1,131 research outputs found
HT2009-88029 THERMAL CONDUCTIVITY OF WATER/CARBON NANOTUBE COMPOSITE SYSTEMS: INSIGHTS FROM MOLECULAR DYNAMICS SIMULATIONS
ABSTRACT The INTRODUCTION Due to their high thermal conductivities, graphene sheets and carbon nanotubes (CNT) are ideal candidates for nextgeneration thermal management device
Phonon Band Structure and Thermal Transport Correlation in a Layered Diatomic Crystal
To elucidate the relationship between a crystal's structure, its thermal
conductivity, and its phonon dispersion characteristics, an analysis is
conducted on layered diatomic Lennard-Jones crystals with various mass ratios.
Lattice dynamics theory and molecular dynamics simulations are used to predict
the phonon dispersion curves and the thermal conductivity. The layered
structure generates directionally dependent thermal conductivities lower than
those predicted by density trends alone. The dispersion characteristics are
quantified using a set of novel band diagram metrics, which are used to assess
the contributions of acoustic phonons and optical phonons to the thermal
conductivity. The thermal conductivity increases as the extent of the acoustic
modes increases, and decreases as the extent of the stop bands increases. The
sensitivity of the thermal conductivity to the band diagram metrics is highest
at low temperatures, where there is less anharmonic scattering, indicating that
dispersion plays a more prominent role in thermal transport in that regime. We
propose that the dispersion metrics (i) provide an indirect measure of the
relative contributions of dispersion and anharmonic scattering to the thermal
transport, and (ii) uncouple the standard thermal conductivity
structure-property relation to that of structure-dispersion and
dispersion-property relations, providing opportunities for better understanding
of the underlying physical mechanisms and a potential tool for material design.Comment: 30 pages, 10 figure
Phonon Transport Across a Vacuum Gap
Phonon transport across a silicon/vacuum-gap/silicon structure is modeled using lattice dynamics calculations and Landauer theory. The phonons transmit thermal energy across the vacuum gap via atomic interactions between the leads. Because the incident phonons do not encounter a classically impenetrable potential barrier, this mechanism is not a tunneling phenomenon. While some incident phonons transmit across the vacuum gap and remain in their original mode, many are annihilated and excite different modes. We show that the heat flux due to phonon transport can be 4 orders of magnitude larger than that due to photon transport predicted from near-field radiation theory
Observation of reduced thermal conductivity in a metal-organic framework due to the presence of adsorbates
Whether the presence of adsorbates increases or decreases thermal conductivity in metal-organic frameworks (MOFs) has been an open question. Here we report observations of thermal transport in the metal-organic framework HKUST-1 in the presence of various liquid adsorbates: water, methanol, and ethanol. Experimental thermoreflectance measurements were performed on single crystals and thin films, and theoretical predictions were made using molecular dynamics simulations. We find that the thermal conductivity of HKUST-1 decreases by 40 â 80% depending on the adsorbate, a result that cannot be explained by effective medium approximations. Our findings demonstrate that adsorbates introduce additional phonon scattering in HKUST-1, which particularly shortens the lifetimes of low-frequency phonon modes. As a result, the system thermal conductivity is lowered to a greater extent than the increase expected by the creation of additional heat transfer channels. Finally, we show that thermal diffusivity is even more greatly reduced than thermal conductivity by adsorption
Limits of dispersoid size and number density in oxide dispersion strengthened alloys fabricated with powder bed fusion-laser beam
Previous work on additively-manufactured oxide dispersion strengthened alloys
focused on experimental approaches, resulting in larger dispersoid sizes and
lower number densities than can be achieved with conventional powder
metallurgy. To improve the as-fabricated microstructure, this work integrates
experiments with a thermodynamic and kinetic modeling framework to probe the
limits of the dispersoid sizes and number densities that can be achieved with
powder bed fusion-laser beam. Bulk samples of a Ni-20Cr 1 wt.\% YO
alloy are fabricated using a range of laser power and scanning velocity
combinations. Scanning transmission electron microscopy characterization is
performed to quantify the dispersoid size distributions across the processing
space. The smallest mean dispersoid diameter (29 nm) is observed at 300 W and
1200 mm/s, with a number density of 1.010 m. The largest
mean diameter (72 nm) is observed at 200 W and 200 mm/s, with a number density
of 1.510 m. Scanning electron microscopy suggests that a
considerable fraction of the oxide added to the feedstock is lost during
