Presentation on theme: "Outline of the report Relevance of the topic Experimental limitations and our solutions Results on permeation measurements of Be coated Eurofer samples."— Presentation transcript:
Outline of the report Relevance of the topic Experimental limitations and our solutions Results on permeation measurements of Be coated Eurofer samples Conclusions and further plans
Activities of the Vacuum lab in relation to hydrogen / deuterium gas interaction on ITER grade: stainelss steel, Be and W Study of hydrogen diffusivity, solubility and surface parameters which deteremine the kinetics, by using: 1) infusion / outgassing techniques (until 2009) or (and) 2) membrane techniques (from late 2009) Any of them requires a careful selection of all experimental details to get reliable results. Measurements consist of time consumtion, failures, non-reproducible runs etc...
Processes involved in the hydrogen isotope retention and recycling in fusion reactor plasma-facing materials. R. A. CAUSEY, J. Nucl. Mater., 300, p.91 (2002). Relevance of permeation of H/D/T in fusion reactors
Experimental setup permeation method using H 2 or D 2 (3 UHV chambers) kovar glass and alumina thimbles: infusion / outgassing techniques (2 UHV chambers)
Instead of applying the more common dynamic method, where the ion current of a characteristic mass number applying mass spectrometer is recorded we improved the static method (gas accumulation), pressure recorded by non-ionizing gauges in a small volume followed by precise mass spectrometry
Classical permeation experiment using hydrogen or deuterium
2 CM heads: 1 mbar & 0.05 mbar FS p min = mbar inverted magnetron All metal UHV system: - pressure rise measurement (vol. 0.5L) - inert gauges - post MS 1 bar CM head Vacuum system for permeation measurements using gas accumulation method
Membrane technique [permeation] determine coefficients like –permeability (P), solubility (S), diffusivity (D) etc. –what is the limiting step for hydrogen migration samples investigated in 2010: –Eurofer steel (40mm diam, 0.5mm thick) dr.R.Lindau, IMF-FZK –Be coated Eurofer by TVA at NILPRP (dr.Lungu) –W coated Eurofer at NILPRP (dr.Ruset) High-Z
Conventional permeation cell to test a membrane in the form of a disc Vacuum side High pressure side Au gasket 40 mm O.D. membrane Massive flanges guarantee uniform load of the seal and seems obvious
Thin-walled permeation cell low background H contribution Vacuum side – p LO High pressure side – p HI, H 2 Au gasket 40 mm O.D. membrane Thin-walled inset ~ 0.3 mm, A ~ 66 cm 2 Flanges are not a part of the UHV All permeation measurements have been performed at 400°C. Be / W coated surface is facing the high pressure (upstream) side.
External outlook of the permeation cell Thin-walled inset – downstream side Upstream side
Results on the cell testing A comparative test was done by two identical AISI316 membranes, 2 h to 400 °C and further heating at 400 °C for 72 hours. QMS analysis of accumulated gases done occasionally. Conventional cell with massive flanges d = 12 mm thick area & A 1 15 cm 2 ; d = 0.3 mm thin area & A 2 20 cm 2, d = 0.5 mm membrane & A cm 2 Thin-walled cell d = 0.3 mm thin area & A 2 66 cm 2, d = 0.5 mm membrane & A cm 2
Comparison of the conventional with the thin-walled cell Outgassing rate at 400°C in the first 24 h
Comparison of the conventional with the thin-walled cell Outgassing rate at 400°C in h
Comparison between conventional and improved cell Sensitivity of the setup increased, approx. 380 times much lower permeation fluxes could be recorded. Preparation time of the cell to achieve this value was substantially shorter, 3 days instead of several weeks. Error due to the permeation of H 2 to the air not observed even at p = 0.1 mbar. Minimum detectable flux j = 1.7×10 -9 mbar L/(s cm 2 ) or 4×10 10 H 2 /(s cm 2 ) V.Nemanic, B.Zajec, M. Zumer, JVST A, 28(4) (2010) 578
