Helium Desorption Research Articles

Retention and Desorption of Hydrogen and Helium in Inner Wall Material Used for The Large Helical Device (LHD)-Simulation experiments using a glow discharge apparatus-

In order to examine the behavior of hydrogen and helium during a glow discharge in Large Helical Device (LHD), simulation experiments using a glow discharge apparatus were conducted. The hydrogen and helium glow discharges were repeated alternately to the wall of a liner installed in the discharge apparatus, and then the retained and reduced gas amounts were measured. The amount of retained helium was approximately 1/4 of retained hydrogen. It was obvious that the retention of hydrogen or helium significantly depended on the discharge sequence. The reduction of retained helium due to hydrogen discharge was very small, whereas hydrogen retention was largely reduced by the helium discharge. The amounts of retained hydrogen and helium decreased with an increase in the liner temperature, whereas the amount of impurity desorption increased with the liner temperature.

Formation, growth and dissociation of He bubbles in Al 2O 3

The formation and dissociation of helium bubbles and helium desorption are investigated in sapphire Al 2O 3(0 0 0 1) implanted with 30 keV He ions to four different doses of 0.1, 0.3, 1.0 and 2.0 × 10 16 ions cm −2. The samples were annealed isochronally up to 1850 K in steps of 100 K. The techniques of Doppler broadening positron beam analysis (PBA) and neutron depth profiling (NDP) were used to investigate defect evolution and helium retention, respectively, during the annealing procedure. It was observed that the maximum bubble volume is found after 1250 K annealing, after which a process of bubble shrinkage sets in. Cross-sectional transmission electron microscopy (XTEM) was performed on the sample that was implanted with the highest-dose (2.0 × 10 16 He ions cm −2) after annealing at 1250 K. It was found that the bubbles are shaped as discs lying parallel with the surface and that the average bubble size is 5.5 nm. In all samples, helium is released mainly at a temperature of 1750 K. The desorption curves were analyzed by means of a permeation model. The activation energy for permeation was found as 4.0 eV.

Helium Accumulation Behavior in Iron Based Model Alloys

Helium desorption from Fe-based model alloys irradiated by energetic helium ions was measured during post-irradiation annealing to investigate the energetics and kinetics of formation and annihilation of helium-related defects. Desorption temperatures were observed to be widely ranged from 450 to 1500 K, indicating that helium is bound to a wide variety of trapping sites such as vacancies and dislocations at various binding states. Such a feature is also observed in fusion ferritic steel. A comparison of helium desorption spectra obtained using Fe, Fe-Cr and Fe-Cr-Ni alloys showed that helium is more strongly trapped in bcc Fe than fcc Fe. It indicates that the long distance migration of helium takes place less frequently in bcc matrix, which may reduce the probability of helium clustering. Fusion ferric steel has a lot of trapping sites for helium such as dislocations, solute atoms, the interface of precipitates, impurities and lath boundaries, and so on, and in addition, it has bct matrix, indicating that most of helium atoms must be dispersed in the matrix and therefore it is difficult for them to cluster as a bubble. This may be a reason for higher helium resistance of the steel.

Thermal annealing behaviour and defect evolution of helium in fully stabilised zirconia

Helium implantations in fully stabilised zirconia have been performed with 30 keV ions with high doses (5.1 × 10 16 and 2.6 × 10 16 cm −2) and low doses (6.3 × 10 15 and 1.7 × 10 15 cm −2). The retained amount of helium and depth profiles have been monitored with neutron depth profiling and the damage distribution with positron beam analysis (PBA) after several annealing steps. The temperature dependent helium release was investigated by thermal helium desorption spectrometry. In the low dose samples, helium is released through diffusion as seen by a broadening of the helium distribution peak. PBA, performed with a two-layer model, shows shrinkage of the damage layer during annealing. For the high dose samples the helium peak does not broaden after annealing. Helium is retained up to high temperatures which is ascribed to bubble formation during thermal annealing. Fitting of the PBA results to a three-layer model shows that the ion implanted layer gets narrower after annealing at 600 K, up to 1000 K the S-parameter is increasing because helium is released from the bubbles, whereby vacancy clusters are left behind.

