Effect of Si implantation on the microstructure of silicon nanocrystals and surrounding SiO2 layer

Si nanocrystals (Si-nc) embedded in a SiO₂ layer have been characterized by means of transmission electron microscopy (TEM) and X-ray photoelectron spectroscopy (XPS). For local Si concentration in excess  8 × 10²¹ Si⁺/cm³, the size of the Si-nc was found to be 3 nm and comparatively homogeneous throughout the whole implanted layer. For local Si concentration in excess of 2.4 × 10²² Si⁺/cm³, the Si-nc diameter ranges from 2 to 12 nm in the sample, the Si-nc in the middle region of the implanted layer being bigger than those near the surface and the bottom of the layer. Also, Si-nc are visible deeper than the implanted depth. Characterization by XPS shows that a large quantity of oxygen was depleted from the first 25 nm in this sample (also visible on TEM image) and most of the SiO₂ bonds have been replaced by Si–O bonds. Experimental and simulation results suggest that a local Si concentration in excess of 3 × 10²¹ Si/cm³ is required for the production of Si-nc.     

Structural characterization of amorphous Fe–Si and its recrystallized layers

We have synthesized amorphous Fe–Si thin layers and investigated their microstructure using transmission electron microscopy (TEM). Si single crystals with (1 1 1) orientation were irradiated with 120 keV Fe⁺ ions to a fluence of 4.0 × 10¹⁷ cm⁻² at cryogenic temperature (120 K), followed by thermal annealing at 1073 K for 2 h. A continuous amorphous layer with a bilayered structure was formed on the topmost layer of the Si substrate in the as-implanted specimen: the upper layer was an amorphous Fe–Si, while the lower one was an amorphous Si. After annealing, the amorphous bilayer crystallized into a continuous β-FeSi₂ thin layer.     

Dynamic Monte Carlo simulation of surface composition change during sputter depth profiling

In the surface analysis methods such as X-ray photoelectron spectroscopy, Auger electron spectroscopy, or secondary-ion mass spectroscopy, sputtering processes are used for depth profiling. However, ion beam bombardment changes the surface composition and the surface structure, which deteriorates the accuracy of the surface analysis and the depth resolution. We studied the preferential sputtering in some materials consisting of two components with different mass ratio by using the dynamic Monte Carlo simulation. Dose dependence of the depth profile of composition is presented. By Ar sputtering, the surface compositions of Au_0.25Cu_0.75, Au_0.67Al_0.33, and Ni_0.5Cu_0.5 alloys changed to Au_0.28Cu_0.72, Au_0.76Al_0.24, and Ni_0.67Cu_0.33, respectively. These results agreed with the experimental data. In order to compare the effect of mass ratio on the preferential sputtering, B–C, Si–C, and W–C systems were investigated. In B–C system, preferential sputtering by 1 and 3 keV Ar ion was negligible. In W–C system, a significant preferential sputtering occurred. In Si–C system, carbon was enriched in the outermost surface layer at a fluence lower than 3×10¹⁶ ions/cm². At higher fluence, the partiality in concentration recovers because of the balance between the enrichment and the preferential sputtering.     

Compatibility of surface-coated steels, refractory metals and ceramics to high temperature lead–bismuth eutectic

Compatibility of cladding material with lead–bismuth eutectic at temperature higher than 650 °C is one of the most crucial issues for feasibility of lead–bismuth-cooled fast reactors with cycle efficiency as high as 40%. In order to search for corrosion-resistant materials with lead–bismuth eutectic at temperature higher than 650 °C, surface-coated steels, some refractory metals and various ceramics were tested by means of stirred-type corrosion test. Lead–bismuth was heated up to 700 °C electrically in an alumina crucible, and oxygen concentration in the lead–bismuth was adequately controlled by injection of argon, steam and hydrogen gas mixture into the lead–bismuth. Specimens of aluminum–iron-alloy-surface-coated steels, refractory metals and ceramics including SiC/SiC composites were immersed in the stirred lead–bismuth for 1000 h. It was found that the surface-coated steels showed good compatibility with the lead–bismuth due to formation of a thin and stable protection layer on the surfaces. Tungsten and molybdenum exhibited high corrosion resistance. On the other hand, niobium is not a reliable material for the high temperature LBE. SiC and Ti₃SiC₂ also exhibited high corrosion resistance. On the other hand, the physical performance of the SiC/SiC composite must be improved especially by minimizing the porosity.     

