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Fabrication technology development and characterization of tritium permeation barriers by a liquid phase method
Fusion Engineering and Design xxx (xxxx) xxx–xxx
Contents lists available at ScienceDirect
Fusion Engineering and Design journal homepage: www.elsevier.com/locate/fusengdes
Fabrication technology development and characterization of tritium permeation barriers by a liquid phase method ⁎
Takumi Chikadaa, , Moeki Matsunagaa, Seira Horikoshia, Jumpei Mochizukia, Hikari Fujitab, ⁎ Yoshimitsu Hishinumac, Kanetsugu Isobed, Takumi Hayashid, , Takayuki Teraib, Yasuhisa Oyaa a
Shizuoka Uniersity, Shizuoka, Japan The University of Tokyo, Tokyo, Japan c National Institute for Fusion Science, Toki, Japan d National Institutes for Quantum and Radiological Science and Technology, Tokai, Japan b
A R T I C L E I N F O
A B S T R A C T
Keywords: Tritium permeation Ceramic coating Lithium-lead Corrosion
Tritium permeation through structural materials is one of critical issues in liquid lithium-lead blanket concepts from the viewpoints of an efficient fuel cycle and radiological safety. Metal oxide coatings have been investigated as tritium permeation barrier and showed high permeation reduction factors. For the application to DEMO reactors, however, corrosion of the coatings by blanket materials is an unavoidable concern. This paper focuses on preparation of three metal oxide, erbium oxide, yttrium oxide, and zirconium oxide coatings by a liquid phase method and comparison of their properties in terms of hydrogen isotope permeability as well as lithium-lead compatibility. The deuterium permeation behavior of the erbium oxide and yttrium oxide coatings was similar, while the zirconium oxide showed a decrease of the permeation flux by further crystallization at lower temperature than the others. The zirconium oxide coating showed the best lithium-lead compatibility among three oxides at up to 600 °C. Deterioration of the coatings after static lithium-lead exposure would be caused by delamination and corrosion. Delamination of the coating would be prevented to control the coatingsubstrate interface. Corrosion of the coatings by formation of ternary oxides or reduction will be the main issue in lithium-lead compatibility at high temperatures.
1. Introduction Tritium permeation through structural materials is one of critical issues in liquid lithium-lead (Li-Pb) blanket concepts from the viewpoints of an efficient fuel cycle and radiological safety. In recent years metal oxide coatings have been investigated as tritium permeation barrier and showed sufficient permeation reduction factors (PRFs, 103–105 at > 500 °C) [1,2]. For the application to DEMO reactors, however, corrosion of the coatings by blanket materials is an unavoidable concern. In the framework of R&D activities of tritium technologies on Broader Approach, this work focuses on preparation of three metal oxide coatings, erbium oxide (Er2O3), yttrium oxide (Y2O3) and zirconium oxide (ZrO2) coatings, by a liquid phase method and comparison of their properties in terms of hydrogen isotope permeability as well as Li-Pb compatibility. Er2O3 and Y2O3 coatings fabricated by both gas and liquid phase methods showed PRFs of more than 103 [3–6]. ZrO2 coating was fabricated by a sol-gel method with electrolytic deposition and also showed a PRF of more than 103 [7].
⁎
Compared with Er2O3 coatings which have been investigated the most in this decade, Y2O3 has a better radioactivation property, and ZrO2 has a coefficient of thermal expansion similar to reduced activation ferritic/ martensitic steels. The goal of this study is to verify all the coating materials reduce hydrogen isotope permeation to a similar extent with the same coating process and investigate the difference in Li-Pb compatibility attributed to reactivity of different chemical compositions. 2. Experimental details 2.1. Coating preparation Reduced activation ferritic/martensitic steel F82H plates (25 × 25 × 0.5 mm3) were mirror-polished on both sides to use as substrates. Er2O3, Y2O3 and ZrO2 coatings were prepared by metal organic decomposition (MOD). Commercially available organic liquids (Kojundo Chemical Laboratory Co., Ltd., Er-03® for Er2O3, Y-03® for Y2O3, and SYM-ZR04® for ZrO2) were used as coating precursors
Corresponding author. E-mail address: [email protected] (T. Chikada).
https://doi.org/10.1016/j.fusengdes.2018.01.054 Received 29 September 2017; Received in revised form 14 January 2018; Accepted 18 January 2018 0920-3796/ © 2018 Elsevier B.V. All rights reserved.
