氘化对KH2PO4晶体微观缺陷影响的正电子湮没研究

采用传统降温法从不同程度氘化(x=0, 0.51, 0.85)的生长溶液中生长氘化KH₂PO₄(KDP) 晶体, 利用正电子湮没技术(正电子寿命谱和多普勒展宽谱)、结合X射线衍射谱(XRD) 结构分析, 对KDP晶体氘化生长的微观缺陷进行了研究, 讨论了氘化程度对晶体内部微观结构特性、缺陷类型和浓度的影响. XRD结果显示晶胞参数a, b值随氘含量的增加而增加, c值无明显变化; 正电子寿命谱结果发现随着氘化浓度的提高, KDP晶体内部中性填隙缺陷以及氧缺陷不断增加, 引起晶体晶格畸变; 氢空位、K空位、杂质替位缺陷不断发生缔合反应形成复合缺陷, 缺陷浓度不断减少; 团簇、微空洞等大尺寸缺陷也在不断发生聚合反应, 缺陷浓度表现为不断减少. 多普勒实验结果表明随着氘化程度的提升, 晶体内部各类缺陷表现为同步变化. 实验结果表明, KDP晶体在低浓度氘化生长(50%以内)下缺陷反应较弱, 而在高浓度氘化(50%以上)下的缺陷反应显著增强.     

Theoretical study on the Rocksalt structured positron annihilation in magnesium oxide

Based on the atomic superposition approximation （ATSUP） and first-principles pseudopotential plane-wave methods, the bulk and Mg mono-vacancy positron lifetime of magnesium oxide were calculated using Arponen-Pajamme and Borofiski-Nieminen positron-annihilation-rate interpolation formula respectively. The calculated values are in good agreement with experimental values and the first-principles method gives more convincing results. The positron annihilation density spectra analysis reveals that positrons mainly annihilate with valence electrons of oxygen atoms when the magnesium-vacancy appears within magnesium oxide.     

SIMION simulation of a slow pulsed positron beam

A new slow pulsed positron beam, including a positron source, a moderator, a chopper, a pre-buncher, a main-buncher and a sample chamber, etc, has been installed and tested. It is necessary to simulate the acceleration, transportation and space focusing of positrons to meet the needs of beam debugging and further positron annihilation experiments. The result from SIMION simulations shows that the radius of the focused positron beam is less than 5 mm, which is further confirmed in our practical debugging process.     

Effect of vacancy charge state on positron annihilation in silicon

The charge-state-dependent lattice relaxation of mono-vacancy in silicon is studied using the first-principles pseu- dopotential plane-wave method. We observe that the structural relaxation for the first-neighbor atoms of the mono-vacancy is strongly dependent on its charge state. The difference in total electron density between with and without charge states in mono-vacancy and its relevant change due to the localized positron are also examined by means of first-principles simu- lation, demonstrating the strong interplay between positron and electron. Our calculations reveal that the positron lifetime decreases with absolute charge value increasing.     

ZIFs纳米晶体中电子偶素的自旋转换

ZIFs晶体由咪唑基桥接单金属离子构成,可通过咪唑酯连接物灵活选取合适的官能团对其结构进行调控,因而被赋予更多新的性质和功能. ZIFs晶体中孔结构及其化学环境与其性能紧密相关.本文采用静置法制备了ZIFs纳米晶体. X射线衍射结果证实制备的晶体为典型的ZIF-8晶体,扫描电子显微镜图可观察到其规则的菱型结构. N₂吸附-脱附测试表明ZIFs晶体具有较大的比表面积和孔容,分别为2966.26 m~2/g和3.01 cm~3/g.随着Co摩尔含量的增大, ZIFs晶体比表面积和孔体积逐渐减小,但是其孔径大小几乎稳定保持在12?左右.而N₂吸附-脱附等温线计算得到的孔径分布未显示咪唑配体组成的六元环的超微孔信息(3.4?).此外,利用正电子湮没寿命和多普勒展宽对晶体的微观结构和表面性能进行了研究.正电子的寿命谱有4个分量.较长寿命τ3,τ4分别是o-Ps在其微孔区域和晶体规则棱角间隙处的湮没寿命.随Co摩尔含量增大,其寿命τ3几乎没有变化,而较长寿命τ4从30.89 ns降至12.57 ns,其对应强度I3,I4也分别从12.93%和8.15%急...     