Helium implantation in silicon: the effects of implantation temperature

Thermal helium desorption spectrometry (THDS) was used to study the effects of implantation temperature on bubble formation in helium implanted silicon at 5×10 16 cm −2 with 50 keV, from RT to 800 °C. Both isochronal and partial annealing schemes have been used and have allowed to raise important process and behaviour. Helium release from bubbles/cavities in silicon is well described by the permeation from cavities to the surface. Entropy effects play a major role in the helium desorption from bubbles. Partial desorption results show that the release of helium by permeation from bubbles/cavities to the surface has an activation energy of 1.8 eV whatever the implantation temperature may be, but also whatever the implantation energy may be. These results are discussed in highlight of the transmission electron microscopy (TEM) observations. As for example, an implantation temperature of 600 °C is needed to create cavities empty of gases. However, with increasing temperature more and more extended defects are formed. Finally at 800 °C, no cavities or bubbles are created but only extended defects.

Adsorption and desorption of helium in aerogels

We have studied the adsorption and desorption of helium in a 95% porosity aerogel. The helium density in the aerogel was directly measured with a capacitive technique, while the pressure and bulk helium density were measured with in situ capacitive gauges. Thermal response is slow in aerogels, so we used a thin sample to minimize the time constant. The combination of high resolution and rapid equilibration allowed us to study the capillary condensation of helium, which in aerogels occurs very close to the bulk saturation pressure. We saw hysteresis between filling and emptying of the pores, even for very slow rates. The hysteresis becomes smaller as the critical temperature is approached. We discuss whether true two phase coexistence can be observed in the helium–aerogel system.

Blister formation and erosion due to blister fracture of SiC or Si

For silicon wafer and silicon carbide, helium ion irradiation followed by annealing was repeated, and the weight loss was measured. The blister formation and its fracture processes were also examined. For Si, many blisters were observed at the surface after the first ion irradiation. After the annealing at 1173 K after the irradiation, small pores formed by helium desorption were observed. With the increase of the cycle number, surface roughness gradually increased. The weight loss due to annealing was about a half of that by ion sputtering. The sputtering yield due to the ion irradiation was approximately twice the reference value in the present experiment, perhaps due to the bubble fracture during the irradiation. For SiC, small blisters were observed on SiC crystals after the ion irradiation. The erosion amount due to only the process of annealing at 1323 K was several times larger than that of ion sputtering, although the sputtering yield due to the ion irradiation was roughly the same as the reference value. The present erosion was significantly large, so that this erosion has to be taken into account regarding the wall lifetime and impurity level of the core plasma.

Surface morphology and helium retention on tungsten exposed to low energy and high flux helium plasma

Surface modification and helium retention on powder metallurgy tungsten and plasma sprayed tungsten coated CFC exposed by low energy (100 eV) high flux (3.83×10 21–1.20×10 22 m −2 s −1) helium plasma have been investigated. Fluence is about 10 26 He m −2. It is found that fine surface morphology change occurs by the helium exposure. There is little difference of the surface modification between the powder metallurgy tungsten and the plasma sprayed tungsten. In the case that the surface morphology do not changed, two helium desorption peak appeared. The amount of desorption of helium is larger than that of deuterium from tungsten under the same exposure condition by heating up to 1273 K by TDS. In the case that fine surface morphology change occurs, desorption peak at high temperature newly appears. Surface modification such as blister is not formed on the surface exposed to helium under the same exposure condition which the blister is formed by deuterium exposure.

Ultrathin Cu Films on Mo(110) Characterized by Helium Implantation

ABSTRACTPoint defects and thermal stability of ultrathin Cu films (3–200 Å) deposited on Mo(110) substrate at 300 K have been characterized using thermal helium desorption spectrometry (THDS). Implantation of the samples (10–100 Å Cu/Mo) with 1000 eV He+ aided in detecting He release from monovacancy in Cu films and desorption of the films (above 1200 K) from Mo substrate. A thickness dependent peak is identified in the helium desorption spectra, which is shown to be due to the process of island formation in the Cu films, using pre-implantation annealing treatments for the 40 Å film. Native defects in the films (3–200 Å) were probed using 75 eV He+ implantation.

Helium implantation defects in SiC: A thermal helium desorption spectrometry investigation

Thermal helium desorption spectrometry was used to characterized helium implantation-induced defects in SiC. 6H–SiC, 4H–SiC, and β–SiC samples were implanted with helium at energies ranging from 100 to 3 keV and doses ranging from 1×1013 to 1×1015 cm−2. They were then subjected to ramp annealing up to 1800 K, with a constant heating rate of 10 K/s. Two groups of peaks contribute to the desorption spectrum: A low-temperature group centered at 600 K and a high-temperature group centered at 1200 K. The evolution of these desorption peaks with implantation dose and energy was studied. The first group (at 600 K) might be attributed to interstitial He and clusters of interstitial He. The second group (at 1200 K) could be related to He release from He-vacancy clusters. A shift of the latter group toward higher temperatures with increasing dose is ascribed to He-vacancy clustering in an Ostwald ripening process. However, the intrinsic properties of the materials used in the study seem to play an important role since different detrapping behaviors were observed for the different samples.

