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Re-emission and thermal desorption of deuterium from plasma sprayed tungsten coatings for application in ASDEX-upgrade
ELSEVIER
Journal of Nuclear Materials 233-237
(1996) 803-808
journal of nuclear materials
Re-emission and thermal desorption of deuterium from plasma sprayed tungsten coatings for application in ASDEX-upgrade C. Garcia-Rosales
a,*, P. Franzen a, H. Plank a, J. Roth ‘, E. Gauthier b
Abstract The trapping and release of deuterium implanted with an energy of 100 eV in wrought and in plasma sprayed tungsten of different manufacture and structure has been investigated by means of re-emission as well as thermal and isothermal desorption spectroscopy. The experimental data for wrought tungsten are compared with model calculations with the PIDAT code in order to estimate the parameters governing diffusion, surface recombination and trapping in tungsten. The amount of retained deuterium in tungsten is of the same order of magnitude as in graphite for the implantation parameters used in this work. The mobile hydrogen concentration in tungsten during the implantation is of the same order of magnitude than the trapped one, being released after the termination of the implantation. The fraction of deuterium trapped to defects increases strongly with the porosity of the samples. The temperature needed for the release of the trapped deuterium (N 600 K) are considerably lower than for graphite, due to the smaller trapping energy (I 1.5 eV).
Beside carbon and beryllium, the high-Z material tungsten is under consideration as plasma facing material for ITER [I]. In order to provide an experimental data base for the ITER divertor, a limited campaign using tungsten as divertor target plate material will be performed at the divertor tokamak ASDEX-Upgrade at the end of 1995. For the technical realisation of this experiment, the present graphite tiles will be coated with plasma sprayed tungsten. The suitability of these coatings for application in ASDEX-Upgrade has been proved by high heat flux tests with power loads up to I6 MW/m2 for a duration of 2 s [2]. One important issue concerning the applicability of plasma sprayed tungsten coatings for plasma facing components is the trapping and release of hydrogenic particles that controls the tritium retention, particle balance and recycling in fusion devices. Plasma sprayed coatings exhibit a relatively high porosity which may influence strongly hydrogen retention. On the other hand, relatively little is known about hydrogen re-emission and trapping
’ Corresponding author. Tel.: + 49-89-32992224; 32992580; e-mail: [email protected]. 0022.31 15/96/$15.00 PII
SOO22-3
tungsten at implantation energies I 100 eV, as will be relevant for the walls of the divertor chamber of ASDEX-Upgrade and ITER. The up to now available data correspond to incident energies of I [3,4] and 7.5 keV
from
1. Introduction
Copyright
I I5(96)00185-7
6 1996 Published
fax: + 49.89-
[5,41. In this work, release and trapping of deuterium implanted into tungsten with an energy of 100 eV measurements by re-emission experiments and thermal desorption spectroscopy (TDS) will be shown. Two different qualities of plasma sprayed tungsten coatings has been investigated and compared to wrought tungsten as reference. Calculations to fit the experimental data on re-emission and thermal desorption of wrought tungsten were performed with the PIDAT (ParticleImplantation, Diffusion And Trapping) computer code [6] in order to estimate rate coefficients for diffusion, recombination and trapping. The results are discussed in comparison with graphite.
