731 research outputs found
Kinetics of natural aging in Al-Mg-Si alloys studied by positron annihilation lifetime spectroscopy
The process of natural aging in pure ternary Al-Mg-Si alloys was studied by
positron annihilation lifetime spectroscopy in real time in order to clarify
the sequence and kinetics of clustering and precipitation. It was found that
natural aging takes place in at least five stages in these alloys, four of
which were directly observed. This is interpreted as the result of complex
interactions between vacancies and solute atoms or clusters. One of the early
stages of positron lifetime evolution coincides with a clustering process
observed by differential scanning calorimetry (DSC) and involves the formation
of a positron trap with \sim 0.200 ns lifetime. In later stages, a positron
trap with a higher lifetime develops in coincidence with the DSC signal of a
second clustering reaction. Mg governs both the kinetics and the lifetime
change in this stage. Within the first 10 min after quenching, a period of
nearly constant positron lifetime was found for those Mg-rich alloys that later
show an insufficient hardness response to artificial aging, the so-called
"negative effect." The various processes observed could be described by two
effective activation energies that were found by varying the aging temperature
from 10 to 37\degree C.Comment: arXiv admin note: same as v2, to correct mistaken v
Depletion of density of states near Fermi energy induced by disorder and electron correlation in alloys
We have performed high resolution photoemission study of substitutionally
disordered alloys Cu-Pt, Cu-Pd, Cu-Ni, and Pd-Pt. The ratios between alloy
spectra and pure metal spectra are found to have dips at the Fermi level when
the residual resistivity is high and when rather strong repulsive
electron-electron interaction is expected. This is in accordance with Altshuler
and Aronov's model which predicts depletion of density of states at the Fermi
level when both disorder and electron correlation are present.Comment: 1 tex file and 4 ps file
An experimental study of columnar crystals using monodisperse microbubbles
We investigate the ordered arrangements of monodisperse microbub bles con ned within narrow cylinders. These foams were imaged using X ray tomography, allowing the 3D positions of the bubbles of the foam to be accurately determined. The structure of these foams closely re semble the minimum energy con guration of hard spheres in cylindrical con nement as found in simulations. For larger ratios, , of cylinder to bubble diameter two and three layered crystals were formed. Each layer of these structures is found to be ordered, with each internal layer resem bling structures found at lower values. The average number of contacts per bubble is seen to increase wit
New foam stabilising additive for alumnium
Liquid metals are mostly made foamable by Ca additions followed by a thickening period. There is a need for an additive that can be easily admixed to an aluminium alloy melt and makes this melt foamable. We have selected aluminium-based grain refiner composites to test their foamability. TiB2, TiC or TiAl3 particles were produced in the melt by flux-assisted melting using fluoride salts. The particle size was kept below 1 im for TiB2 and TiC and around 10µm for TiAl3. The composites were heated to above their melting point (700°C) and were then foamed by either the addition of TiH2 or by injecting gases into the melt directly. Foams were successfully produced using TiB2 and TiC particles, while TiAl3 did not lead to any foam. Foam stability increased from TiB2 to TiC. Ex-situ characterisation of the foams by SEM showed that the particles segregate to the surfaces of the cell walls and lead to almost dense coverages there. Even after dilution of the initial composite a significant amount of foam can be still produced, indicating that these composites are suitable foam stabilizing additive for aluminium alloys and that foams based on small volume fractions of nonmetallic additives can be produced
The rupture of a single liquid aluminium alloy film
Dieser Beitrag ist mit Zustimmung des Rechteinhabers aufgrund einer (DFG geförderten) Allianz- bzw. Nationallizenz frei zugänglich.This publication is with permission of the rights owner freely accessible due to an Alliance licence and a national licence (funded by the DFG, German Research Foundation) respectively.The present study is based on the idea of understanding the rupture of films in metal foams by studying free standing metallic films as a model system. Liquid dynamics, the velocity of the rupturing material as well as the behaviour of ceramic particles inside the melt were analysed optically ex situ and by synchrotron X-ray radiography in situ. It was found that the resistance of films to rupture is mainly based on the interaction between solid particles and an immobile oxide skin, the formation of which depends on the oxygen content of the surrounding atmosphere and the presence of magnesium
