16,515 research outputs found
Formation of Aluminum Particles with Shell Morphology during Pressureless Spark Plasma Sintering of Fe-Al Mixtures: Current-Related or Kirkendall Effect?
A need to deeper understand the influence of electric current on the structure and properties of metallic materials consolidated by Spark Plasma Sintering (SPS) stimulates research on inter-particle interactions, bonding and necking processes in low-pressure or pressureless conditions as favoring technique-specific local effects when electric current passes through the underdeveloped inter-particle contacts. Until now, inter-particle interactions during pressureless SPS have been studied mainly for particles of the same material. In this work, we focused on the interactions between particles of dissimilar materials in mixtures of micrometer-sized Fe and Al powders forming porous compacts during pressureless SPS at 500-650 °C. Due to the chemical interaction between Al and Fe, necks of conventional shape did not form between the dissimilar particles. At the early interaction stages, the Al particles acquired shell morphology. It was shown that this morphology change was not related to the influence of electric current but was due to the Kirkendall effect in the Fe-Al system and particle rearrangement in a porous compact. No experimental evidence of melting or melt ejection during pressureless SPS of the Fe-Al mixtures or Fe and Al powders sintered separately was observed. Porous FeAl-based compacts could be obtained from Fe-40at.%Al mixtures by pressureless SPS at 650 °C
Parity Nonconservation in Odd-isotopes of Single Trapped Atomic Ions
We have estimated the size of the light-shifts due to parity nonconservation
(PNC) interactions in different isotopes of Ba+ and Ra+ ions based on the work
of Fortson [Phys. Rev. Lett. 70, 2383 (1993)]. We have used the nuclear spin
independent (NSI) amplitudes calculated earlier by us [Phys. Rev. Lett. 96,
163003 (2006); Phys. Rev. A 78, 050501(R) (2008)] and we have employed the
third order many-body perturbation theory (MBPT(3)) in this work to estimate
the nuclear spin dependent (NSD) amplitudes in these ions. Ra+ is found to be
more favourable than Ba+ for measuring both the NSI and NSD PNC observables.Comment: 5 pages, 1 tabl
Theoretical studies of the atomic transitions in boron-like ions: Mg VIII, Si X and S XII
In this paper, we have carried out the calculations of the weighted
oscillator strengths and the transition probabilities for a few low-lying
transitions of boron-like ions: Mg VIII, Si X and S XII which are
astrophysically important, particularly, in the atmospheres of the solar
corona. We have employed an all-order relativistic many-body theory called the
relativistic coupled-cluster theory to calculate very precisely these atomic
quantities of astrophysical interest. We have reported for the first time the
transition probabilities for some forbidden transitions which are unavailable
in the literature; either theoretically or experimentally. We also discuss the
physical effects associated with these transitions. Our data can be used for
the identification of spectral lines arising from the coronal atmospheres of
Sun and Sun-like stars having an extended corona.Comment: 8 pages, 4 table
Spectacular Role of Electron Correlation in the Hyperfine Interactions in States in Alkaline Earth Ions
The low-lying n(=3,4,5)d states alkaline earth ions are of vital
importance in a number of different physical applications. The hyperfine
structure constants of these states are characterized by unusually strong
electron correlation effects. Relativistic coupled-cluster theory has been
employed to carry out {\it ab initio} calculations of these constants. The role
of the all order core-polarization effects was found to be decisive in
obtaining good agreement of the results of our calculations with accurate
measurements. The present work is an apt demonstration of the power of the
coupled-cluster method to cope with strongly interacting configurations.Comment: Submitted to Physical Review Letters, 3 figures and 5 table
Relativistic Coupled-Cluster Theory of Atomic Parity Nonconservation: Application to Ba
We report the result of our {\it ab initio} calculation of the parity nonconserving electric dipole transition amplitude in
based on relativistic coupled-cluster theory. Considering
single, double and partial triple excitations, we have achieved an accuracy of
less than one percent. If the accuracy of our calculation can be matched by the
proposed parity nonconservation experiment in Ba for the above
transition,then the combination of the two results would provide an independent
non accelerator test of the Standard Model of particle physics.Comment: 4 pages, 1 figure, Submitted to PR
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