Thermal stability of helium–vacancy clusters in iron

Molecular dynamics calculations were performed to evaluate the thermal stability of helium–vacancy clusters (He n V m ) in Fe using the Ackland Finnis–Sinclair potential, the Wilson–Johnson potential and the Ziegler–Biersack–Littmark–Beck potential for describing the interactions of Fe–Fe, Fe–He and He–He, respectively. Both the calculated numbers of helium atoms, n, and vacancies, m, in clusters ranged from 0 to 20. The binding energies of an interstitial helium atom, an isolated vacancy and a self-interstitial iron atom to a helium–vacancy cluster were obtained from the calculated formation energies of clusters. All the binding energies do not depend much on cluster size, but they primarily depend on the helium-to-vacancy ratio ( n/ m) of clusters. The binding energy of a vacancy to a helium–vacancy cluster increases with the ratio, showing that helium increases cluster lifetime by dramatically reducing thermal vacancy emission. On the other hand, both the binding energies of a helium atom and an iron atom to a helium–vacancy cluster decrease with increasing the ratio, indicating that thermal emission of self-interstitial atoms (SIAs) (i.e. Frenkel-pair production), as well as thermal helium emission, may take place from the cluster of higher helium-to-vacancy ratios. The thermal stability of clusters is decided by the competitive processes among thermal emission of vacancies, SIAs and helium, depending on the helium-to-vacancy ratio of clusters. The calculated thermal stability of clusters is consistent with the experimental observations of thermal helium desorption from α-Fe during post-He-implantation annealing.

Microstructure development and helium behavior in nickel and vanadium base alloys

Transmission electron microscopy and thermal helium desorption spectrometry (TDS) have been used to investigate the influence of alloying elements on helium behavior and bubble microstructure evolution in FCC (Ni+1…7.5 wt%Al) and BCC (V+10…40 wt%Ti) metals. The samples were irradiated by 40-keV He + ions at room temperature up to a fluence of 5×10 20 m −2. Post-irradiation annealings were performed at 1023 K (Ni–Al) and 1075 K (V–Ti) for 1 h. It was shown that alloying elements reduced the bubble size ( d ̄ b ) and increased their density ( ρ b) in both types of alloys. In the Ni–Al alloys the TDS peaks are displaced to higher temperatures with increasing Al concentration in contrast to V–Ti alloys where the TDS peaks are displaced to lower temperatures with increasing Ti content. However in both systems of alloys the effective activation energy for helium desorption grows with alloying element concentration. The results are discussed in terms of alloying element influence on the mechanisms of bubble growth and migration.

High resistance to helium embrittlement in reduced activation martensitic steels

Reduced activation martensitic steels (RAMSs) are the prime candidate structural material for the DEMO reactor and beyond where the material has been considered to suffer severe embrittlement caused by high-dose neutron irradiation and several thousands appm of transmutated helium. However, recent several works show high resistance to helium embrittlement of RAMS. Good performance of RAMS in the presence of rather high concentration of helium is considered to be due to high trapping capacity for helium atoms in the martensitic structure that consists of dislocations, lath boundaries, grain boundaries and carbide/matrix interfaces. To make clear the role of dislocations in trapping helium, thermal helium desorption spectra were measured for iron specimens annealed at different temperatures after cold work. A desorption peak, which increased its height with increasing dislocation density, was observed at around 550 °C, suggesting that dislocations trap helium atoms. A molecular dynamics simulation study for investigating the helium trapping behavior at helium–vacancy complexes suggests that helium is rather strongly bound to the complexes and increases the binding energy of vacancy to the complex, resulting in increasing stability of the complexes at elevated temperatures by reducing thermal emission of vacancies.

Effects of dislocation on thermal helium desorption from iron and ferritic steel

Thermal desorption measurements were performed to investigate helium trapping in α-iron and a reduced activation martensitic steel (RAMS) bombarded at room temperature with mono-energetic He + ions. Incident energies were both 8 keV and 150 eV. Prior to helium implantation, the samples of α-iron were plastically deformed by rolling at room temperature followed by annealing at 673, 873 and 1073 K for 2, 12 and 2 h, respectively. These samples are hereafter called PR1, PR2 and FA, respectively, and the as-rolled sample is called CW. The dislocation densities of the samples decrease in the order of CW, PR1, PR2 and FA. Thermal desorption spectra show a clear peak at around 800 K, only for the CW, PR1, PR2 and RAMS samples, which may correspond to desorption of helium atoms trapped by dislocations. The effects of dislocations on micro-structural evolution in iron and the steel during helium implantation are discussed.

