- Email: [email protected]
Radiation effects on yttria-stabilized zirconia irradiated with He or Xe ions at high temperature
Nuclear Instruments and Methods in Physics Research B 241 (2005) 536–542 www.elsevier.com/locate/nimb
Radiation effects on yttria-stabilized zirconia irradiated with He or Xe ions at high temperature T. Hojo
a,c,* ,
H. Yamamoto b, J. Aihara c, S. Furuno d, K. Sawa c, T. Sakuma a, K. Hojou e
a Department of Science and Engineering, Ibaraki University, Mito, Ibaraki 310-8512, Japan Neutron Science Research Center, Japan Atomic Energy Research Institute, Tokai, Ibaraki 319-1195, Japan Department of Advanced Nuclear Heat Technology, Japan Atomic Energy Research Institute, Oarai, Ibaraki 311-1394, Japan d Research Organization for Information Science and Technology, Tokai, Ibaraki 319-1106, Japan e Office of Planning, Japan Atomic Energy Research Institute, Kashiwa, Chiba 277-0842, Japan b
c
Available online 24 August 2005
Abstract Yttria-stabilized zirconia (YSZ) is a candidate for use as optical and insulating material in a nuclear reactor and/or fusion reactor. In the present study, in situ observations by an electron microscope was performed during helium and xenon ion irradiations of YSZ single crystals. Damage evolution in the YSZ crystals was observed during irradiation with 35 keV He+ with a flux of 5 · 1013 ions/cm2 s at 1073 K, 1273 K and 1473 K. The damage evolution was also observed with 60 keV Xe2+ with a flux of 5 · 1012 ions/cm2 s at 1073 K and 1473 K. In these cases, bubble formation was clearly observed. Dislocation loops were formed by Xe ion irradiation at both temperatures but were not formed by He. This result shows clear dependence of loop formation on the ion species. Dependence of the loop formation on irradiation temperature was also shown in comparison with previous experimental results. 2005 Elsevier B.V. All rights reserved. PACS: 07.78; 61.72.F; 61.80; 61.16.B; 21.60.G Keywords: Electron microscope; Dislocations and other crystal defects; Radiation effects on solids; Transmission electron microscopy; Cluster-models nuclear
1. Introduction * Corresponding author. Address: Department of Science and Engineering, Ibaraki University, Mito, Ibaraki 310-8512, Japan. Tel.: +81 29 282 6360; fax: +81 29 284 3813. E-mail address: [email protected] (T. Hojo).
ZrO2–Y2O3 system material has excellent electrical and mechanical characteristics for use as thermistor material and high ionic conductivity
0168-583X/$ - see front matter 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.nimb.2005.07.102
T. Hojo et al. / Nucl. Instr. and Meth. in Phys. Res. B 241 (2005) 536–542
material. On the other hand, in the field of atomic energy generation, yttria-stabilized cubic zirconia (yttria-stabilized zirconia or YSZ for short) is considered a principal candidate material for rock-like fuels because of its high radiation stability. Rocklike fuels have been proposed for the safe disposal of surplus plutonium from light water reactors and nuclear weapons [1–3]. Important data on radiation stability or its effects, is being reported. No amorphization was observed in stabilized zirconia under neutron or ion irradiation at a high damage level [4–7]. Additionally, even under conditions, such as 1.5 MeV Xe ion irradiation at low temperature, no amorphization was observed [8]. Further, no amorphization was observed after the low energy irradiation under Xe, He and Ne ion [9–11]. These results indicate that stabilized zirconia had strong resistance to irradiation. Also, stabilized zirconia showed negligible swelling of volume under fast neutron irradiation [12,13]. On the other hand, volume swelling up to 4% due to bubble formation was observed after ion irradiation, depending on irradiation temperature and fluence [6]. Only very limited information is available, however, on the swelling of stabilized zirconia due to ion irradiation. For estimating the radiation stability produced by fission products or a-decay, Hojou et al. [11] have studied radiation damage in stabilized zirconia under He or Xe ion irradiations with a transmission electron microscope (TEM) and found no amorphization and volume swelling due to bubble formation after Xe ion irradiation [6,8]. In our previous study [10], we performed irradiation, using 30 keV Ne ions from the fundamental viewpoint. In that study, Ne with mass midway between He and Xe was chosen, and its energy was adjusted so as to produce a large amount of implanted ions and displaced atoms (dpa). However, the ions and atoms were confined within a thin specimen suitable for electron microscope observation. As a result, bubbles were formed mainly on the loop plane at 973 K. Recently, we have performed in situ observation MgAl2O4-stabilized zirconia composite materials irradiated with 30 keV Ne+ ions at room temperature [14]. In this study, stabilized zirconia containing 21% mol of yttria was irradiated with
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two kinds ions of helium (simulation of the alpha ray) and xenon (main FP), at temperatures from 1073 K to 1473 K, close to the environment of actual use. Furthermore, we have examined the results of He or Xe ion irradiation by Sasajima et al. [15] and Ne ion irradiation in our previous study [10] and discuss the differences in defect clusters and bubble formation, depending on ion species and irradiation temperature.
