1,721,049 research outputs found
Introduzione
No full textIntroduzione alla sezione monografica delal Rivista, dedicata al tema "Brasile: Vent'anni di Costituzione democratica
Preface
No full textFew books currently exist that cover such a wide spectrum of topics. The chapters dealing with air pollution from mobile sources, air pollution and health effects and air quality modelling fall into the air pollution category while the ones related to microalgae for carbon dioxide sequestration/biofuels production, fuel cells, and solar energy technology, respectively, can be ascribed to the energy topic. Several technologies to handle a wide spectrum of environmental pollutants are taken into account in numerous chapters. The chapter on biodiversity is clearly related to the conservation issue, while the water pollution subject is tackled by the chapter on water quality monitoring. Finally, a general analysis on green business, as well as a chapter on grid/cloud computing technology for collaborative problem solving and shared resources management conclude the work. Because of its breadth of coverage, this book is particularly useful as a graduate text
Bioactivity enhancement by a ball milling treatment in novel bioactive glass-hydroxyapatite composites produced by spark plasma sintering
Full text linkHydroxyapatite (HA) and a lab-made bioactive glass (BGMS10) are combined (50/50 wt%) in this work, where the effect produced by a ball milling (BM) treatment (0–120 min) prior SPS consolidation on the characteristics of the resulting products is investigated. An extraordinary improvement of the apatite-forming ability during in-vitro test on SPS samples (800 °C/70 MPa/2 min) is obtained using the 30 min BMed mixture. Superior Young's Modulus (122 GPa) and Vickers Hardness (675) were also found compared to unmilled samples (95 GPa and 510, respectively). Microstructural changes induced by BM, with 90 nm HA crystallites size in the bulk composite, and the intimate HA/BGMS10 interfaces established, are the factors mainly responsible for such result. When milling was prolonged to 120 min, samples with relatively lower density, mechanical properties, and in-vitro bioactivity, were produced under the same SPS conditions. The formation of crystalline SiO2 during SPS might be responsible for such behaviour
Combustion synthesis and spark plasma sintering of apatite-tricalcium phosphate nanocomposites
Full text linkA processing route consisting of Spark Plasma Sintering (SPS) of precursor powders prepared by Solution Combustion Synthesis (SCS) is proposed for the first time for the fabrication of bulk nanostructured biphasic calcium phosphates. The apatite phase content in the product obtained by SCS was maximized using a fuel to oxidizer ratio of 1.1. After a post-synthesis air-annealing step conducted a 700 °C/3 h, powders consisted of 83 wt.% of carbonated apatite, with average crystallite size less than 70 nm, and β- and α-TCP (tricalcium phosphate), as secondary phases. Detailed structural analyses evidenced that the original nanostructure was retained after sintering at 900 °C, with the obtainment of nearly 91% dense, apatite-rich, biphasic bioceramics, with grains size of about 100 nm. The developed nanostructured biphasic material is expected to possess a higher resorption rate than standard microcrystalline hydroxyapatite, which makes it preferable for bone tissue regeneration
Recent advances on innovative bioactive glass-hydroxyapatite composites for bone tissue applications: Processing, mechanical properties, and biological performance
Full text linkNew Hydroxyapatite-Bioactive Glass composites, xHA-(1-x)BG (x = 25, 50, and 75 wt %), are developed using HA and BGMS10 glass powders co-milled up to 2 h prior to Spark Plasma Sintering (SPS). Ball milling (BM) promoted the consolidation of HA-rich powders, whereas hindered the densification of 25HA-75BG samples. HA crystallite size is reduced from > 200 nm (unmilled) to 60 (x = 25 %) or 88 nm (x = 75 %) when using 2 h milled mixtures. Glass crystallization occurred in 75HA-25BG samples processed by SPS at 950 °C: a negligeable effect in the amount of the residual amorphous phase (12.3–13.3 wt %) is produced by BM, while changes are observed in the relative content of crystalline phases, with SiO2 increases from 8.5 to 13.1 wt %, whereas α- and β-CaSiO3 correspondingly decrease. Superior Young's modulus and Vickers hardness (130 GPa and 726, respectively) are obtained in HA rich products. Biological tests evidenced that the milling treatment does not determine negative consequences on cells viability
