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Characterization of the irradiation-induced phase transition in the monoclinic polymorph of zirconia
Nuclear Instruments and Methods in Physics Research B xxx (2014) xxx–xxx
Contents lists available at ScienceDirect
Nuclear Instruments and Methods in Physics Research B journal homepage: www.elsevier.com/locate/nimb
Characterization of the irradiation-induced phase transition in the monoclinic polymorph of zirconia R.T. Huang a, Y.H. Shen a, R.H. Huang b, J.Y. Hsu c, H. Niu d, Y.C. Yu e,⇑ a
Institute of Materials Engineering, National Taiwan Ocean University, Keelung 20224, Taiwan Department of Materials Science and Engineering, National Chung Hsing University, Taichung 40227, Taiwan c National Synchrotron Radiation Research Center, Hsinchu 30077, Taiwan d Nuclear Science and Technology Development Center, National Tsing Hua University, Hsinchu 30013, Taiwan e Institute of Physics, Academia Sinica, Taipei 11529, Taiwan b
a r t i c l e
i n f o
Article history: Available online xxxx Keywords: Zirconia Irradiation Proton Implantation Tetragonal TEM
a b s t r a c t Two groups of pure zirconia (ZrO2) specimens with the monoclinic polymorph and distinct nanopowder size ranges of approximately 20–30 nm and 30–60 nm were irradiated separately at room temperature with 100 keV H+ and 3 MeV Fe2+ ions to various fluences. Following TEM observations and XRD analyses, the irradiation with 100 keV protons to a fluence of over 3 1015 ions/cm2 was demonstrated to induce transformation from monoclinic (m) to tetragonal (t) phases in the specimen with the size range of approximately 20–30 nm due to the energy crossover that facilitates the m ? t transformation. However, the transformation did not occur in the irradiated specimen with the other size range even though the fluence of protons was up to 1 1017 ions/cm2. Furthermore, although irradiation with 3 MeV Fe2+ ions brought about low electronic energy loss, the m ? t transformation can be induced under the low fluence of 1 1014 ions/cm2 because a high level of damage defects (oxygen vacancies) may promote the m ? t transformation. Consequently, the largest crystallite size of t-ZrO2 induced by the proton irradiation is approximately 29 nm, as a result of the metastable critical size effect of t-phase zirconia at room temperature. Ó 2014 Elsevier B.V. All rights reserved.
1. Introduction Zirconia (nominally with stoichiometry ZrO2) has become one of the most important ceramic materials over the past several decades because of its superior physical and electrical properties, high ionic conductivity, excellent chemical durability, and low thermal conductivity in a wide range of industrial applications (e.g., as a catalyst, gas sensor, and electrolyte in solid oxide fuel cells, and as a gate dielectric in microelectronics) [1–5]. In addition, the discovery of transformation toughening published in the journal Nature by Garvie et al. [6] proclaimed new high-performance applications of zirconia, ranging from bearing and wear applications to thermal barrier coatings on metal components [7,8]. Moreover, zirconia polymorphs demonstrate excellent radiation tolerance for nuclear applications [9], such that they have been suggested as a promising parent material for use as a fuel matrix in nuclear reactors [10]. However, zirconia exhibits three ⇑ Corresponding author. Address: 128 Sec. 2, Academia Rd, Nankang, Taipei 11529, Taiwan. Tel.: +886 2 27896769; fax: +886 2 27834187. E-mail address: [email protected] (Y.C. Yu).
crystallographic polymorphs as a function of temperature at normal atmospheric pressure [11]: the monoclinic baddeleyite (m-ZrO2) is thermodynamically stable from room temperature to 1170 °C, the tetragonal phase (t-ZrO2) is thermodynamically stable from 1170 to 2370 °C, and the fluorite-type cubic phase (c-ZrO2) is thermodynamically stable from 2370 to 2680 °C (melting point). These polymorphs significantly affect the physical and mechanical properties of zirconia used as fuel or a parent material for nuclear applications. Zirconia phase transition under irradiation has been widely studied under different temperature ranges and the mass and energy of bombarding ions, and these studies have shown that polymorphism plays an important role in the radiation damage evolution of zirconia. In a series of experiments, Benyagoub et al. [12,13] found that irradiation with 135 MeV 58Ni ions, 300 MeV 76 Ge ions, and 250 MeV 127I ions generates a transformation from the monoclinic to the tetragonal phase (hereafter denoted as an m ? t transformation), where primarily electronic stopping exceeding 13 keV/nm dominates over nuclear stopping. Based on transmission electron microscopy (TEM) and Raman spectroscopy examinations, Sickafus et al. [14,15] demonstrated that m-ZrO2
http://dx.doi.org/10.1016/j.nimb.2014.02.081 0168-583X/Ó 2014 Elsevier B.V. All rights reserved.
