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Helium impurity interactions in vanadium and niobium
ELSEVIER
Journal of Nuclear Materials 212-215 (1994) 287-292
Helium impurity interactions in vanadium and niobium A. van Veen, H. Eleveld, M. Clement Interfaculty Reactor Institute, Delft University of Technology, Mekelweg 15, Nl-2629 JB Delft, The Netherlands
Abstract
Thermal helium desorption spectrometry (THDS) has been employed to study the nucleation and growth of helium defect agglomerates in niobium and vanadium. An important difference from other body-centered-cubic metals was the temperature at which helium was released from monovacancy type defects. In MO and W the release takes place at 0.4OT, (T, = melting temperature) but in V and Nb at OZT,,,. This and other observations led to the conclusion that in nearly all interactions of helium with radiation-induced defects, impurities, mainly oxygen, are involved. Impurity levels of 20-30 appm are sufficient to completely suppress the type of helium behaviour that was found in MO and W. Oxygen decorates the helium-vacancy complexes and makes the helium desorption temperature lower, but stabilizes the small defect complexes up to a temperature of about 700 K, beyond this temperature, nucleation of defect clusters is considerably reduced. Oxygen decoration has been observed to reduce the sink strength of the defects for helium trapping.
1. Introduction
The development of low-activation materials (LAM) will be of paramount importance for future energy-producing plasma fusion reactors [l]. Vanadium is the basic constituent in a class of low-activation alloys that are considered long-term candidates for first-wall material. Studies on the mechanical properties of irradiated alloys have shown that impurities and precipitates may play an important role in the observed behavior [2]. Only few basic properties are known for these alloys and the unalloyed metals with regard to small defect nuclei composed of a few helium, vacancies and oxygen impurities. For vanadium, diffusion properties have been reported by Vassen and Jung [3,4]. Results on helium dissociation from helium-filled vacancies and helium-vacancy complexes in vanadium have been reported by Eleveld et al. [5]. The latter study revealed that the presence of oxygen or other impurities in vanadium causes a rather different helium behavior than found for other bee metals, e.g., MO and W from group Via, which can be prepared with much lower impurity concentration. In these metals the heliumvacancy interaction is understood and can be described rather well by atomistic models [6].
In our research, which in the near future will include V-Ti alloys, but first is aimed to understand the role of impurities in relatively pure vanadium, we employed low energy ion irradiation and thermal helium desorption spectrometry (THDS). Although well-defined and reproducible desorption spectra were obtained for vanadium, explanation in terms of vacancies or vacancy clusters filled with helium failed to describe the observed helium behavior. Therefore niobium also was studied. The results obtained for Nb (group Va metal) were in some aspects intermediate between those for molybdenum and vanadium, and thus enabled us to identify better the defect interactions taking place.
2. Experimental
The principles of the THDS technique have been described by Komelsen and van Gorkum [7] and by van Veen et al. [8,9]. A typical experiment proceeds as follows: (1) defect production by keV ions, e.g. He ions, (2) annealing to temperature TA to anneal the defects, (3) subthreshold implantation of low energy helium ions to decorate the defects with helium, and,
0022-3115/94/$07.00 0 1994 Elsevier Science B.V. All rights reserved 0022-3115(93)E0397-R
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A. uan Veen et al. /Journal of Nuclear Materials212-215 (19941 287-292
finally, (4) heating of the sample while monitoring the helium release rate (typical heating rate 10 K/s). The last step generates a desorption spectrum to which desorption peaks can be related to the release from well-defined defect complexes. Analysis of the desorption peaks may yield the number of defects and the dissociation parameters, activation enthalpy, and attempt frequency of the helium dissociation reaction. The samples used in this study were single crystals of vanadium and niobium, both with (110) surface orientation. The quoted purity was 99.999% but, because of the affinity of these metals for impurities like oxygen and nitrogen this purity was probably not maintained in the surface areas during the processing, heating, and ion irradiation cycles of the present experiments.
other reactions of interest have been included. In the following, experiments and assignments are described. 3.1. Helium in monovacancies In helium desorption studies on MO and W, it was shown that the defect population after 1 keV helium irradiation at room temperature consisted of heliumfilled monovacancies, i.e. He,V, of an average degree of filling E = 1 for a dose of 1016 He/m’. Thus a population of complexes is expected composed of V, HeV, HezV, etc. The degre,e of filling increased with the dose. These results were explained by creation of Frenkel pairs (I + V), where I is very mobile, and interstitial He which is also mobile at room temperature. Because the surface acts as a strong sink for both I and He, vacancies are the defects that remain at very low dose. When the vacancy-population increases, trapping of both I and He at vacancies increases. So the V population grows slower than linear because of recombination and annihilation of I and V. But the degree of filling increases by repeated helium trapping at vacancies, e.g., He + V -+ HeV, He + HeV + He,V, etc.
