Effect of irradiation on deuterium reemission and retention in metals

1077

Journal of Nuclear Materials 162-164 (1989) 1077-1081 North-Holland, Amsterdam

EFFECT OF IRRADIATION ON DEUTERIUM

REEMISSION

Tetsuo

IMOTO

TANABE,

Department

Masato

TAKE0

and Shosuke

AND RETENTION

IN METALS

of Nuclear Engineering, Osaka University, Yamadaoka, Suita, Osaka 565, Japan

Key words: irradiation effect, deuterium, reemission, thermal desorption, nickel In order to investigate the effect of irradiation on deuterium reemission and retention in Ni, the reemission and thermal desorption for pre-irradiated Ni under various conditions have been measured employing probing implantation of 8 X lOI D+/m2 (25 keV) at 323 K. After 323 K pre-irradiation, the reemission time becomes longer and the deuterium retention is increased markedly with increasing fluence. This is due to the hydrogen trapping by the defect produced by the pre-irradiation. Although the effect of pre-irradiation above 373 K is not so significant as 323 K pre-irradiation, thermal desorption spectra show the difference. Applying the simple diffusion and trapping theory taking into account the production of the defect (trapping site) during the implantation, the thermal desorption spectra are analyzed and trapping energies of 0.77 eV and 0.91 eV for 323 K pre-irradiation are identified. Because of the lack in understanding of the correlation between the defect structures and the trapping sites, however, the temperature dependence is not well understood.

1. Introduction

2. Experimental

In the previous papers [l-5], we have indicated that the behavior of hydrogen implanted in Ni with rather high energy (around 30 keV) is well interpreted by a simple diffusion and trapping theory. However, the time transient behavior, which is very important because the current tokamak has not a long enough pulse length for the establishment of the steady state, is heavily influenced by sample history, and the defects (both at the surface and in the bulk) produced by energetic hydrogen bombardment work as trapping sites [5] and/or a short circuit path for diffusion [2-31. In the present work we have investigated the effect of irradiation on deuterium reemission and retention in Ni. After Ni specimens were irradiated by 25 keV D+ with various fluences, implanted deuterium was evolved without changing the defect structure by annealing at 500 K (pre-irradiation). After the pre-irradiation, probing implantation with a particular fluence at 323 K was employed and reemission and thermal desorption measurements were carried out. Changing the pre-irradiation fluence, the probing reemission and thermal desorption were compared. Applying the simple diffusion and trapping theory which takes into account the production of the defect (trapping site) during implantation, the effect of pre-irradiation on reemission and thermal desorption are analyzed.

In Fig. 1 the experimental sequence is shown. First the pre-irradiation was carried out at a temperature between 323 and 1000 K with a fluence of 1 X 10” to 2 x 1O22 D’/m2. Then the implanted deuterium was evolved at 473 K for 2 min. During this desorption procedure, it was confirmed by TEM observation that the defect produced by the pre-irradiation was not

0022-3115/89/$03.50 0 Elsevier Science Publishers (North-Holland Physics Publishing Division)

PRE IRRAD.

_DESORP_

MEASUREMENT

ANNEAL.

Fig. 1. Experimental procedure for studying reemission and thermal desorption of pre-irradiated specimen.

B.V.

T. Tanahe et ul. / Ejjeec~ of rrrndiatron on deuterium reemisswn

25keV

D*+NI

at

323K

TIME/s

Fig. 2. Time sequence of the reemission rate at 323 K after pre-irradiation at 323 K with various fluences.

and retention

injected into a pure Ni (99.99%) foil which was mounted on a specimen holder after mechanical polishing and heated by a direct current method. Utilizing a differential pumping technique, the background pressure was kept below lo-” Pa during the implantation. The reemission or thermal desorption rate was determined from a partial pressure rise of D, monitored by a quadrupole mass spectrometer and effective pumping speed of 80 l/s for D,. The maximum error in the reemission rate, mainly originating from neglecting the direct reflection of incident deuterons. was about 15%. The details of the apparatus and the method for determination of the reemission and thermal desorption rate have been described elsewhere [5].