On the effects of implantation temperature in helium implanted silicon

He + ions were implanted into silicon with a fluence of 5×1016 cm−2 at different temperatures ranging from 473 to 1073 K. Samples were analyzed by thermal helium desorption spectroscopy and by transmission electron microscopy. As far as cavity formation is concerned, the behavior can be divided into three stages depending on the implantation temperature. However, it is found that helium release from cavities is governed by a single mechanism regardless of the implantation temperature.

A hot implantation study on the evolution of defects in He ion implanted MgO(1 0 0)

Ion implantation at elevated temperature, so-called hot implantation, was used to study nucleation and thermal stability of the defects. In this work, MgO(1 0 0) single crystal samples were implanted with 30 keV He ions at various implantation temperatures. The implantation doses ranged from 10 14 to 10 16 cm −2. The implantation introduced defects were subsequently studied by thermal helium desorption spectroscopy (THDS) and Doppler broadening positron beam analysis (PBA). The THDS study provides vital information on the kinetics of He release from the sample. PBA technique, being sensitive to the open volume defects, provides complementary information on cavity evolution. The THD study has shown that in most cases helium release is characterised by the activation energy of Q=4.7±0.5 eV with the maximum release temperature of T max=1830 K. By applying first order desorption model the pre-exponent factor is estimated as ν=4.3×10 11 s −1.

Recoil tritium–C 60 interaction: a channel for endohedral encapsulation of tritium in C 60

The introduction of tritium (T), with an initial kinetic energy of 2.7 MeV into the cavity of C 60 using recoil implantation is studied in a 6Li(n,α)T activated homogenized compound matrix of Li 2CO 3 and C 60. Radioactive endohedral T@C 60 could be detected through the liquid scintillation spectrometry coupled with high-pressure liquid chromatography (HPLC). Fast atom bombardment mass spectrometry provided evidence for the retainment of C 60 cage as against radiation damage. Solid state temperature-programmed desorption based on evolved gas showed the desorption of 3He gas (as a β − decay product from T) from the endohedral C 60 cage at a higher temperature than the T 2 and T 2O trapped species at the defect sites and from the ordered crystallographic network of Li 2CO 3, respectively. The non-isothermal kinetics of the helium desorption, as a function of temperature evidenced a diffusion controlled process.

Molecular dynamics study of Cu deposition on Mo and the effects of low-energy ion irradiation

Molecular dynamics simulations were performed to study the growth of Cu films on a Mo(1 1 0) substrate, both by thermal evaporation and by ion-beam assisted deposition (100 eV Ar assistance). The growth mode of the Cu films goes to a 2D–3D–2D transition; the evolving microstructures are also discussed. The influence of the energetic ion impacts is significant, notably affecting wetting phenomena, but also leading to a different film structure. The effects of 75 eV He ion irradiation of a few completed films are also investigated, to help assessing the information on the defect state of the deposited films that is provided by thermal helium desorption experiments.

Influence of Depth in Helium Desorption from Cavities Induced by <sup>3</sup>He Implantation in Silicon

Influence of Depth in Helium Desorption from Cavities Induced by <sup>3</sup>He Implantation in Silicon

Helium implantation defects in SiC studied by thermal helium desorption spectrometry

Thermal helium desorption spectrometry (THDS) was used to characterize helium implantation induced defects in SiC. 6H–SiC and 4H–SiC single crystals were implanted with helium at energies ranging from 100 eV to 3 keV and doses ranging from 10 13 to 10 15 cm −2 . They were then subjected to ramp annealing up to 1800 K, with a constant heating rate of 10 K s −1. To the desorption spectrum contribute two groups of peaks: one group at low temperature centered at 600 K and another group at high temperature centered at 1200 K. The evolution of these desorption peaks with implantation dose and energy has been studied. The first group (at 600 K) might be attributed to interstitial He and clusters of interstitial He. The second group (at 1200 K) could be related to the de-trapping of He from He–vacancies clusters. A shift of the latter group towards higher temperatures observed for increasing dose is ascribed to He–vacancy clustering in an Ostwald ripening process.