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Out-of-pile characterization of Al2O3 coating as electrical insulator
Fusion Engineering and Design 58 – 59 (2001) 719– 723 www.elsevier.com/locate/fusengdes
Out-of-pile characterization of Al2O3 coating as electrical insulator M. Nakamichi *, H. Kawamura Japan Atomic Energy Research Institute, Oarai Research Establishment, 3607, Oarai-machi, Higashi Ibaraki-gun, Ibaraki-ken 311 -1394, Japan
Abstract Al2O3 is one of most promising materials as electrical insulator from a point of high electrical resistivity, etc. In this study, Al2O3 coating as electrical insulator was fabricated by the plasma spraying and its properties without neutron irradiation were investigated. For Al2O3 coating on SS316LN-IG, SS410 by vacuum plasma spraying and 80Ni–20Cr by atmospheric plasma spraying were selected as the undercoating between the ceramic and the metallic substrate. Thermal shock test, electrical resistivity measurement, adhesion test and mechanical impact test were carried out. From the results of this characterization, it was clear that the specimen on which Al2O3 is coated on SS316LN-IG substrate with SS410 undercoating has good thermal shock resistivity. © 2001 Elsevier Science B.V. All rights reserved. Keywords: Electrical insulator; High electrical resistivity; Al2O3; Atmospheric plasma spraying; Vacuum plasma spraying
1. Introduction In a fusion blanket, the use of a ceramic coating on structural material has been considered for electrical insulation. Ceramic coating materials such as Al2O3 [1], MgO·Al2O3 [2], Y2O3 [3], AlN [1], CaO [4], TiN and TiC [5] have been studied in fabrication and properties up to the present. Alumina (Al2O3) is one of candidate materials for insulating coating because it has high electrical [6] and high radiation resistivity. However, crack and peeling occur by difference of thermal expansion between substrate and coating material. Therefore, * Corresponding author. Tel.: + 81-29-264-8417; fax: + 8129-264-8480. E-mail address: [email protected] (M. Nakamichi).
SS410 and 80Ni –20Cr were selected as the undercoating between SS316LN-IG substrate and Al2O3 coating, because thermal expansion coefficient of SS410 and 80Ni –20Cr were close to that of Al2O3. In this study, thermal shock test, metallographical observation, electrical resistivity measurement, adhesion test and mechanical impact test of specimen on which Al2O3 is coated on SS316LN-IG substrate with undercoating were reported.
2. Specimen In this characterization, two kinds of specimen were prepared. In order to coat Al2O3 on SS316LNIG substrate, the undercoating has been tried by two types shown as follows.
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Table 1 Spray conditions of SS410, 80Ni–20Cr and Al2O3 Material Spraying methoda Particle size (mm) Plasma gas Plasma output (kW) Spraying distance (mm) Pressure in chamber (kPa)
SS410 VPS 10–45 Ar+H2 41 275 5.9
80Ni-20Cr APS 10–45 Ar+H2 37 120 –
Al2O3 APS 10–45 Ar+H2 42 120 –
a
VPS, vacuum plasma spraying; APS, atmospheric plasma spraying.
Type 1: vacuum plasma spraying using SS410 Type 2: atmospheric plasma spraying using 80Ni –20Cr No. 1 specimen was undercoated by type 1 on the SS316LN-IG substrate, and no. 2 specimen was undercoated by type 2 on the SS316LN-IG substrate. Al2O3 was coated as a top layer on each undercoating. Coating process of each specimen is shown as follows. The surface of SS316LN-IG substrate (50 mm long, 20 mm wide and 5 mm thick) was degreased and blasted with Al2O3 grit. As to no. 1 specimen, the thickness of the SS410 undercoating deposited by the vacuum plasma spraying was about 150 mm on the substrate. The particle size of the SS410 powder was from 10 to 45 mm. Spraying
conditions of SS410 undercoating are shown in Table 1. As to no. 2 specimen, the thickness of the 80Ni–20Cr undercoating deposited by the atmospheric plasma spraying was about 50 mm on the substrate. The particle size of the 80Ni–20Cr powder was from 10 to 45 mm. Spraying conditions of 80Ni– 20Cr undercoating are shown in Table 1. The thickness of Al2O3 (purity: 99.6 wt.%) coating as a top coating deposited by the atmospheric plasma spraying was about 200 mm. The particle size of Al2O3 powder was from 10 to 45mm. Spraying conditions of the Al2O3 coating are shown in Table 1. Chemical compositions of SS316LN-IG, SS410, 80Ni–20Cr and Al2O3 are shown in Table 2. These undercoatings and the top coating were sprayed by TOCALO Co. Ltd.
