cycles. During multiple cycles, the Co segment remains intact while the Co9S8 exhibits a reversible transformation between Co9S8 and Co nanograins. Our observations provide direct electrochemical behavior and mechanism that govern the CNT-based anode performance in LIBs.
Thin gold films offer intriguing material properties for potential applications including fuel cells, supercapacitors, and electronic and photonic devices. We describe here an ambient filtration ...method that provides a simple and novel way to generate rapidly porous and thin gold films without the need for sophisticated instruments, clean-room environments, and any postgrowth process or sintering steps. Using this approach, we can fabricate highly conductive gold films composed of gold nanoparticles layered atop a matrix of metallic single-walled carbon nanotubes on mixed cellulose ester filter paper within 20 min. These hybrid films (thickness ∼40 nm) exhibit fast electron transfer and excellent electrocatalytic properties that are similar to purchased gold films, but with a larger electroactive surface that lends itself to more sensitive analyte detection. We used the neurotransmitters dopamine and serotonin as benchmark analytes to demonstrate that our hybrid gold films can clearly discriminate the presence of both molecules in a mixture with resolution that greatly exceeds that of either purchased gold slides or electrodeposited gold films. Importantly, we postulate that this new approach could readily be generalized for the rapid fabrication of films from various other metals under ambient conditions, and could also be used as a prelude to transferring the resulting films onto glass or other flexible substrates.
The comprehension of fundamental electrochemical behavior and sodiation mechanism is critical for the design of high-performance electrode materials for sodium-ion (Na-ion) batteries. In this paper, ...the electrochemical sodiation process and microstructure evolution of individual Co sub(9)S sub(8)-filled carbon nanotubes (CNTs) have been directly visualized and studied using in situ transmission electron microscopy. Upon the first sodiation, a reaction front propagates progressively along the filling nanowire, causing the filled CNT to inflate. The filled CNTs behave differently depending on their structures and the magnitude of the sodiation voltage. For a Co sub(9)S sub(8)-filled CNT with an open end, the sodiated Co sub(9)S sub(8) filler shows a substantial axial elongation of 120.8% and a small radial swelling due to the extrusion of CNT walls. In contrast, the closed CNT shows a major radial expansion of 40.6% and a small axial elongation because of the mechanical confinement of the carbon shells. After sodiation, the spacing between the carbon shells increases from 3.4 to 3.8 Aa due to the Na super(+)-ion insertion and the single-crystalline Co sub(9)S sub(8) filler converts to numerous Co nanograins dispersed in a Na sub(2)S matrix. Compared with the gentle microstructure evolution of the CNT under small charging voltage, a strong electrochemical reaction accompanying drastic swelling and fracturing of CNT shells is observed for the CNT electrode under large charging voltage. Our observations provide direct evidence and important insights for the electrochemical process of CNT-based composite materials in Na-ion batteries. Keywords: sodium-ion battery; carbon nanotube; in situ TEM; composite; sodiation
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