- Email: [email protected]
Response of cubic zirconia irradiated with 4-MeV Au ions at high temperature: An X-ray diffraction study
Nuclear Instruments and Methods in Physics Research B 277 (2012) 14–17
Contents lists available at SciVerse ScienceDirect
Nuclear Instruments and Methods in Physics Research B journal homepage: www.elsevier.com/locate/nimb
Response of cubic zirconia irradiated with 4-MeV Au ions at high temperature: An X-ray diffraction study A. Debelle a,⇑, L. Thomé a, A. Boulle b, S. Moll a, F. Garrido a, L. Qasim a, P. Rosza a a b
Centre de Spectrométrie Nucléaire et de Spectrométrie de Masse (CSNSM), Univ. Paris-Sud, CNRS-IN2P3, 91405 Orsay Cedex, France Science des Procédés Céramiques et de Traitements de Surface (SPCTS), CNRS UMR 6638, Centre Européen de la Céramique, 12 rue Atlantis, 87068 Limoges, France
a r t i c l e
i n f o
Article history: Received 31 October 2011 Available online 30 December 2011 Keywords: Ion irradiation Strain YSZ Temperature XRD
a b s t r a c t Yttria-stabilized cubic zirconia single-crystals have been irradiated with 4-MeV Au2+ ions at fluences ranging from 1012 cm2 to 5 1015 cm2 and at three temperatures (room temperature, 500 °C and 800 °C). Evaluation of the irradiation-induced strain has been performed by the X-ray diffraction technique. It is found that, whatever the irradiation temperature, the elastic-strain build-up exhibits two steps. An increase of the (tensile) strain-level is observed in the first step. A drastic strain relaxation occurs at a transition fluence, which defines the beginning of the second step. Increasing the irradiation temperature induces a decrease of the strain level and a shift of the transition fluence towards low fluence. Both effects may be explained by an enhanced defect-clustering rate which occurs already at 500 °C. Ó 2011 Elsevier B.V. All rights reserved.
1. Introduction Most of the materials used in the nuclear industry suffer from extremely severe in-use conditions, irradiation by numerous energetic particles being probably the most detrimental one. Therefore, the behaviour of these materials in such harsh conditions must be precisely known to prevent, or at least to anticipate, the loss of their functional properties. Yttria-stabilised cubic zirconia (YSZ) is envisioned as an inert matrix for the transmutation of minor actinides in nuclear reactors where it will be inherently submitted to various irradiation sources. Extensive studies undertaken from more than a decade to tackle the problem of radiation effects in this material clearly demonstrated its high radiation-resistance (see e.g. Refs. [1,2]). However, almost all works have been carried out at (or below) room temperature (RT), and only very few studies aimed at investigating the radiation damage in YSZ upon irradiation (well) above RT [3–5] (excepting works that dealt with the particular topic of gas bubble precipitation [6,7]). Recently, the response of YSZ irradiated with medium-energy (4 MeV) heavy (Au) ions at RT was comprehensively addressed [8,9]. In the present paper, it is dealt with the effect of the irradiation temperature, namely from RT to 800 °C, on the behaviour of YSZ upon the same irradiation conditions (4-MeV Au ions). For this purpose, characterizations of irradiated crystals have been performed by X-ray diffraction (XRD), a technique that has proven to be relevant for basic studies regarding irradiated materials in
general [10], and nuclear materials such as YSZ [11] and UO2 [12] in particular. 2. Experimental details 2.1. Sample description and irradiation The samples used in this study are commercial (Crystal GmbH company – Berlin), fully stabilized (9.5 mol% Y2O3) cubic {1 0 0}-oriented YSZ single crystals. YSZ has the fluorite structure with a bulk lattice parameter a0 = 0.5145 nm and a density q 5.9 g cm3. YSZ crystals were irradiated at RT, 500 °C and 800 °C with 4-MeV Au2+ ions using the ARAMIS tandem accelerator of the CSNSM (Orsay-France). Irradiations were performed 7° off the normal of the surface crystals to avoid channelling phenomenon, and a beam raster system was used to ensure a uniform ion irradiation; besides, in order to minimise target heating, the ion current during irradiation did not exceed 0.4 lA cm2. The mean projected range of Au particles has been estimated, based on SRIM calculations [13], to be RP 535 nm with a range straggling DRP 140 nm. A broad fluence range has been covered, namely from 1012 up to 5 1015 cm2. The corresponding conversion factor for the displacements per atom (dpa) at the damage peak is 4.5 1015 dpa cm2, as determined by SRIM calculations using threshold displacement energy of 40 eV for both Zr and O sublattices. 2.2. XRD set-up and formalism
