Dynamic hydrogen isotope behavior and its helium irradiation effect in SiC

1. Dynamic hydrogen isotope behavior and its helium irradiation effect in SiC Yasuhisa Oya and Satoru Tanaka The University of Tokyo

2. Objective Structural materials for future fusion reactor Thermal and chemical stability SiC/SiC Composite Low activation Ferritic steel Vanadium alloy From the viewpoint of fusion safety Understanding of hot atom behavior of hydrogen isotopes in fusion reactor circumstance （Under high energy particles irradiation circumstance ） Evaluation of Hydrogen isotope retention behavior and chemical states of SiC by D2+-He+ irradiation by X-ray photoelectron spectroscopy (XPS) and thermal desorption spectroscopy (TDS)

3. Experimental procedure Pretreatment Annealing at 1300K for 10 minutes under the vacuum less than 10-8Pa Energy: 1.0 keV Flux : 1.3 x 1018 D+m-2s-1 Fluence : 1.0x1022 D+ m-2 D2+ irradiation He+ irradiation Energy : 1.3 keV Flux : 1.3 x 1018 D+m-2s-1 Fluence : 0-1.0x1022 D+ m-2 TDS (Thermal desorption spectroscopy) XPS (X-ray photoelectron spectroscopy) X-ray source : Mg-Kα Heating rate : 0.5 K s-1 Temperature : 300-1300 K

4. Sample and experimental system SiC Sample ROICERAM-HSAsahi Glass Co. Ltd. Polycrystalline -SiC(3C-SiC) : Ф10mm×1mmdensity : 3.10 g/cm3 Base pressure ～10-8 Pa The sample can be transferred between TDS chamber and XPS chamber without air exposure. XPS measurements were performed at room temperature.

5. Comparison of TDS spectra from SiC with Si and graphite (a) SiC (b) Si (c) Graphite Thermal desorption spectra of D2 from SiC Two D2 desorption stages 1st stage at 800 K 2nd stage at 1000 K (Deuterium bound to Si) (Deuterium bound to C)

6. TDS spectra of D2 from SiC as a function of D2+ ion fluence D trapping states in SiC : Si-D, C-D Si-D C-D D and He retention as a function of He+ fluence D2 TDS spectra after D2+ irradiation with various ion fluence D retention decreased by He+ pre-irradiation. D retention was not changed by He+ pre-irradiation above the fluence of 0.1 x 1022 He+m-2. D is trapped by Si after saturation of C-D bonds. Si-D is a major chemical state in SiC. D/SiC=0.75

7. XPS spectra of Si and C from SiC as a function of heating temperature (a) C 1s (b) Si 2p By heating above 800 K, the peak position of C 1s was shifted to lower energy side, although that of Si 2p was almost remained in the lower energy side. Both peaks were recovered by heating about 1200 K. Summary of peak positions by heating

8. TDS spectra of D2 from SiC as a function of implantation temperature Implantation temperature dependence on deuterium retention in SiC Comparison of deuterium retention in SiC and graphite By heating the sample at 573 K, the deuterium retention was decreased less than half. However, the deuterium retention was found even above 913 K.

9. D2 and He TDS spectra as a function of He+ post irradiation fluence D2+-He+ irradiation (1.0×1022 He+ m-2) Only D2+ irradiation D2 He D2 desorption mainly consists of two stages. D trapped by Si decreased by He+ post irradiation.

10. Chemical behavior of D2+ irradiated SiC as a function of He+ post irradiation fluence Si 2p C 1s Sensitivity of XPS C 1s : C-D bond and defects Si 2p : mainly defects By D2+ irradiation, C 1s : High energy side Si 2p : Low energy side By He+ irradiation, C 1s : Slight shift toward lower energy side Si 2p : Shift toward lower energy side D was trapped by SiC and some defects would be also introduced. By He+ irradiation,more damaged structures were introduced.

11. Isochronal heating for D2+-He+ irradiated SiC (1) C 1s Si 2p By heating, C 1s was shifted toward low energy side and Si 2p was moved toward high energy side. After the dissociation of C-D bond, the damaged structures would be recovered. Above 1200K, both of C1s and Si2p were shifted to higher energy side. Peak positions of C 1s and Si 2p as a function of heating temperature

12. Isochronal heating for D2+-He+ irradiated SiC (2) π－π* transition XPS spectra of C 1s after heating at 1300K By heating, D was detrapped and SiC structure would be recovered, which led to decrease FWHM. Above 1000K, C was aggregated on the surface and form C=C and/or C-C bonds, which contribute to increase FWHM.

13. 4 4 x 10 x 10 5 5 Peak 1 4.5 4.5 Peak 2 4 4 3.5 3.5 3 3 c/s c/s 2.5 2.5 2 2 1.5 1.5 1 1 0.5 0.5 294 290 286 282 278 294 290 286 282 278 Binding Energy / eV Binding Energy /eV XPS spectra after heating at 1150 K and 1300 K Area ratio Peak1: Peak2 1150 K 45.4: 54.6 1300 K 75.0: 25.0 1300 K 1150 K Peak 1 Peak top: 284.23 eV, FWHM: 1.93 eV　 →C-C bond Peak 2 Peak top: 282.98 eV, FWHM: 1.43 eV　 →C-Si bond C was aggregated by heating at 1300 K.

14. Depth profiling of C 1s and Si 2p after heating at 1300 K Chemical states of C and Si in SiC after heating 1300 K were evaluated by Ar+ sputtering. C was aggregated on the surface. C 1s was largely shifted to lower energy side at the depth of a few nm. Decrease of C=C and/or C-C bond and only Si-C exists in the bulk of SiC Si-C bond is a major chemical state at the depth of ～20nm(irradiation range）. SiC structures were disordered by D2+and He+ irradiation.

15. Conclusions • D2+ irradiation was performed to SiC sample and thereafter, He+ was irradiated to elucidate the correlation between behaviors of hydrogen isotope and the damaged structures. • By He+ irradiation to D2+ irradiated SiC, deuterium retention decreased at the initial stage. D trapped by Si was desorbed, which indicates that the C vacancies interact with irradiated He+. • It can be concluded that the knocked C produced by He+ irradiation was aggregated by heating above 1300 K, which imply that the some C impurity would contaminate the plasma or the tritium breeding materials by contacting the SiC structural (insert) material.