Radiation Effects on Yttria-stabilized ZrO2 Single Crystals
with Helium and Xenon Ions at RT and 923 K.
K. Hojou, T. Hojo, N. Sasajima*, N. Nitani, T. Yamashita, K. Minato
and S. Furuno.
Japan Atomic Energy Research Institute, Tokai-mura, Ibaraki-ken 319-1195, Japan
* National Metrology Institute of Japan, AIST, tukuba-shi, Ibaraki-ken 305-8561, Japan
Abstract.
In situ electron microscope observation were performed to investigate damage evolution in Yttria-stabilized ZrO2
single crystals during 35 keV He+ and 60 keV Xe2+ ions irradiation at room temperature and 923 K, respectively. No
amorphization occurred in the specimens irradiated with the high dose of He and Xe ions at room temperature and 650 °C,
respectively. The bubble densities and size were hardly increased with the fluence of both helium and xenon ions irradiation but
bubbles were found to grow at 923 K. By annealing at 1073 K to 1523 K after Xe ion irradiation, bubbles were observed to
grow, showing bimodal size distribution. On the other hand, in the case of He ion irradiation, bubbles were found to shrink and
decrease in density by annealing at above 1473 K.
INTRODUCTION
New concepts on inert matrix fuels and their burning
in light water reactors (LWRs) have been studied to use
surplus plutonium [1, 3]. Since the spent fuels would
become high level radioactive wastes (HLWs) without
further processing, the rock like fuels should be
designed to satisfy the requirement of a chemical
stability and a high resistance to radiation damage over
a wide temperature range and under severe irradiation
conditions. For the estimation of radiation stability
during reactor irradiation, it is thus important to clarify
the damage structure or damage mechanism of the fuel
materials.
Fission fragments with initial energy of several
ten MeV lose their energy mainly by electron
excitation, resulting in ionization and heat production.
As the kinetic energy of the moving atoms decreases,
they lose their energy by nuclear collision of the hard
sphere kind in which large energy transfer can occur
producing in a high number of displacement. Low
energy ion irradiation, therefore, cause effectively
defect clustering, bubble formation and amorphization.
A12O3 [4-6], Spinel (MgAl2O4) [7-8] and stabilized
ZrO2 [3, 9, 10] are expected to be the candidate
materials because of their high radiation stability. In
particular, extensive data have been recently
accumulated about physical and chemical changes of
irradiation effects on stabilized ZrO2 [11-13]. However,
the radiation effects on ZrO2 under irradiation are not
well known.
In this study to investigate radiation damage in
the rock-like fuels produced by a-particles or fission
fragments, single crystals of stabilized ZrO2 (YSZ)
were irradiated with low energy helium (He) and xenon
(Xe) ion in the transmission electron microscope
(TEM) at room temperature (RT) and 923 K. Helium
ion irradiation is the simulation of a-decay damage and
Xe atoms is one of the most abundant fission
fragments. The irradiated specimens were annealed in
the TEM and annealing effects on these bubbles are
also reported.
EXPERIMENTAL PROCEDURES
Specimens used in the present work were yttriastabilized (10 mol% Y2O3) cubic Zirconia with (111)
orientation produced by Furuuchi Chemical Co. Both
sides of these samples were polished with diamond
paste to mirror-like finish and discs with diameter of 3
mm were cut off from the wafers by an ultrasonic
cutter. Then discs were dimpled to a thickness of 1020 \JL m at the center part of disc using a dimpling
machine. Perforation was achieved by etching with 3
keV Arc ions at an incident angle of 20~ from the
surface of specimen. Final polishing was carried out
with 2 keV Ar+ ions at 15- at RT. All TEM samples
were annealed at 1573 K for 1 hour in air in order to
eliminate the defects induced by ion polishing.
The in-situ observation during ion irradiation was
made using a 200 keV transmission electron
microscope of JEM-2000F type (JEOL) combined with
40 keV ion accelerator (Origin Electric. Co.). In the
TEM specimen chamber, an incident ion was deflected
downward by 30 degrees with electrostatic prism and
implanted at an angle of 60 degrees to the surface of
specimen. The beam was of 35 keV He ions at a flux
of 5xl017 He/m2s and 60 keV Xe2+ ions at a flux of
CP680, Application of Accelerators in Research and Industry: 17th Int'l. Conference, edited by J. L. Duggan and I. L. Morgan
© 2003 American Institute of Physics 0-7354-0149-7/03/$20.00
647
5xl0 16 Xe/m2s. Irradiation was in a total fluence of
l.SxlO 21 He/m2 and l.SxlO 20 Xe/m2, respectively.
