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Optimization of sintering conditions of advanced tritium breeder pebbles fabricated by the emulsion method
G Model
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Optimization of sintering conditions of advanced tritium breeder pebbles fabricated by the emulsion method Tsuyoshi Hoshino ∗ Breeding Functional Materials Development Group, Department of Blanket Systems Research, Rokkasho Fusion Institute, Sector of Fusion Research and Development, Japan Atomic Energy Agency, 2-166 Obuch, Omotedate, Rokkasho-mura, Kamikita-gun, Aomori 039-3212, Japan
h i g h l i g h t s • • • • •
Li2 TiO3 with excess Li (Li2+x TiO3+y ) was developed as an advanced tritium breeder. Li2+x TiO3+y pebbles fabrication by the emulsion method was carried out. Optimum sintering conditions of the Li2+x TiO3+y pebbles were surveyed. Sintering under vacuum and 1% H2 –He atmospheres was optimum condition. The diameter of the sintered pebbles was 1.07 mm, and the grain size was <5 m.
a r t i c l e
i n f o
Article history: Received 21 September 2014 Received in revised form 1 February 2015 Accepted 7 April 2015 Available online xxx Keywords: Advanced tritium breeder Pebble fabrication technique Mass production Li2 TiO3 Li2 TiO3 with excess Li
a b s t r a c t Demonstration power plant (DEMO) reactors require advanced tritium breeders with high thermal stability. Li2 TiO3 with excess Li (Li2+x TiO3+y ) has been developed as a material for an advanced tritium breeder. Considering the tritium release characteristics and the packing factor of the blanket, the optimum pebble diameter and grain size after sintering were determined to be 1 mm and <5 m, respectively. Therefore, optimum sintering conditions were surveyed in the present study to obtain these target values. The predominant factor affecting grain growth is assumed to be the presence of binder in the gel particles. This remaining binder reacts with the excess Li in the Li2+x TiO3+y and Li2 CO3 is generated, which promotes grain growth. To inhibit the generation of Li2 CO3 , calcined Li2+x TiO3+y pebbles were sintered under vacuum and subsequent 1% H2 –He atmosphere conditions. The diameter of the sintered Li2+x TiO3+y pebbles was 1.07 mm, and the average grain size on the surfaces and cross sections was <5 m. The results presented show that Li2+x TiO3+y pebbles with optimum characteristics were successfully fabricated using the emulsion method. © 2015 Elsevier B.V. All rights reserved.
1. Introduction Demonstration power reactors (DEMOs) require advanced tritium breeders that exhibit high stability at high temperatures. Since 2007, the development of an advanced tritium breeder has been undertaken by a joint Japan–EU team in a DEMO R&D operation under the auspices of an International Fusion Energy Research Centre (IFERC) project as part of the Broader Approach activities. This project is expected to continue until 2016. Lithium-containing ternary oxides (Li2 TiO3 , Li4 SiO4 , Li2 ZrO3 , and LiAlO2 ) have been proposed as breeder blanket materials [1]. Among these materials, lithium titanate (Li2 TiO3 ) has been
∗ Tel.: +81 175 71 6703; fax: +81 175 71 6502. E-mail addresses: [email protected], [email protected]
recognized as a prominent candidate material because of its chemical stability, good tritium release, and low-activation characteristics [2–4]. However, the mass of Li2 TiO3 components has been found to decrease with time because of Li evaporation in a H2 atmosphere and Li burn-up [5]. To prevent this mass decrease at high temperatures, Li2 TiO3 with excess Li (Li2+x TiO3+y ) has been developed as an advanced tritium breeder material [6–8]. Li2+x TiO3+y is a non-stoichiometric compound, as modeled by Kleykamp [9]. An advanced tritium breeder incorporating excess Li in single-phase Li2+x TiO3+y is expected to be stable under the prevailing operating conditions, namely in a neutron environment at high temperatures. Pebble fabrication using the emulsion method is one of the promising techniques for the mass production of advanced tritium breeder pebbles, and development of the emulsion method as a fabrication technique for Li2+x TiO3+y pebbles has been pursued [10]. In a previous study, sintered Li2+x TiO3+y pebbles of a 1.4 mm
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diameter were fabricated, and the average grain size on the surfaces and cross sections was determined to be <5 and 5–10 m, respectively. In accordance with considerations of the tritium release characteristics and the packing factor of the blanket, it has been determined that the optimum pebble diameter and grain size after sintering are 1 mm and <5 m [11], respectively. Therefore, the present study has sought to optimize the sintering conditions to obtain these target values.
