Dose rate and temperature effects on blistering phenomena in helium bombarded niobium

Blistering of niobium under bombardment with 5 to 15 keV He ions has been investigated. At these energies there is only one generation of blisters, which are sputtered and give way to a microrelief. A high dose rate of 3–5 mA/cm² has been used to find the energy and temperature dependance of the blistering cut-off effect. At 20° C this cut-off dose increases from about 1 to 3 C/cm² when the energy is increased from 5 to 15 keV. The temperature effect has been investigated with 10 keV ions. The cut-off dose decreased to about 0.2 C/cm² at 700° C. Since in a fusion device, the dose rate may be much smaller than the one used above, the dose rate effect has been investigated. With a dose rate reduced to 0.05 mA/cm² the critical dose for blistering and the cut-off dose have not been found to vary by any large amount.     

Graphite surface erosion by 100 keV helium and hydrogen bombardment

Surface erosion in pyrolytic graphite by 100 keV ⁴He⁺ and 200 keV H⁺₂ ion bombardment has been observed by scanning electron microscopy. The particle fluence ranged from 1 × 10¹⁷ to 5 × 10¹⁸ particles/cm². Although the surface is eroded at 1 × 10¹⁷ particles/cm² in helium bombardment, it is not eroded so heavily even at 5 × 10¹⁷ particles/cm² in hydrogen bombardment. In helium bombardment flaking is significantly observed at 1 × 10¹⁸ particles/cm², and a cone structure appears at 5 × 10¹⁸ particles/cm², which is produced after the first cover flakes off completely. In hydrogen bombardment at 1 × 10¹⁸ particles/cm², many circular blisters are formed which are sputtered off at 5 × 10¹⁸ particles/cm². The surface roughness of the target also affects the erosion.     

Measurement of the depth distribution of light impurities in first-wall materials: He in Nb

We have measured the concentrations and depth profiles of implanted helium in niobium by a method demonstrated previously with hydrogen and lithium in copper. The three targets, bombarded at room temperature with 10 keV He⁺ at doses of 0.01, 0.16 and 0.98 C/cm⁺, were respectively: unblistered; covered with circular blisters; and marked with “microrelief”, without blisters. The corresponding doses retained in the metal were 0.0076, 0.039 and 0.052 C/cm² (i.e.≈3 × 10²² He atoms/cm³) with a 10% normalization uncertainty. The profile shapes did not change much: in particular we did not observe, as the dose increased, an accumulation near the surface, which is receding by erosion (sputtering, blistering). These results show that a mechanism of helium loss starts operating at a dose ?0.16 C/cm², i.e. before the bursting of blisters (if they burst at all), and it is most effective near the surface.     

Erosion yield of Ti from TiC-deposited graphite by hydrogen ion bombardment at high temperatures

Erosion yields of Ti atoms from a TiC-deposited graphite by bombardment with 1 keV hydrogen ion beam of various current densities at 900°C have been investigated by means of the Rutherford backscattering (RBS) technique. It has been observed that the sputtering yields for Ti atoms at 900°C are almost zero below a critical ion flux of 1 × 10¹⁵/cm²·s, compared with the sputtering yield of Ti atoms at room temperature which has been measured to be 1 × 10⁻²atoms/ion. No sputtering of Ti atoms observed at 900°C is explained in terms of self-sustaining coating of the TiC surface with segregated carbon layer. The condition for the self-sustaining coating is discussed.     

Depth distribution and migration of helium in vanadium at elevated temperatures

The depth profiles and retention behavior of ⁴He implanted at 80 keV into vanadium have been investigated in cold rolled bulk and foil samples. The specimens were implanted at temperatures of 100, 400, and 800° C and at fluences up to 1 × 10¹⁸He/cm². Helium depth distributions were found to be Gaussian for the 100° C implants but exhibited a double peak appearance for some of the highest fluence 400° C and 800° C implants. In polished samples blistering and flaking occurred at both 100° C and 400° C implant temperatures for fluences greater than 5 × 10¹⁷He/cm², but little of the implanted helium appeared to have been released at a result of the exfoliation. Conversely, most of the implanted helium was released from 800 C implant areas though no significant blistering or surface perforation was observed. Blistering was observed to occur only on polished samples, through the release behavior was similar to that of the unpolished specimens on which no blistering was observed at any temperature or fluence. The surface condition of samples polished by different techniques was found to exert an influence on the temperature dependence of release and on the character of the blister topography.     

