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Compatibility of insulating ceramic materials with liquid breeders
Fusion Engineering and Design 39 – 40 (1998) 811 – 817
Compatibility of insulating ceramic materials with liquid breeders Takaaki Mitsuyama a, Takayuki Terai b,*, Toshiaki Yoneoka a, Satoru Tanaka a a
Department of Quantum Engineering and Systems Science, Uni6ersity of Tokyo, Bunkyo-ku, Tokyo 113, Japan b Engineering Research Institute, Uni6ersity of Tokyo, Bunkyo-ku, Tokyo 113, Japan
Abstract The development of ceramic coating is one of the most important subjects in liquid blanket R&D. The compatibility of candidate ceramic materials (Y2O3, Al2O3, MgO, 3Al2O3 – MgO, AlN – BN and BN) with liquid metal breeders such as metallic lithium and lithium-lead alloy (Lil7 – Pb83) was investigated at 773 K up to 5 Ms with the change in insulating property. Al2O3 and 3Al2O3 – MgO were severely corroded and dissolved or broken by lithium, while MgO was corroded uniformly with a moderate rate (e.g. 27 mm for 4.8 Ms). The most thermodynamically stable Y2O3 was a little corroded and showed a slight increase in electrical conductivity. On the other hand, all the ceramic materials were not corroded at all by Lil7–Pb83, as predicted from a thermodynamical analysis. AlN – BN and BN corroded by lithium became more fragile because impurities included in the specimens were dissolved in lithium. © 1998 Elsevier Science S.A. All rights reserved.
1. Introduction In a D-T fusion reactor system, tritium must be produced. Liquid metal breeders can be utilized as a coolant as well. In a self-cooled liquid blanket concept, so-called MHD pressure drop caused by the influence of a strong magnetic field on liquid metal flow becomes a problem. As a way to reduce MHD pressure drop, insulating ceramic coating on the duct wall has been proposed. It is also useful as a tritium permeation barrier to decrease tritium leakage through blanket structural materials and as a corrosion barrier to
* Corresponding author. Tel.: +81 3 38122111; fax: + 81 3 58006824; e-mail: [email protected]
protect structural materials from liquid metal breeders [1,2]. In this study, oxides such as yttria (Y2O3), corundum (Al2O3), magnesium oxide (MgO) and spinel (MgAl2O4) and nitrides such as aluminum nitride and boron nitride mixed specimen (AlN– BN) and boron nitride (BN) considered as candidate materials for ceramic coating in liquid blankets were examined in compatibility with liquid breeders such as molten lithium and lithium– lead alloy (Lil7–Pb83). Some specimens of each material were immersed in liquid breeders such as molten lithium and lithium–lead alloy (Lil7– Pb83) and their changes in morphology, weight and electrical resistivity were investigated to clarify the corrosion mechanism and what kind of material is the best for liquid blanket coating.
0920-3796/98/$19.00 © 1998 Elsevier Science S.A. All rights reserved. PII S0920-3796(98)00382-2
T. Mitsuyama et al. / Fusion Engineering and Design 39–40 (1998) 811–817
812
2. Experimental In the examination, the liquid breeder materials were lithium (whose purity was 99.9%) and Lil7– Pb83, and the oxide and nitride specimens were poly-crystalline yttria (Y2O3, 17 × 17 × 1 mm), single-crystalline corundum (Al2O3, f20 ×1 mm), single crystalline magnesium oxide (MgO, 20× 20×1 mm), spinel (f24 × 1 mm) having the composition of 3Al2O3 – MgO instead of stoichiometric compound MgAl2O4, sintered aluminum nitride and boron nitride (AlN–BN, f21 ×1 mm) and sintered boron nitride (BN, f20 ×1 mm). The composition of the Y2O3 specimens was 99.5% Y2O3 +0.5% TiO2, and that of the AlN–BN sintered specimens was AlN:BN = 6:4. The sintered specimens contained the following impurities, Ca B1000, Cr B 10, Fe B60, MgB 10, SiB 100, NiB10 and CB600 ppm and O B 1.5%. The BN specimen also contained B2O3 as an impurity. The experimental apparatus is shown in Fig. 1. The liquid breeder material used in the corrosion test was about 40 – 60 cm3 per batch. In a glove box with Ar gas, the crucible loaded with specimens and liquid breeder material were set in a
Fig. 2. Diagram of thermochemical stability of oxides. Dotted lines represent the cases of solution with several oxygen concentrations.
heating container made of AISI type 316 stainless steel. During the heating at 773 K, Ar gas was made to flow over the liquid. After the corrosion experiment, lithium metal adhering to the corroded specimens was cleaned with water and ethyl-alcohol, and Lil7–Pb83 was cleaned with a mixed solution of acetic acid and hydrogen peroxide and subsequently, water and ethyl-alcohol.
