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Microstructure change and deuterium permeation behavior of erbium oxide coating
Journal of Nuclear Materials 417 (2011) 1241–1244
Contents lists available at ScienceDirect
Journal of Nuclear Materials journal homepage: www.elsevier.com/locate/jnucmat
Microstructure change and deuterium permeation behavior of erbium oxide coating Takumi Chikada a,⇑, Akihiro Suzuki b, Tomohiro Kobayashi c, Hans Maier d, Takayuki Terai e,a, Takeo Muroga f a
Department of Nuclear Engineering and Management, School of Engineering, University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656, Japan Nuclear Professional School, School of Engineering, University of Tokyo, 2-22 Shirakata-shirane, Tokai, Naka, Ibaraki 319-1188, Japan c Institute of Physical and Chemical Research (RIKEN), 2-1 Hirosawa, Wako, Saitama 351-0198, Japan d Max-Planck-Institut für Plasmaphysik, EURATOM Association, Boltzmannstrasse 2, D-85748 Garching, Germany e Institute of Engineering Innovation, School of Engineering, University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656, Japan f Fusion Systems Research Division, Department of Helical Plasma Research, National Institute for Fusion Science, 322-6 Oroshi, Toki, Gifu 509-5292, Japan b
a r t i c l e
i n f o
Article history: Available online 1 January 2011
a b s t r a c t Deuterium permeation measurements and microstructure analyses on erbium oxide coating deposited by filtered arc deposition have been investigated. It is found that the permeation suppression efficiency is proportional to the coating thickness in case of the coating without pores and cracks reaching to the substrate. A phase transformation of the coating during the permeation measurements at 773–973 K has been identified. A grain growth of the coating has occurred during the measurements and permeation suppression efficiency have enhanced according to the grain size. It is suggested that deuterium permeation through the coating is mainly dominated by the crystal grain diffusion. Ó 2011 Elsevier B.V. All rights reserved.
1. Introduction
2. Experimental
An establishment of efficient tritium handling is a key technology for designing fusion power plants. Tritium permeation barrier (TPB) is considered to be essential to reduce loss of fuel and environmental impacts to acceptable levels at fusion blanket systems [1]. In recent decades, studies on thin ceramic coatings have been performed and demonstrated reduction of hydrogen-isotope permeability [2–14]. However, research issues still remain and one of the most important investigations is to clarify tritium permeation mechanism in the coatings. For understanding a precise permeation behavior, both permeation measurements and microstructural analyses of the coatings are significant. Erbium oxide (erbia, Er2O3) is a promising candidate for TPB material because of its high stability under strong reducing atmosphere as well as its capability to suppress hydrogen-isotope permeation [15,16]. In this paper, we focused on deuterium permeation behavior with a microstructure change of the Er2O3 coating fabricated using a physical vapor deposition (PVD) technique. The goal of our study is to reveal the hydrogen-isotope permeation mechanism through the coating deposited under a proper condition which gives a high performance as TPB.
2.1. Deposition of Er2O3 coatings Reduced activation ferritic/martensitic steel F82H (8Cr–2W, Heat No. 9753 42W-4) and JLF-1 (9Cr–2W) were used as substrate materials. Since they showed quite similar deuterium permeation behavior in the previous study [16], two kinds of the substrates were used in the same way. Square plates of 25 mm on a side and 0.5 mm in thickness were mirror polished. Er2O3 coatings were fabricated by a filtered arc deposition device described in Refs. [15,17]. Briefly to describe the scheme, an arc discharge on an erbium metal cathode (99.9% Er) with filtering plasma from metal droplets and introducing oxygen into the main chamber forms an Er2O3 film. The coating was deposited with applying RF-induced bias voltages of 150 V to enhance crystallinity [17]. Deposition time was 5–40 min corresponding 0.3–2.6 lm in thickness of the coating. The substrate was pre-cleaned by argon sputtering before deposition. Substrate temperature during deposition was at room temperature since an oxide layer of the substrate had formed by heating causes peeling of the coating [18]. Furthermore, the oxide layer may influence hydrogen permeation behavior and therefore a precise permeation mechanism in the Er2O3 coating becomes difficult to be clarified.