2. Experimental Two different plasma sprayed (PS) tungsten coatings with different spray conditions and hence different structures (see Table I) were investigated and compared to
by Elsevier Science B.V. All rights reserved
Table I Plasma sprayed samples and their properties Sample
PV-so0
cv-200
Manufactures
Piansee-AG
CEN-Cadarache
Spray atmosphere
Vacuum
Argon
Layer thickness
500 pm
Intcmiediate
Re-PVD
Themlal
layer
treatment
Porosity Surf. roughness
K,
200 pm (IO pm)
-
I4OOO”C.lh &‘)C/r,
I %20%’
ca. 4 I*rn
ca. 18 I*“1
wrought tungsten. Both PS samples were sprayed on a fine grain graphite substrate. The sample PV-500, provided by Plansee AG with a thickness of 500 pm, was sprayed in vacuum and was subjected to a thermal treatment (1400°C for 1 h) after the spray process to achieve a homogeneously recrystallized coating morphology. This sample has a rhenium-containing intemrediate layer (IO pm thickness) deposited by physical vapour deposition between the W-coating and the graphite substrate in order to prevent carbide formation during thermal treatment and high temperature operation. The porosity is of the order of 89%. The sample CV-200 with a thickness of 200 pm was provided by CEN Cadarache and was sprayed in an Ar atmosphere. This sample has no intermediate layer and
was not subjected to a themral treatment. The porosity of this coating is about l5-20%, i.e. considerably higher than the porosity of the PV-500 sample. Fig. 1 shows metallographic sections of the two PS coatings. While the PV-500 sample has a homogeneous, recrystallized structure, the CV-200 sample shows a lamellar structure of molten droplets formed during the plasma spray process with large macro-pores between the lamellae. The re-emission and TDS experiments were performed in the high-current ion source at Garching described in [7]. The samples were implanted with 300 eV D: at room temperature with fluences varying from I X IO” to I X IO” D/cm*. The ion beam current was between 40 and 70 PA equivalent to a flux density of 100 eV deuterons of 2.5 X IO” to 4.4 X IO” D/cm’s, Before implantation the surface was bombarded with 3 keV D_T at 800 K with a fluence 26~ 10’s D/cm’ in order to avoid tungsten oxide layers on the surface. The amount of trapped deuterium was measured by TDS with a maximum temperature of 1400 K and a heating rate of 5 + 0.5 K/s. The samples were heated by electron bombardment from the rear and the temperature was measured by means of an infrared pyrometer. The released particles (HD, D,) during the course of the reemission and TDS experiments were detected with a Balzers 5 I I quadrupole mass spectrometer (QMS). The contribution of HD to the total amount of released D was
PV-500
Fig.
I,
Metallographic
sections of plasma sprayed tungsten coatings on graphite.
C. Garcia-Resales
et al./Journal Temperature
,
.,.
300 600 7-7-7~ 7,
_~
Outgassing
;
,-
100@V0+~W flux: 2xtP O+cm-%’ 300K
0
200
(K) 900
233-237
(1996) 803-808
TDS
3.1. Deuterium release from wrought-W and model calculations
i
600
400
600
Time(s) Fig. 2. Experimental data (0) for deuterium re-emission from wrought-W during deuterium ion bombardment with a flux of 2X IO” D+ crnm2 s-‘, during isothermal release, and during thermal desorption, and results of model calculations with the parameters of Table 2 (solid line).
Fig. 2 shows the released flux from wrought tungsten during 100 eV D+ implantation, during isothermal release after the implantation and during TDS release, as reference for comparison with the plasma sprayed coatings and as basis for model calculations. The TDS spectrum consists of two peaks with peak temperatures of 475 and 850 K. The model of deuterium re-emission and thermal release from tungsten follows the models of deuterium release from graphite [8,9]. The detailed description can be found elsewhere [lo]. Briefly the model assumes that the total concentration of deuterium is given by
c( XJ) = cs(XJ) + The QMS was calibrated by means of the m-emission signal at high implantation fluences (ca. 1 x IO’* D/cm*).
of the parameters
c
i=
negligible.
Table 2 Summary
805
3. Results and discussion
5 Kh
300 K
:
of‘Nucleur Mom-ids
Ct,; I
where c, is the solute concentration, nr the number of traps, and ct.; the deuterium concentration bounded to trap
used for the model calculation
Parameter
Value
Remarks
Surface recombination KC Er
preexp. factor activation energy
> 7.7 X 10-j - 0.59 eV
preexp. factor activation energy
3.5 X IO-’ cm*/s 0.39 eV
cmfi/s
tit
[I21
Diffusion Da J%J
fit
[I21
Trapping Sample properties: n7. R
number of traps
2
from TDS peaks
trapping radius
6.5 A
Eq. (5)
‘A
atomic radius atomic density
1.41 A 6.3 X IO’* W/cm3
1181
energy concentration
0.85 eV 0.01 Traps/W
fit to TDS peak
energy concentration
1.4 eV 0.07 Traps/W
tit to TDS peak
incoming ion flux Gaussian with:
2.5 X IO” D+ cm-*
N*
= 19.3 g/cm3
[ 181
Trap # I :
ET,, I
cr.
