Ion Beam Analysis of the Annealing Behavior of Helium in Ti Films

We present a theoretical calculation finding that a spectrum from ion beam analysis will change at different stopping cross sections. This is more visible at a deeper place in the sample. Helium-contained Ti films annealed at different temperatures are prepared to gain different stopping cross sections whereby the stopping cross section will change with the helium phase states and the pressure of helium bubbles. Then ion beam analysis is used to measure the concentration of helium. It is found that the concentration curve rises greatly after the sample is annealed at 673K which reflects the increasing size of the helium bubble. The results axe consistent with that of positron annihilation radiation spectra which are performed by using a changeable energy positron beam.     

Study of He-induced nano-cavities as sinks of oxygen for forming silicon-on-insulator

In the present work, a Cz-Silicon wafer is implanted with helium ions to produce a buried porous layer, and then thermally annealed in a dry oxygen atmosphere to make oxygen transport into the cavities. The formation of the buried oxide layer in the case of internal oxidation （ITOX） of the buried porous layer of cavities in the silicon sample is studied by positron beam annihilation （PBA）. The cavities are formed by 15 keV He implantation at a fluence of 2×10^16 cm^-2 and followed by thermal annealing at 673 K for 30 min in vacuum. The internal oxidation is carried out at temperatures ranging from 1073 to 1473 K for 2 h in a dry oxygen atmosphere. The layered structures evolved in the silicon are detected by using the PBA and the thicknesses of their layers and nature are also investigated. It is found that rather high temperatures must be chosen to establish a sufficient flux of oxygen into the cavity layer. On the other hand high temperatures lead to coarsening the cavities and removing the cavity layer finally.     

Helium-Implantation-Induced Damage in NHS Steel Investigated by Slow-Positron Annihilation Spectroscopy

Evolutions of defects and helium contained defects produced by atomic displacement and helium deposition with helium implantation at different temperatures in novel high silicon （NHS） steel are investigated by a slow positron beam. Differences of the defect information among samples implanted by helium to a fluence of 1×1017 ions/cm2 at room temperature, 300°C, 450°C and 750°C are discussed. It is found that the mobility of vacancies and vacancy clusters, a recombination of vacancy-type defects and the formation of the He-V complex lead to the occurrence of these differences. At high temperature irradiations, a change of the diffusion mechanism of He atoms/He bubbles might be one of the reasons for the change of the S-parameter.     

High-Temperature Annealing Induced He Bubble Evolution in Low Energy He Ion Implanted 6H-SiC

Bubble evolution in low energy and high dose He-implanted 6 H-SiC upon thermal annealing is studied. The(0001)-oriented 6 H-SiC wafers are implanted with 15 keV helium ions at a dose of 1×10~(17) cm~(-2) at room temperature. The samples with post-implantation are annealed at temperatures of 1073, 1173, 1273, and 1473 K for30 min. He bubbles in the wafers are examined via cross-sectional transmission electron microscopy(XTEM)analysis. The results present that nanoscale bubbles are almost homogeneously distributed in the damaged layer of the as-implanted sample, and no significant change is observed in the He-implanted sample after 1073 K annealing. Upon 1193 K annealing, almost full recrystallization of He-implantation-induced amorphization in 6 H-SiC is observed. In addition, the diameters of He bubbles increase obviously. With continually increasing temperatures to 1273 K and 1473 K, the diameters of He bubbles increase and the number density of lattice defects decreases.The growth of He bubbles after high temperature annealing abides by the Ostwald ripening mechanism. The mean diameter of He bubbles located at depths of 120-135 nm as a function of annealing temperature is fitted in terms of a thermal activated process which yields an activation energy of 1.914+0.236 eV.     

