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Radiation damage effects in cubic-stabilized zirconia irradiated with 72 MeV I+ ions
Nuclear Instruments and Methods in Physics Research B 141 (1998) 358±365
Radiation damage eects in cubic-stabilized zirconia irradiated with 72 MeV I ions Kurt E. Sickafus a,*, Hansjoachim Matzke b, Kazuhiro Yasuda c, Paul Chodak, III a, Richard A. Verrall d, Petru G. Lucuta d,1, H. Robert Andrews d,2, Andrzej Turos e, Rainer Fromknecht f, Neil P. Baker a a
Los Alamos National Laboratory, Materials Science and Techn. Div., Los Alamos, NM 87545, USA b European Commission JRC, Institute for Transuranium Elements, Karlsruhe, Germany c Kyushu University, Fukuoka, Japan d Chalk River Laboratories, Chalk River, Ont., Canada e Soltan Institute for Nuclear Studies, Warsaw, Poland f Forschungszentrum, Karlsruhe INFP, Germany
Abstract Cubic-stabilized zirconia single crystals were irradiated using 72 MeV I ions in the TASSC accelerator facility at Chalk River Laboratory (to simulate a typical U or Pu ®ssion fragment). Irradiations were performed over the ¯uence range 1 ´ 1018 ±5 ´ 1019 ions/m2 , at temperatures of 300, 770, and 1170 K. Damage accumulation was monitored using Rutherford Backscattering Spectroscopy and ion-channeling (RBS/C) techniques. At ambient temperature and at the highest I ¯uence used in these experiments (5 ´ 1019 I /m2 ), RBS/C measurements revealed a rather high degree of lattice disorder. Speci®cally, the dechanneling parameter vmin varied from 80% to greater than 90% over the depth probed by RBS/C (1 lm). Nano-indentation measurements on the same sample indicated decreases in elastic modulus, E, and hardness, H (both by about 9%). These results suggest that an alteration in structure beyond simple defect accumulation occurs under these irradiation conditions. However, transmission electronmicroscopy (TEM) observations and in particular microdiraction measurements failed to reveal any structural transformations in the irradiated material. Ó 1998 Elsevier Science B.V. All rights reserved. PACS: 61.80.Jh; 61.82.Ms Keywords: Radiation damage; Amorphization; Zirconia
1. Introduction *
Corresponding author. Tel.: 505 665 3457; fax: 505 665 9224; e-mail: [email protected]. 1 Present address: ACERAM, Chalk River, Ont., Canada. 2 Present address: Bubble Technologies Industries, Chalk River, Ont., Canada.
Cubic-stabilized zirconia (ZrO2 ±Y2 O3 ) is a candidate inert-matrix, nuclear fuel-form for the destruction of surplus plutonium [1±4] and a candidate actinide host material for nuclear waste
0168-583X/98/$19.00 Ó 1998 Elsevier Science B.V. All rights reserved. PII S 0 1 6 8 - 5 8 3 X ( 9 8 ) 0 0 2 1 7 - 1
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storage [5]. In the former case, the fuels of interest are non-uranium-bearing, so-called non-fertile or inert-matrix fuels [6]. Cubic zirconia is attractive as an actinide host because it is isostructural with the cubic oxides of uranium (UO2 ) and plutonium (PuO2 ) and because actinides are soluble in zirconia. Moreover, rare-earth sesquioxides (Er2 O3 , Dy2 O3 , Gd2 O3 , etc.) are soluble in zirconia [7], so that it is feasible to dope an actinide-bearing zirconia waste-form or fuel-form with rare-earths to serve as depletable neutron absorbers. In a nuclear fuel-form application, zirconia will experience neutron bombardment damage along with damage due to high-energy ®ssion products. In this paper, we examine the radiation damage response of zirconia under ion bombardment, using heavy ions with energies intended to simulate heavy-ion damage due to ®ssion events in actinidebearing fuels. We used 72 MeV I ions to simulate a typical product of a ®ssion event. 2. Experimental procedure Cubic zirconia single crystals were obtained from Zirmat (P. O. Box 365, N. Billerca, MA 01862). Crystals contained Y2 O3 to stabilize the cubic, ¯uorite structure. Compositions of samples were determined using an electron microprobe. All samples were found to contain between 9.5 and 10.7 mol.% Y2 O3 and about 1.0 mol.% HfO2 . Samples were aligned to an (0 0 1)-orientation using a Laue back-re¯ection X-ray diraction camera, then cut to dimensions of approximately 10 mm ´ 10 mm ´ 1 mm. 3 Each sample was then polished on one side to a mirror ®nish. The zirconia crystals were irradiated with 72 MeV I ions in the TASSC accelerator facility at Chalk River Laboratory. Irradiations were performed over the ¯uence range 1 ´ 1018 ±5 ´ 1019 I /m2 , at temperatures of 300, 770, and 1170 K. The I ion ¯ux was maintained at 6 1 ´ 1016 I / m2 s. The irradiated sample area was about 2
3
We also used samples that were cut to a thickness of 0.5 mm, but these samples tended to fracture during the 72 MeV I irradiations, presumably due to thermal shock.
