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In situ luminescence measurement of irradiation defects in ternary lithium ceramics under ion beam irradiation
ELSEVIER
Journal of Nuclear Materials 248 (1997) 132-139
In situ luminescence measurement of irradiation defects in ternary lithium ceramics under ion beam irradiation Kimikazu Moritani a, Hirotake Moriyama
b,,
a Faculty of Engineering, Kyoto University, Yoshida, Sakyo-ku, Kyoto 606-01, Japan b Research Reactor Institute, Kyoto University, Kumatori-cho, Sennan-gun, Osaka 590-04, Japan
Abstract For the performance assessment of fusion reactor solid breeder materials, the production behavior of irradiation defects in some candidate materials of Li 2TIO3, Li e ZrO 3 and Li 2 SnO 3 was studied by an in situ luminescence measurement technique under ion beam irradiation. The luminescence was observed to be composed of multiple luminescence bands and the temperature dependence of the luminescence intensity was measured under He + or H ÷ ion beam irradiation. The production mechanism of irradiation defects was discussed by comparing the present results with those previously obtained for Li20. The effects of irradiation defects on tritium recovery kinetics and material stability were pointed out. © 1997 Elsevier Science B.V.
1. Introduction In a fusion reactor solid blanket system, tritium breeding lithium ceramics are attacked by high energy neutrons and energetic particles from nuclear reactions and severe irradiation damage may be expected. The irradiation behavior of lithium ceramics is thus important for the performance assessment of fusion reactor blanket systems. The effects of irradiation on the tritium release behaviors and microstructural changes of lithium ceramics are particularly important. For a clearer understanding of such effects, we have studied the production behavior of irradiation defects in L i 2 0 [1,2], LiAIO 2 [3], Li2SiO 3 and LiaSiO 4 [4,5] by an in situ luminescence measurement technique under ion beam irradiation. In L i 2 0 and LiA102, it has been confinned that the F + center (an oxygen vacancy trapping an electron) and the F ° center (an oxygen vacancy trapping two electrons), which are commonly observed in ionic crystals, are formed by irradiation. Similarly, the irradia-
* Corresponding author. Fax: + 81-724 51 2424.
tion defects of the E' center (=--Si.), the non-bridging oxygen hole center ( ~ S i - O - ) and the peroxy radical ( ~ S i - O - O • ) are produced in Li2SiO 3 and Li4SiO 4. These defects are considered to play an important role in the tritium behavior. For comparison, the production behavior of irradiation defects in the lithium ceramics of Li2TiO 3, LizZrO 3 and Li2SnO 3 has been studied in the present study.
2. Experimental The lithium ceramics of Li2TiO 3, Li2ZrO 3 and Li2SnO 3 were prepared by solid-state reactions. Powders of T i t 2, ZrO 2 or SnO 2 of reagent grade obtained from Nacalai Tesque were milled with Li2CO 3 in stoichiometric amounts and calcined at 1123 K (950°C) for 8 h. The formation of each compound was confirmed by X-ray diffraction. Pellet-type samples of 10 mm in diameter and about 1 m m in thickness were sintered at 1423 K (1150°C) for 8 h. A He ÷ or H ÷ ion beam, accelerated with a Van de Graaff accelerator, was led to the target sample at 90 °. The size of the ion beam was about 3 m m in diameter and the beam current was monitored. The luminescence from the target sample was led to monochromaters, Ritsu MC-20N
0022-3115/97/$17.00 © 1997 Elsevier Science B.V. All rights reserved. Pll S 0 0 2 2 - 3 1 1 5 ( 9 7 ) 0 0 1 9 4 - 3
K. Moritani, H. Mo riyama / Journal of Nuclear Materials 248 (1997) 132-139 and counted with photo-multipliers, H a m a m a t s u R585. The temperature of the sample holder w a s controlled with an electric heater and a thermocouple and another thermocouple was attached to the sample surface to m o n i t o r its temperature.
133
3. Results 3.1. Analysis o f luminescence bands Figs. 1 - 3 s h o w a set of typical luminescence spectra of Li2TiO3, L i 2 Z r O 3 and L i 2 S n O 3 under H + and He + ion
(b) (a)
• + U2TiO 3 + 2MeV He
25001500 2000"
15oo,
.~ tooo
1000' 500
500
o %',~
0 200
300
400
500
1
~=,,
o
I
I
500
600
Wave length, nm
Wavelength, nm
4000 -
400
300
200
600
Li2TiO ~ + 2MeV H* 4 3 t K)
400
300
3000-
.=_ m
~
2000 -
200 100
1000
0 2OO
I 300
I 400
I 500
0
I 600
200
300
25X103
UzTiO 3 + 2MeV H+ (303K)
600
Ll2"fiOs +
2MeV
He*
4OO
15
300
10
c
5
o
500
500 -
20 o
400 Wave length, nm
Wave length, nm
200
-
100 -
k200
I 300
t 400
I 500
0-
I 600
I 300
200
Wave length, nm
I
400 Wave length, nm
I
500
600
Fig. 1. Luminescence spectra of Li2TiO 3 under (a) H + and (b) He + irradiation.
