- Email: [email protected]
Oxidation resistance of Be12Ti fabricated by plasma-sintering method
Journal of Nuclear Materials 442 (2013) S494–S496
Contents lists available at SciVerse ScienceDirect
Journal of Nuclear Materials journal homepage: www.elsevier.com/locate/jnucmat
Oxidation resistance of Be12Ti fabricated by plasma-sintering method K. Wada a,⇑, K. Munakata a, J.H Kim b, K. Yonehara b, D. Wakai b, M. Nakamichi b a b
Faculty of engineering and resource science, Akita University, 1-1, TegataGakuen, Akita 010-8502, Japan Fusion Research and Development Directorate, Japan Atomic Energy Agency, 2-166, Omotedate, Obuchi, Rokkasho, Kamikita, Aomori 039-3212, Japan
a r t i c l e
i n f o
Article history: Available online 17 May 2013
a b s t r a c t Titanium beryllium intermetallic compounds (beryllides), such as Be12Ti, are alternatives to metallic beryllium as the neutron multiplier for the fusion reactor blanket. Be12Ti is known to have advantages over metallic beryllium, which includes a high melting point, lower swelling and higher chemical stability. This work examined the oxidation resistance of Be12Ti samples fabricated by a plasma-sintering. Metallic beryllium and Be12Ti samples were placed under a flow of an argon gas containing 10,000 ppm of water vapor at the temperature of 1273 K for 24 h. The Be12Ti sample fabricated was shown to have higher oxidation resistance than metallic beryllium. Ó 2013 Published by Elsevier B.V.
1. Introduction Generating tritium to fuel next generation fusion reactors requires the development of a neutron multiplier to be used in the blanket modules. Metallic beryllium has been investigated as a neutron multiplier candidate. However, metallic beryllium is highly reactive with water vapor and oxygen at high temperatures, producing H2 gas and BeO via the following reaction:
Be þ H2 O ! BeO þ H2
ð1Þ
2Be þ O2 ! 2BeO
ð2Þ
H2 is highly explosive, and BeO is harmful to human body. These drawbacks would threaten the safety of fusion reactors, particularly in the case of loss-of-coolant accident (LOCA) in which a coolant line break injects water vapor into the blanket. Therefore, development of new neutron multiplier, which is more chemically stable than metallic beryllium, is necessary. Titanium beryllide is known to have high melting point, lower swelling, and higher chemically stability than metallic beryllium [1]. In particular, compatibility between Be12Ti and SS316LN was evaluated and the advantage of Be12Ti as a neutron multiplier was proven [2]. Be12Ti had been mainly fabricated by not only a hot isostatic pressing (HIP) process but also by casting. HIPing requires repeated, lengthy processing and cast materials have poor composition control. Therefore, these methods require long fabrication periods and high costs. Thus Japan Atomic Energy Agency (JAEA) has suggested a plasma-sintering method for fabrication of the beryllide. This method consists of uniaxial pressing, plasma generation and heating with advantages in ⇑ Corresponding author. Tel.: +81 018 889 2749. E-mail address: [email protected] (K. Wada). 0022-3115/$ - see front matter Ó 2013 Published by Elsevier B.V. http://dx.doi.org/10.1016/j.jnucmat.2013.04.093
terms of easy-to-control and short fabrication period [3,4]. This work investigates the reactivity of a plasma-sintered Be12Ti sample with water vapor.
