Thermal Helium Desorption Spectrometry Research Articles

Noble gas bubbles in bcc metals: Ab initio-based theory and kinetic Monte Carlo modeling

Understanding the interactions of noble gases with metals is of fundamental importance for the design of radiation-resistant structural materials for fission and fusion nuclear reactors. Here we present a unified theory for describing the energetics of He, Ne, Ar, and Kr bubbles in bcc metals in group 5B (V, Nb, Ta), 6B (Cr, Mo, W) and 8B (Fe). Our predictive analytical model is based on the effective-medium and isotropic elasticity theories, and is parameterized using density functional theory (DFT) calculations of small gas-vacancy clusters. By performing kinetic Monte Carlo (KMC) simulations driven by our analytical model, we have predicted the lifetimes of noble gas bubbles and their coarsening by Ostwald ripening. Our most notable finding is the exceptionally higher thermal stability of Ne, Ar and Kr bubbles than He bubbles in bcc metals, conferring them outstanding resistance to Ostwald ripening. The physical origin of the unexpected stability of bubbles formed by large noble gas atoms has been further elucidated. Our theoretical finding is consistent with the experimental observation of He gas bubble superlattice (GBS) coarsening under thermal annealing, and provides new insights on the exceptional stability of fission GBS in bcc U-Mo up to a high homologous temperature of 0.78. The present calculated results also compare favorably with the existing thermal helium desorption spectrometry experiments in the literature.

Regulating the helium bubble nucleation in the titanium tritides by environment temperature during the early aging period

The nucleation of helium bubbles in the early aging stage of tritium storage materials will have an important influence on the bubble growth behavior in the whole of aging. Herein, the nucleation of helium bubbles was adjusted by environment temperature at the first 15 days immediately after preparation of polycrystalline titanium tritide films. The influence of the environment temperature on the nucleation of helium bubbles were studied by X-ray diffraction (XRD), positron annihilation spectroscopy (PAS), thermal helium desorption spectroscopy (THDS) and scanning transmission electron microscopy (STEM). Results demonstrate that the helium bubble nucleation is significantly affected by the environment temperature of −60 °C, 20 °C and 120 °C. The higher the environment temperature, the lower number density of helium bubble in the titanium tritide films. The helium bubble nucleation mechanism is strongly correlated to the helium atom diffusion coefficient that is controlled by the environment temperature. During the following aging, the rate of helium atoms captured by a single helium bubble increases in titanium tritides with low number density bubbles, which accelerates the growth of helium bubbles in the matrix. However, the low number density bubbles also decrease the capture probability of free helium atoms by helium bubbles, resulting in the increase of helium atoms diffusing to infinite size defects such as grain boundaries macroscopically, and accelerating the evolution of helium in titanium tritides.

Defect evolution in ultralow energy, high dose helium implants of silicon performed at elevated temperatures

There is a growing interest in using high dose helium implants to alter point defect populations in silicon. Previous reports have shown that the interaction between helium and vacancies leads to the formation of cavities for medium energy (e.g., 20–100 keV) implants. However, the role of certain factors, such as the proximity of the surface, the damage created by the implant, and the effect of the implant temperature, is not well understood for low energy implants. This study explored a new regime of ultralow energy, elevated temperature implants in order to offer an insight into the effect of these parameters. Transmission electron microscopy (TEM) showed that cavity formation was avoided for 0.5 keV, 450 °C implants up to a dose of 8 × 1016 cm−2. However, extended defects in the form of {311} ribbon-like defects and stacking faults were observed. Quantitative TEM showed that the number of interstitials in these defects was less than 0.2% of the implant dose. In addition, thermal helium desorption spectrometry suggested that only 2% of the implanted He dose was retained in interstitial He and HemVn complexes. A first-order dissociation kinetic model was applied to assess desorption from HemVn, which closely matched energies predicted by density functional theory. This population of excess vacancies and excess interstitials was possibly formed because of incomplete Frenkel pair recombination. Raman spectroscopy showed that the stress from the implant was dominated by the stress from the interstitial-type defects. The evolution of the stress and defects was also explored as a function of post-implant annealing.

