406 research outputs found
Positron trapping and annihilation at interfaces between matrix and cylindrical or spherical precipitates modeled by diffusion-reaction theory
The exact solution of a diffusionreaction model for the trapping and
annihilation of positrons at interfaces of precipitatematrix composites is
presented considering both cylindrical or spherical precipitates.
Diffusion-limitation is taken into account for interfacial trapping from the
surrounding matrix as well as from the interior of the precipitate. Closed-form
expressions are obtained for the mean positron lifetime and for the intensity
of the positron lifetime component associated with the interface-trapped state.
The model contains as special case also positron trapping at extended
open-volume defects like spherical voids or hollow cylinders. This makes the
model applicable to all types of cylindrical- and spherical-shaped extended
defects irrespective of their size and their number density.Comment: To be published in the Proceedings of International Conference on
Positrron Annihilation, 2018; submitted August 16, 2018; Accepted January 8,
2019. AIP Conference Proceeding
Energetics of positron states trapped at vacancies in solids
We report a computational first-principles study of positron trapping at
vacancy defects in metals and semiconductors. The main emphasis is on the
energetics of the trapping process including the interplay between the positron
state and the defect's ionic structure and on the ensuing annihilation
characteristics of the trapped state. For vacancies in covalent semiconductors
the ion relaxation is a crucial part of the positron trapping process enabling
the localization of the positron state. However, positron trapping does not
strongly affect the characteristic features of the electronic structure, e.g.,
the ionization levels change only moderately. Also in the case of metal
vacancies the positron-induced ion relaxation has a noticeable effect on the
calculated positron lifetime and momentum distribution of annihilating
electron-positron pairs.Comment: Submitted to Physical Review B on 17 April 2007. Revised version
submitted on 6 July 200
Continuum theory of vacancy-mediated diffusion
We present and solve a continuum theory of vacancy-mediated diffusion (as
evidenced, for example, in the vacancy driven motion of tracers in crystals).
Results are obtained for all spatial dimensions, and reveal the strongly
non-gaussian nature of the tracer fluctuations. In integer dimensions, our
results are in complete agreement with those from previous exact lattice
calculations. We also extend our model to describe the vacancy-driven
fluctuations of a slaved flux line.Comment: 25 Latex pages, subm. to Physical Review
Spin-glass freezing of maghemite nanoparticles prepared by microwave plasma synthesis
Magnetic properties of 6nm maghemite nanoparticles (prepared by microwave plasma synthesis) have been studied by ac and dc magnetic measurements. Structural characterization includes x-ray diffraction and transmission electron microscopy. The temperature scans of zero field cooled/field cooled (ZFC/FC) magnetization measurements show a maximum at 75 K. The ZFC/FC data are fitted to the Brown-Ne´el relaxation model using uniaxial anisotropy and a log-normal size-distribution function to figure out the effective anisotropy constant K. K turns out to be larger than the anisotropy constant of bulk maghemite. Fitting of the ac susceptibility to an activated relaxation process according to the Arrhenius law provides unphysical values of the spin-flip time and activation energy. A power-law scaling shows a satisfactory fit to the ac susceptibility data and the dynamic critical exponent (zv10) takes value between 4 and 12 which is typical for the spin-glass systems. The temperature dependence of coercivity and exchange bias shows a sharp increase toward low temperatures which is due to enhanced surface anisotropy. The source of this enhanced magnetic anisotropy comes from the disordered surface spins which get frozen at low temperatures. Memory effects and thermoremanent magnetization experiments also support the existence of spin-glass behaviour. All these magnetic measurements signify either magnetic blocking or surface spin-glass freezing at high and low temperatures, respectivel
Disordered and Frustrated Magnetization in Coated MnFe₂O₄ Nanoparticles Prepared by Microwave Plasma Synthesis
Disordered and frustrated magnetization of different surface coated (Cr2O3, Co3O4, ZrO2, and SiO2) MnFe2O4 nanoparticles have been studied using SQUID-magnetometry. Magnetic measurements, such as ZFC/FC and ac-susceptibility evidence surface spin-glass behavior. ZFC/FC curves were also compared with numerical simulation to get information about effective anisotropy constants. Frequency dependent ac susceptibility results were analyzed by using Arrhenius, Vogel Fulcher and dynamic scaling laws to further confirm the spin-glass behavior. It is observed that the strength of surface spins disorder and frustration strongly depends upon the type of the coating material. All these analyses signify that disordered and frustrated surface magnetization in MnFe2O4 nanoparticles greatly depend on the type of the surface coating materials and are useful for controlling the nanoparticle’s magnetism for different practical applications
In Situ Study of Nanoporosity Evolution during Dealloying AgAu and CoPd by Grazing-Incidence Small-Angle X-ray Scattering
Electrochemical dealloying has become a standard technique to produce nanoporous network structures of various noble metals, exploiting the selective dissolution of one component from an alloy. While achieving nanoporosity during dealloying has been intensively studied for the prime example of nanoporous Au from a AgAu alloy, dealloying from other noble-metal alloys has been rarely investigated in the scientific literature. Here, we study the evolution of nanoporosity in the electrochemical dealloying process for both CoPd and AgAu alloys using a combination of in situ grazing-incidence small-angle X-ray scattering (GISAXS), kinetic Monte Carlo (KMC) simulations, and scanning transmission electron microscopy (STEM). When comparing dealloying kinetics, we find a more rapid progression of the dealloying front for CoPd and also a considerably slower coarsening of the nanoporous structure for Pd in relation to Au. We argue that our findings are natural consequences of the effectively higher dealloying potential and the higher interatomic binding energy for the CoPd alloy. Our results corroborate the understanding of electrochemical dealloying on the basis of two rate equations for dissolution and surface diffusion and suggest the general applicability of this dealloying mechanism to binary alloys. The present study contributes to the future tailoring of structural size in nanoporous metals for improved chemical surface activity
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