XRD and AFM study of radiation damage induced by swift heavy ions in Y3Al5O12 single crystals

Defects induced in Y₃Al₅O₁₂ single crystals by swift heavy ions are investigated by X-ray diffraction (XRD) and atomic force microscopy. The irradiation was performed at GANIL with 561 MeV ⁵¹Cr, 466 MeV ¹²⁸Te, and 957 MeV ²⁰⁸Pb ions. The XRD data reveal that the lattice strain increases with increasing electronic stopping power, whereas the hillock parameters (height and diameter) are not influenced by the electronic stopping power. According to our experimental data, for the same mean electronic stopping power, the hillock parameters are more pronounced for the lower range in contrast to swelling measurements. The experimental data show a strong increase in the hillock parameter at higher fluence, indicating the amorphization of Y₃Al₅O₁₂ single crystals.     

后向受激布里渊散射诱导的光学材料破坏机理研究

 区别于传统的受激布里渊散射(SBS)发生器和放大器，提出了一种新型的SBS模型：自供种子光模型 (self Stokes seeding，SSS)。数值求解了SBS耦合波方程组，得到了SBS诱导应力的时空分布。基于SSS建立了高功率激光辐照下光学材料破坏阈值的计算模型，研究了SBS破坏阈值与激光脉宽以及作用区长度的关系。研究发现，SBS作为一种破坏机制，表现为前表面破坏，且破坏阈值与激光脉宽以及作用区长度均成反比。     

Raman spectroscopy study of damage induced in fluorapatite by swift heavy ion irradiations

Raman spectroscopy was used to study the radiation damage of fluorapatite single crystals and sinters. Krypton and iodine ion irradiations were performed at high energies (∼1 MeV amu⁻¹) for fluences ranging between 1 × 10¹¹ and 5 × 10¹³ cm⁻². Evolution of the symmetric stretching mode of the PO₄³⁻ tetrahedral building blocks (strongest Raman mode observed at 965 cm⁻¹) versus ion fluence was investigated. After irradiation, this peak decreases in intensity and a second broader peak appears at lower wavenumber. The well‐resolved peak has been assigned to the crystalline phase, and the broader one to the amorphous phase. The integrated intensity ratios of these two peaks versus fluence are in good agreement with the damage fractions determined by X‐ray diffraction (XRD). Fits of the amorphous fraction versus fluence show that the amorphization mechanisms is dominated by a single‐impact process for iodine ions and by a double‐impact process for krypton ions in the case of single crystals and sinters. For both irradiations, complete amorphization could not be obtained. The amorphous fraction saturates at a maximum value of 88% for sinters and 72% for single crystals. This is attributed to a recrystallization effect which is more important in single crystals than in sinters. For both types of samples, the crystalline peak shifts slightly to a lower wavenumber with fluence, and then shifts back to its initial value for an amorphous fraction larger than 60%. This feature is attributed to a stress relaxation, as shown in the XRD data, which is accompanied by a decrease of the crystalline peak full‐width at half‐maximum. Copyright © 2011 John Wiley & Sons, Ltd.     

Molecular dynamics simulation of atomic hydrogen diffusion in strained amorphous silica

Understanding hydrogen diffusion in amorphous SiO_2(a-SiO_2),especially under strain,is of prominent importance for improving the reliability of semiconducting devices,such as metal-oxide-semiconductor field effect transistors.In this work,the diffusion of hydrogen atom in a-SiO_2 under strain is simulated by using molecular dynamics(MD) with the ReaxFF force field.A defect-free a-SiO_2 atomic model,of which the local structure parameters accord well with the experimental results,is established.Strain is applied by using the uniaxial tensile method,and the values of maximum strain,ultimate strength,and Young's modulus of the a-SiO_2 model under different tensile rates are calculated.The diffusion of hydrogen atom is simulated by MD with the ReaxFF,and its pathway is identified to be a series of hops among local energy minima.Moreover,the calculated diffusivity and activation energy show their dependence on strain.The diffusivity is substantially enhanced by the tensile strain at a low temperature(below 500 K),but reduced at a high temperature(above500 K).The activation energy decreases as strain increases.Our research shows that the tensile strain can have an influence on hydrogen transportation in a-SiO_2,which may be utilized to improve the reliability of semiconducting devices.     