2.2. Characterization of the coatings ⇑ Corresponding author. Tel./fax: +81 3 5841 7420. E-mail addresses: [email protected] (T. Chikada), [email protected] (A. Suzuki). 0022-3115/$ - see front matter Ó 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.jnucmat.2010.12.283
The surface of the coating was examined using a laser microscope. It was the first step to look whether pores and cracks
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T. Chikada et al. / Journal of Nuclear Materials 417 (2011) 1241–1244
reaching to the substrate existed or not for ascertaining the hydrogen permeation mechanism in the dense Er2O3 coating. The crystal structure of the surface which characterizes a crystal phase of the coating was analyzed by X-ray diffraction (XRD). The cross-section of the coating was examined by scanning transmission electron microscopy (STEM). STEM observation was also helpful to evaluate the grain size of the coating by measuring the distance between grain boundaries observed in the coatings. Transmission electron microscopy (TEM) mode was also available for high-resolution observation when the grain size of the coating was too small to determine by STEM mode. The STEM and TEM specimens were prepared by utilizing the focused ion beam (FIB) system. 2.3. Deuterium permeation measurement The deuterium permeation setup applying in this study and the experimental procedure is described in detail in [16]. Two parts of the apparatus divided by a sample were separately evacuated up to about 106 Pa, and then deuterium (purity of 99.995%) was introduced into one side (called upstream) at 104–105 Pa with a leak valve. The coated sample was mounted with the coating facing the upstream for avoiding surface oxidation on uncoated side of the sample by contamination molecules [16]. Measuring partial pressure of deuterium permeating through the sample to another side (called downstream) was performed by quadrupole mass spectrometer (QMS). The downstream chamber was continuously pumped during the experiments and the QMS worked within the range of 1010–105 mol/m2 s for D2 in this method with secondary electron multiplier. Normally, one measurement took approximately for 3 h in total under one temperature condition. When diffusion limits gas permeation through a sample rather than surface reactions, the permeation flux is typically represented by following equation [19]:
Fig. 1. Deuterium permeation flux through the coated samples with different coating thicknesses as a function of the driving pressure. The test temperature is at 773 K. Numbers in parentheses represent the deposition time.
J ¼ Pp0:5=d where J is the permeation flux, P is named permeability which is intrinsic parameter for the sample, p is the driving pressure, and d is the thickness of the sample.
3. Results and discussion 3.1. Thickness dependence Permeation measurements on the coatings with different thicknesses were performed. Deuterium permeation fluxes of the samples are shown in Fig. 1. The permeation flux of the coating of 0.3 lm in thickness is approximately 40 times more than that of 1.3 lm. On the other hand, the flux of the coating of 1.3 lm is twice more than that of 2.6 lm. If the coatings with different thicknesses are perfectly uniform, the permeation flux shall vary inversely with the coating thickness. Therefore, the proportional relation between the coatings of 1.3 and 2.6 lm proves their uniformity. Besides, the sample of 0.3 lm coating has the possibility that defects or microstructural uniformity have played roles as shortcuts for hydrogen permeation. Fig. 2 shows the surface images examined by laser microscopy. Some pores reaching to the substrate are observed on the coating of 0.3 lm in thickness and no pores in that of 1.3 lm. It is expected that deuterium molecules have passed through the pores and diffused into the substrate. When it is presumed the sample of 1.3 lm is perfectly covered with the Er2O3 coating, the fraction of surface coverage of the sample of 0.3 lm can be approximately calculated to be 96.4%. This is considered a reasonable value from the surface image. Consequently, it is important to ensure a proper
Fig. 2. Surface images of the coatings of (a) 0.3 and (b) 1.3 lm in thickness by laser microscopy. The pores penetrating through the coating are seen in (a). Black dots seen in both samples derive from polishing process of the substrates.
thickness of the coating to clarify accurate permeation behavior of hydrogen isotopes when deposited by the PVD method. 3.2. Crystal structure transformation Fig. 3 indicates XRD patterns of the samples with 1.3 lm coating before and after permeation measurements at different maximum temperatures. The maximum temperature has been kept for about 3 h to perform a set of the measurements. The sample deposited at room temperature shows peaks around at 29.7, 30.0,
T. Chikada et al. / Journal of Nuclear Materials 417 (2011) 1241–1244
1243
Fig. 3. XRD spectra of the coatings of 1.3 lm before and after permeation measurements at up to 773, 873, and 973 K.
