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

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$^{-1}$ 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

Gibbs adsorption impact on a nanodroplet shape: modification of Young-Laplace equation

An efficient technique for the evaluation of the Gibbs adsorption of a liquid on a solid substrate is presented. The behavior of a water nanodroplet on a silicon surface is simulated with molecular dynamics. An external field with varying strength is applied on the system to tune the solid-liquid interfacial contact area. A linear dependence of droplet's volume on the contact area is observed. Our modified Young--Laplace equation is used to explain the influence of the Gibbs adsorption on the nanodroplet volume contraction. Fitting of the molecular dynamics results with these of an analytical approach allows us to evaluate the number of atoms per unit area adsorbed on the substrate, which quantifies the Gibbs adsorption. Thus, a threshold of a droplet size is obtained, for which the impact of the adsorption is crucial. Moreover, the presented results can be applied for the evaluation of the adsorption impact on the physical--chemical properties of systems with important surface-to-volume fraction

Revista Praticando Capoeira: materialidade e representações

O presente estudo analisa a revista Praticando Capoeira, entendendo-a como vestígios de uma rede de práticas, capaz de dar a ver a luta de sentidos em que foi produzida e as relações de força que determinaram sua estruturação. O impresso em questão foi produzido pela editora D+T a partir de 1999, vendido principalmente em bancas de revistas até 2009, período em que foram publicadas 44 edições. Por intermédio dos dispositivos editoriais do impresso e pelas representações elaboradas e postas a circular, buscou-se compreender a organização do mundo da capoeira, considerado como um campo social. Uma revista, por sua agilidade de produção e circulação, consegue captar aspectos do cotidiano ao mesmo tempo em que as informações, os embates e as mudanças acontecem. Para trabalharmos com esse objeto, que se converte em fonte de pesquisa, são necessários considerar três elementos distintos, mas articulados, isto é, considerá-lo como suporte de textos, além de atentar para os próprios textos e para o discurso produzido pelo encadeamento de sentidos no ato da leitura. Ancorado na História Cultural e nas reflexões sobre as lutas de representação, a pesquisa entende a coleção de revistas como objetos culturais que guardam as marcas de sua fabricação. Com base na materialidade da revista, trabalha-se com o periódico nos termos de uma arqueologia dos objetos, em busca de pistas, para localizar os sinais das práticas editoriais, capazes de revelar os vestígios das estratégias, utilizadas pelos editores para a organização e regramento da leitura do impresso. A revista Praticando Capoeira é um documento e um monumento sobre o mundo da capoeira, uma testemunha do presente para o futuro, que proporciona a recordação do nosso tempo. As representações estão nos textos e imagens, materializadas no objeto em função de sua fórmula editorial. Identificamos que a revista apresenta a organização do campo com base nos grupos de capoeira e considera a prática da capoeiragem como um dos elementos da identidade brasileira, com potencial esportivo e pedagógico

Thermal conductivity and thermal boundary resistance of nanostructures

International audienceWe present a fabrication process of low-cost superlattices and simulations related with the heat dissipation on them. The influence of the interfacial roughness on the thermal conductivity of semiconductor/semiconductor superlattices was studied by equilibrium and non-equilibrium molecular dynamics and on the Kapitza resistance of superlattice's interfaces by equilibrium molecular dynamics. The non-equilibrium method was the tool used for the prediction of the Kapitza resistance for a binary semiconductor/metal system. Physical explanations are provided for rationalizing the simulation results