Dynamic hydrogen isotope behavior and its helium irradiation effect in SiCYasuhisa Oya and Satoru TanakaThe University of Tokyo

ObjectiveStructural materials for future fusion reactorSiC/SiC CompositeLow activation Ferritic steelVanadium alloyThermal and chemical stabilityUnderstanding of hot atom behavior of hydrogen isotopes in fusion reactor circumstanceUnder high energy particles irradiation circumstance From the viewpoint of fusion safetyEvaluation 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)

Experimental procedurePretreatmentD2+ irradiationHe+ irradiationXPS (X-ray photoelectron spectroscopy)Annealing at 1300K for 10 minutes under the vacuum less than 10-8PaEnergy: 1.0 keVFlux : 1.3 x 1018 D+m-2s-1Fluence : 1.0x1022 D+ m-2Energy : 1.3 keVFlux : 1.3 x 1018 D+m-2s-1Fluence : 0-1.0x1022 D+ m-2TDS (Thermal desorption spectroscopy)X-ray source : Mg-KHeating rate : 0.5 K s-1Temperature : 300-1300 K

Sample and experimental systemROICERAM-HSAsahi Glass Co. Ltd. Polycrystalline -SiC(3C-SiC) : 10mm1mmdensity : 3.10 g/cm3SiC SampleThe sample can be transferred between TDS chamber and XPS chamber without air exposure.XPS measurements were performed at room temperature.Base pressure10-8 Pa

Comparison of TDS spectra from SiC with Si and graphiteThermal desorption spectra of D2 from SiC(a) SiC(b) Si(c) Graphite Two D2 desorption stages1st stage at 800 K2nd stage at 1000 K(Deuterium bound to Si)(Deuterium bound to C)

TDS spectra of D2 from SiC as a function of D2+ ion fluenceD trapping states in SiC : Si-D, C-DSi-DC-DD is trapped by Si after saturation of C-D bonds.Si-D is a major chemical state in SiC.D/SiC=0.75D2 TDS spectra after D2+ irradiation with various ion fluenceD and He retention as a function of He+ fluenceD 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.

XPS spectra of Si and C from SiC as a function of heating temperature(a) C 1s(b) Si 2pBy 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

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

D2 and He TDS spectra as a function of He+ post irradiation fluence D2 desorption mainly consists of two stages.D trapped by Si decreased by He+ post irradiation.Only D2+ irradiationD2+-He+ irradiation(1.01022 He+ m-2)HeD2

Chemical behavior of D2+ irradiated SiC as a function of He+ post irradiation fluenceSi 2pC 1sBy D2+ irradiation,C 1s : High energy sideSi 2p : Low energy sideBy He+ irradiation,C 1s : Slight shift toward lower energy sideSi 2p : Shift toward lower energy sideSensitivity of XPSC 1s : C-D bond and defectsSi 2p : mainly defectsD was trapped by SiC and some defects would be also introduced.By He+ irradiation, more damaged structures were introduced.

Isochronal heating for D2+-He+ irradiated SiC (1)Si 2pC 1sBy 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

Isochronal heating for D2+-He+ irradiated SiC (2)* transitionXPS spectra of C 1s after heating at 1300KBy 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.

XPS spectra after heating at 1150 K and 1300 KPeak 1Peak top: 284.23 eV, FWHM: 1.93 eV C-C bondPeak 2Peak top: 282.98 eV, FWHM: 1.43 eV C-Si bond1300 K1150 KArea ratioPeak1: Peak21150 K45.4: 54.61300 K75.0: 25.0C was aggregated by heating at 1300 K.

Depth profiling of C 1s and Si 2p after heating at 1300 KChemical 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 SiCSi-C bond is a major chemical state at the depth of 20nm(irradiation range. SiC structures were disordered by D2+and He+ irradiation.

ConclusionsD2+ 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.