283 research outputs found
Investigation of the chemical vicinity of crystal defects in ion-irradiated Mg and AZ31 with coincident Doppler broadening spectroscopy
Crystal defects in magnesium and magnesium based alloys like AZ31 are of
major importance for the understanding of their macroscopic properties. We have
investigated defects and their chemical surrounding in Mg and AZ31 on an atomic
scale with Doppler broadening spectroscopy of the positron annihilation
radiation. In these Doppler spectra the chemical information and the defect
contribution have to be thoroughly separated. For this reason samples of
annealed Mg were irradiated with Mg-ions in order to create exclusively
defects. In addition Al- and Zn-ion irradiation on Mg-samples was performed in
order to create samples with defects and impurity atoms. The ion irradiated
area on the samples was investigated with laterally and depth resolved positron
Doppler broadening spectroscopy (DBS) and compared with preceding
SRIM-simulations of the vacancy distribution, which are in excellent agreement.
The investigation of the chemical vicinity of crystal defects in AZ31 was
performed with coincident Doppler broadening spectroscopy (CDBS) by comparing
Mg-ion irradiated AZ31 with Mg-ion irradiated Mg. No formation of
solute-vacancy complexes was found due to the ion irradiation, despite the high
defect mobility.Comment: Submitted to Physical Review B on March 20 20076. Revised version
submitted on September 28 2007. Accepted on October 19 200
Practical implications of GPR investigation using 3D data reconstruction and transmission tomography
Non-destructive investigation using ground penetrating radar is becoming
more popular in the inspection of civil structures. Currently, traditional 2D
imaging is used as a preliminary tool to fi nd possible areas of interest for
more detailed inspection, which can be accomplished by more advanced
techniques like 3D image reconstruction or tomography. In this paper,
a general overview of the work done at University of Minho regarding these
techniques is presented, together with their limitations and advantages over
typical radargrams, with implications for civil engineering applications. For
this purpose, data acquisition on two large masonry walls and one large
concrete specimen have been carried out, using refl ection mode, 3D
reconstruction and transmission tomography. The specimens have been
specially built for non-destructive inspection techniques testing, incorporating
different materials and internal voids. Radar tomography and 3D image
reconstruction techniques provided much more detailed information about
structural integrity and shapes and location of the voids, when compared to
2D imaging originally used for potential target identification.Fundação para a Ciência e a Tecnologia (FCT) - POCTI SFRH/BD/6409/2001"Sustainable Bridges" European project - FP6-PLT-0165
Injection of Positrons into a Dense Electron Cloud in a Magnetic Dipole Trap
The creation of an electron space charge in a dipole magnetic trap and the
subsequent injection of positrons has been experimentally demonstrated.
Positrons (5eV) were magnetically guided from their source and injected into
the trapping field generated by a permanent magnet (0.6T at the poles) using a
cross field E B drift, requiring tailored electrostatic and magnetic
fields. The electron cloud is created by thermionic emission from a tungsten
filament. The maximum space charge potential of the electron cloud reaches
-42V, which is consistent with an average electron density of ()
and a Debye length of () .
We demonstrate that the presence of this space potential does not hamper
efficient positron injection. Understanding the effects of the negative space
charge on the injection and confinement of positrons represents an important
intermediate step towards the production of a confined electron-positron pair
plasma
The Adsorption of H2O on TiO2 and SnO2(110) Studied by First-Principles Calculations
First-principles calculations based on density functional theory and the
pseudopotential method have been used to investigate the energetics of HO
adsorption on the (110) surface of TiO and SnO. Full relaxation of all
atomic positions is performed on slab systems with periodic boundary
conditions, and the cases of full and half coverage are studied. Both molecular
and dissociative (HO OH + H) adsorption are treated,
and allowance is made for relaxation of the adsorbed species to unsymmetrical
configurations. It is found that for both TiO and SnO an unsymmetrical
dissociated configuration is the most stable. The symmetrical molecularly
adsorbed configuration is unstable with respect to lowering of symmetry, and is
separated from the fully dissociated configuration by at most a very small
energy barrier. The calculated dissociative adsorption energies for TiO and
SnO are in reasonable agreement with the results of thermal desorption
experiments. Calculated total and local electronic densities of states for
dissociatively and molecularly adsorbed configurations are presented and their
relation with experimental UPS spectra is discussed
Changes in Cognitive State Alter Human Functional Brain Networks
The study of the brain as a whole system can be accomplished using network theory principles. Research has shown that human functional brain networks during a resting state exhibit small-world properties and high degree nodes, or hubs, localized to brain areas consistent with the default mode network. However, the study of brain networks across different tasks and or cognitive states has been inconclusive. Research in this field is important because the underpinnings of behavioral output are inherently dependent on whether or not brain networks are dynamic. This is the first comprehensive study to evaluate multiple network metrics at a voxel-wise resolution in the human brain at both the whole-brain and regional level under various conditions: resting state, visual stimulation, and multisensory (auditory and visual stimulation). Our results show that despite global network stability, functional brain networks exhibit considerable task-induced changes in connectivity, efficiency, and community structure at the regional level
Advanced structural analysis of a laser additive manufactured Zr-based bulk metallic glass along the build height
Additive manufacturing of bulk metallic glasses (BMGs) has opened this material class to an exciting new range of potential applications, as bulk-scale, net-shaped amorphous components can be fabricated in a single step. However, there exists a critical need to understand the structural details of additive manufactured BMGs and how the glassy structure is linked to the mechanical properties. Here, we present a study of structure and property variations along the build height for a laser powder bed fusion (LPBF) processed Zr-based BMG with composition Zr59.3Cu28.8Nb1.5Al10.4 commercially termed AMZ4, using hardness testing, calorimetry, positron annihilation spectroscopy, synchrotron X-ray diffraction, and transmission electron microscopy. A lower hardness, more rejuvenated glassy structure was found at the bottom of the build compared to the middle region of the build, with the structure and properties of the top region between the two. Such differences could not be attributed to variability in chemical composition or crystallisation; rather, the softer bottom region was found to have a larger medium range order cluster size, attributed to heat dissipation into the build plate during processing, which gave faster cooling rates and less reheating compared to the steady-state middle of the build. However, at the top of the build less reheating occurs compared to the middle, leading to a somewhat softer and less relaxed state
Cold neutral atoms via charge exchange from excited state positronium: a proposal
We present a method for generating cold neutral atoms via charge exchange
reactions between trapped ions and Rydberg positronium. The high charge
exchange reaction cross section leads to efficient neutralisation of the ions
and since the positronium-ion mass ratio is small, the neutrals do not gain
appreciable kinetic energy in the process. When the original ions are cold the
reaction produces neutrals that can be trapped or further manipulated with
electromagnetic fields. Because a wide range of species can be targeted we
envisage that our scheme may enable experiments at low temperature that have
been hitherto intractable due to a lack of cooling methods. We present an
estimate for achievable temperatures, neutral number and density in an
experiment where the neutrals are formed at a milli-Kelvin temperature from
either directly or sympathetically cooled ions confined on an ion chip. The
neutrals may then be confined by their magnetic moment in a co-located magnetic
minimum well also formed on the chip. We discuss general experimental
requirements
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