1,721,136 research outputs found
Porosity effects on nanoporous Au Young's modulus
No full textNanoporous Au is a model system extensively studied to investigate how porosity affects the mechanical behaviour. To fabricate nanoporous Au structures with similar ligament diameter, but different solid volume fractions, we applied tailored dealloying procedures to Ag-Au alloys with Au atomic fraction ranging from 0.20 to 0.41. Their Young's moduli, measured by nanoindentation, correlate with the Au atomic fraction. The scaling law we observe suggests a crucial role of percolation thresholds in the definition of mechanical properties
Coarsening of nanoporous Au during catalytic CO oxidation
No full textThis study focuses on the effects of catalytic activity on nanoporous Au catalysts fabricated by dealloying. Nanoporous Au structures are shown to coarsen with similar kinetics over a broad temperature range. A variation in the apparent activation energy suggests that coarsening is controlled by different rate-limiting steps at high and low temperatures
La povertà in età moderna: «sprone» o «vincolo»?
No full textIl contributo riflette sul dibattito circa la povertà come "virtù", "vizio" o "disgrazia" in età moderna
Hydrogen absorption processes in Mg(2)Ni-based systems: Thermal and mechanochemical kinetics
No full textMg2Ni/Ni and LaMg2Ni alloy powders were exposed to hydrogen under isothermal and
mechanical treatment conditions. In the former case, the amount of hydrogen absorbed
tends with time to a final asymptotic value. Once such a value has been reached, further
hydrogen absorption can be obtained only by submitting the powders to mechanical processing
in the presence of hydrogen. Hydrogen absorption processes under isothermal and
mechanical treatment conditions exhibited different kinetics and their rates have been
compared on a phenomenological basis starting from kinetic evidences. It appeared that
mechanical treatment enhances the rate of hydrogen absorption by four orders of
magnitude as a consequence of a mix of surface area enlargement, temperature rise and local structural excitation processes
A phenomenological kinetic equation for mechanochemical reactions involving highly deformable molecular solids
No full textWith its ability to enable solvent-free chemical reactions, mechanochemistry promises to open new and greener synthetic routes to chemical products of industrial interest. Its practical exploitation requires understanding the relationships between processing variables, powders’ mechanical behaviour, and chemical reactivity. To this aim, rationalizing experimental kinetics is of paramount importance. In this work, we propose a phenomenological kinetic model that could help experimentalists to disentangle the mechanical, chemical, and statistical factors underlying mechanochemical reactions. The model takes into account the statistical nature of ball milling and relates the global kinetic curve that can be obtained experimentally to the deformation and chemical processes that occur on the mesoscopic and microscopic scales during individual impacts. We show that our model equations can satisfactorily best fit experimental datasets, providing information on the underlying mechanochemistry
Symbolic and non-symbolic predictors of number line task in Italian kindergarteners
Full text linkThe number line estimation task (NLE) is often used as a predictor for broader measures of mathematical achievement. In spite of its popularity, it is still not clear whether the task is based on symbolic or non-symbolic numerical competence. In particular, there is only a very limited amount of studies investigating the relationship between NLE performance and symbolic vs. non-symbolic math skills in children who have not yet begun formal schooling. This study investigates the strength of the association between NLE performance and symbolic and non-symbolic tasks in young kindergarteners. Ninety two 5-year-old children completed the NLE task (range 0-100) and a battery of early numerical competence tests including symbolic-lexical tasks, symbolic semantic tasks, and non-symbolic semantic tasks. The relationship between symbolic and non-symbolic early numerical competence and NLE performance was analyzed using a regression model based on the Bayesian Information Criterion (BIC). Results show that only symbolic semantic tasks are significant predictors of NLE performance. These results suggest that symbolic numerical knowledge is involved in number line processing among young children, whilst non-symbolic knowledge is not. This finding brings new data to the debate on the relationship between non-symbolic numeral knowledge and symbolic number processing and supports the evidence of a primary role of symbolic number processing already in young kindergarteners
Mesostructural refinement in the early stages of mechanical alloying
No full textThis study focuses on the early stages of the mechanical alloying of 22 different binary mixtures. In each case, the number of alloyed particles increases linearly with the number of collisions, and the volume of powder effectively processed during a single collision is ∼9.0 × 10−3 mm3. The alloyed particles exhibit a heterogeneous mesostructure with characteristic lengths decreasing exponentially with the number of collisions. The rate of mesostructural refinement is inversely proportional to the difference between the hardnesses of the individual metals
Kinetics of MgH2 formation by ball milling
No full textHydrogen (H2) absorption by metals, metal alloys, and more complex solids is a crucial issue on the road to the efficient H2 storage required by the hydrogen economy. The present study examines the reaction between Mg powder and gaseous H2 under ball milling conditions as a specific case study. Based on accurate kinetic evidence, we show that H2 absorption is governed by a combination of statistical and chemical contributions related, respectively, to the inherent nature of ball milling and the chemistry of the Mg–H2 system. The combination of the two factors determines the shape of the H2 absorption curve. Therefore, chemical information can be only obtained by disentangling the two contributions. To this aim, we propose a general method that can be extended to other chemical transformations activated by ball milling
Riorganizzazione e razionalizzazione della rete degli ambulatori vaccinali della ASL Sassari.
No full textA systematic investigation of the mechanochemical decomposition of Ag oxalate in rod drop experiments
No full textThe present work focuses on the chemical response of Ag oxalate powders to the collision of rods in controlled rod drop experiments performed by using suitably designed reactors. Powder layers of different thickness were submitted to the collision of cylindrical rods with selected radius, mass, and collision velocity. The experimental findings indicate that collisions are able to induce the partial decomposition of Ag oxalate into metallic Ag and gaseous carbon dioxide. The mass of Ag oxalate decomposed is proportional to the collision energy. Being the impact energy the same, the mass decomposed is inversely proportional to the surface area of the powder layer. In addition, it increases irregularly as the thickness of the powder layer increases. The results obtained suggest that rod drop experiments can be profitably used to promote and investigate mechanochemical transformations. © 2012 Elsevier B.V. All rights reserved
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