Table 1 Comparison of luminescence bands Ceramics
Projectile ions
Luminescence bands (nm)
Li20 Li2 SiO 3
He + H÷ He ÷ H+ He + H÷ He + H+ He ÷
(260)
Li2TiO 3 LizZrO 3 Li2SnO 3
(280) (280) (280) 276 + 6 (280) 278 + 7
340 330 (325) 335 + 7 327 + 4 319 + 5 (320) 321 ___4
Note 380 380 380 378 376 371 366 376 371
(510)
_+4 + 3 -t- 5 + 5 + 3 + 4
420 430 423 421 421 416 421 421
+ 3 + 2 _ 3 + 3 + 4 _+ 2
490 490 482 481 483 478 477 485
+ 4 _ 2 -t- 8 + 3 +_ 5 + 4
536 535 541 531 536 540
_+ 4 +_ 3 ___6 ___2 + 7 + 5
Refs. [1,2] Ref. [5] Ref. [4] present present present present present present
K. Moritani, H. Moriyama/ Journal of Nuclear Materials 248 (1997) 132-139
134
beam irradiation. The ordinate represents luminescence intensity corrected for beam current. The peak heights have been observed to be proportional to the beam current and then all of the spectra in the present study are corrected with the beam current for comparison. The luminescence spectra are composed of multiple luminescence bands, as shown in Figs. 1-3. The decomposition of the spectra has been performed by least-squares fittings. Energy-based Gaussian functions were taken for all the luminescence bands and the peak heights and positions were determined. For a proper convergence, the peak width has been assumed to be given by an empirical correlation that y = cx-2 where y denotes the peak width in eV, x the peak position in eV and c the constant. The observed luminescence bands are summarized in Table 1 together with those of L i 2 0 [1] and Li2SiO 3 [4,5].
"•,
(a)
2500u
2000 15001000500
-
0
3O0
400 Wave length, nm
Figs. 4 - 6 show the Arrhenius plots of the luminescence intensity for Li2TiO 3, Li2ZrO 3 and Li2SnO 3 under H + and He + ion beam irradiation. The luminescence intensity corresponds to the peak height determined above. By comparing Figs. 4 - 6 , the observed temperature dependent behaviors are summarized as follows. (1) The temperature dependence of luminescence intensity under He + ion irradiation is different from that under H + ion irradiation; the luminescence intensity under He + ion irradiation rises at temperatures around 700 K while it decreases rather monotonically with increasing temperature under H + irradiation. (2) In the temperature range where the luminescence intensity increases under He + irradiation, luminescence
U2ZrOa + 2MeV He*
(b) sooo 6000 u "~ m
4000
2000
I
I
200
U2ZrO, s + 2MeV H+
3.2. Temperature dependence of luminescence intensity
I
I
500
600
0
I
200
300
1
400 Wavelength. nm
I
I
500
600
Li2ZrO3 + 2MeV He* ~
U
2
Z
I
O
s
2MoV H+
+
4000
30O0 3000 2000
'i
m 2000
"~
m
1000
1000
O~ 2OO
.
.
I
300
.
I
.
400 Wavelength. nm
I
I
500
600
0
I
200
300
I
400 Wavelength. nm
3000 25x103
2O o
~' ®
2500
-
2000
-
15-
•~ t 5 0 0 -
10-
lOOO-
6~
500 -
6
l
200
300
I
400 Wave length, nm
l
I
500
600
0
I
600
Li2ZrO3 ÷ 2MeV He* (311 K)
l
200
J
500
30O
I
400 Wavelength. nm
Fig. 2. Luminescence spectra of Li2 ZrO3 under (a) H + and (b) He + irradiation.
I
I
500
600
K. Moritani, H. Moriyama / Journai of Nuclear Materials 248 (1997) 132-139
(a)
100
(b)
2000
-
~ 1 ~ ,
135 Li2SnO3
+ 2MeV He*
80
E o
60
1500 -
>~ 1000-
E
40 500 -
20 0.
0200
300
400 Wave length, nm
500
600
~-
200
j-
../ I
~~--..~--._..,. [
,~.~ [
I
500
300
400 Wave length, nm
500
300
400 Wave length, nm
500
600
300
400 Wave length, nrn
500
600
350 1000
300 -
8O0
~. 250 -
600
200150-
400
100 200 500 200
300
60X103 -
400 Wavelength, nm
500
~ K U 2 S n O a
O200
600
800
+ 2MeV H*
50)
~ 600 ~u
4019
g
30-
400
20 2O0
10o
%~20O
I
300
I
400 Wave length, nm
I
I
500
600
Fig. 3. Luminescence spectra of Li 2S n O
bands of shorter wave length (shorter than 340 nm) are observed in the spectra. (3) Under H + irradiation, the temperature dependence of the luminescence intensity is different among the materials; the luminescence intensity decreases more rapidly in Li2SnO 3 with increasing temperature than in the other tWO.
4. Discussion 4.1. Luminescence bands and irradiation defects
The luminescence spectra have been decomposed successfully and it is important to address the observed luminescence bands to defect species. Various types of defect species may be considered in the present materials. In the case of Li20 [1,2] and LiA102 [3], for example, the formation of F-type centers, i.e., F + centers and F ° cen-
0
200
3
under (a) H + and (b) He + irradiation.
ters, has been reported in the temperature range up to the blanket operating temperatures. In addition to these, the formation of self-trapped excitons has also been reported below the room temperature [6]. In Li2SiO 3 and Li4SiO4 [4,5], furthermore, the formation of E' centers, non-bridging oxygen hole centers and peroxy radicals is considered and it has been suggested that the luminescence is associated with the formation of peroxy species in these materials in the temperature range from 300 to 850 K. Thus, various types of defect species are considered to be formed in Li2TiO3, Li2ZrO 3 and Li2SnO 3 and it still remains unclear to which defect species the luminescence bands are addressed. Although the luminescence spectra have been decomposed to a number of the luminescence bands, some of these seem to exhibit similar temperature dependences with one another in each material, as shown in Figs. 4-6. Consequently, rather similar origins may be suggested for these luminescence bands. This may be explained by con-
136
K. Moritani, H. Mor~vama / Journal of Nuclear Materials 248 (1997)132-139
(a) los
(b) 10 s (280nm) (325nm) 378nm 423nm 482nm
o z~ o
• • o
Li2"rio 3 + 2MeV H* z~
~x
(280nm) 335nm 376nm 421 nm 481 nm 535nm
o
104
10 +
LI2TiO3 + 2MeV He* :
8_ o o~
--
A
Zo°o°g
~Ae° OSO0 0 o L~O OV V
103
o~ <) ~
V OIVV V
V
•
v
,7
"E --
8 =o
• L II ii I
~^, •
~
o
~
~
Iv
II
102
10 3
._J
10 2
v,7
v
~7
vV~ v
I
e •e
10
~ 1.0
-: °
•
•
101 1.5
2.0
2.5
3.0
3.5
4.0
1.0
1.5
2.0
K "1
1000/T,
2.5
3.0
3.5
4.0
1 0 0 0 / T , K -1
Fig. 4. Arrhenius plots of luminescence intensity of Li2TiO 3 under (a) H + and (b) He + irradiation.