2. Experimental Be and Ti powders with average particle size under 45 lm, were mixed for 60 min by using a mortar grinder RM200 (Retsch, Germany). The mixed ratio was adjusted to the stoichiometric composition of Be12Ti (Be:Ti = 69.3:30.7 wt.%) and this blended powder was sintered with a plasma sintering device KE-PAS III (manufactured by KAKEN Co. Ltd.). Be12Ti discs 20 mm in diameter and 5 mm in thickness, were synthesized by sintering for 40 min at a temperature of 1273 K at 50 MPa. The surface of the plasma -sintered Be12Ti sample is composed of four phases such as Be, Be12Ti, Be17Ti2 and Be2Ti with different fractions. The area fraction of Be in sintered Be12Ti sample surface is 2%. On the other hand, the area fractions of Be12Ti, Be17Ti2 and Be2Ti in the sintered Be12Ti sample surface are 98% [4]. A plasma-sintered Be12Ti sample and a metallic beryllium sample were cut (2 cm diameter and 5 mm thick). Then, each sample was further cut to 3 3 5 mm pieces and polished with 5-lm SiC grit. Fig. 1 shows the experimental apparatus. The metallic beryllium sample and the plasma-sintered Be12Ti sample were placed under a flow of Ar gas containing 10,000 ppm (1%) water vapor. Although the mechanism of hydrogen generation by the reaction between Be12Ti and water vapor at high temperature has not been well understood, it appears that hydrogen is generated by the reaction in Eqs. (1) and (2) when Be12Ti is exposed to water vapor at high temperatures. The temperature of the samples was raised from 313 to 1273 K at 5 K/min. After the reactor temperature reached
K. Wada et al. / Journal of Nuclear Materials 442 (2013) S494–S496
S495
Fig. 1. Experimental apparatus.
1273 K, it was held at 1273 K for 24 h. The flow rate of the Ar gas containing 10,000 ppm of water vapor was 300 cm3/min. The mass gain curve and the concentration of hydrogen generated from the samples were measured with a thermogravimetry-differential thermal analysis and a mass spectrometry instrument manufactured by RIGAKU Co. Ltd. The oxidized sample surface was characterized by means of the X-ray diffraction analysis using Ultima IV, that was also manufactured by RIGAKU Co. Ltd.
3. Results and discussions Fig. 2a and b show mass gain curves and changes in concentration of hydrogen in the outlet stream of the reactor charged with metallic beryllium and the plasma-sintered Be12Ti samples. In this experiment, we calculated hydrogen concentration from the mass gain of the sample by assuming that beryllium which reacted to water vapor turned into BeO. The horizontal and right vertical axes represent duration time and test temperature, respectively. The first left vertical axis and the second left vertical axis represent the concentration of hydrogen generated from the surface of the samples and the mass gain of the samples, respectively. The dash-dot line shows the mass gain of the samples and the dashed line shows the test temperature, respectively. In the case of metallic beryllium, it is known that a catastrophic oxidation reaction takes place under the conditions of high temperature and high water vapor pressure [5]. The weight of the metallic beryllium sample drastically increased and continued to increase for 24 h. The result of the previous studies by Aylmore et al. [6] also indicates that the catastrophic oxidation reaction takes place at temperatures higher than 973 K on the surface of beryllium metal. On the other hand, the mass gain of the Be12Ti sample was very slow. The final mass gain of the metallic beryllium sample was 21.75 mg, which corresponds to approximately 50% of its initial weight (44.63 mg). The final mass gain of the plasma-sintered Be12Ti sample was 1.10 mg, approximately 1.7% of its initial weight (64.18 mg). The average mass gain rate of the metallic beryllium sample and the plasma-sintered Be12Ti sample were 2.8 104 and 6.6 106 mg/sec respectively. The mass gain of the metallic beryllium sample was far greater than that of the plasma-sintered Be12Ti sample. These experimental results suggest that Be12Ti possesses greatly higher oxidation resistance to water vapor compared
Fig. 2. Mass gain curve and hydrogen concentration of (a) the metallic beryllium sample and (b) the Be12Ti sample fabricated by the plasma-sintering method. The dash-dot lines show the mass gain of the samples. The dashed lines show the test temperature. The solid lines show changes in the concentration of hydrogen in the outlet stream of the reactor.