非铁电压电复合陶瓷SrTiO3-Bi12TiO20（ST-BT）的正电子湮没谱学研究

采用传统陶瓷烧结工艺成功制备出新型非铁电压电复合陶瓷SrTiO₃-Bi₁₂TiO₂₀（ST-BT）,利用正电子湮没技术,对ST-BT复合陶瓷烧结过程进行了研究,讨论了烧结过程中材料内部的缺陷变化特征,给出了烧结温度对该复合陶瓷结晶度和缺陷结构的影响.发现烧结温度在860—940℃,烧结时间为3 h的实验条件下,ST-BT复合陶瓷已趋于稳定,出现了大量的单空位型缺陷.烧结温度超过980℃将引起Bi₁₂TiO₂₀相的大量分解,杂相的出现造成缺陷的聚集,形成大尺度的微空洞.实验结果表明,烧结温度在920—940℃的烧结条件下,ST-BT复合陶瓷的结构特性及压电性能均表现出较好的稳定性.     

完美晶体中正电子体寿命计算的几种方法的分析与比较

以正电子寿命为探测对象的正电子湮没寿命谱技术在研究半导体等材料的微缺陷方面得到了广泛的应用,它对晶体的结构类型、缺陷种类以及温度等十分敏感,因此,理论上正电子寿命的快速精确计算与实验数据的结合分析显得尤为重要. 采用中性原子叠加模型、赝势方法和全势方法处理正电子局域势能,有限差分方法自洽求解正电子波函数,局域密度近似和广义梯度近似处理正电子电子关联势和增强因子,以体心立方结构的α-Fe、面心立方结构的Al和复式面心立方结构的Si三种单晶固体为例,分别计算了它们的正电子体寿命,计算值与相应的实验结果和其他计算结果均符合较好. 同时细致分析了这几种方法在电子密度网格点精度、正电子电子关联势和增强因子等方面对正电子体寿命计算的影响,探讨了这几种方法在计算正电子体寿命方面各自的优缺点. 关键词： 正电子体寿命 完美晶体 正电子电子关联势 增强因子     

Self-consistent field method and non-self-consistent field method for calculating the positron lifetime

Many methods are used to calculate the positron lifetime,these methods could be divided into two main types.The first method is atomic superposition approximation method and the second one is the so called energy band calculation method.They are also known as the non-self-consistent field method and self-consistent field method respectively.In this paper,we first introduce the two basic methods and then,we take Si as an example and give our calculation results,these results coincide with our latest experimental results,finally,we discuss the advantages and disadvantages of the two methods.     

Thickness Impacts of Vacancy Defects in Epitaxial La0.7Sr0.3MnO3Thin Films Using Slow Positron Beam

Thickness effects of thin La_0.7Sr_0.3MnO₃ (LSMO) films on (LaAlO₃)_0:3(Sr₂AlTaO₆)_0:7 sub-strates were examined by a slow positron beam technique. Doppler-broadening line shape parameter S was measured as a function of thickness and differnt annealing conditions. Re-sults reveal there could be more than one mechanism to induce vacancy-like defects. It was found that strain-induced defects mainly influence the S value of the in situ oxygen- ambience annealing LSMO thin films and the strain could vanish still faster along with the increase of thickness, and the oxygen-deficient induced defects mainly affect the S value of post-annealing LSMO films.     

Theoretical study on the positron annihilation in Rocksalt structured magnesium oxide

Based on the atomic superposition approximation(ATSUP) and first-principles pseudopotential plane-wave methods,the bulk and Mg mono-vacancy positron lifetime of magnesium oxide were calculated using Arponen-Pajamme and Boron’ski-Nieminen positron-annihilation-rate interpolation formula respectively.The calculated values are in good agreement with experimental values and the first-principles method gives more convincing results.The positron annihilation density spectra analysis reveals that positrons mainly annihilate with valence electrons of oxygen atoms when the magnesium-vacancy appears within magnesium oxide.