Molecular dynamics simulation of wetting on modified amorphous silica surface

The microscopic wetting of water on amorphous silica surfaces has been investigated by molecular dynamics simulation. Different degrees of surface hydroxylation/silanization were considered. It was observed that the hydrophobicity becomes enhanced with an increase in the degree of surface silanization. A continuous transformation from hydrophilicity to hydrophobicity can be attained for the amorphous silica surfaces through surface modification. From the simulation result, the contact angle can exceed 90° when surface silanization percentage is above 50%, showing a hydrophobic character. It is also found that when the percentage of surface silanization is above 70% on the amorphous silica surface, the water contact angle almost remains unchanged (110–120°). This phenomenon is a little different from the wetting behavior on smooth quartz plates in previous experimental report. This change in the wettability on modified amorphous silica surfaces can be interpreted in terms of the interaction between water molecules and the silica surfaces.     

Molecular dynamics study of primary radiation damage in TiVTa concentrated solid-solution alloy

The primary radiation damage in pure V and TiVTa concentrated solid-solution alloy(CSA) was studied using a molecular dynamics method. We have performed displacement cascade simulations to explore the generation and evolution behavior of irradiation defects. The results demonstrate that the defect accumulation and agglomeration in TiVTa CSA are significantly suppressed compared to pure V. The peak value of Frenkel pairs during cascade collisions in TiVTa CSA is much higher than that in pure V du...     

Molecular dynamics simulation of displacement damage in 6H-SiC

ABSTRACT

As a new generation of low-loss components, 6H-SiC is widely used in optoelectronic devices, electronic devices and other fields, especially in high temperature and strong radiation environment. Radiation will lead to a great large of defects generated in the material, then forming permanent displacement damage which will result in performance down or failure. In this paper, the molecular dynamics method was used to study the spatial distribution of defects and the effect of temperature and PKA energy on the various properties of the point defects. The main contributions were the evolution of defects in 6H-SiC crystal, the influence of PKA energy and temperature on the defect distribution and the number of point defect. In this paper, the spatial distribution of point defects in 6H-SiC crystal after PKA incidence was obtained, the recombination rate under four kinds of energy PKA was calculated, and the effect of temperature and incident PKA energy on the number of defects, the rate of the vacancy cluster and the rate of vacancy defects at steady state were investigated. The results show that the number of defects at steady-state increases linearly with the increase of incident PKA energy, the effect of temperature and energy on defects producing and the rate of clusters are insignificant.     

Optical spectroscopy study of damage induced in 4H-SiC by swift heavy ion irradiation

Single crystals of 4H-SiC were irradiated with swift heavy ions (332 MeV Ti, 106 MeV Pb and 2.7 GeV U) in the electronic energy loss regime. The resulting damage was investigated with UV-visible optical absorption spectroscopy and micro-Raman spectroscopy. The evolution of the Raman data with fluence shows an accumulation of isolated point defects without amorphization of the material and a partial recrystallization of the structure, but only at the lowest fluence. Furthermore, the longitudinal optical phonon-plasmon coupling mode disappears upon irradiation, suggesting a strong perturbation of the electronic structure. This evolution is consistent with the optical bandgap decrease and the Urbach edge broadening that was also previously observed for the irradiation with 4 MeV Au ions.     

Lattice damage in InGaN induced by swift heavy ion irradiation

The microstructural responses of In_0.32Ga_0.68N and In_0.9Ga_0.1N films to 2.25 GeV Xe ion irradiation have been investigated using x-ray diffraction, Raman scattering, ion channeling and transmission electron microscopy. It was found that the In-rich In_0.9Ga_0.1N is more susceptible to irradiation than the Ga-rich In_0.32Ga_0.68N. Xe ion irradiation with a fluence of 7× 10¹¹ ions·cm^-2 leads to little damage in In_0.32Ga_0.68N but an obvious lattice expansion in In_0.9Ga_0.1N. The level of lattice disorder in In_0.9Ga_0.1N increases after irradiation, due to the huge electronic energy deposition of the incident Xe ions. However, no Xe ion tracks were observed to be formed, which is attributed to the very high velocity of 2.25 GeV Xe ions. Point defects and/or small defect clusters are probably the dominant defect type in Xe-irradiated In_0.9Ga_0.1N.     

