【作者】 郝颖萍；

【导师】 叶邦角；

【作者基本信息】 中国科学技术大学 ， 粒子物理与原子核物理， 2011， 博士

【摘要】 正电子湮没技术是一门将核物理、核技术应用于固体物理、材料科学、化学、生命科学等学科领域的技术。它以正电子作为探针,通过探测正反物质相遇发生湮没产生的γ光子来研究固体的微观结构信息,主要包括正电子寿命谱仪(PALS)、多普勒展宽谱仪(DBS)、慢正电子束(SPB)等实验技术。该方法最大的特点在于对样品中原子尺度的缺陷极其灵敏,目前已经成材料缺陷研究中不可或缺的工具。本文主要利用正电子湮没技术并结合正电子理论计算及第一性原理计算对铁基超导体SmFeAsO1-xFx的缺陷进行了研究。第一章介绍了正电子湮没谱学及超导物理的基础知识。第二章主要用正电子湮没寿命谱、多普勒展宽谱并结合正电子寿命计算对SmFeAsO1-xFx的缺陷进行了研究。通过测量发现母体和超导样品S参数明显不同,分别反应了母体样品的结构相变和超导样品的超导态转变,S-W曲线良好的线性表明超导相变前后存在的是同一种类型的缺陷；通过寿命谱测量得到两个寿命成分,母体样品为151.6ps和290.3ps,超导样品为161.6ps和316.4ps,样品中的短寿命成分主要来自于正电子自由态湮没。在局域密度近似(LDA)和广义梯度近似(GGA)的基础上,用中性原子叠加-有限差分的方法(SNA-FD)对正电子在SmFeAsO和SmFeAsF单晶中体寿命及单空位寿命进行计算,表明GGA方法计算得到的自由态正电子寿命与正电子寿命谱实验测量的短寿命成分结果符合的较好,根据GGA的结果我们推断寿命测量得到的300ps左右长寿命成分可能来自于正电子在Sm空位中的湮没。在第三章中,我们提出通过定性对比基于密度泛函理论的第一性原理计算得到的态密度与符合多普勒展宽谱来分析费米面附近的电子结构的方法,并对LaOFeAs和SmOFeAs进行了研究。通过Mg、Al、Si的实验测量与理论计算的对比,确认了这种方法的可行性；用CASTEP软件LaOFeAs和SmOFeAs及掺F材料进行了计算,发现掺F以后O-2p电子的态密度明显增加,与常温下SmFeAsO0.82F0.18对SmOFeAs的商谱在(O-4)×10-3moc低动量区间的电子增多的趋势一致。根据理论计算结果,我们可以推断出掺F以后费米面附近的电子态密度(符合多普勒展宽低动量区间)的改变主要受O／F的p电子影响。此结果为研究SmOFeAs为何掺F以后才‘具有超导电性提供了一个可能的思路,同时也为深入分析正电子湮没多普勒展宽谱的动量分布信息提供了一种新的途径。第四章主要运用慢正电子技术对90keV质子辐照下的航天器热控涂层ZnO/Silicone的损伤机理进行了研究。研究表明质子辐照主要是对ZnO/Silicone中Silicone的性能产生了影响,当辐照剂量低于1×1015cm-2时,辐射交联占支配地位,而当辐照剂量高于1×1015cm-2时,则辐射降解占支配地位。更多还原

【Abstract】 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.更多还原