442 research outputs found
Engineering Heteromaterials to Control Lithium Ion Transport Pathways.
Safe and efficient operation of lithium ion batteries requires precisely directed flow of lithium ions and electrons to control the first directional volume changes in anode and cathode materials. Understanding and controlling the lithium ion transport in battery electrodes becomes crucial to the design of high performance and durable batteries. Recent work revealed that the chemical potential barriers encountered at the surfaces of heteromaterials play an important role in directing lithium ion transport at nanoscale. Here, we utilize in situ transmission electron microscopy to demonstrate that we can switch lithiation pathways from radial to axial to grain-by-grain lithiation through the systematic creation of heteromaterial combinations in the Si-Ge nanowire system. Our systematic studies show that engineered materials at nanoscale can overcome the intrinsic orientation-dependent lithiation, and open new pathways to aid in the development of compact, safe, and efficient batteries
Polytropic behavior in the structures of Interplanetary Coronal Mass Ejections
The polytropic process characterizes the thermodynamics of space plasma
particle populations. The polytropic index, , is particularly
important as it describes the thermodynamic behavior of the system by
quantifying the changes in temperature as the system is compressed or expanded.
Using Wind spacecraft plasma and magnetic field data during , we investigate the thermodynamic evolution in 336 Interplanetary
Coronal Mass Ejection (ICME) events. For each event, we derive the index
in the sheath and magnetic ejecta structures, along with the pre-
and post- event regions. We then examine the distributions of all
indices in these four regions and derive the entropic gradient of each, which
is indicative of the ambient heating. We find that in the ICME sheath region,
where wave turbulence is expected to be highest, the thermodynamics takes
longest to recover into the original quasi-adiabatic process, while it recovers
faster in the quieter ejecta region. This pattern creates a thermodynamic
cycle, featuring a near adiabatic value ~ (=5/3)
upstream of the ICMEs, - ~ 0.26 in the sheaths,
- ~ 0.13 in the ICME ejecta, and recovers again to
~ after the passage of the ICME. These results
expose the turbulent heating rates in the ICME plasma: the lower the polytropic
index from its adiabatic value and closer to its isothermal value, the larger
the entropic gradient, and thus, the rate of turbulent heating that heats the
ICME plasma.Comment: 9 pages, 3 figure
Spherical Harmonic Representation of Energetic Neutral Atom Flux Components Observed by IBEX
The Interstellar Boundary Explorer (IBEX) images the heliosphere by observing
energetic neutral atoms (ENAs). The IBEX-Hi instrument onboard IBEX provides
full-sky maps of ENA fluxes produced in the heliosphere and very local
interstellar medium (VLISM) through charge exchange of suprathermal ions with
interstellar neutral atoms. The first IBEX-Hi results showed that in addition
to the anticipated globally distributed flux (GDF), a narrow and bright
emission from a circular region in the sky, dubbed the IBEX ribbon, is visible
in all energy steps. While the GDF is mainly produced in the inner heliosheath,
ample evidence indicates that the ribbon forms outside the heliopause in the
regions where the interstellar magnetic field is perpendicular to the lines of
sight. The IBEX maps produced by the mission team distribute the observations
into rectangle pixels in ecliptic coordinates. The overlap
of the GDF and ribbon components complicates qualitative analyses of each
source. Here, we find the spherical harmonic representation of the IBEX maps,
separating the GDF and ribbon components. This representation describes the ENA
flux components in the sky without relying on any pixelization scheme. Using
this separation, we discuss the temporal evolution of each component over the
solar cycle. We find that the GDF is characterized by larger spatial scale
structures than the ribbon. However, we identify two isolated, small-scale
signals in the GDF region that require further study.Comment: 27 pages, 13 figures, v2 accepted for publication in ApJ
A Discrete Mathematical Model for the Aggregation of \u3ci\u3eβ-Amyloid\u3c/i\u3e
Dementia associated with the Alzheimer\u27s disease is thought to be correlated with the conversion of the β − Amyloid (Aβ) peptides from soluble monomers to aggregated oligomers and insoluble fibrils. We present a discrete-time mathematical model for the aggregation of Aβ monomers into oligomers using concepts from chemical kinetics and population dynamics. Conditions for the stability and instability of the equilibria of the model are established. A formula for the number of monomers that is required for producing oligomers is also given. This may provide compound designers a mechanism to inhibit the Aβ aggregation
Lattice strain effects on the optical properties of MoS2 nanosheets.
