A. Suzuki

T. Muroga

T. Terai

National Institute for Fusion Science (NIFS-Repository)

§5. Properties of Ceramic Coating Material AIN for CTR Liquid Blanket Terai, !-(The University of Tokyo), Suzuki, A., Muroga, T. In fusion reactor designs, liquid blanket concept is a promising one to realize a DEMO reactor of high power density, because it has advantages such as continuous replacement of breeders for reprocessing, no radiation damage for breeders, larger TBR, simpler blanket structure and better thermal transfer than solid blanket concept. On the other hand, it has several critical issues; ( 1) a large MHD pressure drop requires large pump power, in particular in self-cooled designs, (2) liquid breeders have low compatibility with structural materials, and (3) a large amount of tritium may leak to environment due to permeation through tubing walls, particularly in case of Li17-Pb83 blanket concepts. In order to solve these critical issues, a ceramic coating on the surface of tubing materials is proposed. The coating should have high electrical resistivity, high corrosion resistance, low tritium permeability and high thermomechanical integrity: Of course, all these properties are not necessarily required at the same time, and they depend on blanket concept. Aluminum nitride ( AIN) has been already proposed as a coating material for self-cooled Li blankets and some studies have been carried out on preparation, tritium permeability and compatibility with liquid metals. The main impurity in AlN coatings is considered to be oxygen, which may exist as Al20 3. Since Al20 3 is severely corroded and dissolved in liquid lithium, it is necessary to reduce Al203 impurity in AIN coatings. In this study, the compatibility of sintered AIN specimens, whose impurity levels are diiferent, and AIN coatings made on SUS430 substrates with liquid lithium were examined. We examined two kinds of sintered A1N specimens; sintered aluminum nitride (AIN) containing Y20 3 as the sintering aid ( 15 x 15 X 1 mm; 0. 7 5 g) and sintered AIN without Y 203 (15 X 7 x 1 mm; 0. 3 5 g). Generally speaking, some amount of oxygen is included in nitride specimens, which tends to exist as Al20 3 in sintering-aid-free AIN specimens. In case of AIN specimens with Y 20 3 sintering aid, on the other hand, Al20 3 concentration (0. 7 at.%) in the specimens became much lower by the addition of Y203 (Y203: 0.8 at.%) than that in the sintering-aid-free specimens (1 .6 at.%). Therefore, less corrosion is expected 262 in case of specimens contammg Y20 3. The corrosion experiment was .carried out by a static method. In a glove box with Argas, a crucible loaded with specimens and 20-30 g of lithium (99.9% in purity) was set in a heating container made of AISI type 316 stainless steel. At 773K, a flow of Ar gas was introduced over the liquid. After the prescribed time, a corroded specimen was taken out from the crucible and lithium metal adhering to the corroded specimens was cleaned off with water and ethyl alcohol. The corroded specimens were analyzed as follows; ( 1) observation of the appearance, (2) weight change measurement, (3) XRD analysis, and (4) electrical resistivity measurement by two-probe method. The compatibility test of AIN coatings with liquid lithium was also carried out. AIN coatings with less oxide impurity were prepared on SUS430 substrates by the RF sputtering method. In order to improve the bonding between the coating and the substrate, some AIN coated specimens were heated at 1073 K for 100 h. The electric resistivities of the AIN coatings (about 9 ::1.tn) before and after heat treatment were larger than the measurable limit of about 1011 nm. In a glove box with Ar gas, 0. 5 g of lithium was set on the AIN-coated specimen with a SUS430 lid to avoid lithium vaporization, and the specimen was put in the heating container. At 773K, a flow of Ar gas was introduced over the specimen. After 50 h, the specimen was taken out and immersed in water for 3 h to remove the lid and for another one hour to remove lithium on the coatings. After th~ corrosion experiment, the AIN coatings with and without the heat treatment changed their original color of light yellow to metallic gloss with some white parts. Only the SUS430 peaks were observed in the XRD patterns of this specimen. The electric resistivity of this specimen was almost nothing. These mean almost all the A1N coating disappeared in the corrosion experiment to leave only the substrate. Sintered AIN with or without the sintering aid of Y 20 3 examined in lithium at 773K for 1390 h showed the slight decrease of electric resistivity because of the reduction of Al203 impurity, though AIN and Y20 3 components in themselves were subject to no sever corrosion. On the other hand, AlN ceramic coatings on SUS430 wit high resistivity (> 1011 Om) fabricated by the RF sputtering method disappeared in liquid lithium at 77 K in 50 h. This may be because cracks were formed due to the difference in thermal expansion between the coatings and the substrate or the oxide formed between the two was removed by liquid lithium. 

Properties of Ceramic Coating Material AIN for CTR Liquid Blanket

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