3. Results and discussion The desorption measurements were aimed to clarify a number of defect reactions between the elementary defects, i.e. vacancies (V), self-interstitial atoms (I), helium atoms (He) and impurities in V and Nb. The investigated reactions are summarized in Table 1. Also
Table 1 Defect reactions of self-interstitial atoms, vacancies, oxygen, nitrogen and helium in vanadium and niobium. Helium desorption peaks corresponding with the reactions are indicated Vanadium
Reaction
Peak I-1 He
mobile
HeX-rHe+X (X = I, 0, N, C) V + Vmobile + 0
+
L 1,2,3,4
OV
He-surface desorption
Sl s2
OV + 0 + Vmobile 0
--) omobile
HeV + Omobile+ HeOV He,OJ + omobile + He,%,
ED
Peak
(eV1
-8
--) Hemobile
vmobile
Niobium T (K)
100-300 200 >200 <400
L 1,2,3 0.47 (7)
T
ED
(K)
(eV)
- 10 < loo 100-300 280 > 280
s2
620 450 > 480 480-600
0.545 (5)
550 700 > 450
1.90 (15) 1.17 (5)
450-550
+ lv
HeOV + mOmobile-+ HeO,+iVk+r +kL He,O,V + He, - iOV + omobile He,OV -+ He,_,OV+He(nz2) HeOV + OV + He HeNV+NV+He He,O,V, + He, _ ,O,V, + He He desorption from surface oxide
480-600
1 2 3-12 s3
540 580 6OC1200 12001500
1.40 (15)
E, F, G
500-680
H
740
1, J, K 733
7501200 12001500
1.90 (15)
A. van Veen et al. /Joumal
of Nuclear Materials 212-215
(1994) 287-292
289
,
4.0 v$ :!
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i !i E 3 I
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I
,
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600
800
loo0
1200
1400
Temperature (K)
E 2 P
9‘@ 2.5 E n 0 2.0 c 5
0.0 _
Fig. 2. Helium desorption spectra obtained for 1 keV He+ irradiated niobium. The doses used are (a) 6 x 1015 m-*, (b) 1 X 1016 m-*, (c) 5X lOI6 m-*, (d) 1 x 10” m-‘.
1.5
h j
1.0
P .5
0.5 0.0 200
0.2
400
600
600
loo0
1200
1400
Temperature (K)
Fig. 1. (A) Helium desorption spectra obtained for vanadium irradiated with 1 keV He+ at doses of (a) 5~10’~ me2, (b) 1 X 1Ol6 m-‘, (c) 2X 1Or6 m-*, (d) 4x lOI6 m-*, (e) 8x 1016 m-*. (B) Helium desorption spectra as in fig. 1A but at higher doses. Note the difference in scaling. The doses used are (a) 1.2X10” m-*, (b) 1.6x10” m-‘, (c) 2.4~10” mm2, (d) 1 X 10’s m-*.
For vanadium and niobium similar experiments have been performed. The results of dose variations are shown in Figs. la and lb for vanadium and Fig. 2 for niobium, respectively. According to the above description, the desorption peaks appearing first must be ascribed to HeV, thus the peak indicated by 1, 2 for vanadium and by the peak H for niobium. With increase of the dose, peaks that can be ascribed to multiple helium-filled vacancies are observed for niobium, i.e., G, F (He,V and He,V), and E (He, ~ y). The desorption temperature of G, F, and E is lower than H because multiple filling causes a reduction of the helium dissociation energy. Other desorption peaks (not shown) are mentioned in Table 1. A similar result is not observed for vanadium or it must be that release of He,,% corresponds with a peak nearly coinciding with peak 1, 2.