Please cite this article in press as: R.T. Huang et al., Characterization of the irradiation-induced phase transition in the monoclinic polymorph of zirconia, Nucl. Instr. Meth. B (2014), http://dx.doi.org/10.1016/j.nimb.2014.02.081
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Fig. 1. SRIM simulation results for 100 keV H+ ion irradiation of ZrO2. (a) Energy loss as a function of depth for the nuclear and electronic stopping components of the stopping power and (b) collision events (left-hand ordinate) and implanted H+ ion concentration (right-hand ordinate) as a function of depth for the irradiation.
exposed to 340 keV Xe2+ ions at 120 K and to 300 keV Kr2+ ions at approximately 80 K (in the nuclear stopping energy regime), respectively, transformed to t-ZrO2. Valdez et al. [15] showed that 150 keV Ne+ ions irradiation of m-ZrO2 also produces m ? t transformation (in both electronic and nuclear stopping regime). Simeone et al. [16] observed a similar m ? t transformation using 800 keV Bi ions and suggested, based on grazing X-ray diffraction analysis, that the tetragonal phase is localized in the damaged area. Nanocrystalline zirconia is known to have unique mechanical, thermal, and electrical properties compared to its bulk counterparts, and the phase stability of nanosized zirconia can be affected significantly by its grain size [17]. In the considerable body of published literature on the effects of irradiation on bulk zirconia, few systematic studies of the behavior of nanocrystalline zirconia under similar conditions exist. We are aware of one report, in which an m ? t transformation was observed in the monoclinic-dominant ZrO2 with an average grain size of 40–50 nm upon the ion irradiation of 350 keV O+ and 1 MeV Kr2+ at room temperature [18]. Therefore, in this study, we focus on the evolution of pure m-ZrO2 nano particles under 100 keV protons and 3 MeV Fe2+ ions irradiation at room temperature and document an unusual behavior of ZrO2 nanoparticles under size effects in the irradiation-induced m ? t transformation under 100 keV protons irradiation.
Fig. 2. SRIM simulation results for 3 MeV Fe2+ ions irradiation of ZrO2. (a) Energy loss as a function of depth for the nuclear and electronic stopping components of the stopping power and (b) collision events (left-hand ordinate) and implanted Fe2+ ions concentration (right-hand ordinate) as a function of depth for the irradiation.
2. Material and methods Two distinct particle-size ranges of m-ZrO2 powders, approximately 20–30 nm and 30–60 nm (verified by TEM observation), purchased from CERAC Corp. (99.995% pure) were used to prepare the two sample groups to which ion bombardment is executed. First, a small amount of powder with the same size range was added to DI-water, followed by ultrasonication to disperse the particles; then a small drop of suspension was spread repeatedly on a clean silicon chip. The evenly spread specimen was finally dried using an electric hot plate at 60 °C. The thickness of the deposited powder was controlled over 15 lm. To depict the specimen with two distinct particle size ranges of m-ZrO2 powders, the prepared specimens with the smaller particle sizes of approximately 20– 30 nm and with the larger particle sizes of approximately 30– 60 nm were designated as S30 and L30 specimens, respectively. These zirconia specimens were sequentially irradiated with 100 keV H+ ions in the Nuclear Science & Technology Development Center at National Tsing Hua University using a 500 kV High Voltage Engineering Europa ion implanter and with 3 MeV Fe2+ in the Institute of Physics at the Academia Sinica using National Electrostatics Corporation 9SDH-II 3MV Tandem Accelerator. The implantation energy of 100 keV protons, based on the capability of the instrument, was chosen to achieve a relatively large electron
Please cite this article in press as: R.T. Huang et al., Characterization of the irradiation-induced phase transition in the monoclinic polymorph of zirconia, Nucl. Instr. Meth. B (2014), http://dx.doi.org/10.1016/j.nimb.2014.02.081
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Fig. 3. (a) XRD patterns obtained from the L30 specimen irradiated with 3 MeV Fe2+ ions to fluences ranging from 1 1014 to 1 1015 ions/cm2. (b) Narrow scan for (a). (c) XRD patterns obtained from the L30 specimen irradiated with 100 keV H+ ions to fluences ranging from 1 1015 to 1 1017 ions/cm2. (d) Narrow scan for (c). The characteristic peak of irradiation-induced t-ZrO2 did not appear in the XRD spectra recorded from the proton irradiation to the fluences even up to 1 1017 ions/cm2.