Permeation measurements through Be coated Eurofer membranes prepared by TVA Idea and motivation Bulk Be sample tiles tested at JET successfully Inconel coated Be tiles tested successfully Similar tiles may be applied at ITER What is the permeation rate of hydrogen H 2 /D 2 /T 2 through such Be films? Published bulk data scattered! If porous, they may increase the tritium retention in Inconel since both neutral gas and plasma enter its surface easier
JET ITER-like wall experiment ITER 100m 2 Tungsten Low erosion high melting T Negligible T retention Optimise lifetime & T- retention But high Z & melting 700m 2 Beryllium first wall low Z Oxygen getter Optimise plasma performance But large erosion & melting 50 m 2 Graphite CFC Lowish Z No melting in transients Superior heat shock behaviour Optimise heat flux resistance But large erosion & T retention W CFC JET
Published data on solubility and diffusivity for hydrogen (H,D,T) in beryllium published data on diffusivity and solubility very scattered and almost useless for prediction of results (A.A. Pisarev, Fusion Techn., 28, (1995) 1262) no data about hydrogen amount in our samples available a few reports on the same Be quality found as a rough guidance for scheduled measurements
Published data on solubility and diffusivity A. Nikroo, H.W. et al, Fusion Science and Technology, 51, 4 (2007) The only report we could find on Be films reveals that Be shells are porous
Permeation of Be/Eurofer prepared by TVA (SEM) As receivedSlightly polished by alumina (1 m, 300 nm, 60 nm grains) Be thickness 8 m, Eurofer 500 m
Permeation of Be/Eurofer prepared by TVA (AFM) As received RMS = 140 nm Polished by alumina RMS = 2 nm – 6 nm
Permeation of Be/Eurofer - fluxes Sampleflux, qPRF H 2 /(s cm 2 ) ? Results at 1 bar upstream pressure and 400 °C obtained a short period after hydrogen was introduced (5 of 8 samples displayed, 3 were too rough or failed) Is there any reason for great scattering of the flux values?
Additional 2 samples with Ag interlayer below Be: Be coated membranes troublesome for permeation measurements: –Au gasket did not tightly seal the membrane fine polishing –Sealing improved but not leak-tight (~10 -5 mbarL/s) Be film is porous for He between Be and Au. Is it porous for H 2 too? UHV [high pressure side] UHV UHV [high pressure side] Ag film much better, but still leak
Permeation of 2 additional samples Ag film+Be Sampleflux, qPRF H 2 /(s cm 2 ) Results at 1 bar upstream pressure and 400 °C Is there any general reason for scattering of the flux values for 8 µm Be prepared by TVA at the same bias? The behaviour also different from bare membranes!?
Eurofer coated with Be: at t = 0 p HI 0 1 bar time / s j / mbar H 2 cm -2 s -1 Permeation reduction factor (PRF) of steady fluxes Permeation rate vs. time for 4 Be coated membranes (first exposure to 1 bar H 2 ) PRF=13 PRF=22 PRF=42 PRF=110 Large scatter in kinetics (shape of curves) & PRF among identical samples. Subsequent H 2 exposures more consistent but still large variations among samples. Steady flux for bare membrane achieved in 10s
Eurofer coated with Be: permeation is limited by diffusion Diffusion limited regime (DLR) mbar
Eurofer coated with Be: porosity leak cannot be completely eliminated small air leak always present air could oxidize Be layer Intentional oxidation by 1 bar of air (10min) increases the permeation rate for 2-5 times. XPS profile of polished & oxidized Be sample
Eurofer coated with Be: could there be small holes (pinholes) in Be film ? Low-angle (2°) polishing wedge Imperfections do exist, but difficult to find. Sample has ~cm 2