Trap #2: E T.2 cr.2 Source: J S,(x)
center
43 A
std. deviation
23 i
maximum
130A
range
SC’
from experiment from TRIM.SP [ 191
C. Garcia-Rosu1e.s et ai./Journal
806
Table 3 Amount of retained deuterium CV-200 Sample
in and TDS peak temperature
Porosity
Wrought-W
5 1%
PV-500 cv-200
8-9% 15-20%
oJ‘Nucleur Moterids
from wrought-W
act I 2 = 4_rrRDN,c,(c,,, at
-ct.,)
2nd peak
1.6 x lOI 2.6 x lOI 4.4 x 10’6
415 525 612
850 670
- l”U”Li)
(2) nT (3)
and is related
to the
D is the diffusion coefficient, assumed to obey an Arrhenius equation in the whole temperature range, and cri is the concentration of trap i. NA is the atomic density, and R the trapping radius, assumed to be equal for all traps and given by [ 1 I]
(5)
R=3GrA r, being the atomic radius, k,,, is the rate coefficient thermal detrapping and is assumed to be given by k,,, = 10” s-l
X exp
for
E - -$
( 1
E,.; being the trap energy. The boundary condition is given by the flux of deuterium molecules leaving the tungsten surface. The atomic release of deuterium from tungsten [3] is not included in the model due to the relatively low maximum temperature achieved at the TDS experiments. In contrast to graphite, recombination takes place at the surface with the rate coefficient K,. Hence, assuming a semi-infinite sample, the released flux J, is given by, J,=K,cf(x=O)with
KP
K,=-exp fi
(7)
E, being the activation energy. This leads to the boundary condition
ac,-
Z-D
Kr
-ci(x=O)
plasma sprayed coatings
1st peak
- c,,,) - k,,;c ,,,, i = I
S,(x) is the source distribution incoming ion flux J by
and the two investigated
TDS peak temperature
% -S,(x)-D$ at c (47rKDN*c,(cr,, i= I
(1996) 803-808
Trapped deuterium (D/cm’)
i. In contrast to graphite, trapping is assumed to be diffusion limited. The concentrations are varying in space and time according to
-
233-237
PV-500 and
(K)
The differential Eqs. (2) and (3) with the boundary condition given by Eq. (8) can be solved numerically by means of a special version (I .O> of the PIDAT package [6]. The comparison of the model results, calculated with the parameters of Table 2, with the experimental data for the case of wrought-W is shown in Fig. 2. The agreement is good, except for the second TDS peak. However, the experimental data in this region are rather uncertain due to background problems. We are aware that a different set of parameters might lead to the same result, but we tried to keep the number of fitting parameters as low as possible and take as many parameters as possible from literature. As a result of the calculations, the release of deuterium from tungsten during deuterium ion bombardment or thermal treatment is governed by diffusion. Therefore, the recombination rate coefficient given in Table 2 must be regarded as a lower limit and cannot be compared with the experimental achieved data in the literature [5,12,13]. From the calculation it also results that about 80% of the thermally desorbed deuterium in the first TDS peak origins from the solute concentration c, while the second small peak mainly results from trapping by defects. This is in agreement with the observations of Pisarev et al. [5], who also connects the large low temperature peak to deuterium in interstitial solution and the small peaks at higher temperatures with trapping in vacancies. Therefore, the fraction of solute deuterium at the end of implantation is considerably high. 3.2. TDS
measurements
on plasma
sprayed
coatings
Typical TDS spectra of D, molecules released from wrought-W and the two investigated plasma sprayed tungsten coatings CV-200 and PV-500 after implantation at room temperature with a fluence of I X IO’* D/cm2 are presented in Fig. 3. An enhancement of the first peak and a shift to higher temperatures with increasing porosity of the samples is observed, while the second peak is shifted to lower temperatures and disappears in the first peak. The amount of retained deuterium, obtained by integrating the desorption spectra, is given in Table 3 together with the porosity of the samples. The total amount of retained deuterium increases linearly with the porosity, probably due to enhanced trapping in closed pores and grain boundaries and due to the enhanced diffusion range in the
C. Garcia-Resales
1.5 i
:‘; r \
500
700
r?fNucleor
cv-200
1 t
1
300
et al./Jourwl
PV-500 wrought-W
900
1100
Temperature (K) Fig. 3. Thermal desorption of D, molecules from wrought-W ( A) and two different plasma sprayed coatings, CV-200 (0) and PV-500 (Cl) implanted with 100 eV Df ions at room temperature
Muterials
233-237
11996) 803-808
807
compared to the inner surface diffusion in graphite (6 X 10m6 D/cm2 at 300 K). As a result of the model calculation, the trapping zone is about a factor of IO larger than the implantation zone, whereas in graphite almost all deuterium is trapped in the implantation zone. In Fig. 4, the amount of retained deuterium for the sample with the highest porosity, CV-200 is shown as a function of the implanted fluence. The amount of retained deuterium reaches a relatively high level already at fluences I 1 x 10” D/cm’ and increases less steeply with increasing fluence indicating that a relatively high deuterium inventory can be achieved at high fluences due to the fast deuterium diffusion in the bulk. The fast diffusion also leads to a shift of the TDS peak temperature to higher temperatures with increasing implantation fluence due to the larger distance to the surface.