Effects of Y on helium behavior in Y-doped TiH2 films prepared by magnetron sputtering

TiH2 and yttrium-doped TiH2 alloy films with different helium concentrations prepared by magnetron sputtering were investigated using ion beam analysis (IBA), X-ray diffraction (XRD), thermal helium desorption spectrometry (THDS), positron annihilation spectroscopy (PAS) and nanoindentation techniques. It was found that through increasing the concentration of doped Y atoms, strong release peaks of helium will be found at about 820 K. The release at 820 K corresponds to the binding energy of the helium state on grain boundary indicating that the majority of the helium exists in the grain boundaries. In addition, the results of XRD and nanoindentation indicate that the doping of Y in the TiH2 films can improve the hardness and the elastic modulus of the films because of the fine-grain strengthening caused by the decreased size of TiH2 grains.

The evolution of helium from aged Zr tritides: A thermal helium desorption spectrometry study

The evolution of He from Zr-tritides was investigated for aging times up to about 6.5 years using analytical thermal helium desorption spectrometry (THDS). Zr films were deposited onto Mo substrates and then converted into Zr-tritides (ZrT1.70∼1.95) inside a tritiding apparatus loaded with pure tritium gas. During aging, there are at least five forms of He in Zr-tritides, and more than 99% of He atoms are in the form of He bubbles. The isolated He bubbles in lattices begin to link with each other when the He/Zr atom ratio reaches about 0.21, and are connected to grain boundaries or dislocation networks at He concentration of He/Zr ≈ 0.26. An interconnected system of channels decorated by bubbles evolves from the network dislocations, dislocation loops and internal boundaries. These He filled networks are formed completely when the He/Zr atom ratio is about 0.38. Once the He/Zr reached about 0.45, the networks of He bubble penetrate to the film surface and He begins an “accelerated release”. This critical ratio of He to Zr for He accelerated release is much greater than that found previously for Ti-tritides (0.23–0.30). The difference of He retention in Zr-tritides and Ti-tritides was also discussed in this paper.

Detection of helium in irradiated Fe9Cr alloys by coincidence Doppler broadening of slow positron annihilation

An element analysis method, coincidence Doppler broadening spectroscopy of slow positron annihilation, was employed to detect helium in ion-irradiated Fe9Cr alloys. Spectra with higher peak to background ratio were recorded using a two-HPGe detector coincidence measuring system. It means that information in the high-momentum area of the spectra can be used to identify helium in metals. This identification is not entirely dependent on the helium concentration in the specimens, but is related to the structure and microscopic arrangement of atoms surrounding the positron annihilation site. The results of Doppler broadening spectroscopy and transmission electron microscopy show that vacancies and dislocations were formed in ion-irradiated specimens. Thermal helium desorption spectrometry was performed to obtain the types of He traps.

Thermal desorption behavior of helium in aged titanium tritide films

The desorption behavior of helium in TiT(1.5∼1.8)−x3Hex film samples (x = 0.0022–0.22) was investigated by thermal desorption technique in vacuum condition in this paper. The thermal helium desorption spectrometry (THDS) of aging titanium tritide films prepared by electron beam evaporation revealed that, depending on the decayed 3He concentration in the samples, there are more than four states of helium existing in the films. The divided four zones in THDS based on helium states represent respectively: (1) the mobile single helium atoms with low activation energy in all aging samples resulted from the interstitial sites or dissociated from interstitial clusters, loops and dislocations, (2) helium bubbles inside the grain lattices, (3) helium bubbles in the grain boundaries and interconnected networks of dislocations in the helium concentration of 3Hegen/Ti > 0.0094, and (4) helium bubbles near or linked to the film surface by interconnected channel for later aging stage with 3Hegen/Ti > 0.18. The proportion of helium desorption in each zone was estimated, and dissociated energies of helium for different trapping states were given.

Effect of combined doping of Fe and C on the helium behavior in Al before and after aging

Effect of combined doping of Fe and C on the implanted-helium (10keV, 4.0×1021ions/m2) behavior in Al before and after aging for 1year at room temperature has been investigated by scanning electron microscope, transmission electron microscope and thermal helium desorption spectrometry (THDS). The combined doping of Fe and C has a pronounced influence on the surface blistering induced by He ion implantation, and the extent is dependent on the fluence of Fe and C doped. The lower fluence of Fe and higher fluence of C doped, the more resistive to the surface blistering in Al will be. Pipe-shaped structures of high density are produced in the matrix of He implanted Al after the combined doping of Fe and C and then aging at room temperature for 1year. The diameter of pipes increases with Fe fluence. The THDS results show that the thermal helium desorption shifts to the lower temperature range for the Fe, C and He implanted and aged Al samples. With the increase of C fluence, the thermal helium desorption in the lower temperature range will get more prominent. Therefore, aging has a considerable influence on the helium behaviors such as surface blistering, microstructure evolution and thermal desorption in Al, which can deteriorate the irradiation damages induced by He.

