364 research outputs found
Conversion of wooden structures into porous SiC with shape memory synthesis
Synthesis of structured silicon carbide materials can be accomplished using wooden materials as the carbon source, with various silicon impregnation techniques. We have explored the low cost synthesis of SiC by impregnation of carbon from wood with SiO gas at high temperatures, which largely retains the structure of the starting wood (shape memory synthesis). Suitably structured, porous SiC could prove to be an important type of catalyst support material. Shape memory synthesis (SMS) has earlier been tried on high surface area carbon materials. Here we have made an extensive study of SMS on carbon structures obtained from different types of wood. © 2011 Elsevier Ltd and Techna Group S.r.l
Synthesis, characterization, and wear and friction properties of variably structured SiC/Si elements made from wood by molten Si impregnation
We have synthesized pre-shaped SiC/Si ceramic material elements from charcoal (obtained from wood) by impregnation with molten silicon, which takes place in a two-stage process. In the first process, a porous structure of connected micro-crystals of β-SiC is formed, while, in the second process, molten Si totally or partly infiltrates the remaining open regions. This process forms a dense material with cubic (β-)SiC crystallites, of which the majority is imbedded in amorphous Si. The synthesis of preshaped "sprocket" elements demonstrates that desired shapes of such a dense SiC/Si composite ceramic material can be achieved, thus suggesting new industrial applications. The structure and composition of numerous as-synthesized samples were characterized in detail by using a wide range of techniques. Wear and friction properties were also investigated, with polished samples. The properties found for the present samples are very promising for abrasive applications and for new generation brake systems. © 2011 Elsevier Ltd
Quantum beat spectroscopy of repulsive Bose polarons
The physics of impurities in a bosonic quantum environment is a paradigmatic
and challenging many-body problem that remains to be understood in its full
complexity. Here, this problem is investigated for impurities with strong
repulsive interactions based on Ramsey interferometry in a quantum degenerate
gas of 39K atoms. We observe an oscillatory signal that is consistent with a
quantum beat between two co-existing coherent quasiparticle states: the
attractive and repulsive polarons. The interferometric signal allows us to
extract the polaron energies for a wide range of interaction strengths,
complimenting earlier spectroscopic measurements. We furthermore identify
several dynamical regimes towards the formation of the Bose polaron in good
agreement with theory. Our results improve the understanding of quantum
impurities interacting strongly with a bosonic environment, and demonstrate how
quasiparticles as well as short-lived non-equilibrium many-body states can be
probed using Ramsey interferometry
Unraveling the potential co-contributions of cerebral small vessel vasculopathy to the pathogenesis of Alzheimer’s dementia
Variable selection: current practice in epidemiological studies
Selection of covariates is among the most controversial and difficult tasks in epidemiologic analysis. Correct variable selection addresses the problem of confounding in etiologic research and allows unbiased estimation of probabilities in prognostic studies. The aim of this commentary is to assess how often different variable selection techniques were applied in contemporary epidemiologic analysis. It was of particular interest to see whether modern methods such as shrinkage or penalized regression were used in recent publications. Stepwise selection methods remained the predominant method for variable selection in publications in epidemiological journals in 2008. Shrinkage methods were not used in any of the reviewed articles. Editors, reviewers and authors have insufficiently promoted the new, less controversial approaches of variable selection in the biomedical literature, whereas statisticians may not have adequately addressed the method’s feasibility
Determination of the bonding configuration of the metastable molecular oxygen adsorbed on a Si(111)-(7×7) surface
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