processing, due to oxide agglomeration and the ejection of oxide-rich spatter
from the melt pool. After accounting for these losses, the model predictions
for the dispersoid diameter and number density align with the experimental
trends. The results suggest that the mechanism that limits the final number
density is collision coarsening of dispersoids in the melt pool. The modeling
framework is leveraged to propose processing strategies to limit dispersoid
size and increase number density.Comment: Main text: 36 pages, 12 figure
Open and Hidden Charm Production in Heavy Ion Collisions at Ultrarelativistic Energies
We consider the production of the open charm and J/psi mesons in heavy ion
collisions at BNL RHIC. We discuss several recently developed pictures for
J/psi production and argue that a measurement at RHIC energies is crucial for
disentangling these different descriptions.Comment: 19 pages, Latex, 5 PS-figures. v3: Fig.6 is adde
Hybridization from Guest-Host Interactions Reduces the Thermal Conductivity of Metal-Organic Frameworks
We experimentally and theoretically investigate the thermal conductivity and mechanical properties of polycrystalline HKUST-1 metalâorganic frameworks (MOFs) infiltrated with three guest molecules: tetracyanoquinodimethane (TCNQ), 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F-TCNQ), and (cyclohexane-1,4-diylidene)dimalononitrile (H-TCNQ). This allows for modification of the interaction strength between the guest and host, presenting an opportunity to study the fundamental atomic scale mechanisms of how guest molecules impact the thermal conductivity of large unit cell porous crystals. The thermal conductivities of the guest@MOF systems decrease significantly, by on average a factor of 4, for all infiltrated samples as compared to the uninfiltrated, pristine HKUST-1. This reduction in thermal conductivity goes in tandem with an increase in density of 38% and corresponding increase in heat capacity of âŒ48%, defying conventional effective medium scaling of thermal properties of porous materials. We explore the origin of this reduction by experimentally investigating the guest moleculesâ effects on the mechanical properties of the MOF and performing atomistic simulations to elucidate the roles of the mass and bonding environments on thermal conductivity. The reduction in thermal conductivity can be ascribed to an increase in vibrational scattering introduced by extrinsic guest-MOF collisions as well as guest molecule-induced modifications to the intrinsic vibrational structure of the MOF in the form of hybridization of low frequency modes that is concomitant with an enhanced population of localized modes. The concentration of localized modes and resulting reduction in thermal conductivity do not seem to be significantly affected by the mass or bonding strength of the guest species
Energy loss of fast quarks in nuclei
We report an analysis of the nuclear dependence of the yield of Drell-Yan
dimuons from the 800 GeV/c proton bombardment of , C, Ca, Fe, and W
targets. Employing a new formulation of the Drell-Yan process in the rest frame
of the nucleus, this analysis examines the effect of initial-state energy loss
and shadowing on the nuclear-dependence ratios versus the incident proton's
momentum fraction and dimuon effective mass. The resulting energy loss per unit
path length is GeV/fm. This is the first
observation of a nonzero energy loss of partons traveling in nuclear
environment.Comment: 5 pages, including 4 figure
Nuclear Broadening Effects on Hard Prompt Photons at Relativistic Energies
We calculate prompt photon production in high-energy nuclear collisions. We
focus on the broadening of the intrinsic transverse momenta of the partons in
the initial state from nuclear effects, and their influence on the prompt
photon p_t distribution. Comparing to WA98 data from Pb+Pb collisions at SPS
energy we find evidence for the presence of nuclear broadening at high p_t in
this hard process. Below p_t=2.7 GeV the photon distribution is due to small
momentum transfer processes. At RHIC energy, the effect of intrinsic transverse
momentum on the spectrum of prompt photons is less prominent. The region
p_t=3-4 GeV would be the most promising for studying the nuclear broadening
effects at that energy. Below p_t=2-3 GeV the contribution from large momentum
transfers flattens out, and we expect that region to be dominated by soft
contributions.Comment: 19 pages, 3 figures, minor changes, a few references adde
Parton energy loss limits and shadowing in Drell-Yan dimuon production
A precise measurement of the ratios of the Drell-Yan cross section per
nucleon for an 800 GeV/c proton beam incident on Be, Fe and W targets is
reported. The behavior of the Drell-Yan ratios at small target parton momentum
fraction is well described by an existing fit to the shadowing observed in
deep-inelastic scattering. The cross section ratios as a function of the
incident parton momentum fraction set tight limits on the energy loss of quarks
passing through a cold nucleus
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