TC29 Non-loaded SEM by FZJ-IEF sent by dr.Lungu
Eurofer coated with Be: what can be deduced from permeation meas.? PRF = ~ Assume Be film is homogenous and dense (no holes) & DLR: d1P1d1P1 d2P2d2P2 H P -1 R from electr. Similar to parallel resistors and parallel insulation layers. However: H conc. is not continious Permeation coef. P: Time lag L: Single layer: Two layers: Ash, Barrer, Palmer; Brit. J. Appl. Phys., 1965, VOL. 16, p.873 L j must monotonically increase to the steady state value
Eurofer coated with Be: what can be deduced from permeation meas.? PRF = ~ Assume Be film is homogenous and dense (no holes) & DLR: PRF=10 P Be = 3.0x mol H 2 / m 2 s Pa 0.5 PRF=100 P Be = 2.7x mol H 2 / m 2 s Pa 0.5 P = D × S D = ?, S = ? Measured time-lag not reliable to obtain D. Take solubility S from published data: 400°C S Be ~ mol H 2 / cm 3 Pa 0.5, D Be = 1.5x10 -7 cm 2 /s, L 2 = 26s, D Be = 1.4x10 -8 cm 2 /s, L 2 = 96s 400°C D Be / cm 2 s /T / K -1
Permeation results – conclusions and plans Permeation flux was measured on 10 samples Porosity on the microscopic scale was expressed as intight seal (He) on Be/Au interface or in the bulk Be below Au The SEM evidence on porosity (pinholes) given (FZJ and JSI), but confirmation that a low number of pinholes is indeed responsible for the scattering in permeation flux could not be given at this moment W / Be films will be tested and compared to Be and W films comparison to various models is already in progress
Eurofer coated with Be: what can be deduced from permeation meas.? PRF = ~ Assume Be film is homogenous and dense (no holes) & DLR: Measured PRF are in agreement with published range of Be D and S Observed time lag is considerably longer than predicted (26-96 s)
Eurofer coated with Be: what can be deduced from permeation meas.? PRF = ~ Assume Be film is impermeable, permeation occurs only through pinholes H d r – pinhole radius d – substrate thickness (500μm) F – fraction of free surface W. Prins and J.J. Hermans, J. Phys. Chem., 63 (1959) p. 716 pinhole With known PRF we cannot decouple r and pinhole area density n. Pinholes with smaller radius cover lesser fraction of the membrane surface. Equilibrium H concentration in the substrate at the bottom of the pinhole.
Permeation through clean Eurofer Could the air leak, Be porosity and permeation results be related? Eurofer membrane was oxidized in water vapor and subsequently also in the air. Tested at 1 bar (air) & 400 °C. Duration, conditionsPRF Clean Eurofer well outgassed 1 Oxid. H 2 O, 400°C ~ 25 mbar, satur. at 25°C,18h13.6 Oxid.air 400°C1 bar, 18 h0.75 Porous iron oxide even increases the permeation flux, but Cr rich oxide seems to suppress it.
Permeation Be/Eurofer - searching for explanation A few Be coated Eurofer membranes were also oxidized in the air at 1 bar and 400 °C. Results do not express so evident influence of the air, since all samples have been previously (inevitably) exposed to air leaks when a thin but dense oxides could be formed. The expected suppressing of hydrogen flux was not achieved as expected since BeO should be even less permeable than Be It seems that Eurofer is exposed to the air in pinholes or voids in Be Some facts should be verified by a new experiment.
Binding Energy (eV) Normalized Intensity -C1s -Ar2p -Be1s -O1s -O2s -Be1s Surface After sputtering, subsurface region -O1s XPS survey spectra on polished Be sample
XPS spectra Be 1s as a function of depth X-ray photoelectron analysis -XPS XPS: very surface sensitive technique XPS depth profiling (by Ar ion sputtering) => in-depth distribution of elements Be covered by Be-oxide layer Be-oxide layer thickness ~ (35 ± 10) nm, native only 2-3 nm
Permeation of oxidized Be/Eurofer (SEM) Polished oxidized Be heavily oxidized Eurofer