energy loss with a similar penetration depth to the Fe2+ ion implantation. The proton fluences ranged from 1 1015 to 1 1017 ions/cm2, whereas the Fe2+ ion fluences ranged from 1 1014 to 1 1015 ions/cm2. All irradiations were performed at room temperature. Fig. 1 shows the characteristics of 100 keV H+ ions implanting into m-ZrO2, obtained by the program SRIM 2008 [19] (displacement energies of 25 and 28 eV for Zr and O atoms, and a density of 5.68 gm/cm3 were used for these calculation). The estimate of energy loss partitioned into electronic and nuclear components for 100 keV protons in m-ZrO2 is displayed in Fig. 1(a). It is obvious that the value of electronic energy loss is much larger than that of nuclear energy loss. Fig. 1(b) displays the estimate of ballistic damage and implanted proton concentration as a function of depth into the target material of zirconia. It shows that a large amount of ballistic damage appeared around the depth of 550 nm, but with a small value of irradiation-induced collision events (displacement damage). Furthermore, the characteristics of 3 MeV Fe2+ ions implanting into m-ZrO2, in which the estimate of energy loss and ballistic damage and the concentration of implanted Fe2+ ions as a function of depth into the target material of zirconia, are displayed in Fig. 2(a) and (b), respectively. Moreover, to study the evolution of the microstructure, these irradiated specimens were characterized by an X-ray diffractometer (XRD, Panalytical X’Pert Pro MPD) and by transmission electron microscopy (TEM, JEOL 2010). 3. Results and discussion The L30 specimen was first irradiated separately with 100 keV protons and 3 MeV Fe2+ ions to various fluences. Fig. 3(a) shows
the XRD patterns of the L30 specimen irradiated with 3 MeV Fe2+ ions to fluences ranging from 1 1014 to 1 1015 ions/cm2. Following Fe2+ ion irradiation, the characteristic peak of irradiationinduced t-ZrO2 emerged preferentially at approximately 2h = 35.25°, but the peak did not appear in the XRD patterns recorded from the proton irradiation to the fluences even up to 1 1017 ions/cm2, as shown in Fig. 3(c) and (d). The figure also displayed the narrow scan of XRD patterns recorded from 32° to 38° to distinctly identify the characteristic peak of t-ZrO2 exhibiting at 35.25°. Considering Garvie’s finding, in which the t-phase can be found to occur at room temperature only below a critical crystallite size of approximately 30 nm, and the t-ZrO2 was characterized by a small mean crystallite size, large specific surface, and appreciable excess energy [20], the smaller particle size of mZrO2 presumably facilitates the occurrence of t-ZrO2 under proton irradiation due to a large specific surface. Accordingly, the same procedure of ion implantation was subsequently executed on the S30 specimen. Consequently, the t-phase peak induced by the proton irradiation indeed appears in the XRD pattern when the fluence is larger than 3 1015 ions/cm2 and emerges preferentially at approximately 2h = 50.37° (Fig. 4). In addition, the t-phase peak induced by Fe2+ irradiation also appears in the XRD patterns of the S30 specimen, similar to Fig. 3(a). Comparing the results of proton irradiation obtained from the two separate particle size ranges suggests that there is a size effect in the irradiation-induced m ? t transformation of zirconia under proton irradiation with 100 keV. Moreover, these irradiation-induced t-ZrO2 were further characterized by TEM examination. Fig. 5 shows the bright-field and dark-field TEM images taken from the S30 specimen irradiated with 100 keV protons to 3 1015 ions/cm2. Because t-ZrO2 (0 1 1) and (0 1 2) diffraction planes are easily distinguishable from
Please cite this article in press as: R.T. Huang et al., Characterization of the irradiation-induced phase transition in the monoclinic polymorph of zirconia, Nucl. Instr. Meth. B (2014), http://dx.doi.org/10.1016/j.nimb.2014.02.081
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Fig. 4. (a) XRD patterns obtained from the S30 specimen irradiated with 100 keV H+ ions to fluences ranging from 1 1015 to 1 1016 ions/cm2. (b) Narrow scan for (a). Fig. 5. TEM (a) bright-field and (b) dark-field images of the S30 specimen irradiated with 100 keV H+ ions to a fluence of 3 1015 ions/cm2.
m-ZrO2, the dark-field TEM image shown in Fig. 5(b) was obtained from the t-ZrO2 (0 1 1) diffraction plane and the measured size is approximately 28.9 nm. Fig. 6 displays the bright-field TEM images taken from the L30 specimen irradiated with 3 MeV Fe2+ ions to 5 1014 ions/cm2, in which the image was obtained from the ½111 diffraction zone axis of t-ZrO2 showing a single crystal feature and the measured size is approximately 47.0 nm (Fig. 6(b)). Furthermore, to evaluate the irradiation-induced t-ZrO2 particle size, the calculation of recorded XRD spectra based on Gaussian curve fitting and the Scherrer equation [21] and the measurement of the dark-field TEM images obtained from t-ZrO2 (0 1 1) and (0 1 2) diffraction planes were also executed. These evaluated values are shown in Table 1. The irradiation-induced t-ZrO2 particle size seems to approach approximately 29 nm for the irradiation of 100 keV protons used in the S30 specimen. Furthermore, irradiation-induced m ? t transformation under Fe2+ irradiation with 3 MeV may be achieved at a comparatively low fluence. Electronic energy loss released by high energy ions is generally thought to induce significant atomic rearrangement. Benyagoub et al. [13] referred to two competing models, the ‘‘Coulomb explosion’’ and the ‘‘thermal spike’’ mechanism, to account for such a phenomenon, wherein a huge amount of deposited electronic energy loss exceeding an effective threshold near 13 keV nm1 induces a transformation from the monoclinic to the tetragonal phase in zirconia. Table 2 lists the computed irradiation parameters deduced from the program SRIM2008 for all of these experiments. Although the calculated electronic energy loss energies
Fig. 6. TEM bright-field and dark-field images of the L30 specimen irradiated with 3 MeV Fe2+ ions to a fluence of 5 1014 ions/cm2.