sidering the microscopic structures o f the materials. The structural arrangement o f Li2TiO 3, Li2ZrO 3 and L i 2 S n O 3 is NaCl-like. In these compounds, the oxygen atoms form a distorted cubic close-packed network and the cations occupy, in an ordered fashion, all octahedral sites present
(a) 105
in this network [7,8]. The distorted network and the different distribution o f the lithium and the tetravalent cations in the network will be the reason for the different luminescence bands o f similar origins. In Li2TiO 3, for example, the octahedral edges are shared between two Ti ions, two
(b) 10 s LI2ZrO 3 + 2MeV H*
o
104
'
L 276nm 1 319nm / 366nm | 416nm/ 478nm 531nm /
• • o A o 10 4
8_ o
i
i
o
~;" 000
f~ --
+
LI2ZrO3 + 2MeV He*
•
no~ o o
0
103
•
•
.%.'.~ v +'vV~
~
'7
o
103
o
o
~7
V V
V
• °=V ° 0
10s
• • o zx <> v
(280nm) 327nm 371nm 421 nm 483nm 541nm
3.0
3.5
101
102
101 2.0
°o
;
_J
1.5
&e
o
o ~
V
1.0
~.
•~ •
2.5
1000/I", K -I
4.0
1.0
V
IO
~
o o v
V
•
V
V
r
i
r
=
+
1.5
2.0
2.5
3,0
3.5
IO00/T, K -I
Fig. 5. An'henius plots of luminescence intensity of Li2ZrO 3 under (a) H + and (b) He + irradiation.
4.0
K. Moritani, H. Moriyama/ Journal of Nuclear Materials 248 (1997) 132-139 (a) l0 s
(b) • • o
(280nm) (320nrn) 376nm 421nm 477nm 536nrn
a
o 104
v
6
o o o
A
v~ 0 V
!
-r
278nm ] 321 nrn | 371 nm | 421 nm | 4 8 5 nm | 54Onto ]
r
r
Li2SnO 3 + 2 M e V He +
tj
0~ "E ,,, ,~,
8
0
/ °I
,
l°3F~,~o ;
o
,o
v o
._J 102
10 ~ I~00
o v
104
&~7 o
oA a~
• • o A
z~
o
103 8zx
-----r
a a
o oz~ o o¢
10 s
Li2Sn03 + 2MeV H+
137
o
v
VV
V
BV=
V VV
e NIl
•
101 1.5
101 2.0
2.5
3.0
3.5
4.0
1.0
1.5
2.0
1000/T, K -1
2.5
3.0
3.5
4.0
1000/T, K -'~
Fig. 6. Arrhenius plots of luminescence intensity of Li2SnO 3 under (a) H + and (b) He + irradiation.
Li ions, or one Ti and one Li ions and the mean O - O distances for the three categories are 2.58, 3.11 and 2.82 ]~, respectively [8]. These differences will result in the observed variety of the luminescence bands. In addition to these, the structural rearrangement may also be taken into account under irradiation.
(2)
observed especially at higher temperatures. In Li2SiO 3 and Li4SiO4, on the other hand, the observed luminescence is connected with the formation of the peroxy species in which the second-order reactions are less important [4,5]. The present results of Li2TiO 3, Li2ZrO 3 and Li2SnO 3 are considered to be rather similar to those of LieO because of possible participation of the second-order reactions. As shown in Figs. 4-6, the temperature dependences of the luminescence intensity under H + beam irradiation are different from those under He + beam irradiation. Since the specific ionization is much different between both projectile ions, the reaction species concentration in each ion track will be different, and then the second-order reactions will be affected. The observed differences in Figs. 4 - 6 are thus suggesting the participation of such second-order reactions in the production mechanism of irradiation defects in LizTiO 3, LieZrO 3 and LiaSnO 3. By the analogy of the reaction scheme for Li20, a production mechanism of irradiation defects in Li2TiO 3, Li2ZrO 3 and LizSnO 3 is temporarily given as
2Li20 * -~ 2F ° + 02 + 2hv',
(3)
L i z M O 3 --> Li2MO3*,
(F +" O - ) + Li20* ~ 2F ° + 02 + 2 h v ' .
(4)
L i 2 M O ; ~ (F +. 0 - )
4.2. Production mechanism of irradiation defects The production mechanisms of irradiation defects in Li20 [1,2], LiA102 [3], LizSiO 3 and LiaSiO 4 [4,5] have been studied in our previous studies. In the case of Li20, the luminescence associated with the formation of F + centers is observed at lower temperatures while that of F ° centers is observed at higher temperatures. Then the following reaction scheme has been given for the production mechanism of F + and F ° centers: L i 2 0 --~ L i 2 0 *, Li20* ~ (F +' 0 - )
(1) + hv,
Eq. (1) represents the production of excited Li20, L i 2 0 *, by ion beam irradiation. F + centers are considered to be associated with an O - interstitial under Coulomb interaction as (F +- O - ) and to be produced by the first-order reaction, Eq. (2). The second-order reactions, Eqs. (3) and (4), for the formation of F ° centers require some thermal activation and will take place at higher temperatures. This is the reason why the luminescence of F ° centers is
(5) + hv,
(6)
2Li2MO ~ ~ 2F ° + 0 2 + 2hv',
(7)
(F +. O - ) + Li2MO3* ~ 2F ° + 0 2 + 2hv'.
