【作者】 张建东；

【导师】 王铁山；

【作者基本信息】 兰州大学 ， 物理学·粒子物理与原子核物理， 2018， 博士

【摘要】 核能的利用被认为可以解决包括能源危机在内的一系列问题,相应的科学研究也在广泛展开。核能包括裂变能与聚变能,其中裂变核反应除了可提供巨大的能量外,也会产生大量的放射性废物。玻璃固化技术具有处置效率高、适合长期存放等优点,因此成为当前广泛使用的放射性废物处置技术。然而,玻璃固化体的存放时间往往要达到百万年以上,其本身的抗腐蚀性能与稳定性便成为重点关注的问题。另一方面,聚变能由于有着更高的产能效率与更低的放射性污染,也得到了极大的关注。但在其装置的设计与研发中,面向等离子体材料(plasma-facing materials,PFMs)的性能与选择仍需进一步研究。钨材料具有高熔点、低溅射率与低氢同位素(D/T)存留等优点,因此被用于国际热核聚变反应堆(International Thermonuclear Experimental Reactor,ITER)的偏滤器部分。其受到低能、强流的氢同位素离子轰击后的性能变化也被重点关注。这些问题往往最终归结到基础问题的研究上,例如材料中元素的分布、表界面形貌的变化等。目前的实验技术仍不足以解决以上所有的问题,新技术的引入与发展因此而变得十分重要。二次离子质谱(SIMS)是一种仍处于发展中的分析技术。该技术通过分析离子轰击材料表面产生的二次离子与离子团簇,可获得所研究材料中元素的表面质谱、深度分布与表面成像等重要信息。SIMS具有高探测灵敏度(ppm级别)、高质量分辨率(M/ΔM>10000)、高深度分辨率(1-10 nm)与高横向分辨率(约100 nm)等优点。此外,SIMS对样品制备的要求简单,这些优点综合起来,是其它任何技术都无法比拟的。过去几十年来,SIMS在生物、化学与半导体材料领域获得了长足的发展与应用,但在核材料相关领域,SIMS的应用与发展仍处于起步阶段。因此,本论文首次利用ToF-SIMS对面向等离子体材料与玻璃固化体中的基础问题进行进行研究。论文主要分为两部分,一是对氢(H)在多晶钨中的分布与沉积行为的研究,二是对于玻璃固化体元素成像技术的探究。此外,论文也对ToF-SIMS技术的优势与不足进行了探讨。主要内容如下:1.通过6 keV H~+离子注入钨中的实验,研究不同注入条件对H沉积行为的影响。结果表明,钨中气泡与表面肿胀随着注入剂量的增加先后形成。另一方面,40 keV的H~+离子在注量较低时即可有效引起钨表面的肿胀。深度分析表明,尽管40 keV的H~+注入剂量较小,其在注入区的浓度却与6 keV的H~+离子注入较高剂量时接近,由此导致的局部饱和是钨表面形成肿胀的原因。此外,高能H~+离子辐照引起的缺陷会束缚更多的氢离子,也会影响H~+离子向材料内部的进一步扩散。2.利用40 keV H~+离子注入的钨样品进行原位退火分析,探究离子辐照后钨中产生的缺陷性质。结果表明H在200-300℃与400-500℃时有两个明显的释放平台,对应两种不同类型的缺陷,两种缺陷均位于H~+离子射程分布范围内。在对室温下放置130天后的样品进行深度分析与退火后,除发现上述两个热脱附峰外,还观察到了H在室温下的缓慢释放,对应一种较弱的束缚形式。这种束缚遍布整个注入区域,为钨自身的固有缺陷与间隙原子缺陷导致。3.通过比较He~+-H~+辐照、Au~+-H~+辐照与单束氢辐照下的H分布行为,探究不同损伤条件对氢在多晶钨中的分布与沉积的影响。结果表明H的深度分布与预损伤离子的深度分布有关。进一步的离线退火实验表明,Au~+离子与He~+离子辐照产生的缺陷对应的热脱附温度均低于600℃。He还可能占据钨中H的位置,导致H在钨中的浓度不升反降。此外,位于钨靠近表面的位置还有对应更高束缚能的缺陷,这些缺陷可能由H~+离子注入引起,但仍需更多实验验证。4.离子交换与扩散被认为是玻璃固化体腐蚀过程中的重要部分,对应关注的元素有H、Li、B、Na、Si等。目前使用的方法存在表征能力弱,灵敏度较差等难题。ToF-SIMS可以灵敏探测到腐蚀过程中的上述元素,并有着极佳的横向分辨率。对ISG块状玻璃中H~+及B~+、Na~+、Al~+、Ca~+与K~+离子的成像进行探究表明,利用适当电流强度的O溅射束,可有效解决ToF-SIMS成像时存在的H本底过高等问题。该方法对ISG块状玻璃成像的分辨率好于200 nm,对极难实现成像的H元素也有很好的效果。5.由于更不规则的形状与极大的表面积/溶液体积比(S/V),ISG玻璃颗粒中的腐蚀行为更加难以观察。本工作对颗粒状ISG玻璃的表征方法与腐蚀机理进行研究发现,离子交换行为不仅与S/V息息相关,还与腐蚀过程中的应力变化有一定关系。此外,溶液分子通过微米级的缺陷进入玻璃体产生腐蚀,进而导致玻璃碎裂。这也是ISG玻璃颗粒产生复杂腐蚀行为的原因之一。本论文主要使用的分析设备为飞行时间-二次离子质谱仪(ToF-SIMS 5),同时还有其它实验技术的辅助。例如,对于氢在多晶钨中的行为研究借助了扫描隧道显微镜(SEM)、聚焦离子束(FIB)、透射电子显微镜(TEM)与X射线衍射(XRD)等方法。对玻璃腐蚀机理的研究也借助了SEM等相关实验技术。本论文工作充分利用ToF-SIMS的特性与优点,使得氢的深度分布有更高的分辨率与更好的灵敏度,并使玻璃腐蚀层实现包括氢在内的各个元素的高分辨率成像。同时,本工作的分析方法与实验过程也可以引申到其它无机材料的分析中,体现了ToF-SIMS在核材料表界面成像与深度分析中的巨大作用。更多还原

【Abstract】 The use of nuclear energy has been considered as a promising way to solve a series of the problems including energy crisis.Relevant research with the development of nuclear energy is also carried out,of which mostly focusing on the basic aspects of material properties.In Fission industry,the disposal of large amounts of radioactive waste generated by fission reactions is mainly concerned.Vitrification,which is the most widely used technology for radioactive waste disposal,has the advantages of high disposal efficiency and long-term storage.However,the vitrification products often demand stabilities of million years,and the corrosion resistance of the glass is therefore concerned.On the other hand,fusion energy is considered as the most important source for future energy due to its high productivity and low radioactive pollution.However,further research is still needed for the design of plasma-facing materials（PFMs）in the fusion device.Because of its high melting point,low sputtering rate and low hydrogen isotope（D/T）retention,tungsten has been selected as the main component in the divertor part of the International Thermonuclear Experimental Reactor（ITER）,which will be subject to low energy and high density of hydrogen ions.The degradation of the tungsten properties after ion bombardment is also an important issue.Current studies are often attributed to