3. Results and discussion
Fig. 1. Experimental apparatus.
The size of the oxide specimens and their corrosion time are listed in Table 1. The diagrams showing the thermochemical stability of oxides and nitrides are shown in Figs. 2 and 3 [3,4]. Corundum and spinel are less stable than Li2O. Magnesium oxide is more stable than Li2O in the case of the Li–Li2O coexistent system. But if oxygen activity is smaller than that of the Li– Li2O coexistent system, magnesium oxide is less stable than Li2O. Yttria is clearly more stable than Li2O. Regarding nitrides, aluminum nitride and boron nitride are stable in lithium. From
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these results, we can estimate thermodynamically what kind of materials are stable in a given condition. Yttria and the nitrides are expected to be stable in the presence of liquid lithium. In the experimental system, the concentrations of oxygen and nitrogen in Li and Lil7 – Pb83 are unknown, but oxygen and nitrogen contained in Ar gas as impurities reacted with lithium vapor in the upper part of the experimental apparatus to form Li2O and Li3N, which adhered in the inner part of the apparatus. Therefore, it is presumed that liquid Li in the crucible contained little amount of oxygen and nitrogen.
3.1. Yttria specimens immersed in lithium All the corroded yttria specimens changed from the initial white color to black, because of the reduction to hypo-stoichiometry, Y2O3 − x. The yttria specimens were, however, almost sound in dimension except for a small thickness increase (B1%). A typical cross-sectional compositional image of a corroded yttria specimen by EPMA is shown in Fig. 4. The dark thin layer observed on the yttria surface was identified as LiYO2 by
Fig. 3. Diagram of thermochemical stability of nitrides. Dotted lines represent the case of solution with the nitrogen concentration.
Fig. 4. Typical cross-sectional compositional image of a corroded yttria specimen by EPMA.
XRD analysis. The LiYO2 layer gradually increased in its thickness roughly in proportion to immersion time [5]. The electrical resistivity obtained by the Cole– Cole plot using the conventional four-probe method at room temperature is shown in Fig. 5. The electrical resistivity also decreased from its initial value with time. The reason why the electrical resistivity decreased is that yttria specimens were reduced to the hypo-stoichiometric yttria, which had lower electrical resistivity than initial stoichiometric yttria. The value of electrical resistivity of yttria immersed in lithium for 5.1 Ms was in the order of 107 Vm at room temperature.
Fig. 5. Relation of electrical resistivity of corroded yttria specimen at room temperature with corrosion time.
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Fig. 6. Photograph of magnesium oxide specimen corroded for 4.8 Ms.
3.2. Corundum specimens immersed in lithium The corundum specimens were dissolved in molten lithium within 0.7 Ms. The reason is presumably that corundum is thermodynamically unstable against lithium at 773 K. The formation of stable complex oxides such as LiAlO2 against molten lithium was not observed. It was clearly shown that corundum has no corrosion resistance to molten lithium.
3.3. Magnesium oxide specimens immersed in lithium Photograph of magnesium oxide specimens corroded for 4.8 Ms is shown in Fig. 6. The corroded magnesium specimens changed from its initial transparence to white due to surface roughening, and the color change increased with time. The
Fig. 7. Photograph of spinel specimen corroded for 5.1 Ms.
magnesium oxide specimens were locally corroded from their edges and corners. A magnesium oxide single crystal cleaves easily. The specimens were made by the cleavage method except for thin edge parts. Presumably, this corrosion behavior from the edge parts is caused by the existence of many microscopic cracks at their edge parts due to mechanical cutting. The thickness of the specimens corroded for 4.8 Ms was reduced by about 27 mm. From a thermodynamic standpoint, magnesium oxide is stable against lithium at 773 K in the case of the Li– Li2O coexistent system. However, magnesium oxide is unstable against lithium if the oxygen activity is smaller than that of the Li–Li2O coexistent system. The corrosion resistance of magnesium oxide is not sufficient with regard to the postulated conditions of this work. Nevertheless, the electrical resistivity value of the magnesium oxide specimens at room temperature was beyond the measurable limit of 1010 Vm. The XRD pattern of the corroded MgO specimen did not change from that of the uncorroded MgO specimen.