Please cite this article as: Chikada, T., Fusion Engineering and Design (2018), https://doi.org/10.1016/j.fusengdes.2018.01.054
Fusion Engineering and Design xxx (xxxx) xxx–xxx
T. Chikada et al.
without addition of thinner. The preparation parameters were optimized by reference to our previous paper on Y2O3 coatings [8]. The conditions of dipping, drying and pre-heat were same as before. The heat-treatment equipment includes an electric furnace with gas flow controllers connected to gas cylinders. In this system, argon, hydrogen and oxygen of 0–100 sccm (standard cubic centimeter per minute) can be introduced into a quartz tube during heat-treatment. The optimized gas flow is argon and hydrogen with each flow rate of 50 sccm. Er2O3 and Y2O3 coatings were heat-treated at 700 °C, while ZrO2 coatings were at 650 °C due to the formation of visible cracks at 700 °C. The thicknesses of the coatings were 200–300 nm. 2.2. Deuterium permeation measurement Deuterium permeation flux through the sample was measured by a gas-driven deuterium permeation apparatus described elsewhere [3]. The procedure of the permeation experiments was followed by Ref. [4]. Before mounting to the sample holder, the backside of the sample was polished with abrasive papers to remove an oxide layer. The test temperature was set at 400–600 °C, and the measurements were conducted from lower temperature. The driving pressure of deuterium to the upstream was set to 1.00 × 104–8.00 × 104 Pa. The deuterium permeation flux J through a sample with a thickness of d is expressed by the following equation [9]:
J=P
pn d
Fig. 2. Temperature dependence of deuterium permeation flux for Er2O3 and ZrO2 coatings prepared by MOD. Data of uncoated F82H substrate and Y2O3 coating are also presented [6].
concentration in Li-Pb was not controlled. After immersion, the samples were picked up from liquid Li-Pb and then cleansed with a mixture of acetic acid and ethanol. Surfaces of the immersed samples were observed using a field emission scanning electron microscope (FE-SEM, JSM-7100F, JEOL Ltd.) located at National Institute for Fusion Science (NIFS), Japan. Elemental analysis was also examined by energy dispersive X-ray spectroscopy (EDX).
(1)
where P is the permeability and p is the driving pressure. The exponent n can be experimentally estimated with a fitting curve obtained by the permeation fluxes at several driving pressures. When the rate-limiting process is atomic diffusion of deuterium through the solid (diffusion limited regime), the exponent n indicates 0.5. On the other hand, when the rate-limiting process is molecular processes such as molecular diffusion, deuterium adsorption or recombination at the surface (surface limited regime), the exponent n shows unity. In this study, the permeation fluxes of the coatings were compared at the driving pressure of 8.00 × 104 Pa, which is the highest pressure and then has the smallest effect on surface reactions. For the evaluation of TPB efficiency, a permeation reduction factor (PRF) was calculated dividing the permeation flux of a bare substrate by that of a coated one.
