Papers by Keyword: Positron Annihilation

Abstract: Dislocations would be induced after plastic deformation, which might change the mechanical properties of solids. FeCrNi austenitic model alloy and its Mo-diluted alloy were cold rolled with different degree of thickness reduction. Positrons are sensitive to point defects, which are easily trapped and annihilated around the trapping sites. The mean positron lifetimes have been used to estimate the average dislocation concentration in solids. Meanwhile, the trapping efficiency μ was calculated from the lifetime results. The trapping efficiency value is estimated about 3.31×10^-7 cm³s^-1 for FeCrNi alloy and 3.31×10^-7 cm³s^-1 for Mo-diluted alloy, respectively. The increment of the hardness value during plastic deformation is related to the increase of the dislocation density and dislocation pile up in solids.

Abstract: Solution annealed type 316L austenitic stainless steels were irradiated using 2 MeV Fe ions at room temperature. The implanted fluences were 2×10¹² ions/cm² and 1×10¹³ ions/cm², respectively. Variable mono-energetic positron beam was performed to characterize the evolution of microstructure and irradiation induced defects. Results show that large amount of vacancy defects formed after heavy ion irradiation. In which, some of mono-vacancies might migrate to form small-sized clusters at room temperature. After irradiation, implanted Fe atoms mainly be interstitials atoms, but some Fe atoms might recombine with vacancies due to their high mobility, which could decrease the defect concentration, effectively.

Abstract: Nanosized Cu-enriched clusters formed in Cu-containing reactor pressure vessel (RPV) steels during service have a deleterious effect on mechanical properties, which can result in RPV embrittlement and limit reactor operation life. To understand the effect of Cu-enriched clusters (CECs) behavior on mechanical properties, thermal aging at 370°C for times up to 13200 h was performed on the high-copper ferritic steels. The microstructure evolution of CECs was investigated by positron annihilation spectroscopy (PAS) and transmission electron microscope (TEM). The results show that the CECs formed after aging times up to 3000 h, which composition is composed of Fe, Cu, Ni, Mn, and Si, are 9R structures. The CECs lead to precipitation hardening/embrittlement effects. The changes of hardness have a linear relationship with transition temperature shifts.

Abstract: The existing testing method could not accurately detect the defects in inner cavity with ant-nest structure in a part. The reasons are as follows: the shape of inner cavity is irregular, the distribution of wall thickness is uneven and the material composition is complex etc. A new testing method is proposed based on positron annihilation in this paper. The radionuclide solution is injected in inner cavity of the part and then produces positrons which would annihilate within 2 nanoseconds in the polylactide (PLA) material. The information of inner cavity is reflected by counting the γ photons produced by positron annihilation. The maximum likelihood estimation model (MLEM) with average compensation is established according to the detected γ photons. A 2D slice image sequence is obtained by order subset expectation maximization (OSEM) algorithm which solves the MLEM. The targets such as defects in the 2D slice image need to be enhanced by OTSU algorithm. A 3D image is reconstructed by the enhanced 2D slice image sequence which is used to the detection and location of the defects in inner cavity. Two experiments under different environment are conducted which aim at different ant-nest structure parts. The parts are made of different material and made by different manufacture process. A good testing result of the parts with complex shaped inner cavity could be obtained. The results would not change with the change of external temperature or magnetic field and the testing precision can reach to millimeter level.

Abstract: Native defects and ion-implantation induced defects in GaN were studied by means of positron annihilation. Measurements of Doppler broadening spectra of the annihilation radiation for GaN layers grown on Si substrates showed that optically active vacancy-type defects were formed in the layers. Charge transition of the defects due to electron capture occurred when the layers were irradiated by photons with energy above 2.7 eV. It was found that Ti deposition and subsequent annealing introduced vacancy clusters. We also characterized vacancy-type defects in Mg-implanted GaN. The major defect species of vacancies introduced by Mg-implantation was a complex between Ga-vacancy (V_Ga) and nitrogen vacancies (V_Ns). After annealing above 1000C, these defects started to agglomerate, and the major defect species became (V_Ga)₂ coupled with V_Ns. Through this work, we have demonstrated that positron annihilation spectroscopy is a powerful tool for characterizing vacancy-type defects in GaN for power devices applications.