31.5, and 32.2° derived from the monoclinic B-phase Er2O3. On the other hand, the peaks at 29.3 and 31.7° seen in the spectra after measurements represent the cubic C-phase Er2O3. Recently phase transformations of Er2O3 coatings have been investigated and it is suggested that ion bombardment and high internal pressure during particle nucleation induce formation of the B-phase Er2O3 [20– 22]. In this analysis, it is proved that the Er2O3 coating deposited at room temperature starts transformation from the B-phase to the cubic C-phase during the permeation measurements. Moreover, the phase transformation has mainly taken place after the measurement at 873 K and completed at 973 K. 3.3. Deuterium permeation The permeation measurements have been performed on the coating of 1.3 lm in thickness on the substrate. In this set of measurements the sample was not heated more than 523 K before the first measurement. Fig. 4 shows the time-dependent change of the deuterium permeation flux of the coating at 773 and 873 K by repeating tests under the same driving pressures. At 773 K, the permeation flux dropped to 25% from the first measurement to the third when the flux became stable. From the viewpoint of TPB, a reducing factor increased from 200 to 800. At 873 K, the flux dropped again to 60% from the first measurement to the second, and then became stable. No clear decrease of the flux was observed on the measurement at 973 K. It is not believed that these rapid decreases of the flux in a few hours have been caused by surface oxidation of the uncoated side facing to the downstream because impurities in the downstream is quite low and stable during the measurements. Also following the result explained in Section 3.2., the decrease of the flux is considered to have been caused by microstructural transformation of the Er2O3 coating. 3.4. Grain growth The cross-section STEM images of the coatings before and after the permeation measurements at up to 773, 873, and 973 K are represented in Fig. 5. The grain sizes of the coatings after tested
Fig. 4. Temporal change in the deuterium permeation flux of the coating of 1.3 lm on the JLF-1 substrate at 773 and 873 K. The driving pressure corresponding to the rises of the permeation flux has been introduced in sequence: 1.00 104, 2.00 104, 4.00 104, and 8.00 104 Pa.
at each temperature were compared by measuring horizontal distances between the grain boundaries seen in the images. Regarding the coating as deposited, the grain size was estimated by TEM images because it was difficult to measure by the STEM images. The average values of the grain size of the coating as deposited, after the measurements at 773, 873, and 973 K are 20, 90, 250, and 280 nm, respectively. Considering the result that the phase transformation from the B-phase to the C-phase has mainly taken place at 873 K, it is found that grain growth of the Er2O3 coating has induced to enhance deuterium suppression corresponding to the grain size of the coating. The grain growth means a decrease of the grain boundary area in the coating. Therefore, this result also indicates that deuterium transfer through the coating is dominated by grain boundary diffusion rather than lattice diffusion. Additionally, in present study, the influence of the phase transformation is considered to be relatively smaller than that of the grain growth.
4. Summary The permeation mechanism of the Er2O3 coatings deposited at room temperature by filtered arc deposition has been investigated by the deuterium permeation measurements and the microstructural analyses. The coating of 0.3 lm in thickness indicated imperfect coverage and the coatings of 1.3 and 2.6 lm in thickness showed proportional relation in the efficiency of the permeation suppression. The crystal phase of the coatings has transformed from the B-phase to the C-phase during the permeation measurements at 773–973 K. The average grain size of the coating has
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T. Chikada et al. / Journal of Nuclear Materials 417 (2011) 1241–1244
Fig. 5. Bright-field STEM cross-sectional images of the Er2O3 coatings deposited at room temperature: (a) as deposited, after the permeation measurements at (b) 773 K, (c) 873 K, and (d) 973 K.