Preliminary observations of blistering of niobium by 1–15 keV helium ions

We have bombarded at room temperature polycrystalline targets of cold-rolled Marz grade niobium with beams of helium ions of 1, 5, 10 and 15 keV at doses varying from 0.1 to 10.0 C/cm². We have chosen the doses so as to obtain surfaces where the thickness removed by sputtering is respectively less than, comparable to, and greater than the implantation depths. The targets were not further treated beyond mechanical polishing. The vacuum during bombardment was 2 × 10^?8 torr. Blisters of 0.1 to 3.0 μm size have been observed. The blister covered area increases with dose up to a point, but at the higher doses the blisters, if any, are masked by a micro-relief of wide valleys, presumably sputtering-induced, and tiny pock-marks which could be small ruptured helium blisters.     

Room temperature aging effects of blistering and surface roughening after Ar+ ion bombardment on Mo single crystals

300 keV Ar⁺ ions are bombarded on the surface of Mo single crystals to doses of $\text{1 × 10}{}^{17}$, $\text{6.2 × 10}{}^{17}$, $\text{1 × 10}{}^{18}$ and $\text{2.8 × 10}{}^{18}$ ions/cm². After bombardment with $\text{6.2 × 10}{}^{17}$ ions/cm², blistering is observed after room-temperature aging of less than 100 days, in spite of the large sputtering yield. Disappearance of the formed blisters and concurrent surface roughening are observed with further aging. For higher dose bombardments, only surface roughening is observed without prior formation of blisters. Distribution of the injected ions associated with large sputtering yield is derived. From this distribution, the critical amount of injected ions required for blistering to occur is estimated to be equal to or less than $\text{3.4 × 10}{}^{17}$ ions/cm².     

Light particle irradiation effects in Si nanocrystals

Si nanocrystals, formed by Si ion implantation into SiO₂ layers and subsequent annealing at 1150°C, were irradiated at room temperature either with He⁺ions at energies of 30 or 130 keV, or with 400 keV electrons. Transmission electron microscopy (TEM) and photoluminescence (PL) studies were performed. TEM experiments revealed that the Si nanocrystals were ultimately amorphized (for example at ion doses ∼10¹⁶ He cm⁻²) and could not be recrystallized by annealing up to 775°C. This contrasts with previous results on bulk Si, in which electron- and very light ion-irradiation never led to amorphization. Visible photoluminescence, usually ascribed to quantum-size effects in the Si nanocrystals, was found to decrease and vanish after He⁺ ion doses as low as 3 × 10¹²–3 × 10¹³ He cm⁻² (which produce about 1 displacement per nanocrystal). This PL decrease is due to defect-induced non-radiative recombination centers, possibly situated at the Si nanocrystal/SiO₂ interface, and the pre-irradiation PL is restored by a 600°C anneal.     

Enhanced hydrogen trapping due to He ion damage

Single crystals and rolled foil of Mo with and without predamage by 11 or 18 keV⁴ He at room temperature have been injected with 8 or 16 keV H and D isotopes. The H depth profiles and the total D retention were measured by nuclear microanalysis techniques. A strong enhancement in the trapping of hydrogen isotopes after He predamage over Mo without predamage is observed and the H depth profiles scale with the He ion energy. Cold work increases the D trapping in the absence of He damage but after 3 × 10¹⁵ He/cm² predamage little difference remains in the total trapping for single crystals and rolled foil. The release of the D upon annealing due to detrapping occurs primarily between 100 and 450°C. A similar fractional reduction in trapping is observed for elevated temperature D injection as is found in the anneal to corresponding temperatures of room temperature injected samples.     

High dose He+ bombardment of niobium at 800° tO 1400°C

In proposed fusion reactors, the first wall will be bombarded by low energy D⁺, T⁺ and He⁺ ions to very high doses. Room temperature irradiations with 9 keV He⁺ (Roth et al. 1975) showed an initial phase of blistering between 0.05 and 0.25 C/ cm², and a final state of considerable surface roughness at 8 to 112 C/cm²(7 × 10²⁰He⁺/cm²). The present irradiation experiments with 6 keV He⁺ are carried out with polycrystalline Nb foils at temperatures between 800 and 1400°C in order to study the influence of He mobility and of Nb surface diffusion during irradiation. The applied doses range from 2.5 × 10¹⁷He⁺/cm² (blistering) to 5 × 10²⁰He⁺/cm² which corresponds to at least several weeks of reactor operation. The resulting changes of the niobium surface structures are observed by scanning electron microscopy and are pictorially presented in this paper. Mainly, sponge-like open structures are seen to develop at high doses, with increasing physical dimensions at higher temperatures.     

