Positron Annihilation Spectroscopy Study of Micro Defects in Fe-Cu Alloys
|School||Chengdu University of Technology|
|Course||Nuclear Technology and Applications|
|Keywords||positron annihilation micro defects vacancy precipitates|
Micro defects in solids are important because they have a strong influence on the material properties (electrical and optical properties). Many of these defects are on the atomic scale and as difficult to probe experimentally. A number of techniques have been found, such as TEM (Transmission Electron Microscopy), APFIM (atom probe field ion microscopy), and3D-AP (the three-dimensional atom probe). They can give some information in vacancy type defects and nanometer precipitation, although the poor resolution imposes limitation. Many studies of positron interactions with electrons in condensed matter over the last40years have led to the development of PAS (Positron Annihilation Spectroscopy). One of the special roles of PAS is its ability to selectively defect vacancy-type defects. The positron is generally described as a non-destructive probe of electronic structure, annihilation with an electron in the condensed matter giving information on the annihilation site. The trapping of the positron in defects is based on the formation of an attractive potential at open-volume defects, such as vacancies, dislocation and vacancy agglomerates and maybe precipitates.A number of techniques are classified under the general heading of Positron Annihilation Spectroscopy (PAS):These are Positron Annihilation Lifetime Spectroscopy (PALS), Doppler Broadening Spectroscopy (DBS), Angular Correlation of Annihilation Radiation (ACAR), Coincidence Doppler Broadening Spectroscopy (CDBS) and Age-Momentum Correlation technique (AMOC). In PALS, the positron lifetime is registered as the time difference between the emission of1.27MeV γ-quantum generated almost simultaneously with the positron in β+decay of22Na isotope and the one of the0.511MeV annihilation γ-rays. As for DBS and ACAR the conservation of momentum during the annihilation process causes the annihilation radiation to contain information on the electron momentum distribution at the annihilation site. The Doppler-broadened line is characterized by various line-shape parameters such as the "shape" parameter S and "wing" parameter W. These parameters can be used to distinguish different defects by plotting S (valence annihilation) parameter and W (core annihilation) parameter. The CDB was introduced to reduce background and sharpen the instrumental response function. This is also one to study sensitively core annihilation and chemical environment of positron within the defect as a result of extremely low random background at high momentum. The AMOC is basing on an observation of electron momentum distribution in age-domain.After half a century of effort, PAS is considered a sensitive in vacancy-type defects, not sensitive with embedded Nano particles in defect-free material. In this thesis, applying PAS study of micro defects in Fe-Cu alloys. The micro defects and their clusters in Fe-Cu alloys involve vacancies, interstitials, vacancy-Cu pairs, vacancy clusters and Cu precipitates. We can give some information with PAS whether or not sensitive in this micro defects. To understand further, The TCDFT was used to calculate the positron lifetime and positron-electron momentum distributions in micro defects of Fe-Cu alloys.