increased after the permeation measurements at up to 973 K from 20 to 280 nm. The grain growth caused the enhancement of the suppression property and that indicates deuterium permeation in the coating is dominated by crystal grain diffusion. Acknowledgements This work was supported in part by KAKENHI (19055001), Ministry of Education, Culture, Sports, Science and Technology, Japan and Research Fellowships of the Japan Society for the Promotion of Science for Young Scientists. References [1] V.A. Maroni, E.H. Van Deventer, J. Nucl. Mater. 85&86 (1979) 257–269. [2] G.W. Hollenberg, E.P. Simonen, G. Kalinin, A. Terlain, Fusion Eng. Des. 28 (1995) 190–208. [3] A. Perujo, K.S. Forcey, Fusion Eng. Des. 28 (1995) 252–257. [4] A. Perujo, E. Serra, H. Kolbe, T. Sample, J. Nucl. Mater. 233–237 (1996) 1102– 1106. [5] B.A. Kalin, V.L. Yakushin, E.P. Fomina, Fusion Eng. Des. 41 (1998) 119–127. [6] E. Serra, P.J. Kelly, D.K. Ross, R.D. Arnell, J. Nucl. Mater. 257 (1998) 194–198. [7] G. Benamati, C. Chabrol, A. Perujo, E. Rigal, H. Glasbrenner, J. Nucl. Mater. 271&272 (1999) 391–395.
[8] A. Aiello, G. Benamati, M. Chini, C. Fazio, E. Serra, Z. Yao, Fusion Eng. Des. 58–59 (2001) 737–742. [9] T. Sample, A. Perujo, H. Kolbe, B. Mancinelli, J. Nucl. Mater. 283–287 (2000) 1272–1276. [10] A.J. Magielsen, K. Bakker, C. Chabrol, R. Conrad, J.G. van der Laan, E. Rigal, M.P. Stijkel, J. Nucl. Mater. 307–311 (2002) 832–836. [11] A. Aiello, I. Ricapito, G. Benamati, A. Ciampichetti, Fusion Eng. Des. 69 (2003) 245–252. [12] J. Konys, A. Aiello, G. Benamati, L. Giancarli, Fusion Sci. Technol. 47 (2005) 844– 850. [13] T.V. Kulsartov, K. Hayashi, M. Nakamichi, S.E. Afanasyev, V.P. Shestakov, Y.V. Chikhray, E.A. Kenzhin, A.N. Kolbaenkov, Fusion Eng. Des. 81 (2006) 701–705. [14] M. Nakamichi, T.V. Kulsartov, K. Hayashi, S.E. Afanasyev, V.P. Shestakov, Y.V. Chikhray, E.A. Kenzhin, A.N. Kolbaenkov, Fusion Eng. Des. 82 (2007) 2246– 2251. [15] D. Levchuk, S. Levchuk, H. Maier, H. Bolt, A. Suzuki, J. Nucl. Mater. 367–370 (2007) 1033–1037. [16] T. Chikada, A. Suzuki, Z. Yao, D. Levchuk, H. Maier, T. Terai, T. Muroga, Fusion Eng. Des. 84 (2009) 590–592. [17] F. Koch, R. Brill, H. Maier, D. Levchuk, A. Suzuki, T. Muroga, H. Bolt, J. Nucl. Mater. 329–333 (2004) 1403–1406. [18] T. Chikada, A. Suzuki, T. Kobayashi, Z. Yao, D. Levchuk, H. Maier, T. Terai, T. Muroga, Fusion Sci. Technol. 56 (2009) 309–313. [19] K.S. Forcey, D.K. Ross, J.C.B. Simpson, J. Nucl. Mater. 160 (1988) 117–124. [20] M. Tang, P. Lu, J.A. Valdez, K.E. Sickafus, J. Appl. Phys. 99 (2006) 063514. [21] Q. Guo, Y. Zhao, C. Jiang, W.L. Mao, Z. Wang, J. Zhang, Y. Wang, Inorg. Chem. 46 (2007) 6164–6169. [22] C. Adelhelm, T. Pickert, M. Balden, M. Rasinski, T. Plocinski, C. Ziebert, F. Koch, H. Maier, Scr. Mater. 61 (2009) 789–792.