3. Thermal shock resistivity For investigation of the variation of thermal shock resistivity of two kinds of specimen (no. 1 and no. 2), thermal shock test was performed by water quenching from 500, 600, 700 and 800 °C. Four specimens (one specimen per each temperature) were tested. The holding time at each temperature was 30 min. Thermal shock test repeated
Table 2 Chemical components of SS316LN-IG, SS410, 80Ni–20Cr and Al2O3 Chemical compositions (wt.%) SUS316LN-IG C Si 0.029 0.44
Mn 1.64
P 0.012
S 0.009
Ni 12.11
Cr 17.48
Mo 2.56
N 0.067
Co 0.02
Cu 0.09
B 0.0003
Nb B0.01
Ta B0.05
Ti B0.01
Fe Bal.
SUS410 C 0.03
Si 0.88
Mn 0.01
P 0.015
S 0.01
Ni 0.09
Cr 12.4
Fe Bal
80Ni–20Cr C 0.037
Si 1.0
Mn 0.027
Ni Bal.
Cr 19.8
Fe 0.13
Al2O3 Na2O 0.123
SiO2 0.01
Fe2O3 0.01
Al2O3 Bal.
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Fig. 3. Configuration of testing piece for adhesion test.
Fig. 1. Peeling cycles of the Al2O3 coating for thermal shock test. (30) represents that the Al2O3 coating was sound after 30 cycles.
up to 30 cycles from holding temperature to water. The results of visual observation of the Al2O3 coating surface after thermal shock test are shown in Fig. 1. From the results of visual observation of coating surface, no. 1 specimens were sound after 30 cycles of thermal shock test at 500, 600, 700 and 800 °C. No. 2 specimens were sound after 30 cycles of thermal shock test at 500, 600 and 700 °C. However, peeling of Al2O3 coating of no. 2 specimen was observed after ten cycles at 800 °C. The results of metallographical observation of coating cross-section is shown in Fig. 2. From these photographs, it was observed that the porosity in the undercoating of type 1 was higher than that of type 2. Therefore, it was assumed
Fig. 2. Metallographical observation of the coating cross-section.
that the thermal shock resistivity on peeling property of Al2O3 coating from SS316LN-IG substrate improved by the relaxation of thermal stress by pores in the undercoating.
4. Adhesion strength Configuration of testing piece for adhesion test was shown in Fig. 3. Testing pieces with and without coating were joined by the bonding agent. Next, one and the other end of the joined testing piece were pulled at 2.3 mm/min tensile speed. Adhesion was measured at room temperature with three testing pieces. The results of adhesion test were shown in Fig. 4. The average of the adhesion strength of no. 1
Fig. 4. Results of adhesion test.
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M. Nakamichi, H. Kawamura / Fusion Engineering and Design 58–59 (2001) 719–723
From results of electrical resistivity measurement, it was obvious that no. 1 specimen and no. 2 specimen had sufficient electrical resistivity.
6. Mechanical impact resistivity
Fig. 5. Results of electrical resistivity measurement.
specimen and no. 2 specimen were about 71.0 and 82.4 MPa, respectively. From the results of the appearance of the broken-out section in adhesion test, no. 1 specimen and no. 2 specimen were broken-out along the interface between the Al2O3 coating and bonding agent. From the results of the adhesion test, it was clear that no. 1 specimen and no. 2 specimen did not make much difference on adhesion strength.
5. Electrical resistivity Electrical resistivities of no. 1 specimen and no. 2 specimen were measured from room temperature to 800 °C in Ar atmosphere. Ag paste was applied on the Al2O3 coating as an electrode. The applied voltage was DC 500 V. The results of electrical resistivity measurement are shown in Fig. 5. In Fig. 5, reference data also are shown [7,8]. It was assumed that electrical resistivities of no. 1 and no. 2 specimen were higher than that of plasma sprayed Al2O3 coating measured by Pirogov with difference of Al2O3 purity. Therefore, electrical resistivities of no. 1 and no. 2 specimen were close to that of sintered Al2O3 measured by Shikama. Electrical resistivities of no. 1 specimen and no. 2 specimen at room temperature and 800 °C were above 1× 1013 and 5 ×106 V cm, respectively.
