碳同素异形体中的正电子理论

在密度函数理论的基础上,采用中性原子叠加模型和有限差分方法(SNA-FD)计算了石墨,金刚石和C60这三种碳的同素异形体中的正电子分布和湮没情况. 计算表明,在片层结构的石墨晶体中,正电子主要在石墨层间的空隙中湮没,计算出的石墨中的正电子寿命为208ps,与文献中的实验结果210ps符合很好. 在金刚石单晶中,正电子主要在碳原子之间的空隙中存在并发生湮没,计算出的金刚石中的正电子寿命为1159ps,与文献中的实验结果110ps相符合;在面心立方结构的C60晶体中,正电子主要在C60分子球壳内外侧及分子之 关键词： 石墨 金刚石 C60 正电子寿命     

Hg1-xCdxTe晶体缺陷的正电子湮没寿命

利用正电子(e⁺)湮没寿命谱实验研究了Hg_1-xCd_xTe晶体样品的空位缺陷.碲溶剂法生长的样品，不论是n型导电还是p型导电都存在大量的Hg空位.经过合适的退火工艺，p型材料转为n型，同时对正电子的俘获效应减小，表现为正电子湮没平均寿命值减小14—17ps.若退火温度高于350℃，正电子湮没寿命值又增大，表明Hg空位浓度增加.得到HgCdTe中正电子的体寿命为τ_b=272ps.根据正电子湮没寿命和电参数的测量结果，得出 关键词：     

GaAs晶体生长缺陷的正电子湮没研究

本文介绍用正电子湮没寿命和多普勒加宽技术研究750—950°掺Te液相外延生长的和1238℃熔体生长的GaAs晶体生长缺陷. 在800—1238℃生长的晶体中都观测到312±11ps的寿命组分τ₂, 其强度I₂, 多普勒加宽S参加和按捕获模型计算的平均寿命τ都随晶体生长温度增高而增大. 在掺Te外延晶体中, 312ps寿命的正电子陷阱浓度随晶体生长温度增高而线性地增大, 在熔体生长晶体中, 该陷阱浓度低得多, 远偏离以上线性关系. 312ps寿命归因于正电子在Ga空位湮没寿命, 结果显示出掺Te在GaAs晶体中诱导Ga空位.     

TEM纳米云纹法测量纳米碳管变形

提出了透射电子显微镜(TEM)纳米云纹法的新技术,首次将该方法用于单根单壁碳纳米管的残余变形测量。纳米云纹由计算机显示器扫描线与碳纳米管束TEM图像干涉而成。该方法具有纳米级空间分辨率,可直接测量碳纳米管的力学性能。对TEM纳米云纹法的原理进行了详细的阐述,并利用不同管径的单壁碳管束产生了云纹。对直径为7.5nm的弯曲碳管束的残余变形进行测量,直接得到了其中一根直径为1.0nm的单壁碳管的残余变形场。实验结果证明了该方法的可行性。该方法为纳米尺度的碳管力学性能测量提供了新途径。     

单壁碳纳米管力学行为的数字散斑相关法实验研究

通过直接单向拉伸超长单壁碳纳米管束长绳,首次借助高精度数字散斑相关法,并结合显维放大技术,测量了单壁碳纳米管的弹性模量和拉伸强度。试验中观察了单壁碳纳米管束长绳的断裂过程。单壁碳纳米管束长绳通过改进的化学气相沉积技术生成。试验得到单壁碳纳米管的平均杨氏模量为129.0±70.3GPa,平均拉伸强度为1.95±0.56GPa,低于计算值和先前其它文献的试验值。     

单壁碳纳米管受限空间内水的分布

碳纳米管管腔作为分子物质的纳米通道,其储存或输送水的能力具有重要研究价值.为了研究碳纳米管管腔受限空间对水分子团簇结构和分布的影响,本文采用分子动力学方法探究了管径、手性和温度对单壁碳纳米管管腔内水的结构和分布的影响.结果表明:在常温下,管径尺寸范围为1.018—1.253 nm的单壁碳纳米管管内易形成有序的多元环水结构,此范围以外碳纳米管管内难以形成水的有序结构;且随着管径尺寸增大,多元环水呈现由三元环至六元环的结构变化;范德瓦耳斯势分布分析表明,在上述管径范围内,水分子趋向于贴近碳纳米管管壁分布而形成水的有序结构.对比管径尺寸差别较小的碳纳米管,其手性对多元环水结构影响不大.多元环水结构的稳定性表现出温度依赖性,管径较大的碳纳米管内的多元环水的有序结构更易随温度升高而消失.     

