Release of helium from vacancy defects in yttria-stabilized zirconia under irradiation

Fission gas retention or release has a critical impact on the function of advanced nuclear materials. Helium trapping in, and release from, radiation defects induced by neutrons and by α decay in YSZ (yttria-stabilized zirconia) is experimentally simulated using synchronized Zr${}^{+}$ and He${}^{+}$ dual ion beam irradiation. The measured damage profiles consist of two peaks which agree well with the calculated profiles of implantation induced excess point defects. This special implantation related effect has to be carefully considered in the evaluation of experimental investigations which simulate isotropic irradiation effects such as α decay. First-principles calculations show that helium is energetically favorable to be trapped by Zr vacancies in YSZ. Implanted helium alone in YSZ is accumulated in undesirable helium bubbles and results in local surface swelling and lift-off. However, under dual beam irradiation helium is released from vacancy defects and is out-diffused at room temperature. Helium is mobilized by a vacancy-assisted trapping/detrapping mechanism induced by the simultaneous Zr${}^{+}$ ion implantation. This behavior avoids the deleterious helium bubble formation and contributes to the suitable application characteristics of YSZ which result in its excellent radiation hardness.

Figure 1

trim simulation of depth profiles for 2.5 MeV Zr${}^{+}$ and 100 keV He${}^{+}$ implantation into ZrO${}_2$ under an incidence angle of 22.5° (right scale). The irradiation induced depth distributions of excess vacancies ($V_{\mathrm{EX}}$) and excess interstitials ($I_{\mathrm{EX}}$) generated by Zr${}^{+}$ implantation (red circles, left scale) were calculated with the consideration of additional interstitials introduced by the implanted Zr atoms (+1 atoms). The total amount of displacements ($V_{\mathrm{TOT}}$) is also shown (blue squares, leftmost scale).

Figure 2

(a) RBS spectra of YSZ crystals recorded in random and $〈100〉$-axial directions for irradiation by DB, Zr${}^{+}$ implantation and He${}^{+}$ implantation. $d$($x$) is the fitted dechanneling baseline indicated for the Zr${}^{+}$ implantation. The upper scale indicates the depth position. (b) Damage profiles as generated by DB and Zr${}^{+}$ implantation. The profiles are calculated from the corresponding RBS spectra shown in (a). Two damage peak positions are indicated by arrows termed as $R_p$ and ½$R_p$. They are related to the so called ½$R_p$ effect. The sharp peak at channel 750 is due to backscattering from the sample surface.

Figure 3

Strain depth profiles obtained from the high resolution x-ray scattered intensity distribution on YSZ crystals irradiated by (a) DB, Zr${}^{+}$ implantation and (b) He${}^{+}$ implantation. The annealed samples were thermally processed at 800 °C for 1 h. The peak position related to the ½$R_p$ effect for open volume defect formation is marked by an arrow.

Figure 4

Cross-sectional TEM images of a (a) He${}^{+}$ implanted YSZ sample and a (b) DB (Zr${}^{+}$ and He${}^{+}$) implanted sample. In this case helium was implanted to $1\times {10}^{16}$ cm${}^{-2}$ with an ion energy of 260 keV. The length of the arrows indicates the depth range from surface to 500 nm. Black dots are dislocation loops.

Figure 5

High resolution TEM images of the He${}^{+}$ implanted YSZ sample [Fig. 4a] at the depth position of 300 nm. Circles mark small helium bubbles with diameters of about 1 nm shown in (a) under-focus and (b) over-focus.

Figure 6

(a) $S$ parameter data as a function of incident positron energy for YSZ crystals irradiated by DB, single Zr${}^{+}$ and single He${}^{+}$ implantation. The depth corresponding to the positron energy is plotted by the upper scale. An arrow marks the peak position related to the ½$R_p$ effect for open volume defect formation. (b) $W$-$S$ plot of YSZ crystals irradiated by DB, Zr${}^{+}$ and He${}^{+}$ implantation. The deviation of the $W$-$S$ curve from a straight line is marked by a dashed circle.

Figure 7

Coincidence Doppler broadening ratio curves of DB irradiated YSZ (2.5 MeV Zr${}^{+}$ and 260 keV He${}^{+}$ ions) and of single Zr${}^{+}$ ion irradiated YSZ. The positron energy was 12 keV.

Figure 8

AFM images: (a) virgin YSZ surface, (b) single He${}^{+}$ implanted YSZ surface, and (c) DB (Zr${}^{+}$ and He${}^{+}$) implanted YSZ surface. The scanned area is $10\mu \mathrm{m}\times 10\mu \mathrm{m}$. The height scale bar (right) is 20 nm.