fundamental questions,such as the distribution of elements and the changes of the surface topography in materials.However,the experimental technology is still insufficient to solve all the scientific problems.The development of new techniques is therefore very necessary.Secondary ion mass spectrometry（SIMS）is a developing analytical technique,which can obtain important information of materials by analyzing the secondary ions and ion clusters generated by ion bombardment.This technique can be used for both depth profiling and elemental imaging with the properties of excellent detection limit（ppm level）,high mass resolution（M/ΔM>10000）,high depth resolution（1-10 nm）and decent lateral resolution（100 nm）.The sample preparation for SIMS is also simple,which provides obvious convenience for the experiment.All these advantages make SIMS an outstanding one among all the techniques.In the past decades,SIMS has achieved great development in the applications of biology,chemistry and semiconductor materials.However,the use of SIMS in the field of nuclear material is just started.Therefore,it is needed to investigate the basic questions in PFMs and vitrification products using SIMS in this study.This dissertation is divided into two parts,of which one is the study of the hydrogen distribution and the surface topography in ion-irradiated polycrystalline tungsten.The other is the investigation of the elemental imaging of the ISG glass（vitrification products）.The main contents are described as follows:1.Hydrogen distribution in tungsten is affected by different implantation conditions.In this work,the effects of energy and fluence on the surface morphology and H retention of tungsten materials were investigated using H⁺ ions with a fixed beam intensity.The result shows that the critical fluence for bubbling in tungsten was 2×10¹⁷ H⁺/cm² while the critical fluence for blistering caused by 6 keV H⁺ions is¹×10¹⁹ H⁺/cm².On the other hand,40 keV H⁺ions can effectively cause surface blistering when the fluence reaches²×10¹⁷ H⁺/cm².The defects caused by high-energy hydrogen ion irradiation could trap more hydrogen atoms,but also prevent the further diffusion of H into the material.Moreover,H depth distribution indicates that although the fluence of 40 keV H⁺implantation was low,the H concentration was close to that of 6 keV H⁺ions implanting at a higher fluence.The resulting local saturation is the main reason for blistering on the tungsten surface.2.In order to investigate the properties of various defects in tungsten,40keV H⁺ions were implanted into the unannealed polycrystalline tungsten at a fluence of 2×10¹⁷ H⁺/cm².After the implantation,the surface morphology and H depth distribution in tungsten were observed.Moreover,H behavior during isothermal and isochronal annealing was successfully observed with a TOF-SIMS5 instrument.The in-situ analysis shows two release stages at 200-300℃ and400-500℃,which correspond to two different types of defects.Both of them are located at the projected range of implanted H ions.In addition,the same depth analysis and annealing experiments were performed on another sample after 130 days storage at room temperature.In addition to the same two release stages,the slow release of hydrogen at room temperature was found and corresponds to the weaker trapping sites.These defects could extend throughout the implanted area and is caused by inherent defects in the tungsten and interstitial atom defects.3.As the main component of the divertor part in fusion reactor,tungsten is not only bombarded by hydrogen isotopes with low energy and high fluences,but also is irradiated by higher energy He⁺ions and 14.1 MeV neutrons.Therefore,it is necessary to explore the