Hardening of an ODS Ferritic Steel after Helium Implantation and Thermal Annealing

Specimens of an oxide dispersion strengthened(ODS) ferritic steel(15 Cr-4 Al-0.6 Zr-0.1 Ti) are implanted with multiple-energy He ions at room temperature to create a damage plateau of 0.4 dpa for the average(corresponding to an He concentration of about 7000 appm) from the near surface to a depth about 1 um. The specimen is subsequently thermally annealed at 800°C for 1 h in a vacuum so that simple defects can be formed in the as-implanted state that has undergone significant recombination, meanwhile helium bubbles at nano-scale are formed. Hardness of the specimens are tested with the nano-indentation technique. A hardening by 25% is observed. Microstructures of the specimen after irradiation/annealing are investigated with transmission electron microscopy. Helium bubbles are generally located at dislocations and grain boundaries. Using the dispersed barrier strength model, the strength factor of helium bubbles in the ODS ferritic steel is estimated to be between0.1 and 0.26, which is close to that of helium bubbles in austenitic steels.     

Synergistic Effect of Triple Ion Beams on Radiation Damage in CLAM Steel

The synergistic effect of triple ion beams is investigated by simultaneous and scquential irradiations of gold, hydrogen and helium ions on the low activation martensitie steel （CLAM） developed in China. The depth profile measurements of the positron annihilation Doppler broadening S parameter are carried out as a function of slow- positron beam energy to examine the produced radiation damage. The synergistic effect of displacement damage and hydrogen and helium on the formation of radiation damage is clearly observed. In the preset case ,this effect suppresses the radiation damage in the CLAM steel due to the helium and/or hydrogen filling of vacancy clusters.     

Resonances for positron–helium and positron–lithium systems in kappa-distribution plasma

S-wave resonances of positron–helium and positron–lithium systems in kappa-distribution plasmas are investigated using Hylleraas-type wave functions in the framework of the stabilization method. A model potential approach is used to represent the interactions between the outer electron, the positron and the core. The resonance parameters(position and width) of positron–helium and positron–lithium systems below the Ps(2s) threshold are reported as a function of screening parameter and spectral index of plasma.     

Molecular Dynamics Simulations of Helium Behaviour in Titanium Crystals

Molecular dynamics simulations are performed to investigate the behaviour of helium atoms in titanium at a temperature of 30OK. The nucleation and growth of helium bubble has been simulated up to 50 helium atoms. The approach to simulate the bubble growth is to add helium atoms one by one to the bubble and let the system evolve. The titanium cohesion is based on the tight binding scheme derived from the embedded atom method, and the helium-titanium interaction is characterized by fitted potential in the form of a Lennard-Jones function. The pressure in small helium bubbles is approximately calculated. The simulation results show that the pressure will decrease with the increasing bubble size, while increase with the increasing helium atoms. An analytic function about the quantitative relationship of the pressure with the bubble size and number of helium atoms is also fitted.     

Vacancy in 6H—Silicon Carbide Studied by Slow Positron Beam

The defect changes in 6H-SiC after annealing and 10MeV electron irradiation have been studied by using a variable-energy positron beam.It was found that after annealing,the defect concentration in n-type 6H-SiC decreased due to recombination with interstitials.When the sample was annealed at 1400℃ for 30 min in vacuum,a 20-nm thickness Si layer was found on the top of the SiC substrate,this is a direct proof of the Si atoms diffusing to surface when annealed at high temperature stages.After 10MeV electron irradiation,for n-type 6H-SiC,the S parameter increased from 0.4739 to 0.4822,and the relative positron-trapping rate was about 27.878 times of the origin sample,this shows that there are some defects created in n-type 6H-SiC.For p-type 6H-SiC,it is very unclear,this may be because of the opposite charge of vacancy defects.     

