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Autor:
Ravelli, Luca 
Originaltitel:
Improvement of the Pulsed Low Energy Positron System (PLEPS) for complex problems in materials science 
Jahr:
2014 
Typ:
Dissertation 
Einrichtung:
Universität der Bundeswehr München, Fakultät für Luft- und Raumfahrttechnik 
Fakultät:
Fakultät für Luft- und Raumfahrttechnik 
Institut:
LRT 2 - Institut für angewandte Physik und Messtechnik 
Betreuer:
Dollinger, Günther, Prof. Dr. 
Gutachter:
Dollinger, Günther, Prof. Dr.; Brusa, Roberto S., Prof. 
Format:
PDF 
Sprache:
Englisch 
Schlagworte:
Strontiumtitanat ; Fused Silica ; Gitterbaufehler ; Positronenstrahl ; Spektroskopie 
Stichworte:
Positron lifetime, defects, strontium titanate, densified silica glasses 
DDC-Notation:
530.411 
Kurzfassung:
This thesis concerns the application and the improvement of the Pulsed Low Energy Positron System (PLEPS) at the high intensity positron source NEPOMUC at the Munich research reactor FRM-II. This system is used for the defect study in complex materials. Positrons are the ideal probe for non-destructive investigations of vacancy-like defects in matter. The combination of positron lifetime spectroscopy with a pulsed, monochromatic positron beam of variable energy conveys information on the type and the concentration of defects down to the sub-ppm range and their depth-profile with nm resolution. Defect structures in two materials were investigated with PLEPS for this thesis. First, we studied strontium titanate (STO), which is a material of great relevance in modern oxide electronics. The cation vacancies (strontium and titanium vacancies, VSr and VTi, respectively) were identified in STO films deposited by Pulsed Laser Deposition (PLD). It was also shown, that in commercially available STO substrates only titanium vacancies with a concentration of (1.26±0.16) ppm could be detected and that upon annealing in the same conditions as for the PLD procedure a 400 nm thick layer of titanium-oxygen divacancies VTi-O was introduced. The second investigation was performed in permanently densified silica glasses. In combination with XRD measurements the structure evolution upon densification was analyzed. In particular, it was demonstrated that the average inter-tetrahedral void radius measured with PLEPS permits to predict the shift of the first sharp diffraction peak of the static structure factor as a function of the density. In the second part of this work, from the experience gained with PLEPS in the course of this thesis, the limits of the apparatus were analyzed, measures to improve the quality of the positron lifetime spectra measured with PLEPS were identified and tested. Comprehensive simulations were performed to understand the structures in the background of the measured lifetime spectra and possible countermeasures were found. Modifications of the pulsing system allowed to a) improve the time resolution of PLEPS to about 250 ps, b) measure precisely positron lifetime longer than 3 ns, which enhances the capabibility of PLEPS for the determination of free volumes in polymer samples and membranes and c) get rid of disturbing structures in the background of the positron lifetime spectra. Thus, PLEPS in combination with the high intensity positron source NEPOMUC can be considered as the most productive pulsed positron beam for defect depth-profiling in materials currently available world-wide. 
Tag der mündlichen Prüfung:
29.04.2014 
Eingestellt am:
10.06.2014 
Ort:
Neubiberg 
Stadt (Autor):
Trento 
Vorname (Autor):
Luca 
Nachname (Autor):
Ravelli