with a fluence of I X IO’* D/cm*.
presence of pores leading to an access to traps deeper in the sample. The amount of retained deuterium in tungsten is of the same order of magnitude as in graphite for 100 eV D+ implantation (about 2 X lOI D/cm2 at saturation fluences [ 141) in spite of the lower trap energies and trap concentrations. The main difference between graphite and tungsten is the location of recombination. In graphite recombination takes place throughout the whole implantation and diffusion zone at the inner surfaces of the crystallites, being faster than the inner surface diffusion (at temperatures below ca. 1200 K) [9]. The deuterium molecules are released very quickly. Therefore, the solute concentration in graphite is several orders of magnitude lower than the trapped concentration. In the case of tungsten, in contrast, recombination takes place at the geometrical surface. Therefore, a solute concentration comparable to the trapped one is built up during the implantation, in spite of the smaller diffusion coefficient of deuterium in bulk tungsten
Implanted fluence (1017D/cm2) Fig. 4. Deuterium retention in plasma sprayed coating CV-200, and shift of the TDS peak temperature, as a function of the incident fluence.
4. Conclusions The results of this work can be summarized as follows: The amount of retained deuterium in tungsten is of the same order of magnitude as in graphite, for the implantation parameters used in this work (100 eV D+ implantation, fluences up to 1 X 10”). This is in contrast to the behaviour assumed up to now [ 131 and can be attributed to the considerably higher solute concentration in tungsten than in graphite leading to a trapping zone about 10 times larger than the implantation zone, whereas in graphite almost all deuterium is trapped in the implantation zone. In tungsten, the fraction of deuterium trapped to defects increases strongly with the porosity of the samples. The thermal desorption of deuterium from tungsten occurs at temperatures considerably lower than for graphite. However, depending on the porosity: the peak temperature for the release of the larger amount of deuterium is about 475 K for the case of the dense, wrought tungsten, shifting to about 610 K for the most porous sample (CV-200). For graphite peak temperatures are between 900 and 1200 K (see e.g. [9,15- 171). For ITER, tungsten is under consideration as material for the divertor baffle, the divertor dome and the ‘ transparent wall’ [l], hence representing a considerably high surface area which will influence the recycling in the divertor region. These areas will be subject to high fluxes of low energy neutrals (I 100 eV) leading to a high tritium inventory, as the results of this work show, depending on the material system used (dense tungsten or plasma sprayed coatings). However, heating after plasma discharges or plasma operation at wall temperatures of the order of 600-650 K will lead to a complete release of the retained tritium. This release could, however, be strongly influenced by impurities deposited on the surface like carbon, oxygen and boron. Therefore, further investigations are needed in order to study the effect of redeposited layers on hydrogen release from tungsten.
C. Garcia-Rosules
808
et ul./Journul
of‘Nucleur
Acknowledgements We would like to thank Dr. B.M.U. Scherzer for many valuable
discussions
and
Dr.
M.
Mayer
for
performing
in tungsten with the TRIM.SP programm. The technical assistance of W. Ottenberger is greatly acknowledged. calculations
on
the
implantation
range
of
deuterium
References [II G. Janeschitz.
J. Nucl. Mater. 220-222 (1995) 73. Deschka, C. Garcia-Rosales, W. Hohenauer, R. Duwe, E. Gauthier, .I. Linke, M. Lochter, W. Mallner, L. Plijchl, P. Rodhammer and A. Salito, these Proceedings, p. 645. [31J.W. Davis and A.A. Haasz. J. Nucl. Mater. 223 (1995) 312. [41 V. Kh. Alimov and B.M.U. Scherzer, in preparation. J. Nucl. [51 A.A. Pisarev, A.V. Varava and SK. Zhdanov, Mater. 220-222 (I 995) 926. fir [61W. MiYler. Tech. Rep. IPP 9/44 (Max-Plan&Institut Plasmaphysik, 1983). J. Roth and W. Ottenberger, I71 W. E&stein, C. Garcia-Rosales, Sputtering data, Tech. Rep. IPP 9/82 (Max-Plan&-Institut tir Plasmaphysik, 1993).
I21S.
