Search results for "Crystallization"
showing 10 items of 774 documents
Dynamic self-assembly of non-Brownian spheres studied by molecular dynamics simulations.
2015
Granular self-assembly of confined non-Brownian spheres under gravity is studied by molecular dynamics simulations. Starting from a disordered phase, dry or cohesive spheres organize, by vibrational annealing, into body-centered-tetragonal or face-centered-cubic structures, respectively. During the self-assembling process, isothermal and isodense points are observed. The existence of such points indicates that both granular temperature and packing fraction undergo an inversion process that may be in the core of crystal nucleation. Around the isothermal point, a sudden growth of granular clusters having the maximum coordination number takes place, indicating the outcome of a first-order phas…
Confined crystallization of a HKUST-1 metal–organic framework within mesostructured silica with enhanced structural resistance towards water
2017
A HKUST-1 metal–organic framework was crystallized in the NH2-modified mesostructured silica FDU-12 in order to improve its structural stability upon water exposure. In-depth structural characterization studies of the designed composite confirmed successful formation of the MOF phase within the ordered spherical mesopores of the silica matrix. In spite of the confinement within the cavities, MOF exhibits full accessibility for the adsorbed gas molecules. In contrast to the bulk HKUST-1, which undergoes slow phase transition in a humid environment, the structural integrity of the HKUST-1 in the humid-protective matrix remains unchanged even after immersion and stirring in water at elevated t…
Protein crystallization: universal thermodynamic vs. specific effects of PEG
2008
The interest of nucleation of protein crystals and aggregates (including oligomerization) spans from basic physics theory all the way to biophysics, nanophysics, clinical sciences, biotechnologies, food technologies and polymer–solvent interactions. Understanding nucleation within a theoretical framework capable of providing quantitative predictions and control of nucleation rates, or even the very occurrence of crystallization, is a long-sought goal of remarkable relevance to each of the above fields. A large amount of work has been aimed at such goal, but success has been so far rather limited. Work at our laboratory has more recently highlighted a direct link between nucleation rates and…
Transitions of tethered chain molecules under tension
2014
An applied tension force changes the equilibrium conformations of a polymer chain tethered to a planar substrate and thus affects the adsorption transition as well as the coil-globule and crystallization transitions. Conversely, solvent quality and surface attraction are reflected in equilibrium force-extension curves that can be measured in experiments. To investigate these effects theoretically, we study tethered chains under tension with Wang-Landau simulations of a bond-fluctuation lattice model. Applying our model to pulling experiments on biological molecules we obtain a good description of experimental data in the intermediate force range, where universal features dominate and finite…
Time scale of protein aggregation dictated by liquid-liquid demixing
2003
The growing impact of protein aggregation pathologies, together with the current high need for extensive information on protein structures are focusing much interest on the physics underlying the nucleation and growth of protein aggregates and crystals. Sickle Cell Hemoglobin (HbS), a point-mutant form of normal human Hemoglobin (HbA), is the first recognized and best-studied case of pathologically aggregating protein. Here we reanalyze kinetic data on nucleation of deoxy-HbS aggregates by referring them to the (concentration-dependent) temperature Ts characterizing the occurrence of the phase transition of liquid-liquid demixing (LLD) of the solution. In this way, and by appropriate scalin…
Gradients in physical parameters in zoned felsic magma bodies: Implications for evolution and eruptive withdrawal
1990
Abstract Five diverse, well documented, chemically zoned magmas have been chosen from the literature to demonstrate the extent and patterns of density and viscosity gradients in zoned magma chambers. The patterns are used to assess implications for development of zonation, and withdrawal dynamics and preservation of systematic chemical variations in the final pyroclastic deposit. These examples are: Bishop Tuff, California (high-silica rhyolite); Los Humeros, Mexico (calc-alkaline rhyolite to andesite); Fogo A, Azores (trachyte); Laacher See, Eifel (phonolite) and Tenerife, Canary Islands (phonolite). It was necessary to make several simplifying assumptions in order to calculate viscosity a…
Influence of the crystallization process on the photoelectrochemical behaviour of anodic TiO2 films
1997
On the basis of kinetic and photoelectrochemical studies we show that the formation of amorphous or strongly disordered TiO2 films on electropolished titanium rods can occur upon anodization in 0.5 M H2SO4 solution in a range of thickness which depends on the anodization rate. This finding is confirmed both by the changes in the shape of the photocurrent vs. potential curves with the energy of the incident photons, and by the impedance behaviour of the junction. Our data indicate that TiO2 films having different degree of disorder are formed depending on the anodization rate and oxide thickness. Crystalline films are formed at very low growth rates since very low thicknesses. Amorphous or s…
Excited state absorption and energy-transfer mechanisms of up-conversion luminescence in Er3+-doped oxyfluoride glass ceramics at different temperatu…
2010
Abstract Oxyfluoride silicate glass SiO2–Al2O3–Na2CO3–NaF–LaF3–ErF3 was synthesized. The glass transition and crystallization temperatures were determined by differential thermal analysis. Glass ceramics containing LaF3:Er3+ crystallites of size ∼20 nm were formed in the glass matrix after the heat treatment of the precursor glass in the vicinity of the crystallization temperature. Up-conversion luminescence, excitation spectra as well as time-resolved up-conversion luminescence of the glass and glass ceramics were studied at different temperatures. The up-conversion transients showed that at room temperature the dominant mechanism of the up-conversion luminescence in the glass ceramics is …
Luminescence properties of Eu, RE3+ (RE = Dy, Sm, Tb) co-doped oxyfluoride glasses and glass–ceramics
2017
Abstract Eu, RE3+ (RE = Dy, Sm, Tb) co-doped SiO2–CaF2–Al2O3–CaO oxyfluoride glasses have been prepared by the melt quenching method. By heating the precursor glasses at 670 °C for 1 h, glass–ceramics containing CaF2 nanocrystallites were obtained. DTA and XRD measurements were performed. Excitation spectra of the prepared samples as well as photoluminescence spectra at the excitation of 355 nm and 450 nm were measured, CIE colour coordinates and colour rendering indexes were found. In the glass–ceramics the presence of Eu2+ ions was observed by a broad emission band in the 400–500 nm spectral range. The most promising doping combinations for white light luminophores within our research are…
Deep center luminescence versus surface preparation of ZnSe single crystals
2001
A close relationship between the photoluminescence emissions labeled Y and S, related to dislocations and extended structural defects, and the preparation of the surface state of ZnSe single crystals before PL (photoluminescence) measurements has been established. The samples were obtained by solid-phase recrystallization under different pressure conditions. An easy method for achieving good quality surfaces with a very significant reduction of such Y and S PL emissions is proposed.