Contents lists available at ScienceDirect
Journal of Nuclear Materials journal homepage: www.elsevier.com/locate/jnucmat
Microstructure change and deuterium permeation behavior of erbium oxide coating Takumi Chikada a,⇑, Akihiro Suzuki b, Tomohiro Kobayashi c, Hans Maier d, Takayuki Terai e,a, Takeo Muroga f a
Department of Nuclear Engineering and Management, School of Engineering, University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656, Japan Nuclear Professional School, School of Engineering, University of Tokyo, 2-22 Shirakata-shirane, Tokai, Naka, Ibaraki 319-1188, Japan c Institute of Physical and Chemical Research (RIKEN), 2-1 Hirosawa, Wako, Saitama 351-0198, Japan d Max-Planck-Institut für Plasmaphysik, EURATOM Association, Boltzmannstrasse 2, D-85748 Garching, Germany e Institute of Engineering Innovation, School of Engineering, University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656, Japan f Fusion Systems Research Division, Department of Helical Plasma Research, National Institute for Fusion Science, 322-6 Oroshi, Toki, Gifu 509-5292, Japan b
a r t i c l e
i n f o
Article history: Available online 1 January 2011
a b s t r a c t Deuterium permeation measurements and microstructure analyses on erbium oxide coating deposited by filtered arc deposition have been investigated. It is found that the permeation suppression efficiency is proportional to the coating thickness in case of the coating without pores and cracks reaching to the substrate. A phase transformation of the coating during the permeation measurements at 773–973 K has been identified. A grain growth of the coating has occurred during the measurements and permeation suppression efficiency have enhanced according to the grain size. It is suggested that deuterium permeation through the coating is mainly dominated by the crystal grain diffusion. Ó 2011 Elsevier B.V. All rights reserved.
1. Introduction
2. Experimental
An establishment of efficient tritium handling is a key technology for designing fusion power plants. Tritium permeation barrier (TPB) is considered to be essential to reduce loss of fuel and environmental impacts to acceptable levels at fusion blanket systems [1]. In recent decades, studies on thin ceramic coatings have been performed and demonstrated reduction of hydrogen-isotope permeability [2–14]. However, research issues still remain and one of the most important investigations is to clarify tritium permeation mechanism in the coatings. For understanding a precise permeation behavior, both permeation measurements and microstructural analyses of the coatings are significant. Erbium oxide (erbia, Er2O3) is a promising candidate for TPB material because of its high stability under strong reducing atmosphere as well as its capability to suppress hydrogen-isotope permeation [15,16]. In this paper, we focused on deuterium permeation behavior with a microstructure change of the Er2O3 coating fabricated using a physical vapor deposition (PVD) technique. The goal of our study is to reveal the hydrogen-isotope permeation mechanism through the coating deposited under a proper condition which gives a high performance as TPB.
2.1. Deposition of Er2O3 coatings Reduced activation ferritic/martensitic steel F82H (8Cr–2W, Heat No. 9753 42W-4) and JLF-1 (9Cr–2W) were used as substrate materials. Since they showed quite similar deuterium permeation behavior in the previous study [16], two kinds of the substrates were used in the same way. Square plates of 25 mm on a side and 0.5 mm in thickness were mirror polished. Er2O3 coatings were fabricated by a filtered arc deposition device described in Refs. [15,17]. Briefly to describe the scheme, an arc discharge on an erbium metal cathode (99.9% Er) with filtering plasma from metal droplets and introducing oxygen into the main chamber forms an Er2O3 film. The coating was deposited with applying RF-induced bias voltages of 150 V to enhance crystallinity [17]. Deposition time was 5–40 min corresponding 0.3–2.6 lm in thickness of the coating. The substrate was pre-cleaned by argon sputtering before deposition. Substrate temperature during deposition was at room temperature since an oxide layer of the substrate had formed by heating causes peeling of the coating [18]. Furthermore, the oxide layer may influence hydrogen permeation behavior and therefore a precise permeation mechanism in the Er2O3 coating becomes difficult to be clarified.
