Characterization and erosion of metal-containing carbon layers

Transcription

1 Characterization and erosion of metal-containing carbon layers Martin Balden Max-Planck-Institut für Plasmaphysik, EURATOM Association, D Garching, Germany Materials Research Division (MF)

2 Outline Introduction Importance of mixed layers Why are we interested in metal-containing amorphous carbon layers (a-c:me)? Characterization Which material systems are investigated? How are specimens produced? Which characterization techniques are applied to obtain which material property? Chemical erosion by hydrogen impact Total erosion yield of carbon: IBA Chemical erosion yield (CD 4 ): QMS Conclusion

3 Introduction Plasma-facing materials: ITER design (Mix) SEM first wall: Be (low Z, O getter) bulk plasma divertor wall: W (low erosion) target plates: Carbon (CFC) + thermal shock resistance - chemical erosion - tritium inventory I3: Counsell JET: ITER-like wall project I10-13: Matthews, Hirai, Neu, Lungu, P06, P50-52

4 Introduction: Mixed materials & doping Chemical erosion of carbon by hydrogen source term: re-deposited C layers containing tritium material mix (Be, W, C, steels, ) erosion / deposition mixed layers: properties -erosion - H-retention - composition C rich - thermal stability - microstructure (I8: Doerner) Reduction of chemical erosion by doping mechanism

5 Introduction: Metal-containing carbon films to simulate mixed layers magnetron sputtered metal-containing carbon films well characterized model materials laboratory erosion studies on a-c:me films focused on W + C (additional: Ti, V, Zr) P30 Galilea

6 Introduction: Investigation strategy Investigation of mixed layers composition, distribution in depth, lateral homogeneity, impurities, crystallinity, chemical state, film morphology characterization on - atomic level - nanometer scale - micrometer scale chemical erosion XAS XRD RBS SEM energy & mass-separated hydrogen ion beam: Length scale Å nm µm TEM AFM + total erosion yield: MeV ion beam analysis (weight loss; in-situ) + chemical erosion yield: mass spectrometry

7 Outline Production and Characterization

8 Production of a-c:me layers metal-doped amorphous carbon layers: - magnetron sputter deposition dual source, 300 K C 200 nm - single and triple layers on graphite or Si erosion thermal treatment, diffusion µm no surface-substrate influence C x M y C Graphite or Si 300 nm 300 nm - dopant (W, Ti, V, Zr) - concentrations (0-20 %) annealing: K (0.25 & 2 h) (e.g. carbide formation, diffusion) Triple layer: 13 at% W 1 µm SEM, cross section pure C 13 at% W in C pure C graphite (substrate) composition, homogeneity, morphology

9 Composition, distribution & impurities (RBS) at% V-doped C layer as-deposited simulated on C as-deposited simulated on Si depth C 0.82 V 0.18 C or Si 520 nm Intensity (a.u.) C interface 4 MeV 4 He 165 x 40 C surface Si interface V interface Ar interface O surface Vsurface W interface dopant homogeneity O < 1 at% Ar ~ 1 at% released by annealing W < 0.01 at% unusual Energy (MeV) H-RBS

10 Layer morphology (SEM, AFM) Triple layer: 13 at% V surface no significant changes by annealing pure C 13 at% V in C pure C graphite Single layer: 7.5 at% Zr on Si 30 nm 1 µm homogeneous, columnar growth structures size triggered by substrate roughness 4 µm no change in columnar growth by dopant 4.4 µm 0 nm

11 Outline Chemical bonding of metals and crystallinity

12 Local atomic environment (XAS: EXAFS) EXAFS: Extended X-ray absorption fine structure X-ray absorption Photo electrons Normalized absorption coefficient EXAFS spectrum Interference / Oscillations EXAFS Information about the local atomic environment of the absorbing atom (Neighboring atoms N, distance R, disorder σ)

13 Local atomic environment (XAS: EXAFS) Normalized absorption coefficient 1 Zr edge 1 Zr-doped ZrC Zr 14 at% Zr Zr ZrC 1300 K, 1/4 h 1100 K, 2 h 900 K, 1/4 h 700 K, 2 h references as-deposited (300 K) C C 0.86 Zr 0.14 C Graphite Doping with Ti, V & Zr 210 nm 260 nm 330 nm as-deposited :- amorphous / disorder - non-metallic state carbide structure already at 1100 K (qualitative quantitative) X-ray energy (ev) Exception W-doping: - as-deposited : higher order - annealed: only slight changes