Materials
233-237
(19961
803-808
[8] W. Miiller and B.M.U. Scherzer, J. Appl. Phys. 64 (10) (I 988) 4860. 191 A.A. Haasz, P. Franzen, J.W. Davis, S. Chiu and C.S. Pitcher, J. Appl. Phys. 77 (1995) 66. [lOI P. Franzen, B.M.U. Scherzer, V.Kh. Alimov and C. GarciaRosales, (1995) in preparation. [l II G. Duesing, H. Hemmerich, W. Sassin and W. Schilling, Proc. Int. Conf. Vacancies Interstitials Metals (1970) 246. for Plasma-Surface In[I21 K.L. Wilson, In: Data Compendium teractions, (IAEA, Vienna, 1984). [I31 K.L. Wilson, R. Bastasz, R.A. Causey, D.K. Brice, B.L. Doyle, W.R. Wampler, W. Miiller, B.M.U. Scherzer and T. Tanabe, In: Atomic and Plasma-Material Interaction Data for Fusion (IAEA, Vienna, 1991). J. Roth, R. Behrisch, J. Bohdansky, W. [I41 G. Staudenmaier, E&stein, P. Staib, S. Matteson and S.K. Erents, J. NucI. Mater. 84 (1979) 149. J. Roth and R. Behrisch, J. Nucl. Mater. [I51 C. Garcia-Rosales, 212-215 (1994) I21 1. [I61 J.A. Sawicki, J. Roth and L.M. Howe, J. Nucl. Mater. 162-164 (1989) 1019. [171 V. Philipps, E. Vietzke, M. Erdweg and K. Flaskamp, J. Nucl. Mater. I455 147 (1987) 292. fur Chemiker und Physiker, [I81 D’ans-Lax, Taschenbuch (Springer, Berlin, 1978). [191 W. E&stein, Mater. Sci. 10 (1991) I.
Journal of Nuclear Materials 233-237
(1996) 803-808
journal of nuclear materials
Re-emission and thermal desorption of deuterium from plasma sprayed tungsten coatings for application in ASDEX-upgrade C. Garcia-Rosales
a,*, P. Franzen a, H. Plank a, J. Roth ‘, E. Gauthier b
Abstract The trapping and release of deuterium implanted with an energy of 100 eV in wrought and in plasma sprayed tungsten of different manufacture and structure has been investigated by means of re-emission as well as thermal and isothermal desorption spectroscopy. The experimental data for wrought tungsten are compared with model calculations with the PIDAT code in order to estimate the parameters governing diffusion, surface recombination and trapping in tungsten. The amount of retained deuterium in tungsten is of the same order of magnitude as in graphite for the implantation parameters used in this work. The mobile hydrogen concentration in tungsten during the implantation is of the same order of magnitude than the trapped one, being released after the termination of the implantation. The fraction of deuterium trapped to defects increases strongly with the porosity of the samples. The temperature needed for the release of the trapped deuterium (N 600 K) are considerably lower than for graphite, due to the smaller trapping energy (I 1.5 eV).
Beside carbon and beryllium, the high-Z material tungsten is under consideration as plasma facing material for ITER [I]. In order to provide an experimental data base for the ITER divertor, a limited campaign using tungsten as divertor target plate material will be performed at the divertor tokamak ASDEX-Upgrade at the end of 1995. For the technical realisation of this experiment, the present graphite tiles will be coated with plasma sprayed tungsten. The suitability of these coatings for application in ASDEX-Upgrade has been proved by high heat flux tests with power loads up to I6 MW/m2 for a duration of 2 s [2]. One important issue concerning the applicability of plasma sprayed tungsten coatings for plasma facing components is the trapping and release of hydrogenic particles that controls the tritium retention, particle balance and recycling in fusion devices. Plasma sprayed coatings exhibit a relatively high porosity which may influence strongly hydrogen retention. On the other hand, relatively little is known about hydrogen re-emission and trapping
’ Corresponding author. Tel.: + 49-89-32992224; 32992580; e-mail: [email protected]. 0022.31 15/96/$15.00 PII
SOO22-3
tungsten at implantation energies I 100 eV, as will be relevant for the walls of the divertor chamber of ASDEX-Upgrade and ITER. The up to now available data correspond to incident energies of I [3,4] and 7.5 keV
from
1. Introduction
Copyright
I I5(96)00185-7
6 1996 Published
fax: + 49.89-
[5,41. In this work, release and trapping of deuterium implanted into tungsten with an energy of 100 eV measurements by re-emission experiments and thermal desorption spectroscopy (TDS) will be shown. Two different qualities of plasma sprayed tungsten coatings has been investigated and compared to wrought tungsten as reference. Calculations to fit the experimental data on re-emission and thermal desorption of wrought tungsten were performed with the PIDAT (ParticleImplantation, Diffusion And Trapping) computer code [6] in order to estimate rate coefficients for diffusion, recombination and trapping. The results are discussed in comparison with graphite.