Effect of Annealing on Ferroelectric Properties of Nanometre BaTiO3 Ceramics Prepared by High Pressure Sintering Method

Dense nanocrystalline BaTiO3 ceramics with a grain size of 5Onto are prepared under 6 GPa at 1273K using a high pressure sintering method. The sintered bulk is uniform and the relative density is above 97%. We anneal the ceramic samples in oxygen with various temperatures and for the annealing, several broadened peaks can be observed at different times without apparent grain growth. After about 378K（ by dielectric measurements. However, these peaks are very different from those of coarser-grained ceramics. It is indicated that both the elimination of oxygen vacancies and the release of residual stresses caused by high pressure greatly improve the overall ferroelectric properties of BaTiO3 ceramics. The observation of nearly linear polarization hysteresis loop after anneal provides the solid evidence of ferroelectricity in these nano-sized BaTiO3 ceramics. It is believed that the absence of 90° domains and the existence of poor-permittivity nonferroelectric grain boundaries contribute to the slim loop.     

Evolution,migration and clustering of helium-vacancy complexes in RAFM steel- depth resolved positron annihilation Doppler broadening study

Indian Reduced Activation Ferritic Martensitic steel is implanted with 130 keV helium ions to a fluence of 5 × 10¹⁴ and 1 × 10¹⁶ ions/cm² and investigated using positron annihilation spectroscopy. The samples were characterised by defect sensitive S and W-parameters using depth resolved slow positron beam. A dose dependency is observed in the nucleation and growth of helium bubbles with annealing temperature. An experimental evidence for the migration of smaller helium-vacancy complexes is observed via the variation in thickness/width of irradiated layer with temperature. The S–W plot clearly shows the regions corresponding to defect annealing, bubble nucleation and growth.     

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.     

In-situ TEM observation of the evolution of helium bubbles in Mo during He+ irradiation and post-irradiation annealing

The evolution of helium bubbles in purity Mo was investigated by in-situ transmission electron microscopy (TEM) during 30 keV He⁺ irradiation (at 673 K and 1173 K) and post-irradiation annealing (after 30 keV He⁺ irradiation with the fluence of 5.74×10¹⁶ He⁺/cm² at 673 K). Both He⁺ irradiation and subsequently annealing induced the initiation, aggregation, and growth of helium bubbles. Temperature had a significant effect on the initiation and evolution of helium bubbles. The higher the irradiation temperature was, the larger the bubble size at the same irradiation fluence would be. At 1173 K irradiation, helium bubbles nucleated and grew preferentially at grain boundaries and showed super large size, which would induce the formation of microcracks. At the same time, the geometry of helium bubbles changed from sphericity to polyhedron. The polyhedral bubbles preferred to grow in the shape bounded by {100} planes. After statistical analysis of the characteristic parameters of helium bubbles, the functions between the average size, number density of helium bubbles, swelling rate and irradiation damage were obtained. Meanwhile, an empirical formula for calculating the size of helium bubbles during the annealing was also provided.     

Positron annihilation study of helium clustering in alpha irradiated copper

Abstract

Helium clustering in alpha irradiated copper has been investigated by positron annihilation spectroscopy. Pure copper samples have been homogeneously helium implanted using a cyclotron, yielding helium concentrations of 100 appm and 400 appm. Post-implantation positron lifetime and Doppler broadened annihilation lineshape measurements have been carried out on these Cu samples as a function of isochronal annealing temperature. An annealing stage observed in the isochronal annealing curve viz., a marked reduction in the resolved lifetime τ₂ and an increase of its intensity I ₂, is explained as due to the formation of helium bubble embryos. At higher annealing temperatures, τ₂ corresponding to helium bubbles increases and saturates while its intensity I ₂ decreases, indicating an increase in the size of the bubble with a concomitant decrease in the bubble concentration. This stage is interpreted to be the bubble growth stage. From an analysis of positron lifetime parameters in the growth stage, helium stom density, bubble size and bubble concentration have been deduced at various annealing temperatures. The bubble characteristics are found to be affected by the helium dose. The present results on direct helium implanted Cu are compared with those of our earlier study on n-irradiated Cu-B, where helium was introduced using ¹⁰B(n, α)⁷ Li reaction.     