3.4. Spinel specimens immersed in lithium The corroded spinel specimens were broken while they were immersed in lithium. Especially a part of the specimens corroded for 0.7 and 2.8 Ms could not be taken from lithium. A photograph of spinel specimen corroded for 5.1 Ms is shown in Fig. 7. The corroded spinel specimens changed from the initial transparence to white or grey due to surface roughening. The spinel specimens were severely corroded except the rim and the small center part. The surface of the severely corroded part was very rough, and the thickness of the part was decreased considerably. The spinel specimens corroded over 2.8 Ms had many small holes through the part. XRD patterns of uncorroded and the corroded specimens are shown in Fig. 8. By XRD analysis, magnesium oxide peak was weakly observed on the corroded spinel specimens. In view of the result, the reason why the part was corroded severely is that the alumina
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Fig. 8. XRD patterns of spinel specimens (a) uncorroded and (b) corroded for 5.1 Ms.
component enriched in the part was dissolved in molten lithium. Magnesium oxide as a corrosion-resistant material is superior to the molten lithium. The magnesium oxide on the spinel surface is, however, not protective against lithium and it is estimated to be porous. The electrical resistivity values of the spinel specimens corroded below 1.3 Ms remained large beyond the measurable limit of about 1010 Vm.
3.5. All oxide specimens immersed in lithium-lead alloy None of oxide specimens (Y2O3, Al2O3, MgO and MgAl2O4) immersed in lithium-lead alloy were corroded except a little change in surface color This result is reasonable from a thermodynamic consideration i.e. that lithium activity decreased so much in a lithium-lead alloy.
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Fig. 9. Photograph of AlN-BN specimens uncorroded and corroded for 0.6, 1.8 and 5.1 Ms.
3.6. Aluminum nitride and boron nitride mixed specimens immersed in lithium In the experiment, three specimens were immersed in lithium for 0.6, 1.8 and 5.1 Ms. A photograph of uncorroded and corroded AlN– BN specimens is shown in Fig. 9. The AlN–BN specimen corroded for 0.6 Ms did not change in its size but slightly changed in its surface color. The specimen corroded for 1.8 Ms had many cracks, with a large change in its surface color and shape. The specimen corroded for 5.1 Ms did not change in its appearance but was covered with lithium when the specimen was taken from lithium. When the lithium was cleaned with water, the specimen broke into pieces in water as shown in Fig. 9.
Fig. 10. Photograph of BN specimens uncorroded and corroded for 0.6 Ms.
The weight of the specimen corroded for 0.6 Ms decreased by 1.8% as compared with its initial value and that of the specimen corroded for 1.8 Ms decreased by 9.5%. By XRD analysis, only aluminium oxide disappear on the corroded specimens. The electrical resistivity value of the AlN– BN specimen corroded for 0.6 Ms remained larger than the measurable limit of about 1010 Vm at room temperature. It was estimated that its degradation from the initial value was not large. Aluminum nitride and boron nitride are thermodynamically stable against lithium at 773 K and the appearance of the corroded specimens did not change when they were taken from lithium. Accordingly, it is considered that the most part of aluminum nitride and boron nitride was not dissolved in lithium and only aluminum oxide as an impurity was dissolved in lithium, so that the specimens became fragile. The specimens corroded for 5.1 Ms broke into pieces in water due to the stress generated by reaction between water and lithium. It is expected that the AlN–BN specimens including no impurities unstable against lithium have high corrosion resistance against lithium.