⇑ Corresponding author. E-mail address: [email protected] (A. Debelle). 0168-583X/$ - see front matter Ó 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.nimb.2011.12.047
XRD measurements have been performed with two different apparatus. Crystals irradiated at RT were analysed with a
A. Debelle et al. / Nuclear Instruments and Methods in Physics Research B 277 (2012) 14–17
four-circle Seifert XRD-3000 goniometer using a line focus Cu Xray source and equipped with a Ge (2 2 0) double-crystal monochromator providing a parallel and monochromatic (CuKa1 radiation, k = 0.15406 nm) incident X-ray beam. A 0.07 mm detector slit allowed a 2h angular resolution of 0.01°. Specimens irradiated at high temperature were characterised with a Philips X’Pert PRO MRD diffractometer also equipped with a standard Cu tube. An intense and monochromatic beam was obtained by using a multilayer mirror behind the tube followed by a four-crystal monochromator (4Ge220) in asymmetric configuration; the resulting primary-beam divergence was 18 arcsec (0.005°). A three-bounce crystal analyzer (3Ge220) was used to further limit the detector acceptance (this configuration is usually referred to as ‘triple-axis configuration’). In both cases (room and high temperature), symmetric h–2h scans and reciprocal space maps (RSMs) were recorded in the vicinity of the (4 0 0) Bragg reflection (2h 73.575°) of the fluorite-type zirconia structure. The formalism used in this paper is the one presented in Ref. [10], and is based on the work of Dederichs with regard to the treatment of diffuse X-ray scattering [14]: (i) KN and K// are the normal (out-ofplane) and parallel (in-plane) components of the scattering vector K (2sinh/k), respectively; (ii) H(4 0 0) refers to the reciprocal lattice vector for the (4 0 0) reflection; (iii)qN is defined as KN H(4 0 0) and represents the deviation from the reciprocal lattice vector (sometimes called reduced scattering vector); (iv) (qN/H(4 0 0)) is equal to the elastic strain in the direction normal to the surface of irradiated
15
samples; (v) (DK///H(4 0 0)) indicates the width (here in degrees) of the reciprocal lattice point in the transverse direction.
3. Results and discussion Fig. 1 displays h–2h scans recorded in the vicinity of the (4 0 0) reflection for virgin and irradiated YSZ single crystals at different temperatures i.e. at RT (Fig. 1(a)), 550 °C (Fig. 1(b)) and 800 °C (Fig. 1(c)). All spectra exhibit an intense narrow peak on the high-angle side which corresponds to the unirradiated part of samples; this finding was expected since the depth probed by X-rays in this configuration reaches 4 lm (1/e attenuation-length criterion), which is much greater than the damaged thickness (1 lm). In the following, this peak will be considered as an internal strain gauge that allows quantifying the irradiation-induced normal elastic strain, eN = qN/H(4 0 0) (see Refs. [10,11]). A signal at lower 2h angle is also recorded for irradiated crystals. This observation indicates that the irradiated layers are characterised by an average lattice parameter larger than that of a virgin crystal. It has been shown that the formation of interstitial-type defects during irradiation must be at the origin of this lattice expansion [8]. Furthermore, this signal is characterised by specific features that clearly indicate the presence of a dilatation gradient along the direction normal to the sample surface [10,11]. Simulations of selected experimental XRD data, following the procedure
Fig. 1. h–2h scans recorded in the vicinity of the (0 0 4) reflection for virgin and irradiated YSZ crystals at increasing Au-ion fluences and at (a) RT, (b) 500 °C and (c) 800 °C. Labels correspond to the ion fluences (expressed in cm2). Curves are shifted vertically for visualisation purposes.
16
A. Debelle et al. / Nuclear Instruments and Methods in Physics Research B 277 (2012) 14–17
Fig. 3. Variation as a function of the Au-ion fluence of the maximum elastic strain (emax N ) measured in YSZ crystals irradiated at different temperatures. Symbols are experimental data and lines are drawn for visualisation purposes.
Fig. 2. (a) Experimental h–2h scans (symbols) on the (4 0 0) Bragg planes for a virgin YSZ specimen and for a crystal irradiated at 2 1014 cm2 at 800 °C; solid lines represent the corresponding simulated curves obtained from calculations performed in the framework of the procedure described in Ref. [11]. (b) Comparison of the damage depth distribution obtained from channelling measurements in a YSZ crystal irradiated at 2 1014 cm2 at 800 °C and of the disorder depth profile (Debye–Waller factor, see Refs. [11] and [15]) as derived from the simulation displayed in (a).