2. Experimental procedures Yttria-stabilized cubic zirconia (YSZ) specimens containing 21% mol of yttria were used in this study. Irradiations and in situ observations were performed with JEM-2000F TEM, coupled directly to a 40 kV ion accelerator. In this instrument, the ion beam was incident to the specimen surface at an angle of 60. The 35 keV He ions at a flux of 5 · 1013 ions/cm2 s and 60 keV Xe2+ ions at a flux of 5 · 1012 ions/cm2 s were used. Irradiations had a total fluence of 1.8 · 1017 ions/cm2 and 1.8 · 1016 ions/cm2, respectively. Under these irradiation conditions, the average range and damage peak depth of 35 keV He+ ion and 60 keV Xe2+ ion were estimated by TRIM calculation [16].
3. Results Fig. 1 shows sequential electron micrographs of damage evolution in YSZ irradiated with 35 keV He+ ions with a flux of 5 · 1013 ions/cm2 s at 1073 K. In the upper parts of the Fig. 1(b)–(d), the micrographs expanded to four times were inserted. At this temperature, bubbles were observed to be formed homogeneously, even in comparatively thin regions. Bubbles were not observed at a fluence of 1.8 · 1016 ions/cm2, as shown in Fig. 1(a). Bubbles were observed at a fluence of 3 · 1016 ions/cm2, as shown in the expanded part of Fig. 1(b). Bubbles increased in size and number density with increasing fluence, as shown Fig. 1(c) and (d). At a fluence of 1.8 · 1017 ions/cm2, number density was about 7.6 · 104/lm2, and its size reached 2.8 nm in diameter.
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T. Hojo et al. / Nucl. Instr. and Meth. in Phys. Res. B 241 (2005) 536–542
Fig. 1. Sequential electron micrographs of damage evolution in stabilized zirconia irradiated with 35 keV He+ ions with a flux of 5 · 1013 ions/cm2 s at 1073 K, with the following fluences: (a) 1.8 · 1016 ions/cm2, (b) 3 · 1016 ions/cm2, (c) 9 · 1016 ions/cm2 (d) 1.8 · 1017 ions/cm2.
Fig. 2 shows sequential electron micrographs of damage evolution in stabilized zirconia irradiated with 35 keV He+ ions with a flux of 5 · 1013 ions/cm2 s at 1273 K. In the upper parts of the Fig. 2(b) and (c), the micrographs expanded four times were inserted. At this temperature, bubbles were observed to be formed homogeneously even in comparatively thin region.
Bubbles were observed within a fluence of 1.8 · 1016 ions/cm2, as shown in Fig. 2(b). It was proven that the bubbles were formed with less fluence at this temperature than that at 1073 K. The size of bubbles increased with increasing fluence. At a fluence of 1.8 · 1017 ions/cm2, bubble size showed bimodal distribution with diameter peaks of 3.3 nm and 5.6 nm. The number density
Fig. 2. Sequential electron micrographs of damage evolution in stabilized zirconia irradiated with 35 keV He+ ions with a flux of 5 · 1013 ions/cm2 s at 1273 K, with the following fluences: (a) 9 · 1015 ions/cm2, (b) 1.8 · 1016 ions/cm2, (c) 9 · 1016 ions/cm2 (d) 1.8 · 1017 ions/cm2.
T. Hojo et al. / Nucl. Instr. and Meth. in Phys. Res. B 241 (2005) 536–542
of bubbles increased remarkably with the increase of fluence to 9 · 1016 ions/cm2, but over this fluence, density hardly increased at all. The number density of bubbles labeled at about 4.8 · 104/lm2. Fig. 3 shows sequential electron micrographs of damage evolution in stabilized zirconia irradiated with 35 keV He+ ions with a flux of 5 · 1013 ions/cm2 s at 1473 K. In the upper part of the Fig. 3(b), with an expansion of the micrograph, the bubble was inserted. Thick places and thin places were clearly distinguished in the case of increased fluence. A thick place is shown in Fig. 3(c) and (e) and a thin place is shown in Fig. 3(d) and (f). We inserted an enlargement in the right lower part of Fig. 3(b) so that bubbles can be seen more easily. With regard to the growth of bubbles, we observed clearer differences between thin and thick regions with increasing fluence. We found bubble formation and growth suppressed in thin regions at 1473 K. Thin regions at fluences of 9 · 1016 ions/cm2 and 1.8 · 1017 ions/cm2 are shown in Fig. 3(d) and (f) and thick regions were shown Fig. 3(c) and (e). Bubble formation was observed at a fluence of 1.5 · 1016 ions/cm2 as shown in Fig. 3(b). The size and number density
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of bubbles increased with increasing fluence as shown in Fig. 3(c) and (e). The size of bubbles is 7 nm and the number density is 1.2 · 103/lm2 in thick regions at a fluence of 1.8 · 1017 ions/cm2. As shown in Fig. 3(d) and (f), the bubble size grows but the number density hardly increases at all in the thin region. Fig. 3(c)–(f) show that bubbles are in a bimodal distribution and have facets. We did not observe loop formation after He irradiation under present irradiation conditions. Fig. 4 shows sequential electron micrographs of damage evolution in stabilized zirconia irradiated with 60 keV Xe2+ ions with a flux of 5 · 1012 ions/cm2 s at 1073 K. In the upper part of the Fig. 4(a), the micrograph, expanded four times, was inserted. Bubbles were observed at a fluence of 9 · 1014 ions/cm2, as shown in the expanded part of Fig. 4(a). Bubbles increased in size with increasing fluence and their maximum size was about 4.5 nm in diameter at a fluence of 1.8 · 1016 ions/cm2, as shown in Fig. 4(d). The number density of bubbles becomes saturated where the fluence is 4.5 · 1015 ions/cm2, and above this saturation, the density decreases with increasing fluence. Dislocation
Fig. 3. Sequential electron micrographs of damage evolution in stabilized zirconia irradiated with 35 keV He+ ions with a flux of 5 · 1013 ions/cm2 s at 1473 K, with the following fluences: (a) 9 · 1015 ions/cm2, (b) 1.5 · 1016 ions/cm2, (c,d) 9 · 1016 ions/cm2 (e,f) 1.8 · 1017 ions/cm2.