基于北京慢正电子强束流的微脉冲化系统设计

慢正电子湮没寿命测量是研究材料表面微观缺陷的重要分析方法．束团化系统是实现慢正电子湮没寿命测量的核心部件, 主要由斩波、聚束两部分组成, 它可以将随时间连续分布的束流束团化, 从而获得正电子湮没寿命测量的时间起点及满足时间分辨率要求的束团．本文以粒子动力学计算为基础, 完成了束团化系统的物理设计, 其时间分辨率设计值为150ps(FWHM)．     

化合物半导体材料的正电子寿命计算

在局域密度理论（LDA）和广义梯度理论（GGA）的基础上计算了ZnO,GaN,GaAs,SiC和InP五种化合物半导体材料中的正电子湮没信息,包括化合物半导体材料中的自由态正电子的湮没寿命;还有不同类型空位（单空位,双空位）附近俘获的束缚态正电子密度分布和湮没率分布,以及束缚态正电子的湮没寿命. 关键词： 半导体 正电子寿命     

元素周期表中元素单晶基于局域密度理论的正电子寿命计算

在局域密度近似理论（LDA）的基础上用中性原子叠加模型和有限插分方法（SNA-FD）计算了元素周期表中各种元素单晶的正电子体寿命和单空位寿命.分析了不同结构的单晶中自由正电子的分布信息和湮没信息.元素单晶的正电子寿命计算值与文献中的实验测量值相符合,表明LDA基础上的SNA-FD方法可以作为单晶中正电子湮没理论计算的有效研究手段. 关键词： 局域密度近似理论 正电子寿命     

用正电子研究NaCl在NaY沸石上的固溶过程

用正电子湮没谱学研究NaCl与NaY沸石机械混合后, NaCl在NaY中的固溶扩散过程.分别测量不同质量比的NaCl/NaY［(1—20)%］经500℃烘烤1h，NaCl/NaY(15%)经不同温度烘烤1h,以及NaCl/NaY(15%)经500℃烘烤不同时间后的正电子寿命谱.所有寿命谱都出现了5个寿命分量, 其中第3，4，5寿命分别与β笼、超笼及沸石微粒界面空洞的大小和数量相关.实验表明正电子湮没谱学能敏感地表征NaCl在NaY中的固溶扩散过程. 关键词： 正电子湮没谱学 氯化钠 沸石     

温度对单壁碳纳米管端口结构的影响

利用紧束缚势分子动力学模拟方法，研究了温度在2000—3500 K之间单壁碳纳米管端口结构的变化趋势.研究表明，温度对整个管端口结构起关键作用，计算表明温度在3000K和3500K下碳管两端口在15ps时间尺度内依次闭合，温度高易于使理想单壁碳管端口封闭，且端口封闭导致碳管系统能量的降低.由于Armchair型碳纳米管与相同半径的Zigzag型碳纳米管相比有相对低的应力能，导致Armchair型碳纳米管更易形成端口封闭的结构. 关键词： 碳纳米管 紧束缚势     

A compact positron annihilation lifetime spectrometer

Using LYSO scintillator coupled on HAMAMATSU R9800(a fast photomultiplier)to form the small size γ-ray detectors,a compact lifetime spectrometer has been built for the positron annihilation experiments.The system time resolution FWHM=193 ps and the coincidence counting rate -8 cps/μCi were achieved.A lifetime value of 219±1 ps of positron annihilation in well annealed Si was tested,which is in agreement with the typical values published in the previous lectures.     

POSITRON ANNIHILATION IN CARBON NANOTUBE POWDERS

Positron lifetime spectra have been measured in two kinds of carbon nanotube powders as a function of temperature range between 32 and 296 K. It has been found that all spectra are essentially temperature-independent in the above temperature range. The results of analysis show that there are three components in the powders of carbon nanotube with an average diameter of 30 nm, and four components in the powders of carbon nanotube with typical diameters of around 15 nm. The average values of lifetime components obtained at various temperatures are about 220, 390 ps, and 2.0 ns for the former, and about 140, 300, 650 ps and 6.4 ns for the latter.     