359
mm in diameter. Damage accumulation was assessed by Rutherford Backscattering Spectroscopy and ion-channeling (RBS/C) at INFP, FZK Karlsruhe, using a 2 MeV He beam aligned along a á0 0 1ñ-orientation. Samples were coated with thin layers of gold, except where small (2 mm diam.) masks were placed over the areas of interest. This served to minimize charging problems during RBS/C analyses. Nano-indentation was used to investigate the mechanical properties of irradiated zirconia samples. In these experiments, a Nano Indenter â II instrument at Los Alamos National Laboratory was used to determine the Young's modulus (E) and the hardness (H), by the continuous stiness method [8], a dynamic method in which applied load and indenter displacement are continuously increased and monitored in each indentation test. A fused silica sample was used as a control and tested at the outset of each experiment. Some of the irradiated zirconia crystals were prepared in cross section for examination by transmission electron microscopy (TEM). The radiation-induced microstructures were examined in a Philips CM-30 electron microscope operating at 300 kV. Bright-®eld (BF) imaging and microdiffraction techniques were used for analyses. The electronic and displacive components of energy loss for 72 MeV I ions in zirconia were estimated from Monte Carlo simulations using the TRIM binary collision code [9] (speci®cally, using TRIM-96.01). Results of these simulations are shown in Fig. 1. For the calculations shown in Fig. 1, we used a density of 5.96 g/cm3 for yttriacubic-stabilized zirconia (from JCPDS ®le 301468 [10], for composition Zr0:85 Y0:15 O1:93 , close to the composition of our samples), and a threshold displacement energy of 40 eV for all elements. TRIM simulations indicate that the range of 72 MeV I ions in zirconia is 6.8 lm and the longitudinal straggling of the ions is 510 nm. The peak concentration of I per 1019 I/m2 dose in the irradiated zirconia is about 0.014 at.%, while each ion produces about 32,000 net atomic displacements. For the largest ion dose used in this study, 5 ´ 1019 I/m2 , TRIM simulations indicate that the peak damage level (averaged over cation and anion sublattices) is about 7.7 dpa (at a depth of
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K.E. Sickafus et al. / Nucl. Instr. and Meth. in Phys. Res. B 141 (1998) 358±365
19 keV/nm/ion. This electronic loss component of the stopping decreases approximately linearly over most of the ion range. 3. Results & discussion
Fig. 1. The electronic and displacive components of energy loss for 72 MeV I ions in yttria-stabilized cubic zirconia (density, 5.96 g/cm3 and composition, Zr0:85 Y0:15 O1:93 ), estimated from Monte Carlo simulations using the TRIM binary collision code. A threshold displacement energy of 40 eV was used for all elements.
about 6.7 lm). Due to ionization mechanisms, these swift ions are subjected to large electronic losses over much of their range. TRIM simulations indicate that the magnitude of the electronic stopping power at the zirconia sample surface is about
Fig. 2 shows RBS/C spectra obtained from yttria-stabilized cubic zirconia single crystals irradiated with 72 MeV I ions to a ¯uence of 1 ´ 1019 I/m2 , at temperatures of 300, 770, and 1170 K. The spectra exhibit some features similar to those observed in our previous work on cubic zirconia crystals irradiated with 350±400 keV Xe ions [11,12]. The measurements here show prominent surface dechanneling peaks extending to a depth of about 200 nm for irradiation temperatures of 300 and 770 K (i.e., from approximately channel number 415 to channel 375 in Fig. 2), followed by a uniform region with a higher degree of lattice order, as evidenced by the lower ion backscattered yield from depths below say, 200 nm (below channel 375). A measure of lattice disorder is vmin , the ratio of the backscattered ion yield in the channel orientation, ((0 0 1) in this case), versus the ion yield in a ``random'' orientation (sample tilted about 15° away from (0 0 1)). Beneath the surface damage peak, i.e. at depth just below 200 nm, the
Fig. 2. RBS/C spectra obtained from yttria-stabilized cubic zirconia single crystals irradiated with 72 MeV I ions to a ¯uence of 1 ´ 1019 I/m2 , at temperatures of 300, 770, and 1170 K.
K.E. Sickafus et al. / Nucl. Instr. and Meth. in Phys. Res. B 141 (1998) 358±365
spectra in Fig. 2 indicate that vmin for samples irradiated at 300 and 770 K is about 63%, while at 1170 K, vmin is about 25%. This suggests that there are no discernible dierences in thermal damage recovery mechanisms in zirconia for temperatures below 770 K, while new thermally activated mechanisms are initiated by 1170 K. This observation is consistent with our earlier studies in which we observed no dierence in radiation damage accumulation in zirconia at irradiation temperatures of about 170 and 300 K. Also, at the irradiation dose in Fig. 2 (1 ´ 1019 I/m2 ), the backscattered ion yield never approaches the random level, indicating that the lattice is disordered but not amorphized at this dose (at least over the ®rst micron or so, the depth over which RBS/C backscattering information is obtained). Fig. 3 shows RBS/C spectra for yttria-stabilized zirconia single crystals irradiated at 300 K to ¯uences of 1 ´ 1018 , 1 ´ 1019 , and 5 ´ 1019 I/m2 . The dechanneling at the highest ion dose is surprisingly high. For zirconia at the highest irradiation dose, the measured vmin behind the surface damage peak is about 90%. This is signi®cant because in our previous studies using 350±400 kV Xe ions to high doses, we observed vmin to saturate at about 70% [11,12], far below the vmin values observed here. The results in Fig. 3 suggest that zirconia crystals
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exposed to high ¯uence, swift ion irradiation, suer rather extensive microstructural changes and possibly even succumb to a crystal structure transformation. The large magnitude of ion dechanneling observed here has several possible origins: (a) an accumulation of a high concentration of point defects and defect aggregates; (b) a polygonized microstructure; (c) a polycrystalline microstructure; (d) a rotation of the implanted layer relative to the substrate; (e) the onset of a phase transformation, particularly an amorphization transformation. The experiments described below were performed in an eort to distinguish between these possibilities. Nano-indentation experiments were performed on the zirconia crystals irradiated at 300 K to determine the eects of I irradiation on mechanical properties. Fig. 4 shows nano-indentation measurements of the elastic modulus (Fig. 4(a)) and the hardness (Fig. 4(b)) for ZrO2 ±Y2 O3 irradiated to ¯uences of 1 ´ 1018 , 1 ´ 1019 , and 5 ´ 1019 I/ m2 . Data points represent the mean values of the Young's Modulus (E) and the hardness (H), at speci®c indentation depths, based on ten separate load-displacement tests per sample. The roll-o in E and H values at indentation depths less than 30 nm is due to limitations associated with characterizing the geometry (contact area) of the indenter tip at extremely shallow indentation depths. The
Fig. 3. RBS/C spectra for yttria-stabilized zirconia single crystals irradiated at 300 K with 72 MeV I ions to ¯uences of 1 ´ 1018 , 1 ´ 1019 , and 5 ´ 1019 I/m2 .
