Mechanism of Formation of Monodispersed Colloids by Aggregation of Nanosize Precursors

It has been experimentally established in numerous cases that precipitation of monodispersed colloids from homogeneous solutions is a complex process. Specifically, it was found that in many systems nuclei, produced rapidly in a supersaturated solution, grow to nanosize primary particles (singlets), which then coagulate to form much larger final colloids in a process dominated by irreversible capture of these singlets. This paper describes a kinetic model that explains the formation of dispersions of narrow size distribution in such systems. Numerical simulations of the kinetic equations, with experimental model parameter values, are reported. The model was tested for a system involving formation of uniform spherical gold particles by reduction of auric chloride in aqueous solutions. The calculated average size, the width of the particle size distribution, and the time scale of the process, agreed reasonably well with the experimental values.Comment: 38 pages in plain TeX and 7 JPG figure

In vivo Analysis of Choroid Plexus Morphogenesis in Zebrafish

BACKGROUND: The choroid plexus (ChP), a component of the blood-brain barrier (BBB), produces the cerebrospinal fluid (CSF) and as a result plays a role in (i) protecting and nurturing the brain as well as (ii) in coordinating neuronal migration during neurodevelopment. Until now ChP development was not analyzed in living vertebrates due to technical problems. METHODOLOGY/PRINCIPAL FINDINGS: We have analyzed the formation of the fourth ventricle ChP of zebrafish in the GFP-tagged enhancer trap transgenic line SqET33-E20 (Gateways) by a combination of in vivo imaging, histology and mutant analysis. This process includes the formation of the tela choroidea (TC), the recruitment of cells from rhombic lips and, finally, the coalescence of TC resulting in formation of ChP. In Notch-deficient mib mutants the first phase of this process is affected with premature GFP expression, deficient cell recruitment into TC and abnormal patterning of ChP. In Hedgehog-deficient smu mutants the second phase of the ChP morphogenesis lacks cell recruitment and TC cells undergo apoptosis. CONCLUSIONS/SIGNIFICANCE: This study is the first to demonstrate the formation of ChP in vivo revealing a role of Notch and Hedgehog signalling pathways during different developmental phases of this process

p–T Phase Diagram of Phosphorus Revisited

International audienceMelting and phase transitions of phosphorus allotropes have been studied in the 2-11 GPa pressure range using in situ electrical resistivity measurements and synchrotron X-ray diffraction. It was found that the melting curve of rhombohedral phosphorus exhibits positive slope of 58(2) K/GPa and crosses the rhombohedral-simple cubic phase boundary (negative slope of -420(70) K/GPa) at about 8.3 GPa and 1490 K. The melting slope of simple cubic phosphorus can be estimated as +300(90) K/GPa. Calculations of phosphorus phase equilibria have been performed using our experimental and literature data within the framework of the phenomenological thermodynamics. The constructed equilibrium p-T phase diagram accurately describes the experimentally observed phase relations in solid and liquid phosphorus up to 12 GPa and 2000 K

Phase Diagram of the B–BN System at Pressures up to 24 GPa: Experimental Study and Thermodynamic Analysis

Phase relations in the B–BN system have been studied ex situ and in situ at pressures 2–20 GPa and temperatures up to 2800 K. The evolution of topology of the B–BN phase diagram has been investigated up to 24 GPa using models of phenomenological thermodynamics with interaction parameters derived from our experimental data on phase equilibria at high pressures and high temperatures. There are two thermodynamically stable boron subnitrides in the system i.e. B$_{13}$N$_2$ and B$_{50}$N$_2$. Above 16.5 GPa, the $B_{50}N_2 ⇄ L + B_{13}N_2$ peritectic reaction transforms to the solid-phase reaction of $B_{50}N_2$ decomposition into tetragonal boron $(t′-B_{52})$ and B$_{13}$N$_2$, while the incongruent type of B$_{13}$N$_2$ melting changes to the congruent type only above 23.5 GPa. The constructed phase diagram provides fundamentals for directed high-pressure synthesis of superhard phases in the B–BN system

Kinetics of cBN crystallization in the Li3N−BN\mathrm{Li_{3}N-BN} system at 6.6 GPa

Kinetics of cBN crystallization from the Li3N-BN melt being in equilibrium with hBN has been studied in situ at 6.6 GPa in the 1720–1820 K range using diffraction of synchrotron radiation. The process under consideration has been found to have the activation energy of 115 ± 5 kJ mol−1 and to be controlled by BN diffusion in the melt. Kinetics data might be best fitted by a model that assumes an instantaneous nucleation in the initial stage of crystallization and nucleation at constant rate when hBN-to-cBN conversion degree is higher than 0.2. Using the model or regular solutions, the maximum solubilities of hBN and cBN in the melt of the Li3N-BN system were calculated, which allowed the BN diffusion coefficient in the melt at 6.6 GPa and 1770 K to be evaluated at (3 ± 1) × 10−7cm2 s−1