3. Results Deuterium permeation fluxes of Er2O3 and ZrO2 coatings are shown in Fig. 2. Fitting lines from permeation data of the F82H substrate and the Y2O3 coated sample were also presented [6]. Two lines for the Y2O3 coating indicate that the trend of permeation flux changed on the measurement at 600 °C due to the further crystallization of the coating. The Er2O3 coated sample showed a trend similar to the Y2O3, although the permeation flux at 600 °C was an order of magnitude lower, and the PRF reached 2900. Another remark is that the pressure exponent changed from 0.63–0.76 to 0.50 after the decrease of permeation flux, which is also a similar trend. The permeation flux of the ZrO2 coated sample decreased at 500 °C and became comparable with the fitting line after further crystallization. On the measurement at 600 °C, however, the permeation flux discontinuously increased, indicating deterioration of the coating. Surface SEM micrographs of the Er2O3, Y2O3 and ZrO2 coatings after Li-Pb exposure are shown in Figs. 3–5, respectively. Approximately 30% of the coated area was lost for the Er2O3 coating after exposure at 400 °C for 100 h, and more than 50% at 500 °C for 100 h. The atomic ratio of Er/Fe on the coatings detected by EDX was 0.20 before exposure and 0.12–0.22 after exposure at 500 °C for 100 h, indicating partial loss of Er by Li-Pb exposure. Er2O3 was not detected on the whole sample surface after exposure at more than 550 °C for 100 h. The Y2O3 coating lost its coated area by more than 50% after exposure at 400 °C for 100 h, and more than 90% at 500 °C; therefore, the immersion tests at higher temperatures and longer duration were not conducted. The ZrO2 coating showed a clearly different behavior. More than 90% of the coated area remained after exposure up to at 500 °C for 500 h, and approximately 70% remained at 600 °C for 100 h. The atomic ratio of Zr/Fe detected by EDX was 0.29 before and after exposure at 500 °C for 500 h, and 0.16 after exposure at 600 °C for 100 h.
2.3. Li-Pb compatibility test The coated samples were cut into small specimens (approximately 10 × 10 × 0.5 mm3) and winded by an iron (Fe) wire to easily soak in and pick up from liquid Li-Pb. The experimental arrangement for static Li-Pb exposure tests is shown in Fig. 1. Li-Pb was synthesized from 99.9% Li and 99.999% Pb with the atomic rate of 15.7:84.3 under argon atmosphere in a glove box. Li-Pb of approximately 20 cm3 was used for each test. A sample soaked in Li-Pb was encapsulated in a stainless steel holder, and then was set in a muffle furnace. The exposure temperature and time were at 400–600 °C and 100–500 h, respectively. An oxygen monitor set in the glove box indicated less than 0.1% during synthesizing and loading Li-Pb, although oxygen
4. Discussion Fig. 1. Sample arrangement for Li-Pb immersion test.
The ZrO2 coating showed a different deuterium permeation 2
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Fig. 3. Surface SEM micrographs of Er2O3 coating samples: (a) before Li-Pb exposure, (b) after exposure at 400 °C, and (c) 500 °C for 100 h.
Fig. 4. Surface SEM micrographs of Y2O3 coating samples: (a) before Li-Pb exposure, (b) after exposure at 400 °C, and (c) 500 °C for 100 h.
Fig. 5. Surface SEM micrographs of ZrO2 coating samples: (a) before Li-Pb exposure, (b) after exposure at 500 °C for 500 h, and (c) at 600 °C for 100 h.
exposure or the damage at during cleansing caused massive delamination of the coatings. In the previous study, the Er2O3 coating prepared by the MOD method showed low PRFs when it had a thick ironchromium oxide layer formed during the coating process [5]. In this case, oxygen in Li-Pb would be supplied to the substrate through micropores on the coating or the substrate-coating interface, leading to the deterioration of coating adhesion. In order to maintain adhesion of the coating, pretreatment of the RAFM substrate surface will be effective: the formation of a dense chromium oxide (Cr2O3) layer to prevent the formation of iron oxide (Fe3O4) which has high coefficients of thermal expansion. The other deterioration process is corrosion by Li-Pb. After exposure at higher temperatures as shown in Figs. 3(c) and 5(c), the atomic ratio of Er/Fe or Zr/Fe by EDX analysis clearly decreased, indicating that the coating became thinner. The Er2O3 coating would be corroded through formation of the ternary oxide LiErO2 and then detachment from the coating [13]. On the other hand, the ZrO2 coating seemed not to form a ternary oxide with Li, but might be corroded through a redox reaction with Li at higher temperatures. Also in terms of corrosion resistance, the ZrO2 coating showed the best performance in the operation temperature range at below 600 °C.
