正电子湮没谱学在金属材料氢/氦行为研究中的应用

用于核反应堆的金属结构材料中氢/氦泡的前躯体——(氢/氦)-空位复合体的形成受到温度、辐照剂量等多方面因素的影响,研究其在材料中的形成和演化行为对气泡形核的理解及先进核反应堆材料的发展起着至关重要的作用.然而,受到分辨率的局限,这种原子尺度的微结构很难用电镜等常规方法进行表征,以致于该问题的研究上可利用的数据相对较少.正电子湮没谱学是一种研究材料中微观缺陷的特色表征方法,近些年来慢正电子束流和新型核探测谱仪技术的不断发展以及基于慢束发展起来的多种实验测试方法的改进,使正电子湮没技术应用已拓展到金属材料中氢/氦行为的研究领域,在金属材料表面氢/氦辐照损伤的研究中发挥了重要作用.本文结合国内外相关进展以及本课题组的一些研究成果评述了正电子湮没谱学在金属材料氢氦行为研究中的应用,着重讨论了正电子湮没寿命谱、多普勒展宽谱、符合多普勒展宽三种测量方法在如下金属材料氢/氦行为研究中的优势:1)氢/氦气泡尺寸和浓度的估算; 2)高能氢/氦离子辐照损伤缺陷及缺陷的退火、时效的演化行为; 3)不同形变程度样品中氢/氦与形变缺陷的相互作用; 4)不同能量或剂量氢/氦离子辐照对材料造成的损伤以及氢氦协同作用...     

高性能贮氢电极合金

在简要介绍有关贮氢合金及镍－氢化物（Ｎｉ／ＭＨ）电池概念的基础上，回顾了ＡＢ５型和ＡＢ２型Ｌａｖｅｓ相贮氢电极合金的发展历史，综述了国内外有关上述两种贮氢电极合金的最新发展，讨论了提高贮氢电极合金综合电化学性能的各种方法．     

表面修饰纳米TiO2的贮氢合金电极的光充电行为

采用水解-沉淀法制备了锐钛矿结构的纳米级TiO2,研究了表面修饰TiO2的贮氢合金电极的光充电、循环伏安及交流阻抗特性.结果表明,表面未修饰TiO2的贮氢合金电极在光照下电极电位基本无变化,而表面修饰TiO2的贮氢合金电极在光照下,电极电位向负方向偏移,可达-0.835V,表明在光照射条件下电极表面有氢原子形成.电化学阻抗谱的结果也表明,表面修饰电极在光照时表面有吸附氢存在,并存在氢原子向贮氢合金内部的扩散过程.扫描电镜观察表明,表面修饰TiO2的贮氢合金电极在光充电后产生的氢原子被贮氢合金吸收引起膨胀,导致表面出现大量微裂纹.     

机械合金化贮氢钛铜非晶合金的结构研究

用机械球磨方法制得了数种不同组成的钛铜非晶合金。用Ｘ射线粉末衍射，扫描电子显微镜及扩展Ｘ射线吸收精细结构研究了它们的结构。得出了用机械合金化制得的钛铜非晶合金的结构与用液相急冷法制得的相同合金的结构不同，形成两侧分别为钛与铜金属薄膜，中间夹以非晶钛铜合金的夹心饼干式结构。过长的球磨时间，会使温度升高而产生结构弛豫，在合金属层内形成微细的钛、铜金属微粒。     

充氚不锈钢中氦行为的实验研究

对充氚和未充氚的抗氢-2不锈钢(HR-2)样品进行退火处理,利用正电子湮没寿命谱技术以及金相检验技术探讨不锈钢中氦和微缺陷的相互作用行为.未充氚样品中,影响正电子寿命值的主要因素为杂质元素在晶界的析出.充氚样品实验中,退火温度小于300℃时,正电子寿命值的增加说明了氦泡的形成过程为非热形成,通过“冲出位错环”机制形成及长大;退火温度在300~600℃之间,充氚样品正电子寿命值的降低以及He的跃迁概率的计算结果,说明He原子通过热迁移至晶界;退火温度大于600℃时,热平衡空位浓度的计算结果以及正电子寿命值的增加说明热平衡空位开始发挥作用.     