Quantitative investigation of precipitate growth during ageing of Al-(Mg,Si) alloys by energy-filtered electron diffraction
Energetics and stability of nanostructured amorphous carbon
Monte Carlo simulations, supplemented by ab initio calculations, shed light
into the energetics and thermodynamic stability of nanostructured amorphous
carbon. The interaction of the embedded nanocrystals with the host amorphous
matrix is shown to determine in a large degree the stability and the relative
energy differences among carbon phases. Diamonds are stable structures in
matrices with sp^3 fraction over 60%. Schwarzites are stable in low-coordinated
networks. Other sp^2-bonded structures are metastable.Comment: 11 pages, 7 figure
Demixing, remixing and cellular networks in binary liquids containing colloidal particles
We present a confocal-microscopy study of demixing and remixing in binary
liquids containing colloidal particles. First, particle-stabilized emulsions
have been fabricated by nucleation and growth of droplets upon cooling from the
single-fluid phase. We show that their stability mainly derives from
interfacial particles; the surplus of colloids in the continuous phase possibly
provides additional stability. Upon heating these emulsions, we have observed
the formation of polyhedral cellular networks of colloids, just before the
system remixes. Given a suitable liquid-liquid composition, the initial
emulsions cross the binary-liquid symmetry line due to creaming. Therefore,
upon heating, the droplets do not shrink and they remain closely packed. The
subsequent network formation relies on a delicate balance between the Laplace
pressure and the pressure due to creaming/remixing. As high concentrations of
colloids in the cell walls inhibit film thinning and rupture, the networks can
be stabilized for more than 30 minutes. This opens up an avenue for their
application in the fabrication of advanced materials.Comment: http://dx.doi.org/10.1039/b918002
In situ radiographic investigation of de lithiation mechanisms in a tin electrode lithium ion battery.
The lithiation and delithiation mechanisms of multiple Sn particles in a customized flat radiography cell were investigated by in amp; 8197;situ synchrotron radiography. For the first time, four de lithiation phenomena in a Sn electrode battery system are highlighted 1 amp; 8197;the de lithiation behavior varies between different Sn particles, 2 amp; 8197;the time required to lithiate individual Sn particles is markedly different from the time needed to discharge the complete battery, 3 amp; 8197;electrochemical deactivation of originally electrochemically active particles is reported, and 4 amp; 8197;a change of electrochemical behavior of individual particles during cycling is found and explained by dynamic changes of de lithiation pathways amongst particles within the electrode. These unexpected findings fundamentaly expand the understanding of the underlying de lithiation mechanisms inside commercial lithium ion batteries LIBs and would open new design principles for high performance next generation LIB
Parquet approach to nonlocal vertex functions and electrical conductivity of disordered electrons
A diagrammatic technique for two-particle vertex functions is used to
describe systematically the influence of spatial quantum coherence and
backscattering effects on transport properties of noninteracting electrons in a
random potential. In analogy with many-body theory we construct parquet
equations for topologically distinct {\em nonlocal} irreducible vertex
functions into which the {\em local} one-particle propagator and two-particle
vertex of the coherent-potential approximation (CPA) enter as input. To
complete the two-particle parquet equations we use an integral form of the Ward
identity and determine the one-particle self-energy from the known irreducible
vertex. In this way a conserving approximation with (Herglotz) analytic
averaged Green functions is obtained. We use the limit of high spatial
dimensions to demonstrate how nonlocal corrections to the (CPA)
solution emerge. The general parquet construction is applied to the calculation
of vertex corrections to the electrical conductivity. With the aid of the
high-dimensional asymptotics of the nonlocal irreducible vertex in the
electron-hole scattering channel we derive a mean-field approximation for the
conductivity with vertex corrections. The impact of vertex corrections onto the
electronic transport is assessed quantitatively within the proposed mean-field
description on a binary alloy.Comment: REVTeX 19 pages, 9 EPS diagrams, 6 PS figure
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