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Fig. 4. Sequential electron micrographs of damage evolution in stabilized zirconia irradiated with 60 keV Xe2+ ions with a flux of 5 · 1012 ions/cm2 s at 1073 K, with the following fluences: (a) 9 · 1014 ions/cm2, (b) 4.5 · 1015 ions/cm2, (c) 9 · 1015 ions/cm2 (d) 1.8 · 1016 ions/cm2.
loops were formed at a fluence of 4.5 · 1015 ions/ cm2 as shown in Fig. 4(b). The loops soon became tangled with each other with increasing fluence. At a fluence of 1.8 · 1016 ions/cm2, the bubbles grow but the number density is decreased as shown Fig. 4(d).
Fig. 5 shows sequential electron micrographs of damage evolution in stabilized zirconia irradiated with 60 keV Xe2+ ions with a flux of 5 · 1012 ions/cm2 s at 1473 K. In the upper parts of Fig. 5(a) and (b), the micrographs expanded four times were inserted.
Fig. 5. Sequential electron micrographs of damage evolution in stabilized zirconia irradiated with 60 keV Xe2+ ions with a flux of 5 · 1012 ions/cm2 s at 1473 K, with the following fluences: (a) 9 · 1014 ions/cm2, (b) 3 · 1015 ions/cm2, (c) 9 · 1015 ions/cm2 (d) 1.8 · 1016 ions/cm2.
T. Hojo et al. / Nucl. Instr. and Meth. in Phys. Res. B 241 (2005) 536–542
Bubbles were observed at a fluence of 9 · 1014 ions/cm2, as shown in the expanded part of Fig. 5(a). Bubbles increased in size with increasing fluence, and their maximum size was about 5.6 nm in diameter at a fluence of 1.8 · 1016 ions/ cm2 as shown in Fig. 5(d). The number density of bubbles is saturated where the fluence is 4.5 · 1015 ions/cm2, and above that, density decreases with increasing fluence. Dislocation loops were formed at a fluence of 3 · 1015 ions/cm2, as shown in Fig. 5(b). The loops soon became tangled with each other with increasing fluence. In the case of Xe ion irradiation, we cannot observe clear differences in the morphology of damage evolution occurring at 1073 K and 1473 K irradiation.