behavior in comparison with the Er2O3 and Y2O3 coatings. In particular, the temperature of further crystallization is significant for the practical use as a TPB [6]. The permeation flux through the ZrO2 coating decreased at 500 °C, while those of the Er2O3 and Y2O3 coatings decreased at 600 °C. This temperature difference might be critical because the upper limit temperature of RAFM steels is 550 °C [10]. For the Er2O3 and Y2O3 coatings, modification of the preparation process to ensure a better crystallization should be considered if an actual system requires a PRF of more than 100 in the operation temperature at 500 °C. On the other hand, the ZrO2 coating will exert an efficient reduction of tritium permeation in the operation temperature at 500 °C. However, the PRF dropped to 100 at 600 °C, indicating the deterioration of the coating including crack formation and delamination. In this case, a risk lies under abnormal high temperature conditions. One of promising modification techniques is multi-layer formation. An increased number of interfaces allows to add additional solution and diffusion steps which increase PRFs [11]. In addition, the multi-layer structure insures against deterioration of PRFs by crack formation because the permeation flux does not increase much unless a crack extends through the whole multi-layer coating [12]. Two major deterioration processes of the coatings are assumed after Li-Pb exposure. Delamination due to weakened adhesion to the substrate would be the main case for the Er2O3 and Y2O3 coatings at lower temperature exposure. The edges of the coatings are sharp as shown in Fig. 4(b), suggesting the thermal stress in the cooling process after
5. Conclusions Three metal oxide Er2O3, Y2O3 and ZrO2 coatings were prepared by 3
Fusion Engineering and Design xxx (xxxx) xxx–xxx
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a liquid phase method, and deuterium permeability and Li-Pb compatibility were compared. The Er2O3 coating showed a trend in deuterium permeation similar to the Y2O3 coating. A PRF of the Er2O3 coating was about 100 at the first measurements at lower than 600 °C, and then increased up to 2900. In the case of the ZrO2 coating, the PRF increased up to 1000 at 550 °C; however, decreased to 100 at 600 °C, indicating deterioration of the coating. In Li-Pb exposure tests, the Er2O3 and Y2O3 coatings showed the massive delamination at 400 and 500 °C, while most part of the ZrO2 coating remained after exposure at 500 °C for 500 h. Corrosion by formation of a ternary oxide LiErO2 or reduction of ZrO2 by Li might occur during exposure at higher temperatures. The ZrO2 coating showed the best performance as a corrosion-resistance TPB below 600 °C among the three materials. Control of the coating-substrate interface is important to prevent delamination of the coatings.
materials, Nucl. Fusion 57 (092007) (2017) 60. [2] F. Yang, et al., Tritium permeation characterization of Al2O3/FeAl coatings as tritium permeation barriers on 321 type stainless steel containers, J. Nucl. Mater. 478 (2016) 144–148. [3] T. Chikada, et al., Deuterium permeation behavior of erbium oxide coating on austenitic, ferritic, and ferritic/martensitic steels, Fusion Eng. Des. 84 (2009) 590–592. [4] T. Chikada, et al., Surface behaviour in deuterium permeation through erbium oxide coatings, Nucl. Fusion 51 (063023) (2011) 5. [5] T. Chikada, et al., Microstructure control and deuterium permeability of erbium oxide coating on ferritic/martensitic steels by metal-organic decomposition, Fusion Eng. Des. 85 (2010) 1537–1541. [6] T. Chikada, et al., Crystallization and deuterium permeation behaviors of yttrium oxide coating prepared by metal organic decomposition, Nucl. Mater. Energy 9 (2016) 529–534. [7] Y. Hatano, et al., Fabrication of ZrO2 coatings on ferritic steel by wet-chemical methods as a tritium permeation barrier, Phys. Scr. T145 (014044) (2011) 5. [8] J. Mochizuki, et al., Deuterium permeation behavior of tritium permeation barrier coating containing carbide nanoparticles, Fusion Eng. Des. 124 (2017) 1073–1076. [9] E. Serra, et al., Influence of traps on the deuterium behaviour in the low activation martensitic steels F82H and Batman, J. Nucl. Mater. 245 (1997) 108–114. [10] H. Tanigawa, et al., Status and key issues of reduced activation ferritic/martensitic steels as the structural material for a DEMO blanket, J. Nucl. Mater. 417 (2011) 9–15. [11] S. Horikoshi, et al., Deuterium permeation and retention behaviors in erbium oxideiron multilayer coatings, Fusion Eng. Des. 124 (2017) 1086–1090. [12] J. Mochizuki, et al., Preparation and characterization of Er2O3-ZrO2 multi-layer coating for tritium permeation barrier by metal organic decomposition, 13th International Symposium on Fusion Nuclear Technology, September 25–29, Kyoto, Japan, 2017. [13] M. Nagura, et al., LiErO2 formation on Er2O3 in static and natural convection lithium, Fusion Eng. Des. 84 (2009) 1384–1387.
