† Corresponding author. E-mail:
Project supported by the National Magnetic Confinement Fusion Energy Research Project, Ministry of Science and Technology of China (Grant No. 2015GB109002), the Innovation Fund of Postgraduate, Xihua University, China (Grant No. ycjj2018017), and the National Natural Science Foundation of China (Grant No. 21401173).
The retention and release of deuterium in W–2%Y2O3 composite materials and commercially pure tungsten after they have been implanted by deuterium plasma (flux ∼ 3.71 × 1021 D/m2⋅s, energy ∼ 25 eV, and fluence up to 1.3 × 1026 D/m2) are studied. The results show that the total amount of deuterium released from W–2%Y2O3 is 5.23 × 1020 D/m2(2.5 K/min), about 2.5 times higher than that from the pure tungsten. Thermal desorption spectra (TDS) at different heating rates (2.5 K/min–20 K/min) reveal that both W and W–2%Y2O3 have two main deuterium trapped sites. For the low temperature trap, the deuterium desorption activation energy is 0.85 eV (grain boundary) in W, while for high temperature trap, the desorption activation energy is 1.57 eV (vacancy) in W and 1.73 eV (vacancy) in W–2%Y2O3.
Tungsten has been considered as one of the candidate PFMs for ITER because of its favorable physical and chemical properties, such as high thermal conductivity, high melting point, high-temperature mechanics performance, low physical and chemical sputtering yields, and no chemical reaction with hydrogen.[1–7] However, the brittleness of W material leads PFM to likely undergo crack failure when the transient event occurs.[8]
W–Y2O3 alloys can effectively impede the recovery and recrystallization at high temperature, and have low ductile–brittle transition temperature (DBTT) compared with pure tungsten.[12,13] Tan et al.[14] prepared and studied the mechanical properties and microstructure changes of W–Y2O3 alloy under helium irradiation, the results showed that W–Y2O3 have the trend of lattice distortion, polycrystal and phase transformation. Yao et al.[15] studied the surface damage of W–Y2O3 irradiated by helium. The results showed that the grain orientation of W–Y2O3 irradiated by helium ion changes obviously and helium bubbles gather near the surface phase interface. The surface damage and hydrogen isotope retention of W–Y2O3 exposed to deuterium plasma have been investigated, the experimental results of Tan[16] showed that the retention of deuterium is related to the texture and laminated microstructure of the irradiated surface of the sample. The results of Zhao[17] showed that the blisters and deuterium retention of W–Y2O3 are strongly dependent on the injection temperature. However, the desorption activation energy of deuterium in W–Y2O3 is rarely reported.
In this work, based on the research of Yao et al., the deuterium desorption activation energy in W–2%Y2O3 composite material and commercial pure tungsten after being exposed to deuterium plasma are experimentally determined by thermal desorption spectroscopy. The differences in deuterium retention and release between W–2%Y2O3 and W are discussed.
W–2%vol Y2O3 was supplied by Hefei University of Technology, which was prepared by using the wet chemistry combined with plasma sintering,[18,19] and the pure-W was obtained from ATTL www.tlwm.cn) with a purity of 99.95 wt%. All samples were hot rolled with the rolling ratio of 50%, and then were fabricated into square pieces with 10 mm×10 mm×1 mm from the same manufacturing batch. The surfaces were mechanically polished to a mirror-like finish then ultrasonically cleaned in acetone and ethanol. Finally, all samples were annealed at 1273 K under vacuum better than 5 × 10−5 Pa for 1 h to relieve residual stresses generated in the grinding and polishing process.
Deuterium ion irradiation was conducted by using comprehensive ECR plasma for tritium (CEPT) generating device in the Science and Technology on Surface Physics and Chemistry Laboratory.
The ion implantation direction is perpendicular to the rolling direction (RD)–transverse direction (TD) surface of the material (as shown in Fig.