Abstract: Croconic acid is the first single molecular organic ferroelectric material exhibiting very high spontaneous polarization (~ 20 μC/cm²) at room temperature. Maximizing polarization depends on minimizing void defects in croconic acid crystals. In this experiment, the change in void defects upon the thermal treatment is characterized using positron annihilation lifetime spectroscopy. Both defect void size and intensity are measured, and their dependence upon the thermal treatment duration is studied. In addition, the relation between the void defect and ferroelectric hysteresis of croconic acid is established.

Abstract: A brief report is given on recent studies of the atomistic processes during charging of battery cathode material Li_xCoO₂ by means of magnetometry and positron annihilation. A set-up for operando magnetometry is implemented which, for the first time, allowed to continuously monitor the distinct variation of the magnetic susceptibility χ of Li_xCoO₂ which occurs during consecutive charging and discharging cycles. The variation of χ with Li⁺ content in the concentration range 1>x≥0.77 arises from a variation of the electronic density of states and from electronic correlation effects. The χ (x)-behaviour for x<0.77 shows that oxygen is involved in the charging process. Positron annihilation reveals vacancy-type defects on the Li-sublattice, the size of which increases with Li-extraction. Indication for Li-reordering at the reversibility limit of Li extraction is found which correlates with χ (x)-variations in this concentration regime. First measurements on Li_xCoO₂ thin-films performed at the positron beam line NEPOMUC of FRM II at the Heinz Maier-Leibnitz neutron source will be presented.

Abstract: In this Chapter, we review knowledge about diffusion in quasi-crystalline alloys (quasicrystals). In Section 1 we first remind the reader of some major aspects of the quasi-crystalline state and in Section 2 we introduce phase diagrams with quasi-crystalline phases, for which detailed diffusion studies are available. We mention in Section 3 the more common experimental methods for diffusion studies. The diffusive motion of atoms in quasi-crystalline alloys can be studied by the same techniques used for crystalline metallic alloys and intermetallics – measurements of radiotracer diffusion and diffusion of stable isotopes and solute atoms by SIMS profiling. The best-studied quasi-crystalline alloys are icosahedral AlPdMn, icosahedral ZnMgRE (RE = rare earth metal), and decagonal AlNiCo. The major diffusion results for these quasicrystals are reviewed in Sections 4, 5, and 6. Section 7 is devoted to the pressure dependence of diffusion in quasicrystals and to a comparison of the activation volumes with those of crystalline metals. Positron annihilation studies are also mentioned, which together with activation volumes for diffusion strongly favour a vacancy mechanism in quasicrystals. The major results and conclusions are summarized in Section 8.

Abstract: In the present work, positron annihilation spectroscopy was employed for investigation of hydrogen-induced defects in Pd. Well annealed Pd samples were electrochemically charged with hydrogen and development of defects during hydrogen loading was investigated. At low concentrations (α-phase, x_H < 0.017 H/Pd) hydrogen loading introduced vacancies. When the hydrogen concentration exceeds 0.017 H/Pd, particles of hydrogen rich α’-phase are formed. This generates dislocations in the sample in order to accommodate the volume mismatch between the α and the α’-phase. Moreover, additional vacancies are introduced into the sample by crossing dislocations. Vickers hardness testing revealed that absorbed hydrogen causes hardening of the sample. In the α-phase region the hardness increases due to solid solution hardening caused by dissolved hydrogen. At higher hydrogen concentrations when dislocations were created hardness increases due to strain hardening caused by dislocations.

Abstract: This paper presents the results of a defect structure investigation in commercially pure titanium alloy after hydrogen charging in a gaseous atmosphere at the temperature of 873 K up to the concentration of 5.1 at. %. Structure of samples was studied by positron lifetime, Doppler broadening and X-ray diffraction spectrometry. Several processes, corresponding to the different ranges of hydrogen concentrations were revealed. It was shown that hydrogen, penetrating in the material, expands its crystal lattice, initiates formation of vacancy-like defects of different dimensions and reacts with the last ones, forming the defect-hydrogen complexes.