than metallic beryllium. The amount of hydrogen concentration of the metallic beryllium sample was 24 times higher than that of the plasma-sintered Be12Ti sample. The solid lines in the figures show changes in the concentration of hydrogen in the outlet stream of the reactor. In the case of the metallic beryllium sample, a considerable amount of hydrogen was produced throughout the experiment. The amount of hydrogen generated from each 1 cm2 of the surface of the metallic beryllium sample was 1.06 103 mol. On the other hand, in the case of the plasma-sintered Be12Ti sample, the amount of hydrogen generated from each 1 cm2 of the surface of the plasma-sintered
S496
K. Wada et al. / Journal of Nuclear Materials 442 (2013) S494–S496 6x10 5
6x105
(a)
4x10 5 3x10 5 2x10 5
Be BeO
(b)
5x105
Intensity (cps)
Intensity (cps)
5x10 5
Be
4x105 3x105 2x105 1x105
1x10 5
0
0
20
40
60
80
20
100
40
4
80
100
4
6x10
6x10
Be
(c)
4
5x10
Be12Ti
4
4
5x10
Intensity (cps)
Intensity (cps)
60
2 (degree)
2 (degree)
4x10
4
3x10
4
2x10
4
Be12Ti BeO
(d)
4
4x10
4
3x10
4
2x10
4
1x10
1x10
0
20
0
40
60
80
100
20
40
60
80
100
2 (degree)
2 (degree)
Fig. 3. X-ray diffraction pattern of (a) the metallic beryllium sample before the experiment, (b) the metallic beryllium sample after the experiment, (c) the Be12Ti sample before the experiment and (d) the Be12Ti sample after the experiment. (a and b): Full scale is 6 105. (c and d): Full scale is 6 104.
Be12Ti sample was only 9.58 105 mol. In previous studies, similar experiments on Be12Ti samples manufactured by a HIP method were carried out [7,8]. The results suggest that the amount of hydrogen generated from the Be12Ti sample manufactured by HIPing was three times as large as that generated from the plasma-sintered Be12Ti sample. Thus, the plasma-sintered Be12Ti sample is considered to have been more uniformly prepared than the Be12Ti sample manufactured by the HIP method. Fig. 3a and b shows the X-ray diffraction patterns of the metallic beryllium sample before and after exposure to the Ar gas containing 10,000 ppm of water vapor, respectively. Several diffraction peaks arose (see Fig. 3b), and most of them were attributed to BeO (marked with closed triangle). After the experiment, a large amount of white product was observed on the surface of the metallic beryllium sample. Furthermore, in the case of the metallic beryllium sample, volume expansion and peel-off of this white product was observed. Fig. 3c shows the diffraction pattern of the Be12Ti sample before exposure to the Ar gas containing 10,000 ppm of water vapor. For this sample, most of diffraction peaks were attributed to Be12Ti (marked with closed square), and diffraction peaks related to metal oxides were not observed. Fig. 3d shows the diffraction pattern of the Be12Ti sample after exposure to the Ar gas containing 10,000 ppm of water vapor. In the case of the Be12Ti sample, white products only appeared as limited spots on the surface, and neither swelling nor peel-off of the surface product was observed. Diffraction peaks related to BeO were observed on the sample surface, whereas the intensity of the diffraction peak attributed to BeO was quite small compared with the oxidized metallic beryllium sample. The results of this analysis and the experiments shown above indicate that the Be12Ti sample possesses high oxidation resistance.
5. Conclusion The oxidation resistance of the Be12Ti sample, fabricated by a plasma-sintering method, under water vapor was investigated. A metallic beryllium sample and a Be12Ti sample were placed under an Ar gas containing 10,000 ppm of water vapor at the temperature of 1273 K for 24 h. Catastrophic oxidation reaction was not observed for the Be12Ti sample. The mass gain and the amount of hydrogen generated from the plasma-sintered Be12Ti sample were much smaller than those from the metallic beryllium sample. Furthermore, in the case of the metallic beryllium sample, volume expansion and peel-off of the surface products were observed. X-ray diffraction analysis clarified that this white product on the beryllium and the beryllide sample was BeO and the intensity of the BeO for the beryllide was much smaller than that for beryllium. Acknowledgments This work was supported by the Japan Atomic Energy Agency under the Joint Work contract 23K131, as a part of Broader Approach activities. References [1] [2] [3] [4] [5] [6] [7] [8]
H. Kawamura et al., Fusion Eng. Des. 61–62 (2002) 391–397. H. Kawamura et al., J. Nucl. Mater. 307–311 (2002) 638–642. M. Nakamichi et al., Fusion Eng. Des. 86 (2011) 2262–2264. M. Nakamichi et al., J. Nucl. Mater. 417 (2011) 765–768. D.A. Petti et al., J. Nucl. Mater. 283–287 (2000). D.W. Aylmore et al., J. Nucl. Mater. 3 (2) (1961) 190–200. K. Munakata et al., Fusion Eng. Des. 75–79 (2005) 997–1002. K. Munakata et al., J. Nucl. Mater. 329–333 (2004) 1357–1360.