Experimental determination of the production mechanism for noncharacteristic radiation in swift heavy-ion collisions

Absolute differential cross sections of non-characteristic radiation emitted in energetic heavy-ion collisions (E _P =3.92–48 MeV,Z=11–22) can only be explained in the framework of quasimolecular radiation. All other processes, particularly bremsstrahlung mechanisms are found to be negligible. For high projectile and photon energies the one-step quasimolecular radiation is dominant in solid targets, whereas the two-step process contributes mainly at low projectile and photon energies.     

Molecular dynamics study of self-diffusion in Zr

We employed a recently developed semi-empirical Zr potential to determine the diffusivities in hcp and bcc Zr via molecular dynamics simulation. The point defect concentration was determined directly from molecular dynamics (MD) simulation rather than from theoretical methods using T = 0 calculations. Our MD simulation indicates that the diffusion proceeds via the interstitial mechanism in hcp Zr, and both vacancy and interstitial mechanisms contribute to diffusivity in bcc Zr. The agreement with the experimental data is excellent for hcp Zr and rather good for bcc Zr at high temperatures, but there is considerable disagreement at low temperatures.     

熔石英亚表面划痕激光诱导损伤阈值实验研究

 用原子力显微镜和光学显微镜观测酸蚀后熔石英亚表面划痕,并根据形貌特征将其分为Boussinesq-point-force crack(BPFC)、Hertzian-conical scratch(HCS)和Plastic indent(PI)三类,测试了各类划痕的损伤阈值,讨论了激光损伤机制。结果表明锐度较大的BPFC损伤阈值不超过2.0 J/cm<>2;深度小于1 μm的 HCS阈值可达2.6 J/cm²;形变较大的PI阈值至2.8 J/cm2,形变较小的PI的激光损伤阈值与无缺陷材料相当。BPFC 和深度超过1 μm的HCS是导致熔石英损伤阈值低的主要因素。     

Role of thermodynamics in the shape transformation of embedded metal nanoparticles induced by swift heavy-ion irradiation

Swift heavy-ion irradiation of elemental metal nanoparticles (NPs) embedded in amorphous SiO(2) induces a spherical to rodlike shape transformation with the direction of NP elongation aligned to that of the incident ion. Large, once-spherical NPs become progressively more rodlike while small NPs below a critical diameter do not elongate but dissolve in the matrix. We examine this shape transformation for ten metals under a common irradiation condition to achieve mechanistic insight into the transformation process. Subtle differences are apparent including the saturation of the elongated NP width at a minimum sustainable, metal-specific value. Elongated NPs of lesser width are unstable and subject to vaporization. Furthermore, we demonstrate the elongation process is governed by the formation of a molten ion-track in amorphous SiO(2) such that upon saturation the elongated NP width never exceeds the molten ion-track diameter.     

Molecular dynamics study of ice structural evolution

Molecular dynamics simulation is employed to study the structural evolution of low density amorphous ice during its compression from one atmosphere to 2.5 GPa. Calculated results show that high density amorphous ice is formed at an intermediate pressure of -1.0 GPa; the O-O-O bond angle ranges from 83° to 113°, and the O-H… O bond is bent from 112° to 160°. Very high density amorphous ice is obtained by quenching to 80 K and decompressing the ice to ambient pressure from 160 K/1.3 GPa or 160 K/1.7 GPa; and the next-nearest O-O length is found to be 0.310 nm, just 0.035 nm beyond the nearest O-O distance of 0.275 nm.     