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Fig. 4. Nano-indentation measurements of (a) elastic (Young's) modulus, E, and (b) the hardness, H, for ZrO2 ±Y2 O3 irradiated at 300 K with 72 MeV I ions to ¯uences of 1 ´ 1018 , 1 ´ 1019 , and 5 ´ 1019 I/m2 .
standard deviation in the mean values of E and H (not shown in Fig. 4) decrease sharply with increasing indentation depth. For example, in the case of an unirradiated zirconia crystal, the fractional standard deviation of E (i.e., the standard deviation of E divided by the mean value of E) falls from 29% at an indentation depth of 6 nm to 3.5% by an indentation depth of 35 nm. Similarly, the fractional standard deviation of H varies from
67% at 5 nm indentation depth to 6.6% at 35 nm indentation depth. For these reasons, it is best to compare values of E and H for dierent samples at indentation depths greater than about 30 nm. The E values in Fig. 4(a) for both unirradiated and irradiated zirconia samples range between 270±290 GPa, for indentation depths of about 100 nm. These values of E are higher than the reported value of 228 GPa for the Young's modulus of cubic-stabilized zirconia (at 293 K) containing 11.1 mol.% Y2 O3 (equivalent to Zr0:8002 Y0:1998 O1:900 , a slightly higher concentration of Y2 O3 than in our crystals) [13]. We do not have an explanation for this discrepancy. Nevertheless, we observe a decrease in E by almost 7% from about 290 to 270 GPa (as measured at 100 nm indenter displacement), upon ion irradiation to a ¯uence of 5 ´ 1019 I/m2 . The fractional standard deviation of E at this indenter depth is about 5%, so the uncertainty in E is less than the observed softening of the modulus due to ion irradiation. This is intriguing because this change occurs in the near-surface region where the displacement damage level is only about 0.9 dpa at this dose (the peak damage level of 7.7 dpa occurs at 6.7 lm depth, far below the depth sampled by the Nano indenter). In a previous study using 370 keV Xe ions, we observed no change in E to a peak damage level of 34 dpa [14]. 4 Since the elastic modulus represents an intrinsic materials' property, the decrease in E observed here may re¯ect some crystal structure transformation, e.g. partial amorphization. The hardness data in Fig. 4(b) is similarly intriguing. The hardness (H) values in Fig. 4(b) for both unirradiated and irradiated zirconia samples range between 21.0±22.3 GPa, for indentation depths of about 100 nm. For iodine ¯uences between 0.1±1 ´ 1019 I/m2 , H is unchanged by irradiation (within the statistical accuracy of the nanoindentation technique). But by 5 ´ 1019 I/m2 ¯uence, H is observed to decrease by about 6% (as measured at 100 nm displacement), similar to the decrease in E. The fractional standard deviation of H at this indenter depth is about 4.5%, so again
4 The range of 370 keV Xe ions is shallow enough that the peak damage region is sampled by the Nano indenter.
K.E. Sickafus et al. / Nucl. Instr. and Meth. in Phys. Res. B 141 (1998) 358±365
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Fig. 5. Cross-sectional transmission electron micrograph of a ZrO2 ±Y2 O3 single crystal, irradiated with 72 MeV I ions to a ¯uence of 5 ´ 1019 I/m2 at 300 K. The BF image was obtained from a region near the end-of-range of the I ions. Inset microdiraction patterns indicate that both the irradiated defected region and the unirradiated underlying substrate have the same crystal symmetry and orientation.
the uncertainty in H is less than the measured softening due to ion irradiation. Contrary to this result, in our previous study we observed a small (5%) increase in H upon ion irradiation. The irradiation-induced softening observed here is perplexing, and once again is compatible with a structural transformation such as partial amorphization. Fig. 5 shows a cross-sectional, BF transmission electron micrograph of a ZrO2 ±Y2 O3 single crystal, irradiated to a ¯uence of 5 ´ 1019 I/m2 at 300
K. 5 The area imaged in Fig. 5 is near the endof-range of the I ions and the inset microdiraction patterns were obtained from both a heavily 5 This particular sample was obtained from a zirconia boule doped with ceria (CeO2 ). Electron microprobe analyses indicated that the ceria concentration in this material was below the detectability limit of the analytical probe ( 6 0.04 mol.% CeO2 ). We have not yet established whether this dopant aects the damage response of zirconia.