Spark plasma sintering, mechanical and in-vitro behavior of a novel Sr- and Mg-containing bioactive glass for biomedical applications
Full text linkThe so-called BGMS10, a bioactive glass containing 10 mol.% SrO and 10 mol.% MgO, displays a low inclination to crystallize, as confirmed by its high activation energy (538.9 kJ/mol). Such peculiar aspect and the beneficial use of SPS allow for the obtainment of 99.7 % dense and fully amorphous products at 750 °C. The incipient crystallization in the glass is observed when temperature is increased to 850 °C, while 95 wt. % crystallized ceramics are produced at 950 °C. Main crystalline phases are α- and β-CaSiO3, with grain size of 89 and 97 nm, respectively. Glass crystallization is accompanied by Young's modulus increase from 90.92 to 98.38 GPa. On the other hand, partially crystallized samples (850 °C) exhibit higher Vickers hardness (718.8) compared to fully crystallized ones (619.8), which show lower density (98.6 %). In-vitro tests in SBF indicate that the silica-gel film preceding apatite nucleation is mostly formed on the amorphous substrate region
Modeling of Electric Current Assisted Sintering: An extended fluid-like approach for the description of powders rheological behavior
No full textA general theoretical framework for investigating the rheological behavior of powder undergoing electric current assisted consolidation is proposed in this work. The most relevant phenomena occurring during ECAS process are taken into account. Consolidating powders are assumed to behave as a viscous fluid. Specifically, powder viscosity is expressed by the Bird–Carreau–Yasuda's model, which includes the power-law creep description typically adopted in the ECAS modeling literature. The proposed stress-strain constitutive law allows one avoiding a priori assumptions regarding the powder rheological behavior. The resulting system of differential equations cannot be solved analytically, so that the finite-element method (FEM) numerical technique is adopted. A numerical investigation of the effect of some representative model parameters on powder consolidation is performed and the possible different rheological regimes correspondingly taking place are identified. Influence of the applied mechanical load and zero-shear rate viscosity is studied and a suitable approach to map out the powder rheological behavior is presented. It is found that powder rheological response is not only determined by the intrinsic material properties but also depends upon the process operating conditions. As an example, high values of mechanical load along with low values of zero-shear rate viscosity induce a nonlinear viscous flow while Newtonian (linear) flow takes place for low values of mechanical load associated to high values of zero-shear rate viscosity. The proposed model should be considered as a first attempt to extend the rheological theory of sintering recently proposed in the field of electric current assisted consolidation to a broader range of shear rates, which, in turn, extends the application of this theory to a wider variety of materials and operating conditions
From individual to equimolar binary transition metal diborides: The case of (Nb0.5M0.5)B2, M=Ti, Hf
Full text link(Nb0.5Ti0.5)B2 and (Nb0.5Hf0.5)B2 in bulk form were produced by Spark Plasma Sintering (SPS) from powders obtained by Self-propagating High-temperature Synthesis. The carbothermal reduction of oxide contaminants with graphite promoted the conversion of secondary phases into equimolar diborides, and improved samples densification, particularly for (Nb0.5Hf0.5)B2 (from 89.7 % to 97.4 %). A single-phase solid solution was obtained for both systems at 2050 °C by SPS, while longer holding times (40 instead of 20 min) were needed for the Hf-containing ceramic. The latter system showed Vickers hardness (18.47 GPa) and Young's modulus (586 GPa) slightly better than HfB2. NbB2 and (Nb0.5Hf0.5)B2 were the most sensitive systems to the oxidation environment, with the formed oxide scales detached from samples at 600 and 800 °C, respectively, due to volume changes accompanying phase transitions in the generated niobium oxides. Conversely, the formation of a stable mixed oxide (TiNb2O7) made (Nb0.5Ti0.5)B2 more resistant to oxidation, compared to TiB2 and NbB2
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