Positron and thermal desorption studies on He ion implanted nuclear graphite

The positron beam Doppler Broadening (DB) and Thermal Helium Desorption Spectroscopy (THDS) techniques are applied to study the behavior of radiation induced point defects in IG-110 nuclear graphite. The defects are introduced by irradiation at room temperature with 200keV He+ at doses ranging from 1015 to 1017 He/cm2. In the thermal desorption spectroscopy, the release of He+ is observed between 500K and 800K. With increasing He+ implantation dose, the fraction of He+ desorbed decreases from 27% to 3%. In the DB-curves showing the S parameter values versus positron implantation depth, the derived vacancy type defect distribution are in accordance with those obtained by SRIM calculations. Subsequent annealing of the implanted samples in steps of 100K for 5 minutes up to 1200K shows a distinct decrease of the S parameter value to its reference value between 500K and 700K. This temperature interval corresponds to the literature values for single vacancy migration energy of 1-2eV.

Quantifying He-point defect interactions in Fe through coordinated experimental and modeling studies of He-ion implanted single-crystal Fe

Understanding the effects of helium on the microstructural evolution and mechanical properties of structural materials are among the most challenging issues in fusion materials research. In this work, we combine thermal helium desorption spectroscopy (THDS) with positron annihilation spectroscopy (PAS) and a spatially dependent cluster dynamics model to investigate the energetics of helium-point defect interactions in helium-implanted single-crystal iron. The combination of modeling and thermal desorption measurements allows identification of the binding energies of small He–V clusters, the migration energy of single vacancy and possible mechanisms (e.g., shrinkage of He3V2 clusters) responsible for measured Helium desorption peaks, and the effect of impurities (e.g., carbon) on these values. Furthermore, the model predicts the depth dependence of the helium and helium–vacancy clusters as a function of time and temperature during the thermal desorption measurement. Here, we report the THDS measurement results as a function of He implantation energy from 10 to 40keV at a fluence level of 1×1015He/cm2, along with selected PAS measurements. The experimental results are compared to the modeling predictions to evaluate the extent to which self-consistent values of the He-point defect binding and interaction energies and diffusivities can explain the data.

Thermal Annealing Behavior of Helium in Ti Films Deposited by Magnetron Sputtering

Helium contents of up to 30at.% are prepared in sputter-deposited Ti films. Isochronal annealing behaviors of helium including the depth profiles and the evolution of helium bubbles in the films at different temperatures are examined by ion beam analysis including Rutherford backscattering spectrometry (RBS) and elastic recoil detection analysis (ERDA), as well as thermal helium desorption spectroscopy (THDS). It is found that the energy spreading induced by structural inhomogeneities in the spectra of RBS and ERDA as well as the increment in the width of spectra occurs, which corresponds to the change of stopping cross-section of helium atoms in the Ti film due to the change of physical-state of helium in the evolution of helium bubble. The ion beam analysis on the helium evolution is consistent with the THDS measurement. Ion beam technique opens interesting possibilities in the characterizing on the growth of helium bubbles.

Effect of Fe and C doping on the thermal release of helium from aluminum

The effect of Fe and C doping on the thermal release of helium from Al implanted with 10 keV, 4.0 × 10 21 ion/m 2 He at room temperature (RT) has been investigated by thermal helium desorption spectrometry (THDS) and transmission electron microscope (TEM). The results show that Fe and C doping have significant impact on the release of helium from Al and the extent depends on the doping fluence. Proper fluence of Fe and C doping would lead to the retardation of the release of helium from Al, while excessive fluence of Fe and C doping would result in more desorption peaks and the release of helium in lower temperature ranges. Fe and C doping have different influence on the release of helium from Al, and the difference is related with the secondary phases forming in the samples.

Modeling spatially dependent kinetics of helium desorption in BCC iron following He ion implantation

One of the most important tasks in fusion materials research is to study the effect of helium on the microstructure and mechanical property evolution of structural materials. Thermal helium desorption spectrometry (THDS), through measuring the outward He surface flux as a function of temperature (or time), provides indirect information about the kinetics and energetics of helium transport and trapping/detrapping which is important for developing a predictive assessment for the life performance of fusion reactors. Nevertheless, THDS data interpretation is not straightforward, particularly when a broad temperature regime and a high He concentration are concerned. Here we present results from a spatially-dependent rate theory modeling framework in coordination with our previous THDS experiments on single crystal iron implanted with 4He + ions at 5 or 10 keV to fluences of 10 18 or 10 19 He/m 2. The model incorporates both temporally and spatially dependent diffusion, trapping, and detrapping (emission) kinetics for both implantation process and post-implantation thermal annealing. Possible desorption sequences/mechanisms are discussed.