Ion beam synthesis and recrystallization of amorphous SiGe/SiC structures

Si_1−xGe_x amorphous layers implanted with different doses of carbon (between 5 × 10¹⁵ and 2 × 10¹⁷ cm⁻² and annealed at 700°C and 900°C have been analyzed by Raman and Infrared spectroscopies, electron microscopy and Auger electron spectroscopy. The obtained data show the synthesis of amorphous SiC by implanting at the highest doses. In these cases, recrystallization only occurs at the highest annealing temperature (900°C). The structure of the synthesized SiC strongly depends on the implantation dose, in addition to the anneal temperature. For the highest dose (2 × 10¹⁷ cm⁻²), crystalline β-SiC is formed. Finally, a strong migration of Ge towards the Si substrate is observed from the region where SiC precipitation occurs.     

用于ICF的非晶SiC微球制备及微观结构研究

以四甲基硅烷、反式二丁烯和氢气为工作气源,采用化学气相沉积-高温热解法成功制备了壁厚约21μm的非晶SiC微球。利用能量色散X射线光谱仪、X射线光电子能谱仪、X射线衍射仪、Raman光谱仪、扫描电子显微镜、白光干涉仪和X射线照相机对SiC微球的化学成分、结晶状态、表面形貌与粗糙度以及密度与球形度等进行了测量和分析。结果表明:在无氧环境下,通过450~900℃的高温热解及致密化可将在聚α甲基苯乙烯芯轴上沉积的掺硅碳氢聚合物涂层转变成致密的SiC微球。SiC微球呈非晶态,其C/Si原子比约为1.3,主要含有C—Si键和C=C键,微观结构呈无规则状且颗粒分布均匀,密度、球形度和壁厚均匀性分别为2.62 g/cm~3、99.8%和96.8%。     

Influence of polytypism on elementary processes of ion-beam-induced defect production in SiC

Classical molecular dynamics (MD) simulations are performed to investigate elementary ion-beam-induced defect production in 3C– and 4H–SiC. A modified Tersoff potential is used to model the interactions between the atoms. For cases where the C and Si primary knockon atoms (PKAs) start parallel or antiparallel to the [0 0 0 1] direction, the threshold PKA energy for defect formation as well as the final defect configuration and its formation energy are determined. The elementary defects observed in 3C–SiC and 4H–SiC differ significantly whereas the corresponding threshold PKA energies and the formation energies of the configurations are mostly similar. In 4H–SiC new sites for C and Si interstitials are found: one site is situated between two C₃Si₃ hexagonal rings, the other between a C₃ and an Si₃ trigonal ring.     

Structural changes of SiC under electron-beam irradiation: Temperature dependence

Electron-beam-irradiation effects on silicon carbide (SiC) was investigated as a function of the irradiated temperatures. Single crystalline 6H-SiC was irradiated with 300 kV electrons at temperatures ranging from −170 to 250 °C. It was found that amorphous SiC is induced at −170 °C and room temperature, while crystalline Si is formed at 250 °C with a high electron fluence. It is considered that preferential knock-on displacement of C atoms and damage recovery play an important role in the formation of the amorphous SiC and crystalline Si.     