Mg2 Ni贮氢合金的结构、性能与改性

Mg2Ni合金以价格低廉、高能量密度,而引起人们的广泛关注。本文对Mg2Ni贮氢合金的结构、性能特别是电化学性能,以及常用的改性方法进行综述。     

Mg2Ni纳米贮氢合金电极的电化学性能

测定了Mg2Ni纳米贮氢合金电极在扫描速率份别为10，20，40，80，160mV／s时的循环伏安曲线。从曲线上读出不同扫描速率时的阴极峰电流Ipc，阳极峰电流Ipa，阴极峰电势Epc，阳极峰电势Epa，数据列于表1。作Ipc-v^1/2关系图，如图1所示。     

Mg2Ni纳米贮氢合金电极的制备技术

以机械合金化法制备的Mg2Ni纳米粉末为原料采用以下3种方法制备电化学测试用电极：将Mg2Ni粉末与黏结剂的混合粉末调制成浆状涂敷于泡沫镍基体的两面，以一定的压力压制成片，形成三明治式夹层电极；利用模具先将混合粉末压制成圆片状，然后将其夹在两块泡沫镍基片中间以一定的压力压制成包覆型电极；利用模具先将混合粉末压制成圆片状，直接在片材表面焊上极耳，制作成圆片型电极。制备好的3种电极如图1所示。     

贮氢材料及其纳米化

 70年代席卷全球的石油危机促使各国开始寻求新型的、来源充足的能源;随着环境意识的加强,人类迫切需要一种清洁能源。氢能作为一种来源丰富、清洁、高能量密度的能源(见表1),早就引起了人们的关注。不过由于氢是一种密度很低的气体,传统氢气贮运以气体、液体状态为主。气态贮运贮存量小,液态贮运需消耗大量液化热,而且需要杜瓦瓶等附属设备;它们的安全性能均很差。1970年荷兰菲利普实验室发现ＬａＮｉ5材料具有可逆吸放氢的特性,贮氢体积密度高于液氢(见表2),安全性能很高;他们还指出贮氢材料具有广阔的开发前景,从而引起全球贮氢材料研究的浪潮。     

离子束技术在金属中氦行为研究中的应用

结合我们近期的研究工作，本文介绍和讨论了离子束技术在金属中氦行为研究中的应用，还介绍和讨论了氦在金属中的基本特性，如氦的捕获、迁移和氦泡结构等．     

大气环境下氦气等离子体中化学过程数值模拟

建立一个大气化学程序用以模拟大气压下（地面附近）或低压（高空）情况下混合气体（氦气中混入少量空气）中产生等离子体后的化学过程。给出了电子寿命和主要带电粒子的随时间演化图。电子寿命在大气压下要长于大气等离子体，但在低压情况下这个结论不成立。电子数密度在一很长一段时间内不服从指数衰减规律。     

掺He钛膜中He泡的演化研究

本文利用慢正电子束分析（SPBA）和透射电子显微镜（TEM）方法研究了钛膜内He相关缺陷的演化．实验结果表明：室温下,钛膜中的He主要形成均匀的小He泡．随着He浓度的增大,He泡的密度相应地增加．在973K高温退火后,在含高浓度He的钛膜样品中观测到大He泡的出现．实验结果表明迁移合并和较大He泡周围的级联融合共同作用导致了大He泡的形成．     

Optical potential approach for positron scattering by metastable 2~3S state of helium

The momentum space coupled channels optical(CCO) method for positron scattering has been extended to study the scattering of positrons by metastable helium for impact energies in the range from the positronium threshold up to high energies. Both the positronium formation and ionization continuum channels are included in the calculations via a complex equivalent local potential. The positronium formation, ionization, elastic and 2~3S–2~3P excitation, and total scattering cross sections are all presented and compared with the available information.     

Evolution of helium bubbles in nickel-based alloy by post-implantation annealing

Nickel-based alloys have been considered as candidate structural materials used in generation IV nuclear reactors serving at high temperatures. In the present study, alloy 617 was irradiated with 180-keV helium ions to a fluence of 3.6×10¹⁷ ions/cm² at room temperature. Throughout the cross-section transmission electron microscopy (TEM) image, numerous over-pressurized helium bubbles in spherical shape are observed with the actual concentration profile a little deeper than the SRIM predicted result. Post-implantation annealing was conducted at 700 ℃ for 2 h to investigate the bubble evolution. The long-range migration of helium bubbles occurred during the annealing process, which makes the bubbles of the peak region transform into a faceted shape as well. Then the coarsening mechanism of helium bubbles at different depths is discussed and related to the migration and coalescence (MC) mechanism. With the diffusion of nickel atoms slowed down by the alloy elements, the migration and coalescence of bubbles are suppressed in alloy 617, leading to a better helium irradiation resistance.     