Please cite this article in press as: R.T. Huang et al., Characterization of the irradiation-induced phase transition in the monoclinic polymorph of zirconia, Nucl. Instr. Meth. B (2014), http://dx.doi.org/10.1016/j.nimb.2014.02.081
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Table 1 Evaluation of the irradiation-induced t-ZrO2 crystallite size from the recorded XRD spectra calculation based on Gaussian curve fits and Scherrer equation and the measurement of the dark-field TEM images. Source
Sample
Energy
Fluence
m?t
XRD evaluation (nm) (I)
TEM measurement (nm) (II)
Fe2+
L30
3 MeV
1 1015 5 1014 3 1014 1 1014
Yes Yes Yes Yes
27.7 28.6 29.0 28.9
28.0–48.7 29.1–47.0 29.9–46.8 29.5–40.8
H+
S30
100 KeV
1 1016 5 1015 3 1015 1 1015
Yes Yes Yes Yes
28.5 28.8 27.8 –
26.6–29.9 26.1–29.7 24.8–29.4 –
Table 2 Computed irradiation parameters deduced from the program SRIM2008 for all these experiments. Ion species +
H H+ Fe+
Energy (MeV) 0.1 1.5 3
Ion range (lm) 0.55 16.9 0.92
are both lower than 13 keV nm1, the irradiation-induced m ? t transformation under Fe2+ irradiation with 3 MeV and under proton irradiation with 100 keV to an m-ZrO2 particle size less than 30 nm could be achieved. Upon the irradiation-induced m ? t transformation under Fe2+ irradiation with 3 MeV, the released electronic energy loss energy is transported by electronic heat conduction to a larger volume and consequently leads to electron lattice interactions which transfer heat to the lattice atoms, thereby giving rise to a ‘‘thermal spike’’ for the target atoms. However, the irradiated phase likely contains a high level of damage defects (oxygen vacancies), which helps promote the m ? t transformation by allowing for the expansion of the ZrAO distance and the increase of the Zr coordination [13,17] under low electronic energy loss. As for the irradiation-induced m ? t transformation under proton irradiation with 100 keV to an m-ZrO2 particle size of less than 30 nm, an energy crossover [20] can play a significant role in the m ? t transformation, in which the thermodynamically metastable phase in the bulk is energetically favorable as the grain size decreases below a critical value. Hence, the average sizes of the irradiation-induced t-ZrO2 particle approach approximately 29 nm, due to their metastable critical sizes after irradiation. Furthermore, Pitcher et al. [22] reported that at larger size regimes, monoclinic ZrO2 has the lowest total energy; as the size decreases to nanosized regimes, their specific surface and energy increase. Upon proton irradiation with 100 keV, the irradiation-induced damage defects are much smaller than the Fe2+ irradiation with 3 MeV. The irradiation-induced m ? t transformation is dependent mostly on the deposited electronic energy loss, but the energy loss is not enough to induce the m ? t transformation for large-size zirconia (over 30 nm). Conversely, because energy crossover helps trigger the m ? t transformation, the irradiation-induced m ? t transformation under proton irradiation with 100 keV to an mZrO2 particle size of less than 30 nm could be achieved at a fluence level of over 3 1015 ions/cm2. 4. Conclusions In summary, irradiation with 100 keV protons to a fluence of over 3 1015 ions/cm2 can induce the m ? t phase transformation in the specimen with the size range of approximately 20–30 nm, due to the energy crossover which benefits the promotion of transformation. However, the transformation does not occur in the irradiated specimen with the other size range even though the fluence is up to 1 1017 ions/cm2. Furthermore, although irradiation with
Electronic energy loss (keV nm1) 3
0.2 10 0.058 103 3.016 103
Nuclear energy loss (eV nm1) 0.352 103 0.04057 103 492.3 103
3 MeV Fe2+ ions brings about low electronic energy loss, the m ? t transformation is exhibited under the low fluence of 1 1014 ions/ cm2 because a high level of damage defects (oxygen vacancies) could be beneficial to triggering the m ? t transformation. Consequently, the largest crystallite sizes of t-ZrO2 induced by proton irradiation approach approximately 29 nm as a result of the metastable critical size of t-phase zirconia at room temperature.
Acknowledgments The authors gratefully acknowledge the support of this study by the National Science Council of Republic of China (NSC-99-2221E-019-010-MY3) and the assistance in SEM and TEM examination given by the sophisticated instrument user center of National Taiwan Ocean University.