Radiation defects induced by helium implantation in gold-based alloys investigated by positron annihilation spectroscopy

The formation of gas bubbles in metallic materials may result in drastic degradation of in-service properties. In order to investigate this effect in high density and medium-low melting temperature (T _M) alloys, positron annihilation spectroscopy measurements were performed on helium-implanted gold–silver solid solutions after isochronal annealing treatments. Three recovery stages are observed, attributed to the migration and elimination of defects not stabilized by helium atoms, helium bubble nucleation and bubble growth. Similarities with other metals are found for the recovery stages involving bubble nucleation and growth processes. Lifetime measurements indicate that He implantation leads to the formation of small and over-pressurized bubbles that generate internal stresses in the material. A comprehensive picture is drawn for possible mechanisms of helium bubble evolution. Two values of activation energy (0.26 and 0.53 eV) are determined below and above 0.7T _M, respectively, from the variation of the helium bubble radius during the bubble growth stage. The migration and coalescence mechanism, which accounts for these very low activation energies, controls the helium bubble growth.     

Vacancy trapping and helium decoration in Sb ion-implanted tungsten studied by Mössbauer spectroscopy

Mössbauer effect measurements have been performed using sources of¹¹⁹Sb implanted in W without and with post-implanted helium. Each of the sources was subjected to an isochronal annealing sequence in order to study vacancy trapping, helium decoration and recovery of damage. Four sites have been identified for Sb implanted in tungsten; one of these corresponds with substitutional Sb atoms, two others are assigned to Sb atoms associated with vacancies, while the last one can be either vacancy or impurity associated. The development of site occupation as a function of annealing temperature is in accordance with the one-interstitial model. Injection of 2·10¹⁶ He/cm² leads to nucleation of helium bubbles. Helium atoms that are released from these bubbles at about 1300 K are retrapped by Sb atoms to form new bubbles.     

高密度氦相变的分子动力学研究

采用分子动力学方法结合对关联函数分析计算了0–1000 K范围内氦的固–液相变曲线, 与实验数据的对比显示, 在0–500 K之间与实验数据符合很好, 500 K以上还没有相应的实验数据. 另外, 计算了钛金属中不同尺寸氦泡的压强, 并与高密度氦的固–液相变曲线进行了对比. 结果显示, 在低温条件下, 随着温度的降低, 钛晶体中可能会出现固态氦泡; 在300 K以上不会存在固态氦泡.     

Helium implanted CuHf as studied by TDPAC and positron lifetime measurements

¹⁸¹Ta time differential perturbed angular correlation (TDPAC) and positron lifetime measurements were carried out on homogeneously α-implanted CuHf samples. TDPAC measurements indicate the trapping of vacancy clusters and helium associated defect complexes by Hf atoms. The presence of helium-vacancy complexes and helium stabilised voids has been identified by positron lifetime measurements. Further the nucleation and growth stages of helium bubbles have been identified. TDPAC and positron lifetime measurements indicate that Hf atoms act as heterogeneous nucleating centers for helium bubbles. Hf atoms are found to suppress the bubble growth in CuHf as indicated by the results of positron lifetime measurements.     

The density of helium in bubbles in implanted materials: Results from VUV absorption and eel spectroscopy

Abstract

The novel application of vacuum ultra-violet absorption spectroscopy and electron energy loss spectroscopy to helium bubbles in metals is presented. These measurements, carried out on thin aluminium films containing different concentrations of helium and various bubble size distributions, were aimed at determining the density (and thus pressure) of helium in bubbles by observing the shift and broadening of the IS-2P transition in the helium. The data coupled with a theoretical model developed by the authors (see following paper) indicate densities as high as 10²³ He cm^?3 for specimens containing small bubbles. Data are also presented on the effect that annealing and cooling have on these spectra. The annealing experiments give rise to fairly complex changes in absorption peak structure but with a general shift towards the unperturbed resonance line. The cooling experiment gives rise to a further shift and a narrowing of the absorption spectrum on cooling to 77 K which is tentatively identified as the liquid/solid transition in the helium. Finally, fluorescence spectrum of an Al/He specimen excited with low energy electrons is presented.