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defected region near the end-of-range and from a pristine substrate region beyond the I ion range. The BF image in Fig. 5 indicates that the irradiated material contains a high concentration of defects, primarily dislocations. No evidence for void formation was found by TEM (void and bubble formation have been observed in 60 keV Xe ion irradiations of cubic zirconia at irradiation temperatures between 300 and 1473 K [4]). The microdiraction pattern from the irradiated region in Fig. 5 indicates that the material is crystalline and by comparison to the diraction pattern from the unirradiated region, it remains epitaxial with the underlying substrate. Similar microdiraction patterns (not shown) were observed across the entire irradiated layer. TEM failed to reveal evidence for a structural transformation. No evidence was found for polygonization; nor for polycrystallinity; nor for rotation of the implanted layer relative to the substrate; nor for a structural transformation such as amorphization. Perhaps the high magnitude of the RBS/C He ion dechanneling is due simply to a high concentration of defects and defect clusters in the irradiated material. By comparison to earlier ion irradiation damage studies on zirconia and similar ¯uorite-structured materials, zirconia crystals irradiated with 72 MeV I experience radiation damage eects that are somewhat inconsistent with previous studies and consequently, the observations are somewhat dicult to interpret. As mentioned previously, Xe-ion irradiations of zirconia to doses representing far more displacement damage than in these experiments, produced far less retained damage (even at cryogenic temperatures) as measured by RBS/C [11,12,15] and by TEM [3], and no radiation-induced changes in mechanical properties were observed [14]. RBS/C measurements of radiation damage accumulation in high-dose, Xeion-irradiated calcium ¯uorite (CaF2 ) [16] and uranium dioxide (UO2 ) [17], both isostructural compounds to cubic zirconia, exhibited less damage accumulation than the zirconia crystals in these experiments. Also, structural transformations such as amorphization do not occur in UO2 to ion irradiation damage levels of at least 20 dpa [18]. In the experiments presented here, some of the RBS/C results (Fig. 3) hinted at irradiation-in-
duced amorphization of cubic zirconia and moreover, a structural transformation was suggested by radiation-induced changes in the intrinsic property, E (Fig. 4). However, no radiation-induced transformation could be con®rmed by TEM (Fig. 5). The major distinction of the irradiation experiments presented here is the very high electronic stopping power of the 72 MeV I irradiation species (Fig. 1). Perhaps for cubic zirconia, lattice recovery and damage annealing mechanisms are not eective in the swift ion (®ssion) energy regime in which the electronic/nuclear stopping power ratio is far greater than 1. Experiments are underway to assess the eects of variations in the electronic/nuclear stopping power ratio on damage evolution in cubic zirconia. 4. Conclusions 72 MeV I ion irradiations were performed on yttria-stabilized cubic zirconia single crystals at temperatures ranging from 300±1170 K. The 72 MeV I ions were chosen as they possess mass and energy typical of a ®ssion fragment; also, this ion is characterized by a large electronic stopping power when it enters a solid. Using RBS/C analyses, damage accumulation was found to saturate at much higher values (at temperatures below 770 K) compared to measurements in previous ion irradiation studies using 350±400 keV Xe ion irradiations (ions in the high nuclear stopping regime). Nano-indentation experiments revealed softening, i.e. a decrease in the elastic modulus and hardness by about 6±7% for the highest dose irradiations (5 ´ 1019 I/m2 at 300 K). This could indicate a structural change such as partial amorphization, but such a transformation was not con®rmed by cross-sectional TEM observations. The latter observations revealed crystalline, highly defected material throughout the I -irradiated region. Acknowledgements The authors wish to thank M.G. Snow and R.G. Warren of Los Alamos National Laboratory for providing electron microprobe analyses of the
K.E. Sickafus et al. / Nucl. Instr. and Meth. in Phys. Res. B 141 (1998) 358±365
zirconia crystals used for this study. Also, the authors from Los Alamos National Laboratory acknowledge the support provided by the Department of Energy, Oce of Basic Energy Sciences, Division of Materials Science. References [1] H. Akie, T. Muromura, H. Takano, S. Matsuura, Nucl. Technol. 107 (1994) 182. [2] C. Degueldre, U. Kasemeyer, F. Botta, G. Ledergerber, Mat. Res. Soc. Symp. Proc. 412 (1996) 15. [3] C. Degueldre, P. Heimgartner, G. Ledergerber, N. Sasajima, K. Hojou, T. Muromura, L. Wang, W. Gong, R. Ewing, Mat. Res. Soc. Symp. Proc. 439 (1997) 625. [4] N. Sasajima, T. Matsui, K. Hojou, S. Furuno, H. Otsu, K. Izui, T. Muromura, these proceedings (REI-9), Nucl. Instr. and Meth. B 141 (1998) 487. [5] K.E. Sickafus, P. Chodak, III, R.J. Hanrahan, Jr., K. J. McClellan, J.N. Mitchell, C.J. Wetteland, K. Yasuda, Hj. Matzke, R.A. Verrall, K.B. Ramsey, H.T. Blair, K. Chidester, N. Yu, Los Alamos Report # LA-UR-971002, J. Mat. Res., accepted for publication. [6] Hj. Matzke, Nucl. Instr. and Meth. B 116 (1996) 121.