References [1] R. Mueller, R. Jossen, S.E. Pratsinis, M. Watson, M.K. Akhtar, J. Am. Ceram. Soc. 87 (2004) 197. [2] X. Song, A. Sayari, Catal. Rev. Sci. Eng. 38 (1996) 329. [3] J.W. Schwank, M. DiBattista, MRS Bull. 24 (1999) 44. [4] S. Park, J.M. Vohs, R.J. Gorte, Nature 404 (2000) 265. [5] A.A. Demkov, Phys. Status Solidi 226 (2001) 57. [6] R.C. Garvie, R.H.J. Hannink, R.T. Pascoe, Nature 258 (1975) 703. [7] P.K. Wright, A.G. Evans, Curr. Opin. Solid State Mater. Sci. 4 (1999) 255. [8] X. Zhou, I. Balachov, D.D. Macdonald, Corros. Sci. 40 (1998) 1349. [9] F.W. Clinard, D.L. Rohr, W. Ranken, J. Am. Ceram. Soc. 60 (1977) 287. [10] V.M. Oversby, C.C. McPheeters, C. Degueldre, J.M. Paratte, J. Nucl. Mater. 245 (1997) 17. [11] E.C. Subbarao, in: A.H. Heuer, L.W. Hobbs (Eds.), Science and Technology of Zirconia, The American Ceramic Society Inc., Columbus, OH, 1981, p. 1. [12] A. Benyagoub, F. Levesque, F. Couvreur, C. Gibert-Mougel, C. Dufour, E. Paumier, Appl. Phys. Lett. 77 (2000) 3197. [13] A. Benyagoub, Nucl. Instr. Meth. B 206 (2003) 132. [14] K.E. Sickafus, H. Matzke, T. Hartmann, K. Yasuda, J.A. Valdez, P. Chodak III, M. Nastasi, R.A. Verrall, J. Nucl. Mater. 274 (1999) 66. [15] J.A. Valdez, M. Tang, C. Zhenhuan, M.I. Peters, K.E. Sickafus, Nucl. Instr. Meth. B 218 (2004) 103. [16] D. Simeone, G. Baldinozzi, D. Gosset, S. Le Caër, L. Mazerolles, Rev. Adv. Mater. Sci. 10 (2005) 118. [17] S. Shukla, S. Seal, Int. Mater. Rev. 50 (2005) 45. [18] F. Lu, J. Zhang, M. Huang, F. Namavar, R.C. Ewing, J. Lian, J. Phys. Chem. C 115 (2011) 7193. [19] J.F. Ziegler, SRIM2008.05,
Please cite this article in press as: R.T. Huang et al., Characterization of the irradiation-induced phase transition in the monoclinic polymorph of zirconia, Nucl. Instr. Meth. B (2014), http://dx.doi.org/10.1016/j.nimb.2014.02.081
Contents lists available at ScienceDirect
Nuclear Instruments and Methods in Physics Research B journal homepage: www.elsevier.com/locate/nimb
Characterization of the irradiation-induced phase transition in the monoclinic polymorph of zirconia R.T. Huang a, Y.H. Shen a, R.H. Huang b, J.Y. Hsu c, H. Niu d, Y.C. Yu e,⇑ a
Institute of Materials Engineering, National Taiwan Ocean University, Keelung 20224, Taiwan Department of Materials Science and Engineering, National Chung Hsing University, Taichung 40227, Taiwan c National Synchrotron Radiation Research Center, Hsinchu 30077, Taiwan d Nuclear Science and Technology Development Center, National Tsing Hua University, Hsinchu 30013, Taiwan e Institute of Physics, Academia Sinica, Taipei 11529, Taiwan b
a r t i c l e
i n f o
Article history: Available online xxxx Keywords: Zirconia Irradiation Proton Implantation Tetragonal TEM
a b s t r a c t Two groups of pure zirconia (ZrO2) specimens with the monoclinic polymorph and distinct nanopowder size ranges of approximately 20–30 nm and 30–60 nm were irradiated separately at room temperature with 100 keV H+ and 3 MeV Fe2+ ions to various fluences. Following TEM observations and XRD analyses, the irradiation with 100 keV protons to a fluence of over 3 1015 ions/cm2 was demonstrated to induce transformation from monoclinic (m) to tetragonal (t) phases in the specimen with the size range of approximately 20–30 nm due to the energy crossover that facilitates the m ? t transformation. However, the transformation did not occur in the irradiated specimen with the other size range even though the fluence of protons was up to 1 1017 ions/cm2. Furthermore, although irradiation with 3 MeV Fe2+ ions brought about low electronic energy loss, the m ? t transformation can be induced under the low fluence of 1 1014 ions/cm2 because a high level of damage defects (oxygen vacancies) may promote the m ? t transformation. Consequently, the largest crystallite size of t-ZrO2 induced by the proton irradiation is approximately 29 nm, as a result of the metastable critical size effect of t-phase zirconia at room temperature. Ó 2014 Elsevier B.V. All rights reserved.
1. Introduction Zirconia (nominally with stoichiometry ZrO2) has become one of the most important ceramic materials over the past several decades because of its superior physical and electrical properties, high ionic conductivity, excellent chemical durability, and low thermal conductivity in a wide range of industrial applications (e.g., as a catalyst, gas sensor, and electrolyte in solid oxide fuel cells, and as a gate dielectric in microelectronics) [1–5]. In addition, the discovery of transformation toughening published in the journal Nature by Garvie et al. [6] proclaimed new high-performance applications of zirconia, ranging from bearing and wear applications to thermal barrier coatings on metal components [7,8]. Moreover, zirconia polymorphs demonstrate excellent radiation tolerance for nuclear applications [9], such that they have been suggested as a promising parent material for use as a fuel matrix in nuclear reactors [10]. However, zirconia exhibits three ⇑ Corresponding author. Address: 128 Sec. 2, Academia Rd, Nankang, Taipei 11529, Taiwan. Tel.: +886 2 27896769; fax: +886 2 27834187. E-mail address: [email protected] (Y.C. Yu).