(8)
Following this reaction mechanism, two types of luminescence bands are considered; one is the luminescence associated with the formation of F + centers and the other is of F ° centers. The observed luminescence bands are thus classified into the two types. It is remembered here that the
138
K. Moritani, H. Moriyama / Journal of Nuclear Materials 248 (1997) 132-139
Table 2 Properties of ceramic breeder materials Ceramics
Density (g cm -3 )
Li density (gcm -3)
Melting point (°C)
Li ~O Li2 SiO3 Li 2TIO3(/~ ) Li2ZrO 3 Li~ SnO 3
2.023 2.52 3.43 4.15 4.99
0.94 0.36 0.43 0.38 0.38
1423 1200 1535 1615 < tO00
Then tritium is released in the form of T 2 or T20 to the sweep gas. By taking this reaction scheme, some predictions have been made and the observations in the BEATRIX-II experiment have successfully been interpreted [9]. It is important that irradiation defects participate in the reaction mechanisms at such high temperatures, because material stability will be affected as well as tritium release kinetics. In the case of H2-containing sweep gas, F ° and 0 2 are considered to react with H 2 at the grain surface as
luminescence bands of shorter wave length (shorter than 340 nm) are observed more clearly at high temperatures under He + irradiation. By considering the possible participation of the secondary reactions, these bands may be attributed to F ° centers. On the other hand, the luminescence bands of longer wave length (longer than 380 rim) are observed irrespective of temperature under both H + and He + irradiation and may be of F + centers. For some details, it is interesting to compare the temperature dependences of the luminescence intensity of Li2TiO 3, LizZrO 3 and Li2SnO 3 under H + beam irradiation. In these materials, the luminescence intensity decreases rather monotonically with increasing temperature under H + irradiation. As shown in Figs. 4 - 6 , however, the intensity is found to decrease more rapidly in Li2SnO 3 than in the other two. One of the reasons may due to different rates of the back reaction of Eq. (5) or different rates of non-radiative transition of L i a M O 3. It seems that the rate is more sensitive to temperature in Li2SnO 3 whose melting point is the lowest as shown in Table 2. 4.3. Possible effects on tritium recovery and material stabili O,
The production behavior of irradiation defects in LizTiO 3, Li2ZrO 3 and LizSnO 3 is similar to those in L i 2 0 and the irradiation defects have been observed to be formed up to blanket operating temperatures. The effects of irradiation defects on tritium recovery and material stability will thus be considered in these materials. For the assessment of tritium release behavior of L i 2 0 under irradiation, a model reaction scheme has been presented by taking into account the interactions of irradiation defects with tritium [9]. Tritium is produced in L i 2 0 grains by nuclear reactions and stabilized in the chemical states of T+(LiOT) or T - ( L i T ) . The tritium species interacts with the defect products of F ° centers and O 2 molecules and diffuses to the grain surface, where the following surface reactions take place: LiT + LiT ~ 2Li + T 2 ~,
(9)
LiT + LiOT ~ L i 2 0 + T 2 1' ,
(10)
LiOT + LiOT ~ L i 2 0 + T201".
(11)
F 0 + H 2 --> 2LiH,
(12)
02 -~- 2H 2 ~ 2 H 2 0 1".
(13)
Due to Eq. (13), the moisture concentration of H 2 0 will increase with increasing H 2 concentration in the sweep gas, as observed in the BEATRIX-II, and then the material degradation will be accelerated in the presence of H~. Further studies are suggested accordingly.
5. Conclusions (1) Multiple luminescence bands have been observed in the in situ luminescence measurement of Li 2TIO3, Li 2ZrO3 and Li2SnO 3 under H + and He + ion beam irradiation. Although various types of defect species are considered to be formed in these materials, the distorted network and the different distribution of the lithium and the tetravalent cations in the network will be a possible reason for the different luminescence bands of similar origins. (2) An interesting temperature dependence of the luminescence intensity has been observed; the luminescence intensity under He + ion irradiation rises at the temperatures around 700 K while it decreases rather monotonically with increasing temperature under H + irradiation. In the temperature range where the luminescence intensity rises under He + irradiation, the luminescence bands of shorter wave length (shorter than 340 nm) are observed more clearly in the spectra. From these findings, it has been suggested that the production mechanism of irradiation defects in these materials is similar to that in Li20. (3) Irradiation defects are formed even at the blanket operating temperatures, and the effects on tritium recovery and material stability have been pointed out by considering the participation of F ° centers and 0 2 molecules in the reaction mechanisms. These species may react with hydrogen in the sweep gas and accelerate the degradation of the materials.
Acknowledgements The authors wish to thank Mr K. Yoshida, Kyoto University, for his kind encouragements.
K. MoritanL H. Moriyama / Journal of Nuclear Materials 248 (1997) 132-139
References [1] Y. Asaoka, H. Moriyama, K. Iwasaki, K. Moritani, Y. Ito, J. Nucl. Mater. 183 (1991) 174. [2] Y. Asaoka, H. Moriyama, Y. Ito, Fusion Technol. 21 (1992) 1944. [3] Y. Asaoka, H. Moriyama, K. Iwasaki, K. Moritani, Y. Ito, J. Nucl. Mater. 191-194 (1992) 268. [4] H. Moriyama, T. Nagae, K. Moritani, Y. Ito, in: Fusion Technology 1992, eds. C. Fen'o, M. Gasparotto and H. Knoepfel (North-Holland, Amsterdam, 1993) p. 1434.
139
[5] H. Moriyama, T. Nagae, K. Moritani, Y. Ito, Nucl. Instrum. Methods B91 (1994) 317. [6] V. Grishmanov, S. Tanaka, Proc. of 4th Int. Workshop on Ceramic Breeder Blanket Interactions, Kyoto, Oct. 9-1 l, 1995, p. 153. [7] J.L. Hodeau, M. Marezio, A. Santoro, R.S. Roth, J. Solid State Chem. 45 (1982) 170. [8] J.F. Dorrian, R.E. Newnham, Mater. Res. Bull. 4 (1969) 179. [9] H. Moriyama, T. Kurasawa, J. Nucl. Mater. 212-215 (1994) 932.