3. Results modified nor annealed out. With probing implantation of fluence of 8 X 1019 D+t/m’ at 323 K, the reemission and thermal desorption were investigated. In order to avoid the diffusive release of deuterium after implantation, the specimen temperature was immediately ramped up at a rate of 10 K/s. After this procedure the specimen was fully annealed at above 1300 K for more than 1 h, which was indispensable for obtaining reproducible results in the next run. When the integrated fluence exceeded 1 X 1O23 D+/m2, the specimen was exchanged with a new one. All procedures were carried out in an ultrahigh vacuum system with a base pressure of better than 1 x lo-’ Pa. A mass-analyzed 25 keV Dt beam was

Fig. 2 shows time sequences of the reemission rate at 323 K (50 o C) for the pre-irradiated specimen at 323 K with various fluences. One can clearly see that the reemission rate is reduced with increasing pre-irradiation fluence and most of the probing deuterium is trapped in the specimen pre-irradiated with 1.4 x lOI* D+/m’. Correspondingly, retained deuterium, which is given by an integration of the thermal desorption spectra with time, increases as seen in fig. 3(a). The first peak around 360 K slightly decreases with increasing pre-irradiation fluence, while the second peak around 420 K increases markedly. The thermal desorption spectra without pre-irradiation are also given in fig. 3 (b)

25keVD+-,NI

I25 keV

D-N1

at

at

323K

/-, /_.

323K

8 .lO’gD’.m’

i

i

II \, 1

!f

__

:; I,

ot 323K

-

,.~

-_,

_-.

j

FLUENCE 8x10”

I<

/:’ PRE-IRRADIATION

IMPLANTED

I ,\,LI

_~~~.

L5.19 , L x 102’

~---

LOxlO”

- -

\

D’,d

1 7.1o22

L1 ~lO~~D’.ni~ 3-

LOO

3

500

desorption

1

/

LOO

500

TEMPERATURE/

TEMPERATURE/K

Fig. 3. (a) Thermal

30(

spectra

for pre-irradiated specimen at 323 K with various fluences. spectra with implanted fluence for fully annealed specimen.

600

K

(b) Change

of thermal

desorption

1079

T. Tanabe et al. / Effect of irradiation on deuterium reemission and retention

25keV

d+Ni

25 keV D*+Ni 8.1dg

at 323K D’.rrl*

,/-\ _._._.

_..__

573K 773K 973K

1

30 TIME/s

-

ANNEALED

I\

PRE-IRRADIATION L.5 x ld’ D’.n-? . .. ...

323 K 373K

---_._ _._.

L73K 57 3 K

.._..__

773K 973 K

60

Fig. 4. Time sequence of the reemission rate at 323 K after pre-irradiation with a fluence of 4.5 X 10’ D’/m* at various temperatures. where both peaks are seen to increase with implanted

fluence. In fig. 4 reemission rates are compared with different temperature pre-irradiation of 4.5 X lo’* D’/m’. Although the effect of pre-irradiation above 373 K (100’ C) on the reemission is not so appreciable as for 323 K pre-irradiation, the reemission is somewhat different. The effect of the pre-irradiation temperature change is clearly seen in thermal desorption spectra given in fig. 5. The 573 K pre-irradiation, for example, shows a smaller retention corresponding to an earlier saturation in the reemission rate, and the 773 K preirradiation shows quite difference thermal desorption

TEMPERATURE/K

Fig. 5. Thermal desorption spectra for pre-irradiation specimen with a fluence of 4.5 X 102r D+/m* at various temperatures.

spectra as in the case of the reemission. At 973 K, the effect of pre-irradiation is not significant and the retention is rather smaller than that of fully annealed one.

4. Discussion Because the probing fluence (8 x lOI D’/m2) is not large enough to occupy all trapping sites produced by the pre-irradiation at 323 K, deuterium must be preferentially trapped at higher binding trapping sites

Fig. 6. TEM micrograph of Ni irradiated by 25 keV D+ with 3 X lo*’ ions/m* at (a) 300 K, (b) 600 K and (c) 750 K (Niwase [6]). The growth of dislocation loops is clearly seen.