3.7. Boron nitride specimens immersed in lithium The boron nitride specimens were immersed in lithium for 0.6, 1.8 and 5.1 Ms. A photograph of boron nitride specimens uncorroded and corroded for 0.6 Ms is shown in Fig. 10. The boron nitride specimen corroded for 0.6 Ms did not change its appearance, but was covered with lithium when the specimen was taken from lithium. When the lithium adhered to the specimen and was cleaned with water a part of the specimen was broken and the left part became fragile. The boron nitride specimens corroded for 1.8 and 5.1 Ms did not change its apparent form either, but the specimens broke into pieces when they were immersed in water. In view of the result, it is considered that boron nitride was not dissolved in lithium but impurities were dissolved in lithium, so that the boron nitride specimen became fragile, like AlN– BN specimens. The reason why boron nitride specimens were corroded more than AlN–BN specimens is that boron nitride has a higher wettability to lithium than the AlN–BN specimen.
T. Mitsuyama et al. / Fusion Engineering and Design 39–40 (1998) 811–817
4. Conclusion The oxide and the nitride specimens were corroded by lithium or a lithium-lead alloy at 773 K. The yttria specimens corroded by lithium hardly changed their size but the electrical resistivity were slightly decreased. The corundum specimens were dissolved in lithium within 0.7 Ms. The magnesium oxide specimens corroded by lithium were almost sound in dimension and did not change electrical resistivity. The spinel specimen corroded by lithium had many small holes through them. On the other hand, all the oxide specimens had good corrosion resistance against lithium-lead alloy. The AlN– BN specimens and the boron nitride specimens corroded by lithium became fragile because impurities included in the specimens were
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817
dissolved in lithium. Accordingly it is expected that the AlN–BN specimens and boron nitride specimens containing no unstable impurities against lithium have good compatibility with lithium.
References [1] S. Malang, Fusion Eng. Des. 16 (1991) 95 – 109. [2] S. Malang, H.U. Borgstedt, F.H. Farnum, K. Natesan, I.V. Vitkovski, Fusion Eng. Des. 27 (1995) 570 – 586. [3] R.N. Singh, J. Am. Ceram. Soc. 59 (1976) 112 – 115. [4] The Japan Society of Calorimetry and Thermal Analysis, MALT (Materials oriented little thermodynamic data base), Kagakugijutsusha, 1985. [5] T. Terai, T. Yoneoka, H. Tanaka, A. Suzuki, S. Tanaka, M. Nakamichi, H. Kawamura, K. Miyajima, Y. Harada, J. Nucl. Mater. 233 – 237 (1996) 1421.
Compatibility of insulating ceramic materials with liquid breeders Takaaki Mitsuyama a, Takayuki Terai b,*, Toshiaki Yoneoka a, Satoru Tanaka a a
Department of Quantum Engineering and Systems Science, Uni6ersity of Tokyo, Bunkyo-ku, Tokyo 113, Japan b Engineering Research Institute, Uni6ersity of Tokyo, Bunkyo-ku, Tokyo 113, Japan
Abstract The development of ceramic coating is one of the most important subjects in liquid blanket R&D. The compatibility of candidate ceramic materials (Y2O3, Al2O3, MgO, 3Al2O3 – MgO, AlN – BN and BN) with liquid metal breeders such as metallic lithium and lithium-lead alloy (Lil7 – Pb83) was investigated at 773 K up to 5 Ms with the change in insulating property. Al2O3 and 3Al2O3 – MgO were severely corroded and dissolved or broken by lithium, while MgO was corroded uniformly with a moderate rate (e.g. 27 mm for 4.8 Ms). The most thermodynamically stable Y2O3 was a little corroded and showed a slight increase in electrical conductivity. On the other hand, all the ceramic materials were not corroded at all by Lil7–Pb83, as predicted from a thermodynamical analysis. AlN – BN and BN corroded by lithium became more fragile because impurities included in the specimens were dissolved in lithium. © 1998 Elsevier Science S.A. All rights reserved.