allowed cutting the slurry flow with oil from the oil-filled syringe. The size of the resulting spherical tritium breeder gel particles was controlled by adjusting the oil and slurry flow speeds. The gel particles were deposited in an oil-filled container. The gel particles obtained were subsequently calcined in air at 873 K for 5 h and sintered under several conditions in an effort to control the grain size.
2. Experimental
The diameter and sphericity were measured by optical microscopy. The density of the sintered pebbles was obtained by the mercury intrusion technique. The grain size was analyzed by scanning electron microscopy (SEM). The actual Li/Ti mole ratio was analyzed by inductively coupled plasma-atomic emission spectroscopy (ICP-AES). The crush load was measured using a universal testing machine. The crystal structure was analyzed by powder X-ray diffraction (XRD).
2.1. Raw material preparation In this application of the emulsion method, gel particles are produced by a single-phase Li2+x TiO3+y slurry and oil in a granulator. As such, it is essential to ensure the single-phase nature of the slurry. During solution phase synthesis with Li and Ti alcohol solutions, the excess Li was found to be incorporated into Li4 TiO4 , which is chemically more unstable than Li2 TiO3 . Thus, the processing steps of a new solid-state reaction were developed [12]. Accordingly, powders of LiOH·H2 O and H2 TiO3 in a Li/Ti molecular ratio of 2.2 are mixed. The mixtures are continuously rotated at room temperature for 48 h in a polyethylene container, and gradually transform into a gel by solid-state reaction. After drying, the gel is calcined at 773 K for 5 h in air and sintered at 1373 K for 5 h in N2 . The sintered Li2+x TiO3+y powder is then used as the raw material of the emulsion method. 2.2. Emulsion method The emulsion method is illustrated in Fig. 1. The granulator employed was comprised of two syringes arranged in a T-shaped flow path. One syringe was filled with oil and the other with a Li2+x TiO3+y slurry composed of the raw material processed according to the procedure described in Section 2.1. The two flow lines from the syringes were connected in a T-shaped flow path, which
Fig. 1. Illustrations, photographs, and a schematic of Li2+x TiO3+y pebble fabrication by the emulsion method.
2.3. Characterizations
3. Results and discussion 3.1. Pebble fabrication The predominant factor affecting grain growth is assumed to be the presence of binder in the gel particles. The remaining binder reacts with Li in Li2+x TiO3+y and generates Li2 CO3 , which promotes grain growth. To inhibit the generation of Li2 CO3 , the calcined Li2+x TiO3+y pebbles were sintered in vacuum at 1373 K for 2 h within a covered crucible. The Li/Ti molar ratio of the sintered pebbles was nearly equivalent to that of the starting material, and the crystal structure of the sintered pebbles did not change during the fabrication process. The average grain sizes on the surfaces and cross sections of the sintered Li2+x TiO3+y pebbles were both less than 5 m, as shown in Fig. 2.
Fig. 2. Photographs of Li2+x TiO3+y pebbles fabricated by vacuum sintering and SEM images reflecting the grain size at a representative surface and cross section.