described in details elsewhere [11], are displayed in Fig. 2. From these simulations, both the strain and disorder (i.e. static Debye– Waller factor [11,14]) depth distributions can be retrieved. Fig. 2(a) presents experimental data (symbols) for a YSZ crystal irradiated at 800 °C at a fluence of 2 1014 cm2 and the corresponding simulation (solid line); in Fig. 2(b) the disorder depth profile [15] (Debye–Waller factor) derived from the simulation is displayed, along with the damage profile as determined by channelling measurements. Results readily indicate that the disorder depth profile, as extracted from XRD, and the damage distribution, as obtained from channelling, are very similar, at least at low irradiation fluence. Consequently, radiation defects detected by channelling are closely related to the development of the measured elastic strain. For all curves displayed in Fig. 1, a Bragg diffraction signal (i.e. a coherent scattering, by opposition to diffuse scattering, see e.g. Ref. [16]), arising from the damaged part of crystals, is readily observed at low fluence. This result reveals the development, in the first step of the damage build-up, of an irradiation-induced elastic strain whose level varies with the fluence and also with the irradiation temperature. Then, at a transition fluence that depends on the irradiation temperature, a second damage step occurs, where Bragg diffraction is superseded by diffuse scattering. It has been demonstrated that this dramatic change can be explained by
considering that the irradiated layer does not anymore respond elastically to the radiation damage but on the contrary becomes plastically deformed, leading to a relaxation of the elastic strain [8,12,17]. This mechanism is attributed to the re-arrangement of small defect clusters into extended defects such as dislocations. Fig. 3 presents the variation, for the three investigated temperatures, of the maximum normal elastic strain (emax N ) exhibited by irradiated layers as a function of the irradiation fluence. As expected from the XRD data displayed in Fig. 1, these elastic-strain kinetics show a similar behaviour whatever the irradiation fluence: (i) in the first irradiation stage, an increase of the (tensile) elastic strain is readily observed at low fluence and, (ii) with increasing fluence, a drastic strain relaxation occurs (the elastic strain has been arbitrarily fixed to zero since it is not possible to evaluate it precisely for this step). However, a huge difference appears in the elasticstrain level: it exceeds 0.5% after irradiation at RT, while it barely reaches 0.1% at 500 °C and is even less, 0.08%, at 800 °C. This result suggests that the defect density decreases with increasing temperature (assuming that the defect nature is similar whatever the irradiation temperature, an assumption that appears to be correct according to the preliminary transmission electron microscopy results). Another remarkable difference due to the change of the irradiation temperature lies in the occurrence of the transition between the two steps: the higher the temperature, the earlier the transition. This statement is supported by additional XRD results presented in Fig. 4 that shows three RSMs recorded for YSZ crystals irradiated at the same fluence (1015 cm2) but at different temperature. The map obtained for the sample irradiated at RT (Fig. 4(a)) is characteristic of the first step of the damage build-up. This map exhibits a streak along the normal component of the scattering vector (KN), indicating a strain gradient along the surface normal direction, consistently with the h–2h scans displayed in Fig. 1; this strain likely arises from the formation of small irradiation-induced defect clusters, as suggested by the fact that the scattered intensity along the in-plane component (K//) is confined to a small region. On the contrary, the RSM recorded for the crystal irradiated at 500 °C (Fig. 4(b)) is characteristic of the second step of the damage buildup: it shows a smaller streak along KN (consistent with a partial strain relaxation) but also a significant diffuse scattering signal evidenced by a smearing along K//. This feature is even more pronounced at 800 °C (Fig. 4(c)). The appearance of this diffuse scattering suggests a change in the strain build-up mechanism and hence, according to the above mentioned mechanism the formation of extended defects. Consequently, it is readily demonstrated that the damage kinetics is accelerated when the irradiation temperature is increased. A tentative explanation of this result is given below.
A. Debelle et al. / Nuclear Instruments and Methods in Physics Research B 277 (2012) 14–17
17
Fig. 4. Reciprocal space maps recorded in the vicinity of the (4 0 0) Bragg reflection for YSZ crystals irradiated with 4-MeV Au ions at 1015 cm2 and at (a) RT, (b) 500 °C and (c) 800 °C.