4. Discussion The results of these experiments show that bubbles were formed independently of ion species and irradiation temperatures. We must refer to other research results in order to know if this result is universal. Sasajima et al. irradiated polycrystalline MgAl2O4-stabilized zirconia (containing 12% mol yttria) composites with 60 keV Xe ions at a flux of 5 · 1012 ions/cm2 s and 35 keV He ions at a flux of 5 · 1013 ions/cm2 s at room temperature and 923 K [14]. The results showed that bubbles were formed in zirconia crystalline grains under these irradiation conditions. Our previous study showed that in stabilized zirconia (containing 21% mol yttria) irradiated with 30 keV Ne ions with a flux of 5 · 1013 ions/cm2 s at room temperature 923 K and 1473 K, bubbles were formed at each irradiation temperature [10]. Considering our experimental results and other data, we can conclude that it is universal that bubbles are formed by inert gas irradiation from room temperature to 1473 K. With regard to dislocation loop formation, the present experimental results show clear differences between the two ion species. In He irradiation, loops are not formed at 1073 K and 1473 K. In Xe irradiation, loops are formed both at 1073 K and 1473 K. A difference with regard to loop formation between the two-irradiation temperatures was not seen in this experiment. In order to exam-
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ine more minutely irradiation conditions, which enable loops to form, we need to study the relation between irradiation conditions and loop formation. According to the results of Hojou et al. [11], loops are formed at room temperature by both 35 keV He and 60 keV Xe irradiation. However, the loops are not formed at 923 K after 35 keV He irradiation. On the other hand, the loops were formed both at room temperature and 923 K. Our previous study [10] showed that the loops are formed at room temperature and 923 K but not at 1473 K after 30 keV Ne irradiation. Considering our previous results and other data, we conclude that loops are formed by He, Ne and Xe irradiations at room temperature. At 973 K, loops are not formed by He irradiation but they are formed by Ne and Xe irradiations. At 1473 K, loops are formed by Xe irradiation but are not formed by He and Ne irradiations. The results show that loop formation depends on the ion species at high temperatures. When we examine the dependence of loop formation on irradiation temperature in the same ion species, we find that loops are formed from room temperature to 1473 K by Xe irradiation. Loops are formed at 923 K but are not formed at 1473 K by Ne irradiation. Loops are formed at room temperature but are not formed above 923 K in He irradiation. Thus, there is a dependence of loop formation on temperature, at least in the case of He and Ne irradiation. That is, loops are not formed at high temperatures in these two irradiation species. The number of interstitial atoms flowing into a loop must be larger than that of atoms released from a loop for its growth. The number of interstitial atoms flowing into a loop can be considered to be approximately proportional to the product of multiplying density of interstitial atoms near the loop by mobility of interstitial atoms near the loop. The number of atoms released from a loop should increase with rising temperature. We examined the difference of loop formation in the two ion species, as shown in this experiment. The difference of dpa was calculated by TRIM between He and Xe. According to TRIM calculation, the dpa of Xe is about 10 times larger than that of He, which means that the number of lattice atoms displaced by Xe irradiation is 10 times
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larger than that of atoms displaced by He irradiation. Thus, it can be considered that such large number of displaced atoms are strongly enhanced by the loop formation. In the case of Xe irradiation at 1073 K or 1473 K, the number of interstitial atoms flowing into a dislocation loop is larger than the number of atoms released from the loop, owing to the large number of displaced atoms. This behavior enhances the loop formation and growth. In case of He irradiation, the amount of interstitial atoms flowing into the loop is smaller than that of atoms released from the loop, due to the small amount of displaced atoms. The loops are, therefore, hardly formed at all. These results show that the difference of the number of displaced atoms affects loop formation. The dependence of loop formation on irradiation temperatures can be explained as follows. In ion irradiation, the number of atoms released from a loop increases with rising temperature. The mobility of interstitial atoms increases with rising temperature, which affects the number of atoms flowing into a loop. Because a specimen suitable for electron microscope observation is thin, the amount of interstitial atoms leaving from the specimen surface increases with increasing mobility, decreasing the density of interstitial atoms. The mobility of interstitial atoms contributing to loop growth increases with rising irradiation temperature, while the density decreases due to surface evaporation. The number of atoms released from the loop increases with rising temperature. On the basis of obtained results, it can be concluded that loops tend to be formed up to a certain temperature at the point where the number of atoms released from loop is larger than that of atoms flowing into loop.
5. Summary The present results show that bubbles are formed and grow in size and number with increasing ion fluence under the present irradiation conditions of this study. Referring to other experimental results, we can conclude that bubble formation does not depend on from room temperature to 1473 K for inert gas irradiation.
With regard to dislocation loop formation, the obtained results showed clear differences between the two ion species. It can be considered that the dependence of loop formation on ion species stems essentially from the difference of dpa per second between the two ion species. Considering our results and others, we found that there is also dependence of loop formation on irradiation temperature.