Journal of Nuclear Materials 248 (1997) 132-139
In situ luminescence measurement of irradiation defects in ternary lithium ceramics under ion beam irradiation Kimikazu Moritani a, Hirotake Moriyama
b,,
a Faculty of Engineering, Kyoto University, Yoshida, Sakyo-ku, Kyoto 606-01, Japan b Research Reactor Institute, Kyoto University, Kumatori-cho, Sennan-gun, Osaka 590-04, Japan
Abstract For the performance assessment of fusion reactor solid breeder materials, the production behavior of irradiation defects in some candidate materials of Li 2TIO3, Li e ZrO 3 and Li 2 SnO 3 was studied by an in situ luminescence measurement technique under ion beam irradiation. The luminescence was observed to be composed of multiple luminescence bands and the temperature dependence of the luminescence intensity was measured under He + or H ÷ ion beam irradiation. The production mechanism of irradiation defects was discussed by comparing the present results with those previously obtained for Li20. The effects of irradiation defects on tritium recovery kinetics and material stability were pointed out. © 1997 Elsevier Science B.V.
1. Introduction In a fusion reactor solid blanket system, tritium breeding lithium ceramics are attacked by high energy neutrons and energetic particles from nuclear reactions and severe irradiation damage may be expected. The irradiation behavior of lithium ceramics is thus important for the performance assessment of fusion reactor blanket systems. The effects of irradiation on the tritium release behaviors and microstructural changes of lithium ceramics are particularly important. For a clearer understanding of such effects, we have studied the production behavior of irradiation defects in L i 2 0 [1,2], LiAIO 2 [3], Li2SiO 3 and LiaSiO 4 [4,5] by an in situ luminescence measurement technique under ion beam irradiation. In L i 2 0 and LiA102, it has been confinned that the F + center (an oxygen vacancy trapping an electron) and the F ° center (an oxygen vacancy trapping two electrons), which are commonly observed in ionic crystals, are formed by irradiation. Similarly, the irradia-
* Corresponding author. Fax: + 81-724 51 2424.
tion defects of the E' center (=--Si.), the non-bridging oxygen hole center ( ~ S i - O - ) and the peroxy radical ( ~ S i - O - O • ) are produced in Li2SiO 3 and Li4SiO 4. These defects are considered to play an important role in the tritium behavior. For comparison, the production behavior of irradiation defects in the lithium ceramics of Li2TiO 3, LizZrO 3 and Li2SnO 3 has been studied in the present study.
2. Experimental The lithium ceramics of Li2TiO 3, Li2ZrO 3 and Li2SnO 3 were prepared by solid-state reactions. Powders of T i t 2, ZrO 2 or SnO 2 of reagent grade obtained from Nacalai Tesque were milled with Li2CO 3 in stoichiometric amounts and calcined at 1123 K (950°C) for 8 h. The formation of each compound was confirmed by X-ray diffraction. Pellet-type samples of 10 mm in diameter and about 1 m m in thickness were sintered at 1423 K (1150°C) for 8 h. A He ÷ or H ÷ ion beam, accelerated with a Van de Graaff accelerator, was led to the target sample at 90 °. The size of the ion beam was about 3 m m in diameter and the beam current was monitored. The luminescence from the target sample was led to monochromaters, Ritsu MC-20N
0022-3115/97/$17.00 © 1997 Elsevier Science B.V. All rights reserved. Pll S 0 0 2 2 - 3 1 1 5 ( 9 7 ) 0 0 1 9 4 - 3
K. Moritani, H. Mo riyama / Journal of Nuclear Materials 248 (1997) 132-139 and counted with photo-multipliers, H a m a m a t s u R585. The temperature of the sample holder w a s controlled with an electric heater and a thermocouple and another thermocouple was attached to the sample surface to m o n i t o r its temperature.
133
3. Results 3.1. Analysis o f luminescence bands Figs. 1 - 3 s h o w a set of typical luminescence spectra of Li2TiO3, L i 2 Z r O 3 and L i 2 S n O 3 under H + and He + ion
(b) (a)
• + U2TiO 3 + 2MeV He
25001500 2000"
15oo,
.~ tooo
1000' 500
500
o %',~
0 200
300
400
500
1
~=,,
o
I
I
500
600
Wave length, nm
Wavelength, nm
4000 -
400
300
200
600
Li2TiO ~ + 2MeV H* 4 3 t K)
400
300
3000-
.=_ m
~
2000 -
200 100
1000
0 2OO
I 300
I 400
I 500
0
I 600
200
300
25X103
UzTiO 3 + 2MeV H+ (303K)
600
Ll2"fiOs +
2MeV
He*
4OO
15
300
10
c
5
o
500
500 -
20 o
400 Wave length, nm
Wave length, nm
200
-
100 -
k200
I 300
t 400
I 500
0-
I 600
I 300
200
Wave length, nm
I
400 Wave length, nm
I
500
600
Fig. 1. Luminescence spectra of Li2TiO 3 under (a) H + and (b) He + irradiation.
Table 1 Comparison of luminescence bands Ceramics
Projectile ions
Luminescence bands (nm)
Li20 Li2 SiO 3
He + H÷ He ÷ H+ He + H÷ He + H+ He ÷
(260)
Li2TiO 3 LizZrO 3 Li2SnO 3
(280) (280) (280) 276 + 6 (280) 278 + 7
340 330 (325) 335 + 7 327 + 4 319 + 5 (320) 321 ___4
Note 380 380 380 378 376 371 366 376 371
(510)
_+4 + 3 -t- 5 + 5 + 3 + 4
420 430 423 421 421 416 421 421
+ 3 + 2 _ 3 + 3 + 4 _+ 2
490 490 482 481 483 478 477 485
+ 4 _ 2 -t- 8 + 3 +_ 5 + 4
536 535 541 531 536 540
_+ 4 +_ 3 ___6 ___2 + 7 + 5
Refs. [1,2] Ref. [5] Ref. [4] present present present present present present
K. Moritani, H. Moriyama/ Journal of Nuclear Materials 248 (1997) 132-139
134
beam irradiation. The ordinate represents luminescence intensity corrected for beam current. The peak heights have been observed to be proportional to the beam current and then all of the spectra in the present study are corrected with the beam current for comparison. The luminescence spectra are composed of multiple luminescence bands, as shown in Figs. 1-3. The decomposition of the spectra has been performed by least-squares fittings. Energy-based Gaussian functions were taken for all the luminescence bands and the peak heights and positions were determined. For a proper convergence, the peak width has been assumed to be given by an empirical correlation that y = cx-2 where y denotes the peak width in eV, x the peak position in eV and c the constant. The observed luminescence bands are summarized in Table 1 together with those of L i 2 0 [1] and Li2SiO 3 [4,5].