Yasuda et al. [4] and Ryazanov et al. [18] showed that, in irradiated YSZ crystals, there exists a critical size above which the defect nature changes, namely from defect clusters to a dislocation network. This transition is typically that observed upon the present irradiation conditions (see Refs. [8,9]). Hence, it is conjectured that the temperature increase induces a higher defect mobility leading to an enhanced defect-clustering process. In this case, the critical size above which the defect nature evolves should be reached at lower fluence when the irradiation temperature is raised. In addition, an increased defect-clustering rate could also account for the decrease of the defect density. It is worth noting that this effect is already operational at 500 °C, since the results obtained at this temperature are much closer to those found at 800 °C than at RT. 4. Conclusion Zirconia single crystals with the cubic structure have been irradiated with 4-MeV Au ions over a broad fluence range and at different temperatures (RT, 500 °C and 800 °C). These specimens have been characterised by XRD. The elastic-strain variation as a function of the ion fluence shows an overall similar behaviour whatever the irradiation temperature. In the first irradiation step, a tensile strain develops; it is shown that the irradiated layers exhibit a strain depth profile that follows the damage distribution determined by channelling measurements. Then, above a transition fluence, the strain is relaxed, revealing a second step in the damage accumulation process. Beyond this apparent similar behaviour, differences are evidenced when the irradiation temperature is changed. It is observed that the strain level decreases and the damage build-up is accelerated with increasing irradiation temperature. These results may be accounted for by an enhanced defect-clustering rate, induced by a higher defect mobility, which is due to the temperature increase. This effect is already effective at 500 °C. This behaviour upon ion irradiation at elevated temperature appears to be unusual (compared to as other ceramic oxides such as e.g. STO [19] or MgO [20] where it is observed an efficient dynamic annealing process), and additional data are required to obtain a comprehensive understanding. For this purpose, an extensive
study combining channelling, X-ray diffraction and transmission electron microscopy is currently underway. Acknowledgements Authors would like to acknowledge the SEMIRAMIS staff for carrying out ion irradiation. XRD measurements on the P analytical diffractometer have been performed at the nanocenter CTU-IEFMinerve that is partially funded by the ‘‘Conseil Général de l’Essonne’’. References [1] K.E. Sickafus, Hj. Matzke, Th. Hartmann, K. Yasuda, J.A. Valdez, P. Chodak III, M. Nastasi, R.A. Verall, J. Nucl. Mater. 274 (1999) 66. [2] L. Thomé, J. Fradin, J. Jagielski, A. Gentils, S.E. Enescu, F. Garrido, Eur. Phys. J. Appl. Phys. 24 (2003) 37. [3] K.E. Sickafus, H. Matzke, K. Yasuda, I.I.I.P. Chodak, R.A. Verrall, P.G. Lucuta, H.R. Andrews, A. Turos, R. Fromknecht, N.P. Baker, Nucl. Instr. Meth. B 141 (1998) 358. [4] K. Yasuda, C. Kinoshita, S. Matsumura, A.I. Ryazanov, J. Nucl. Mater. 319 (2003) 74. [5] L. Vincent, L. Thome, F. Garrido, O. Kaitasov, F. Houdelier, J. Appl. Phys. 104 (2008) 114904–114908. [6] T. Hojo, J. Aihara, K. Hojou, S. Furuno, H. Yamamoto, N. Nitani, T. Yamashita, K. Minato, T. Sakuma, J. Nucl. Mater. 319 (2003) 81. [7] T. Hojo, H. Yamamoto, J. Aihara, S. Furuno, K. Sawa, T. Sakuma, K. Hojou, Nucl. Instr. Meth. B 241 (2005) 536. [8] S. Moll, L. Thomé, G. Sattonnay, A. Debelle, L. Vincent, F. Garrido, J. Jagielski, J. Appl. Phys. 106 (2009) 073509. [9] A. Debelle, S. Moll, B. Décamps, L. Thomé, G. Sattonnay, F. Garrido, I. Jozwik, J. Jagielski, Scripta Mater. 63 (2010) 665. [10] A. Debelle, A. Declémy, Nucl. Instr. Meth. B 268 (2010) 1460. [11] A. Boulle, A. Debelle, J. Appl. Cryst. 43 (2010) 1046. [12] A. Debelle, A. Boulle, F. Garrido, L. Thomé, J. Mater. Sci. 46 (2011) 4683. [13] J.F. Ziegler, J. P. Biersack, U. Littmark, The Stopping and Range of Ions in Solids, Pergamon, New York, 1985. Available at: www.srim.org. [14] P.H. Dederichs, J. Phys. F: Metal. Phys. 3 (1973) 471. [15] It must be noted that in the present case, we plotted ð1 expLH Þ, LH being the Debye–Waller factor, in order to compare this latter with the damage fraction determined by channeling. [16] A. Boulle, R. Guinebretière, A. Dauger, J. Phys. D Appl. Phys. 38 (2005) 3907. [17] V.S. Speriosu, B.M. Paine, M.-A. Nicolet, H.L. Glass, Appl. Phys. Lett. 40 (1982) 604. [18] A.I. Ryazanov, K. Yasuda, C. Kinoshita, A.V. Klaptsov, J. Nucl. Mater. 307–311 (2002) 918. [19] Y. Li, R.J. Liu, W.-K. Chu, T.J. Tate, Phys. Rev. B 57 (1998) 5668. [20] I.O. Usov, J.A. Valdeza, K.E. Sickafus, NIM B 269 (2011) 288.