References [1] H. Akie, T. Muromura, H. Takano, S. Matsuura, Nucl. Technol. 107 (1994) 182. [2] N. Nitani, H. Akie, H. Takano, T. Ohmichi, T. Muromura, in: Proceedings of the PSI Workshop on Advanced Fuel Cycle, PSI, Switzerland, 18–19 September, 1995, p. 118. [3] C. Dugueldre, U. Kasemeyer, F. Botta, G. Ledergerber, Mater. Res. Soc. Symp. Proc. 412 (1996) 15. [4] E.L. Fleischer, M.G. Norton, M.A. Zaleski, W. Hertl, C.B. Carter, J.W. Mayer, J. Mater. Res. 6 (1991) 1905. [5] N. Yu, K.E. Sickafus, P. Kodali, M. Nastasi, J. Nucl. Mater. 244 (1997) 266. [6] N. Sasajima, T. Matsui, K. Hojou, S. Furuno, H. Otsu, K. Izui, T. Muromura, Nucl. Instr. and Meth. B 141 (1998) 487. [7] K.E. Sickafus, Hj. Matzke, K. Yasuda, P. Chodak III, R.A. Verrall, P.G. Lucuta, H.R. Andrew, A. Turos, R. Fromknecht, N.P. Baker, Nucl. Instr. and Meth. B 141 (1998) 358. [8] C. Degueldre, P. Heimgartener, G. Ledergerber, N. Sasajima, K. Hojou, T. Muromura, L. Wang, W. Gong, R. Ewing, Mater. Res. Soc. Symp. Proc. 439 (1997) 625. [9] N. Sasajima, T. Mastui, K. Hojou, S. Furuno, H. Otsu, K. Izui, T. Muromura, Nucl. Instr. and Meth. B 141 (1998) 487. [10] T. Hojo, J. Aihara, K. Hojou, S. Furuno, N. Nitani, T. Yamashita, K. Minato, T. Sakuma, J. Nucl. Mater. 319 (2003) 81. [11] K. Hojou, T. Hojo, N. Sasajima, N. Nitani, T. Yamashita, K. Minato, S. Furuno, in: Proceedings of 17th International Conference on the Application of Accelerators in Research and Industry, 2003, p. 647. [12] F.W. Clinard Jr., G.F. Hurley, L.W. Hobbs, J. Nucl. Mater. 108–109 (1982) 655. [13] F.W. Clinard Jr., D.L. Rohr, W.A. Ranken, J. Am. Ceram. Soc. 60 (1977) 287. [14] T. Hojo, H. Yamamoto, J. Aihara, S. Furuno, K. Hojou, K. Sawa, T. Sakuma, Nucl. Instr. and Meth. B, in press. [15] N. Sasajima, T. Matui, S. Furuno, T. Shiratori, K. Hojou, Nucl. Instr. and Meth. B 166–167 (2002) 250. [16] J.F. Ziegler, J.P. Biersack, U. Littmark, The Stopping and Range of Ions in Solids, Pergamon Press, New York, 1985.
Radiation effects on yttria-stabilized zirconia irradiated with He or Xe ions at high temperature T. Hojo
a,c,* ,
H. Yamamoto b, J. Aihara c, S. Furuno d, K. Sawa c, T. Sakuma a, K. Hojou e
a Department of Science and Engineering, Ibaraki University, Mito, Ibaraki 310-8512, Japan Neutron Science Research Center, Japan Atomic Energy Research Institute, Tokai, Ibaraki 319-1195, Japan Department of Advanced Nuclear Heat Technology, Japan Atomic Energy Research Institute, Oarai, Ibaraki 311-1394, Japan d Research Organization for Information Science and Technology, Tokai, Ibaraki 319-1106, Japan e Office of Planning, Japan Atomic Energy Research Institute, Kashiwa, Chiba 277-0842, Japan b
c
Available online 24 August 2005
Abstract Yttria-stabilized zirconia (YSZ) is a candidate for use as optical and insulating material in a nuclear reactor and/or fusion reactor. In the present study, in situ observations by an electron microscope was performed during helium and xenon ion irradiations of YSZ single crystals. Damage evolution in the YSZ crystals was observed during irradiation with 35 keV He+ with a flux of 5 · 1013 ions/cm2 s at 1073 K, 1273 K and 1473 K. The damage evolution was also observed with 60 keV Xe2+ with a flux of 5 · 1012 ions/cm2 s at 1073 K and 1473 K. In these cases, bubble formation was clearly observed. Dislocation loops were formed by Xe ion irradiation at both temperatures but were not formed by He. This result shows clear dependence of loop formation on the ion species. Dependence of the loop formation on irradiation temperature was also shown in comparison with previous experimental results. 2005 Elsevier B.V. All rights reserved. PACS: 07.78; 61.72.F; 61.80; 61.16.B; 21.60.G Keywords: Electron microscope; Dislocations and other crystal defects; Radiation effects on solids; Transmission electron microscopy; Cluster-models nuclear
1. Introduction * Corresponding author. Address: Department of Science and Engineering, Ibaraki University, Mito, Ibaraki 310-8512, Japan. Tel.: +81 29 282 6360; fax: +81 29 284 3813. E-mail address: [email protected] (T. Hojo).