Argon ion impact desorption cross sections of chlorine and carbon from molybdenum

Total desorption cross sections have been measured for Cl (σ_Cl) and C(σ_C) on molybdenum by argon ion bombardment for an incidence angle of 60° from the surface normal. For the bombardment an ion gun with low current density (i₀ ~ 1 × 10 ^?7 A cm^?2) at low system pressure (~10^?9 Torr) was used. The detection was performed by AES and the data were sensitivity factor corrected. The AES analysis of the surface after adsorption showed that Mo, C and Cl contributed to more than 94% of the atomic composition. With known i₀, it is possible to obtain σ from the adsorbate signal vs ion bombardment time curve. For ion energies between 0.2 keV to 1.0 keV the measured value for σ_Cl and σ_C are 0.5?3 × 10^?15 cm² and 0.2?4 × 10^?15 cm², respectively. The possible effects of the surface roughness due to prebombardment are discussed.     

Measurement of chemical sputtering yields of various types of carbon

Measurements of the chemical sputtering during the bombardment of pyrolytic graphite, isotropic carbon and glassy carbon with 0.1–6 keV hydrogen ions have been made in the temperature rarige of room temperature to 700°C. The maximum production rate occurs at 1 keV for the incident energy and 525°C for the target temperature in all types of carbon. Energy and temperature dependences of chemical sputtering of carbon are not affected by the structures of the carbon. The reason is ascribed to radiation damage of the surface of the carbon. The dose dependence of the methane production rate was influenced by the hydrogen concentration in a target prior to bombardment, but the steady rate was obtained after the target was bombarded with protons at a dose of more than $\text{1 × 10}{}^{18}\text{H}{}^{\mathrm{+}}\text{/cm}{}^2$.     

Severe surface spalling and blistering in Mo Single crystals subjected to high fluence He bombardments

Single crystals of Mo were implanted at room temperature with 200 keV He⁺ ions to fluences high enough to produce two layers of exfoliation. The first layer of exfoliation occurs by the entire spalling of the crystal surface at a critical fluence of ≈ 10¹⁸/cm². This is followed by subsequent blistering and blister rupturing on the already spalled surface. The exfoliation mechanisms are discussed with relation to the microstructure which develops during implantation and the attendant property changes. The implanted regions were studied by SEM, TEM, and by ion beam channeling.     

Physical and chemical sputtering of graphite and SiC by hydrogen and helium in the energy range of 600 to 7500 eV

The erosion of pyrolytic graphite and silicon carbide due to the bombardment with monoenergetic hydrogen ions with energies of 600 to 7500 eV has been investigated in the temperature range of near room temperature to 750°C. The erosion yield of SiC is about 10^?2 and shows no pronounced temperature dependence. In contrast to SiC the erosion yield of pyrolytic graphite shows a maximum at a temperature of about 600°C. The ratio of the maximum erosion yield to that at room temperature depends on the energy of the hydrogen ions and increases from about 11 at 3000 eV to 32 at 670 eV. The production of CH₄ during the bombardment of the graphite has been found proportional to the erosion yield. When graphite was bombarded with He ions no hydrocarbon production and no temperature dependence of the erosion yield could be observed. The results are compared with values for the erosion yields of carbon by thermal atomic hydrogen taken from literature.     

Helium irradiation of Ni-(Zr or Nb) metallic glasses: Blistering,flaking and bubble formation

NiZr and NiNb are potential metallic glasses for nuclear applications. We have studied blistering, flaking and bubble formation in Ni₆₄Zr₃₆, Ni₃₃Zr₆₇ and Ni₆₀Nb₄₀ glasses under helium ion bombardment at room temperature. The effect of projectile energy (50–150 keV), total dose (0.01–10 × 10¹⁸ ions/cm²), dose rate (10–100 μA/cm²) and thermal crystallization on critical dose for blistering and/or flaking, average blister diameter and development of surface topography was examined. The surface damage effects in metallic glasses were in general similar to those for crystalline materials with the notable exception that the critical dose values are higher by about 50–100% for metallic glasses. TEM investigations revealed irradiation induced partial crystallization of Ni₃₃Zr₆₇ glass. Metallic glasses Ni₆₄Zr₃₆ and Ni₆₀Nb₄₀ were stable under helium irradiation and have also shown very high resistance against blistering and/or flaking. The possible mechanisms of helium trapping in metallic glasses are also discussed.     