Al2O3 coating receive mechanical impact as an electromagnetic force at ITER plasma disruption. In this test, a steel hammer was dropped on the surface of the Al2O3 coating as mechanical impact. Dropping distance was 0.2 m, impact energy was about 14 kJ/m2. Mechanical impact test repeated up to 30,000 cycles. Impact energy and maximum cyclic number at ITER plasma disruption were about 7 kJ/m2 and 10,000cycles, respectively. Mechanical impact resistivities of No.1 specimen and No.2 specimen were measured at room temperature, 150 and 250 °C. Electrical resistance of the Al2O3 coating was measured each 1000 or 2000 cycles. The applied voltage was DC 500 V. The results of mechanical impact test are shown in Figs. 6 and 7. No. 1 specimen and no. 2 specimen at each temperature were sound after 30,000 cycles. Peeling of the Al2O3 coating of each specimen did not occurred from results of visual observation. Electrical resistance of no. 1 specimen and no. 2 specimen were constant above 1× 1012 V at each temperature. From results of the mechanical impact test, it was clear that no. 1 specimen and no.
Fig. 6. Results of mechanical impact test (no. 1 specimen).
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(2) From the results of the adhesion test, it was clear that each specimen did not make much difference on adhesion strength. (3) From the results of electrical resistivity measurement, it was clear that each specimen have sufficient electrical resistivity. (4) From the results of mechanical impact test, it was obvious that the mechanical impact resistivity of each specimen was satisfactory for the ITER design guideline.
Acknowledgements Fig. 7. Results of mechanical impact test (no. 2 specimen).
2 specimen have sufficient mechanical impact resistivity.
The authors gratefully acknowledge K. Tani and T. Teratani of TOCALO Co. Ltd. for the coating fabrication and useful discussions.
References 7. Conclusion Alumina (Al2O3) coatings fabricated on the surface of SS316LN-IG substrate with two types of undercoatings were investigated for thermal shock resistivity, adhesion strength, electrical resistivity and mechanical impact resistivity. No.1 specimen was undercoated by type 1 on the SS316LN-IG substrate. On the other hand, no. 2 specimen was undercoated by type 2 on the SS316LN-IG substrate. And Al2O3 was coated as a top coating on the each undercoating. The following conclusions were drawn. (1) From results of thermal shock test, it was clear that no. 1 specimen is superior to no. 2 specimen for thermal shock resistivity on peeling property of Al2O3 coating. It was assumed that the thermal shock resistivity of no. 1 specimen improved by the relaxation of thermal stress because of higher porosity in the undercoating.
[1] S. Malang, et al., Comparison of lithium and the eutectic lead– lithium alloy, two candidate liquid metal breeder materials for self-cooled blankets, Fusion Eng. Des. 27 (1995) 399 – 406. [2] M. Nakamichi, et al., Trial fabrication and preliminary characterization of MgO·Al2O3 coating, J. Nucl. Mater. 233 – 237 (1996) 1427 – 1430. [3] M. Nakamichi, et al., Characterization of Y2O3 coating for liquid blanket, J. Nucl. Mater. 248 (1997) 165 – 169. [4] J.-H. Park, et al., CaO insulator coatings and self-healing of defects on V – Cr– Ti alloys in liquid lithium system, J. Nucl. Mater. 233-237 (1996) 476 – 481. [5] A. Perujo, K. Forcey, Tritium permeation barriers for fusion technology, Fusion Eng. Des. 28 (1995) 252 – 257. [6] A.A. Bauer, J.L. Bates, Battelle Mem. Inst. Rept., 1930, (Jul. 31, 1974). [7] T. Shikama, Electrical degradation of ceramic insulators due to dynamic irradiation effects, Mater. Trans. JIM 36 (1996) 997 – 1003. [8] H. Ohno, Development of new materials and their testing on in-situ electrical properties in neutron environment, JAERI symposium on high energy neutron source for material research and development, (1989) 243 – 261.