Acknowledgments This work was conducted in the framework of Broader Approach activity collaboration program with National Institutes for Quantum and Radiological Science and Technology, Japan. This work was supported by JSPS KAKENHI Grant Number 15H05562, and the general collaboration research with National Institute for Fusion Science (NIFS15KEMF070). References [1] Ch. Linsmeier, et al., Development of advanced high heat flux and plasma-facing
4
Contents lists available at ScienceDirect
Fusion Engineering and Design journal homepage: www.elsevier.com/locate/fusengdes
Fabrication technology development and characterization of tritium permeation barriers by a liquid phase method ⁎
Takumi Chikadaa, , Moeki Matsunagaa, Seira Horikoshia, Jumpei Mochizukia, Hikari Fujitab, ⁎ Yoshimitsu Hishinumac, Kanetsugu Isobed, Takumi Hayashid, , Takayuki Teraib, Yasuhisa Oyaa a
Shizuoka Uniersity, Shizuoka, Japan The University of Tokyo, Tokyo, Japan c National Institute for Fusion Science, Toki, Japan d National Institutes for Quantum and Radiological Science and Technology, Tokai, Japan b
A R T I C L E I N F O
A B S T R A C T
Keywords: Tritium permeation Ceramic coating Lithium-lead Corrosion
Tritium permeation through structural materials is one of critical issues in liquid lithium-lead blanket concepts from the viewpoints of an efficient fuel cycle and radiological safety. Metal oxide coatings have been investigated as tritium permeation barrier and showed high permeation reduction factors. For the application to DEMO reactors, however, corrosion of the coatings by blanket materials is an unavoidable concern. This paper focuses on preparation of three metal oxide, erbium oxide, yttrium oxide, and zirconium oxide coatings by a liquid phase method and comparison of their properties in terms of hydrogen isotope permeability as well as lithium-lead compatibility. The deuterium permeation behavior of the erbium oxide and yttrium oxide coatings was similar, while the zirconium oxide showed a decrease of the permeation flux by further crystallization at lower temperature than the others. The zirconium oxide coating showed the best lithium-lead compatibility among three oxides at up to 600 °C. Deterioration of the coatings after static lithium-lead exposure would be caused by delamination and corrosion. Delamination of the coating would be prevented to control the coatingsubstrate interface. Corrosion of the coatings by formation of ternary oxides or reduction will be the main issue in lithium-lead compatibility at high temperatures.
1. Introduction Tritium permeation through structural materials is one of critical issues in liquid lithium-lead (Li-Pb) blanket concepts from the viewpoints of an efficient fuel cycle and radiological safety. In recent years metal oxide coatings have been investigated as tritium permeation barrier and showed sufficient permeation reduction factors (PRFs, 103–105 at > 500 °C) [1,2]. For the application to DEMO reactors, however, corrosion of the coatings by blanket materials is an unavoidable concern. In the framework of R&D activities of tritium technologies on Broader Approach, this work focuses on preparation of three metal oxide coatings, erbium oxide (Er2O3), yttrium oxide (Y2O3) and zirconium oxide (ZrO2) coatings, by a liquid phase method and comparison of their properties in terms of hydrogen isotope permeability as well as Li-Pb compatibility. Er2O3 and Y2O3 coatings fabricated by both gas and liquid phase methods showed PRFs of more than 103 [3–6]. ZrO2 coating was fabricated by a sol-gel method with electrolytic deposition and also showed a PRF of more than 103 [7].