Please cite this article in press as: T. Hoshino, Optimization of sintering conditions of advanced tritium breeder pebbles fabricated by the emulsion method, Fusion Eng. Des. (2015), http://dx.doi.org/10.1016/j.fusengdes.2015.04.023
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However, the average grain size of the surfaces was found to be larger than that of the cross sections. Any residual Li2 CO3 on the pebble surface will generate grain growth during high temperature use. Therefore, annealing tests of the Li2+x TiO3+y pebbles sintered in vacuum was carried out. 3.2. Annealing test Fig. 3 shows the apparatus employed for the annealing tests. Li2+x TiO3+y pebbles were installed in a tubular furnace and were annealed in a 1% H2 –He atmosphere at 1173 K for 4 and 14 days. After annealing, the grain size of the annealed Li2+x TiO3+y pebbles was measured by SEM (Fig. 4). The average grain size on the surfaces of the annealed Li2+x TiO3+y pebbles was observed to increase with increasing annealing time. Finally, the grain size was >15 m after 14 days. This indicates the presence of residual Li2 CO3 on the surface of the Li2+x TiO3+y pebbles fabricated by vacuum sintering. Thus, optimized sintering conditions were surveyed. 3.3. Optimization of sintering conditions The binder includes carbon (C) and oxygen (O), and CO2 is generated by pyrolysis of the binder. Generation of residual Li2 CO3 is likely to be caused by the reaction of this CO2 and the excess Li in Li2+x TiO3+y during sintering. Therefore, the control of the oxygen potential during sintering is important to inhibit the generation of Li2 CO3 . The oxygen potential in a 1% H2 –He atmosphere is very low. Therefore, the calcined Li2+x TiO3+y pebbles were sintered in vacuum at 1073 K for 3 h, and subsequently, in a 1% H2 –He atmosphere at 1323 K for 5 h. Fig. 5a shows SEM images of the surfaces and cross sections of the sintered Li2+x TiO3+y pebbles. Unfortunately, residual Li2 CO3 Fig. 5. SEM images of the surfaces and cross sections of Li2+x TiO3+y pebbles sintered under vacuum and subsequent 1% H2 –He atmosphere conditions.
Fig. 3. Apparatus employed for the annealing tests of Li2+x TiO3+y pebbles fabricated by vacuum sintering.
was observed on the surface of the sintered Li2+x TiO3+y pebbles. To accelerate CO2 removal during sintering, the sintering process was again applied to calcined Li2+x TiO3+y pebbles with the crucible cover removed. Fig. 5b shows the results of sintering under vacuum and subsequent 1% H2 –He atmosphere conditions without a crucible cover. Residual Li2 CO3 was not observed on the surfaces of the resulting Li2+x TiO3+y pebbles by SEM, and the average grain sizes on the surfaces and cross sections of the improved Li2+x TiO3+y pebbles were both <5 m. Although Li vaporization during sintering is expected to vary depending on the presence or absence of the crucible cover, the ratio of Li/Ti was not changed, as shown in Table 1. Furthermore, the improved Li2+x TiO3+y pebbles were also subjected to annealing tests. Fig. 6 shows SEM images of the surfaces and cross sections of the improved Li2+x TiO3+y pebbles before and after annealing for 14 days. No residual Li2 CO3 was observed, nor was grain growth observed from the SEM images over the course of annealing. The crush load of annealed Li2+x TiO3+y pebbles was found to largely retain the value exhibited prior to annealing. Table 2 summarizes the characteristics of the improved Li2+x TiO3+y pebbles produced by the emulsion method and sintered under vacuum and subsequent 1% H2 –He atmosphere conditions. The XRD patterns of the improved Li2+x TiO3+y pebbles matched Table 1 Li/Ti ratios of Li2+x TiO3+y pebbles sintered under vacuum and subsequent 1% H2 –He atmosphere conditions. Ratio of Li/Ti
Fig. 4. The grain structure of Li2+x TiO3+y pebbles at a representative surface after annealing. The average grain size is observed to be >15 m after 14 days.