Influence of nitrogen ion implantation energies on surface chemical bonding structure and mechanical properties of nitrogen-implanted silicon carbide ceramics

Nitrogen ions were implanted into silicon carbide ceramics (N⁺-implanted SiC) at different ions energies. The surface chemical bonding structure of N⁺-implanted SiC ceramics were investigated by using X-ray photoelectron spectroscopy (XPS). The hardness of N⁺-implanted SiC ceramics was measured using nano-indenter, and the friction and wear properties of the N⁺-implanted SiC/SiC tribopairs were studied using ball-on-disk type tribo-meter in water lubrication. The wear tracks were observed using non-contact surface profilometer and scanning electron microscope (SEM). The results showed that the surface roughness of N⁺-implanted SiC ceramic was higher than that of SiC ceramic, and some chemical bonds such as Si–N, C–C, CN and C–N bonds were formed in N⁺-implanted layer besides Si–C bonds. In comparison of SiC ceramic’s hardness, the hardness of N⁺-implanted SiC ceramics at 30 and 50 keV was higher while that at 65 keV was lower. Under water lubrication, the friction coefficient and the specific wear rates for the N⁺-implanted SiC/SiC tribopairs were all lower than those of the SiC/SiC tribopairs, and displayed the lowest values at 50 keV. According to XPS analysis, it was concluded that the high wear resistance and low friction coefficient for the N⁺-implanted SiC/SiC tribopairs were attributed to the formation of carbon rich composite on the surface of N⁺-implanted SiC ceramics.     

Vacancy defects induced in the track region of 132 MeV C irradiated SiC

Positron annihilation lifetime spectroscopy (PALS) and electron paramagnetic resonance (EPR) have been used in this work to investigate vacancy defects induced in the track region of 132 MeV ¹²C irradiated silicon carbide. Irradiations have been performed at room temperature at a fluence of 2.5 × 10¹⁴ cm⁻² in N-low doped 6H–SiC and 3C–SiC monocrystals. Silicon monovacancies have been detected in both polytypes using EPR. Their charge state and concentration have been determined in the track and cascade region of the C⁺ ions. PALS measurements performed as a function of temperature have shown the presence of V_Si–C divacancies in the track region for both polytypes.     

Determination of stoichiometry in silicon carbide materials using elastic backscattering spectrometry

Elastic backscattering spectrometry (EBS) was performed on SiC materials, using ⁴He particles at energies ranging from 2 to 4 MeV, in order to establish the energy values that lead to an accurate measurement of the Si/C ratio. Analysis of the random yield of “bulk” SiC single crystals indicates that energy values of 3.25 and 3.75 MeV are the most suitable for chemical composition determination; backscattering yield of carbon is enhanced compared to the yield measured at 2 MeV, while the excitation of strong resonances above 3.75 MeV are suppressed. Random backscattering yield measurements were then carried out at an energy of 3.25 MeV on unhydrogenated SiC thin films grown on Si(1 0 0), by pulsed laser deposition, at different substrate temperatures. The Si and C atomic concentrations in the films were determined with an uncertainty of 1% and little interference from the underlying substrate. The films were found to be stoichiometric with a Si/C ratio of 1.03 ± 0.05, independent of deposition temperature, which indicates that the films were grown under congruent ablation conditions. The analysis proved to be applicable to both amorphous and crystalline SiC layers, as confirmed by the results obtained for films deposited at 400 and 950 °C, respectively.     

Electron microscopy and Rutherford backscattering spectrometry characterisation of 6H SiC samples implanted with He

6H SiC single crystals were implanted at room temperature with 1 MeV He⁺ up to a fluence of 2 × 10¹⁷ at./cm². RBS-channeling analysis with a 2 MeV He⁺ beam indicated the formation of extended defects or the generation of point defects at a constant concentration over a depth of about 1 μm. Electron microscopy characterisation revealed the presence of two amorphous buried layers at depths of about 1.75 and 4.8 μm. They are due to the implantation and to the analysing RBS beam, respectively. No extended planar or linear faults were found in the region between the surface and the first amorphous layer. However, at the surface, a 50 nm thick amorphous layer was observed in which crystalline inclusions were embedded. Electron diffraction and HREM data of the inclusions were typical for diamond. These inclusions were even found in the crystalline SiC material below this layer, however at a reduced density.     