crystallographic polymorphs as a function of temperature at normal atmospheric pressure [11]: the monoclinic baddeleyite (m-ZrO2) is thermodynamically stable from room temperature to 1170 °C, the tetragonal phase (t-ZrO2) is thermodynamically stable from 1170 to 2370 °C, and the fluorite-type cubic phase (c-ZrO2) is thermodynamically stable from 2370 to 2680 °C (melting point). These polymorphs significantly affect the physical and mechanical properties of zirconia used as fuel or a parent material for nuclear applications. Zirconia phase transition under irradiation has been widely studied under different temperature ranges and the mass and energy of bombarding ions, and these studies have shown that polymorphism plays an important role in the radiation damage evolution of zirconia. In a series of experiments, Benyagoub et al. [12,13] found that irradiation with 135 MeV 58Ni ions, 300 MeV 76 Ge ions, and 250 MeV 127I ions generates a transformation from the monoclinic to the tetragonal phase (hereafter denoted as an m ? t transformation), where primarily electronic stopping exceeding 13 keV/nm dominates over nuclear stopping. Based on transmission electron microscopy (TEM) and Raman spectroscopy examinations, Sickafus et al. [14,15] demonstrated that m-ZrO2
http://dx.doi.org/10.1016/j.nimb.2014.02.081 0168-583X/Ó 2014 Elsevier B.V. All rights reserved.
Please cite this article in press as: R.T. Huang et al., Characterization of the irradiation-induced phase transition in the monoclinic polymorph of zirconia, Nucl. Instr. Meth. B (2014), http://dx.doi.org/10.1016/j.nimb.2014.02.081
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Fig. 1. SRIM simulation results for 100 keV H+ ion irradiation of ZrO2. (a) Energy loss as a function of depth for the nuclear and electronic stopping components of the stopping power and (b) collision events (left-hand ordinate) and implanted H+ ion concentration (right-hand ordinate) as a function of depth for the irradiation.
exposed to 340 keV Xe2+ ions at 120 K and to 300 keV Kr2+ ions at approximately 80 K (in the nuclear stopping energy regime), respectively, transformed to t-ZrO2. Valdez et al. [15] showed that 150 keV Ne+ ions irradiation of m-ZrO2 also produces m ? t transformation (in both electronic and nuclear stopping regime). Simeone et al. [16] observed a similar m ? t transformation using 800 keV Bi ions and suggested, based on grazing X-ray diffraction analysis, that the tetragonal phase is localized in the damaged area. Nanocrystalline zirconia is known to have unique mechanical, thermal, and electrical properties compared to its bulk counterparts, and the phase stability of nanosized zirconia can be affected significantly by its grain size [17]. In the considerable body of published literature on the effects of irradiation on bulk zirconia, few systematic studies of the behavior of nanocrystalline zirconia under similar conditions exist. We are aware of one report, in which an m ? t transformation was observed in the monoclinic-dominant ZrO2 with an average grain size of 40–50 nm upon the ion irradiation of 350 keV O+ and 1 MeV Kr2+ at room temperature [18]. Therefore, in this study, we focus on the evolution of pure m-ZrO2 nano particles under 100 keV protons and 3 MeV Fe2+ ions irradiation at room temperature and document an unusual behavior of ZrO2 nanoparticles under size effects in the irradiation-induced m ? t transformation under 100 keV protons irradiation.
Fig. 2. SRIM simulation results for 3 MeV Fe2+ ions irradiation of ZrO2. (a) Energy loss as a function of depth for the nuclear and electronic stopping components of the stopping power and (b) collision events (left-hand ordinate) and implanted Fe2+ ions concentration (right-hand ordinate) as a function of depth for the irradiation.
2. Material and methods Two distinct particle-size ranges of m-ZrO2 powders, approximately 20–30 nm and 30–60 nm (verified by TEM observation), purchased from CERAC Corp. (99.995% pure) were used to prepare the two sample groups to which ion bombardment is executed. First, a small amount of powder with the same size range was added to DI-water, followed by ultrasonication to disperse the particles; then a small drop of suspension was spread repeatedly on a clean silicon chip. The evenly spread specimen was finally dried using an electric hot plate at 60 °C. The thickness of the deposited powder was controlled over 15 lm. To depict the specimen with two distinct particle size ranges of m-ZrO2 powders, the prepared specimens with the smaller particle sizes of approximately 20– 30 nm and with the larger particle sizes of approximately 30– 60 nm were designated as S30 and L30 specimens, respectively. These zirconia specimens were sequentially irradiated with 100 keV H+ ions in the Nuclear Science & Technology Development Center at National Tsing Hua University using a 500 kV High Voltage Engineering Europa ion implanter and with 3 MeV Fe2+ in the Institute of Physics at the Academia Sinica using National Electrostatics Corporation 9SDH-II 3MV Tandem Accelerator. The implantation energy of 100 keV protons, based on the capability of the instrument, was chosen to achieve a relatively large electron
Please cite this article in press as: R.T. Huang et al., Characterization of the irradiation-induced phase transition in the monoclinic polymorph of zirconia, Nucl. Instr. Meth. B (2014), http://dx.doi.org/10.1016/j.nimb.2014.02.081
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3
Fig. 3. (a) XRD patterns obtained from the L30 specimen irradiated with 3 MeV Fe2+ ions to fluences ranging from 1 1014 to 1 1015 ions/cm2. (b) Narrow scan for (a). (c) XRD patterns obtained from the L30 specimen irradiated with 100 keV H+ ions to fluences ranging from 1 1015 to 1 1017 ions/cm2. (d) Narrow scan for (c). The characteristic peak of irradiation-induced t-ZrO2 did not appear in the XRD spectra recorded from the proton irradiation to the fluences even up to 1 1017 ions/cm2.