1064 nm激光致熔石英损伤的数值模拟研究

为了研究1064 nm激光对增透熔石英的热应力损伤机理以及等离子体分布等效应。基于热传导和气体动力学理论,探究了毫秒激光对增透熔石英的热应力损伤和致燃损伤的理论模型,利用comsol软件模拟了1064 nm激光作用增透熔石英时材料内部的热损伤、应力损伤以及激光支持燃烧波,模拟结果表明在激光光斑半径区域内,温升较为明显,形成较大的温度梯度,激光作用区域受热膨胀,其余区域会对膨胀发生抵制,因此材料内部产生应力,其中上表面的径向应力、环向应力在激光光斑边缘附近达到最大值,应力损伤应该先从辐照中心点或激光光斑边缘附近产生,同时发现等离子体的传播是稳态的,燃烧波的最大速度发生在最初时刻,并随着扩散时间逐渐变低。     

1064 nm激光致熔石英损伤的数值模拟研究

为了研究1064 nm激光对增透熔石英的热应力损伤机理以及等离子体分布等效应。基于热传导和气体动力学理论,探究了毫秒激光对增透熔石英的热应力损伤和致燃损伤的理论模型,利用comsol软件模拟了1064 nm激光作用增透熔石英时材料内部的热损伤、应力损伤以及激光支持燃烧波,模拟结果表明在激光光斑半径区域内,温升较为明显,形成较大的温度梯度,激光作用区域受热膨胀,其余区域会对膨胀发生抵制,因此材料内部产生应力,其中上表面的径向应力、环向应力在激光光斑边缘附近达到最大值,应力损伤应该先从辐照中心点或激光光斑边缘附近产生,同时发现等离子体的传播是稳态的,燃烧波的最大速度发生在最初时刻,并随着扩散时间逐渐变低。     

STM and AFM observations of damage produced by swift Ne and Kr ions in graphite

Radiation damage of highly oriented pyrolitic graphite (HOPG) samples have been investigated following irradiation with 215 MeV Ne and 209 MeV Kr ions, available at U-400 cyclotron, Dubna. A freshly cleaved HOPG surface was irradiated perpendicularly to the sample surface (c plane). A low ion irradiation dose was used (10¹² ions/cm²) in order to avoid damage overlap. Scanning tunneling microscopy (STM) and atomic force microscopy (AFM) are useful methods allowing direct observation of surface defects. The observations were made after irradiation without any further sample preparation. The experimental results are compared to computer simulations (TRIM code) and primary knonked-on atomic spectrum calculations (LET code). Clear distinction can be made between surface features attributed to nuclear stopping effects and defects owing to electronic stopping mechanisms.     

Thermal redistribution of indium in amorphous silicon layers

A new impurity redistribution mechanism is reported for low temperature annealing (525°C) of (100) Si samples implanted with high indium doses. The redistribution is a strong function of implant dose and is believed to be stress related.     

辐照损伤对钨晶格热导率影响的分子动力学研究

钨是最具应用前景的面向等离子体候选材料,但核聚变堆内强烈的辐照环境会使钨的近表面区域产生辐照损伤,进而影响其关键的导热性能.本文构建了包含辐照损伤相关缺陷的晶体钨模型,并采用非平衡分子动力学的方法定量研究了这些缺陷对钨导热性能的影响.结果表明,随中子辐射能量的增加,晶体内部留下的Frenkel缺陷数目增多进而导致钨的晶格热导率降低;间隙原子比空位更易于向晶界偏聚,且钨中的间隙钨原子与空位相比,使晶格热导率下降程度更大.纳米级氦气泡导致晶格热导率的显著降低,气孔率为2.1%时晶格热导率降至完美晶体的约25%.这些不同的缺陷造成不同程度的周围晶格扭曲,增加了声子散射几率,是导致晶格热导率下降的根源.     

BaTiO3铁电体中辐射位移效应的分子动力学模拟

运用基于壳模型的分子动力学方法研究了BaTiO₃铁电体中的辐射位移效应.采用O原子作为初级击出原子,模拟了当初级击出原子能量为1 keV时体系内缺陷的产生和演化.模拟结果表明,当入射方向为[001]时,体系内产生的缺陷最多.在所有缺陷中,以O缺陷的含量为最高,达80%以上.同时,这些缺陷的产生并不显著改变体系的自发极化强度,对体系的极化翻转过程也基本没有影响.在外电场作用下,观察到了显著的缺陷迁移. 关键词： 分子动力学 3铁电体')" href="#">BaTiO₃铁电体 辐射位移效应