Irradiation was performed at room temperature and 923
K. From the calculation using TRIM code, the average
range and damage peak depth of ions in ZrO2 irradiated
with 35 keV He ions were estimated to be 169 nm and
130 nm and 60 keV Xe ions were estimated to be 19
nm and 11 nm, respectively, using a density of
5.48x103 Kg/m3 and a displacement energy of 60 eV.
The damage rates at the peak damage level are l.lxlO" 3
dpa/s for He ion irradiation and l.lxlO" 2 dpa/a for Xe
ion irradiations, respectively. All the irradiations were
carried out for one hour. From this calculation, about
30% to 40% of the implanted He ions and 100% of the
Xe ions were considered to stop within the specimens
in the observed area of thickness of about 100 nm.
These helium and xenon atoms are considered to
contribute to the bubble formation.
To examine the temperature dependence of
radiation damage and bubble formation, YSZ
specimens were irradiated with 35 keV He + and 60
keV Xe2+ ions at the room temperature and 923 K.
After ion irradiations, to understand the annealing
behavior the specimens were annealed in the TEM and
bubbles growth process was examined. Temperature
was raised stepwise at 373 K intervals up to 1473 K
and then to 1523 K in a Gatan hot stage of single tilts
type. At each annealing stage specimens were annealed
for 10 min up to 1173 K and then annealed for 20 min
from 1273 K to 1473 K.
homogeneously formed at an early stage of the
irradiation at a fluence of 6xl019 He/m2. Total dpa of
this fluence was estimated to be about 0.5 dpa. Both
the number density and the size of defect clusters
increased with the increasing fluence up to 3xl020
He/m2 , as shown in Fig.l (b). As the irradiation
proceeded, the defect cluster increased gradually,
tangled and the number density decreased.
Bubbles were formed at the fluence near 9x10,20
He/m2 , as shown in Fig. 1 (c). For this fluence, the
total dpa was estimated to be about 8 dpa. As the
irradiation proceeded these bubbles continued to
increase in number density but the size of bubbles did
not increase to more than about 2.5 nm diameter at a
fluence of l.SxlO 21 He/m2, as shown in Fig. 1 (d).
Amorphization did not occur as shown by the selected
area electron diffraction patterns.
Fig. 2 shows the damage evolution in YSZ during
60 keV Xe2+ ions irradiation at room temperature. Very
small defect clusters less than 1 nm with dot contrast
were formed at a fluence of 6xl018 Xe/m2, as shown in
Fig. 2 (a). Total dpa of this fluence was estimated to
be about 1.3 dpa. Defect clusters increased with the
increasing fluence up to 3x1019 Xe/m2 and then
bubbles were observed at same fluence, as shown in
Fig. 2 (b). The density of bubbles increased with
increasing fluence but the size of bubbles did not
increase to more than about 1 nm diameter at a fluence
of l.SxlO 20 Xe/m2, as shown in Fig. 2 (d).
Amorphization did not observed by the selected area
electron diffraction patterns of YSZ irradiated with Xe
ion to a fluence of l.SxlO 20 Xe/m2 at RT, as shown in
Fig. 2 (d) and total dpa of this fluence was about 40
dpa. We did not indicate that the YSZ transforms into
another crystalline phase upon ion irradiation, even at
very large dpa.
EXPERIMENTAL RESULTS
Effect of He+ and Xe2+ Ions Irradiation
at Room Temperature
Fig. 1 shows the damage evolution in YSZ during
He+ ion irradiation at room temperature. Very small
defect clusters less than 1 nm with weak contrast were
FIGURE 1 TEM image of (111) plane of stabilized ZrO2 during 35 keV He+ ion irradiation at room temperature,
(a) 6xl019 He/m2, (b) 3xl020 He/m2, (c) 9xl020 He/m2 (d) l.SxlO 21 He/m2
648
FIGURE 2 TEM image of (111) plane of stabilized ZrO2 during 60 keV Xe2+ ion irradiation at room temperature,
(a) 6xl018 Xe/m2, (b) 3xl019 Xe/m2, (c) 6xl019 Xe/m2 (d) 9xl019 Xe/m2, (e) l.SxlO20 Xe/m2
Effect of He+ and Xe2+ Ions Irradiation
at 923 K
helium atoms may diffuse towards the surface of thin
specimen easily due to its high mobility and are
considered to escape from the surface, resulting in the
reduction of bubble diameter and the decrease of
babbles density.
Furthermore, in this temperature
range no change in crystal structure can be observed
from the electron diffraction patters.