Defects of ultrathin Cu films on Mo(1 1 0) studied by thermal helium desorption spectrometry

Defects of ultrathin Cu films (3–200 Å) deposited on Mo(1 1 0) at room temperature by e-beam evaporation in ultra-high vacuum are investigated using thermal helium desorption spectrometry. The samples are analysed with both 75 and 1000 eV He+ implantation. Cu films transform into islands on annealing and the temperature of this transformation is strongly thickness dependent. Helium release from defects close to the surface of the Cu films (∼450 K), from monovacancies in as-deposited Cu film (50–200 Å), and from defects close to the interface (800–1050 K) are identified. Annealing of monovacancies is mainly responsible for reduction in helium trapping in a 20 Å Cu film prior to film islanding. There is an indication of retrapping of helium released from the first 5 Å layer of the film in the defects located in the overlayers (5–95 Å). Helium in the Cu films survives until the desorption temperature of the film (1200–1300 K).

Study of defects and thermal stability of ultrathin Cu films on Ta(110) and Ta(100) by thermal helium desorption spectrometry

Thermal helium desorption spectrometry has been used to study the interaction of helium with defects in Cu films (5–300 Å) deposited on Ta(110) and Ta(100) single crystals by ultrahigh vacuum electron beam evaporation. The thermal stability of the Cu films was also investigated. Cu films on Ta(110) and Ta(100) at room temperature are metastable and on heating, the films transform into islands. The temperature at which this takes place is strongly dependent on the Cu film thickness and for a given thickness (> 40 Å) occurs at a lower temperature on Ta(100) than on Ta(110). The activation energy for island formation is 1.6 ± 0.4 eV for 50 Å Cu/Ta(110) and 0.8 ± 0.1 eV for 100 Å Cu/Ta(100) obtained by Kissinger analysis. The geometry of the Cu islands resulting from annealing 50 Å Cu films at 1000 K for 10 s depends strongly on the Ta substrate orientation. There is evidence for the stressed states of both the Cu films and the Ta substrates. Helium release from monovacancies and vacancy clusters in Cu films (> 75 Å) on Ta(110) and Ta(100) was detected at ~ 750 K and ~ 800–1000 K respectively. The sublimation of the Cu films from the Ta substrates could be observed by the release of retained helium at ~ 1300 K.

Effects of interstitial impurity on behavior of helium-defect complexes in vanadium studied by THDS

The effects of helium on macroscopic mechanical properties in vanadium alloys have been studied rather extensively. However, the evolution processes of helium-defect complexes, especially during the early stage, is not clearly understood. This is mainly because of the complexity of helium behavior due to the large amount of interstitial impurities (C, N, O, etc.) in vanadium. THDS (thermal helium desorption spectrometry) was performed to examine the nature and behavior of helium-defect complexes. The observed desorption peaks were assumed to be concerned with vacancy type defects. In vanadium, most of vacancies are decorated by interstitial impurities, and implanted helium produces He n V n X-type defects (X = C, N, O). The desorption peaks of 570 K, 690 K and 940 K were assigned as He n OV, He n OV 2 and He n OV 4, respectively. The population of these peaks increased with oxygen concentration. On the other hand, some peaks which were independent of impurity concentration were deduced as impurity free defect clusters such as He n V n .

Thermal helium desorption spectrometry of helium-implanted iron

Thermal helium desorption spectrometry (THDS) is employed to study the kinetics and energetics of helium in iron implanted with 100 keV He to 1 × 10 11, 1 × 10 13, and 1 × 10 15 He/cm 2. While no clear desorption signals are observed for the two lower dose samples, preliminary results reveal that the majority of the He atoms desorb at ∼1000 °C and at >1100 °C for the 1 × 10 15 He/cm 2 sample. Both conventional reaction model and Johnson–Mehl–Avrami (JMA) transformation model are utilized to analyze the ∼1000 °C desorption event. The measured signal can be reproduced very well by combining a first-order and a higher order ( n ∼ 5.8) JMA fits. This event is observed repetitively during successive heating–cooling cycles. The success of the higher order JMA model and the repetitive appearance strongly suggest that this event is mainly associated with α ↔ γ phase transformation, while the dissociation of HeV clusters may also be involved. The observation of spurious peaks which may affect future studies is also reported.