Substitution effects on the hydrogen storage behavior of AB2 alloys by first principles

The hydrogen storage behavior of the TiCr₂ and ZrCr₂ alloys substituted with the third components (Zr, V, Fe, Ni) have been studied using first-principles calculations. The change of the hydrogen absorption energies caused by metal doping is arising from the charge transfer among the doped alloys interior. Zr and V atoms devoted abundant electrons, leading to a great enhancement of the H absorption energy, while Fe and Ni atoms always accepted electrons, yielding a remarkable decrease of the H absorption energy. The hydrogen diffusion energy barrier is closely correlated with the geometry effect rather than the electronic structure.     

Surface damage and topography of erbium metal films implanted with helium to high fluences

Topographical and expansion effects which occur as a result of implanting erbium thin films with helium up to fluences of 1.5 × 10¹⁸ He⁺/cm² are described. There exists an inverse relationship between critical dose and annealing temperature with respect to the formation of surface bubbles. Post implantation annealing at or below 400°C is found to strongly reduce implantation induced expansion for doses less than 3.5 × 10¹⁷ He⁺/cm², but is observed to result in increased expansion above this dose. At temperatures above 400°C, expansion is increased for all doses investigated. Details of bubble development in the implanted layer are discussed and the manner in which surface bubbles develop from enlarged subsurface bubbles is illustrated.     

Mössbauer study of Mg–Ni(Fe) alloys processed as materials for solid state hydrogen storage

Mg–Ni–Fe magnesium-rich intermetallic compounds were prepared following two distinct routes. A Mg₈₈Ni₁₁Fe₁ sample (A) was prepared by melt spinning Mg–Ni–Fe pellets and then by high-energy ball milling for 6 h the obtained ribbons. A (MgH₂)₈₈Ni₁₁Fe₁ sample (B) was obtained by high-energy ball milling for 20 h a mixture of Ni, Fe and MgH₂ powders in the due proportions. A SPEX8000 shaker mill with a 10:1 ball to powder ratio was used for milling in argon atmosphere. The samples were submitted to repeated hydrogen absorption/desorption cycles in a Sievert type gas–solid reaction controller at temperatures in the range 520?÷?590 K and a maximum pressure of 2.5 MPa during absorption. The samples were analysed before and after the hydrogen absorption/desorption cycles by X-ray diffraction and Mössbauer spectroscopy. The results concerning the hydrogen storage properties of the studied compounds are discussed in connection with the micro-structural characteristics found by means of the used analytical techniques. The improved kinetics of hydrogen desorption for sample A, in comparison to sample B, has been ascribed to the different behaviour of iron atoms in the two cases, as proved by Mössbauer spectroscopy. In fact, iron results homogeneously distributed in sample A, partly at the Mg₂Ni grain boundaries, with catalytic effect on the gas–solid reaction; in sample B, instead, iron is dispersed inside the hydride powder as metallic iron or superparamagnetic iron.     

Pseudo-potential caculations of positron annihilation rates in bubbles of helium contained in the metals Mg,Ag and Au

Abstract

A pseudo-potential technique has been applied to calculate the annihilation rates for positrons in bubbles of helium gas in gold and silver. The positron pseudo-wavefunction in each case was found to be in a surface state of the metal, the exact form of which displayed a slight dependance on the density of the helium gas. The annihilation rate of the positron was the sum of a nearly constant surface rate and a strongly density dependant annihilation rate with the helium electrons. There is a trend for annihilation with the helium to be greater in metals with low free electron densities, the helium in such metals resting closer to the metal surface thus increasing the overlap of the positron wavefunction with the helium.     

金属钨中氦行为的分子动力学模拟

采用分子动力学方法模拟了氦在金属钨中的扩散聚集行为. 首先,建立了氦与钨原子间相互作用势,短程部分采用ZBL势形式,长程部分采用从头算法数据,实现了两者之间的平滑连接. 通过计算氦在钨中不同间隙位的形成能发现,单个氦原子更易存在于金属钨中的四面体间隙位,这与最新的研究成果是一致的. 在400-1200 K的温度范围内,考察了氦原子在金属钨中的扩散行为,获得了扩散迁移能,其值介于实验值和从头算法结果之间. 最后,研究了氦的聚集行为,从能量的角度考察了氦团簇形成初期的生长机理. 研究发现,在氦团簇形成初期,氦团簇对氦的结合能随着氦团簇的生长有逐渐增大的趋势,说明氦团簇吸收氦的能力逐渐增强. 关键词： 氦扩散 氦团簇 辐照损伤 分子动力学模拟