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[7] A. Rouanet, Rev. Int. Hautes Temper. et Refract. 8 (1971) 161. [8] W.C. Oliver, G.M. Pharr, J. Mat. Res. 7 (1992) 1564. [9] J.F. Ziegler, J.P. Biersack, U. Littmark, The Stopping and Range of Ions in Solids, Pergamon Press, New York, 1985. [10] International Committee for Diraction Data, Powder Diraction File (Joint Committee on Powder Diraction Standards, Philadelphia, PA, 1974 ± present). [11] N. Yu, K.E. Sickafus, P. Kodali, M. Nastasi, J. Nucl. Mat. 244 (1997) 266. [12] K. Yasuda, M. Nastasi, K.E. Sickafus, C.J. Maggiore, N. Yu, Proc. 13th Int. Conf. on Ion Beam Analysis, Lisbon, Portugal, 1997, Nucl. Instr. and Meth. B 136±138 (1998) 499. [13] H.M. Kandil, J.D. Greiner, J.F. Smith, J. Am. Ceram. Soc. 67 (1984) 341. [14] K.E. Sickafus, C.J. Wetteland, N.P. Baker, N. Yu, R. Devanathan, M. Nastasi, N. Bordes, Mat. Sci. Eng. A, in press. [15] E.L. Fleischer, M.G. Norton, M.A. Zaleski, W. Hertl, C.B. Carter, J.W. Mayer, J. Mat. Res. 6 (1991) 1905. [16] N. Yu, M. Nastasi, K.E. Sickafus, K. Yasuda, J.R. Tesmer, Nucl. Instr. and Meth. B 127/128 (1997) 591. [17] Hj. Matzke, Nucl. Instr. and Meth. B 116 (1996) 121. [18] Hj. Matzke, J. Nucl. Mat. 190 (1992) 101.
Radiation damage eects in cubic-stabilized zirconia irradiated with 72 MeV I ions Kurt E. Sickafus a,*, Hansjoachim Matzke b, Kazuhiro Yasuda c, Paul Chodak, III a, Richard A. Verrall d, Petru G. Lucuta d,1, H. Robert Andrews d,2, Andrzej Turos e, Rainer Fromknecht f, Neil P. Baker a a
Los Alamos National Laboratory, Materials Science and Techn. Div., Los Alamos, NM 87545, USA b European Commission JRC, Institute for Transuranium Elements, Karlsruhe, Germany c Kyushu University, Fukuoka, Japan d Chalk River Laboratories, Chalk River, Ont., Canada e Soltan Institute for Nuclear Studies, Warsaw, Poland f Forschungszentrum, Karlsruhe INFP, Germany
Abstract Cubic-stabilized zirconia single crystals were irradiated using 72 MeV I ions in the TASSC accelerator facility at Chalk River Laboratory (to simulate a typical U or Pu ®ssion fragment). Irradiations were performed over the ¯uence range 1 ´ 1018 ±5 ´ 1019 ions/m2 , at temperatures of 300, 770, and 1170 K. Damage accumulation was monitored using Rutherford Backscattering Spectroscopy and ion-channeling (RBS/C) techniques. At ambient temperature and at the highest I ¯uence used in these experiments (5 ´ 1019 I /m2 ), RBS/C measurements revealed a rather high degree of lattice disorder. Speci®cally, the dechanneling parameter vmin varied from 80% to greater than 90% over the depth probed by RBS/C (1 lm). Nano-indentation measurements on the same sample indicated decreases in elastic modulus, E, and hardness, H (both by about 9%). These results suggest that an alteration in structure beyond simple defect accumulation occurs under these irradiation conditions. However, transmission electronmicroscopy (TEM) observations and in particular microdiraction measurements failed to reveal any structural transformations in the irradiated material. Ó 1998 Elsevier Science B.V. All rights reserved. PACS: 61.80.Jh; 61.82.Ms Keywords: Radiation damage; Amorphization; Zirconia
1. Introduction *
Corresponding author. Tel.: 505 665 3457; fax: 505 665 9224; e-mail: [email protected]. 1 Present address: ACERAM, Chalk River, Ont., Canada. 2 Present address: Bubble Technologies Industries, Chalk River, Ont., Canada.