Contents lists available at SciVerse ScienceDirect
Nuclear Instruments and Methods in Physics Research B journal homepage: www.elsevier.com/locate/nimb
Response of cubic zirconia irradiated with 4-MeV Au ions at high temperature: An X-ray diffraction study A. Debelle a,⇑, L. Thomé a, A. Boulle b, S. Moll a, F. Garrido a, L. Qasim a, P. Rosza a a b
Centre de Spectrométrie Nucléaire et de Spectrométrie de Masse (CSNSM), Univ. Paris-Sud, CNRS-IN2P3, 91405 Orsay Cedex, France Science des Procédés Céramiques et de Traitements de Surface (SPCTS), CNRS UMR 6638, Centre Européen de la Céramique, 12 rue Atlantis, 87068 Limoges, France
a r t i c l e
i n f o
Article history: Received 31 October 2011 Available online 30 December 2011 Keywords: Ion irradiation Strain YSZ Temperature XRD
a b s t r a c t Yttria-stabilized cubic zirconia single-crystals have been irradiated with 4-MeV Au2+ ions at fluences ranging from 1012 cm2 to 5 1015 cm2 and at three temperatures (room temperature, 500 °C and 800 °C). Evaluation of the irradiation-induced strain has been performed by the X-ray diffraction technique. It is found that, whatever the irradiation temperature, the elastic-strain build-up exhibits two steps. An increase of the (tensile) strain-level is observed in the first step. A drastic strain relaxation occurs at a transition fluence, which defines the beginning of the second step. Increasing the irradiation temperature induces a decrease of the strain level and a shift of the transition fluence towards low fluence. Both effects may be explained by an enhanced defect-clustering rate which occurs already at 500 °C. Ó 2011 Elsevier B.V. All rights reserved.
1. Introduction Most of the materials used in the nuclear industry suffer from extremely severe in-use conditions, irradiation by numerous energetic particles being probably the most detrimental one. Therefore, the behaviour of these materials in such harsh conditions must be precisely known to prevent, or at least to anticipate, the loss of their functional properties. Yttria-stabilised cubic zirconia (YSZ) is envisioned as an inert matrix for the transmutation of minor actinides in nuclear reactors where it will be inherently submitted to various irradiation sources. Extensive studies undertaken from more than a decade to tackle the problem of radiation effects in this material clearly demonstrated its high radiation-resistance (see e.g. Refs. [1,2]). However, almost all works have been carried out at (or below) room temperature (RT), and only very few studies aimed at investigating the radiation damage in YSZ upon irradiation (well) above RT [3–5] (excepting works that dealt with the particular topic of gas bubble precipitation [6,7]). Recently, the response of YSZ irradiated with medium-energy (4 MeV) heavy (Au) ions at RT was comprehensively addressed [8,9]. In the present paper, it is dealt with the effect of the irradiation temperature, namely from RT to 800 °C, on the behaviour of YSZ upon the same irradiation conditions (4-MeV Au ions). For this purpose, characterizations of irradiated crystals have been performed by X-ray diffraction (XRD), a technique that has proven to be relevant for basic studies regarding irradiated materials in
general [10], and nuclear materials such as YSZ [11] and UO2 [12] in particular. 2. Experimental details 2.1. Sample description and irradiation The samples used in this study are commercial (Crystal GmbH company – Berlin), fully stabilized (9.5 mol% Y2O3) cubic {1 0 0}-oriented YSZ single crystals. YSZ has the fluorite structure with a bulk lattice parameter a0 = 0.5145 nm and a density q 5.9 g cm3. YSZ crystals were irradiated at RT, 500 °C and 800 °C with 4-MeV Au2+ ions using the ARAMIS tandem accelerator of the CSNSM (Orsay-France). Irradiations were performed 7° off the normal of the surface crystals to avoid channelling phenomenon, and a beam raster system was used to ensure a uniform ion irradiation; besides, in order to minimise target heating, the ion current during irradiation did not exceed 0.4 lA cm2. The mean projected range of Au particles has been estimated, based on SRIM calculations [13], to be RP 535 nm with a range straggling DRP 140 nm. A broad fluence range has been covered, namely from 1012 up to 5 1015 cm2. The corresponding conversion factor for the displacements per atom (dpa) at the damage peak is 4.5 1015 dpa cm2, as determined by SRIM calculations using threshold displacement energy of 40 eV for both Zr and O sublattices. 2.2. XRD set-up and formalism
⇑ Corresponding author. E-mail address: [email protected] (A. Debelle). 0168-583X/$ - see front matter Ó 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.nimb.2011.12.047