1. Introduction In a D-T fusion reactor system, tritium must be produced. Liquid metal breeders can be utilized as a coolant as well. In a self-cooled liquid blanket concept, so-called MHD pressure drop caused by the influence of a strong magnetic field on liquid metal flow becomes a problem. As a way to reduce MHD pressure drop, insulating ceramic coating on the duct wall has been proposed. It is also useful as a tritium permeation barrier to decrease tritium leakage through blanket structural materials and as a corrosion barrier to
* Corresponding author. Tel.: +81 3 38122111; fax: + 81 3 58006824; e-mail: [email protected]
protect structural materials from liquid metal breeders [1,2]. In this study, oxides such as yttria (Y2O3), corundum (Al2O3), magnesium oxide (MgO) and spinel (MgAl2O4) and nitrides such as aluminum nitride and boron nitride mixed specimen (AlN– BN) and boron nitride (BN) considered as candidate materials for ceramic coating in liquid blankets were examined in compatibility with liquid breeders such as molten lithium and lithium– lead alloy (Lil7–Pb83). Some specimens of each material were immersed in liquid breeders such as molten lithium and lithium–lead alloy (Lil7– Pb83) and their changes in morphology, weight and electrical resistivity were investigated to clarify the corrosion mechanism and what kind of material is the best for liquid blanket coating.
0920-3796/98/$19.00 © 1998 Elsevier Science S.A. All rights reserved. PII S0920-3796(98)00382-2
T. Mitsuyama et al. / Fusion Engineering and Design 39–40 (1998) 811–817
812
2. Experimental In the examination, the liquid breeder materials were lithium (whose purity was 99.9%) and Lil7– Pb83, and the oxide and nitride specimens were poly-crystalline yttria (Y2O3, 17 × 17 × 1 mm), single-crystalline corundum (Al2O3, f20 ×1 mm), single crystalline magnesium oxide (MgO, 20× 20×1 mm), spinel (f24 × 1 mm) having the composition of 3Al2O3 – MgO instead of stoichiometric compound MgAl2O4, sintered aluminum nitride and boron nitride (AlN–BN, f21 ×1 mm) and sintered boron nitride (BN, f20 ×1 mm). The composition of the Y2O3 specimens was 99.5% Y2O3 +0.5% TiO2, and that of the AlN–BN sintered specimens was AlN:BN = 6:4. The sintered specimens contained the following impurities, Ca B1000, Cr B 10, Fe B60, MgB 10, SiB 100, NiB10 and CB600 ppm and O B 1.5%. The BN specimen also contained B2O3 as an impurity. The experimental apparatus is shown in Fig. 1. The liquid breeder material used in the corrosion test was about 40 – 60 cm3 per batch. In a glove box with Ar gas, the crucible loaded with specimens and liquid breeder material were set in a
Fig. 2. Diagram of thermochemical stability of oxides. Dotted lines represent the cases of solution with several oxygen concentrations.
heating container made of AISI type 316 stainless steel. During the heating at 773 K, Ar gas was made to flow over the liquid. After the corrosion experiment, lithium metal adhering to the corroded specimens was cleaned with water and ethyl-alcohol, and Lil7–Pb83 was cleaned with a mixed solution of acetic acid and hydrogen peroxide and subsequently, water and ethyl-alcohol.
3. Results and discussion
Fig. 1. Experimental apparatus.
The size of the oxide specimens and their corrosion time are listed in Table 1. The diagrams showing the thermochemical stability of oxides and nitrides are shown in Figs. 2 and 3 [3,4]. Corundum and spinel are less stable than Li2O. Magnesium oxide is more stable than Li2O in the case of the Li–Li2O coexistent system. But if oxygen activity is smaller than that of the Li– Li2O coexistent system, magnesium oxide is less stable than Li2O. Yttria is clearly more stable than Li2O. Regarding nitrides, aluminum nitride and boron nitride are stable in lithium. From
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these results, we can estimate thermodynamically what kind of materials are stable in a given condition. Yttria and the nitrides are expected to be stable in the presence of liquid lithium. In the experimental system, the concentrations of oxygen and nitrogen in Li and Lil7 – Pb83 are unknown, but oxygen and nitrogen contained in Ar gas as impurities reacted with lithium vapor in the upper part of the experimental apparatus to form Li2O and Li3N, which adhered in the inner part of the apparatus. Therefore, it is presumed that liquid Li in the crucible contained little amount of oxygen and nitrogen.
3.1. Yttria specimens immersed in lithium All the corroded yttria specimens changed from the initial white color to black, because of the reduction to hypo-stoichiometry, Y2O3 − x. The yttria specimens were, however, almost sound in dimension except for a small thickness increase (B1%). A typical cross-sectional compositional image of a corroded yttria specimen by EPMA is shown in Fig. 4. The dark thin layer observed on the yttria surface was identified as LiYO2 by
Fig. 3. Diagram of thermochemical stability of nitrides. Dotted lines represent the case of solution with the nitrogen concentration.