"Strain engineering" in functional materials has been widely explored to tailor the physical properties of electronic materials and improve their electrical and/or optical properties. Here, we exploit both in plane and out of plane uniaxial tensile strains in MoS2 to modulate its band gap and engineer its optical properties. We utilize X-ray diffraction and cross-sectional transmission electron microscopy to quantify the strains in the as-synthesized MoS2 nanosheets and apply measured shifts of Raman-active modes to confirm lattice strain modification of both the out-of-plane and in-plane phonon vibrations of the MoS2 nanosheets. The induced band gap evolution due to in-plane and out-of-plane tensile stresses is validated by photoluminescence (PL) measurements, promising a potential route for unprecedented manipulation of the physical, electrical and optical properties of MoS2
Investigating the IBEX Ribbon Structure a Solar Cycle Apart
A Ribbon of enhanced energetic neutral atom (ENA) emissions was discovered by
the Interstellar Boundary Explorer (IBEX) in 2009, redefining our understanding
of the heliosphere boundaries and the physical processes occurring at the
interstellar interface. The Ribbon signal is intertwined with that of a
globally distributed flux (GDF) that spans the entire sky. To a certain extent,
Ribbon separation methods enabled examining its evolution independent of the
underlying GDF. Observations over a full solar cycle revealed the Ribbon's
evolving nature, with intensity variations closely tracking those of the solar
wind (SW) structure after a few years delay accounting for the SW-ENA recycling
process. In this work, we examine the Ribbon structure, namely, its ENA fluxes,
angular extent, width, and circularity properties for two years, 2009 and 2019,
representative of the declining phases of two adjacent solar cycles. We find
that, (i) the Ribbon ENA fluxes have recovered in the nose direction and south
of it down to ~ 25{\deg} (for energies below 1.7 keV) and not at mid and high
ecliptic latitudes; (ii) The Ribbon width exhibits significant variability as a
function of azimuthal angle; (iii) Circularity analysis suggests that the 2019
Ribbon exhibits a statistically consistent radius with that in 2009. The
Ribbon's partial recovery is aligned with the consensus of a heliosphere with
its closest point being southward of the nose region. The large variability of
the Ribbon width as a function of Azimuth in 2019 compared to 2009 is likely
indicative of small-scale processes within the Ribbon.Comment: 5 figure
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A fast-settling folded-cascode CMOS operational amplifier
A fast-settling CMOS operational amplifier using the
folded-cascode topology has been designed. The design is
based on a theory developed by Yang et al for the optimum
settling time of two-pole systems for switched-capacitor
applications. The theory states that the two poles are
separated by a given factor in order to obtain a minimum
small-signal settling time. The design is achieved by
performing a complete small-signal analysis to gain a
better understanding of the frequency and time responses
of the folded-cascode op-amp. SPICE simulation results
are in good agreement with the theoretical predictions
First IBEX observations of the terrestrial plasma sheet and a possible disconnection event
The Interstellar Boundary Explorer (IBEX) mission has recently provided the first all-sky maps of energetic neutral atoms (ENAs) emitted from the edge of the heliosphere as well as the first observations of ENAs from the Moon and from the magnetosheath stagnation region at the nose of the magnetosphere. This study provides the first IBEX images of the ENA emissions from the nightside magnetosphere and plasma sheet. We show images from two IBEX orbits: one that displays typical plasma sheet emissions, which correlate reasonably well with a model magnetic field, and a second that shows a significant intensification that may indicate a near-Earth (similar to 10 R(E) behind the Earth) disconnection event. IBEX observations from similar to 0.5-6 keV indicate the simultaneous addition of both a hot (several keV) and colder (similar to 700 eV) component during the intensification; if IBEX directly observed magnetic reconnection in the magnetotail, the hot component may signify the plasma energization
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