![]() | Fig. 1. (a) RD, TD, and ND represent rolling direction, transverse direction, and normal direction, respectively, and (b) schematic diagram of plasma implantation direction. |
The detailed conditions of the deuterium ion irradiation are listed in Table
![]() | Table 1. Detailed experiment conditions for pure-W and W–Y2O3. . |
After the samples are irradiated by deuterium ions, the deuterium retention and desorption activation energy in the sample were obtained by using thermal desorption spectroscopy device[20] in the Science and Technology on Surface Physics and Chemistry Laboratory. The pipe which the ions passed through was baked for 72 h to remove residual gas in the material prior to the TDS experiment. The samples were heated by linearly increasing the temperature up to 1273 K at different heating rates (Table
![]() | Table 2. Heating rate for pure-W and W–Y2O3 in TDS experiment. . |
Figures
![]() | Fig. 2. EBSD images of (a) W and (b) W–Y2O3 before being irradiated by deuterium ions, (c) legend for panels (a) and (b), SEM images of (d) W and (e) W–Y2O3 after being irradiated by deuterium ions. |
The surface morphology of W and W–Y2O3 irradiated by deuterium ions confirm this point (Figs.
![]() | Fig. 5. Plots of desorption activation energies of LTT and HTT of W and W–Y2O3 by least squares fitting, where line is least square fitting. |
Pan et al.[21] showed that the potential barrier for H ions to diffuse into the surface is larger than that into the bulk phase, and H ions preferentially accumulate on the W (111) surface compared with on the bulk phase, and W (110) is the resistance surface for the formation of H blisters. Figures
Figure
![]() | Table 3. Lattice constants of pure-W and W–Y2O3 before and after being irradiated. . |
Figures
The TDS spectra show the macroscopic characteristics of deuterium released from the whole sample, which is formed by coupling the different deuterium trap release peaks in the sample. The TDS spectra of W–Y2O3 and W have obvious shoulder peaks near 600 K, indicating that there is a corresponding low temperature deuterium trap in the sample. The whole release peak can be divided into the coupling of the release peaks of the low temperature traps (LTTs) and the high temperature traps (HTTs) (Figs.
The TDS peak of W and W–Y2O3 are in accordance with Gauss distribution. According to the results of the reaction kinetics in differential thermal analysis of Kissinger,[22] the desorption activation energy can be calculated from the TDS spectra with different heating rates by the following formula:
Since β and Tp are known, the evoluation of desorption activation energy Ea of deuterium ion from a trapping site can be calculated from the slope of the
According to the experimental and modeling results (see Table
![]() | Table 4. Desorption activation energy of deuterium in W. . |
It is worth mentioning that some deuterium atoms even are desorbed from pure W at very high temperature (1200 K) (Fig.
![]() | Fig. 6. NRA determined deuterium depth profile (topmost 2000 nm) for W and W–Y2O3 exposed to deuterium plasma. |
The results of Zhou et al.[31] showed that without considering the effect of temperature, most of the H atoms occupy the interstitial positions in the bulk phase, while only some H atoms occupy the defects related to the vacancies. Therefore, most of the deuterium atoms in W–Y2O3 are mainly concentrated at the interstitial positions near the surface layer, while most of the deuterium atoms in W are distributed at the interstitial positions in the whole bulk phase, which may be the reason why the deuterium release peak at the interstitial position in W–Y2O3 is observed in the TDS spectrum, but the deuterium release peak at the interstitial position in W cannot be observed obviously.
Figure
The deuterium retention and release in Y2O3 doped by tungsten, exposed to high-flux low-energy plasma, is studied. The total number of deuterium atoms released from W–Y2O3 is 5.23 × 1020 D/m2 (2.5 K/min), about 2.5 times higher than that from pure tungsten. The W–Y2O3 has two main deuterium trapped sites. For low temperature traps, the desorption activation energy is 0.39 eV while for high temperature traps (vacancy), the desorption activation energy is 1.73 eV. Both low temperature traps and high temperature traps make important contributions to trapping D.
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