Despite its ubiquitous presence in the built environment, concrete's molecular-level properties are only recently being explored using experimental and simulation studies. Increasing societal ...concerns about concrete's environmental footprint have provided strong motivation to develop new concrete with greater specific stiffness or strength (for structures with less material). Herein, a combinatorial approach is described to optimize properties of cement hydrates. The method entails screening a computationally generated database of atomic structures of calcium-silicate-hydrate, the binding phase of concrete, against a set of three defect attributes: calcium-to-silicon ratio as compositional index and two correlation distances describing medium-range silicon-oxygen and calcium-oxygen environments. Although structural and mechanical properties correlate well with calcium-to-silicon ratio, the cross-correlation between all three defect attributes reveals an indentation modulus-to-hardness ratio extremum, analogous to identifying optimum network connectivity in glass rheology. We also comment on implications of the present findings for a novel route to optimize the nanoscale mechanical properties of cement hydrate.
Different techniques have been used to characterize the physical and chemical structure of the red coral calcitic skeleton. A section normal to the axis of the skeleton shows a medullar zone ...surrounded by a circular domain composed of concentric rings. Growth rings are revealed by the cyclic variation of organic matter (OM) and Mg/Ca ratio. These growth rings are annual; thus, both OM and Mg/Ca ratio can be used to date red coral colonies. Growth rings display wavelets. The internal structure of each wavelet results from the stacking of layers with tortuous interfaces. Tortuosity is due to the presence of microprotuberances. Interfaces between layers may display sharp discontinuities indicative of interruption of the mineralizing process. SEM and TEM studies show that each layer is made of (1) fibers, organized or not in fan-shaped structures; and (2) submicrometer (apparently mono-) crystalline units. Fibers are superstructures made of submicrometer units possibly assembled by an oriented aggregation mechanism. HRTEM studies show that in spite of displaying single-crystal scattering behavior, the submicrometer crystalline units are made of 2-5 nm nanograins again possibly aggregated by a mechanism of oriented attachment. Thus, submicrometer crystalline units and polycrystalline fibers can be both defined as mesocrystals. The red coral skeleton is a hierarchically organized organic-inorganic composite that exhibits porosity and structural and compositional order on length scales from the nanoscale to the macroscale.
Surface interfaces of thin magnesium oxide films elaborated onto Si(100)–(2×1) substrates were characterized using scanning tunneling microscopy and spectroscopy, Auger electron spectroscopy, atomic ...force microscopy, and high-resolution transmission electron microscopy. We report that a flat and highly homogeneous magnesium oxide with well-defined interfaces could be grown at room temperature (RT) by repeating alternate adsorption of Mg atomic monolayer and O2 on Si(100). RT oxidation process of the first Mg monolayer plays a crucial role as driving force allowing a partial decomposition of amorphous ultra-thin Mg2Si at the Mg/Si interface to form more magnesium oxide in the surface. This process induces crystallization of the interfacial Mg2Si thin film and then gives arise to an unexpected MgOx/Mg2Si(11–1)/Si(100) heterostructure. MgOx monolayer displays a band gap of about 6eV and exhibits a weak RMS roughness on large areas.
Concentration gradients in calcium are common in metamorphic or magmatic garnets and can be used to determine the timescales of geological processes. However, the kinetics of Ca diffusion in garnet ...is poorly constrained and experimental studies have to date yielded widely varying diffusion coefficients. In this paper, we describe a new method for generating diffusion profiles in garnet. We incorporated polished and compositionally homogeneous garnet seeds in a finely ground powder of clinopyroxene and garnet. During the experiments (1.3 GPa, 1,050-1,250 degrees C, and fO2 = the graphite-O2 buffer), the mineral powder partially melted, recrystallized, and formed a 10-50 mcm wide overgrowth zone of compositionally distinct garnet around the seeds. Long duration experiments generated measurable relaxation profiles at these seed/overgrowth interfaces. We performed analytical transmission electron microscope traverses across the interfaces in each experiment. Thirteen usable compositional profiles were obtained with characteristic distances of diffusion ranging from 300 to 1,000 nm. From these profiles, Ca-(Fe, Mg) interdiffusion coefficients were retrieved using an analytical solution for the diffusion equation and the data were cast in an Arrhenius relation. Linear regression of the data yields an activation energy Q Ca-(Fe, Mg) equal to 188 +/- 48 kJ mol-1 and a frequency factor D 0 equal to 6.6 x 10-14 m2 s-1. Within the compositional range studied, the composition of garnet has no major effect on the Ca-(Fe, Mg) interdiffusion coefficient. The very slow diffusion rate of Ca is in agreement with natural observations indicating that Ca diffuses more slowly than Fe and Mg. The Ca diffusion coefficients derived from this study are not model-dependent and can be used to determine the durations of geological events from Ca relaxation profiles in natural garnets. PUBLICATION ABSTRACT
Various sources of bibliographic information regarding red lead transformations in artworks are reviewed. Typical examples of red lead use in mural paintings, either as a pure colour or as an ...admixture with other pigments, are reported. Several cases of red lead discolourations are described here; they underline some debates regarding the alteration process. The role of environmental factors, such as lightening, humidity, pollution or microbial contamination, is emphasized. Recent investigations show that both composition and micro-structural features of red lead grains, which result from the manufacturing process, are determining factors in the pigment alteration process.