At doses 2 1013 cm-*, release peaks 3, 4, 5, 6, 7 develop in the temperature range 700-1000 K. At very high dose, peaks 8, 9, 10, 11 develop at temperatures up to 1200 K. Similar effects are observed also for niobium but at a somewhat higher dose. The high temperature peaks are attributed to helium-vacancy clustering. The experimental results, in particular those for Nb which are similar for MO and W, are nevertheless not consistent with the earlier description of damage buildup. It is assumed for vanadium (and known for niobium) that monovacancies are mobile at room temperature [lO,ll] so that with all defects and parti-
50
100
150
200
250
300
350
Temperature(K)
Fig. 3. Peak population of the peaks 1, 2 ascribed to HeOV dissociation in vanadium versus the temperature during irradiation with 1 keV He+ ions. The solid and dotted curves correspond with results of model calculations on migration of irradiation-induced vacancies while a concentration of 20 appm 0 is present. The dose rate amounted to 5 X 1014 He m-* s-l.
A. uan Veen et al. /Journal
290
of Nuclear Materials 212-215
(1994) 287-292
20 -
Nb (110)
“f
4
l5
f E 8
6
.gj
5
.
H
v
E+F+G
lo-
8 % a”
699k
,
5-
0
200
l I
4
300
400
500
COO
700
550K
ECHJ !
Annaal temperature (K)
Fig. 4. Peak populations of the peaks H and E+F+ G versus
4566
y+L
the annealing temperature for niobium preirradiated with
(
1 x lOI6 He+ m-*, ion energy 3 keV. After the annealing the defects have been probed with 1 x 1017 He+ m-* at 100 eV ion energy. The solid curve is the result of model calculations including the effect of oxygen mobility at 500 K and OV dissociation at 700 K. The dotted line indicates the H-peak population without annealing and without helium.
3
9
298K
I
12OK
cles involved being mobile, buildup of stable defect clusters at room temperature would have been suppressed considerably.
I
:
IC)O
Temperature (K)
3.2. Vacancies immobilized by oxygen
The observation of damage retention at room temperature has led to a number of implantation experiments for which the implantation temperature was varied or a postirradiation anneal was applied. The reactions V + Vmobile and Vmobile+ 0 + OV are demonstrated by implantation of a low dose of 1 keV helium at varying temperature. The population of the 1, 2 peak (550 K) versus the annealing temperature is shown in Fig. 3. A partial reduction of the peak population is observed at - 200 K which is ascribed to loss of vacancies because they become mobile at this temperature. The stage at 100 K is related with the mobility of interstitial helium, but will not be discussed here. Some 60% of the vacancies appear to survive the annealing at 200 K, and therefore must be immobilized
v 10
Q o! d * 9
0 .. ‘0
bl
&..
8?J E b& c .g 13.
y. .
i
6-
desorption spectra obtained for 1 keV of vanadium at the indicated irradiation dose of 1X10t7 m-‘. (b) Temperature populations for spectra in Fig. 5a.
4+5+6+7+8
.
L
.. i
a i% 4e: E 2 I” 2-
4 itr V.
.Y Fig. 5. (a) Helium He+-ion irradiation temperatures for a dependence of peak
1+2
.
*. *. .? i .. a.
.*
.
Total
V
. ‘0,.
0 0
200
i.
-. i*a
‘%.
400
600
Irradiation temperature (K)
800
A. van Veen et al. /Journal of Nuclear Materials212-215 (1994) 287-292
by impurity trapping (oxygen). Modelling of the process with depth-dependent rate and diffusion theory yields a theoretical curve for the annealing stage with a diffusion coefficient D, v = 1 X lo-’ exp{ - 0.47 ( f O.O2WeV]/kT] m2 s- ’ with k Boltzmann’s constant. The concentration of oxygen that was assumed to explain the vacancy retention was - 20 appm. In an earlier article we showed that the formed VO defects dissociated at 450 K [5]. This was demonstrated by helium probing during annealing of the VO defects. For niobium the results of an annealing experiment are shown in Fig. 4. After irradiation with a low dose of 3 keV He ions, annealing and helium probing (200 eV> was performed. Peak populations ascribed to oxygen-decorated vacancies show two recovery stages at 500 and 750 K, the latter is ascribed to VO dissociation (see also Table 1). Fitting of the recovery stage indicates a first order process with dissociation energy 2.qf0.15) eV and attempt frequency 1013 s-l.