Photoluminescence and positron annihilation spectroscopic investigation on a H(+) irradiated ZnO single crystal

Low temperature photoluminescence and room temperature positron annihilation spectroscopy have been employed to investigate the defects incorporated by 6?MeV H(+) ions in a hydrothermally grown ZnO single crystal. Prior to irradiation, the emission from donor bound excitons is at 3.378?eV (10?K). The irradiation creates an intense and narrow emission at 3.368?eV (10?K). The intensity of this peak is nearly four times that of the dominant near band edge peak of the pristine crystal. The characteristic features of the 3.368?eV emission indicate its origin as a 'hydrogen at oxygen vacancy' type defect. The positron annihilation lifetime measurement reveals a single component lifetime spectrum for both the unirradiated (164?±?1?ps) and irradiated crystal (175?±?1?ps). It reflects the fact that the positron lifetime and intensity of the new irradiation driven defect species are a little higher compared to those in the unirradiated crystal. However, the estimated defect concentration, even considering the high dynamic defect annihilation rate in ZnO, comes out to be ～4?×?10(17)?cm(-3) (using SRIM software). This is a very high defect concentration compared to the defect sensitivity of positron annihilation spectroscopy. A probable reason is the partial filling of the incorporated vacancies (positron traps), which in ZnO are zinc vacancies. The positron lifetime of ～175?ps (in irradiated ZnO) is consistent with recent theoretical calculations for partially hydrogen-filled zinc vacancies in ZnO. Passivation of oxygen vacancies by hydrogen is also reflected in the photoluminescence results. A possible reason for such vacancy filling (at both Zn and O sites) due to irradiation has also been discussed.     

Calculation of the sizes of individual few-walled carbon nanotubes and their bundles

The temperature dependences of the number of nucleating single-walled and few-walled nanotubes and their diameter have been determined over a wide range of model parameters in the framework of the problem regarding the nucleation of carbon nanotubes from catalytic particles supersaturated with carbon. It has been demonstrated that, initially, individual nanotubes nucleate and grow and, then, they can be joined together into bundles. The mechanism of the formation of bundles in the proposed model follows from the quantum-chemical analysis of the steady-state growth of nanotubes at the level of release of individual carbon atoms. During the growth, the axis of the nanotube rotates about the normal to the surface of the catalytic particle. This leads to the cross-linking of nanotubes into bundles. The characteristic diagram of the regions of the existence of individual single-walled, few-walled, and multiwalled nanotubes and their bundles has been constructed as a function of the temperature and the size of catalytic particles.     

Recovery of deformed iron studied by positrons

The isochronal annealing of two plastically deformed irons of 99.998% and 99.86% purities is studied by positron lifetime and annihilation line shape measurements. The results show that trapping of positrons is caused by dislocations. At around 300° C a recovery process attributed to a rearrangement of dislocation structure is observed in the pure but not in the impure iron. The positron lifetime in deformed and recrystallized state is found to be 167 ps and 117 ps, respectively.     

Visible and near-infrared excited-state dynamics of single-walled carbon nanotubes

Excited-state lifetimes of isolated single-walled semiconducting carbon nanotubes (SWNTs) have been measured for the first time; these excited states, observed over the 400- to 1800-nm spectral domain, possess lifetimes that range from several ps to more than 100 ps. Sub-ps to ps decay components are assigned to relaxation in SWNT bundles. Interrogation of the samples with different SWNT mean diameters further confirms the dependence of the excited-state lifetime on roll-up vector. The ratio of fast and slow decaying component contributions in the first van Hove band can be viewed as a measure of the bundle content. PACS 78.67.Ch; 78.47.+p; 61.46.+w; 73.22.-f     

Helium bubbles in neutron-irradiated copper-boron studied by positron annihilation

The influence of helium, introduced by the ¹⁰B(n, α)⁷Li reaction, on the evolution of defect structure in copper containing a few hundred ppm boron has been studied by detailed positron lifetime and two-photon angular correlation measurements, supplemented by TEM studies. In the as-irradiated state of Cu-B, two lifetime components have been resolved. The shorter lifetime, τ₁, = 167 ps of 97% intensity, has been understood as due to positron trapping at small helium-vacancy complexes, while the longer lifetime τ₂ = 450 ps of 3% intensity is explained as due to helium-free voids. Marked changes in the annihilation characteristics observed at 670 K are interpreted in terms of the nucleation of microbubbles, controlled by thermally activated helium migration to vacancy traps. Corroborative evidence for the onset of helium clustering is obtained from the change in the average size of positron traps as deduced from the smearing of the measured angular correlation spectra. Helium bubbles and helium-free voids coexisting in the system have been distinguished by a three-component analysis of the lifetime spectra. Bubbles are found to be stable beyond the temperature of dissociation of voids. The size and concentration of bubbles, determined independently by TEM measurements, are in accordance with the positron annihilation results in the growth stage. The observed positron lifetime at higher annealing temperatures has been analysed by relating the annihilation rate to helium atom density and helium pressures in bubbles evaluated. These pressures are in satisfactory agreement with the estimates of equilibrium pressures, leading to the conclusion that bubble relaxation occurs by the mechanism of thermal vacancy condensation.