2. Experimental Two different plasma sprayed (PS) tungsten coatings with different spray conditions and hence different structures (see Table I) were investigated and compared to
by Elsevier Science B.V. All rights reserved
Table I Plasma sprayed samples and their properties Sample
PV-so0
cv-200
Manufactures
Piansee-AG
CEN-Cadarache
Spray atmosphere
Vacuum
Argon
Layer thickness
500 pm
Intcmiediate
Re-PVD
Themlal
layer
treatment
Porosity Surf. roughness
K,
200 pm (IO pm)
-
I4OOO”C.lh &‘)C/r,
I %20%’
ca. 4 I*rn
ca. 18 I*“1
wrought tungsten. Both PS samples were sprayed on a fine grain graphite substrate. The sample PV-500, provided by Plansee AG with a thickness of 500 pm, was sprayed in vacuum and was subjected to a thermal treatment (1400°C for 1 h) after the spray process to achieve a homogeneously recrystallized coating morphology. This sample has a rhenium-containing intemrediate layer (IO pm thickness) deposited by physical vapour deposition between the W-coating and the graphite substrate in order to prevent carbide formation during thermal treatment and high temperature operation. The porosity is of the order of 89%. The sample CV-200 with a thickness of 200 pm was provided by CEN Cadarache and was sprayed in an Ar atmosphere. This sample has no intermediate layer and
was not subjected to a themral treatment. The porosity of this coating is about l5-20%, i.e. considerably higher than the porosity of the PV-500 sample. Fig. 1 shows metallographic sections of the two PS coatings. While the PV-500 sample has a homogeneous, recrystallized structure, the CV-200 sample shows a lamellar structure of molten droplets formed during the plasma spray process with large macro-pores between the lamellae. The re-emission and TDS experiments were performed in the high-current ion source at Garching described in [7]. The samples were implanted with 300 eV D: at room temperature with fluences varying from I X IO” to I X IO” D/cm*. The ion beam current was between 40 and 70 PA equivalent to a flux density of 100 eV deuterons of 2.5 X IO” to 4.4 X IO” D/cm’s, Before implantation the surface was bombarded with 3 keV D_T at 800 K with a fluence 26~ 10’s D/cm’ in order to avoid tungsten oxide layers on the surface. The amount of trapped deuterium was measured by TDS with a maximum temperature of 1400 K and a heating rate of 5 + 0.5 K/s. The samples were heated by electron bombardment from the rear and the temperature was measured by means of an infrared pyrometer. The released particles (HD, D,) during the course of the reemission and TDS experiments were detected with a Balzers 5 I I quadrupole mass spectrometer (QMS). The contribution of HD to the total amount of released D was
PV-500
Fig.
I,
Metallographic
sections of plasma sprayed tungsten coatings on graphite.
C. Garcia-Resales
et al./Journal Temperature
,
.,.
300 600 7-7-7~ 7,
_~
Outgassing
;
,-
100@V0+~W flux: 2xtP O+cm-%’ 300K
0
200
(K) 900
233-237
(1996) 803-808
TDS
3.1. Deuterium release from wrought-W and model calculations
i
600
400
600
Time(s) Fig. 2. Experimental data (0) for deuterium re-emission from wrought-W during deuterium ion bombardment with a flux of 2X IO” D+ crnm2 s-‘, during isothermal release, and during thermal desorption, and results of model calculations with the parameters of Table 2 (solid line).
Fig. 2 shows the released flux from wrought tungsten during 100 eV D+ implantation, during isothermal release after the implantation and during TDS release, as reference for comparison with the plasma sprayed coatings and as basis for model calculations. The TDS spectrum consists of two peaks with peak temperatures of 475 and 850 K. The model of deuterium re-emission and thermal release from tungsten follows the models of deuterium release from graphite [8,9]. The detailed description can be found elsewhere [lo]. Briefly the model assumes that the total concentration of deuterium is given by
c( XJ) = cs(XJ) + The QMS was calibrated by means of the m-emission signal at high implantation fluences (ca. 1 x IO’* D/cm*).
of the parameters
c
i=
negligible.