With crucible cover Without crucible cover
2.13 2.14
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fabrication by the emulsion method has been shown to be a promising technique for their mass production. In accordance with considerations of tritium release characteristics and the packing factor of the breeder blanket, the optimum diameter was determined to be 1 mm, with an optimum grain size after sintering of less than 5 m. Therefore, optimization of the pebble sintering conditions was carried out to obtain the target value. The diameter of the pebbles is easily controlled by adjusting the flow speeds of the Li2+x TiO3+y slurry and oil. However, the grain size is difficult to control. The predominant factor affecting grain growth is assumed to be the presence of binder in the gel particles, where the remaining binder reacts with Li in Li2+x TiO3+y and generates Li2 CO3 . To inhibit the grain growth of the sintered Li2+x TiO3+y pebbles, the sintering temperature and atmosphere were optimized. Grain growth was inhibited by sintering under vacuum at 1073 K for 3 h and subsequently, in a 1% H2 –He atmosphere at 1323 K for 5 h without a crucible cover. The diameter, sphericity, density, grain size, molar ratio (Li/Ti), crystal structure, and crush load of the improved Li2+x TiO3+y pebbles were determined to approach optimum values. The results suggest that sintering under vacuum and subsequent 1% H2 –He atmosphere conditions represents the optimum sintering condition for Li2+x TiO3+y pebbles. Acknowledgments Fig. 6. SEM images of the surfaces and cross sections of the improved Li2+x TiO3+y pebbles before and after annealing for 14 days. Table 2 Characteristics of the improved Li2+x TiO3+y pebbles produced by the emulsion method. Diameter (mm) Sphericity Density (%T.D.) Grain size (m) Ratio of Li/Ti Crush load (N) Crystal structure
1.07 1.01 83.5 <5 2.13 31 Single-phase Li2 TiO3
the pattern of Li2 TiO3 listed in the JCPDF card [13], indicating that the improved Li2+x TiO3+y pebbles were the non-stoichiometric compound Li2+x TiO3+y . The sphericity of the improved Li2+x TiO3+y pebbles was determined from optical microscopy to be 1.1. The density was 83.5% T.D. and nearly satisfied the optimum density of 85% T.D. The data presented indicates that sintering under vacuum and subsequent 1% H2 –He atmosphere conditions were effective for minimizing the grain growth of the sintered Li2+x TiO3+y pebbles fabricated by the emulsion method. 4. Conclusion Li2 TiO3 with excess Li (Li2+x TiO3+y ) has been developed as a material for an advanced tritium breeder, and Li2+x TiO3+y pebble
This report has been prepared within the framework of the Agreement between the Government of Japan and the European Atomic Energy Community for The Joint Implementation of The Broader Approach Activities in the Field of Fusion Energy Research. The author would like to thank all the contributors to the IFERC projects in Japan as well as all the members of the IFERC Project Team. References [1] L. Giancarli, V. Chuyanov, M. Abdou, M. Akiba, B.G. Hong, R. Lässer, et al., J. Nucl. Mater. 367–370 (2007) 1271–1280. [2] N. Roux, J. Avon, A. Floreancig, J. Mougin, B. Rasneur, S. Ravel, J. Nucl. Mater. 233–237 (1996) 1431–1435. [3] N. Roux, S. Tanaka, C. Johnson, R. Verrall, Fusion Eng. Des. 41 (1998) 31–38. [4] J.P. Kopasz, J.M. Miller, C.E. Johnson, J. Nucl. Mater. 212–215 (1994) 927–931. [5] T. Hoshino, M. Dokiya, T. Terai, Y. Takahashi, M. Yamawaki, Fusion Eng. Des. 61–62 (2002) 353–360. [6] T. Hoshino, M. Yasumoto, K. Tsuchiya, K. Hayashi, H. Nishimura, A. Suzuki, et al., Fusion Eng. Des. 82 (2007) 2269–2273. [7] T. Hoshino, K. Kato, Y. Natori, M. Nakamura, K. Sasaki, K. Hayashi, et al., Fusion Eng. Des. 84 (2009) 956–959. [8] H. Tanigawa, T. Hoshino, Y. Kawamura, M. Nakamichi, K. Ochiai, M. Akiba, et al., Nucl. Fusion 49 (2009) 055021. [9] H. Kleykamp, Fusion Eng. Des. 61 (2002) 361–366. [10] T. Hoshino, M. Nakamichi, Fusion Eng. Des. 87 (2012) 486–492. [11] T. Hanada, M. Nishikawa, T. Kanazawa, H. Yamasaki, N. Yamashita, S. Fukada, J. Nucl. Mater. 417 (2011) 735–738. [12] T. Hoshino, F. Oikawa, Fusion Eng. Des. 86 (2011) 2172–2175. [13] PDF#33-0831, PDF-Card, Material Data Inc.