3.3. Oxygen trapping by helium-vacancy
complexes
The first annealing stage in Fig. 3 has been ascribed by us to the mobility of oxygen in niobium. The fitted activation energy amounts to 1.2( 5 0.1) eV, D,,, = 8 x lo-* m-* s-l, and an oxygen concentration of 30 appm. This activation energy corresponds well with the literature value of oxygen migration in vanadium [12]. The annealing stage cannot be ascribed to vacancy migration; as already quoted, vacancies in niobium migrate already at room temperature. Therefore it is proposed that multiple oxygen atoms can decorate the vacancies or helium-vacancy complexes in niobium so that vacancies then act as nucleation centers for growth of oxide precipitates. The effect of oxygen decoration is a reduced trapping of helium. Similarly in another experiment, it was observed that reduction reactions of the type HeV + I + He take place in as-irradiated niobium, but little reduction seems possible after annealing to 630 K. Apparently the defect has been modified by oxygen trapping. Experiments in vanadium on oxygen trapping were conducted by implantation of 200 eV He. The sample was then annealed and desorbed in two ways, i.e., (1) by ramp annealing at 10 K/s and by (2) a 10 min anneal at 480 K followed by the same ramp anneal. Higher temperature peaks contributed to the spectrum on use of the latter procedure. Apparently, the anneal time was sufficient for the oxygen atoms to cluster around the HeV defects modifying them to give a stronger binding for helium. In the table the reactions for oxide formation are indicated. We assume that during this process self interstitial atoms (SIA or I) are emitted.
291
3.4. Oxygen and nitrogen implantation In another experiment, oxygen was implanted in vanadium and a postannealing treatment was applied at 575 K. In the helium desorption spectra, a peak at 600 K and other high temperature peaks were observed which we ascribe to oxide precipitates. Similar experiments with nitrogen implantation gave desorption at slightly different temperatures. 3.5. Helium irradiation at varying temperatures in vanadium The spectra and peak populations in vanadium are given for desorption after irradiation at temperatures varying from 120 to 700 K in Fig. 5a and Fig. 5b, respectively. The dominant effect is the strong reduction of retained helium at 550 K. We ascribe this in line with the forementioned results that the HeOV defect (see Table 1) dissociates at this temperature and therefore the nucleation sites for formation of extended He-O-V clusters are strongly reduced. Larger clusters or helium bubbles will probably grow, but their nucleation is much more difficult. Details of interest are the partial reduction of the peak 1, 2 on annealing at 200 K due to mobility of vacancies and the growth of helium-vacancy cluster peaks (4, 5, 6, 7, 8) above 200 K.
4. Summary and conclusion Helium desorption has been used to study defect behavior in ion-irradiated vanadium and niobium. It appears that, at temperatures above which vacancies become mobile (200-300 K), the vacancies become decorated with oxygen. At temperatures for mobility of oxygen (> 450 K), oxygen decoration becomes more pronounced and modifies the trapping behavior of helium and self-interstitial atoms. Important defects essential for nucleation of defect clusters appear to be vacancies decorated with impurity atoms. In vanadium beyond 600 K, nucleation changes from finely dispersed nuclei to a low density of nucleation sites. Acknowledgement
Mr. K.T. Westerduin is acknowledged in performing the THDS experiments.
for his help
References
[l] S. Yano, M. Tada and H. Matsui, J. Nucl. mater. 179-181 (1991) 779. [2] H.M. Chung and D.L. Smith, J. Nucl. Mater. 191-194 (1992) 942.
A. uan Veen et al. /Journal
292 [3] R. Vassen,
[4] [5] [6]
[7]
of Nuclear Materials 212-215
H, Trinkaus and P. Jung, Phys. Rev. B44 (1991) 4806. P. Jung, in Diffusion Processes in Nuclear Materials, ed. R.P. Agarwala (Elsevier, Amsterdam, 1992) pp. 236-269. H. Eleveld, A. van Veen, M. Clement and M. de Moor, Plasma Devices and Operations 3 (1994) 65. A. van Veen, in Fundamental Aspects of Inert Gases in Solids, eds. SE. Donnelly and J.H. Evans, Nato ASI series B, Physics vol. 279 (Plenum, New York, 1991) pp. 41-57. E.V. Kornelsen and A.A. van Gorkum, Vacuum 31(1981) 99.