Fig. 4. Typical cross-sectional compositional image of a corroded yttria specimen by EPMA.
XRD analysis. The LiYO2 layer gradually increased in its thickness roughly in proportion to immersion time [5]. The electrical resistivity obtained by the Cole– Cole plot using the conventional four-probe method at room temperature is shown in Fig. 5. The electrical resistivity also decreased from its initial value with time. The reason why the electrical resistivity decreased is that yttria specimens were reduced to the hypo-stoichiometric yttria, which had lower electrical resistivity than initial stoichiometric yttria. The value of electrical resistivity of yttria immersed in lithium for 5.1 Ms was in the order of 107 Vm at room temperature.
Fig. 5. Relation of electrical resistivity of corroded yttria specimen at room temperature with corrosion time.
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Fig. 6. Photograph of magnesium oxide specimen corroded for 4.8 Ms.
3.2. Corundum specimens immersed in lithium The corundum specimens were dissolved in molten lithium within 0.7 Ms. The reason is presumably that corundum is thermodynamically unstable against lithium at 773 K. The formation of stable complex oxides such as LiAlO2 against molten lithium was not observed. It was clearly shown that corundum has no corrosion resistance to molten lithium.
3.3. Magnesium oxide specimens immersed in lithium Photograph of magnesium oxide specimens corroded for 4.8 Ms is shown in Fig. 6. The corroded magnesium specimens changed from its initial transparence to white due to surface roughening, and the color change increased with time. The
Fig. 7. Photograph of spinel specimen corroded for 5.1 Ms.
magnesium oxide specimens were locally corroded from their edges and corners. A magnesium oxide single crystal cleaves easily. The specimens were made by the cleavage method except for thin edge parts. Presumably, this corrosion behavior from the edge parts is caused by the existence of many microscopic cracks at their edge parts due to mechanical cutting. The thickness of the specimens corroded for 4.8 Ms was reduced by about 27 mm. From a thermodynamic standpoint, magnesium oxide is stable against lithium at 773 K in the case of the Li– Li2O coexistent system. However, magnesium oxide is unstable against lithium if the oxygen activity is smaller than that of the Li–Li2O coexistent system. The corrosion resistance of magnesium oxide is not sufficient with regard to the postulated conditions of this work. Nevertheless, the electrical resistivity value of the magnesium oxide specimens at room temperature was beyond the measurable limit of 1010 Vm. The XRD pattern of the corroded MgO specimen did not change from that of the uncorroded MgO specimen.
3.4. Spinel specimens immersed in lithium The corroded spinel specimens were broken while they were immersed in lithium. Especially a part of the specimens corroded for 0.7 and 2.8 Ms could not be taken from lithium. A photograph of spinel specimen corroded for 5.1 Ms is shown in Fig. 7. The corroded spinel specimens changed from the initial transparence to white or grey due to surface roughening. The spinel specimens were severely corroded except the rim and the small center part. The surface of the severely corroded part was very rough, and the thickness of the part was decreased considerably. The spinel specimens corroded over 2.8 Ms had many small holes through the part. XRD patterns of uncorroded and the corroded specimens are shown in Fig. 8. By XRD analysis, magnesium oxide peak was weakly observed on the corroded spinel specimens. In view of the result, the reason why the part was corroded severely is that the alumina
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Fig. 8. XRD patterns of spinel specimens (a) uncorroded and (b) corroded for 5.1 Ms.
component enriched in the part was dissolved in molten lithium. Magnesium oxide as a corrosion-resistant material is superior to the molten lithium. The magnesium oxide on the spinel surface is, however, not protective against lithium and it is estimated to be porous. The electrical resistivity values of the spinel specimens corroded below 1.3 Ms remained large beyond the measurable limit of about 1010 Vm.
3.5. All oxide specimens immersed in lithium-lead alloy None of oxide specimens (Y2O3, Al2O3, MgO and MgAl2O4) immersed in lithium-lead alloy were corroded except a little change in surface color This result is reasonable from a thermodynamic consideration i.e. that lithium activity decreased so much in a lithium-lead alloy.
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Fig. 9. Photograph of AlN-BN specimens uncorroded and corroded for 0.6, 1.8 and 5.1 Ms.
