Computer simulation of crystallization kinetics with non-Poisson distributed nuclei

The influence of non-uniform distribution of nuclei on crystallization kinetics of amorphous materials is investigated. This case cannot be described by the well-known Johnson-Mehl-Avrami (JMA) equation, which is only valid under the assumption of a spatially homogeneous nucleation probability. The results of computer simulations of crystallization kinetics with nuclei distributed according to a cluster and a hardcore distribution are compared with JMA kinetics. The effects of the different distributions on the so-called Avrami exponent $n$ are shown. Furthermore, we calculate the small-angle scattering curves of the simulated structures which can be used to distinguish experimentally between the three nucleation models under consideration.Comment: 14 pages including 7 postscript figures, uses epsf.sty and ioplppt.st

Predominant Functional Expression of Kv1.3 by Activated Microglia of the Hippocampus after Status epilepticus

BACKGROUND:Growing evidence indicates that the functional state of microglial cells differs according to the pathological conditions that trigger their activation. In particular, activated microglial cells can express sets of Kv subunits which sustain delayed rectifying potassium currents (Kdr) and modulate differently microglia proliferation and ability to release mediators. We recently reported that hippocampal microglia is in a particular activation state after a status epilepticus (SE) and the present study aimed at identifying which of the Kv channels are functionally expressed by microglia in this model. METHODOLOGY/PRINCIPAL FINDINGS:SE was induced by systemic injection of kainate in CX3CR1(eGFP/+) mice and whole cell recordings of fluorescent microglia were performed in acute hippocampal slices prepared 48 h after SE. Microglia expressed Kdr currents which were characterized by a potential of half-maximal activation near -25 mV, prominent steady-state and cumulative inactivations. Kdr currents were almost abolished by the broad spectrum antagonist 4-Aminopyridine (1 mM). In contrast, tetraethylammonium (TEA) at a concentration of 1 mM, known to block Kv3.1, Kv1.1 and 1.2 subunits, only weakly reduced Kdr currents. However, at a concentration of 5 mM which should also affect Kv1.3 and 1.6, TEA inhibited about 30% of the Kdr conductance. Alpha-dendrotoxin, which selectively inhibits Kv1.1, 1.2 and 1.6, reduced only weakly Kdr currents, indicating that channels formed by homomeric assemblies of these subunits are not important contributors of Kdr currents. Finally, agitoxin-2 and margatoxin strongly inhibited the current. CONCLUSIONS/SIGNIFICANCE:These results indicate that Kv1.3 containing channels predominantly determined Kdr currents in activated microglia after SE

Decomposition kinetics of bulk amorphous Zr41Ti14Cu12.5Ni10Be22.5 alloys studied by computer simulation and small angle neutron scattering

The decomposition behaviour during annealing in the undercooled liquid regime of Zr41Ti14Cu12.5Ni10Be22.5 is studied by means of small-angle neutron scattering (SANS). In this temperature range, a microstructure consisting of precipitates of an amorphous phase is formed. The scattering curves show a characteristic maximum which becomes more and more intense with increasing annealing time until a saturation state is reached. All scattering curves at different annealing times obey a universal scaling law by scaling the intensity and the q axis by the intensity and the position of the maximum, respectively. In order to interpret the scattering curves, the scattering behaviour of a system of interacting particles and its time evolution are simulated. The centres of the precipitates are modelled (either analytically or numerically) by a hard-core point field, i.e. they have a minimum distance from each other (`excluded volume'). At each of these points, a nucleus is created according to a given nucleation rate, either instantaneously (all nuclei start growing at the same time) or continuously (in each time interval, a constant number of nuclei starts growing). Variation of the parameters of the hard-core point field, the nucleation mechanism and the growth law for the precipitates and comparison of the calculated and measured small-angle scattering curves allow the following interpretation: The amorphous precipitates are distributed according to a hard-core structure and the nucleation is continuous. The growth of the precipitates is diffusion controlled (a ∝ t 1/2).</jats:p

Analysis of SANS data with models of stochastic geometry

Variable random and partially disordered germ grain models are generated both by analytical mathematical methods and by computer simulations. The small angle scattering intensities of the models are calculated and the experimental SANS data obtained from Si3N4 ceramics and from bulk amorphous metallic alloys are interpreted in terms of the model

Der Einfluss objektorientierter Organisationsformen auf die Gestaltung absatzpolitischer Entscheidungsprozesse

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