XRD measurements have been performed with two different apparatus. Crystals irradiated at RT were analysed with a
A. Debelle et al. / Nuclear Instruments and Methods in Physics Research B 277 (2012) 14–17
four-circle Seifert XRD-3000 goniometer using a line focus Cu Xray source and equipped with a Ge (2 2 0) double-crystal monochromator providing a parallel and monochromatic (CuKa1 radiation, k = 0.15406 nm) incident X-ray beam. A 0.07 mm detector slit allowed a 2h angular resolution of 0.01°. Specimens irradiated at high temperature were characterised with a Philips X’Pert PRO MRD diffractometer also equipped with a standard Cu tube. An intense and monochromatic beam was obtained by using a multilayer mirror behind the tube followed by a four-crystal monochromator (4Ge220) in asymmetric configuration; the resulting primary-beam divergence was 18 arcsec (0.005°). A three-bounce crystal analyzer (3Ge220) was used to further limit the detector acceptance (this configuration is usually referred to as ‘triple-axis configuration’). In both cases (room and high temperature), symmetric h–2h scans and reciprocal space maps (RSMs) were recorded in the vicinity of the (4 0 0) Bragg reflection (2h 73.575°) of the fluorite-type zirconia structure. The formalism used in this paper is the one presented in Ref. [10], and is based on the work of Dederichs with regard to the treatment of diffuse X-ray scattering [14]: (i) KN and K// are the normal (out-ofplane) and parallel (in-plane) components of the scattering vector K (2sinh/k), respectively; (ii) H(4 0 0) refers to the reciprocal lattice vector for the (4 0 0) reflection; (iii)qN is defined as KN H(4 0 0) and represents the deviation from the reciprocal lattice vector (sometimes called reduced scattering vector); (iv) (qN/H(4 0 0)) is equal to the elastic strain in the direction normal to the surface of irradiated
15
samples; (v) (DK///H(4 0 0)) indicates the width (here in degrees) of the reciprocal lattice point in the transverse direction.
3. Results and discussion Fig. 1 displays h–2h scans recorded in the vicinity of the (4 0 0) reflection for virgin and irradiated YSZ single crystals at different temperatures i.e. at RT (Fig. 1(a)), 550 °C (Fig. 1(b)) and 800 °C (Fig. 1(c)). All spectra exhibit an intense narrow peak on the high-angle side which corresponds to the unirradiated part of samples; this finding was expected since the depth probed by X-rays in this configuration reaches 4 lm (1/e attenuation-length criterion), which is much greater than the damaged thickness (1 lm). In the following, this peak will be considered as an internal strain gauge that allows quantifying the irradiation-induced normal elastic strain, eN = qN/H(4 0 0) (see Refs. [10,11]). A signal at lower 2h angle is also recorded for irradiated crystals. This observation indicates that the irradiated layers are characterised by an average lattice parameter larger than that of a virgin crystal. It has been shown that the formation of interstitial-type defects during irradiation must be at the origin of this lattice expansion [8]. Furthermore, this signal is characterised by specific features that clearly indicate the presence of a dilatation gradient along the direction normal to the sample surface [10,11]. Simulations of selected experimental XRD data, following the procedure
Fig. 1. h–2h scans recorded in the vicinity of the (0 0 4) reflection for virgin and irradiated YSZ crystals at increasing Au-ion fluences and at (a) RT, (b) 500 °C and (c) 800 °C. Labels correspond to the ion fluences (expressed in cm2). Curves are shifted vertically for visualisation purposes.
16
A. Debelle et al. / Nuclear Instruments and Methods in Physics Research B 277 (2012) 14–17
Fig. 3. Variation as a function of the Au-ion fluence of the maximum elastic strain (emax N ) measured in YSZ crystals irradiated at different temperatures. Symbols are experimental data and lines are drawn for visualisation purposes.
Fig. 2. (a) Experimental h–2h scans (symbols) on the (4 0 0) Bragg planes for a virgin YSZ specimen and for a crystal irradiated at 2 1014 cm2 at 800 °C; solid lines represent the corresponding simulated curves obtained from calculations performed in the framework of the procedure described in Ref. [11]. (b) Comparison of the damage depth distribution obtained from channelling measurements in a YSZ crystal irradiated at 2 1014 cm2 at 800 °C and of the disorder depth profile (Debye–Waller factor, see Refs. [11] and [15]) as derived from the simulation displayed in (a).