"•,
(a)
2500u
2000 15001000500
-
0
3O0
400 Wave length, nm
Figs. 4 - 6 show the Arrhenius plots of the luminescence intensity for Li2TiO 3, Li2ZrO 3 and Li2SnO 3 under H + and He + ion beam irradiation. The luminescence intensity corresponds to the peak height determined above. By comparing Figs. 4 - 6 , the observed temperature dependent behaviors are summarized as follows. (1) The temperature dependence of luminescence intensity under He + ion irradiation is different from that under H + ion irradiation; the luminescence intensity under He + ion irradiation rises at temperatures around 700 K while it decreases rather monotonically with increasing temperature under H + irradiation. (2) In the temperature range where the luminescence intensity increases under He + irradiation, luminescence
U2ZrOa + 2MeV He*
(b) sooo 6000 u "~ m
4000
2000
I
I
200
U2ZrO, s + 2MeV H+
3.2. Temperature dependence of luminescence intensity
I
I
500
600
0
I
200
300
1
400 Wavelength. nm
I
I
500
600
Li2ZrO3 + 2MeV He* ~
U
2
Z
I
O
s
2MoV H+
+
4000
30O0 3000 2000
'i
m 2000
"~
m
1000
1000
O~ 2OO
.
.
I
300
.
I
.
400 Wavelength. nm
I
I
500
600
0
I
200
300
I
400 Wavelength. nm
3000 25x103
2O o
~' ®
2500
-
2000
-
15-
•~ t 5 0 0 -
10-
lOOO-
6~
500 -
6
l
200
300
I
400 Wave length, nm
l
I
500
600
0
I
600
Li2ZrO3 ÷ 2MeV He* (311 K)
l
200
J
500
30O
I
400 Wavelength. nm
Fig. 2. Luminescence spectra of Li2 ZrO3 under (a) H + and (b) He + irradiation.
I
I
500
600
K. Moritani, H. Moriyama / Journai of Nuclear Materials 248 (1997) 132-139
(a)
100
(b)
2000
-
~ 1 ~ ,
135 Li2SnO3
+ 2MeV He*
80
E o
60
1500 -
>~ 1000-
E
40 500 -
20 0.
0200
300
400 Wave length, nm
500
600
~-
200
j-
../ I
~~--..~--._..,. [
,~.~ [
I
500
300
400 Wave length, nm
500
300
400 Wave length, nm
500
600
300
400 Wave length, nrn
500
600
350 1000
300 -
8O0
~. 250 -
600
200150-
400
100 200 500 200
300
60X103 -
400 Wavelength, nm
500
~ K U 2 S n O a
O200
600
800
+ 2MeV H*
50)
~ 600 ~u
4019
g
30-
400
20 2O0
10o
%~20O
I
300
I
400 Wave length, nm
I
I
500
600
Fig. 3. Luminescence spectra of Li 2S n O
bands of shorter wave length (shorter than 340 nm) are observed in the spectra. (3) Under H + irradiation, the temperature dependence of the luminescence intensity is different among the materials; the luminescence intensity decreases more rapidly in Li2SnO 3 with increasing temperature than in the other tWO.
4. Discussion 4.1. Luminescence bands and irradiation defects
The luminescence spectra have been decomposed successfully and it is important to address the observed luminescence bands to defect species. Various types of defect species may be considered in the present materials. In the case of Li20 [1,2] and LiA102 [3], for example, the formation of F-type centers, i.e., F + centers and F ° cen-
0
200
3
under (a) H + and (b) He + irradiation.
ters, has been reported in the temperature range up to the blanket operating temperatures. In addition to these, the formation of self-trapped excitons has also been reported below the room temperature [6]. In Li2SiO 3 and Li4SiO4 [4,5], furthermore, the formation of E' centers, non-bridging oxygen hole centers and peroxy radicals is considered and it has been suggested that the luminescence is associated with the formation of peroxy species in these materials in the temperature range from 300 to 850 K. Thus, various types of defect species are considered to be formed in Li2TiO3, Li2ZrO 3 and Li2SnO 3 and it still remains unclear to which defect species the luminescence bands are addressed. Although the luminescence spectra have been decomposed to a number of the luminescence bands, some of these seem to exhibit similar temperature dependences with one another in each material, as shown in Figs. 4-6. Consequently, rather similar origins may be suggested for these luminescence bands. This may be explained by con-
136
K. Moritani, H. Mor~vama / Journal of Nuclear Materials 248 (1997)132-139
(a) los
(b) 10 s (280nm) (325nm) 378nm 423nm 482nm
o z~ o
• • o
Li2"rio 3 + 2MeV H* z~
~x
(280nm) 335nm 376nm 421 nm 481 nm 535nm
o
104
10 +
LI2TiO3 + 2MeV He* :
8_ o o~
--
A
Zo°o°g
~Ae° OSO0 0 o L~O OV V
103
o~ <) ~
V OIVV V
V
•
v
,7
"E --
8 =o
• L II ii I
~^, •
~
o
~
~
Iv
II
102
10 3
._J
10 2
v,7
v
~7
vV~ v
I
e •e
10
~ 1.0
-: °
•
•
101 1.5
2.0
2.5
3.0
3.5
4.0
1.0
1.5
2.0
K "1
1000/T,
2.5
3.0
3.5
4.0
1 0 0 0 / T , K -1
Fig. 4. Arrhenius plots of luminescence intensity of Li2TiO 3 under (a) H + and (b) He + irradiation.