ZrO2–Y2O3 system material has excellent electrical and mechanical characteristics for use as thermistor material and high ionic conductivity
0168-583X/$ - see front matter 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.nimb.2005.07.102
T. Hojo et al. / Nucl. Instr. and Meth. in Phys. Res. B 241 (2005) 536–542
material. On the other hand, in the field of atomic energy generation, yttria-stabilized cubic zirconia (yttria-stabilized zirconia or YSZ for short) is considered a principal candidate material for rock-like fuels because of its high radiation stability. Rocklike fuels have been proposed for the safe disposal of surplus plutonium from light water reactors and nuclear weapons [1–3]. Important data on radiation stability or its effects, is being reported. No amorphization was observed in stabilized zirconia under neutron or ion irradiation at a high damage level [4–7]. Additionally, even under conditions, such as 1.5 MeV Xe ion irradiation at low temperature, no amorphization was observed [8]. Further, no amorphization was observed after the low energy irradiation under Xe, He and Ne ion [9–11]. These results indicate that stabilized zirconia had strong resistance to irradiation. Also, stabilized zirconia showed negligible swelling of volume under fast neutron irradiation [12,13]. On the other hand, volume swelling up to 4% due to bubble formation was observed after ion irradiation, depending on irradiation temperature and fluence [6]. Only very limited information is available, however, on the swelling of stabilized zirconia due to ion irradiation. For estimating the radiation stability produced by fission products or a-decay, Hojou et al. [11] have studied radiation damage in stabilized zirconia under He or Xe ion irradiations with a transmission electron microscope (TEM) and found no amorphization and volume swelling due to bubble formation after Xe ion irradiation [6,8]. In our previous study [10], we performed irradiation, using 30 keV Ne ions from the fundamental viewpoint. In that study, Ne with mass midway between He and Xe was chosen, and its energy was adjusted so as to produce a large amount of implanted ions and displaced atoms (dpa). However, the ions and atoms were confined within a thin specimen suitable for electron microscope observation. As a result, bubbles were formed mainly on the loop plane at 973 K. Recently, we have performed in situ observation MgAl2O4-stabilized zirconia composite materials irradiated with 30 keV Ne+ ions at room temperature [14]. In this study, stabilized zirconia containing 21% mol of yttria was irradiated with
537
two kinds ions of helium (simulation of the alpha ray) and xenon (main FP), at temperatures from 1073 K to 1473 K, close to the environment of actual use. Furthermore, we have examined the results of He or Xe ion irradiation by Sasajima et al. [15] and Ne ion irradiation in our previous study [10] and discuss the differences in defect clusters and bubble formation, depending on ion species and irradiation temperature.
2. Experimental procedures Yttria-stabilized cubic zirconia (YSZ) specimens containing 21% mol of yttria were used in this study. Irradiations and in situ observations were performed with JEM-2000F TEM, coupled directly to a 40 kV ion accelerator. In this instrument, the ion beam was incident to the specimen surface at an angle of 60. The 35 keV He ions at a flux of 5 · 1013 ions/cm2 s and 60 keV Xe2+ ions at a flux of 5 · 1012 ions/cm2 s were used. Irradiations had a total fluence of 1.8 · 1017 ions/cm2 and 1.8 · 1016 ions/cm2, respectively. Under these irradiation conditions, the average range and damage peak depth of 35 keV He+ ion and 60 keV Xe2+ ion were estimated by TRIM calculation [16].
3. Results Fig. 1 shows sequential electron micrographs of damage evolution in YSZ irradiated with 35 keV He+ ions with a flux of 5 · 1013 ions/cm2 s at 1073 K. In the upper parts of the Fig. 1(b)–(d), the micrographs expanded to four times were inserted. At this temperature, bubbles were observed to be formed homogeneously, even in comparatively thin regions. Bubbles were not observed at a fluence of 1.8 · 1016 ions/cm2, as shown in Fig. 1(a). Bubbles were observed at a fluence of 3 · 1016 ions/cm2, as shown in the expanded part of Fig. 1(b). Bubbles increased in size and number density with increasing fluence, as shown Fig. 1(c) and (d). At a fluence of 1.8 · 1017 ions/cm2, number density was about 7.6 · 104/lm2, and its size reached 2.8 nm in diameter.
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T. Hojo et al. / Nucl. Instr. and Meth. in Phys. Res. B 241 (2005) 536–542
Fig. 1. Sequential electron micrographs of damage evolution in stabilized zirconia irradiated with 35 keV He+ ions with a flux of 5 · 1013 ions/cm2 s at 1073 K, with the following fluences: (a) 1.8 · 1016 ions/cm2, (b) 3 · 1016 ions/cm2, (c) 9 · 1016 ions/cm2 (d) 1.8 · 1017 ions/cm2.
Fig. 2 shows sequential electron micrographs of damage evolution in stabilized zirconia irradiated with 35 keV He+ ions with a flux of 5 · 1013 ions/cm2 s at 1273 K. In the upper parts of the Fig. 2(b) and (c), the micrographs expanded four times were inserted. At this temperature, bubbles were observed to be formed homogeneously even in comparatively thin region.
Bubbles were observed within a fluence of 1.8 · 1016 ions/cm2, as shown in Fig. 2(b). It was proven that the bubbles were formed with less fluence at this temperature than that at 1073 K. The size of bubbles increased with increasing fluence. At a fluence of 1.8 · 1017 ions/cm2, bubble size showed bimodal distribution with diameter peaks of 3.3 nm and 5.6 nm. The number density
Fig. 2. Sequential electron micrographs of damage evolution in stabilized zirconia irradiated with 35 keV He+ ions with a flux of 5 · 1013 ions/cm2 s at 1273 K, with the following fluences: (a) 9 · 1015 ions/cm2, (b) 1.8 · 1016 ions/cm2, (c) 9 · 1016 ions/cm2 (d) 1.8 · 1017 ions/cm2.