2.2. Characterization of the coatings ⇑ Corresponding author. Tel./fax: +81 3 5841 7420. E-mail addresses: [email protected] (T. Chikada), [email protected] (A. Suzuki). 0022-3115/$ - see front matter Ó 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.jnucmat.2010.12.283
The surface of the coating was examined using a laser microscope. It was the first step to look whether pores and cracks
1242
T. Chikada et al. / Journal of Nuclear Materials 417 (2011) 1241–1244
reaching to the substrate existed or not for ascertaining the hydrogen permeation mechanism in the dense Er2O3 coating. The crystal structure of the surface which characterizes a crystal phase of the coating was analyzed by X-ray diffraction (XRD). The cross-section of the coating was examined by scanning transmission electron microscopy (STEM). STEM observation was also helpful to evaluate the grain size of the coating by measuring the distance between grain boundaries observed in the coatings. Transmission electron microscopy (TEM) mode was also available for high-resolution observation when the grain size of the coating was too small to determine by STEM mode. The STEM and TEM specimens were prepared by utilizing the focused ion beam (FIB) system. 2.3. Deuterium permeation measurement The deuterium permeation setup applying in this study and the experimental procedure is described in detail in [16]. Two parts of the apparatus divided by a sample were separately evacuated up to about 106 Pa, and then deuterium (purity of 99.995%) was introduced into one side (called upstream) at 104–105 Pa with a leak valve. The coated sample was mounted with the coating facing the upstream for avoiding surface oxidation on uncoated side of the sample by contamination molecules [16]. Measuring partial pressure of deuterium permeating through the sample to another side (called downstream) was performed by quadrupole mass spectrometer (QMS). The downstream chamber was continuously pumped during the experiments and the QMS worked within the range of 1010–105 mol/m2 s for D2 in this method with secondary electron multiplier. Normally, one measurement took approximately for 3 h in total under one temperature condition. When diffusion limits gas permeation through a sample rather than surface reactions, the permeation flux is typically represented by following equation [19]:
Fig. 1. Deuterium permeation flux through the coated samples with different coating thicknesses as a function of the driving pressure. The test temperature is at 773 K. Numbers in parentheses represent the deposition time.
J ¼ Pp0:5=d where J is the permeation flux, P is named permeability which is intrinsic parameter for the sample, p is the driving pressure, and d is the thickness of the sample.
3. Results and discussion 3.1. Thickness dependence Permeation measurements on the coatings with different thicknesses were performed. Deuterium permeation fluxes of the samples are shown in Fig. 1. The permeation flux of the coating of 0.3 lm in thickness is approximately 40 times more than that of 1.3 lm. On the other hand, the flux of the coating of 1.3 lm is twice more than that of 2.6 lm. If the coatings with different thicknesses are perfectly uniform, the permeation flux shall vary inversely with the coating thickness. Therefore, the proportional relation between the coatings of 1.3 and 2.6 lm proves their uniformity. Besides, the sample of 0.3 lm coating has the possibility that defects or microstructural uniformity have played roles as shortcuts for hydrogen permeation. Fig. 2 shows the surface images examined by laser microscopy. Some pores reaching to the substrate are observed on the coating of 0.3 lm in thickness and no pores in that of 1.3 lm. It is expected that deuterium molecules have passed through the pores and diffused into the substrate. When it is presumed the sample of 1.3 lm is perfectly covered with the Er2O3 coating, the fraction of surface coverage of the sample of 0.3 lm can be approximately calculated to be 96.4%. This is considered a reasonable value from the surface image. Consequently, it is important to ensure a proper
Fig. 2. Surface images of the coatings of (a) 0.3 and (b) 1.3 lm in thickness by laser microscopy. The pores penetrating through the coating are seen in (a). Black dots seen in both samples derive from polishing process of the substrates.
thickness of the coating to clarify accurate permeation behavior of hydrogen isotopes when deposited by the PVD method. 3.2. Crystal structure transformation Fig. 3 indicates XRD patterns of the samples with 1.3 lm coating before and after permeation measurements at different maximum temperatures. The maximum temperature has been kept for about 3 h to perform a set of the measurements. The sample deposited at room temperature shows peaks around at 29.7, 30.0,
T. Chikada et al. / Journal of Nuclear Materials 417 (2011) 1241–1244
1243
Fig. 3. XRD spectra of the coatings of 1.3 lm before and after permeation measurements at up to 773, 873, and 973 K.