⁎
Compared with Er2O3 coatings which have been investigated the most in this decade, Y2O3 has a better radioactivation property, and ZrO2 has a coefficient of thermal expansion similar to reduced activation ferritic/ martensitic steels. The goal of this study is to verify all the coating materials reduce hydrogen isotope permeation to a similar extent with the same coating process and investigate the difference in Li-Pb compatibility attributed to reactivity of different chemical compositions. 2. Experimental details 2.1. Coating preparation Reduced activation ferritic/martensitic steel F82H plates (25 × 25 × 0.5 mm3) were mirror-polished on both sides to use as substrates. Er2O3, Y2O3 and ZrO2 coatings were prepared by metal organic decomposition (MOD). Commercially available organic liquids (Kojundo Chemical Laboratory Co., Ltd., Er-03® for Er2O3, Y-03® for Y2O3, and SYM-ZR04® for ZrO2) were used as coating precursors
Corresponding author. E-mail address: [email protected] (T. Chikada).
https://doi.org/10.1016/j.fusengdes.2018.01.054 Received 29 September 2017; Received in revised form 14 January 2018; Accepted 18 January 2018 0920-3796/ © 2018 Elsevier B.V. All rights reserved.
Please cite this article as: Chikada, T., Fusion Engineering and Design (2018), https://doi.org/10.1016/j.fusengdes.2018.01.054
Fusion Engineering and Design xxx (xxxx) xxx–xxx
T. Chikada et al.
without addition of thinner. The preparation parameters were optimized by reference to our previous paper on Y2O3 coatings [8]. The conditions of dipping, drying and pre-heat were same as before. The heat-treatment equipment includes an electric furnace with gas flow controllers connected to gas cylinders. In this system, argon, hydrogen and oxygen of 0–100 sccm (standard cubic centimeter per minute) can be introduced into a quartz tube during heat-treatment. The optimized gas flow is argon and hydrogen with each flow rate of 50 sccm. Er2O3 and Y2O3 coatings were heat-treated at 700 °C, while ZrO2 coatings were at 650 °C due to the formation of visible cracks at 700 °C. The thicknesses of the coatings were 200–300 nm. 2.2. Deuterium permeation measurement Deuterium permeation flux through the sample was measured by a gas-driven deuterium permeation apparatus described elsewhere [3]. The procedure of the permeation experiments was followed by Ref. [4]. Before mounting to the sample holder, the backside of the sample was polished with abrasive papers to remove an oxide layer. The test temperature was set at 400–600 °C, and the measurements were conducted from lower temperature. The driving pressure of deuterium to the upstream was set to 1.00 × 104–8.00 × 104 Pa. The deuterium permeation flux J through a sample with a thickness of d is expressed by the following equation [9]:
J=P
pn d
Fig. 2. Temperature dependence of deuterium permeation flux for Er2O3 and ZrO2 coatings prepared by MOD. Data of uncoated F82H substrate and Y2O3 coating are also presented [6].
concentration in Li-Pb was not controlled. After immersion, the samples were picked up from liquid Li-Pb and then cleansed with a mixture of acetic acid and ethanol. Surfaces of the immersed samples were observed using a field emission scanning electron microscope (FE-SEM, JSM-7100F, JEOL Ltd.) located at National Institute for Fusion Science (NIFS), Japan. Elemental analysis was also examined by energy dispersive X-ray spectroscopy (EDX).