T. Hojo et al. / Nucl. Instr. and Meth. in Phys. Res. B 241 (2005) 536–542
of bubbles increased remarkably with the increase of fluence to 9 · 1016 ions/cm2, but over this fluence, density hardly increased at all. The number density of bubbles labeled at about 4.8 · 104/lm2. Fig. 3 shows sequential electron micrographs of damage evolution in stabilized zirconia irradiated with 35 keV He+ ions with a flux of 5 · 1013 ions/cm2 s at 1473 K. In the upper part of the Fig. 3(b), with an expansion of the micrograph, the bubble was inserted. Thick places and thin places were clearly distinguished in the case of increased fluence. A thick place is shown in Fig. 3(c) and (e) and a thin place is shown in Fig. 3(d) and (f). We inserted an enlargement in the right lower part of Fig. 3(b) so that bubbles can be seen more easily. With regard to the growth of bubbles, we observed clearer differences between thin and thick regions with increasing fluence. We found bubble formation and growth suppressed in thin regions at 1473 K. Thin regions at fluences of 9 · 1016 ions/cm2 and 1.8 · 1017 ions/cm2 are shown in Fig. 3(d) and (f) and thick regions were shown Fig. 3(c) and (e). Bubble formation was observed at a fluence of 1.5 · 1016 ions/cm2 as shown in Fig. 3(b). The size and number density
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of bubbles increased with increasing fluence as shown in Fig. 3(c) and (e). The size of bubbles is 7 nm and the number density is 1.2 · 103/lm2 in thick regions at a fluence of 1.8 · 1017 ions/cm2. As shown in Fig. 3(d) and (f), the bubble size grows but the number density hardly increases at all in the thin region. Fig. 3(c)–(f) show that bubbles are in a bimodal distribution and have facets. We did not observe loop formation after He irradiation under present irradiation conditions. Fig. 4 shows sequential electron micrographs of damage evolution in stabilized zirconia irradiated with 60 keV Xe2+ ions with a flux of 5 · 1012 ions/cm2 s at 1073 K. In the upper part of the Fig. 4(a), the micrograph, expanded four times, was inserted. Bubbles were observed at a fluence of 9 · 1014 ions/cm2, as shown in the expanded part of Fig. 4(a). Bubbles increased in size with increasing fluence and their maximum size was about 4.5 nm in diameter at a fluence of 1.8 · 1016 ions/cm2, as shown in Fig. 4(d). The number density of bubbles becomes saturated where the fluence is 4.5 · 1015 ions/cm2, and above this saturation, the density decreases with increasing fluence. Dislocation
Fig. 3. Sequential electron micrographs of damage evolution in stabilized zirconia irradiated with 35 keV He+ ions with a flux of 5 · 1013 ions/cm2 s at 1473 K, with the following fluences: (a) 9 · 1015 ions/cm2, (b) 1.5 · 1016 ions/cm2, (c,d) 9 · 1016 ions/cm2 (e,f) 1.8 · 1017 ions/cm2.
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Fig. 4. Sequential electron micrographs of damage evolution in stabilized zirconia irradiated with 60 keV Xe2+ ions with a flux of 5 · 1012 ions/cm2 s at 1073 K, with the following fluences: (a) 9 · 1014 ions/cm2, (b) 4.5 · 1015 ions/cm2, (c) 9 · 1015 ions/cm2 (d) 1.8 · 1016 ions/cm2.
loops were formed at a fluence of 4.5 · 1015 ions/ cm2 as shown in Fig. 4(b). The loops soon became tangled with each other with increasing fluence. At a fluence of 1.8 · 1016 ions/cm2, the bubbles grow but the number density is decreased as shown Fig. 4(d).
Fig. 5 shows sequential electron micrographs of damage evolution in stabilized zirconia irradiated with 60 keV Xe2+ ions with a flux of 5 · 1012 ions/cm2 s at 1473 K. In the upper parts of Fig. 5(a) and (b), the micrographs expanded four times were inserted.
Fig. 5. Sequential electron micrographs of damage evolution in stabilized zirconia irradiated with 60 keV Xe2+ ions with a flux of 5 · 1012 ions/cm2 s at 1473 K, with the following fluences: (a) 9 · 1014 ions/cm2, (b) 3 · 1015 ions/cm2, (c) 9 · 1015 ions/cm2 (d) 1.8 · 1016 ions/cm2.
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Bubbles were observed at a fluence of 9 · 1014 ions/cm2, as shown in the expanded part of Fig. 5(a). Bubbles increased in size with increasing fluence, and their maximum size was about 5.6 nm in diameter at a fluence of 1.8 · 1016 ions/ cm2 as shown in Fig. 5(d). The number density of bubbles is saturated where the fluence is 4.5 · 1015 ions/cm2, and above that, density decreases with increasing fluence. Dislocation loops were formed at a fluence of 3 · 1015 ions/cm2, as shown in Fig. 5(b). The loops soon became tangled with each other with increasing fluence. In the case of Xe ion irradiation, we cannot observe clear differences in the morphology of damage evolution occurring at 1073 K and 1473 K irradiation.