(1994) 287-292
[8] A. van Veen, Mater. Sci. Forum 15-18 (1987) 3. [9] A. van Veen, A. Warnaar and L.M. Caspers, Vacuum 30 (1980) 109. [lo] H. Schultz, in Point Defects and Defect Interactions in Metals, eds. .I. Takamura, M. Doyama and M. Kiritani (1977) p. 183. [ll] K. Faber and H. Schultz, Radiat. Eff. 31 (1977) 157. [12] E. Fromm and E. Gebhardt, Gase und Kohlenstoff in Metallen (Springer, Berlin, 1976).
Journal of Nuclear Materials 212-215 (1994) 287-292
Helium impurity interactions in vanadium and niobium A. van Veen, H. Eleveld, M. Clement Interfaculty Reactor Institute, Delft University of Technology, Mekelweg 15, Nl-2629 JB Delft, The Netherlands
Abstract
Thermal helium desorption spectrometry (THDS) has been employed to study the nucleation and growth of helium defect agglomerates in niobium and vanadium. An important difference from other body-centered-cubic metals was the temperature at which helium was released from monovacancy type defects. In MO and W the release takes place at 0.4OT, (T, = melting temperature) but in V and Nb at OZT,,,. This and other observations led to the conclusion that in nearly all interactions of helium with radiation-induced defects, impurities, mainly oxygen, are involved. Impurity levels of 20-30 appm are sufficient to completely suppress the type of helium behaviour that was found in MO and W. Oxygen decorates the helium-vacancy complexes and makes the helium desorption temperature lower, but stabilizes the small defect complexes up to a temperature of about 700 K, beyond this temperature, nucleation of defect clusters is considerably reduced. Oxygen decoration has been observed to reduce the sink strength of the defects for helium trapping.
1. Introduction
The development of low-activation materials (LAM) will be of paramount importance for future energy-producing plasma fusion reactors [l]. Vanadium is the basic constituent in a class of low-activation alloys that are considered long-term candidates for first-wall material. Studies on the mechanical properties of irradiated alloys have shown that impurities and precipitates may play an important role in the observed behavior [2]. Only few basic properties are known for these alloys and the unalloyed metals with regard to small defect nuclei composed of a few helium, vacancies and oxygen impurities. For vanadium, diffusion properties have been reported by Vassen and Jung [3,4]. Results on helium dissociation from helium-filled vacancies and helium-vacancy complexes in vanadium have been reported by Eleveld et al. [5]. The latter study revealed that the presence of oxygen or other impurities in vanadium causes a rather different helium behavior than found for other bee metals, e.g., MO and W from group Via, which can be prepared with much lower impurity concentration. In these metals the heliumvacancy interaction is understood and can be described rather well by atomistic models [6].
In our research, which in the near future will include V-Ti alloys, but first is aimed to understand the role of impurities in relatively pure vanadium, we employed low energy ion irradiation and thermal helium desorption spectrometry (THDS). Although well-defined and reproducible desorption spectra were obtained for vanadium, explanation in terms of vacancies or vacancy clusters filled with helium failed to describe the observed helium behavior. Therefore niobium also was studied. The results obtained for Nb (group Va metal) were in some aspects intermediate between those for molybdenum and vanadium, and thus enabled us to identify better the defect interactions taking place.
2. Experimental
The principles of the THDS technique have been described by Komelsen and van Gorkum [7] and by van Veen et al. [8,9]. A typical experiment proceeds as follows: (1) defect production by keV ions, e.g. He ions, (2) annealing to temperature TA to anneal the defects, (3) subthreshold implantation of low energy helium ions to decorate the defects with helium, and,
0022-3115/94/$07.00 0 1994 Elsevier Science B.V. All rights reserved 0022-3115(93)E0397-R
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A. uan Veen et al. /Journal of Nuclear Materials212-215 (19941 287-292
finally, (4) heating of the sample while monitoring the helium release rate (typical heating rate 10 K/s). The last step generates a desorption spectrum to which desorption peaks can be related to the release from well-defined defect complexes. Analysis of the desorption peaks may yield the number of defects and the dissociation parameters, activation enthalpy, and attempt frequency of the helium dissociation reaction. The samples used in this study were single crystals of vanadium and niobium, both with (110) surface orientation. The quoted purity was 99.999% but, because of the affinity of these metals for impurities like oxygen and nitrogen this purity was probably not maintained in the surface areas during the processing, heating, and ion irradiation cycles of the present experiments.
