Enhanced Radiation Tolerance of Tungsten Nanoparticles to He Ion Irradiation

Abstract

Materials exposed to plasmas in magnetic confinement nuclear reactors will accumulate radiation-induced defects and energetically implanted gas atoms (from the plasma and transmutations), of which insoluble helium (He) is likely to be the most problematic. The large surface-area-to-volume ratio exhibited by nanoporous materials provides an unsaturable sink with the potential to continuously remove both point defects and He. This property enhances the possibilities for these materials to be tailored for high radiation-damage resistance. In order to explore the potential effect of this on the individual ligaments of nanoporous materials, we present results on the response of tungsten (W) nanoparticles (NPs) to 15 keV He ion irradiation. Tungsten foils and various sizes of NPs were ion irradiated concurrently and imaged in-situ via transmission electron microscopy at 750 °C. Helium bubbles were not observed in NPs with diameters less than 20 nm but did form in larger NPs and the foils. No dislocation loops or black spot damage were observed in any NPs up to 100 nm in diameter but were found to accumulate in the W foils. These results indicate that a nanoporous material, particularly one made up of ligaments with characteristic dimensions of 30 nm or less, is likely to exhibit significant resistance to He accumulation and structural damage and, therefore, be highly tolerant to radiation.

Keywords: helium bubbles; in-situ TEM; nanoporous materials; plasma-facing materials; radiation tolerance; tungsten nanoparticles.

Figure 1

SRIM calculations modified using the SICMod code for a circular cross-section of spherical W NPs irradiated with 15 keV He to 1.1 × 10¹⁷ ions·cm⁻² showing the damage density for diameters of (a) 20 nm and (b) 80 nm. (The colour scale units of damage density apply to both images.)

Figure 2

Bright-field TEM images showing the distribution of bubbles in W NPs of different sizes after irradiation with 15 keV He to a fluence of 9.6 × 10¹⁶ ions·cm⁻² at 750 °C showing: (a) a distribution of NPs of different sizes dispersed at the edge of the electropolished region of a W foil before irradiation; (b) 800 nm overfocus; and (c) 800 nm underfocus after irradiation. Small bubbles appear as black spots in overfocus and white spots in underfocus.

Figure 3

BF-TEM images comparing bubbles in a 35 nm diameter W NP (a–c) and foil (d–f) as a function of fluence taken at 400 nm overfocus. The arrows in (a–c) highlight the growth of a bubble in the NP as the fluence increases. (Scale marker in (a,d) applies to all images). Figure (g) shows the relationship between bubble density and fluence for NPs with diameters of 20 ± 5 and 50 ± 5 nm in the foil.

Figure 4

Underfocus BF-TEM images for W NPs: (a) unirradiated and (b) irradiated to a fluence of 1.1 × 10¹⁷ ions·cm⁻² showing the bubble distributions in NPs of different sizes. The relationship between bubble density and particle size for W NPs with different diameters irradiated to a fluence of 1.1 × 10¹⁷ ions·cm⁻² is shown in (c). (The scale marker in (a) applies to both images.)

Figure 5

Distribution of bubble sizes in W irradiated with 15 keV He to a fluence of 1.1 × 10¹⁷ ions·cm⁻²: (a) 20–35 nm NPs (sample = 80 bubbles in 20 NPs); (b) 40–55 nm (sample = 200 bubbles in 20 NPs); (c) 60–80 nm (sample = 280 bubbles in 28 NPs); and (d) the foils (sample = 280 bubbles). (e) Plot of the size dependence of swelling due to bubbles for W NPs of different diameters.

Figure 6

Plots of: (a) calculations and experimental data [49,50] for density as a function of pressure for He at 625 °C and 750 °C; and (b) relationship between He concentration (based on resolvable bubbles) and NP volume at a fluence of 1.1 × 10¹⁷ ions·cm⁻².

Figure 7

TEM images of: (a) NPs of different sizes and W foil irradiated to 1.1 × 10¹⁷ ions·cm⁻²; (b) enlarged area of the foil after tilting >15° to be close to a different zone axis to that shown in (a) to avoid any possible g.b invisibility criteria; and (c) enlarged area of NP after equivalent tilting of >15°. Note the complete absence of dislocation loops in the NPs evident in (a,c).