14 Crystallinity of layers (XRD): annealing 5 4 (111) VC 13 at% V (200) VC C C 0.87 V 0.13 C 210 nm 280 nm 320 nm Diffusion: <20 nm (RBS) Intensity (a.u.) K, 2 h K, 1/4 h K, 2 h K, 1/4 h 1 as-deposited (300 K) 9 nm 7 nm 5 nm 4 nm Graphite Doping with Ti, V & Zr atomically deposition crystallite growth carbide formation after annealing (carbon-poor TiC, VC, ZrC) crystallite size increasing with T and t Estimation of crystallite size with Scherrer s formula Cu K α fixed incident angle: 2 exception W-doping Scattering angle

15 Crystallinity of W-doped layers (XRD) 200 WC (001) W 2 C(100) WC (100) W 2 C(002) W 2 C(101) 15 at% W C 0.85 W 0.15 Graphite 500 nm Intensity (cps) annealed at 1100 K "as-deposited" already as-deposited show peak sub-carbide (W 2 C) instead of carbide (WC) more pronounced after annealing 50 pure substrate w/o layer Cu K α fixed incident angle: Scattering angle (degree) grain size: ~ 2 nm (always) (estimation with Scherrer s formula)

16 Crystallinity of W-doped layers (XRD) 200 WC (001) W 2 C(100) WC (100) Annealed at 1700 K W 2 C(002) W 2 C(101) 15 at% W C 0.85 W 0.15 Graphite 500 nm Intensity (cps) annealed at 1100 K "as-deposited" already as-deposited show peak sub-carbide (W 2 C) instead of carbide (WC) more pronounced after annealing 50 TEM pure substrate w/o layer Cu K α fixed incident angle: Scattering angle (degree) grain size: ~ 2 nm (always) (estimation with Scherrer s formula) Annealing at 1700 K WC larger grains >20 nm

17 Outline Chemical erosion

18 Erosion Experiments: High Current Ion Source Experimental setup mono-energetic mass-separated ion beam: 30 ev, 200 ev, 1 kev D (D 3+ ) flux: ~10 19 D/m 2 s temperature controlled (RT-1450 K) Experimental procedure annealing to 1100 K carbide grains: W 2 nm, other 4-8 nm D bombardment of a-c:me - D energy: 30 ev / 200 ev - temperature: fixed RT / ~750 K D/m 2 two types of stepwise increased: RT K measurements D/m 2 per step, total <10 23 D/m 2

19 Concept of Erosion Yield Metal-containing amorphous carbon films (a-c:me) Erosion conditions Structure of a-c:me Erosion yield (Y) total amount of eroded C total erosion yield Y IBA Y Chem + Y Phys produced CD 4 chemical erosion yield Y CD4 ion beam analysis (IBA) of erosion spot analysis of gas phase by mass spectrometry (QMS) total reduction mechanism Y = C D removed incident Influence of doping on produced hydrocarbons expected reflected in amount of produced CD 4

20 Results: Erosion Yield IBA / QMS 0.3 Pure carbon films 30 ev 200 ev 1000 ev Total erosion yield (C/D) Relative CD 4 production yield Y IBA YCD4 QMS = Y Chem + Y Phys RT 750 K RT 750 K 850 K

21 Results: Erosion Yield IBA / QMS 0.3 Pure carbon films 30 ev 200 ev 1000 ev Total erosion yield (C/D) Relative CD 4 production yield Y IBA YCD4 QMS = Y Chem + Y Phys RT 750 K RT 750 K 850 K Scaling: CD 4 production for 1 kev D at T max equal Y Chem ( c = 1 ) Y Chem = c Y CD4 c = D: Roth 83: 3 Mech 98: ~2 (200 ev) H: Roth 83: 4 Yamada 87: 2 Davis 88: 1.2 Mech 97: ~2 (200 ev) Y hot > Y RT (well known) Y CD4 always smaller than Y IBA less CD 4 produced compared to 1 kev at T max