described in details elsewhere [11], are displayed in Fig. 2. From these simulations, both the strain and disorder (i.e. static Debye– Waller factor [11,14]) depth distributions can be retrieved. Fig. 2(a) presents experimental data (symbols) for a YSZ crystal irradiated at 800 °C at a fluence of 2 1014 cm2 and the corresponding simulation (solid line); in Fig. 2(b) the disorder depth profile [15] (Debye–Waller factor) derived from the simulation is displayed, along with the damage profile as determined by channelling measurements. Results readily indicate that the disorder depth profile, as extracted from XRD, and the damage distribution, as obtained from channelling, are very similar, at least at low irradiation fluence. Consequently, radiation defects detected by channelling are closely related to the development of the measured elastic strain. For all curves displayed in Fig. 1, a Bragg diffraction signal (i.e. a coherent scattering, by opposition to diffuse scattering, see e.g. Ref. [16]), arising from the damaged part of crystals, is readily observed at low fluence. This result reveals the development, in the first step of the damage build-up, of an irradiation-induced elastic strain whose level varies with the fluence and also with the irradiation temperature. Then, at a transition fluence that depends on the irradiation temperature, a second damage step occurs, where Bragg diffraction is superseded by diffuse scattering. It has been demonstrated that this dramatic change can be explained by
considering that the irradiated layer does not anymore respond elastically to the radiation damage but on the contrary becomes plastically deformed, leading to a relaxation of the elastic strain [8,12,17]. This mechanism is attributed to the re-arrangement of small defect clusters into extended defects such as dislocations. Fig. 3 presents the variation, for the three investigated temperatures, of the maximum normal elastic strain (emax N ) exhibited by irradiated layers as a function of the irradiation fluence. As expected from the XRD data displayed in Fig. 1, these elastic-strain kinetics show a similar behaviour whatever the irradiation fluence: (i) in the first irradiation stage, an increase of the (tensile) elastic strain is readily observed at low fluence and, (ii) with increasing fluence, a drastic strain relaxation occurs (the elastic strain has been arbitrarily fixed to zero since it is not possible to evaluate it precisely for this step). However, a huge difference appears in the elasticstrain level: it exceeds 0.5% after irradiation at RT, while it barely reaches 0.1% at 500 °C and is even less, 0.08%, at 800 °C. This result suggests that the defect density decreases with increasing temperature (assuming that the defect nature is similar whatever the irradiation temperature, an assumption that appears to be correct according to the preliminary transmission electron microscopy results). Another remarkable difference due to the change of the irradiation temperature lies in the occurrence of the transition between the two steps: the higher the temperature, the earlier the transition. This statement is supported by additional XRD results presented in Fig. 4 that shows three RSMs recorded for YSZ crystals irradiated at the same fluence (1015 cm2) but at different temperature. The map obtained for the sample irradiated at RT (Fig. 4(a)) is characteristic of the first step of the damage build-up. This map exhibits a streak along the normal component of the scattering vector (KN), indicating a strain gradient along the surface normal direction, consistently with the h–2h scans displayed in Fig. 1; this strain likely arises from the formation of small irradiation-induced defect clusters, as suggested by the fact that the scattered intensity along the in-plane component (K//) is confined to a small region. On the contrary, the RSM recorded for the crystal irradiated at 500 °C (Fig. 4(b)) is characteristic of the second step of the damage buildup: it shows a smaller streak along KN (consistent with a partial strain relaxation) but also a significant diffuse scattering signal evidenced by a smearing along K//. This feature is even more pronounced at 800 °C (Fig. 4(c)). The appearance of this diffuse scattering suggests a change in the strain build-up mechanism and hence, according to the above mentioned mechanism the formation of extended defects. Consequently, it is readily demonstrated that the damage kinetics is accelerated when the irradiation temperature is increased. A tentative explanation of this result is given below.
A. Debelle et al. / Nuclear Instruments and Methods in Physics Research B 277 (2012) 14–17
17
Fig. 4. Reciprocal space maps recorded in the vicinity of the (4 0 0) Bragg reflection for YSZ crystals irradiated with 4-MeV Au ions at 1015 cm2 and at (a) RT, (b) 500 °C and (c) 800 °C.