31.5, and 32.2° derived from the monoclinic B-phase Er2O3. On the other hand, the peaks at 29.3 and 31.7° seen in the spectra after measurements represent the cubic C-phase Er2O3. Recently phase transformations of Er2O3 coatings have been investigated and it is suggested that ion bombardment and high internal pressure during particle nucleation induce formation of the B-phase Er2O3 [20– 22]. In this analysis, it is proved that the Er2O3 coating deposited at room temperature starts transformation from the B-phase to the cubic C-phase during the permeation measurements. Moreover, the phase transformation has mainly taken place after the measurement at 873 K and completed at 973 K. 3.3. Deuterium permeation The permeation measurements have been performed on the coating of 1.3 lm in thickness on the substrate. In this set of measurements the sample was not heated more than 523 K before the first measurement. Fig. 4 shows the time-dependent change of the deuterium permeation flux of the coating at 773 and 873 K by repeating tests under the same driving pressures. At 773 K, the permeation flux dropped to 25% from the first measurement to the third when the flux became stable. From the viewpoint of TPB, a reducing factor increased from 200 to 800. At 873 K, the flux dropped again to 60% from the first measurement to the second, and then became stable. No clear decrease of the flux was observed on the measurement at 973 K. It is not believed that these rapid decreases of the flux in a few hours have been caused by surface oxidation of the uncoated side facing to the downstream because impurities in the downstream is quite low and stable during the measurements. Also following the result explained in Section 3.2., the decrease of the flux is considered to have been caused by microstructural transformation of the Er2O3 coating. 3.4. Grain growth The cross-section STEM images of the coatings before and after the permeation measurements at up to 773, 873, and 973 K are represented in Fig. 5. The grain sizes of the coatings after tested
Fig. 4. Temporal change in the deuterium permeation flux of the coating of 1.3 lm on the JLF-1 substrate at 773 and 873 K. The driving pressure corresponding to the rises of the permeation flux has been introduced in sequence: 1.00 104, 2.00 104, 4.00 104, and 8.00 104 Pa.
at each temperature were compared by measuring horizontal distances between the grain boundaries seen in the images. Regarding the coating as deposited, the grain size was estimated by TEM images because it was difficult to measure by the STEM images. The average values of the grain size of the coating as deposited, after the measurements at 773, 873, and 973 K are 20, 90, 250, and 280 nm, respectively. Considering the result that the phase transformation from the B-phase to the C-phase has mainly taken place at 873 K, it is found that grain growth of the Er2O3 coating has induced to enhance deuterium suppression corresponding to the grain size of the coating. The grain growth means a decrease of the grain boundary area in the coating. Therefore, this result also indicates that deuterium transfer through the coating is dominated by grain boundary diffusion rather than lattice diffusion. Additionally, in present study, the influence of the phase transformation is considered to be relatively smaller than that of the grain growth.
4. Summary The permeation mechanism of the Er2O3 coatings deposited at room temperature by filtered arc deposition has been investigated by the deuterium permeation measurements and the microstructural analyses. The coating of 0.3 lm in thickness indicated imperfect coverage and the coatings of 1.3 and 2.6 lm in thickness showed proportional relation in the efficiency of the permeation suppression. The crystal phase of the coatings has transformed from the B-phase to the C-phase during the permeation measurements at 773–973 K. The average grain size of the coating has
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T. Chikada et al. / Journal of Nuclear Materials 417 (2011) 1241–1244
Fig. 5. Bright-field STEM cross-sectional images of the Er2O3 coatings deposited at room temperature: (a) as deposited, after the permeation measurements at (b) 773 K, (c) 873 K, and (d) 973 K.
increased after the permeation measurements at up to 973 K from 20 to 280 nm. The grain growth caused the enhancement of the suppression property and that indicates deuterium permeation in the coating is dominated by crystal grain diffusion. Acknowledgements This work was supported in part by KAKENHI (19055001), Ministry of Education, Culture, Sports, Science and Technology, Japan and Research Fellowships of the Japan Society for the Promotion of Science for Young Scientists. References [1] V.A. Maroni, E.H. Van Deventer, J. Nucl. Mater. 85&86 (1979) 257–269. [2] G.W. Hollenberg, E.P. Simonen, G. Kalinin, A. Terlain, Fusion Eng. Des. 28 (1995) 190–208. [3] A. Perujo, K.S. Forcey, Fusion Eng. Des. 28 (1995) 252–257. [4] A. Perujo, E. Serra, H. Kolbe, T. Sample, J. Nucl. Mater. 233–237 (1996) 1102– 1106. [5] B.A. Kalin, V.L. Yakushin, E.P. Fomina, Fusion Eng. Des. 41 (1998) 119–127. [6] E. Serra, P.J. Kelly, D.K. Ross, R.D. Arnell, J. Nucl. Mater. 257 (1998) 194–198. [7] G. Benamati, C. Chabrol, A. Perujo, E. Rigal, H. Glasbrenner, J. Nucl. Mater. 271&272 (1999) 391–395.
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