Table 2 Summary
805
3. Results and discussion
5 Kh
300 K
:
of‘Nucleur Mom-ids
Ct,; I
where c, is the solute concentration, nr the number of traps, and ct.; the deuterium concentration bounded to trap
used for the model calculation
Parameter
Value
Remarks
Surface recombination KC Er
preexp. factor activation energy
> 7.7 X 10-j - 0.59 eV
preexp. factor activation energy
3.5 X IO-’ cm*/s 0.39 eV
cmfi/s
tit
[I21
Diffusion Da J%J
fit
[I21
Trapping Sample properties: n7. R
number of traps
2
from TDS peaks
trapping radius
6.5 A
Eq. (5)
‘A
atomic radius atomic density
1.41 A 6.3 X IO’* W/cm3
1181
energy concentration
0.85 eV 0.01 Traps/W
fit to TDS peak
energy concentration
1.4 eV 0.07 Traps/W
tit to TDS peak
incoming ion flux Gaussian with:
2.5 X IO” D+ cm-*
N*
= 19.3 g/cm3
[ 181
Trap # I :
ET,, I
cr.
Trap #2: E T.2 cr.2 Source: J S,(x)
center
43 A
std. deviation
23 i
maximum
130A
range
SC’
from experiment from TRIM.SP [ 191
C. Garcia-Rosu1e.s et ai./Journal
806
Table 3 Amount of retained deuterium CV-200 Sample
in and TDS peak temperature
Porosity
Wrought-W
5 1%
PV-500 cv-200
8-9% 15-20%
oJ‘Nucleur Moterids
from wrought-W
act I 2 = 4_rrRDN,c,(c,,, at
-ct.,)
2nd peak
1.6 x lOI 2.6 x lOI 4.4 x 10’6
415 525 612
850 670
- l”U”Li)
(2) nT (3)
and is related
to the
D is the diffusion coefficient, assumed to obey an Arrhenius equation in the whole temperature range, and cri is the concentration of trap i. NA is the atomic density, and R the trapping radius, assumed to be equal for all traps and given by [ 1 I]
(5)
R=3GrA r, being the atomic radius, k,,, is the rate coefficient thermal detrapping and is assumed to be given by k,,, = 10” s-l
X exp
for
E - -$
( 1
E,.; being the trap energy. The boundary condition is given by the flux of deuterium molecules leaving the tungsten surface. The atomic release of deuterium from tungsten [3] is not included in the model due to the relatively low maximum temperature achieved at the TDS experiments. In contrast to graphite, recombination takes place at the surface with the rate coefficient K,. Hence, assuming a semi-infinite sample, the released flux J, is given by, J,=K,cf(x=O)with
KP
K,=-exp fi
(7)
E, being the activation energy. This leads to the boundary condition
ac,-
Z-D
Kr
-ci(x=O)
plasma sprayed coatings
1st peak
- c,,,) - k,,;c ,,,, i = I
S,(x) is the source distribution incoming ion flux J by
and the two investigated
TDS peak temperature
% -S,(x)-D$ at c (47rKDN*c,(cr,, i= I
(1996) 803-808
Trapped deuterium (D/cm’)
i. In contrast to graphite, trapping is assumed to be diffusion limited. The concentrations are varying in space and time according to
-
233-237
PV-500 and
(K)
The differential Eqs. (2) and (3) with the boundary condition given by Eq. (8) can be solved numerically by means of a special version (I .O> of the PIDAT package [6]. The comparison of the model results, calculated with the parameters of Table 2, with the experimental data for the case of wrought-W is shown in Fig. 2. The agreement is good, except for the second TDS peak. However, the experimental data in this region are rather uncertain due to background problems. We are aware that a different set of parameters might lead to the same result, but we tried to keep the number of fitting parameters as low as possible and take as many parameters as possible from literature. As a result of the calculations, the release of deuterium from tungsten during deuterium ion bombardment or thermal treatment is governed by diffusion. Therefore, the recombination rate coefficient given in Table 2 must be regarded as a lower limit and cannot be compared with the experimental achieved data in the literature [5,12,13]. From the calculation it also results that about 80% of the thermally desorbed deuterium in the first TDS peak origins from the solute concentration c, while the second small peak mainly results from trapping by defects. This is in agreement with the observations of Pisarev et al. [5], who also connects the large low temperature peak to deuterium in interstitial solution and the small peaks at higher temperatures with trapping in vacancies. Therefore, the fraction of solute deuterium at the end of implantation is considerably high. 3.2. TDS
measurements
on plasma
sprayed
coatings
Typical TDS spectra of D, molecules released from wrought-W and the two investigated plasma sprayed tungsten coatings CV-200 and PV-500 after implantation at room temperature with a fluence of I X IO’* D/cm2 are presented in Fig. 3. An enhancement of the first peak and a shift to higher temperatures with increasing porosity of the samples is observed, while the second peak is shifted to lower temperatures and disappears in the first peak. The amount of retained deuterium, obtained by integrating the desorption spectra, is given in Table 3 together with the porosity of the samples. The total amount of retained deuterium increases linearly with the porosity, probably due to enhanced trapping in closed pores and grain boundaries and due to the enhanced diffusion range in the