other reactions of interest have been included. In the following, experiments and assignments are described. 3.1. Helium in monovacancies In helium desorption studies on MO and W, it was shown that the defect population after 1 keV helium irradiation at room temperature consisted of heliumfilled monovacancies, i.e. He,V, of an average degree of filling E = 1 for a dose of 1016 He/m’. Thus a population of complexes is expected composed of V, HeV, HezV, etc. The degre,e of filling increased with the dose. These results were explained by creation of Frenkel pairs (I + V), where I is very mobile, and interstitial He which is also mobile at room temperature. Because the surface acts as a strong sink for both I and He, vacancies are the defects that remain at very low dose. When the vacancy-population increases, trapping of both I and He at vacancies increases. So the V population grows slower than linear because of recombination and annihilation of I and V. But the degree of filling increases by repeated helium trapping at vacancies, e.g., He + V -+ HeV, He + HeV + He,V, etc.
3. Results and discussion The desorption measurements were aimed to clarify a number of defect reactions between the elementary defects, i.e. vacancies (V), self-interstitial atoms (I), helium atoms (He) and impurities in V and Nb. The investigated reactions are summarized in Table 1. Also
Table 1 Defect reactions of self-interstitial atoms, vacancies, oxygen, nitrogen and helium in vanadium and niobium. Helium desorption peaks corresponding with the reactions are indicated Vanadium
Reaction
Peak I-1 He
mobile
HeX-rHe+X (X = I, 0, N, C) V + Vmobile + 0
+
L 1,2,3,4
OV
He-surface desorption
Sl s2
OV + 0 + Vmobile 0
--) omobile
HeV + Omobile+ HeOV He,OJ + omobile + He,%,
ED
Peak
(eV1
-8
--) Hemobile
vmobile
Niobium T (K)
100-300 200 >200 <400
L 1,2,3 0.47 (7)
T
ED
(K)
(eV)
- 10 < loo 100-300 280 > 280
s2
620 450 > 480 480-600
0.545 (5)
550 700 > 450
1.90 (15) 1.17 (5)
450-550
+ lv
HeOV + mOmobile-+ HeO,+iVk+r +kL He,O,V + He, - iOV + omobile He,OV -+ He,_,OV+He(nz2) HeOV + OV + He HeNV+NV+He He,O,V, + He, _ ,O,V, + He He desorption from surface oxide
480-600
1 2 3-12 s3
540 580 6OC1200 12001500
1.40 (15)
E, F, G
500-680
H
740
1, J, K 733
7501200 12001500
1.90 (15)
A. van Veen et al. /Joumal
of Nuclear Materials 212-215
(1994) 287-292
289
,
4.0 v$ :!
Nb(llO)
0.8 3.0
z 0
i !i E 3 I
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I
I
,
/
600
800
loo0
1200
1400
Temperature (K)
E 2 P
9‘@ 2.5 E n 0 2.0 c 5
0.0 _
Fig. 2. Helium desorption spectra obtained for 1 keV He+ irradiated niobium. The doses used are (a) 6 x 1015 m-*, (b) 1 X 1016 m-*, (c) 5X lOI6 m-*, (d) 1 x 10” m-‘.
1.5
h j
1.0
P .5
0.5 0.0 200
0.2
400
600
600
loo0
1200
1400
Temperature (K)
Fig. 1. (A) Helium desorption spectra obtained for vanadium irradiated with 1 keV He+ at doses of (a) 5~10’~ me2, (b) 1 X 1Ol6 m-‘, (c) 2X 1Or6 m-*, (d) 4x lOI6 m-*, (e) 8x 1016 m-*. (B) Helium desorption spectra as in fig. 1A but at higher doses. Note the difference in scaling. The doses used are (a) 1.2X10” m-*, (b) 1.6x10” m-‘, (c) 2.4~10” mm2, (d) 1 X 10’s m-*.
