1,720,998 research outputs found
Adsorption of Paraffin Vapour on Oxidized Molybdenum Substrates at Nano and Micro Scales
No full textSputtered oxidized molybdenum surfaces were exposed at room temperature for different times to paraffin vapors obtained at 150◦C. Scanning polarization force microscopy (SPFM), optical and confocal microscopy were used to characterize the surfaces. The condensed morphologies are complex and strongly dependent upon the quantity of vapor molecules deposited on the substrate surface. A thin paraffin film is initially formed
and quite uniform nano-height drops are nucleated randomly over it within 10–20 s time exposures. Their average contact angle ranged between 1◦ + –2.5◦. Further vapor deposition led to a more complex regime where nano-height drops do not show a clear interface with the film, while micro-sized drops do. The tangent approximation method adopted by Salmeron and Xu for the nano-drop regimes was extended to the micro-sized drop
regime obtaining an averaged effective contact angle equal to 4◦ + –5◦. Both nano-height and micro-sized drops shape and effective contact angles have been discussed taking into account their interactions between the film and the drops
Role of the water matric potential (M) and of equilibrium water content (EWC) on the water self-diffusion coefficient and on the oxygen permeability in hydrogel contact lenses
No full textThis paper illustrates a new thermodynamic and kinetic model that describes the relationship between the water self-diffusion
coefficient, D* w/g; in hydrogel contact lenses, in terms of water matric potential ψM and equilibrium water content (EWC).
Experimental measurements on commercial contact lenses yield water thermodynamic activity values ranging between 0.996 and
0.999. The corresponding CM matric potential is, respectively, between -8 and -2 J/mol at temperature 35°C. Comparison between water self-diffusion coefficients derived in this paper and those suggested by other authors shows that our values are greater (25%–50%) than the previous ones. The impact of this model on the nature of the oxygen permeability, ∏; in the lenses has been evaluated and the changes of ∏ with ψM and EWC are predicted and compared with direct experimental measurements. For the contact lenses investigated, the oxygen permeability turns out to be only a quadratic function of equilibrium water content, despite the fact that the fraction of the ‘‘free’’ water molecules can be as high as 50%
Percolation threshold of nano kaolin powders dispersed in liquid polybutadiene and stiffness of styrene-butadiene rubbers with nano kaolin fillers
No full textLiquid-like H2O adsorption layers to catalyze the Ca(OH)2/CO2 solid–gas reaction and to form a non-protective solid product layer at 20°C
No full textSolid products and rate-limiting step in the thermal half decomposition of natural dolomite in a CO2 (g) atmosphere.
No full textNatural dolomite powders obtained from caves which give unusual high resistance building materials, have been decomposed in a Knudsen cell at high CO2 pressures in the temperature range of 913-973 K. XRD traces for the final solid products, after the first half thermal decomposition, have shown, that beside the XRD patterns for the calcite and MgO, the existence of a new structure with major peaks at 2teta equal to 38.5 and 65°. This finding has been ascribed to a solid solution of MgO in calcite. The kinetic analysis of the TG curves yield a total apparent entalpy for the decomposition equal to 440 +/- 10 kJmol-1 for a range of fraction decomposed varying between 0.2 and 0.7. This value is much closer to the theoretical expected at 950 K value DeltaH = 486 kJmol-1 for the dolomite decompositio in CO2 environment, where CaO, MgO and oxides of solid solution can be the solid reaction products. The rate determining step is the transport of CO2 across the reacting interface through an high activated thermal process due to solid state diffusion of CO32- in the bulk and/or the grain boundaries phases of CaCO3 and/or of the solid solution. The microstructure evolution of the solid products follows a shear-transformation mechanism. At temperatures below 943K, porous product particles are characterized by a monomodal barrow pore size distribution around 0.05 micron. At higher temperatures, a critical level of tensions inside the particles is reached and a bimodal pore size distribution around 1 and 0.05 micron is formed
Rheological properties of silica and kaolin dispersions in mineral paraffinic oil: effect of water films adsorbed on the solid particles
No full textPiezoresistance behavior of silicone-graphite composites in the proximity of the electric percolation threshold
No full textComposites of a silicone matrix charged with graphite powder of micrometric size in volume fractions around the electric percolation threshold (25–35%) have been investigated with regard to their piezoresistance properties. The elastic modulus changes with graphite content, reaching a maximum at 30 vol.%. From measurements of electric resistivity, the percolation threshold was determined as 31 vol.% at a compressive strain of 2%. The threshold value was found to be dependent on the applied compressive strain so that an insulator in the unstrained condition may become a conductor when subjected to a small pressure. The property can be exploited for contact sensors. Further, the electric resistance of a composite, charged a little beyond the percolation threshold, is also strain dependent, according to an equation of the type R = R0exp(βε), where β was found to be about 51.5. This value of corresponds to a very high electric sensitivity of the material to an applied strain and makes it a candidate for application as a logarithmic strain transducer. Owing to the visco-elastic behavior of the elastomer matrix, there is a retardation of the electric response on unloading of about 2 s. The electric response to an applied stress follows an exponential law on loading and undergoes a corresponding retardation on unloading
Immersion Calorimetric Method to Study Lime Surfaces Impurities and Particles Aggregate Morphologies
No full textAluminum hydroxide microstructural units in gelled media aged, or nonaged, with alcohol and water
No full textAluminum hydroxide gelled media dispersed and then aged in alcohol (ES) or in water (WS) were prepared with a solid phase concentration equal to 0.45% v/v. A model for microstructural units dispersed in the liquid phase has been derived. The beneficial action of alcohol in giving xerogel powders with higher specific surface area and higher porosity has been explained on the basis that alcohol produces changes in the corresponding dispersed microstructural units. The effect of the aging time in alcohol and in water was evaluated, and the evolution of all parameters was described
Calcium carbonate binding mechanisms in the setting of calcium and calcium-magnesium putty-limes
No full textCalcium and calcium magnesium putty-limes (C-L and C-M-L) were characterized, through SEM, Porosimeter and Chemical Analysis, to obtain information on the microstructure of their solid-phases. Irregular agglomerates of portlandite grains and large acicular crystallites of brucite are differently interconnected to form a cellular solid matrix dispersed into the saturated aqueous solution of the hydroxides. The setting of these putty-limes was followed also in a thermobalance with a thermostatic chamber designed to keep the temperature in the range 15–30 °C. The tests were done at a constant humidity of 80%, under isothermal conditions, and a wet and carbonated N 2 flux with 1% of CO 2 was added. The microstructure of the final samples was characterized by the usual methods of investigation at micro–macro scale. Drying kinetics and the related shrinkage processes were discussed on the basis of a modified Kelvin equation, which predicts for the C-L lime putty a larger shrinkage (50.3%) than for C-M-L (35.5%). The binding mechanisms of the calcium carbonate in the strengthening of the putty-lime systems is due to the interconnected texture formed by the calcium carbonate fine crystallites formed during the precipitation process. The scientific reasons for this microstructure evolution have been discussed and explained on the basis of experimental data and theory, leading to a better understanding of the complex relationships between drying, shrinkage and the chemical processes occurring in the setting of putty-lime
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