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Positron Lifetime (positron + lifetime)
Terms modified by Positron Lifetime Selected AbstractsPositron annihilation spectroscopic study of hydrothermal grown n-type zinc oxide single crystalPHYSICA STATUS SOLIDI (C) - CURRENT TOPICS IN SOLID STATE PHYSICS, Issue 10 2007C. W. Hui Abstract Positron lifetime and coincidence Doppler broadening spectroscopic (CDBS) measurements were carried out to study the defects in two hydrothermal (HT) grown ZnO single crystal samples (HT1 and HT2) obtained from two companies. Single component model could offer good fittings to the room temperature spectra of HT1 and HT2, with the positron lifetimes equal to 199 ps and 181 ps respectively. These two lifetime components were associated with saturated positron trapping into two VZn -related defects with different microstructures. The positron lifetimes of HT1 was found to be temperature independent. For the HT2 sample, the positron lifetime remained unchanged with T > 200 K and decreased with decreasing temperature as T<200K. This could be explained by the presence of an additional positron trap having similar electronic environment to that of the delocalized state and competing in trapping positrons with the 181 ps component at low temperatures. Positron-electron autocorrelation function, which was the fingerprint of the annihilation site, was extracted from the CDBS spectrum. The obtained autocorrelation functions of HT1 and HT2 at room temperature, and HT2 at 50 K had features consistent with the above postulates that the 181 ps and the 199 ps components had distinct microstructures and the low temperature positron trap existed in HT2. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] Temperature dependence of positronium lifetime in cylindrical poresPHYSICA STATUS SOLIDI (C) - CURRENT TOPICS IN SOLID STATE PHYSICS, Issue 10 2007R. Zaleski Abstract Positron Annihilation Lifetime Spectroscopy (PALS) results for ordered mesoporous silica sieves (MCM-41 and its fibre form MSF) are used to verify predictions of the Extended Tao-Eldrup (ETE) model. Cylindrical pores (R , 1,2 nm) in these materials have well defined geometry and size, thus the structure of MCM-41 and MSF reproduces the model assumptions. Positron lifetimes were measured in the range 100-500K. In MCM-41, due to low intensity of pore related ortho-positronium component, it is difficult to determine the component's lifetime precisely. Accuracy of the lifetime fitting is better for MSF, where temperature dependence is in a good agreement with the ETE model results. For comparison, the ETE model predictions fit the lifetime of the pore related component in the Vycor sample only for T > 200 K. Below this value the lifetime exceeds the model predictions by over 30%. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] Natural Aging in Al-Mg-Si Alloys , A Process of Unexpected Complexity,ADVANCED ENGINEERING MATERIALS, Issue 7 2010John Banhart The natural aging behavior of pure ternary Al-Mg-Si alloys is investigated by measuring hardness, electrical resistivity and positron lifetime, as well as carrying out thermal analysis and atom probe microscopy. It is found that several distinct temporal stages of natural aging can be distinguished in which one of these quantities shows a characteristic behavior and that these times coincide for many of these measurements. The rate of change in the measured data is correlated with proposed solute dynamics during natural aging for both aging that takes place prior to artificial aging (natural pre-aging) and after artificial underaging (natural secondary aging) heat treatments. Controlling factors for solute dynamics are discussed. [source] Vacancy-type defects and electronic structure of perovskite-oxide SrTiO3 from positron annihilationPHYSICA STATUS SOLIDI (A) APPLICATIONS AND MATERIALS SCIENCE, Issue 2 2006A. S. Hamid Abstract The vacancy-type defects in Nb-doped SrTiO3 and in undoped SrTiO3, annealed in H2 flow, were investigated by means of positron lifetime and 2D angular correlation of annihilation radiation (ACAR) experiments. The calculations of the lifetime of positron were performed by using atomic superposition (AT-SUP) method. The results showed that positrons annihilate from a free state in the Nb-doped SrTiO3. The trapping centers in the annealed sample were found to be oxygen vacancies VO associated with relaxation of the surrounding ions. Moreover, the momentum distributions of the samples studied were correlated to the variation of their electronic structure. It was proposed from the drastic change in the momentum distribution upon introduction of VO, that 2D-ACAR technique is a sensitive tool for acquiring information on the electronic and bond structure of the perovskite-oxides. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] Effect of magnetic field on the corrosion of iron and St20 steel as studied by positron annihilationPHYSICA STATUS SOLIDI (B) BASIC SOLID STATE PHYSICS, Issue 7 2010Ryszard Pietrzak Abstract Lifetime spectra of positron annihilation in iron and St20 steel corroded in the water solution of HCl and in vapour above its surface have been investigated with emphasis on the effect of corrosion time and environment (solution or vapour) as well as of the presence and orientation of the magnetic field on the measured positron annihilation parameters. All the registered positron lifetime spectra consist of two components. For corroded samples the intensities of the two components as well as the corresponding values of the mean positron lifetime, differ from those for uncorroded material. The corrosion-induced changes in the positron annihilation parameters depend on the corrosion environment (solution or vapour) and on the presence and direction of external magnetic field. [source] Positron preferential annihilation in Bi,Sb alloysPHYSICA STATUS SOLIDI (B) BASIC SOLID STATE PHYSICS, Issue 10 2009Jerzy Dryzek Abstract We report the measurements of the positron annihilation characteristics, i.e., positron lifetime and coincidence Doppler broadening spectra performed on five Bi,Sb alloys having a Bi content between 12.5 and 81,at.