1080

T. Tanabe et al. / Effeci of irradiation on deuterlum wemission

as seen in fig. 3(a). On the other hand, the defects produced during the probing implantation at 323 K might be superior to those produced by the pre-irradiation at 373 and 473 K. This is the reason why the effects of 373 and 473 K pre-irradiation are hardly seen in fig. 4. Niwase et al. in their TEM study of Ni irradiated with deuteron [6] have shown that the number density of defect clusters (mainly interstitial loops) is markedly reduced with increasing implantation temperature as seen in fig. 6. Such interstitial loops are possibly the origin of the trapping sites. However, it is difficult to interpret why the preirradiation effect at 773 K is more marked than those at 373 and 473 K. Since 773 K pre-irradiation results in quite different reemission and thermal desorption, it is likely that the trapping sites produced by 773 K irradiation are different from those produced at lower temperatures (see fig. 6). This might be correlated with vacancy migration in Ni which becomes appreciable above 600 K [7]. One should note that the permeation spike appeared in energetic hydrogen (plasma and/or ion) driven permeation through Ni is also dominant in these temperature range [l-3]. Earlier saturation in the reemission with 573 and 773 K pre-irradiation might be also correlated with the appearence of the permeation spike. At elevated temperatures, interstitials and vacancies are easily extinguished, and the effect of irradiation is very small as is the case in the 973 K pre-irradiation. According to the diffusion and trapping theory [8], hydrogen concentration during the thermal desorption process is expressed by, ac(x,

2) = _,a+_,

at

aqx, at

r) _ art.

t).

ax2 f)

= DC(x,

+T(x,

(1)

t)[T,O(x)- T,(x, t)]/#

f>v,exp(-(E,+E,)/kT), (2)

where T,(x, t), T’(x), A;, v,, E, and E, are the concentration of trapped hydrogen, the density of the initial trapping sites, the jump distances, the trapping frequency, the trapping energy for the i th trap and the activation energy of diffusion, respectively. In previous work [5], three trapping energies of 0.5, 0.7 and 0.9 eV were obtained for 300 K irradiation employing D = 3.9 x lo-’ exp (-0.38/RT) m* s-t [9], X, = 0.25 nm, v, = 10’3 s-l and T,‘(x) given by a damage distribution calculated with the TRIM code. Similar to most of the desorption studies, the previous thermal desorption was carried out some time after the beam was stopped, and

and retention

%l

N.

DIFFUSION

‘E

ill0 u

b. TRAPPING

MODEL

3 2

B 7

!

z. ,“ ~\,\ \ I \ ,’ ’ \\

TRAP

-

1 110’9

_.._.

2.10’9 3 .,o’g

---

5.10’9

--

TEMPERATURE

DENSITY 077eV

/

K

I ni’l 091eV

5.10’9

500

Fig. 7. Simulated thermal desorption spectra given by a numerical solution of eq. (I), supposing number densities of two trapping sites with trapping energies of 0.77 and 0.91 eV increase with pre-irradiation fluence (see text).

some deuterium was released before the desorption measurement, changing the deuterium distribution in the specimen [5]. Here, the temperature ramp has been immediately started after the beam was stopped in order to avoid a change in the deuterium distribution. Two trapping sites with trapping energies of 0.77 and 0.91 eV for 323 K irradiation are identified by using the same parameters as the previous ones. (The detailed analysis will be published in a separate paper [lo].) Although these two trapping energies agree well with the two higher trapping energies of the previous work, low energy trapping site [11,12] is not identified. Since the reemission measurements are carried out at a little higher temperature (373 K) than RT, deuterium trapped at the lower energy sites is immediately detrapped. In addition, the present thermal desorption includes a large amount of diffusive release of deuterium from the interstitial sites immediately after the beam is turned off. which has not been accounted in the earlier measurement and hinders the deuterium release from the low trapping energy sites. This early release of deuterium might be correlated with dynamic retention during the discharge and have a important role in hydrogen recycling. In order’to simulate the pre-irradiation effect on the the& .besorption spectra [fig. 3(a)], both trapping sites are supposed to increase with the pre-irradiation fluence and ‘C@calated results are given in fig. 7. One can see a nice correspondence between Fig. 3(a) and fig. 7 where it is shown that the peak temperature shifts higher with increasing number density of the trapping site and that the first peak tends to saturate while the

