3,085 research outputs found
Size and morphology control of ultrafine refractory complex oxide crystals
High-temperature complex oxides are of considerable interest as their applications cover a broad spectrum from catalytic to optical technology. Indeed, new exciting opportunities might emerge if these high-temperature complex oxides, in which structure crystallization is achieved at temperatures T > 1000 °C, could be synthesized as nonaggregated, ultrafine building blocks. In general, such refractory complex oxide particles are difficult to synthesize as ultrafine crystals because of the strong driving force available for sintering and coarsening in this high-temperature range. This paper reports a new synthetic process for the preparation of nonaggregated, ultrafine barium hexa-aluminate, BaO, 6Al2O3, (BHA), and Ba0.9Eu0.1MgAl10O17, (BAM) crystals in which structure crystallization occurs around 1300 °C. Our process is based on the Ba2+ and Al3+ ions high-temperature controlled diffusion from carbon−inorganic hybrid compounds prepared from soft chemistry routes. Control of morphology of these refractory complex aluminates displaying nanoplatelets morphology was achieved via the tailoring of high-temperature diffusion lengths of the various cations involved in the formation of these ultrafine refractory crystals
The role of long-range forces in the phase behavior of colloids and proteins
The phase behavior of colloid-polymer mixtures, and of solutions of globular
proteins, is often interpreted in terms of a simple model of hard spheres with
short-ranged attraction. While such a model yields a qualitative understanding
of the generic phase diagrams of both colloids and proteins, it fails to
capture one important difference: the model predicts fluid-fluid phase
separation in the metastable regime below the freezing curve. Such demixing has
been observed for globular proteins, but for colloids it appears to be
pre-empted by the appearance of a gel. In this paper, we study the effect of
additional long-range attractions on the phase behavior of spheres with
short-ranged attraction. We find that such attractions can shift the
(metastable) fluid-fluid critical point out of the gel region. As this
metastable critical point may be important for crystal nucleation, our results
suggest that long-ranged attractive forces may play an important role in the
crystallization of globular proteins. However, in colloids, where refractive
index matching is often used to switch off long-ranged dispersion forces,
gelation is likely to inhibit phase separation.Comment: EURO-LATEX, 6 pages, 2 figure
Crystallization in Glassy Suspensions of Hard Ellipsoids
We have carried out computer simulations of overcompressed suspensions of
hard monodisperse ellipsoids and observed their crystallization dynamics. The
system was compressed very rapidly in order to reach the regime of slow,
glass-like dynamics. We find that, although particle dynamics become
sub-diffusive and the intermediate scattering function clearly develops a
shoulder, crystallization proceeds via the usual scenario: nucleation and
growth for small supersaturations, spinodal decomposition for large
supersaturations.
In particular, we compared the mobility of the particles in the regions where
crystallization set in with the mobility in the rest of the system. We did not
find any signature in the dynamics of the melt that pointed towards the
imminent crystallization events
Faceting and branching in 2D crystal growth
The official published version of the Article can be accessed from the link below - Copyright @ 2011 APSUsing atomic scale time-dependent density functional calculations we confirm that both diffusion-controlled and diffusionless crystallization modes exist in simple 2D systems. We provide theoretical evidence that a faceted to nonfaceted transition is coupled to these crystallization modes, and faceting is governed by the local supersaturation at the fluid-crystalline interface. We also show that competing modes of crystallization have a major influence on mesopattern formation. Irregularly branched and porous structures are emerging at the crossover of the crystallization modes. The proposed branching mechanism differs essentially from dendritic fingering driven by diffusive instability.This work has been supported by the EU FP7
Collaborative Project ENSEMBLE under Grant
Agreement NMP4-SL-2008-213669 and by the
Hungarian Academy of Sciences under Contract
No. OTKA-K-62588
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