Cubic-stabilized zirconia (ZrO2 ±Y2 O3 ) is a candidate inert-matrix, nuclear fuel-form for the destruction of surplus plutonium [1±4] and a candidate actinide host material for nuclear waste
0168-583X/98/$19.00 Ó 1998 Elsevier Science B.V. All rights reserved. PII S 0 1 6 8 - 5 8 3 X ( 9 8 ) 0 0 2 1 7 - 1
K.E. Sickafus et al. / Nucl. Instr. and Meth. in Phys. Res. B 141 (1998) 358±365
storage [5]. In the former case, the fuels of interest are non-uranium-bearing, so-called non-fertile or inert-matrix fuels [6]. Cubic zirconia is attractive as an actinide host because it is isostructural with the cubic oxides of uranium (UO2 ) and plutonium (PuO2 ) and because actinides are soluble in zirconia. Moreover, rare-earth sesquioxides (Er2 O3 , Dy2 O3 , Gd2 O3 , etc.) are soluble in zirconia [7], so that it is feasible to dope an actinide-bearing zirconia waste-form or fuel-form with rare-earths to serve as depletable neutron absorbers. In a nuclear fuel-form application, zirconia will experience neutron bombardment damage along with damage due to high-energy ®ssion products. In this paper, we examine the radiation damage response of zirconia under ion bombardment, using heavy ions with energies intended to simulate heavy-ion damage due to ®ssion events in actinidebearing fuels. We used 72 MeV I ions to simulate a typical product of a ®ssion event. 2. Experimental procedure Cubic zirconia single crystals were obtained from Zirmat (P. O. Box 365, N. Billerca, MA 01862). Crystals contained Y2 O3 to stabilize the cubic, ¯uorite structure. Compositions of samples were determined using an electron microprobe. All samples were found to contain between 9.5 and 10.7 mol.% Y2 O3 and about 1.0 mol.% HfO2 . Samples were aligned to an (0 0 1)-orientation using a Laue back-re¯ection X-ray diraction camera, then cut to dimensions of approximately 10 mm ´ 10 mm ´ 1 mm. 3 Each sample was then polished on one side to a mirror ®nish. The zirconia crystals were irradiated with 72 MeV I ions in the TASSC accelerator facility at Chalk River Laboratory. Irradiations were performed over the ¯uence range 1 ´ 1018 ±5 ´ 1019 I /m2 , at temperatures of 300, 770, and 1170 K. The I ion ¯ux was maintained at 6 1 ´ 1016 I / m2 s. The irradiated sample area was about 2
3
We also used samples that were cut to a thickness of 0.5 mm, but these samples tended to fracture during the 72 MeV I irradiations, presumably due to thermal shock.
359
mm in diameter. Damage accumulation was assessed by Rutherford Backscattering Spectroscopy and ion-channeling (RBS/C) at INFP, FZK Karlsruhe, using a 2 MeV He beam aligned along a á0 0 1ñ-orientation. Samples were coated with thin layers of gold, except where small (2 mm diam.) masks were placed over the areas of interest. This served to minimize charging problems during RBS/C analyses. Nano-indentation was used to investigate the mechanical properties of irradiated zirconia samples. In these experiments, a Nano Indenter â II instrument at Los Alamos National Laboratory was used to determine the Young's modulus (E) and the hardness (H), by the continuous stiness method [8], a dynamic method in which applied load and indenter displacement are continuously increased and monitored in each indentation test. A fused silica sample was used as a control and tested at the outset of each experiment. Some of the irradiated zirconia crystals were prepared in cross section for examination by transmission electron microscopy (TEM). The radiation-induced microstructures were examined in a Philips CM-30 electron microscope operating at 300 kV. Bright-®eld (BF) imaging and microdiffraction techniques were used for analyses. The electronic and displacive components of energy loss for 72 MeV I ions in zirconia were estimated from Monte Carlo simulations using the TRIM binary collision code [9] (speci®cally, using TRIM-96.01). Results of these simulations are shown in Fig. 1. For the calculations shown in Fig. 1, we used a density of 5.96 g/cm3 for yttriacubic-stabilized zirconia (from JCPDS ®le 301468 [10], for composition Zr0:85 Y0:15 O1:93 , close to the composition of our samples), and a threshold displacement energy of 40 eV for all elements. TRIM simulations indicate that the range of 72 MeV I ions in zirconia is 6.8 lm and the longitudinal straggling of the ions is 510 nm. The peak concentration of I per 1019 I/m2 dose in the irradiated zirconia is about 0.014 at.%, while each ion produces about 32,000 net atomic displacements. For the largest ion dose used in this study, 5 ´ 1019 I/m2 , TRIM simulations indicate that the peak damage level (averaged over cation and anion sublattices) is about 7.7 dpa (at a depth of
360
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19 keV/nm/ion. This electronic loss component of the stopping decreases approximately linearly over most of the ion range. 3. Results & discussion
Fig. 1. The electronic and displacive components of energy loss for 72 MeV I ions in yttria-stabilized cubic zirconia (density, 5.96 g/cm3 and composition, Zr0:85 Y0:15 O1:93 ), estimated from Monte Carlo simulations using the TRIM binary collision code. A threshold displacement energy of 40 eV was used for all elements.
about 6.7 lm). Due to ionization mechanisms, these swift ions are subjected to large electronic losses over much of their range. TRIM simulations indicate that the magnitude of the electronic stopping power at the zirconia sample surface is about
Fig. 2 shows RBS/C spectra obtained from yttria-stabilized cubic zirconia single crystals irradiated with 72 MeV I ions to a ¯uence of 1 ´ 1019 I/m2 , at temperatures of 300, 770, and 1170 K. The spectra exhibit some features similar to those observed in our previous work on cubic zirconia crystals irradiated with 350±400 keV Xe ions [11,12]. The measurements here show prominent surface dechanneling peaks extending to a depth of about 200 nm for irradiation temperatures of 300 and 770 K (i.e., from approximately channel number 415 to channel 375 in Fig. 2), followed by a uniform region with a higher degree of lattice order, as evidenced by the lower ion backscattered yield from depths below say, 200 nm (below channel 375). A measure of lattice disorder is vmin , the ratio of the backscattered ion yield in the channel orientation, ((0 0 1) in this case), versus the ion yield in a ``random'' orientation (sample tilted about 15° away from (0 0 1)). Beneath the surface damage peak, i.e. at depth just below 200 nm, the
Fig. 2. RBS/C spectra obtained from yttria-stabilized cubic zirconia single crystals irradiated with 72 MeV I ions to a ¯uence of 1 ´ 1019 I/m2 , at temperatures of 300, 770, and 1170 K.