sidering the microscopic structures o f the materials. The structural arrangement o f Li2TiO 3, Li2ZrO 3 and L i 2 S n O 3 is NaCl-like. In these compounds, the oxygen atoms form a distorted cubic close-packed network and the cations occupy, in an ordered fashion, all octahedral sites present
(a) 105
in this network [7,8]. The distorted network and the different distribution o f the lithium and the tetravalent cations in the network will be the reason for the different luminescence bands o f similar origins. In Li2TiO 3, for example, the octahedral edges are shared between two Ti ions, two
(b) 10 s LI2ZrO 3 + 2MeV H*
o
104
'
L 276nm 1 319nm / 366nm | 416nm/ 478nm 531nm /
• • o A o 10 4
8_ o
i
i
o
~;" 000
f~ --
+
LI2ZrO3 + 2MeV He*
•
no~ o o
0
103
•
•
.%.'.~ v +'vV~
~
'7
o
103
o
o
~7
V V
V
• °=V ° 0
10s
• • o zx <> v
(280nm) 327nm 371nm 421 nm 483nm 541nm
3.0
3.5
101
102
101 2.0
°o
;
_J
1.5
&e
o
o ~
V
1.0
~.
•~ •
2.5
1000/I", K -I
4.0
1.0
V
IO
~
o o v
V
•
V
V
r
i
r
=
+
1.5
2.0
2.5
3,0
3.5
IO00/T, K -I
Fig. 5. An'henius plots of luminescence intensity of Li2ZrO 3 under (a) H + and (b) He + irradiation.
4.0
K. Moritani, H. Moriyama/ Journal of Nuclear Materials 248 (1997) 132-139 (a) l0 s
(b) • • o
(280nm) (320nrn) 376nm 421nm 477nm 536nrn
a
o 104
v
6
o o o
A
v~ 0 V
!
-r
278nm ] 321 nrn | 371 nm | 421 nm | 4 8 5 nm | 54Onto ]
r
r
Li2SnO 3 + 2 M e V He +
tj
0~ "E ,,, ,~,
8
0
/ °I
,
l°3F~,~o ;
o
,o
v o
._J 102
10 ~ I~00
o v
104
&~7 o
oA a~
• • o A
z~
o
103 8zx
-----r
a a
o oz~ o o¢
10 s
Li2Sn03 + 2MeV H+
137
o
v
VV
V
BV=
V VV
e NIl
•
101 1.5
101 2.0
2.5
3.0
3.5
4.0
1.0
1.5
2.0
1000/T, K -1
2.5
3.0
3.5
4.0
1000/T, K -'~
Fig. 6. Arrhenius plots of luminescence intensity of Li2SnO 3 under (a) H + and (b) He + irradiation.
Li ions, or one Ti and one Li ions and the mean O - O distances for the three categories are 2.58, 3.11 and 2.82 ]~, respectively [8]. These differences will result in the observed variety of the luminescence bands. In addition to these, the structural rearrangement may also be taken into account under irradiation.
(2)
observed especially at higher temperatures. In Li2SiO 3 and Li4SiO4, on the other hand, the observed luminescence is connected with the formation of the peroxy species in which the second-order reactions are less important [4,5]. The present results of Li2TiO 3, Li2ZrO 3 and Li2SnO 3 are considered to be rather similar to those of LieO because of possible participation of the second-order reactions. As shown in Figs. 4-6, the temperature dependences of the luminescence intensity under H + beam irradiation are different from those under He + beam irradiation. Since the specific ionization is much different between both projectile ions, the reaction species concentration in each ion track will be different, and then the second-order reactions will be affected. The observed differences in Figs. 4 - 6 are thus suggesting the participation of such second-order reactions in the production mechanism of irradiation defects in LizTiO 3, LieZrO 3 and LiaSnO 3. By the analogy of the reaction scheme for Li20, a production mechanism of irradiation defects in Li2TiO 3, Li2ZrO 3 and LizSnO 3 is temporarily given as
2Li20 * -~ 2F ° + 02 + 2hv',
(3)
L i z M O 3 --> Li2MO3*,
(F +" O - ) + Li20* ~ 2F ° + 02 + 2 h v ' .
(4)
L i 2 M O ; ~ (F +. 0 - )
4.2. Production mechanism of irradiation defects The production mechanisms of irradiation defects in Li20 [1,2], LiA102 [3], LizSiO 3 and LiaSiO 4 [4,5] have been studied in our previous studies. In the case of Li20, the luminescence associated with the formation of F + centers is observed at lower temperatures while that of F ° centers is observed at higher temperatures. Then the following reaction scheme has been given for the production mechanism of F + and F ° centers: L i 2 0 --~ L i 2 0 *, Li20* ~ (F +' 0 - )
(1) + hv,
Eq. (1) represents the production of excited Li20, L i 2 0 *, by ion beam irradiation. F + centers are considered to be associated with an O - interstitial under Coulomb interaction as (F +- O - ) and to be produced by the first-order reaction, Eq. (2). The second-order reactions, Eqs. (3) and (4), for the formation of F ° centers require some thermal activation and will take place at higher temperatures. This is the reason why the luminescence of F ° centers is
(5) + hv,
(6)
2Li2MO ~ ~ 2F ° + 0 2 + 2hv',
(7)
(F +. O - ) + Li2MO3* ~ 2F ° + 0 2 + 2hv'.