%. The strong effect of the preferential annihilation of positrons with the electrons of the Bi atoms has been revealed in these alloys. [source] Atomic structure and positron lifetime in the metallic glass Zr55Cu30Ni5Al10PHYSICA STATUS SOLIDI (C) - CURRENT TOPICS IN SOLID STATE PHYSICS, Issue 10 2007K. Sugita Abstract Zr-based metallic glasses have superior characteristics such as mechanical strength, corrosion resistance and precision casting ability. From positron lifetime measurements, a negative temperature dependence of the mean positron lifetime above room temperature was reported and ascribed to the presence of shallow traps. However, the trapping sites remain unknown under the present circumstances. To get a further understanding of the experimental positron lifetime value, the positron density distribution and the lifetime in the Zr-based metallic glass Zr55Cu30Ni5Al10 have been calculated. The calculation shows that the positrons are annihilated inhomogeneously and most positrons are annihilated preferentially around Cu/Al. These results indicate that the positron lifetime not exactly reflects the total free volume. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] Positron annihilation spectroscopic study of hydrothermal grown n-type zinc oxide single crystalPHYSICA STATUS SOLIDI (C) - CURRENT TOPICS IN SOLID STATE PHYSICS, Issue 10 2007C. W. Hui Abstract Positron lifetime and coincidence Doppler broadening spectroscopic (CDBS) measurements were carried out to study the defects in two hydrothermal (HT) grown ZnO single crystal samples (HT1 and HT2) obtained from two companies. Single component model could offer good fittings to the room temperature spectra of HT1 and HT2, with the positron lifetimes equal to 199 ps and 181 ps respectively. These two lifetime components were associated with saturated positron trapping into two VZn -related defects with different microstructures. The positron lifetimes of HT1 was found to be temperature independent. For the HT2 sample, the positron lifetime remained unchanged with T > 200 K and decreased with decreasing temperature as T<200K. This could be explained by the presence of an additional positron trap having similar electronic environment to that of the delocalized state and competing in trapping positrons with the 181 ps component at low temperatures. Positron-electron autocorrelation function, which was the fingerprint of the annihilation site, was extracted from the CDBS spectrum. The obtained autocorrelation functions of HT1 and HT2 at room temperature, and HT2 at 50 K had features consistent with the above postulates that the 181 ps and the 199 ps components had distinct microstructures and the low temperature positron trap existed in HT2. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] Nanocrystalline alloys studied by positron annihilation lifetime spectroscopyPHYSICA STATUS SOLIDI (C) - CURRENT TOPICS IN SOLID STATE PHYSICS, Issue 10 2007Martin Petriska Abstract Nanocrystalline alloys present a new class of materials that exhibit interesting magnetic properties. These stem primarily from the presence of crystalline grains with the size of several tens of nanometers embedded in a residual amorphous matrix. In this paper, we employ positron annihilation spectrometry to investigate the relation between positron lifetime and microstructure of the NANOPERM-type metallic Fe76Mo8Cu1B15 alloy. The behaviour of positron lifetime with respect to the annealing temperature is discussed for samples, which were prepared by one hour thermal treatment in a vacuum at temperatures ranging from 330 up to 700 °C. Samples in amorphous (as-quenched) state were analysed, too. The results of this work confirm that the optimum annealing temperature of 550 °C correlates with the minimum of free volume and coincide with the temperature at which superior magnetic properties are observed. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] Positron lifetime calculations for stacking fault tetrahedra in copperPHYSICA STATUS SOLIDI (C) - CURRENT TOPICS IN SOLID STATE PHYSICS, Issue 10 2007S. Van Petegem Abstract We investigate the positron response of simulated stacking fault tetrahedra in Cu. We find that in both the regular and truncated stacking fault tetrahedra positron lifetimes are about 10-15 ps larger than the bulk lifetime. This is an indication that the vacancy-like defects observed in neutron irradiated Cu by positron annihilation lifetime measurements do not arise during the formation of the stacking fault tetrahedra but rather during the interaction of the stacking fault tetrahedra with other defects. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] Positron annihilation spectroscopic study of hydrothermal grown n-type zinc oxide single crystalPHYSICA STATUS SOLIDI (C) - CURRENT TOPICS IN SOLID STATE PHYSICS, Issue 10 2007C. W. Hui Abstract Positron lifetime and coincidence Doppler broadening spectroscopic (CDBS) measurements were carried out to study the defects in two hydrothermal (HT) grown ZnO single crystal samples (HT1 and HT2) obtained from two companies. Single component model could offer good fittings to the room temperature spectra of HT1 and HT2, with the positron lifetimes equal to 199 ps and 181 ps respectively. These two lifetime components were associated with saturated positron trapping into two VZn -related defects with different microstructures. The positron lifetimes of HT1 was found to be temperature independent. For the HT2 sample, the positron lifetime remained unchanged with T > 200 K and decreased with decreasing temperature as T<200K. This could be explained by the presence of an additional positron trap having similar electronic environment to that of the delocalized state and competing in trapping positrons with the 181 ps component at low temperatures. Positron-electron autocorrelation function, which was the fingerprint of the annihilation site, was extracted from the CDBS spectrum. The obtained autocorrelation functions of HT1 and HT2 at room temperature, and HT2 at 50 K had features consistent with the above postulates that the 181 ps and the 199 ps components had distinct microstructures and the low temperature positron trap existed in HT2. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] | ||||||||||