energy loss with a similar penetration depth to the Fe2+ ion implantation. The proton fluences ranged from 1 1015 to 1 1017 ions/cm2, whereas the Fe2+ ion fluences ranged from 1 1014 to 1 1015 ions/cm2. All irradiations were performed at room temperature. Fig. 1 shows the characteristics of 100 keV H+ ions implanting into m-ZrO2, obtained by the program SRIM 2008 [19] (displacement energies of 25 and 28 eV for Zr and O atoms, and a density of 5.68 gm/cm3 were used for these calculation). The estimate of energy loss partitioned into electronic and nuclear components for 100 keV protons in m-ZrO2 is displayed in Fig. 1(a). It is obvious that the value of electronic energy loss is much larger than that of nuclear energy loss. Fig. 1(b) displays the estimate of ballistic damage and implanted proton concentration as a function of depth into the target material of zirconia. It shows that a large amount of ballistic damage appeared around the depth of 550 nm, but with a small value of irradiation-induced collision events (displacement damage). Furthermore, the characteristics of 3 MeV Fe2+ ions implanting into m-ZrO2, in which the estimate of energy loss and ballistic damage and the concentration of implanted Fe2+ ions as a function of depth into the target material of zirconia, are displayed in Fig. 2(a) and (b), respectively. Moreover, to study the evolution of the microstructure, these irradiated specimens were characterized by an X-ray diffractometer (XRD, Panalytical X’Pert Pro MPD) and by transmission electron microscopy (TEM, JEOL 2010). 3. Results and discussion The L30 specimen was first irradiated separately with 100 keV protons and 3 MeV Fe2+ ions to various fluences. Fig. 3(a) shows
the XRD patterns of the L30 specimen irradiated with 3 MeV Fe2+ ions to fluences ranging from 1 1014 to 1 1015 ions/cm2. Following Fe2+ ion irradiation, the characteristic peak of irradiationinduced t-ZrO2 emerged preferentially at approximately 2h = 35.25°, but the peak did not appear in the XRD patterns recorded from the proton irradiation to the fluences even up to 1 1017 ions/cm2, as shown in Fig. 3(c) and (d). The figure also displayed the narrow scan of XRD patterns recorded from 32° to 38° to distinctly identify the characteristic peak of t-ZrO2 exhibiting at 35.25°. Considering Garvie’s finding, in which the t-phase can be found to occur at room temperature only below a critical crystallite size of approximately 30 nm, and the t-ZrO2 was characterized by a small mean crystallite size, large specific surface, and appreciable excess energy [20], the smaller particle size of mZrO2 presumably facilitates the occurrence of t-ZrO2 under proton irradiation due to a large specific surface. Accordingly, the same procedure of ion implantation was subsequently executed on the S30 specimen. Consequently, the t-phase peak induced by the proton irradiation indeed appears in the XRD pattern when the fluence is larger than 3 1015 ions/cm2 and emerges preferentially at approximately 2h = 50.37° (Fig. 4). In addition, the t-phase peak induced by Fe2+ irradiation also appears in the XRD patterns of the S30 specimen, similar to Fig. 3(a). Comparing the results of proton irradiation obtained from the two separate particle size ranges suggests that there is a size effect in the irradiation-induced m ? t transformation of zirconia under proton irradiation with 100 keV. Moreover, these irradiation-induced t-ZrO2 were further characterized by TEM examination. Fig. 5 shows the bright-field and dark-field TEM images taken from the S30 specimen irradiated with 100 keV protons to 3 1015 ions/cm2. Because t-ZrO2 (0 1 1) and (0 1 2) diffraction planes are easily distinguishable from
Please cite this article in press as: R.T. Huang et al., Characterization of the irradiation-induced phase transition in the monoclinic polymorph of zirconia, Nucl. Instr. Meth. B (2014), http://dx.doi.org/10.1016/j.nimb.2014.02.081
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Fig. 4. (a) XRD patterns obtained from the S30 specimen irradiated with 100 keV H+ ions to fluences ranging from 1 1015 to 1 1016 ions/cm2. (b) Narrow scan for (a). Fig. 5. TEM (a) bright-field and (b) dark-field images of the S30 specimen irradiated with 100 keV H+ ions to a fluence of 3 1015 ions/cm2.
