145 research outputs found
Thermal boundary conductance across rough interfaces probed by molecular dynamics
In this article, we report the influence of the interfacial roughness on the
thermal boundary conductance between two crystals, using molecular dynamics. We
show evidence of a transition between two regimes, depending on the interfacial
roughness: when the roughness is small, the boundary conductance is constant
taking values close to the conductance of the corresponding planar interface.
When the roughness is larger, the conductance becomes larger than the planar
interface conductance, and the relative increase is found to be close to the
increase of the interfacial area. The cross-plane conductivity of a
superlattice with rough interfaces is found to increase in a comparable amount,
suggesting that heat transport in superlattices is mainly controlled by the
boundary conductance. These observations are interpreted using the wave
characteristics of the energy carriers. We characterize also the effect of the
angle of the asperities, and find that the boundary conductance displayed by
interfaces having steep slopes may become important if the lateral period
characterizing the interfacial profile is large enough. Finally, we consider
the effect of the shape of the interfaces, and show that the sinusoidal
interface displays the highest conductance, because of its large true
interfacial area. All these considerations are relevant to the optimization of
nanoscale interfacial energy transport
Thermal transport in 2D and 3D nanowire networks
We report on thermal transport properties in 2 and 3 dimensions
interconnected nanowire networks (strings and nodes). The thermal conductivity
of these nanostructures decreases in increasing the distance of the nodes,
reaching ultra-low values. This effect is much more pronounced in 3D networks
due to increased porosity, surface to volume ratio and the enhanced
backscattering at 3D nodes compared to 2D nodes. We propose a model to estimate
the thermal resistance related to the 2D and 3D interconnections in order to
provide an analytic description of thermal conductivity of such nanowire
networks; the latter is in good agreement with Molecular Dynamic results
CO adsorption on metal surfaces: a hybrid functional study with plane wave basis set
We present a detailed study of the adsorption of CO on Cu, Rh, and Pt (111)
surfaces in top and hollow sites. The study has been performed using the local
density approximation, the gradient corrected functional PBE, and the hybrid
Hartree-Fock density functionals PBE0 and HSE03 within the framework of
generalized Kohn-Sham density functional theory using a plane-wave basis set.
As expected, the LDA and GGA functionals show a tendency to favor the hollow
sites, at variance with experimental findings that give the top site as the
most stable adsorption site. The PBE0 and HSE03 functionals reduce this
tendency. In fact, they predict the correct adsorption site for Cu and Rh but
fail for Pt. But even in this case, the hybrid functional destabilizes the
hollow site by 50 meV compared to the PBE functional. The results of the total
energy calculations are presented along with an analysis of the projected
density of states.Comment: 32 pages, 6 tables, 3 figures. (Re)Submitted to Phys. Rev. B; LDA
results added in the tables; minor changes in the tex
Impact of Screw and Edge Dislocation on the Thermal Conductivity of Nanowires and Bulk GaN
We report on thermal transport properties of wurtzite GaN in the presence of
dislocations, by using molecular dynamics simulations. A variety of isolated
dislocations in a nanowire configuration were analyzed and found to reduce
considerably the thermal conductivity while impacting its temperature
dependence in a different manner. We demonstrate that isolated screw
dislocations reduce the thermal conductivity by a factor of two, while the
influence of edge dislocations is less pronounced. The relative reduction of
thermal conductivity is correlated with the strain energy of each of the five
studied types of dislocations and the nature of the bonds around the
dislocation core. The temperature dependence of the thermal conductivity
follows a physical law described by a T variation in combination with an
exponent factor which depends on the material's nature, the type and the
structural characteristics of the dislocation's core. Furthermore, the impact
of the dislocations density on the thermal conductivity of bulk GaN is
examined. The variation and even the absolute values of the total thermal
conductivity as a function of the dislocation density is similar for both types
of dislocations. The thermal conductivity tensors along the parallel and
perpendicular directions to the dislocation lines are analyzed. The discrepancy
of the anisotropy of the thermal conductivity grows in increasing the density
of dislocations and it is more pronounced for the systems with edge
dislocations
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