Yasuda et al. [4] and Ryazanov et al. [18] showed that, in irradiated YSZ crystals, there exists a critical size above which the defect nature changes, namely from defect clusters to a dislocation network. This transition is typically that observed upon the present irradiation conditions (see Refs. [8,9]). Hence, it is conjectured that the temperature increase induces a higher defect mobility leading to an enhanced defect-clustering process. In this case, the critical size above which the defect nature evolves should be reached at lower fluence when the irradiation temperature is raised. In addition, an increased defect-clustering rate could also account for the decrease of the defect density. It is worth noting that this effect is already operational at 500 °C, since the results obtained at this temperature are much closer to those found at 800 °C than at RT. 4. Conclusion Zirconia single crystals with the cubic structure have been irradiated with 4-MeV Au ions over a broad fluence range and at different temperatures (RT, 500 °C and 800 °C). These specimens have been characterised by XRD. The elastic-strain variation as a function of the ion fluence shows an overall similar behaviour whatever the irradiation temperature. In the first irradiation step, a tensile strain develops; it is shown that the irradiated layers exhibit a strain depth profile that follows the damage distribution determined by channelling measurements. Then, above a transition fluence, the strain is relaxed, revealing a second step in the damage accumulation process. Beyond this apparent similar behaviour, differences are evidenced when the irradiation temperature is changed. It is observed that the strain level decreases and the damage build-up is accelerated with increasing irradiation temperature. These results may be accounted for by an enhanced defect-clustering rate, induced by a higher defect mobility, which is due to the temperature increase. This effect is already effective at 500 °C. This behaviour upon ion irradiation at elevated temperature appears to be unusual (compared to as other ceramic oxides such as e.g. STO [19] or MgO [20] where it is observed an efficient dynamic annealing process), and additional data are required to obtain a comprehensive understanding. For this purpose, an extensive
study combining channelling, X-ray diffraction and transmission electron microscopy is currently underway. Acknowledgements Authors would like to acknowledge the SEMIRAMIS staff for carrying out ion irradiation. XRD measurements on the P analytical diffractometer have been performed at the nanocenter CTU-IEFMinerve that is partially funded by the ‘‘Conseil Général de l’Essonne’’. References [1] K.E. Sickafus, Hj. Matzke, Th. Hartmann, K. Yasuda, J.A. Valdez, P. Chodak III, M. Nastasi, R.A. Verall, J. Nucl. Mater. 274 (1999) 66. [2] L. Thomé, J. Fradin, J. Jagielski, A. Gentils, S.E. Enescu, F. Garrido, Eur. Phys. J. Appl. Phys. 24 (2003) 37. [3] K.E. Sickafus, H. Matzke, K. Yasuda, I.I.I.P. Chodak, R.A. Verrall, P.G. Lucuta, H.R. Andrews, A. Turos, R. Fromknecht, N.P. Baker, Nucl. Instr. Meth. B 141 (1998) 358. [4] K. Yasuda, C. Kinoshita, S. Matsumura, A.I. Ryazanov, J. Nucl. Mater. 319 (2003) 74. [5] L. Vincent, L. Thome, F. Garrido, O. Kaitasov, F. Houdelier, J. Appl. Phys. 104 (2008) 114904–114908. [6] T. Hojo, J. Aihara, K. Hojou, S. Furuno, H. Yamamoto, N. Nitani, T. Yamashita, K. Minato, T. Sakuma, J. Nucl. Mater. 319 (2003) 81. [7] T. Hojo, H. Yamamoto, J. Aihara, S. Furuno, K. Sawa, T. Sakuma, K. Hojou, Nucl. Instr. Meth. B 241 (2005) 536. [8] S. Moll, L. Thomé, G. Sattonnay, A. Debelle, L. Vincent, F. Garrido, J. Jagielski, J. Appl. Phys. 106 (2009) 073509. [9] A. Debelle, S. Moll, B. Décamps, L. Thomé, G. Sattonnay, F. Garrido, I. Jozwik, J. Jagielski, Scripta Mater. 63 (2010) 665. [10] A. Debelle, A. Declémy, Nucl. Instr. Meth. B 268 (2010) 1460. [11] A. Boulle, A. Debelle, J. Appl. Cryst. 43 (2010) 1046. [12] A. Debelle, A. Boulle, F. Garrido, L. Thomé, J. Mater. Sci. 46 (2011) 4683. [13] J.F. Ziegler, J. P. Biersack, U. Littmark, The Stopping and Range of Ions in Solids, Pergamon, New York, 1985. Available at: www.srim.org. [14] P.H. Dederichs, J. Phys. F: Metal. Phys. 3 (1973) 471. [15] It must be noted that in the present case, we plotted ð1 expLH Þ, LH being the Debye–Waller factor, in order to compare this latter with the damage fraction determined by channeling. [16] A. Boulle, R. Guinebretière, A. Dauger, J. Phys. D Appl. Phys. 38 (2005) 3907. [17] V.S. Speriosu, B.M. Paine, M.-A. Nicolet, H.L. Glass, Appl. Phys. Lett. 40 (1982) 604. [18] A.I. Ryazanov, K. Yasuda, C. Kinoshita, A.V. Klaptsov, J. Nucl. Mater. 307–311 (2002) 918. [19] Y. Li, R.J. Liu, W.-K. Chu, T.J. Tate, Phys. Rev. B 57 (1998) 5668. [20] I.O. Usov, J.A. Valdeza, K.E. Sickafus, NIM B 269 (2011) 288.