m-ZrO2, the dark-field TEM image shown in Fig. 5(b) was obtained from the t-ZrO2 (0 1 1) diffraction plane and the measured size is approximately 28.9 nm. Fig. 6 displays the bright-field TEM images taken from the L30 specimen irradiated with 3 MeV Fe2+ ions to 5 1014 ions/cm2, in which the image was obtained from the ½111 diffraction zone axis of t-ZrO2 showing a single crystal feature and the measured size is approximately 47.0 nm (Fig. 6(b)). Furthermore, to evaluate the irradiation-induced t-ZrO2 particle size, the calculation of recorded XRD spectra based on Gaussian curve fitting and the Scherrer equation [21] and the measurement of the dark-field TEM images obtained from t-ZrO2 (0 1 1) and (0 1 2) diffraction planes were also executed. These evaluated values are shown in Table 1. The irradiation-induced t-ZrO2 particle size seems to approach approximately 29 nm for the irradiation of 100 keV protons used in the S30 specimen. Furthermore, irradiation-induced m ? t transformation under Fe2+ irradiation with 3 MeV may be achieved at a comparatively low fluence. Electronic energy loss released by high energy ions is generally thought to induce significant atomic rearrangement. Benyagoub et al. [13] referred to two competing models, the ‘‘Coulomb explosion’’ and the ‘‘thermal spike’’ mechanism, to account for such a phenomenon, wherein a huge amount of deposited electronic energy loss exceeding an effective threshold near 13 keV nm1 induces a transformation from the monoclinic to the tetragonal phase in zirconia. Table 2 lists the computed irradiation parameters deduced from the program SRIM2008 for all of these experiments. Although the calculated electronic energy loss energies
Fig. 6. TEM bright-field and dark-field images of the L30 specimen irradiated with 3 MeV Fe2+ ions to a fluence of 5 1014 ions/cm2.
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Table 1 Evaluation of the irradiation-induced t-ZrO2 crystallite size from the recorded XRD spectra calculation based on Gaussian curve fits and Scherrer equation and the measurement of the dark-field TEM images. Source
Sample
Energy
Fluence
m?t
XRD evaluation (nm) (I)
TEM measurement (nm) (II)
Fe2+
L30
3 MeV
1 1015 5 1014 3 1014 1 1014
Yes Yes Yes Yes
27.7 28.6 29.0 28.9
28.0–48.7 29.1–47.0 29.9–46.8 29.5–40.8
H+
S30
100 KeV
1 1016 5 1015 3 1015 1 1015
Yes Yes Yes Yes
28.5 28.8 27.8 –
26.6–29.9 26.1–29.7 24.8–29.4 –
Table 2 Computed irradiation parameters deduced from the program SRIM2008 for all these experiments. Ion species +
H H+ Fe+
Energy (MeV) 0.1 1.5 3
Ion range (lm) 0.55 16.9 0.92
are both lower than 13 keV nm1, the irradiation-induced m ? t transformation under Fe2+ irradiation with 3 MeV and under proton irradiation with 100 keV to an m-ZrO2 particle size less than 30 nm could be achieved. Upon the irradiation-induced m ? t transformation under Fe2+ irradiation with 3 MeV, the released electronic energy loss energy is transported by electronic heat conduction to a larger volume and consequently leads to electron lattice interactions which transfer heat to the lattice atoms, thereby giving rise to a ‘‘thermal spike’’ for the target atoms. However, the irradiated phase likely contains a high level of damage defects (oxygen vacancies), which helps promote the m ? t transformation by allowing for the expansion of the ZrAO distance and the increase of the Zr coordination [13,17] under low electronic energy loss. As for the irradiation-induced m ? t transformation under proton irradiation with 100 keV to an m-ZrO2 particle size of less than 30 nm, an energy crossover [20] can play a significant role in the m ? t transformation, in which the thermodynamically metastable phase in the bulk is energetically favorable as the grain size decreases below a critical value. Hence, the average sizes of the irradiation-induced t-ZrO2 particle approach approximately 29 nm, due to their metastable critical sizes after irradiation. Furthermore, Pitcher et al. [22] reported that at larger size regimes, monoclinic ZrO2 has the lowest total energy; as the size decreases to nanosized regimes, their specific surface and energy increase. Upon proton irradiation with 100 keV, the irradiation-induced damage defects are much smaller than the Fe2+ irradiation with 3 MeV. The irradiation-induced m ? t transformation is dependent mostly on the deposited electronic energy loss, but the energy loss is not enough to induce the m ? t transformation for large-size zirconia (over 30 nm). Conversely, because energy crossover helps trigger the m ? t transformation, the irradiation-induced m ? t transformation under proton irradiation with 100 keV to an mZrO2 particle size of less than 30 nm could be achieved at a fluence level of over 3 1015 ions/cm2. 4. Conclusions In summary, irradiation with 100 keV protons to a fluence of over 3 1015 ions/cm2 can induce the m ? t phase transformation in the specimen with the size range of approximately 20–30 nm, due to the energy crossover which benefits the promotion of transformation. However, the transformation does not occur in the irradiated specimen with the other size range even though the fluence is up to 1 1017 ions/cm2. Furthermore, although irradiation with
Electronic energy loss (keV nm1) 3
0.2 10 0.058 103 3.016 103
Nuclear energy loss (eV nm1) 0.352 103 0.04057 103 492.3 103
3 MeV Fe2+ ions brings about low electronic energy loss, the m ? t transformation is exhibited under the low fluence of 1 1014 ions/ cm2 because a high level of damage defects (oxygen vacancies) could be beneficial to triggering the m ? t transformation. Consequently, the largest crystallite sizes of t-ZrO2 induced by proton irradiation approach approximately 29 nm as a result of the metastable critical size of t-phase zirconia at room temperature.
Acknowledgments The authors gratefully acknowledge the support of this study by the National Science Council of Republic of China (NSC-99-2221E-019-010-MY3) and the assistance in SEM and TEM examination given by the sophisticated instrument user center of National Taiwan Ocean University.
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