Thermal stability and brazing characteristics of Zr0.7−xMxBe0.3 (M=Ti or Nb) ternary amorphous filler metals

The effects of Ti or Nb substitution on the thermal stability and brazing characteristics of Zr_0.7−xM_xBe_0.3 (M=Ti or Nb) ternary amorphous alloys were investigated in order to improve properties of Zr–Be binary amorphous alloy as a new filler metal for joining zirconium alloy. The Zr_0.7−xM_xBe_0.3 (M=Ti or Nb; 0x0.1) ternary amorphous alloys were produced by melt-spinning method. In the selected compositional range, the thermal stability of Zr_0.7−xTi_xBe_0.3 and Zr_0.7−xNb_xBe_0.3 amorphous alloys are improved by the substitution of titanium or niobium for zirconium. As the Ti and Nb content increases, the crystallization temperatures increase from 610°C to 717°C and 610°C to 678°C, respectively. These amorphous alloys were put into practical use in joining bearing pads on zircaloy cladding sheath. Using Zr–Ti–Be amorphous alloys as filler metals, smooth interface and spherical primary particles (proeutectic phase) appear in the brazed layer, which is the similar microstructure of using Zr_0.7Be_0.3 binary amorphous alloys. In the case of Zr–Nb–Be amorphous alloys, Ni-precipitated Zr phase that may cause some degradation in ductility and corrosion-resistance is formed at both sides of the brazed layer.     

The crystal structure of oxides grown on Zr–20Nb alloy

Oxides that were grown on Zr–20Nb in water at 300°C for 3 d, or in air at 400°C for 2 h were characterized by analytical electron microscopy. In both oxides, a similar microstructure was observed and similar electron diffraction patterns and high resolution lattice images were obtained. Analyses of the results showed that the crystal structure of the oxides was identical to that of an incommensurate modulated Nb₂Zr_x−2O_2x+1 phase, with x ≈ 10.     

Spectroscopic and microscopic study of the corrosion of iron–silicon steel by lead–bismuth eutectic (LBE) at elevated temperatures

The performance of iron–silica alloys with different silicon composition was evaluated after exposure to an isothermal bath of lead–bismuth eutectic (LBE). Four alloys were evaluated: pure iron, Fe–1.24%Si, Fe–2.55%Si and Fe–3.82%Si. The samples were exposed to LBE in a dynamic corrosion cell for periods from 700 to 1000 h at a temperature of 550 °C. After exposure, the thickness and composition of the oxide layer were examined using optical microscopy, scanning electron microscopy (SEM) and X-ray photoelectron spectrometry (XPS), including sputter depth profiling. Particular attention was paid to the role, spatial distribution, and chemical speciation of silicon. Low-binding-energy silicon (probably silicates or ) was found in the oxide; while elemental silicon (Si) was found in the metal as expected, and silica (SiO₂) was found at the bottom of the oxide layer, consistent with the formation of a layer between the oxide and the metal. Alloys with low concentrations of Si contained only silicate in the oxide. Alloys with higher concentrations of Si contained a layer of silica at the boundary between the oxide and the bulk metal. All of the alloys examined showed signs of oxide failure. This study has implications for the role of silicon in the stability of the oxide layer in the corrosion of steel by LBE.     

Atomistic simulation of collision cascades in zircon

Defect production in energetic collision cascades in zircon has been studied by molecular dynamics simulation using a partial charge model combined with the Ziegler–Biersack–Littmark potential. Energy dissipation, defect accumulation, Si–O–Si polymerization and Zr coordination number were examined for 10 keV and 30 keV U recoils simulated in the constant NVE ensemble. For both energies an amorphous core was produced with features similar to that of melt quenched zircon. Disordered Si ions in this core were polymerized with an average degree of polymerization of 1.5, while disordered Zr ions showed a coordination number of about 6 in agreement with EXAFS results. These results suggest that nano-scale phase separation into silica- and zirconia-rich regions occurs in the amorphous core.     

