36 research outputs found
Reduced-Order Model to Predict Thermal Conductivity of Dimensionally-Confined Materials
Predicting nanoscale thermal transport in dielectrics requires models, such
as the Boltzmann transport equation (BTE), that account for phonon boundary
scattering in structures with complex geometries. Although the BTE has been
validated against several key experiments, its computational expense limits its
applicability. Here, we demonstrate the use of an analytic reduced-order model
for predicting the thermal conductivity in dimensionally confined materials,
i.e., monolithic and porous thin films, and rectangular and cylindrical
nanowires. The approach uses the recently developed "Ballistic Correction
Model" (BCM) which accounts for materials' full distribution of phonon
mean-free-paths. The model is validated against BTE simulations for a selection
of base materials, obtaining excellent agreement. By furnishing a precise yet
easy-to-use prediction of thermal transport in nanostructures, our work strives
to accelerate the identification of materials for energy-conversion and
thermal-management applications.Comment: This article may be downloaded for personal use only. Any other use
requires prior permission of the author and AIP Publishing. This article
appeared in S. A. Hosseini et al., Appl. Phys. Lett. 122 (26): 262202 (2023)
and may be found at https://doi.org/10.1063/5.014979
Heat current anticorrelation effects leading to thermal conductivity reduction in nanoporous Si
Prevailing nanostructuring strategies focus on increasing phonon scattering and reducing the mean-free-path of phonons across the spectrum. In nanoporous Si materials, for example, boundary scattering reduces thermal conductivity drastically. In this work, we identify an unusual anticorrelated specular phonon scattering effect which can result in additional reductions in thermal conductivity of up to âŒ80% for specific nanoporous geometries. We further find evidence that this effect has its origin in heat trapping between large pores with narrow necks. As the heat becomes trapped between the pores, phonons undergo multiple specular reflections such that their contribution to the thermal conductivity is partly undone. We find this effect to be wave-vector dependent at low temperatures. We use large-scale molecular-dynamics simulations, wave-packet analysis, as well as an analytical model to illustrate the anticorrelation effect, evaluate its impact on thermal conductivity, and detail how it can be controlled to manipulate phonon transport in nanoporous materials
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Role of seta angle and flexibility in the gecko adhesion mechanism
A model is developed to describe the reversible nature of gecko dry adhesion. The central aspect of this model is that the seta can be easily peeled away from the contacting surface by a small moment at the contact tip. It is shown that this contact condition is very sensitive, but can result in robust adhesion if individual setae are canted and highly flexible. In analogy to the âcone of friction,â we consider the âadhesion regionââthe domain of normal and tangential forces that maintain adhesion. Results demonstrate that this adhesion region is highly asymmetric enabling the gecko to adhere under a variety of loading conditions associated with scuttling horizontally, vertically, and inverted. Moreover, under each of these conditions, there is a low energy path to de-adhesion. In this model, obliquely canted seta (as possessed by geckos) rather than vertically aligned fibers (common in synthetic dry adhesive) provides the most robust adhesion
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Reversible intercalation of methyl viologen as a dicationic charge carrier in aqueous batteries.
The interactions between charge carriers and electrode structures represent one of the most important considerations in the search for new energy storage devices. Currently, ionic bonding dominates the battery chemistry. Here we report the reversible insertion of a large molecular dication, methyl viologen, into the crystal structure of an aromatic solid electrode, 3,4,9,10-perylenetetracarboxylic dianhydride. This is the largest insertion charge carrier when non-solvated ever reported for batteries; surprisingly, the kinetic properties of the (de)insertion of methyl viologen are excellent with 60% of capacity retained when the current rate is increased from 100âmA g-1 to 2000âmA g-1. Characterization reveals that the insertion of methyl viologen causes phase transformation of the organic host, and embodies guest-host chemical bonding. First-principles density functional theory calculations suggest strong guest-host interaction beyond the pure ionic bonding, where a large extent of covalency may exist. This study extends the boundary of battery chemistry to large molecular ions as charge carriers and also highlights the electrochemical assembly of a supramolecular system
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Surprising behaviour during dissipation and collision of flexural waves in carbon nanotubes
The manuscript reports on simulations of the intrinsic dissipation of standing and traveling
flexural vibrations in carbon nanotubes. It is found that extended traveling and standing waves
exhibit anomalous dissipation, during which the excited modes experience massive damping
that is triggered by the accumulation of energy in special gateway modes. In the second part of
this work the attenuation of traveling flexural wave packets is examinedâincluding the
collisions between wave packets. Surprisingly, these wave packets show markedly different
dissipation behaviour from extended waves with the same wavelength and amplitude.
Moreover, the wave packet collisions are seen to be sensitive to the direction of collision,
hinting at temperature gradient induced reduction of the thermal conductivity. Following the
cascade of energy as it dissipates it is seen that scattering of energy into other flexural modes
has little effect on the net energy flux, while dissipation into non-flexural modes is thermally
resistive
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Resonant behavior in heat transfer across weak molecular interfaces
Molecular dynamics (MD) simulations are used to study, in detail, the transfer of thermal (vibrational) energy between objects with discrete vibrational spectra to those with a semi-continuum of spectra. The transfer of energy is stochastic and strongly dependent on the instantaneous separation between the bodies. The insight from the MD simulations can be captured with a simple classical model that agrees well with quantum models. This model can be used to optimize systems for efficient frequency selective energy transfer, which can be used in designing a chemical sensor through nanomechanical resonance spectroscopy.Article Copyright 2013 American Institute of Physics. This article may be downloaded for personal use only. Any other use requires prior permission of the author and the American Institute of Physics. This is the publisherâs final pdf. The published article can be found at: http://scitation.aip.org/content/aip/journal/jap
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Predicting Capacity of Hard Carbon Anodes in Sodium-Ion Batteries Using Porosity Measurements
We report an inverse relationship between measurable porosity values and reversible capacity from sucrose-derived hard carbon as an anode for sodium-ion batteries (SIBs). Materials with low measureable pore volumes and surface areas obtained through Nâ sorption yield higher reversible capacities. Conversely, increasing measurable porosity and specific surface area leads to sharp decreases in reversible capacity. Utilizing a low porosity material, we thus are able to obtain a reversible capacity of 335 mAh gâ»Âč. These findings suggest that sodium-ion storage is highly dependent on the absence of pores detectable through Nâ sorption in sucrose-derived carbon