Please cite this article in press as: T. Hoshino, Optimization of sintering conditions of advanced tritium breeder pebbles fabricated by the emulsion method, Fusion Eng. Des. (2015), http://dx.doi.org/10.1016/j.fusengdes.2015.04.023
ARTICLE IN PRESS
FUSION-7893; No. of Pages 4
Fusion Engineering and Design xxx (2015) xxx–xxx
Contents lists available at ScienceDirect
Fusion Engineering and Design journal homepage: www.elsevier.com/locate/fusengdes
Optimization of sintering conditions of advanced tritium breeder pebbles fabricated by the emulsion method Tsuyoshi Hoshino ∗ Breeding Functional Materials Development Group, Department of Blanket Systems Research, Rokkasho Fusion Institute, Sector of Fusion Research and Development, Japan Atomic Energy Agency, 2-166 Obuch, Omotedate, Rokkasho-mura, Kamikita-gun, Aomori 039-3212, Japan
h i g h l i g h t s • • • • •
Li2 TiO3 with excess Li (Li2+x TiO3+y ) was developed as an advanced tritium breeder. Li2+x TiO3+y pebbles fabrication by the emulsion method was carried out. Optimum sintering conditions of the Li2+x TiO3+y pebbles were surveyed. Sintering under vacuum and 1% H2 –He atmospheres was optimum condition. The diameter of the sintered pebbles was 1.07 mm, and the grain size was <5 m.
a r t i c l e
i n f o
Article history: Received 21 September 2014 Received in revised form 1 February 2015 Accepted 7 April 2015 Available online xxx Keywords: Advanced tritium breeder Pebble fabrication technique Mass production Li2 TiO3 Li2 TiO3 with excess Li
a b s t r a c t Demonstration power plant (DEMO) reactors require advanced tritium breeders with high thermal stability. Li2 TiO3 with excess Li (Li2+x TiO3+y ) has been developed as a material for an advanced tritium breeder. Considering the tritium release characteristics and the packing factor of the blanket, the optimum pebble diameter and grain size after sintering were determined to be 1 mm and <5 m, respectively. Therefore, optimum sintering conditions were surveyed in the present study to obtain these target values. The predominant factor affecting grain growth is assumed to be the presence of binder in the gel particles. This remaining binder reacts with the excess Li in the Li2+x TiO3+y and Li2 CO3 is generated, which promotes grain growth. To inhibit the generation of Li2 CO3 , calcined Li2+x TiO3+y pebbles were sintered under vacuum and subsequent 1% H2 –He atmosphere conditions. The diameter of the sintered Li2+x TiO3+y pebbles was 1.07 mm, and the average grain size on the surfaces and cross sections was <5 m. The results presented show that Li2+x TiO3+y pebbles with optimum characteristics were successfully fabricated using the emulsion method. © 2015 Elsevier B.V. All rights reserved.