Contents lists available at SciVerse ScienceDirect
Journal of Nuclear Materials journal homepage: www.elsevier.com/locate/jnucmat
Oxidation resistance of Be12Ti fabricated by plasma-sintering method K. Wada a,⇑, K. Munakata a, J.H Kim b, K. Yonehara b, D. Wakai b, M. Nakamichi b a b
Faculty of engineering and resource science, Akita University, 1-1, TegataGakuen, Akita 010-8502, Japan Fusion Research and Development Directorate, Japan Atomic Energy Agency, 2-166, Omotedate, Obuchi, Rokkasho, Kamikita, Aomori 039-3212, Japan
a r t i c l e
i n f o
Article history: Available online 17 May 2013
a b s t r a c t Titanium beryllium intermetallic compounds (beryllides), such as Be12Ti, are alternatives to metallic beryllium as the neutron multiplier for the fusion reactor blanket. Be12Ti is known to have advantages over metallic beryllium, which includes a high melting point, lower swelling and higher chemical stability. This work examined the oxidation resistance of Be12Ti samples fabricated by a plasma-sintering. Metallic beryllium and Be12Ti samples were placed under a flow of an argon gas containing 10,000 ppm of water vapor at the temperature of 1273 K for 24 h. The Be12Ti sample fabricated was shown to have higher oxidation resistance than metallic beryllium. Ó 2013 Published by Elsevier B.V.
1. Introduction Generating tritium to fuel next generation fusion reactors requires the development of a neutron multiplier to be used in the blanket modules. Metallic beryllium has been investigated as a neutron multiplier candidate. However, metallic beryllium is highly reactive with water vapor and oxygen at high temperatures, producing H2 gas and BeO via the following reaction:
Be þ H2 O ! BeO þ H2
ð1Þ
2Be þ O2 ! 2BeO
ð2Þ
H2 is highly explosive, and BeO is harmful to human body. These drawbacks would threaten the safety of fusion reactors, particularly in the case of loss-of-coolant accident (LOCA) in which a coolant line break injects water vapor into the blanket. Therefore, development of new neutron multiplier, which is more chemically stable than metallic beryllium, is necessary. Titanium beryllide is known to have high melting point, lower swelling, and higher chemically stability than metallic beryllium [1]. In particular, compatibility between Be12Ti and SS316LN was evaluated and the advantage of Be12Ti as a neutron multiplier was proven [2]. Be12Ti had been mainly fabricated by not only a hot isostatic pressing (HIP) process but also by casting. HIPing requires repeated, lengthy processing and cast materials have poor composition control. Therefore, these methods require long fabrication periods and high costs. Thus Japan Atomic Energy Agency (JAEA) has suggested a plasma-sintering method for fabrication of the beryllide. This method consists of uniaxial pressing, plasma generation and heating with advantages in ⇑ Corresponding author. Tel.: +81 018 889 2749. E-mail address: [email protected] (K. Wada). 0022-3115/$ - see front matter Ó 2013 Published by Elsevier B.V. http://dx.doi.org/10.1016/j.jnucmat.2013.04.093
terms of easy-to-control and short fabrication period [3,4]. This work investigates the reactivity of a plasma-sintered Be12Ti sample with water vapor.
2. Experimental Be and Ti powders with average particle size under 45 lm, were mixed for 60 min by using a mortar grinder RM200 (Retsch, Germany). The mixed ratio was adjusted to the stoichiometric composition of Be12Ti (Be:Ti = 69.3:30.7 wt.%) and this blended powder was sintered with a plasma sintering device KE-PAS III (manufactured by KAKEN Co. Ltd.). Be12Ti discs 20 mm in diameter and 5 mm in thickness, were synthesized by sintering for 40 min at a temperature of 1273 K at 50 MPa. The surface of the plasma -sintered Be12Ti sample is composed of four phases such as Be, Be12Ti, Be17Ti2 and Be2Ti with different fractions. The area fraction of Be in sintered Be12Ti sample surface is 2%. On the other hand, the area fractions of Be12Ti, Be17Ti2 and Be2Ti in the sintered Be12Ti sample surface are 98% [4]. A plasma-sintered Be12Ti sample and a metallic beryllium sample were cut (2 cm diameter and 5 mm thick). Then, each sample was further cut to 3 3 5 mm pieces and polished with 5-lm SiC grit. Fig. 1 shows the experimental apparatus. The metallic beryllium sample and the plasma-sintered Be12Ti sample were placed under a flow of Ar gas containing 10,000 ppm (1%) water vapor. Although the mechanism of hydrogen generation by the reaction between Be12Ti and water vapor at high temperature has not been well understood, it appears that hydrogen is generated by the reaction in Eqs. (1) and (2) when Be12Ti is exposed to water vapor at high temperatures. The temperature of the samples was raised from 313 to 1273 K at 5 K/min. After the reactor temperature reached
K. Wada et al. / Journal of Nuclear Materials 442 (2013) S494–S496
S495
Fig. 1. Experimental apparatus.