Suppression of sputtering of nickel by coverage with self-sustaining thin segregated carbon layers

Sputtering of two-layered films composed of nickel (~5000 Å) and nickel carbide (~1500 Å) at 600° C by 5 keV Ar⁺ bombardment on the nickel side has been studied using Rutherford backscattering of 1.3 MeV H⁺ ions. It is found that the removal rate of nickel atoms from specimens is dependent on ion current density and that the removal rate of nickel atoms is very much smaller than that of carbon atoms when the ion current density is low. During ion bombardments at a low current density carbon segregation by a thickness of nearly two monolayers is observed at the nickel surface. Thus suppression of the removal rate of nickel atoms is ascribed to coverage of the nickel surface with segregated carbon atoms which are continuously supplied by diffusion through the nickel film from the carbide layer.     

Temperature dependence of trapping and depth profiles of 6 to 15 kev deuterium in carbon

Depth profiles of 30 keV D⁺₂ and 20 keV D⁺₂ implanted into edge and basal-oriented pyrolytic graphite have been measured by means of the D(³He,α)H nuclear reaction in the temperature range of 300 to 800 K. At room temperature deuterium concentrations up to 30 at.% are found in a surface layer corresponding to the range of the ions. The measured depth profiles do not fully agree either with calculated range profiles or with the damage profiles, but are determined by the two together. At higher temperatures the deuterium concentrations decrease and the profiles broaden. At room temperature the amount of trapped deuterium increases linearly with dose below 10¹⁸ deuterons/cm². The trapping coefficient is roughly 60%. At 5 × 10¹⁸ deuterons/cm² the amount of trapped deuterium in the probed layer (~4000 Å) reaches saturation and the trapping coefficient becomes zero. The saturation value decreases with increasing temperature and increases with increasing energy.     

Annealing of ion implanted 4H–SiC in the temperature range of 100–800 °C analysed by ion beam techniques

Ion implantation induced damage formation and subsequent annealing in 4H–SiC in the temperature range of 100–800 °C has been investigated. Silicon Carbide was implanted at room temperature with 200 keV ⁴⁰Ar ions with two implantation fluences of 4 × 10¹⁴ and 2 × 10¹⁵ ions/cm². The samples were characterized by Rutherford backscattering and nuclear reaction analysis techniques in channeling mode using 2.00 and 4.30 MeV ⁴He ion beams for damage buildup and recovery in the Si and C sublattices, respectively. At low ion fluence, the restoration of the Si sublattice is evident already at 200 °C and a considerable annealing step occurs between 300 and 400 °C. Similar results have been obtained for the C sublattice using the nuclear resonance reaction for carbon, ¹²C(α,α)¹²C at 4.26 MeV. For samples implanted with the higher ion fluence, no significant recovery is observed at these temperatures.     

Surface damage of metallic glasses by argon ion bombardment

The surface damage of metallic glasses Fe₄₀Ni₄₀P₁₄B₆, Fe₄₀Ni₃₈Mo₄B₁₈, Fe₄₀Ni₄₀B₂₀, Fe₈₀B₂₀ and Ni₆₄Zr₃₆ under argon ion bombardment at room temperature has been investigated. Blister formation was observed in the dose range of 1 × 10¹⁷ to 1 × 10¹⁸ ions/cm². At higher doses, blisters disappeared with concurrent roughening of the bombarded surface. Erosional features like cones and pyramids are not observed. Argon induced blisters also disappeared on room temperature aging without any further bombardment after blister formation. Post-irradiation annealing at high temperatures (673 and 873 K) resulted in blister formation, severe surface exfoliation and pin-hole formation. The precipitation of the implanted argon into bubbles is also observed.     

Ion bombardment of insulators

Some of the effects which were observed when some stable inorganic insulators were subjected to ion bombardment are described. Most of the results were for bombardment at room temperature with the light ions H⁺, D⁺, He⁺, having energies in the neighborhood of 100 keV. Phenomena associated with destruction of the surface: stress, deformation, fracture, and blistering, are emphasized.