1. Introduction Demonstration power reactors (DEMOs) require advanced tritium breeders that exhibit high stability at high temperatures. Since 2007, the development of an advanced tritium breeder has been undertaken by a joint Japan–EU team in a DEMO R&D operation under the auspices of an International Fusion Energy Research Centre (IFERC) project as part of the Broader Approach activities. This project is expected to continue until 2016. Lithium-containing ternary oxides (Li2 TiO3 , Li4 SiO4 , Li2 ZrO3 , and LiAlO2 ) have been proposed as breeder blanket materials [1]. Among these materials, lithium titanate (Li2 TiO3 ) has been
∗ Tel.: +81 175 71 6703; fax: +81 175 71 6502. E-mail addresses: [email protected], [email protected]
recognized as a prominent candidate material because of its chemical stability, good tritium release, and low-activation characteristics [2–4]. However, the mass of Li2 TiO3 components has been found to decrease with time because of Li evaporation in a H2 atmosphere and Li burn-up [5]. To prevent this mass decrease at high temperatures, Li2 TiO3 with excess Li (Li2+x TiO3+y ) has been developed as an advanced tritium breeder material [6–8]. Li2+x TiO3+y is a non-stoichiometric compound, as modeled by Kleykamp [9]. An advanced tritium breeder incorporating excess Li in single-phase Li2+x TiO3+y is expected to be stable under the prevailing operating conditions, namely in a neutron environment at high temperatures. Pebble fabrication using the emulsion method is one of the promising techniques for the mass production of advanced tritium breeder pebbles, and development of the emulsion method as a fabrication technique for Li2+x TiO3+y pebbles has been pursued [10]. In a previous study, sintered Li2+x TiO3+y pebbles of a 1.4 mm
http://dx.doi.org/10.1016/j.fusengdes.2015.04.023 0920-3796/© 2015 Elsevier B.V. All rights reserved.
Please cite this article in press as: T. Hoshino, Optimization of sintering conditions of advanced tritium breeder pebbles fabricated by the emulsion method, Fusion Eng. Des. (2015), http://dx.doi.org/10.1016/j.fusengdes.2015.04.023
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diameter were fabricated, and the average grain size on the surfaces and cross sections was determined to be <5 and 5–10 m, respectively. In accordance with considerations of the tritium release characteristics and the packing factor of the blanket, it has been determined that the optimum pebble diameter and grain size after sintering are 1 mm and <5 m [11], respectively. Therefore, the present study has sought to optimize the sintering conditions to obtain these target values.
allowed cutting the slurry flow with oil from the oil-filled syringe. The size of the resulting spherical tritium breeder gel particles was controlled by adjusting the oil and slurry flow speeds. The gel particles were deposited in an oil-filled container. The gel particles obtained were subsequently calcined in air at 873 K for 5 h and sintered under several conditions in an effort to control the grain size.
2. Experimental
The diameter and sphericity were measured by optical microscopy. The density of the sintered pebbles was obtained by the mercury intrusion technique. The grain size was analyzed by scanning electron microscopy (SEM). The actual Li/Ti mole ratio was analyzed by inductively coupled plasma-atomic emission spectroscopy (ICP-AES). The crush load was measured using a universal testing machine. The crystal structure was analyzed by powder X-ray diffraction (XRD).
2.1. Raw material preparation In this application of the emulsion method, gel particles are produced by a single-phase Li2+x TiO3+y slurry and oil in a granulator. As such, it is essential to ensure the single-phase nature of the slurry. During solution phase synthesis with Li and Ti alcohol solutions, the excess Li was found to be incorporated into Li4 TiO4 , which is chemically more unstable than Li2 TiO3 . Thus, the processing steps of a new solid-state reaction were developed [12]. Accordingly, powders of LiOH·H2 O and H2 TiO3 in a Li/Ti molecular ratio of 2.2 are mixed. The mixtures are continuously rotated at room temperature for 48 h in a polyethylene container, and gradually transform into a gel by solid-state reaction. After drying, the gel is calcined at 773 K for 5 h in air and sintered at 1373 K for 5 h in N2 . The sintered Li2+x TiO3+y powder is then used as the raw material of the emulsion method. 2.2. Emulsion method The emulsion method is illustrated in Fig. 1. The granulator employed was comprised of two syringes arranged in a T-shaped flow path. One syringe was filled with oil and the other with a Li2+x TiO3+y slurry composed of the raw material processed according to the procedure described in Section 2.1. The two flow lines from the syringes were connected in a T-shaped flow path, which
Fig. 1. Illustrations, photographs, and a schematic of Li2+x TiO3+y pebble fabrication by the emulsion method.