Celotno besedilo
Dostopno za:
BFBNIB, DOBA, GIS, IJS, IZUM, KILJ, KISLJ, NUK, PILJ, PNG, SAZU, UILJ, UKNU, UL, UM, UPUK
We propose a theoretical approach of nucleation, growth and ageing processes, relevant for precipitation in solution, vapor condensation and crystallization in a simple melt. It is based on the ...classical nucleation theory, on a size-dependent (algebraic) growth law allowing growth, resorption and ripening of particles (or droplets) simultaneously, and on conservation laws akin to a thermodynamically closed system. Compared to popular population balance models, it explicitly keeps track of the time evolution of
any particle nucleated in the system. This yields a thorough knowledge of the particle population, which not only allows the crystal size distribution to be computed as a by-product, but permits a deeper understanding of the dynamics of these disequilibrated systems. The evolution of the system is followed through the time dependence of a control parameter
S
(
t
)
, and the changes in the instantaneous particle population. The model yields a set of highly non-linear coupled equations, which, aside from the initial conditions, are functions of only three irreducible dimensionless parameters. This assures that a common general scenario exists for the dynamics of these systems, as shown in the following paper in this issue.
We simulate the dynamics of nucleation, growth and resorption of particles in an initially supersaturated closed medium, according to the approach worked out in the preceding paper, which applies to ...precipitation in solution, condensation in the vapor phase or crystallization from a melt. We find that the formation of the new phase obeys a generic scenario, involving mainly three stages, whatever the values of the parameters of the model: a phase of pure nucleation during which the supersaturation decreases, a plateau characterized by a quasi-equilibrium between nucleation and (positive) growth on the one hand, and resorption on the other hand, and an ageing stage ending with a single particle in thermodynamic equilibrium with its surrounding medium. During the plateau regime, very complex dynamical effects take place, which may be disentangled thanks to the full knowledge of the particle population obtained by our approach. We emphasize that the particles present in the system in the long time regime are not those formed at
t
=
0
, but rather during some (usually) short time interval(s) on the plateau. This point should be kept in mind when using crystal size distribution (CSD) to date events which have occurred in the past.
The physical properties of the hydrous phyllosilicate lizardite have been investigated by atomistic simulation using the GULP code based on transferable semi-empirical interatomic potentials. ...Lizardite behavior was first investigated during structure relaxation at room temperature. The Helmholtz free energy is minimum for an equilibrium structure that is in agreement with experiment. The bulk, shear, and Young modulii for lizardite were calculated along with the Poisson ratio. From the shear and bulk modulii, we also calculated translational and longitudinal acoustic wave velocities that are important quantities for tectonophysics models. As expected, lizardite is stiffer in the a direction parallel to the layers than in the c perpendicular direction; the variation of the unit cell parameters with pressure is in good agreement with experiment. The cohesive energy between two successive layers along c direction was calculated at 0.33 eV (i.e., 0.11 eV per OH bond) in good agreement with recent ab initio calculations. Upon pressure and temperature variations, we evidenced that structural changes are mainly pressure induced; pressure being accommodated by a decrease of the c parameter up to 10 GPa. We also found that the change of slope in the derivative of the c cell parameter with respect to pressure occurring around 2 GPa originates from the bending of the interlayer hydroxyl groups with respect to the layer normal direction.
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