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spectra in Fig. 2 indicate that vmin for samples irradiated at 300 and 770 K is about 63%, while at 1170 K, vmin is about 25%. This suggests that there are no discernible dierences in thermal damage recovery mechanisms in zirconia for temperatures below 770 K, while new thermally activated mechanisms are initiated by 1170 K. This observation is consistent with our earlier studies in which we observed no dierence in radiation damage accumulation in zirconia at irradiation temperatures of about 170 and 300 K. Also, at the irradiation dose in Fig. 2 (1 ´ 1019 I/m2 ), the backscattered ion yield never approaches the random level, indicating that the lattice is disordered but not amorphized at this dose (at least over the ®rst micron or so, the depth over which RBS/C backscattering information is obtained). Fig. 3 shows RBS/C spectra for yttria-stabilized zirconia single crystals irradiated at 300 K to ¯uences of 1 ´ 1018 , 1 ´ 1019 , and 5 ´ 1019 I/m2 . The dechanneling at the highest ion dose is surprisingly high. For zirconia at the highest irradiation dose, the measured vmin behind the surface damage peak is about 90%. This is signi®cant because in our previous studies using 350±400 kV Xe ions to high doses, we observed vmin to saturate at about 70% [11,12], far below the vmin values observed here. The results in Fig. 3 suggest that zirconia crystals
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exposed to high ¯uence, swift ion irradiation, suer rather extensive microstructural changes and possibly even succumb to a crystal structure transformation. The large magnitude of ion dechanneling observed here has several possible origins: (a) an accumulation of a high concentration of point defects and defect aggregates; (b) a polygonized microstructure; (c) a polycrystalline microstructure; (d) a rotation of the implanted layer relative to the substrate; (e) the onset of a phase transformation, particularly an amorphization transformation. The experiments described below were performed in an eort to distinguish between these possibilities. Nano-indentation experiments were performed on the zirconia crystals irradiated at 300 K to determine the eects of I irradiation on mechanical properties. Fig. 4 shows nano-indentation measurements of the elastic modulus (Fig. 4(a)) and the hardness (Fig. 4(b)) for ZrO2 ±Y2 O3 irradiated to ¯uences of 1 ´ 1018 , 1 ´ 1019 , and 5 ´ 1019 I/ m2 . Data points represent the mean values of the Young's Modulus (E) and the hardness (H), at speci®c indentation depths, based on ten separate load-displacement tests per sample. The roll-o in E and H values at indentation depths less than 30 nm is due to limitations associated with characterizing the geometry (contact area) of the indenter tip at extremely shallow indentation depths. The
Fig. 3. RBS/C spectra for yttria-stabilized zirconia single crystals irradiated at 300 K with 72 MeV I ions to ¯uences of 1 ´ 1018 , 1 ´ 1019 , and 5 ´ 1019 I/m2 .
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Fig. 4. Nano-indentation measurements of (a) elastic (Young's) modulus, E, and (b) the hardness, H, for ZrO2 ±Y2 O3 irradiated at 300 K with 72 MeV I ions to ¯uences of 1 ´ 1018 , 1 ´ 1019 , and 5 ´ 1019 I/m2 .
standard deviation in the mean values of E and H (not shown in Fig. 4) decrease sharply with increasing indentation depth. For example, in the case of an unirradiated zirconia crystal, the fractional standard deviation of E (i.e., the standard deviation of E divided by the mean value of E) falls from 29% at an indentation depth of 6 nm to 3.5% by an indentation depth of 35 nm. Similarly, the fractional standard deviation of H varies from
67% at 5 nm indentation depth to 6.6% at 35 nm indentation depth. For these reasons, it is best to compare values of E and H for dierent samples at indentation depths greater than about 30 nm. The E values in Fig. 4(a) for both unirradiated and irradiated zirconia samples range between 270±290 GPa, for indentation depths of about 100 nm. These values of E are higher than the reported value of 228 GPa for the Young's modulus of cubic-stabilized zirconia (at 293 K) containing 11.1 mol.% Y2 O3 (equivalent to Zr0:8002 Y0:1998 O1:900 , a slightly higher concentration of Y2 O3 than in our crystals) [13]. We do not have an explanation for this discrepancy. Nevertheless, we observe a decrease in E by almost 7% from about 290 to 270 GPa (as measured at 100 nm indenter displacement), upon ion irradiation to a ¯uence of 5 ´ 1019 I/m2 . The fractional standard deviation of E at this indenter depth is about 5%, so the uncertainty in E is less than the observed softening of the modulus due to ion irradiation. This is intriguing because this change occurs in the near-surface region where the displacement damage level is only about 0.9 dpa at this dose (the peak damage level of 7.7 dpa occurs at 6.7 lm depth, far below the depth sampled by the Nano indenter). In a previous study using 370 keV Xe ions, we observed no change in E to a peak damage level of 34 dpa [14]. 4 Since the elastic modulus represents an intrinsic materials' property, the decrease in E observed here may re¯ect some crystal structure transformation, e.g. partial amorphization. The hardness data in Fig. 4(b) is similarly intriguing. The hardness (H) values in Fig. 4(b) for both unirradiated and irradiated zirconia samples range between 21.0±22.3 GPa, for indentation depths of about 100 nm. For iodine ¯uences between 0.1±1 ´ 1019 I/m2 , H is unchanged by irradiation (within the statistical accuracy of the nanoindentation technique). But by 5 ´ 1019 I/m2 ¯uence, H is observed to decrease by about 6% (as measured at 100 nm displacement), similar to the decrease in E. The fractional standard deviation of H at this indenter depth is about 4.5%, so again