2.3. Characterizations
3. Results and discussion 3.1. Pebble fabrication The predominant factor affecting grain growth is assumed to be the presence of binder in the gel particles. The remaining binder reacts with Li in Li2+x TiO3+y and generates Li2 CO3 , which promotes grain growth. To inhibit the generation of Li2 CO3 , the calcined Li2+x TiO3+y pebbles were sintered in vacuum at 1373 K for 2 h within a covered crucible. The Li/Ti molar ratio of the sintered pebbles was nearly equivalent to that of the starting material, and the crystal structure of the sintered pebbles did not change during the fabrication process. The average grain sizes on the surfaces and cross sections of the sintered Li2+x TiO3+y pebbles were both less than 5 m, as shown in Fig. 2.
Fig. 2. Photographs of Li2+x TiO3+y pebbles fabricated by vacuum sintering and SEM images reflecting the grain size at a representative surface and cross section.
Please cite this article in press as: T. Hoshino, Optimization of sintering conditions of advanced tritium breeder pebbles fabricated by the emulsion method, Fusion Eng. Des. (2015), http://dx.doi.org/10.1016/j.fusengdes.2015.04.023
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However, the average grain size of the surfaces was found to be larger than that of the cross sections. Any residual Li2 CO3 on the pebble surface will generate grain growth during high temperature use. Therefore, annealing tests of the Li2+x TiO3+y pebbles sintered in vacuum was carried out. 3.2. Annealing test Fig. 3 shows the apparatus employed for the annealing tests. Li2+x TiO3+y pebbles were installed in a tubular furnace and were annealed in a 1% H2 –He atmosphere at 1173 K for 4 and 14 days. After annealing, the grain size of the annealed Li2+x TiO3+y pebbles was measured by SEM (Fig. 4). The average grain size on the surfaces of the annealed Li2+x TiO3+y pebbles was observed to increase with increasing annealing time. Finally, the grain size was >15 m after 14 days. This indicates the presence of residual Li2 CO3 on the surface of the Li2+x TiO3+y pebbles fabricated by vacuum sintering. Thus, optimized sintering conditions were surveyed. 3.3. Optimization of sintering conditions The binder includes carbon (C) and oxygen (O), and CO2 is generated by pyrolysis of the binder. Generation of residual Li2 CO3 is likely to be caused by the reaction of this CO2 and the excess Li in Li2+x TiO3+y during sintering. Therefore, the control of the oxygen potential during sintering is important to inhibit the generation of Li2 CO3 . The oxygen potential in a 1% H2 –He atmosphere is very low. Therefore, the calcined Li2+x TiO3+y pebbles were sintered in vacuum at 1073 K for 3 h, and subsequently, in a 1% H2 –He atmosphere at 1323 K for 5 h. Fig. 5a shows SEM images of the surfaces and cross sections of the sintered Li2+x TiO3+y pebbles. Unfortunately, residual Li2 CO3 Fig. 5. SEM images of the surfaces and cross sections of Li2+x TiO3+y pebbles sintered under vacuum and subsequent 1% H2 –He atmosphere conditions.
Fig. 3. Apparatus employed for the annealing tests of Li2+x TiO3+y pebbles fabricated by vacuum sintering.