For vanadium and niobium similar experiments have been performed. The results of dose variations are shown in Figs. la and lb for vanadium and Fig. 2 for niobium, respectively. According to the above description, the desorption peaks appearing first must be ascribed to HeV, thus the peak indicated by 1, 2 for vanadium and by the peak H for niobium. With increase of the dose, peaks that can be ascribed to multiple helium-filled vacancies are observed for niobium, i.e., G, F (He,V and He,V), and E (He, ~ y). The desorption temperature of G, F, and E is lower than H because multiple filling causes a reduction of the helium dissociation energy. Other desorption peaks (not shown) are mentioned in Table 1. A similar result is not observed for vanadium or it must be that release of He,,% corresponds with a peak nearly coinciding with peak 1, 2.
At doses 2 1013 cm-*, release peaks 3, 4, 5, 6, 7 develop in the temperature range 700-1000 K. At very high dose, peaks 8, 9, 10, 11 develop at temperatures up to 1200 K. Similar effects are observed also for niobium but at a somewhat higher dose. The high temperature peaks are attributed to helium-vacancy clustering. The experimental results, in particular those for Nb which are similar for MO and W, are nevertheless not consistent with the earlier description of damage buildup. It is assumed for vanadium (and known for niobium) that monovacancies are mobile at room temperature [lO,ll] so that with all defects and parti-
50
100
150
200
250
300
350
Temperature(K)
Fig. 3. Peak population of the peaks 1, 2 ascribed to HeOV dissociation in vanadium versus the temperature during irradiation with 1 keV He+ ions. The solid and dotted curves correspond with results of model calculations on migration of irradiation-induced vacancies while a concentration of 20 appm 0 is present. The dose rate amounted to 5 X 1014 He m-* s-l.
A. uan Veen et al. /Journal
290
of Nuclear Materials 212-215
(1994) 287-292
20 -
Nb (110)
“f
4
l5
f E 8
6
.gj
5
.
H
v
E+F+G
lo-
8 % a”
699k
,
5-
0
200
l I
4
300
400
500
COO
700
550K
ECHJ !
Annaal temperature (K)
Fig. 4. Peak populations of the peaks H and E+F+ G versus
4566
y+L
the annealing temperature for niobium preirradiated with
(
1 x lOI6 He+ m-*, ion energy 3 keV. After the annealing the defects have been probed with 1 x 1017 He+ m-* at 100 eV ion energy. The solid curve is the result of model calculations including the effect of oxygen mobility at 500 K and OV dissociation at 700 K. The dotted line indicates the H-peak population without annealing and without helium.
3
9
298K
I
12OK
cles involved being mobile, buildup of stable defect clusters at room temperature would have been suppressed considerably.
I
:
IC)O
Temperature (K)
3.2. Vacancies immobilized by oxygen
The observation of damage retention at room temperature has led to a number of implantation experiments for which the implantation temperature was varied or a postirradiation anneal was applied. The reactions V + Vmobile and Vmobile+ 0 + OV are demonstrated by implantation of a low dose of 1 keV helium at varying temperature. The population of the 1, 2 peak (550 K) versus the annealing temperature is shown in Fig. 3. A partial reduction of the peak population is observed at - 200 K which is ascribed to loss of vacancies because they become mobile at this temperature. The stage at 100 K is related with the mobility of interstitial helium, but will not be discussed here. Some 60% of the vacancies appear to survive the annealing at 200 K, and therefore must be immobilized
v 10
Q o! d * 9
0 .. ‘0
bl
&..
8?J E b& c .g 13.
y. .
i
6-
desorption spectra obtained for 1 keV of vanadium at the indicated irradiation dose of 1X10t7 m-‘. (b) Temperature populations for spectra in Fig. 5a.
4+5+6+7+8
.
L
.. i
a i% 4e: E 2 I” 2-
4 itr V.
.Y Fig. 5. (a) Helium He+-ion irradiation temperatures for a dependence of peak
1+2
.
*. *. .? i .. a.
.*
.
Total
V
. ‘0,.
0 0
200
i.
-. i*a
‘%.