In the case of Xe2+ ions irradiation at 923 K, very
small defect clusters which seemed to be small
dislocation loops were formed within early irradiation
time and its total dpa was about 0.3 dpa at fluence of
l.SxlO 18 Xe/m2, as shown in Fig. 4 (a). They increased
gradually and tangled, as the irradiation proceeded. On
the one hand, bubbles were formed at the fluence near
6xl018 Xe/m , as shown in Fig. 4 (b). As the irradiation
proceeded these bubbles continued to increase in
number density and these size continuously grew from
Fig. 3 shows the damage evolution in YSZ during
He+ ion irradiation at 923 K. The dislocation loop-like
contrast observed at a fluence of 3xl019 He/m , as
shown in Fig. 3 (a). Bubbles were observed at a
fluence of 9xl020 He/m2 ,as shown in Fig. 3 (c). The
size of bubbles increased with increasing fluence and
the density of bubbles were found to decrease with
increasing fluence, as shown in Fig. 3 (d). In
comparison with the diameter of bubbles formed with a
fluence of 1.4xl021 He/m2 at RT, the diameter of
bubbles at 923 K was almost the same as the diameter
of bubble formation at RT. But bubbles were generally
expected to grow larger at high temperature than low
temperature.
As this reason, at high temperature
FIGURE 3 TEM image of (111) plane of stabilized ZrO2 during 35 keV He+ ion irradiation at 923 K.
(a) 9xl019 He/m2, (b) 3xl020 He/m2, (c) 9xl020 He/m2 (d) 1.4xl021 He/m2, (e) l.SxlO21 He/m2
649
FIGURE 4 TEM image of (111) plane of stabilized ZrO2 during 60 keV Xe2+ ion irradiation at 923 K.
(a) 1.5xl018 Xe/m2, (b) 6xl018 Xe/m2, (c) 1.4xl020 Xe/m2 (d) l.SxlO20 Xe/m2
1.5 to 3.5 nm at fluence of l.SxlO 20 Xe/m2, as shown in
Fig.4(d).
In the irradiation with Xe ion to a fluence of
l.SxlO 20 Xe/m2 at RT and 923 K,
however,
amorphization could not be observed from the electron
diffraction patterns. In the case of A12O3 [5,6] and
MgAl2O4[8], amorphization was confirmed in the
specimens irradiated with Xe2+ ions at RT. It can be
reasonably said, therefore, YSZ is very resistant against
ion irradiation.
the density of bubbles began to decrease by the
annealing to 1373 K, as shown in Fig. 5 (c) and by
annealing above 1473 K, the size of bubbles grew to 3
nm diameter about two times of 923 K by coalescence
with each other. Furthermore, bubble was found to
shrink and decrease in density at 1523 K, as shown in
Fig. 5 (e).
Fig. 6 showed the annealing behavior in specimen
irradiated with 60 keV Xe24" ions to a fluence of
l.SxlO 20 Xe/m2 at 923 K. By the annealing above
1373 K, some of the small bubbles began to grow large
by coalescence with each other, resulting in the
bimodal size distribution, as shown in Fig. 6 (c).
Annealing Effects after Irradiation
After the irradiation at 923 K to the fluence of
l.SxlO21 He/m2 and l.SxlO 20 Xe/m2 in YSZ, the
annealing experiments were performed at 1173 K, 1273
K, 1373 K, 1473 K and 1523 K, as shown in Fig. 5 and
Fig. 6, respectively. In the case of He+ ion irradiation ,
DISCUSSION
One of the remarkable results obtained in the present
irradiation experiments is that dislocation loops form in
FIGURE 5 TEM image of (111) plane of stabilized ZrO2 annealed after 35 keV He+ ion irradiation with a fluence of
l.SxlO21 He/m2 at 923 K. (a) 1173 K, (b) 1273 K, (c) 1373 K, (d) 1473 K, (e) 1523 K
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FIGURE 6 TEM image of (111) plane of stabilized ZrO2 annealed after 60 keV Xe2+ ion irradiation with a fluence of
l.SxlO20 Xe/m2at 923 K. (a) 1073 K, (b) 1173 K, (c) 1273 K, (d)1373 K, (e) 1473 K
early stage of irradiation and no amorphization occurs
in YSZ irradiated with the high dose of He and Xe ions
at room temperature and 923 K, even at very large dpa.
This fact indicates that irradiation-induced interstitials
are easily mobile at RT and quick to recombine with
vacancies by short range migration, thus resulting in
keeping the crystalline structure after high dose
irradiation.
The second remarkable result is that the bubbles
densities increased but the size hardly increased with
increasing the fluence of both helium and xenon ions
irradiation at room temperature. The mobility of
vacancy and/or implanted atoms was low at RT. This
result suggests that bubbles nucleate by homogeneous
fashion. It is considered that a large number of
effective trapping centers for the implanted helium and
xenon atoms are presented in the crystal and act as
effective nuclei for bubble formation. These trapping
centers increased with increasing of a fluence. These
trapping centers are considered to consist of the
vacancies, vacancy clusters or pressurized bubbles
formed by ion irradiation and/or oxygen vacancies due
to the addition of Y3+ into ZrO2 [11,12]. Increasing
trapping centers depressed the bubble growth. As
results of Xe ion irradiation at RT and 923 K, some of
the trapping centers allow gas atoms to escape from the
traps, and then gas atoms migrate to other stronger traps
or stable complexes such as small bubbles, resulting in
the growth of bubbles, as shown in Fig. 4(b) to (d).