C. Garcia-Resales
1.5 i
:‘; r \
500
700
r?fNucleor
cv-200
1 t
1
300
et al./Jourwl
PV-500 wrought-W
900
1100
Temperature (K) Fig. 3. Thermal desorption of D, molecules from wrought-W ( A) and two different plasma sprayed coatings, CV-200 (0) and PV-500 (Cl) implanted with 100 eV Df ions at room temperature
Muterials
233-237
11996) 803-808
807
compared to the inner surface diffusion in graphite (6 X 10m6 D/cm2 at 300 K). As a result of the model calculation, the trapping zone is about a factor of IO larger than the implantation zone, whereas in graphite almost all deuterium is trapped in the implantation zone. In Fig. 4, the amount of retained deuterium for the sample with the highest porosity, CV-200 is shown as a function of the implanted fluence. The amount of retained deuterium reaches a relatively high level already at fluences I 1 x 10” D/cm’ and increases less steeply with increasing fluence indicating that a relatively high deuterium inventory can be achieved at high fluences due to the fast deuterium diffusion in the bulk. The fast diffusion also leads to a shift of the TDS peak temperature to higher temperatures with increasing implantation fluence due to the larger distance to the surface.
with a fluence of I X IO’* D/cm*.
presence of pores leading to an access to traps deeper in the sample. The amount of retained deuterium in tungsten is of the same order of magnitude as in graphite for 100 eV D+ implantation (about 2 X lOI D/cm2 at saturation fluences [ 141) in spite of the lower trap energies and trap concentrations. The main difference between graphite and tungsten is the location of recombination. In graphite recombination takes place throughout the whole implantation and diffusion zone at the inner surfaces of the crystallites, being faster than the inner surface diffusion (at temperatures below ca. 1200 K) [9]. The deuterium molecules are released very quickly. Therefore, the solute concentration in graphite is several orders of magnitude lower than the trapped concentration. In the case of tungsten, in contrast, recombination takes place at the geometrical surface. Therefore, a solute concentration comparable to the trapped one is built up during the implantation, in spite of the smaller diffusion coefficient of deuterium in bulk tungsten
Implanted fluence (1017D/cm2) Fig. 4. Deuterium retention in plasma sprayed coating CV-200, and shift of the TDS peak temperature, as a function of the incident fluence.
4. Conclusions The results of this work can be summarized as follows: The amount of retained deuterium in tungsten is of the same order of magnitude as in graphite, for the implantation parameters used in this work (100 eV D+ implantation, fluences up to 1 X 10”). This is in contrast to the behaviour assumed up to now [ 131 and can be attributed to the considerably higher solute concentration in tungsten than in graphite leading to a trapping zone about 10 times larger than the implantation zone, whereas in graphite almost all deuterium is trapped in the implantation zone. In tungsten, the fraction of deuterium trapped to defects increases strongly with the porosity of the samples. The thermal desorption of deuterium from tungsten occurs at temperatures considerably lower than for graphite. However, depending on the porosity: the peak temperature for the release of the larger amount of deuterium is about 475 K for the case of the dense, wrought tungsten, shifting to about 610 K for the most porous sample (CV-200). For graphite peak temperatures are between 900 and 1200 K (see e.g. [9,15- 171). For ITER, tungsten is under consideration as material for the divertor baffle, the divertor dome and the ‘ transparent wall’ [l], hence representing a considerably high surface area which will influence the recycling in the divertor region. These areas will be subject to high fluxes of low energy neutrals (I 100 eV) leading to a high tritium inventory, as the results of this work show, depending on the material system used (dense tungsten or plasma sprayed coatings). However, heating after plasma discharges or plasma operation at wall temperatures of the order of 600-650 K will lead to a complete release of the retained tritium. This release could, however, be strongly influenced by impurities deposited on the surface like carbon, oxygen and boron. Therefore, further investigations are needed in order to study the effect of redeposited layers on hydrogen release from tungsten.
C. Garcia-Rosules
808
et ul./Journul
of‘Nucleur
Acknowledgements We would like to thank Dr. B.M.U. Scherzer for many valuable
discussions
and
Dr.
M.
Mayer
for
performing
in tungsten with the TRIM.SP programm. The technical assistance of W. Ottenberger is greatly acknowledged. calculations
on
the
implantation
range
of
deuterium
References [II G. Janeschitz.
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