Out-of-pile characterization of Al2O3 coating as electrical insulator M. Nakamichi *, H. Kawamura Japan Atomic Energy Research Institute, Oarai Research Establishment, 3607, Oarai-machi, Higashi Ibaraki-gun, Ibaraki-ken 311 -1394, Japan
Abstract Al2O3 is one of most promising materials as electrical insulator from a point of high electrical resistivity, etc. In this study, Al2O3 coating as electrical insulator was fabricated by the plasma spraying and its properties without neutron irradiation were investigated. For Al2O3 coating on SS316LN-IG, SS410 by vacuum plasma spraying and 80Ni–20Cr by atmospheric plasma spraying were selected as the undercoating between the ceramic and the metallic substrate. Thermal shock test, electrical resistivity measurement, adhesion test and mechanical impact test were carried out. From the results of this characterization, it was clear that the specimen on which Al2O3 is coated on SS316LN-IG substrate with SS410 undercoating has good thermal shock resistivity. © 2001 Elsevier Science B.V. All rights reserved. Keywords: Electrical insulator; High electrical resistivity; Al2O3; Atmospheric plasma spraying; Vacuum plasma spraying
1. Introduction In a fusion blanket, the use of a ceramic coating on structural material has been considered for electrical insulation. Ceramic coating materials such as Al2O3 [1], MgO·Al2O3 [2], Y2O3 [3], AlN [1], CaO [4], TiN and TiC [5] have been studied in fabrication and properties up to the present. Alumina (Al2O3) is one of candidate materials for insulating coating because it has high electrical [6] and high radiation resistivity. However, crack and peeling occur by difference of thermal expansion between substrate and coating material. Therefore, * Corresponding author. Tel.: + 81-29-264-8417; fax: + 8129-264-8480. E-mail address: [email protected] (M. Nakamichi).
SS410 and 80Ni –20Cr were selected as the undercoating between SS316LN-IG substrate and Al2O3 coating, because thermal expansion coefficient of SS410 and 80Ni –20Cr were close to that of Al2O3. In this study, thermal shock test, metallographical observation, electrical resistivity measurement, adhesion test and mechanical impact test of specimen on which Al2O3 is coated on SS316LN-IG substrate with undercoating were reported.
2. Specimen In this characterization, two kinds of specimen were prepared. In order to coat Al2O3 on SS316LNIG substrate, the undercoating has been tried by two types shown as follows.
0920-3796/01/$ - see front matter © 2001 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 0 - 3 7 9 6 ( 0 1 ) 0 0 5 4 3 - 9
M. Nakamichi, H. Kawamura / Fusion Engineering and Design 58–59 (2001) 719–723
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Table 1 Spray conditions of SS410, 80Ni–20Cr and Al2O3 Material Spraying methoda Particle size (mm) Plasma gas Plasma output (kW) Spraying distance (mm) Pressure in chamber (kPa)
SS410 VPS 10–45 Ar+H2 41 275 5.9
80Ni-20Cr APS 10–45 Ar+H2 37 120 –
Al2O3 APS 10–45 Ar+H2 42 120 –
a
VPS, vacuum plasma spraying; APS, atmospheric plasma spraying.
Type 1: vacuum plasma spraying using SS410 Type 2: atmospheric plasma spraying using 80Ni –20Cr No. 1 specimen was undercoated by type 1 on the SS316LN-IG substrate, and no. 2 specimen was undercoated by type 2 on the SS316LN-IG substrate. Al2O3 was coated as a top layer on each undercoating. Coating process of each specimen is shown as follows. The surface of SS316LN-IG substrate (50 mm long, 20 mm wide and 5 mm thick) was degreased and blasted with Al2O3 grit. As to no. 1 specimen, the thickness of the SS410 undercoating deposited by the vacuum plasma spraying was about 150 mm on the substrate. The particle size of the SS410 powder was from 10 to 45 mm. Spraying
conditions of SS410 undercoating are shown in Table 1. As to no. 2 specimen, the thickness of the 80Ni–20Cr undercoating deposited by the atmospheric plasma spraying was about 50 mm on the substrate. The particle size of the 80Ni–20Cr powder was from 10 to 45 mm. Spraying conditions of 80Ni– 20Cr undercoating are shown in Table 1. The thickness of Al2O3 (purity: 99.6 wt.%) coating as a top coating deposited by the atmospheric plasma spraying was about 200 mm. The particle size of Al2O3 powder was from 10 to 45mm. Spraying conditions of the Al2O3 coating are shown in Table 1. Chemical compositions of SS316LN-IG, SS410, 80Ni–20Cr and Al2O3 are shown in Table 2. These undercoatings and the top coating were sprayed by TOCALO Co. Ltd.
