Study Of SmFeAsO_1-xF_x Defect By Positron Annihilation Techniques

Positron annihilation technique (PAT) is a method that applies nuclear physics and analysis technology to solid physics, materials science, chemistry, biology etc. Using positron as a probe, it can provide a good approach to study the property of solid by detecting theγray which is generated by electron-positron annihilation. PAT mainly contains Positron annihilation lifetime spectroscopy (PALS), Doppler broadening spectroscopy (DBS), Slow positron beam (SPB) and so on. The greatest advantage of this technique lies in its sensitivity to atomic-scale defects in the samples. Thus it has become an indispensable implement in the study of defects in materials.In this paper, positron annihilation lifetime, doppler broadening and theoretical calculation of positron annihilation characteristics combined with Density functional theory (DFT) based first-principle calculation are used to study the defect in iron superconductor of SmFeAsO1-xFx.In the first chapter, a brief introduction of the basic knowledge of positron annihilation spectroscopy and superconductivity physics is given.In the second chapter, the defects of iron-based high-temperature superconductor SmFeAsO1-xFx are studied by PALS, DBS combined with the calculation of positron lifetime. The temperature dependence of S-parameter shows a remarkable difference between the parent and superconductor, which indicates the structural phase transition for parent and superconductivity transition for superconductor. The well linearity of S-W plot determines only one-type defects through the superconducting transition. SmFeAsO and SmFeAsO0.82F0.18 polycrystalline samples are studied by PALS at room temperature and the results show two lifetime component, which are 151.6ps and 290.3ps for the parent sample and 161.6ps and 316.4ps for the superconducting sample. It should be noted that the shorter lifetime component chiefly comes from positron free annihilation in the bulk. Based on local density approximation (LDA) and general gradient approximation (GGA), positron bulk lifetimes and positron monovacancy lifetimes of perfect SmFeAsO and SmFeAsF crystals are calculated by the Superposed-Neutral-Atom model and Finite-Difference method (SNA-FD). The calculated positron bulk lifetime by GGA method is in agreement well with that of the shorter lifetime component measured by positron lifetime experiment. Therefore, according to the result of GGA, we conclude that the approximate 300ps lifetime component may come from the positron annihilation in Sm vacancy. In the third chapter, we propose a new mothod to analyse the electronic momentum distribution of coincidence doppler broadening spectrum (DBS), that is, compare the density of state (DOS) calculated by Density functional theory (DFT) based first-principle calculation with the electronic momentum distribution of DBS. According to this idea, the electronic structure of LaOFeA and SmOFeAs are studied. The feasibility of this method is confirmed by comparing the experiment spectra of Mg, Al, Si with the DOS of DFT. CASTEP is used to calculate the electronic band structures and DOS of LaFeAsO1-xFx and SmFeAsO1-xFx, and the result shows that an increase in the partial denstiy of state of O-2p near the Fermi surface after F-doping. According to our CDB at room temperature, an increase in the region of [(0-4)×10-3m0c] is also observed after F-doping. Therefore, we can conclude that the O/F-2p plays an important role in the change of DOS near the Fermi surface. This result provides a way to research the superconductivity of SmFeAsO1-xFx superconductor, meanwhile, a new method is proposed for analysis the positron annihilation CDB spectrum.In the fourth chapter, the degradation mechanism of spacecraft thermal control coating ZnO/Silicone under 90keV proton irradiation is studied by slow positron beam. The result shows that the primary degradation caused by proton irradiation is derived from silicone, when irradiation dose is below 1×1015cm-2, the radiation crosslink dominates. However, when the radiation dose is greater than 1×1015cm-2, the radiation degradation dominates.