(1)
where P is the permeability and p is the driving pressure. The exponent n can be experimentally estimated with a fitting curve obtained by the permeation fluxes at several driving pressures. When the rate-limiting process is atomic diffusion of deuterium through the solid (diffusion limited regime), the exponent n indicates 0.5. On the other hand, when the rate-limiting process is molecular processes such as molecular diffusion, deuterium adsorption or recombination at the surface (surface limited regime), the exponent n shows unity. In this study, the permeation fluxes of the coatings were compared at the driving pressure of 8.00 × 104 Pa, which is the highest pressure and then has the smallest effect on surface reactions. For the evaluation of TPB efficiency, a permeation reduction factor (PRF) was calculated dividing the permeation flux of a bare substrate by that of a coated one.
3. Results Deuterium permeation fluxes of Er2O3 and ZrO2 coatings are shown in Fig. 2. Fitting lines from permeation data of the F82H substrate and the Y2O3 coated sample were also presented [6]. Two lines for the Y2O3 coating indicate that the trend of permeation flux changed on the measurement at 600 °C due to the further crystallization of the coating. The Er2O3 coated sample showed a trend similar to the Y2O3, although the permeation flux at 600 °C was an order of magnitude lower, and the PRF reached 2900. Another remark is that the pressure exponent changed from 0.63–0.76 to 0.50 after the decrease of permeation flux, which is also a similar trend. The permeation flux of the ZrO2 coated sample decreased at 500 °C and became comparable with the fitting line after further crystallization. On the measurement at 600 °C, however, the permeation flux discontinuously increased, indicating deterioration of the coating. Surface SEM micrographs of the Er2O3, Y2O3 and ZrO2 coatings after Li-Pb exposure are shown in Figs. 3–5, respectively. Approximately 30% of the coated area was lost for the Er2O3 coating after exposure at 400 °C for 100 h, and more than 50% at 500 °C for 100 h. The atomic ratio of Er/Fe on the coatings detected by EDX was 0.20 before exposure and 0.12–0.22 after exposure at 500 °C for 100 h, indicating partial loss of Er by Li-Pb exposure. Er2O3 was not detected on the whole sample surface after exposure at more than 550 °C for 100 h. The Y2O3 coating lost its coated area by more than 50% after exposure at 400 °C for 100 h, and more than 90% at 500 °C; therefore, the immersion tests at higher temperatures and longer duration were not conducted. The ZrO2 coating showed a clearly different behavior. More than 90% of the coated area remained after exposure up to at 500 °C for 500 h, and approximately 70% remained at 600 °C for 100 h. The atomic ratio of Zr/Fe detected by EDX was 0.29 before and after exposure at 500 °C for 500 h, and 0.16 after exposure at 600 °C for 100 h.
2.3. Li-Pb compatibility test The coated samples were cut into small specimens (approximately 10 × 10 × 0.5 mm3) and winded by an iron (Fe) wire to easily soak in and pick up from liquid Li-Pb. The experimental arrangement for static Li-Pb exposure tests is shown in Fig. 1. Li-Pb was synthesized from 99.9% Li and 99.999% Pb with the atomic rate of 15.7:84.3 under argon atmosphere in a glove box. Li-Pb of approximately 20 cm3 was used for each test. A sample soaked in Li-Pb was encapsulated in a stainless steel holder, and then was set in a muffle furnace. The exposure temperature and time were at 400–600 °C and 100–500 h, respectively. An oxygen monitor set in the glove box indicated less than 0.1% during synthesizing and loading Li-Pb, although oxygen
4. Discussion Fig. 1. Sample arrangement for Li-Pb immersion test.
The ZrO2 coating showed a different deuterium permeation 2
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Fig. 3. Surface SEM micrographs of Er2O3 coating samples: (a) before Li-Pb exposure, (b) after exposure at 400 °C, and (c) 500 °C for 100 h.
Fig. 4. Surface SEM micrographs of Y2O3 coating samples: (a) before Li-Pb exposure, (b) after exposure at 400 °C, and (c) 500 °C for 100 h.