3. Thermal shock resistivity For investigation of the variation of thermal shock resistivity of two kinds of specimen (no. 1 and no. 2), thermal shock test was performed by water quenching from 500, 600, 700 and 800 °C. Four specimens (one specimen per each temperature) were tested. The holding time at each temperature was 30 min. Thermal shock test repeated
Table 2 Chemical components of SS316LN-IG, SS410, 80Ni–20Cr and Al2O3 Chemical compositions (wt.%) SUS316LN-IG C Si 0.029 0.44
Mn 1.64
P 0.012
S 0.009
Ni 12.11
Cr 17.48
Mo 2.56
N 0.067
Co 0.02
Cu 0.09
B 0.0003
Nb B0.01
Ta B0.05
Ti B0.01
Fe Bal.
SUS410 C 0.03
Si 0.88
Mn 0.01
P 0.015
S 0.01
Ni 0.09
Cr 12.4
Fe Bal
80Ni–20Cr C 0.037
Si 1.0
Mn 0.027
Ni Bal.
Cr 19.8
Fe 0.13
Al2O3 Na2O 0.123
SiO2 0.01
Fe2O3 0.01
Al2O3 Bal.
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Fig. 3. Configuration of testing piece for adhesion test.
Fig. 1. Peeling cycles of the Al2O3 coating for thermal shock test. (30) represents that the Al2O3 coating was sound after 30 cycles.
up to 30 cycles from holding temperature to water. The results of visual observation of the Al2O3 coating surface after thermal shock test are shown in Fig. 1. From the results of visual observation of coating surface, no. 1 specimens were sound after 30 cycles of thermal shock test at 500, 600, 700 and 800 °C. No. 2 specimens were sound after 30 cycles of thermal shock test at 500, 600 and 700 °C. However, peeling of Al2O3 coating of no. 2 specimen was observed after ten cycles at 800 °C. The results of metallographical observation of coating cross-section is shown in Fig. 2. From these photographs, it was observed that the porosity in the undercoating of type 1 was higher than that of type 2. Therefore, it was assumed
Fig. 2. Metallographical observation of the coating cross-section.
that the thermal shock resistivity on peeling property of Al2O3 coating from SS316LN-IG substrate improved by the relaxation of thermal stress by pores in the undercoating.
4. Adhesion strength Configuration of testing piece for adhesion test was shown in Fig. 3. Testing pieces with and without coating were joined by the bonding agent. Next, one and the other end of the joined testing piece were pulled at 2.3 mm/min tensile speed. Adhesion was measured at room temperature with three testing pieces. The results of adhesion test were shown in Fig. 4. The average of the adhesion strength of no. 1
Fig. 4. Results of adhesion test.
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M. Nakamichi, H. Kawamura / Fusion Engineering and Design 58–59 (2001) 719–723
From results of electrical resistivity measurement, it was obvious that no. 1 specimen and no. 2 specimen had sufficient electrical resistivity.
6. Mechanical impact resistivity
Fig. 5. Results of electrical resistivity measurement.
specimen and no. 2 specimen were about 71.0 and 82.4 MPa, respectively. From the results of the appearance of the broken-out section in adhesion test, no. 1 specimen and no. 2 specimen were broken-out along the interface between the Al2O3 coating and bonding agent. From the results of the adhesion test, it was clear that no. 1 specimen and no. 2 specimen did not make much difference on adhesion strength.
5. Electrical resistivity Electrical resistivities of no. 1 specimen and no. 2 specimen were measured from room temperature to 800 °C in Ar atmosphere. Ag paste was applied on the Al2O3 coating as an electrode. The applied voltage was DC 500 V. The results of electrical resistivity measurement are shown in Fig. 5. In Fig. 5, reference data also are shown [7,8]. It was assumed that electrical resistivities of no. 1 and no. 2 specimen were higher than that of plasma sprayed Al2O3 coating measured by Pirogov with difference of Al2O3 purity. Therefore, electrical resistivities of no. 1 and no. 2 specimen were close to that of sintered Al2O3 measured by Shikama. Electrical resistivities of no. 1 specimen and no. 2 specimen at room temperature and 800 °C were above 1× 1013 and 5 ×106 V cm, respectively.
