Positron affinities for elemental metals
ABSTRACT The relevant quantity in the comparison of the absolute positron energy levels in different materials is the sum of the internal electron and positron chemical potentials, i.e. the sum of the Fermi level and the bottom of the lowest positron band relative to a common, well-defined reference energy. This sum is defined as the positron affinity. The positron affinity reflects the preference of the positron for different components in heterostructures made of different materials and the preference between the host matrix and precipitates in alloys. Moreover, the affinity is closely related to the positron work function and positronium formation potential which are important parameters in the slow-positron-beam experiments. The authors have determined the positron affinity for the alkaline and alkaline-earth metals, 3d-, 4d-, and 5d-transition metal series, and for some metals on the right in the Periodic Table. The diamond structure semiconductors are also considered. The determination is based on the self-consistent electron structure calculations and the subsequent calculation of the positron band structure within the local-density approximation. The trends are studied and interpreted along the different columns and rows of the Periodic Table. The results are also compared with available experiments.
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ABSTRACT: The influence of the addition of C to the Fe-Mn-Si-Cr-Ni base material is investigated at room temperature. Steel samples were deformed during a tensile experiment up to a strain of 17%. Light optical microscopy (OM) and x-ray diffraction (XRD) gave information about the different micro-structural phases that exist in the deformed and the undeformed alloys. The evolution of the defect structure is followed by positron annihilation techniques such as Doppler broadening of annihilation radiation spectroscopy (DBAR) and the positron annihilation lifetime spectroscopy (PALS). During deformation a martensitic ε-phase is induced. The size of the martensite plates increases with increasing deformation.Journal of Materials Engineering and Performance 18(5):575-581. · 0.92 Impact Factor
Dataset: carbon femn asm
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ABSTRACT: Microstructural evolution during nanocrystallization of amorphous metallic glass Fe68.5Si18.5Nb3B9Cu1 has been studied. The effect of microstructural features of nanocrystalline phases on soft magnetic properties have been evaluated and rationalized with existing theoretical models. The TEM and XRD studies have shown the presence of single Fe3Si nanocrystalline phase at 550 °C (size range 13–16 nm) and three nanocrystalline phases Fe3Si, Fe3B, and Fe2B at 800 °C (size range 18–100 nm). Increase in annealing time at 800 °C resulted in decomposition of metastable Fe3B phase to equilibrium Fe2B phase. Positron annihilation spectroscopy revealed the presence of nanovoids in amorphous samples. Theoretical estimations showed that these nanovoids were having free volume equivalent to that of a vacancy defect consisting of five or more atoms vacancy cluster present in the amorphous samples. Nature of interfaces associated with nanocrystalline phases could be characterized using positron annihilation spectroscopy. This study showed that metallic nanoparticles have very low concentration of thermal vacancies. Effects of nature of phases, particle density, and nanoparticle size on saturation magnetization and coercivity have been studied.Journal of Nanoparticle Research 01/2012; 15(1):1-14. · 2.18 Impact Factor