1273 K, it was held at 1273 K for 24 h. The flow rate of the Ar gas containing 10,000 ppm of water vapor was 300 cm3/min. The mass gain curve and the concentration of hydrogen generated from the samples were measured with a thermogravimetry-differential thermal analysis and a mass spectrometry instrument manufactured by RIGAKU Co. Ltd. The oxidized sample surface was characterized by means of the X-ray diffraction analysis using Ultima IV, that was also manufactured by RIGAKU Co. Ltd.
3. Results and discussions Fig. 2a and b show mass gain curves and changes in concentration of hydrogen in the outlet stream of the reactor charged with metallic beryllium and the plasma-sintered Be12Ti samples. In this experiment, we calculated hydrogen concentration from the mass gain of the sample by assuming that beryllium which reacted to water vapor turned into BeO. The horizontal and right vertical axes represent duration time and test temperature, respectively. The first left vertical axis and the second left vertical axis represent the concentration of hydrogen generated from the surface of the samples and the mass gain of the samples, respectively. The dash-dot line shows the mass gain of the samples and the dashed line shows the test temperature, respectively. In the case of metallic beryllium, it is known that a catastrophic oxidation reaction takes place under the conditions of high temperature and high water vapor pressure [5]. The weight of the metallic beryllium sample drastically increased and continued to increase for 24 h. The result of the previous studies by Aylmore et al. [6] also indicates that the catastrophic oxidation reaction takes place at temperatures higher than 973 K on the surface of beryllium metal. On the other hand, the mass gain of the Be12Ti sample was very slow. The final mass gain of the metallic beryllium sample was 21.75 mg, which corresponds to approximately 50% of its initial weight (44.63 mg). The final mass gain of the plasma-sintered Be12Ti sample was 1.10 mg, approximately 1.7% of its initial weight (64.18 mg). The average mass gain rate of the metallic beryllium sample and the plasma-sintered Be12Ti sample were 2.8 104 and 6.6 106 mg/sec respectively. The mass gain of the metallic beryllium sample was far greater than that of the plasma-sintered Be12Ti sample. These experimental results suggest that Be12Ti possesses greatly higher oxidation resistance to water vapor compared
Fig. 2. Mass gain curve and hydrogen concentration of (a) the metallic beryllium sample and (b) the Be12Ti sample fabricated by the plasma-sintering method. The dash-dot lines show the mass gain of the samples. The dashed lines show the test temperature. The solid lines show changes in the concentration of hydrogen in the outlet stream of the reactor.
than metallic beryllium. The amount of hydrogen concentration of the metallic beryllium sample was 24 times higher than that of the plasma-sintered Be12Ti sample. The solid lines in the figures show changes in the concentration of hydrogen in the outlet stream of the reactor. In the case of the metallic beryllium sample, a considerable amount of hydrogen was produced throughout the experiment. The amount of hydrogen generated from each 1 cm2 of the surface of the metallic beryllium sample was 1.06 103 mol. On the other hand, in the case of the plasma-sintered Be12Ti sample, the amount of hydrogen generated from each 1 cm2 of the surface of the plasma-sintered
S496
K. Wada et al. / Journal of Nuclear Materials 442 (2013) S494–S496 6x10 5
6x105
(a)
4x10 5 3x10 5 2x10 5
Be BeO
(b)
5x105
Intensity (cps)
Intensity (cps)
5x10 5
Be
4x105 3x105 2x105 1x105
1x10 5
0
0
20
40
60
80
20
100
40
4
80
100
4
6x10
6x10
Be
(c)
4
5x10
Be12Ti
4
4
5x10
Intensity (cps)
Intensity (cps)
60
2 (degree)
2 (degree)
4x10
4
3x10
4
2x10
4
Be12Ti BeO
(d)
4
4x10
4
3x10
4
2x10
4
1x10
1x10
0
20
0
40
60
80
100
20
40
60
80
100
2 (degree)
2 (degree)
Fig. 3. X-ray diffraction pattern of (a) the metallic beryllium sample before the experiment, (b) the metallic beryllium sample after the experiment, (c) the Be12Ti sample before the experiment and (d) the Be12Ti sample after the experiment. (a and b): Full scale is 6 105. (c and d): Full scale is 6 104.