was observed on the surface of the sintered Li2+x TiO3+y pebbles. To accelerate CO2 removal during sintering, the sintering process was again applied to calcined Li2+x TiO3+y pebbles with the crucible cover removed. Fig. 5b shows the results of sintering under vacuum and subsequent 1% H2 –He atmosphere conditions without a crucible cover. Residual Li2 CO3 was not observed on the surfaces of the resulting Li2+x TiO3+y pebbles by SEM, and the average grain sizes on the surfaces and cross sections of the improved Li2+x TiO3+y pebbles were both <5 m. Although Li vaporization during sintering is expected to vary depending on the presence or absence of the crucible cover, the ratio of Li/Ti was not changed, as shown in Table 1. Furthermore, the improved Li2+x TiO3+y pebbles were also subjected to annealing tests. Fig. 6 shows SEM images of the surfaces and cross sections of the improved Li2+x TiO3+y pebbles before and after annealing for 14 days. No residual Li2 CO3 was observed, nor was grain growth observed from the SEM images over the course of annealing. The crush load of annealed Li2+x TiO3+y pebbles was found to largely retain the value exhibited prior to annealing. Table 2 summarizes the characteristics of the improved Li2+x TiO3+y pebbles produced by the emulsion method and sintered under vacuum and subsequent 1% H2 –He atmosphere conditions. The XRD patterns of the improved Li2+x TiO3+y pebbles matched Table 1 Li/Ti ratios of Li2+x TiO3+y pebbles sintered under vacuum and subsequent 1% H2 –He atmosphere conditions. Ratio of Li/Ti
Fig. 4. The grain structure of Li2+x TiO3+y pebbles at a representative surface after annealing. The average grain size is observed to be >15 m after 14 days.
With crucible cover Without crucible cover
2.13 2.14
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fabrication by the emulsion method has been shown to be a promising technique for their mass production. In accordance with considerations of tritium release characteristics and the packing factor of the breeder blanket, the optimum diameter was determined to be 1 mm, with an optimum grain size after sintering of less than 5 m. Therefore, optimization of the pebble sintering conditions was carried out to obtain the target value. The diameter of the pebbles is easily controlled by adjusting the flow speeds of the Li2+x TiO3+y slurry and oil. However, the grain size is difficult to control. The predominant factor affecting grain growth is assumed to be the presence of binder in the gel particles, where the remaining binder reacts with Li in Li2+x TiO3+y and generates Li2 CO3 . To inhibit the grain growth of the sintered Li2+x TiO3+y pebbles, the sintering temperature and atmosphere were optimized. Grain growth was inhibited by sintering under vacuum at 1073 K for 3 h and subsequently, in a 1% H2 –He atmosphere at 1323 K for 5 h without a crucible cover. The diameter, sphericity, density, grain size, molar ratio (Li/Ti), crystal structure, and crush load of the improved Li2+x TiO3+y pebbles were determined to approach optimum values. The results suggest that sintering under vacuum and subsequent 1% H2 –He atmosphere conditions represents the optimum sintering condition for Li2+x TiO3+y pebbles. Acknowledgments Fig. 6. SEM images of the surfaces and cross sections of the improved Li2+x TiO3+y pebbles before and after annealing for 14 days. Table 2 Characteristics of the improved Li2+x TiO3+y pebbles produced by the emulsion method. Diameter (mm) Sphericity Density (%T.D.) Grain size (m) Ratio of Li/Ti Crush load (N) Crystal structure
1.07 1.01 83.5 <5 2.13 31 Single-phase Li2 TiO3
the pattern of Li2 TiO3 listed in the JCPDF card [13], indicating that the improved Li2+x TiO3+y pebbles were the non-stoichiometric compound Li2+x TiO3+y . The sphericity of the improved Li2+x TiO3+y pebbles was determined from optical microscopy to be 1.1. The density was 83.5% T.D. and nearly satisfied the optimum density of 85% T.D. The data presented indicates that sintering under vacuum and subsequent 1% H2 –He atmosphere conditions were effective for minimizing the grain growth of the sintered Li2+x TiO3+y pebbles fabricated by the emulsion method. 4. Conclusion Li2 TiO3 with excess Li (Li2+x TiO3+y ) has been developed as a material for an advanced tritium breeder, and Li2+x TiO3+y pebble
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Please cite this article in press as: T. Hoshino, Optimization of sintering conditions of advanced tritium breeder pebbles fabricated by the emulsion method, Fusion Eng. Des. (2015), http://dx.doi.org/10.1016/j.fusengdes.2015.04.023