4. Discussion The results of these experiments show that bubbles were formed independently of ion species and irradiation temperatures. We must refer to other research results in order to know if this result is universal. Sasajima et al. irradiated polycrystalline MgAl2O4-stabilized zirconia (containing 12% mol yttria) composites with 60 keV Xe ions at a flux of 5 · 1012 ions/cm2 s and 35 keV He ions at a flux of 5 · 1013 ions/cm2 s at room temperature and 923 K [14]. The results showed that bubbles were formed in zirconia crystalline grains under these irradiation conditions. Our previous study showed that in stabilized zirconia (containing 21% mol yttria) irradiated with 30 keV Ne ions with a flux of 5 · 1013 ions/cm2 s at room temperature 923 K and 1473 K, bubbles were formed at each irradiation temperature [10]. Considering our experimental results and other data, we can conclude that it is universal that bubbles are formed by inert gas irradiation from room temperature to 1473 K. With regard to dislocation loop formation, the present experimental results show clear differences between the two ion species. In He irradiation, loops are not formed at 1073 K and 1473 K. In Xe irradiation, loops are formed both at 1073 K and 1473 K. A difference with regard to loop formation between the two-irradiation temperatures was not seen in this experiment. In order to exam-
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ine more minutely irradiation conditions, which enable loops to form, we need to study the relation between irradiation conditions and loop formation. According to the results of Hojou et al. [11], loops are formed at room temperature by both 35 keV He and 60 keV Xe irradiation. However, the loops are not formed at 923 K after 35 keV He irradiation. On the other hand, the loops were formed both at room temperature and 923 K. Our previous study [10] showed that the loops are formed at room temperature and 923 K but not at 1473 K after 30 keV Ne irradiation. Considering our previous results and other data, we conclude that loops are formed by He, Ne and Xe irradiations at room temperature. At 973 K, loops are not formed by He irradiation but they are formed by Ne and Xe irradiations. At 1473 K, loops are formed by Xe irradiation but are not formed by He and Ne irradiations. The results show that loop formation depends on the ion species at high temperatures. When we examine the dependence of loop formation on irradiation temperature in the same ion species, we find that loops are formed from room temperature to 1473 K by Xe irradiation. Loops are formed at 923 K but are not formed at 1473 K by Ne irradiation. Loops are formed at room temperature but are not formed above 923 K in He irradiation. Thus, there is a dependence of loop formation on temperature, at least in the case of He and Ne irradiation. That is, loops are not formed at high temperatures in these two irradiation species. The number of interstitial atoms flowing into a loop must be larger than that of atoms released from a loop for its growth. The number of interstitial atoms flowing into a loop can be considered to be approximately proportional to the product of multiplying density of interstitial atoms near the loop by mobility of interstitial atoms near the loop. The number of atoms released from a loop should increase with rising temperature. We examined the difference of loop formation in the two ion species, as shown in this experiment. The difference of dpa was calculated by TRIM between He and Xe. According to TRIM calculation, the dpa of Xe is about 10 times larger than that of He, which means that the number of lattice atoms displaced by Xe irradiation is 10 times
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larger than that of atoms displaced by He irradiation. Thus, it can be considered that such large number of displaced atoms are strongly enhanced by the loop formation. In the case of Xe irradiation at 1073 K or 1473 K, the number of interstitial atoms flowing into a dislocation loop is larger than the number of atoms released from the loop, owing to the large number of displaced atoms. This behavior enhances the loop formation and growth. In case of He irradiation, the amount of interstitial atoms flowing into the loop is smaller than that of atoms released from the loop, due to the small amount of displaced atoms. The loops are, therefore, hardly formed at all. These results show that the difference of the number of displaced atoms affects loop formation. The dependence of loop formation on irradiation temperatures can be explained as follows. In ion irradiation, the number of atoms released from a loop increases with rising temperature. The mobility of interstitial atoms increases with rising temperature, which affects the number of atoms flowing into a loop. Because a specimen suitable for electron microscope observation is thin, the amount of interstitial atoms leaving from the specimen surface increases with increasing mobility, decreasing the density of interstitial atoms. The mobility of interstitial atoms contributing to loop growth increases with rising irradiation temperature, while the density decreases due to surface evaporation. The number of atoms released from the loop increases with rising temperature. On the basis of obtained results, it can be concluded that loops tend to be formed up to a certain temperature at the point where the number of atoms released from loop is larger than that of atoms flowing into loop.
5. Summary The present results show that bubbles are formed and grow in size and number with increasing ion fluence under the present irradiation conditions of this study. Referring to other experimental results, we can conclude that bubble formation does not depend on from room temperature to 1473 K for inert gas irradiation.
With regard to dislocation loop formation, the obtained results showed clear differences between the two ion species. It can be considered that the dependence of loop formation on ion species stems essentially from the difference of dpa per second between the two ion species. Considering our results and others, we found that there is also dependence of loop formation on irradiation temperature.
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