Be12Ti sample was only 9.58 105 mol. In previous studies, similar experiments on Be12Ti samples manufactured by a HIP method were carried out [7,8]. The results suggest that the amount of hydrogen generated from the Be12Ti sample manufactured by HIPing was three times as large as that generated from the plasma-sintered Be12Ti sample. Thus, the plasma-sintered Be12Ti sample is considered to have been more uniformly prepared than the Be12Ti sample manufactured by the HIP method. Fig. 3a and b shows the X-ray diffraction patterns of the metallic beryllium sample before and after exposure to the Ar gas containing 10,000 ppm of water vapor, respectively. Several diffraction peaks arose (see Fig. 3b), and most of them were attributed to BeO (marked with closed triangle). After the experiment, a large amount of white product was observed on the surface of the metallic beryllium sample. Furthermore, in the case of the metallic beryllium sample, volume expansion and peel-off of this white product was observed. Fig. 3c shows the diffraction pattern of the Be12Ti sample before exposure to the Ar gas containing 10,000 ppm of water vapor. For this sample, most of diffraction peaks were attributed to Be12Ti (marked with closed square), and diffraction peaks related to metal oxides were not observed. Fig. 3d shows the diffraction pattern of the Be12Ti sample after exposure to the Ar gas containing 10,000 ppm of water vapor. In the case of the Be12Ti sample, white products only appeared as limited spots on the surface, and neither swelling nor peel-off of the surface product was observed. Diffraction peaks related to BeO were observed on the sample surface, whereas the intensity of the diffraction peak attributed to BeO was quite small compared with the oxidized metallic beryllium sample. The results of this analysis and the experiments shown above indicate that the Be12Ti sample possesses high oxidation resistance.
5. Conclusion The oxidation resistance of the Be12Ti sample, fabricated by a plasma-sintering method, under water vapor was investigated. A metallic beryllium sample and a Be12Ti sample were placed under an Ar gas containing 10,000 ppm of water vapor at the temperature of 1273 K for 24 h. Catastrophic oxidation reaction was not observed for the Be12Ti sample. The mass gain and the amount of hydrogen generated from the plasma-sintered Be12Ti sample were much smaller than those from the metallic beryllium sample. Furthermore, in the case of the metallic beryllium sample, volume expansion and peel-off of the surface products were observed. X-ray diffraction analysis clarified that this white product on the beryllium and the beryllide sample was BeO and the intensity of the BeO for the beryllide was much smaller than that for beryllium. Acknowledgments This work was supported by the Japan Atomic Energy Agency under the Joint Work contract 23K131, as a part of Broader Approach activities. References [1] [2] [3] [4] [5] [6] [7] [8]
H. Kawamura et al., Fusion Eng. Des. 61–62 (2002) 391–397. H. Kawamura et al., J. Nucl. Mater. 307–311 (2002) 638–642. M. Nakamichi et al., Fusion Eng. Des. 86 (2011) 2262–2264. M. Nakamichi et al., J. Nucl. Mater. 417 (2011) 765–768. D.A. Petti et al., J. Nucl. Mater. 283–287 (2000). D.W. Aylmore et al., J. Nucl. Mater. 3 (2) (1961) 190–200. K. Munakata et al., Fusion Eng. Des. 75–79 (2005) 997–1002. K. Munakata et al., J. Nucl. Mater. 329–333 (2004) 1357–1360.