(8)
Following this reaction mechanism, two types of luminescence bands are considered; one is the luminescence associated with the formation of F + centers and the other is of F ° centers. The observed luminescence bands are thus classified into the two types. It is remembered here that the
138
K. Moritani, H. Moriyama / Journal of Nuclear Materials 248 (1997) 132-139
Table 2 Properties of ceramic breeder materials Ceramics
Density (g cm -3 )
Li density (gcm -3)
Melting point (°C)
Li ~O Li2 SiO3 Li 2TIO3(/~ ) Li2ZrO 3 Li~ SnO 3
2.023 2.52 3.43 4.15 4.99
0.94 0.36 0.43 0.38 0.38
1423 1200 1535 1615 < tO00
Then tritium is released in the form of T 2 or T20 to the sweep gas. By taking this reaction scheme, some predictions have been made and the observations in the BEATRIX-II experiment have successfully been interpreted [9]. It is important that irradiation defects participate in the reaction mechanisms at such high temperatures, because material stability will be affected as well as tritium release kinetics. In the case of H2-containing sweep gas, F ° and 0 2 are considered to react with H 2 at the grain surface as
luminescence bands of shorter wave length (shorter than 340 nm) are observed more clearly at high temperatures under He + irradiation. By considering the possible participation of the secondary reactions, these bands may be attributed to F ° centers. On the other hand, the luminescence bands of longer wave length (longer than 380 rim) are observed irrespective of temperature under both H + and He + irradiation and may be of F + centers. For some details, it is interesting to compare the temperature dependences of the luminescence intensity of Li2TiO 3, LizZrO 3 and Li2SnO 3 under H + beam irradiation. In these materials, the luminescence intensity decreases rather monotonically with increasing temperature under H + irradiation. As shown in Figs. 4 - 6 , however, the intensity is found to decrease more rapidly in Li2SnO 3 than in the other two. One of the reasons may due to different rates of the back reaction of Eq. (5) or different rates of non-radiative transition of L i a M O 3. It seems that the rate is more sensitive to temperature in Li2SnO 3 whose melting point is the lowest as shown in Table 2. 4.3. Possible effects on tritium recovery and material stabili O,
The production behavior of irradiation defects in LizTiO 3, Li2ZrO 3 and LizSnO 3 is similar to those in L i 2 0 and the irradiation defects have been observed to be formed up to blanket operating temperatures. The effects of irradiation defects on tritium recovery and material stability will thus be considered in these materials. For the assessment of tritium release behavior of L i 2 0 under irradiation, a model reaction scheme has been presented by taking into account the interactions of irradiation defects with tritium [9]. Tritium is produced in L i 2 0 grains by nuclear reactions and stabilized in the chemical states of T+(LiOT) or T - ( L i T ) . The tritium species interacts with the defect products of F ° centers and O 2 molecules and diffuses to the grain surface, where the following surface reactions take place: LiT + LiT ~ 2Li + T 2 ~,
(9)
LiT + LiOT ~ L i 2 0 + T 2 1' ,
(10)
LiOT + LiOT ~ L i 2 0 + T201".
(11)
F 0 + H 2 --> 2LiH,
(12)
02 -~- 2H 2 ~ 2 H 2 0 1".
(13)
Due to Eq. (13), the moisture concentration of H 2 0 will increase with increasing H 2 concentration in the sweep gas, as observed in the BEATRIX-II, and then the material degradation will be accelerated in the presence of H~. Further studies are suggested accordingly.
5. Conclusions (1) Multiple luminescence bands have been observed in the in situ luminescence measurement of Li 2TIO3, Li 2ZrO3 and Li2SnO 3 under H + and He + ion beam irradiation. Although various types of defect species are considered to be formed in these materials, the distorted network and the different distribution of the lithium and the tetravalent cations in the network will be a possible reason for the different luminescence bands of similar origins. (2) An interesting temperature dependence of the luminescence intensity has been observed; the luminescence intensity under He + ion irradiation rises at the temperatures around 700 K while it decreases rather monotonically with increasing temperature under H + irradiation. In the temperature range where the luminescence intensity rises under He + irradiation, the luminescence bands of shorter wave length (shorter than 340 nm) are observed more clearly in the spectra. From these findings, it has been suggested that the production mechanism of irradiation defects in these materials is similar to that in Li20. (3) Irradiation defects are formed even at the blanket operating temperatures, and the effects on tritium recovery and material stability have been pointed out by considering the participation of F ° centers and 0 2 molecules in the reaction mechanisms. These species may react with hydrogen in the sweep gas and accelerate the degradation of the materials.
Acknowledgements The authors wish to thank Mr K. Yoshida, Kyoto University, for his kind encouragements.
K. MoritanL H. Moriyama / Journal of Nuclear Materials 248 (1997) 132-139
References [1] Y. Asaoka, H. Moriyama, K. Iwasaki, K. Moritani, Y. Ito, J. Nucl. Mater. 183 (1991) 174. [2] Y. Asaoka, H. Moriyama, Y. Ito, Fusion Technol. 21 (1992) 1944. [3] Y. Asaoka, H. Moriyama, K. Iwasaki, K. Moritani, Y. Ito, J. Nucl. Mater. 191-194 (1992) 268. [4] H. Moriyama, T. Nagae, K. Moritani, Y. Ito, in: Fusion Technology 1992, eds. C. Fen'o, M. Gasparotto and H. Knoepfel (North-Holland, Amsterdam, 1993) p. 1434.
139
[5] H. Moriyama, T. Nagae, K. Moritani, Y. Ito, Nucl. Instrum. Methods B91 (1994) 317. [6] V. Grishmanov, S. Tanaka, Proc. of 4th Int. Workshop on Ceramic Breeder Blanket Interactions, Kyoto, Oct. 9-1 l, 1995, p. 153. [7] J.L. Hodeau, M. Marezio, A. Santoro, R.S. Roth, J. Solid State Chem. 45 (1982) 170. [8] J.F. Dorrian, R.E. Newnham, Mater. Res. Bull. 4 (1969) 179. [9] H. Moriyama, T. Kurasawa, J. Nucl. Mater. 212-215 (1994) 932.