22 Comparison Erosion Yield IBA / QMS ev 200 ev 30 ev 200 ev 1 kev Y IBA Y CD4 Total erosion yield (C/D) Relative production yield CD pure carbon films 6% Ti 3% W 2% Zr 6% Ti 3% W 2% Zr 7% Ti 4% W 7% Ti 4% W RT RT RT 740 K 620 K 740 K RT RT 760 K 750 K RT 750 K RT 750 K 850 K

23 Total erosion yield Y IBA : Effect of doping Y IBA,dopant / Y IBA,C Carbon erosion yield Zr W Ti dopant 6%, 7% Ti 3%, 4% W 2% Zr reduced total erosion for all doped samples reduction most prominent at high temperatures and 30 ev (for Ti only 5 %) reduction changes with metal content / type metal erosion observed for 200 ev RT 750 hot K 750 hot K RT 30 ev 30 ev 200 ev 200 ev comparable to yields for carbides

24 Comparison Erosion Yield QMS / IBA relative Y CD4 / Y IBA CD 4 production equal to 1 kev D, T max (Y Chem = Y CD4 ) Ti W Zr C dopant 6%, 7% Ti 3%, 4% W 2% Zr pure carbon chem + phys chem RT hot hot RT 30 ev 30 ev 200 ev 200 ev Y CD4 /Y IBA < 1 for pure C more C x D y / radicals ratio always higher for a-c:me compared to C 200 ev: Y CD4 /Y IBA 1 more C x D y / radicals 30 ev: Y CD4 /Y IBA 1 more CD 4 production compare to published factors (1.2-4) mainly CD 4

25 Temperature dependence of chemical erosion (QMS) Doping with W (V, Ti, Zr) mass- & energy-separated ion beam (30 & 200 ev/d) detection of volatile erosion species (CD 4 ) CD 4 production yield - strong reduction at elevated temperatures - enhancement at RT total yield reduced CD 4 production yield (CD 4 /D) ev D 3 at% W-doped layer C 0.97 W 0.03 Si 1500 nm pure C layer graphite Temperature (K)

26 Temperature dependence of chemical erosion (QMS) Doping with W (V, Ti, Zr) mass- & energy-separated ion beam (30 & 200 ev/d) detection of volatile erosion species (CD 4 ) CD 4 production yield - strong reduction at elevated temperatures - enhancement at RT total yield reduced distribution of erosion products changed jump CD 4 production yield (CD 4 /D) ev D 3 at% W-doped layer RT 620 K Reduction by factor of 15 Reduction by factor of 7! Temperature (K)

27 Temperature dependence of 200 ev D RT Reduction by factor of 3 jump Relative CD 4 production yield ev D pure C layer graphite 4 at% W-doped C layer 750 K Reduction by factor of Temperature (K)

28 Ratio of QMS signal: Temperature dependence Signal ratio of mass 18 to mass ev D W Zr C 30 ev D Temperature (K) Methane cracking D Ratio mass 18 / mass 20: - indicator of CD 3 or cracking of heavier C x D y - changes of ratio change in distribution of erosion species 200 ev: - only methane at T max - C: comparable to 1 kev - doping: changes at high T 30 ev: - mainly methane at RT - doping: changes at high T more CD 4 at RT

29 Conclusion production and characterization of nano-structured metal-containing amorphous carbon films (0 20 at% Me) as model for mixed re-deposited layers chemical erosion strongly affected by doping - total erosion yield drastically reduced (factor of ~10 at elevated temperatures; at RT at least factor of 3) - doping changes distribution of erosion species to more CD 4 production (30 ev) using only the signature of one species leads to wrong erosion yields (e.g. spectroscopic investigations on strongly polluted surface), but CD 4 is may be still indicator for variations

30 Discussion / Questions Contributors: Ch. Adelhelm, E. de Juan Pardo, J. Roth, S. Lindig, A. Herrmann, B. Cieciwa, I. Quintana Alonso, B. Dubiel, E. Welter

31 The end

32 Deuterium retention Deuterium content determined by NRA 25 Deuteium retention (10 20 at/m 2 ) literature average range of implanted D in C 1 kev 25 nm 200 ev 5.5 nm 30 ev 1.2 nm fixed erosion condition fluence D/m kev 200 ev 200 ev 30 ev 30 ev RT RT hot RT hot