314 research outputs found
Nucleation and Growth of GaN/AlN Quantum Dots
We study the nucleation of GaN islands grown by plasma-assisted
molecular-beam epitaxy on AlN(0001) in a Stranski-Krastanov mode. In
particular, we assess the variation of their height and density as a function
of GaN coverage. We show that the GaN growth passes four stages: initially, the
growth is layer-by-layer; subsequently, two-dimensional precursor islands form,
which transform into genuine three-dimensional islands. During the latter
stage, island height and density increase with GaN coverage until the density
saturates. During further GaN growth, the density remains constant and a
bimodal height distribution appears. The variation of island height and density
as a function of substrate temperature is discussed in the framework of an
equilibrium model for Stranski-Krastanov growth.Comment: Submitted to PRB, 10 pages, 15 figure
ComĂ©rcio Justo como EstratĂ©gia de Internacionalização de Pequenos NegĂłcios: Empoderamento para as Rendeiras da Arte Renascença no Semi-Ărido Pernambucano
A pesquisa foi realizada com o fito de avaliar as reais possibilidades das mulheres que produzem rendas no agreste pernambucano virem a internacionalizar sua produção se beneficiando do comĂ©rcio justo como forma de adquirir maior autonomia frente aos atravessadores que exploram naquela regiĂŁo a mĂŁo-de-obra local. A metodologia utilizada consistiu em repertoriar as principais dificuldades da comunidade rendeira nos municĂpios de Pesqueira, Poção e adjacĂȘncias que respondem por mais de 80% da renda renascença produzida no Brasil e em contra-ponto, compreender, mediante pesquisa de campo, os mecanismos que regem o comĂ©rcio justo na Europa, em particular na França, onde foram contactados no perĂodo de maio de 2008 Ă julho de 2009 inĂșmeros agentes atuando na comercialização de produtos do comĂ©rcio alternativo. Em conclusĂŁo, descobriu-se que dentre os atores da filiĂšre Ă©quitable os principais influenciadores eram os importadores europeus que mantinham relaçÔes estreitas com as comunidades produtoras nos paises do Sul. Para as rendeiras do agreste, ficou claro que suas melhores chances em participar do movimento encontram-se na moda Ă©tica (peças confeccionadas com algodĂŁo orgĂąnico) em plena expansĂŁo na Europa.ComĂ©rcio justo, internacionalização, intermediĂĄrios, renda renascença
Unveiling the impact of the effective particles distribution on strengthening mechanisms: A multiscale characterization of Mg+Y2O3 nanocomposites
International audienceMost models used to account for the hardening of nanocomposites only consider a global volume fraction of particles which is a simplified indicator that overlooks the particles size and spatial distribution. The current study aims at quantifying the effect of the real experimental particles spatial and size distribution on the strengthening of a magnesium based nanocomposites reinforced with Y 2 O 3 particles processed by Friction Stir Processing (FSP). X-ray tomographic 3-D images allowed to identify the best FSP parameters for the optimum nanocomposite. A detailed analysis indicates that the observed hardening is mainly due to Orowan strengthening and the generation of geometrically necessary dislocations (GND) due to thermal expansion coefficients (CTE) mismatch between magnesium and Y 2 O 3 particles. A multiscale characterization coupling 3D X-ray laboratory, synchrotron nanoholotomography and transmission electron microscopy (TEM) has been used to investigate particles size and spatial distribution over four orders of magnitude in length scales. Two dedicated micromechanical models for the two strengthening mechanisms are applied on the experimental particle fields taking into account the real particles size and spatial distribution, and compared to classical models based on average data. This required to develop a micromechanical model for CTE mismatch hardening contribution. This analysis reveals that the contribution from CTE mismatch is decreased by a factor two when taking into account the real distribution of particles instead of an average volume fraction
The interplay of microscopic and mesoscopic structure in complex networks
Not all nodes in a network are created equal. Differences and similarities
exist at both individual node and group levels. Disentangling single node from
group properties is crucial for network modeling and structural inference.
Based on unbiased generative probabilistic exponential random graph models and
employing distributive message passing techniques, we present an efficient
algorithm that allows one to separate the contributions of individual nodes and
groups of nodes to the network structure. This leads to improved detection
accuracy of latent class structure in real world data sets compared to models
that focus on group structure alone. Furthermore, the inclusion of hitherto
neglected group specific effects in models used to assess the statistical
significance of small subgraph (motif) distributions in networks may be
sufficient to explain most of the observed statistics. We show the predictive
power of such generative models in forecasting putative gene-disease
associations in the Online Mendelian Inheritance in Man (OMIM) database. The
approach is suitable for both directed and undirected uni-partite as well as
for bipartite networks
Deciphering the connectivity structure of biological networks using MixNet
<p>Abstract</p> <p>Background</p> <p>As biological networks often show complex topological features, mathematical methods are required to extract meaningful information. Clustering methods are useful in this setting, as they allow the summary of the network's topology into a small number of relevant classes. Different strategies are possible for clustering, and in this article we focus on a model-based strategy that aims at clustering nodes based on their connectivity profiles.</p> <p>Results</p> <p>We present MixNet, the first publicly available computer software that analyzes biological networks using mixture models. We apply this method to various networks such as the <it>E. coli </it>transcriptional regulatory network, the macaque cortex network, a foodweb network and the <it>Buchnera aphidicola </it>metabolic network. This method is also compared with other approaches such as module identification or hierarchical clustering.</p> <p>Conclusion</p> <p>We show how MixNet can be used to extract meaningful biological information, and to give a summary of the networks topology that highlights important biological features. This approach is powerful as MixNet is adaptive to the network under study, and finds structural information without any a priori on the structure that is investigated. This makes MixNet a very powerful tool to summarize and decipher the connectivity structure of biological networks.</p
Particle-induced morphological modification of Al alloy equiaxed dendrites revealed by sub-second in situ microtomography
The study of dendritic growth is a challenging topic at the heart of intense research in material science. Understanding such processes is of prime importance as it helps predicting the final microstructure governing material properties. In the specific case of the design of metal-matrix nanocomposites (MMNCs), the addition of nano-sized particles inside the metallic melt increases the complexity as their influence on the growth morphology of dendrites is not yet fully understood. In the present experimental study, we use in situ X-ray tomography imaging with very high temporal resolution (0.35 s per 3D image) coupled with in situ ultrasonic melt homogenisation to record, in 3D and real time, the free growth at high cooling rates (~2 K.s-1) of equiaxed dendrites in an AA6082 alloy containing Y2O3 nanoparticles. The careful 3D analysis of the dendrite morphologies as well as their solidification dynamics reveals that in the case of well-dispersed particles, dendrite equiaxed growth occurs through complex hyper-branched morphologies. Such behaviour is believed to arise from particle-induced modification of the solidification processes at the origin of multiple splitting, branching and curving mechanisms of the dendrite arms. These results shed light on long-standing empirical and modelling statements and open new ways for direct investigation of equiaxed growth in metallic alloys and composites.European Commission in the 7th Framework Program (contract FP7-NMP3-LA-2012-280421) ExoMet Project by the European Space Agency and by the individual partner organizations; ESRF-MA1876 long term project
- âŠ