Fig. 5. Surface SEM micrographs of ZrO2 coating samples: (a) before Li-Pb exposure, (b) after exposure at 500 °C for 500 h, and (c) at 600 °C for 100 h.
exposure or the damage at during cleansing caused massive delamination of the coatings. In the previous study, the Er2O3 coating prepared by the MOD method showed low PRFs when it had a thick ironchromium oxide layer formed during the coating process [5]. In this case, oxygen in Li-Pb would be supplied to the substrate through micropores on the coating or the substrate-coating interface, leading to the deterioration of coating adhesion. In order to maintain adhesion of the coating, pretreatment of the RAFM substrate surface will be effective: the formation of a dense chromium oxide (Cr2O3) layer to prevent the formation of iron oxide (Fe3O4) which has high coefficients of thermal expansion. The other deterioration process is corrosion by Li-Pb. After exposure at higher temperatures as shown in Figs. 3(c) and 5(c), the atomic ratio of Er/Fe or Zr/Fe by EDX analysis clearly decreased, indicating that the coating became thinner. The Er2O3 coating would be corroded through formation of the ternary oxide LiErO2 and then detachment from the coating [13]. On the other hand, the ZrO2 coating seemed not to form a ternary oxide with Li, but might be corroded through a redox reaction with Li at higher temperatures. Also in terms of corrosion resistance, the ZrO2 coating showed the best performance in the operation temperature range at below 600 °C.
behavior in comparison with the Er2O3 and Y2O3 coatings. In particular, the temperature of further crystallization is significant for the practical use as a TPB [6]. The permeation flux through the ZrO2 coating decreased at 500 °C, while those of the Er2O3 and Y2O3 coatings decreased at 600 °C. This temperature difference might be critical because the upper limit temperature of RAFM steels is 550 °C [10]. For the Er2O3 and Y2O3 coatings, modification of the preparation process to ensure a better crystallization should be considered if an actual system requires a PRF of more than 100 in the operation temperature at 500 °C. On the other hand, the ZrO2 coating will exert an efficient reduction of tritium permeation in the operation temperature at 500 °C. However, the PRF dropped to 100 at 600 °C, indicating the deterioration of the coating including crack formation and delamination. In this case, a risk lies under abnormal high temperature conditions. One of promising modification techniques is multi-layer formation. An increased number of interfaces allows to add additional solution and diffusion steps which increase PRFs [11]. In addition, the multi-layer structure insures against deterioration of PRFs by crack formation because the permeation flux does not increase much unless a crack extends through the whole multi-layer coating [12]. Two major deterioration processes of the coatings are assumed after Li-Pb exposure. Delamination due to weakened adhesion to the substrate would be the main case for the Er2O3 and Y2O3 coatings at lower temperature exposure. The edges of the coatings are sharp as shown in Fig. 4(b), suggesting the thermal stress in the cooling process after
5. Conclusions Three metal oxide Er2O3, Y2O3 and ZrO2 coatings were prepared by 3
Fusion Engineering and Design xxx (xxxx) xxx–xxx
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a liquid phase method, and deuterium permeability and Li-Pb compatibility were compared. The Er2O3 coating showed a trend in deuterium permeation similar to the Y2O3 coating. A PRF of the Er2O3 coating was about 100 at the first measurements at lower than 600 °C, and then increased up to 2900. In the case of the ZrO2 coating, the PRF increased up to 1000 at 550 °C; however, decreased to 100 at 600 °C, indicating deterioration of the coating. In Li-Pb exposure tests, the Er2O3 and Y2O3 coatings showed the massive delamination at 400 and 500 °C, while most part of the ZrO2 coating remained after exposure at 500 °C for 500 h. Corrosion by formation of a ternary oxide LiErO2 or reduction of ZrO2 by Li might occur during exposure at higher temperatures. The ZrO2 coating showed the best performance as a corrosion-resistance TPB below 600 °C among the three materials. Control of the coating-substrate interface is important to prevent delamination of the coatings.
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Acknowledgments This work was conducted in the framework of Broader Approach activity collaboration program with National Institutes for Quantum and Radiological Science and Technology, Japan. This work was supported by JSPS KAKENHI Grant Number 15H05562, and the general collaboration research with National Institute for Fusion Science (NIFS15KEMF070). References [1] Ch. Linsmeier, et al., Development of advanced high heat flux and plasma-facing
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