工作压强对SiC空心微球结构及性能的影响

采用化学气相沉积（CVD）-高温热解法,在不同工作压强条件下,制备了惯性约束聚变靶用SiC空心微球。利用X射线光电子能谱仪、扫描电子显微镜、白光干涉仪、X射线照相机对SiC空心微球的成分、表面形貌、表面粗糙度、球形度以及壁厚均匀性进行了测试与分析。研究结果表明：随工作压强的增加,SiC空心微球的表面均方根粗糙度先减小后增加,当工作压强为15 Pa时,表面均方根粗糙度达到最小值98 nm;随工作压强的增加,SiC空心微球的球形度未发生明显变化,且均优于97%;而壁厚均匀性则随工作压强的增加先增加后减小,当压强为15 Pa时,壁厚均匀性可达95%。     

Electronic excitation effects in CeO2 under irradiations with high-energy ions of typical fission products

In order to understand the formation mechanism of a crystallographic re-structuring in the periphery region of high-burnup nuclear fuel pellets, named as “rim structure”, information on the accumulation process of radiation damage and fission products (FPs), as well as high-density electronic excitation effects by FPs, are needed. In order to separate each of these processes and understand the high-density electronic excitation effects, 70–210 MeV FP ion (Xe^10–14+, I⁷⁺ and Zr⁹⁺) irradiation studies on CeO₂, as a simulation of fluorite ceramics of UO₂, have been done at a tandem accelerator of JAEA-Tokai and the microstructure changes were determined by transmission electron microscope (TEM). Measurements of the diameter of ion tracks, which are caused by high-density electronic excitation, have clarified that the effective area of electronic excitation by high-energy fission products is around 5–7 nm  and the square of the track diameter tends to follow linear function of the electronic stopping power (S_e). Prominent changes are hardly observed in the microstructure up to 400 °C. After overlapping of ion tracks, the elliptical deformation of diffraction spots is observed, but the diffraction spots are maintained at higher fluence. These results indicate that the structure of CeO₂ is still crystalline and not amorphous. Under ion tracks overlapping heavily (>1 × 10¹⁵ ions/cm²), surface roughness, with characteristic size of the roughness around 1 μm, is observed and similar surface roughness has also been observed in light-water reactor (LWR) fuels.     

Structural evolution in Fe ion implanted Si upon thermal annealing

We have performed high-dose Fe ion implantation into Si and characterized ion-beam-induced microstructures as well as annealing-induced ones using transmission electron microscopy (TEM) and grazing-incidence X-ray diffraction (GIXRD). Single crystals of Si(1 0 0) substrate were irradiated at 623 K with 120 keV Fe⁺ ions to a fluence of 4 × 10¹⁷ cm⁻². The irradiated samples were then annealed in a vacuum furnace at temperatures ranging from 773 K to 1073 K. Cross-sectional TEM observations and GIXRD measurements revealed that a layered structure is formed in the as-implanted specimen with ε-FeSi, β-FeSi₂ and damaged Si, as component layers. A continuous β-FeSi₂ layer was formed on the topmost layer of the Si substrate after thermal annealing.     

Structural change of carbon nanotubes produced by Si ion beam irradiation

Multi-walled carbon nanotubes (MWCNTs) were irradiated by 40 keV Si ion beam with different doses. The structural change of the MWCNTs was revealed by transmission electron microscopy, high-resolution transmission electron microscopy and Raman spectroscopy. The structural characterization after irradiation shows that the formation of amorphous carbon nanowires proceeds through two periods, carbon nanotube – semi-solid amorphous carbon nanowire with hollow structure – solid amorphous carbon nanowire. Based on the interaction between energetic particles and carbon nanotubes, the structural transformation process and corresponding mechanisms are discussed. A model is presented to illustrate the structural change of carbon nanotubes with increased irradiation dose.