By annealing at 1073 K to 1473 K after Xe ion
irradiation, bubbles were observed to grow, showing
bimodal size distribution. On the other hand, in the case
of He ion irradiation ( annealing up to 1523 K ),
bubbles were found to shrink and decrease in density
by annealing at above 1473 K.
As results of
annealing after He ion irradiation and the experiments
of the thermal helium desorption measurements [12],
the interaction of the self-interstitial atoms with heliumvacancy complexes may suggests that is weaker than
xenon-vacancy complexes. But the strength of the
interaction may depend on the size of these complexes
which depend on the condition of ion-irradiation. This
consideration may give a basic idea to account for the
present observed results that bubble growth occurs
during annealing at 1273 K to 1523 K after Xe ion
irradiation.
CONCLUSIONS
The remarkable results for Yttria-stabilized ZrO2
observed in the present work are summarized as
follows.
(1) No amorphization were observed in YSZ irradiated
with 35 keV He+ ion to the high fluence of l.SxlO 21
He/m2 ( about 16 dpa ) and 60 keV Xe24" ion to the
fluence of l.SxlO20 Xe/m2 ( about 40 dpa ) at room
temperature and 923 K, respectively.
(2) The bubble densities increased and size were hardly
increased with the fluence of both helium and xenon
ions irradiation at room temperature. But the bubbles
were found to grow during Xe ion irradiation at923 K.
(3) In the case of Xe2+ ion irradiation, bubbles were
observed to grow by annealing above 1373 K after
irradiation. On the other hand, in the case of He+ ion
irradiation, bubbles were found to shrink and decrease
in density by annealing at above 1473 K.
REFERENCES
1. Akie, H., Muromura, T., Takano, H., and Matsuura, H., Nucl.
Technol 107 (1994) pp.182-192.
2. Nitani, N., Akie, H., Takano, H., Ohmichi, T., and Muromura, T.,
in: Proceedings ofPSI workshop on Advanced Fuel Cycl, PSI,
Switzerland, 18-19 September 1995, pp.118-128.
3. Degueldre, C., Pouchon, M., Dobeli, M., Sickafus, K., Hojou, K.,
Ledergerber, G.andAbolhassani-Dadras, S., J. Nucl Mater., 289
(2001) pp.115-121.
4. Lee, W. E., Jenkins, M.L., and Pells, G.P., Philos. Mag. A 51
651
(1985)pp.639-659.
5. Y. Katano, K. Hojou, T. Nakazawa, T. Aruga, D. Yamaki and K.
Noda, Nucl. Instr. and Meth. B 141 (1998) pp.411-418.
6. Sasajima, N., Matsui, T., Furuno, S., Hojou,, and Otsu, H., Nucl.
Instr. and Meth. B 148 (1999)pp.745-751.
7. Clinard, F.W., Hurley, Jr. G.F., and Hobbs, L. W, J. Nucl. Mater.,
108&109(1982)pp.655-670.
8. Furuno, S., Sasajima, N. , Hojou, K., Izui, K., Otsu, H.,
Muromura, T., and Matsui, T., Nucl. Instr. and Meth. B
127&128(1997)pp.l81-185.
9. Sickafus, K.E., Matzke, Hj., Yasuda, K., Chdak III, P., Verrall,
R.A., Lucuta, P.O., Andrews, H.R., Turos, A., Fromknecht, R.,
and Baker, N.P., Nucl. Instr. and Meth. B 141 (1998) pp.358365.
10. Sasajima, N., Matui, T., Hojou, K., Furuno, S., Otsu, H., Izui,
K.,and Muromura, T., Nucl. Instr. and Meth. B 141 (1998)
pp.487-493.
11. Sasajima, N., Matui, T., Furuno, S., Shiratori, T., and Hojou, K.,
Nucl. Instr. and Meth. B 166-167 (2002) pp.250-255.
12. Damen, P.M.G., Matzuke, Hj., Ronchi, C., Hiernaut, J.-P., Wiss,
T., Fromknecht, R., van Veen, A., and Labohm, F., Nucl Instr.
and Meth. B 191 (2002) pp.571-576.
13. Utsunomiya, S., Wang, L.M.,and Ewing, R.C., Nucl. Instr. and
Meth. B 191 (2002) pp.600-605.
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