4 The range of 370 keV Xe ions is shallow enough that the peak damage region is sampled by the Nano indenter.
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Fig. 5. Cross-sectional transmission electron micrograph of a ZrO2 ±Y2 O3 single crystal, irradiated with 72 MeV I ions to a ¯uence of 5 ´ 1019 I/m2 at 300 K. The BF image was obtained from a region near the end-of-range of the I ions. Inset microdiraction patterns indicate that both the irradiated defected region and the unirradiated underlying substrate have the same crystal symmetry and orientation.
the uncertainty in H is less than the measured softening due to ion irradiation. Contrary to this result, in our previous study we observed a small (5%) increase in H upon ion irradiation. The irradiation-induced softening observed here is perplexing, and once again is compatible with a structural transformation such as partial amorphization. Fig. 5 shows a cross-sectional, BF transmission electron micrograph of a ZrO2 ±Y2 O3 single crystal, irradiated to a ¯uence of 5 ´ 1019 I/m2 at 300
K. 5 The area imaged in Fig. 5 is near the endof-range of the I ions and the inset microdiraction patterns were obtained from both a heavily 5 This particular sample was obtained from a zirconia boule doped with ceria (CeO2 ). Electron microprobe analyses indicated that the ceria concentration in this material was below the detectability limit of the analytical probe ( 6 0.04 mol.% CeO2 ). We have not yet established whether this dopant aects the damage response of zirconia.
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defected region near the end-of-range and from a pristine substrate region beyond the I ion range. The BF image in Fig. 5 indicates that the irradiated material contains a high concentration of defects, primarily dislocations. No evidence for void formation was found by TEM (void and bubble formation have been observed in 60 keV Xe ion irradiations of cubic zirconia at irradiation temperatures between 300 and 1473 K [4]). The microdiraction pattern from the irradiated region in Fig. 5 indicates that the material is crystalline and by comparison to the diraction pattern from the unirradiated region, it remains epitaxial with the underlying substrate. Similar microdiraction patterns (not shown) were observed across the entire irradiated layer. TEM failed to reveal evidence for a structural transformation. No evidence was found for polygonization; nor for polycrystallinity; nor for rotation of the implanted layer relative to the substrate; nor for a structural transformation such as amorphization. Perhaps the high magnitude of the RBS/C He ion dechanneling is due simply to a high concentration of defects and defect clusters in the irradiated material. By comparison to earlier ion irradiation damage studies on zirconia and similar ¯uorite-structured materials, zirconia crystals irradiated with 72 MeV I experience radiation damage eects that are somewhat inconsistent with previous studies and consequently, the observations are somewhat dicult to interpret. As mentioned previously, Xe-ion irradiations of zirconia to doses representing far more displacement damage than in these experiments, produced far less retained damage (even at cryogenic temperatures) as measured by RBS/C [11,12,15] and by TEM [3], and no radiation-induced changes in mechanical properties were observed [14]. RBS/C measurements of radiation damage accumulation in high-dose, Xeion-irradiated calcium ¯uorite (CaF2 ) [16] and uranium dioxide (UO2 ) [17], both isostructural compounds to cubic zirconia, exhibited less damage accumulation than the zirconia crystals in these experiments. Also, structural transformations such as amorphization do not occur in UO2 to ion irradiation damage levels of at least 20 dpa [18]. In the experiments presented here, some of the RBS/C results (Fig. 3) hinted at irradiation-in-
duced amorphization of cubic zirconia and moreover, a structural transformation was suggested by radiation-induced changes in the intrinsic property, E (Fig. 4). However, no radiation-induced transformation could be con®rmed by TEM (Fig. 5). The major distinction of the irradiation experiments presented here is the very high electronic stopping power of the 72 MeV I irradiation species (Fig. 1). Perhaps for cubic zirconia, lattice recovery and damage annealing mechanisms are not eective in the swift ion (®ssion) energy regime in which the electronic/nuclear stopping power ratio is far greater than 1. Experiments are underway to assess the eects of variations in the electronic/nuclear stopping power ratio on damage evolution in cubic zirconia. 4. Conclusions 72 MeV I ion irradiations were performed on yttria-stabilized cubic zirconia single crystals at temperatures ranging from 300±1170 K. The 72 MeV I ions were chosen as they possess mass and energy typical of a ®ssion fragment; also, this ion is characterized by a large electronic stopping power when it enters a solid. Using RBS/C analyses, damage accumulation was found to saturate at much higher values (at temperatures below 770 K) compared to measurements in previous ion irradiation studies using 350±400 keV Xe ion irradiations (ions in the high nuclear stopping regime). Nano-indentation experiments revealed softening, i.e. a decrease in the elastic modulus and hardness by about 6±7% for the highest dose irradiations (5 ´ 1019 I/m2 at 300 K). This could indicate a structural change such as partial amorphization, but such a transformation was not con®rmed by cross-sectional TEM observations. The latter observations revealed crystalline, highly defected material throughout the I -irradiated region. Acknowledgements The authors wish to thank M.G. Snow and R.G. Warren of Los Alamos National Laboratory for providing electron microprobe analyses of the
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