T. Tanabe et al. / Effect of irradiation on deuterium reemissicmnnd retention

second peak continues to grow. It is noteworthy that the peak shift in the thermal desotption originates from the change of the trap density and not from that of the trapping energy which is usually considered to be the origin of the peak shift. Thus the change of the thermal desorption due to the preirradiation at 323 K is basically interpreted by the increase of trapping sites with fluence. However, it is difficult to simulate the complex temperature dependence with the fixed parameters given above, because both the trapping site and trapping energies must be changed according to the mo~fication of defect structure with the implanted temperature [6]. (See fig. 6.) It is often claimed that a surface effect might be dominant in the desorption process [13,14]. Recently, however, Chorkendorf et al. [15] have clearly shown that the total amount of surface chemisorbed deuterium is negligibly small and trap formation plays a more important role. In addition, the desorption of implanted hydrogen is found to be strongly forward-peak along the crystal surface normal suggesting no surface barrier, which has also been reported by Comsa et al. 1161 for permeated hydrogen. Therefore the modification (defect formation) of the subsurface layer or the bulk must be seriously considered in hydrogen recycling. The defect structure, unfortunately, is highly dependent on the irradiation temperature [6] and is not well correlated with the trapping sites. Much work must be done for understanding the behavior of hydrogen implanted in metals and probing implantation at higher temperature will help the trap analysis.

5. Conclusions In the present work we have investigated the effect of irradiation on hydrogen reemission and retention in Ni. After Ni specimens were bombarded by 25 keV D’ with various fluences, implanted deuterium was desorbed without changing the defect structure by annealing at 500 K (pre-irradiation), After the prebombardment we have employed probing implantation with certain fluences at 323 K and measured the reemission and thermal desorption. Changing the prebombardment conditions, the probing reemission and thermal desorption were compared and the results obtained are as follows. At 323 K pre-irradiation, the reemission time becomes longer and the desorbed deuterium is increased markedly with increasing fluence. This is due to hydro-

1081

gen trapping by the defect produced by the pre-irradiation. The effect of pre-irradiation above 373 K on the reemission is not so significant as at 323 K. Thermal desorption spectra, however, show the difference. Especially 773 K pre-irradiation reveals unique desorption spectrum, probably due to the defect structure change owing to vacancy migration. The numerical analysis by a simple diffusion and trapping theory taking into account the production of the defect (trapping site) during the implantation gives two trapping sites with trapping energies of 0.77 and 0.91 eV for 323 K irradiation and is successful for interpretation of thermal desorption changes with increasing pre-irradiation fluence. However the complexity in the temperature dependence is not interpreted well with the fixed parameters, because of the lack in understanding of the correlation between the trapping sites and the defect structures which change with implanted temperature.

References [l] T. Tauabe, N. Saitoh and S. Imoto, J. Nucl. Mater. 103 & 104 (1981) 483. [2] T. Tanabe, Y. Furuyama and S. Imoto, J. Nucl. Mater. 122 & 123 (1984) 1563. [3] T. Tanabe, Y. Furuyama, H. Hirano and S. Imoto, J. Nucl. Mater. 128 & 129 (1984) 641. [4] T. Tanabe, Y. Furuyama and S. Imoto, J. Nucl. Mater. 145-147 (1987) 305. [5] T. Tauabe, H. Hirano and S. Imoto, J. Nucl. Mater. 151 (1987) 38. [6] T. Niwase, Thesis Osaka University (1986) and submitted to J. Nucl. Mater. [7] M. Kiritani and H. Takata, J. Nucl. Mater. 69 & 70 (1978) 277. [8] M.I. Baskes, Sandia National Laboratories reports SANDSOand SAND 83-8231. [9] T. Tanabe, Y. Furuyama, H. Hirano and S. Imoto, Trans. Japan. Inst. Met. 28 (1987) 706. [lo] T. Tanabe, M. Takeo and S. Imoto, to be published. [ll] F. Besenbacher, J. Bottiger and S.M. Myers, J. Appl. Phys. 53 (1982) 3547 and 3536. [12] W.A. Wampler and S.M. Myers, Nucl. Instr. and Meth. B7/8 (1985) 76. [13] F.E. Waelbroeck, P. Wienhold and J. Winter, J. Nucl. Mater. 111 & 112 (1982) 185. (141 D. Presinger, P. Borgensen, W. Moeller and B.M.U. Schemer, Nucl. Instr. and Meth. B9 (1985) 270. [15] I. Chorkendorf, J.N. Russell, Jr. and J.T. Yates, Jr., Surf. Sci. 182 (1987) 375. 1161 G. Comsa and R. David, Surf. Sci. Rep. 5 (1985) 145.