Investigations of helium incorporated into a film deposited by magnetron sputtering

Helium-containing titanium film was prepared by magnetron sputtering in a gas mixture of helium and argon. This method distinctly differs from traditional helium introduction into metals. A high concentration of helium up to 45 at.% and a uniform distribution in the bulk were shown by ion beam analysis. The process of helium incorporation is considered in which the helium particles are energetically backscattered from the target, fly through the glow discharge zone and are consequently embedded into the growing film. It is virtually a continuous sub-threshold-energy implantation during film deposition based on the plasma technique. The limited partial pressure of argon is to maintain the discharge and sputter the titanium target. The helium content incorporated can be controlled conveniently by adjusting the pressure of the working gases. X-ray diffraction, transmission electron microscopy, Doppler broadening of positron beam analysis and thermal helium desorption spectroscopy revealed the microstructure of titanium films, the state of helium bubbles, the defect characteristic related to helium and thermal release behaviour of helium from the titanium film.

Defects and morphological changes in nanothin Cu films on polycrystalline Mo analyzed by thermal helium desorption spectrometry

Thermal helium desorption spectrometry (THDS) has been used for the investigation of defects and thermal stability of thin Cu films (5–200Å) deposited on a polycrystalline Mo substrate in ultrahigh vacuum. These films are metastable at room temperature. On heating, the films transform into islands, giving rise to a relatively broad peak in the helium desorption spectra. The temperature of this island formation is dependent on film thickness, being 417K for 10Å and 1100K for a 200Å film. The activation energy for island formation was found to be 0.3±0.1eV for 75Å film. Grain boundaries have a strong effect on island formation. The defect concentration in the as-deposited films is ∼5×10−4, for films thicker than 50Å and more for thinner films. Helium release from monovacancies was identified in the case of a 200Å film. Helium release was also seen during sublimation of the Cu film (∼1350K). Overlayer experiments were used to identify helium trapped close to the film surface. An increase of the substrate temperature during deposition resulted in a film that had already formed islands. Argon-ion assistance (250eV) during film deposition with an ion/atom ratio of ∼0.1 resulted in a significant enhancement of helium trapping in the films. The argon concentration in the films was found to be 10−3. The temperature of island formation was increased due to argon-ion assistance. The helium and argon desorption spectra are found to be similar, which is due to most of the helium becoming trapped in the defects created by the argon beam. The role of the Mo surface in affecting the defects at the film-substrate interface is investigated. The effect of variation of helium fluence and helium implantation energy is also considered. The present THDS results of Cu/poly-Mo are compared to those of Cu∕Mo(100) and Cu∕Mo(100) reported earlier.

Defects and morphological changes in nanothin Cu films on Mo(100) studied by thermal helium desorption spectrometry

Thermal helium desorption spectrometry has been used for a comprehensive experimental investigation of vacancy-related defects and thermal stability of nanothin Cu films (10–300Å) deposited on a Mo(100) substrate at 300K in UHV. In the helium desorption spectra (300–2000K), a peak is observed that shows that the Cu films are highly unstable. Postdeposition annealing treatments show that this peak is due to the thermally-induced island formation of the copper films. This process is strongly dependent on film thickness; its temperature ranging from 429K for a 10-Å film to 1312K for a 300-Å film, which is not simply due to the considerable change of stress with film thickness. Between 70 and 75Å, there is a distinct narrowing of the peak shape, indicating a change in the kinetics. For a 100-Å film lying in the large thickness range, an activation energy E=1.2±0.1eV was determined. The island-formation process is defect-mediated, as suggested by the fact that the temperature is lowered by an increase of the helium implantation energy and fluence. The island formation during deposition at an elevated substrate temperature occurs at a lower temperature than during postdeposition annealing. The helium released from monovacancies (E=1.8±0.2eV) and vacancy clusters in the Cu films and during high-temperature evaporation of the Cu films from the Mo substrate (Esubl=3.5±0.5eV) is also identified. The x-ray diffraction measurements on 60 and 200-Å films show that they are both fcc Cu(100) films with identical macrostrains; the 200-Å film has a sharper texture than the 60-Å film. The orientation relation between the film and the substrate was found to be (100)[100]fccCu‖(100)[110]bccMo.