Al2O3 coating receive mechanical impact as an electromagnetic force at ITER plasma disruption. In this test, a steel hammer was dropped on the surface of the Al2O3 coating as mechanical impact. Dropping distance was 0.2 m, impact energy was about 14 kJ/m2. Mechanical impact test repeated up to 30,000 cycles. Impact energy and maximum cyclic number at ITER plasma disruption were about 7 kJ/m2 and 10,000cycles, respectively. Mechanical impact resistivities of No.1 specimen and No.2 specimen were measured at room temperature, 150 and 250 °C. Electrical resistance of the Al2O3 coating was measured each 1000 or 2000 cycles. The applied voltage was DC 500 V. The results of mechanical impact test are shown in Figs. 6 and 7. No. 1 specimen and no. 2 specimen at each temperature were sound after 30,000 cycles. Peeling of the Al2O3 coating of each specimen did not occurred from results of visual observation. Electrical resistance of no. 1 specimen and no. 2 specimen were constant above 1× 1012 V at each temperature. From results of the mechanical impact test, it was clear that no. 1 specimen and no.
Fig. 6. Results of mechanical impact test (no. 1 specimen).
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(2) From the results of the adhesion test, it was clear that each specimen did not make much difference on adhesion strength. (3) From the results of electrical resistivity measurement, it was clear that each specimen have sufficient electrical resistivity. (4) From the results of mechanical impact test, it was obvious that the mechanical impact resistivity of each specimen was satisfactory for the ITER design guideline.
Acknowledgements Fig. 7. Results of mechanical impact test (no. 2 specimen).
2 specimen have sufficient mechanical impact resistivity.
The authors gratefully acknowledge K. Tani and T. Teratani of TOCALO Co. Ltd. for the coating fabrication and useful discussions.
References 7. Conclusion Alumina (Al2O3) coatings fabricated on the surface of SS316LN-IG substrate with two types of undercoatings were investigated for thermal shock resistivity, adhesion strength, electrical resistivity and mechanical impact resistivity. No.1 specimen was undercoated by type 1 on the SS316LN-IG substrate. On the other hand, no. 2 specimen was undercoated by type 2 on the SS316LN-IG substrate. And Al2O3 was coated as a top coating on the each undercoating. The following conclusions were drawn. (1) From results of thermal shock test, it was clear that no. 1 specimen is superior to no. 2 specimen for thermal shock resistivity on peeling property of Al2O3 coating. It was assumed that the thermal shock resistivity of no. 1 specimen improved by the relaxation of thermal stress because of higher porosity in the undercoating.
[1] S. Malang, et al., Comparison of lithium and the eutectic lead– lithium alloy, two candidate liquid metal breeder materials for self-cooled blankets, Fusion Eng. Des. 27 (1995) 399 – 406. [2] M. Nakamichi, et al., Trial fabrication and preliminary characterization of MgO·Al2O3 coating, J. Nucl. Mater. 233 – 237 (1996) 1427 – 1430. [3] M. Nakamichi, et al., Characterization of Y2O3 coating for liquid blanket, J. Nucl. Mater. 248 (1997) 165 – 169. [4] J.-H. Park, et al., CaO insulator coatings and self-healing of defects on V – Cr– Ti alloys in liquid lithium system, J. Nucl. Mater. 233-237 (1996) 476 – 481. [5] A. Perujo, K. Forcey, Tritium permeation barriers for fusion technology, Fusion Eng. Des. 28 (1995) 252 – 257. [6] A.A. Bauer, J.L. Bates, Battelle Mem. Inst. Rept., 1930, (Jul. 31, 1974). [7] T. Shikama, Electrical degradation of ceramic insulators due to dynamic irradiation effects, Mater. Trans. JIM 36 (1996) 997 – 1003. [8] H. Ohno, Development of new materials and their testing on in-situ electrical properties in neutron environment, JAERI symposium on high energy neutron source for material research and development, (1989) 243 – 261.