Microstructural changes in copper-graphite-alumina nanocomposites produced by mechanical alloying.

Microstructural features of nanostructured copper-matrix composites produced via high-energy milling were studied. Copper-graphite-alumina batches were planetary ball milled up to 16 h; copper-graphite batches were also prepared under the same conditions to evaluate the effect of contamination from the milling media. The microstructure of the produced materials was characterized by field emission gun scanning electron microscopy/energy-dispersive spectroscopy and related to Raman, X-ray diffraction, and particle size analysis results. Results showed that alumina was present in all milled powders. However, size reduction was effective at shorter times in the copper-graphite-alumina system. In both cases the produced powders were nanostructured, containing graphite and alumina nanoparticles homogeneously distributed in the copper matrix, especially for longer milling times and in the presence of added alumina. Copper crystallite size was significantly affected above 4 h milling; nanographite size decreased and incipient amorphization occurred. A minimum size of 15 nm was obtained for the copper crystallite copper-alumina-graphite composite powders, corresponding to 16 h of milling. Contamination from the media became more significant above 8 h. Results suggest that efficient dispersion and bonding of graphite and alumina nanoparticles in the copper matrix is achieved, envisioning high conductivity, high strength, and thermal stability.

Microstructural characterization of concrete prepared with recycled aggregates.

Several authors have reported the workability, mechanical properties, and durability of concrete produced with construction waste replacing the natural aggregate. However, a systematic microstructural characterization of recycled aggregate concrete has not been reported. This work studies the use of fine recycled aggregate to replace fine natural aggregate in the production of concrete and reports the resulting microstructures. The used raw materials were natural aggregate, recycled aggregate obtained from a standard concrete, and Portland cement. The substitution extent was 0, 10, 50, and 100 vol%; hydration was stopped at 9, 24, and 96 h and 28 days. Microscopy was focused on the cement/aggregate interfacial transition zone, enlightening the effect of incorporating recycled aggregate on the formation and morphology of the different concrete hydration products. The results show that concretes with recycled aggregates exhibit typical microstructural features of the transition zone in normal strength concrete. Although overall porosity increases with increasing replacement, the interfacial bond is apparently stronger when recycled aggregates are used. An addition of 10 vol% results in a decrease in porosity at the interface with a corresponding increase of the material hardness. This provides an opportunity for development of increased strength Portland cement concretes using controlled amounts of concrete waste.

Mechanochemical synthesis of hydroxyapatite using cuttlefish bone and chicken eggshell as calcium precursors.

This work explores the possibility of synthesising hydroxyapatite via mechanochemical route using biogenic calcium carbonate sources, namely calcite in chicken eggshell and aragonite in cuttlebone. The calcium source and orthophosphoric acid in ratio complying with Ca/P = 1.67 were submitted to high-energy ball milling with transferred energy values in the 1.6 to 123.0 kWh/g range, in the presence of 6.4 wt% water. Results show that increasing transferred energy results in CaCO3 → DCPD → HA reaction sequence when the used calcium source is cuttlebone, and in CaCO3 → DCPD → DCPA → HA when eggshell is used. The produced calcium orthophosphates exist in delimited energy transfer ranges; HA forms monophasic regions at the highest transferred energy ranges tested. 52.5 kWh/g is the minimum energy value for hydroxyapatite formation starting from eggshell, while only 6.2 kWh/g is required starting from cuttlebone. Calcium orthophosphate dimensionality depends on the supplied energy and on the starting CaCO3 polymorph, and includes nanospheres and nanoplates, and more complex flower-like geometries built from individual nanoparticles. Milling maps were built to systematise the effect of initial CaCO3 polymorph and transferred milling energy on the conditions for hydroxyapatite mechanochemical formation. Obtained results demonstrate the potential of chicken eggshell and cuttlefish bone as natural precursors to produce hydroxyapatite, and the ability of high-energy milling as the corresponding processing route. This indicates an opportunity window for the development of reliable, scalable, fast and cost-effective HA production method.

A study on the aqueous dispersion mechanism of CuO powders using Tiron.

Tiron is an anionic surface active agent, known to render extremely efficient results in the dispersion of metallic oxides such as Al(2)O(3), ZrO(2) and TiO(2). Its dispersion mechanism upon these oxides outcomes from efficient complexation of the corresponding metallic cation, leading to chemisorption of the dispersant molecule upon the oxide surface. In the present work, the effect of Tiron on the stabilization of CuO particles in aqueous suspension is studied to clarify the dispersant's role in the change of colloidal properties and its underlying stabilization mechanism, and also to quantify processing conditions for the oxide. The molecule's dispersing effectiveness was studied through adsorption, rheological and electrophoretic measurements, using both bare and surface charge modified copper oxide particles. The dispersant significantly decreases CuO suspensions' viscosity, increases particles' surface charge and strongly displaces the point of zero charge into the acidic direction. Dispersant/CuO interaction was assessed through FTIR analysis. It was concluded that, at the natural suspensions pH, the change of colloidal properties takes place through an inner-sphere complexation mechanism of the dispersant molecule alcohol groups upon hydroxyl groups at the oxide surface. This mechanism explains the obtained results, which demonstrate Tiron's high dispersion ability upon CuO particles in aqueous medium.