400
600
Irradiation temperature (K)
800
A. van Veen et al. /Journal of Nuclear Materials212-215 (1994) 287-292
by impurity trapping (oxygen). Modelling of the process with depth-dependent rate and diffusion theory yields a theoretical curve for the annealing stage with a diffusion coefficient D, v = 1 X lo-’ exp{ - 0.47 ( f O.O2WeV]/kT] m2 s- ’ with k Boltzmann’s constant. The concentration of oxygen that was assumed to explain the vacancy retention was - 20 appm. In an earlier article we showed that the formed VO defects dissociated at 450 K [5]. This was demonstrated by helium probing during annealing of the VO defects. For niobium the results of an annealing experiment are shown in Fig. 4. After irradiation with a low dose of 3 keV He ions, annealing and helium probing (200 eV> was performed. Peak populations ascribed to oxygen-decorated vacancies show two recovery stages at 500 and 750 K, the latter is ascribed to VO dissociation (see also Table 1). Fitting of the recovery stage indicates a first order process with dissociation energy 2.qf0.15) eV and attempt frequency 1013 s-l.
3.3. Oxygen trapping by helium-vacancy
complexes
The first annealing stage in Fig. 3 has been ascribed by us to the mobility of oxygen in niobium. The fitted activation energy amounts to 1.2( 5 0.1) eV, D,,, = 8 x lo-* m-* s-l, and an oxygen concentration of 30 appm. This activation energy corresponds well with the literature value of oxygen migration in vanadium [12]. The annealing stage cannot be ascribed to vacancy migration; as already quoted, vacancies in niobium migrate already at room temperature. Therefore it is proposed that multiple oxygen atoms can decorate the vacancies or helium-vacancy complexes in niobium so that vacancies then act as nucleation centers for growth of oxide precipitates. The effect of oxygen decoration is a reduced trapping of helium. Similarly in another experiment, it was observed that reduction reactions of the type HeV + I + He take place in as-irradiated niobium, but little reduction seems possible after annealing to 630 K. Apparently the defect has been modified by oxygen trapping. Experiments in vanadium on oxygen trapping were conducted by implantation of 200 eV He. The sample was then annealed and desorbed in two ways, i.e., (1) by ramp annealing at 10 K/s and by (2) a 10 min anneal at 480 K followed by the same ramp anneal. Higher temperature peaks contributed to the spectrum on use of the latter procedure. Apparently, the anneal time was sufficient for the oxygen atoms to cluster around the HeV defects modifying them to give a stronger binding for helium. In the table the reactions for oxide formation are indicated. We assume that during this process self interstitial atoms (SIA or I) are emitted.
291
3.4. Oxygen and nitrogen implantation In another experiment, oxygen was implanted in vanadium and a postannealing treatment was applied at 575 K. In the helium desorption spectra, a peak at 600 K and other high temperature peaks were observed which we ascribe to oxide precipitates. Similar experiments with nitrogen implantation gave desorption at slightly different temperatures. 3.5. Helium irradiation at varying temperatures in vanadium The spectra and peak populations in vanadium are given for desorption after irradiation at temperatures varying from 120 to 700 K in Fig. 5a and Fig. 5b, respectively. The dominant effect is the strong reduction of retained helium at 550 K. We ascribe this in line with the forementioned results that the HeOV defect (see Table 1) dissociates at this temperature and therefore the nucleation sites for formation of extended He-O-V clusters are strongly reduced. Larger clusters or helium bubbles will probably grow, but their nucleation is much more difficult. Details of interest are the partial reduction of the peak 1, 2 on annealing at 200 K due to mobility of vacancies and the growth of helium-vacancy cluster peaks (4, 5, 6, 7, 8) above 200 K.
4. Summary and conclusion Helium desorption has been used to study defect behavior in ion-irradiated vanadium and niobium. It appears that, at temperatures above which vacancies become mobile (200-300 K), the vacancies become decorated with oxygen. At temperatures for mobility of oxygen (> 450 K), oxygen decoration becomes more pronounced and modifies the trapping behavior of helium and self-interstitial atoms. Important defects essential for nucleation of defect clusters appear to be vacancies decorated with impurity atoms. In vanadium beyond 600 K, nucleation changes from finely dispersed nuclei to a low density of nucleation sites. Acknowledgement
Mr. K.T. Westerduin is acknowledged in performing the THDS experiments.
for his help
References
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