3.6. Aluminum nitride and boron nitride mixed specimens immersed in lithium In the experiment, three specimens were immersed in lithium for 0.6, 1.8 and 5.1 Ms. A photograph of uncorroded and corroded AlN– BN specimens is shown in Fig. 9. The AlN–BN specimen corroded for 0.6 Ms did not change in its size but slightly changed in its surface color. The specimen corroded for 1.8 Ms had many cracks, with a large change in its surface color and shape. The specimen corroded for 5.1 Ms did not change in its appearance but was covered with lithium when the specimen was taken from lithium. When the lithium was cleaned with water, the specimen broke into pieces in water as shown in Fig. 9.
Fig. 10. Photograph of BN specimens uncorroded and corroded for 0.6 Ms.
The weight of the specimen corroded for 0.6 Ms decreased by 1.8% as compared with its initial value and that of the specimen corroded for 1.8 Ms decreased by 9.5%. By XRD analysis, only aluminium oxide disappear on the corroded specimens. The electrical resistivity value of the AlN– BN specimen corroded for 0.6 Ms remained larger than the measurable limit of about 1010 Vm at room temperature. It was estimated that its degradation from the initial value was not large. Aluminum nitride and boron nitride are thermodynamically stable against lithium at 773 K and the appearance of the corroded specimens did not change when they were taken from lithium. Accordingly, it is considered that the most part of aluminum nitride and boron nitride was not dissolved in lithium and only aluminum oxide as an impurity was dissolved in lithium, so that the specimens became fragile. The specimens corroded for 5.1 Ms broke into pieces in water due to the stress generated by reaction between water and lithium. It is expected that the AlN–BN specimens including no impurities unstable against lithium have high corrosion resistance against lithium.
3.7. Boron nitride specimens immersed in lithium The boron nitride specimens were immersed in lithium for 0.6, 1.8 and 5.1 Ms. A photograph of boron nitride specimens uncorroded and corroded for 0.6 Ms is shown in Fig. 10. The boron nitride specimen corroded for 0.6 Ms did not change its appearance, but was covered with lithium when the specimen was taken from lithium. When the lithium adhered to the specimen and was cleaned with water a part of the specimen was broken and the left part became fragile. The boron nitride specimens corroded for 1.8 and 5.1 Ms did not change its apparent form either, but the specimens broke into pieces when they were immersed in water. In view of the result, it is considered that boron nitride was not dissolved in lithium but impurities were dissolved in lithium, so that the boron nitride specimen became fragile, like AlN– BN specimens. The reason why boron nitride specimens were corroded more than AlN–BN specimens is that boron nitride has a higher wettability to lithium than the AlN–BN specimen.
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4. Conclusion The oxide and the nitride specimens were corroded by lithium or a lithium-lead alloy at 773 K. The yttria specimens corroded by lithium hardly changed their size but the electrical resistivity were slightly decreased. The corundum specimens were dissolved in lithium within 0.7 Ms. The magnesium oxide specimens corroded by lithium were almost sound in dimension and did not change electrical resistivity. The spinel specimen corroded by lithium had many small holes through them. On the other hand, all the oxide specimens had good corrosion resistance against lithium-lead alloy. The AlN– BN specimens and the boron nitride specimens corroded by lithium became fragile because impurities included in the specimens were
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dissolved in lithium. Accordingly it is expected that the AlN–BN specimens and boron nitride specimens containing no unstable impurities against lithium have good compatibility with lithium.
References [1] S. Malang, Fusion Eng. Des. 16 (1991) 95 – 109. [2] S. Malang, H.U. Borgstedt, F.H. Farnum, K. Natesan, I.V. Vitkovski, Fusion Eng. Des. 27 (1995) 570 – 586. [3] R.N. Singh, J. Am. Ceram. Soc. 59 (1976) 112 – 115. [4] The Japan Society of Calorimetry and Thermal Analysis, MALT (Materials oriented little thermodynamic data base), Kagakugijutsusha, 1985. [5] T. Terai, T. Yoneoka, H. Tanaka, A. Suzuki, S. Tanaka, M. Nakamichi, H. Kawamura, K. Miyajima, Y. Harada, J. Nucl. Mater. 233 – 237 (1996) 1421.