hydrogen behavior in tungsten under different damage conditions.This work compares the hydrogen distribution behavior of He⁺-H⁺ irradiation,Au⁺-H⁺ irradiation and single hydrogen ions irradiation.The results show that hydrogen depth distribution is related to the pre-damaged levels.For higher damage,higher hydrogen retention in tungsten could be found.Further ex-situ observation shows that the binding energy of Au⁺and He⁺ion induced-defects is related to vacancy and vacancy clusters.The retention of H caused by Au⁺ and He⁺ ions is different,which might be caused by the different distribution of pre-implanted ions.Moreover,He also occupies the H position in the tungsten,which may affect the concentration of H in different ways.This work also indicates higher trapping energy related to the defects that may be caused by H⁺ ion implantation,but more experimental evidence is needed.4.Ion exchange and interdiffusion are two important processes in the glass corrosion behavior.Important exchanged ions such as H⁺and B⁺,Na⁺,Al⁺,Ca⁺,and K⁺ are difficult to characterize by most of the currently used methods.The detection limit and image resolution are also not ideal enough.The elemental imaging of H elements is more difficult to achieve.ToF-SIMS can detect light elements and has excellent lateral resolution and is an important method in glass corrosion research.However,it is still needed to investigate the optimized settings during the experimental process.The first part of this work explored the imaging of H⁺and B⁺,Na⁺,Al⁺,Ca⁺,and K⁺ ions in ISG bulk glass.The rise of vacuum and H background during the analysis were discussed,and the ways to solve these problems were also proposed.Finally,the elemental imaging of corrosion layers in the glass was successfully achieved.This method is also very effective for H,which is extremely difficult to image,and the lateral resolution can reach as well as 200 nm.5.In the second part of the glass etching work,we discussed the corrosion behavior of ISG particles.Due to the irregular shape and the large surface area/solution volume ratio（S/V）,the corrosion behavior of ISG glass particles is very complex.This work provides a way for observing the various corrosion behaviors in the ISG particles.It is observed that the ion exchange behavior is not only closely related to the S/V,but also related to the stress change during the corrosion process.In addition,solution molecules will enter the glass through micron-sized defects and cause more serious corrosion,which in turn causes glass fragmentation.This is one of the reasons for its complex corrosion behavior.The main instrument in this dissertation is a time-of-flight secondary ion mass spectrometry（ToF-SIMS 5）.Other experimental instruments were also used as supplemental tools.For example,the study of the behavior of hydrogen in polycrystalline tungsten has been assisted by scanning tunneling microscopy（SEM）,focused ion beam（FIB）,transmission electron microscopy（TEM）and X-ray diffraction（XRD）.The research on the corrosion mechanism of glass also utilizes related techniques such as SEM.This dissertation makes full use of the characteristics and advantages of ToF-SIMS,which provides H distributions with high resolutions and better sensitivities.Moreover,it enables the high-resolution imaging for various elements（including H）of the corrosion layers in the glass.The analytical method and experimental process of this work can also be extended to the analysis of other inorganic materials.This indicates the importance of ToF-SIMS in the interface imaging and depth analysis of nuclear materials.Finally,the development prospects of this technology in the fields of material analysis are also discussed.更多还原