@misc{oai:ir.soken.ac.jp:00000521, author = {姚, 振宇 and ヤオ, チェンユ and YAO, Zhenyu}, month = {2016-02-17, 2016-02-17}, note = {Among the concepts for breeding blanket of fusion reactors, self-cooled Li/V blanket, in which vanadium alloy acts as structural material and liquid lithium as both tritium breeder and coolant, is thought to be particularly attractive. Lithium has high tritium breeding capability, good heat transfer and negligible neutron damage. In addition vanadium alloys have low induced radioactivity, good compatibility with lithium, good resistance to neutron damage and high strength at high temperature. These advantages lead to a simple structural design, no use of neutron multiplier beryllium with enough tritium breeding rate, and continuous handling of chemistry for the breeder.
　　One of the critical issues for liquid Li/V-alloy blanket concept is large magneto-hydrodynamic (MHD) pressure drop against pumping system while liquid lithium flows in blanket ducts in a strong magnetic field. A promising way to mitigate the pressure drop is to apply electrically insulating coating (oxide or nitride ceramics) onto inner wall of the structural ducts.
　　In-situ formation of oxide coating onto vanadium alloys in liquid lithium is quite attractive because the process will make it possible to deposit coatings onto complex surfaces after fabrication of the components, and it offers the possibility to heal cracks of coatings without disassembling the components. In this method, the vanadium alloy substrate pre-charged with oxygen is exposed in liquid lithium doped with metal, where the oxygen reacts with the metal to form an insulating oxide coating on the vanadium alloys.
　　Among candidate coating materials, Er₂O₃ ceramics showed good compatibility with liquid Li, a high electrical resistivity, and a high stability in air with moisture, and thus is regarded as a promising candidate material for the insulating coating.
　　The purpose of this study is to develop an in-situ Er₂O₃ insulating coating method to V-4Cr-4Ti (NIFS-HEAT-2) that is one of the reference vanadium alloys.
　　For this purpose, the first step is to pre-charge oxygen with proper level and to distribute in the limited depth of V-4Cr-4Ti, without degrading mechanical property of the substrate. In the present study, the oxygen behavior in V-4Cr-4Ti during oxidation and subsequent annealing were investigated first at all. V-4Cr-4Ti alloy was oxidized in flowing Ar with impurity of oxygen, nitrogen and moisture for 0.5-8h and subsequently annealed in vacuum for 16h at 700℃. By oxidation for supplying enough oxygen, the oxygen of 1000-9000ppm was introduced into V-4Cr-4Ti with little nitrogen, where oxygen was concentrated in a region near the surface. The weight gain obeyed a parabolic law indicating that the oxidation process was controlled by thermo-diffusion rate of oxygen in V-4Cr-4Ti. During the annealing for keeping the oxygen in the substrate, oxygen was homogeneously diffused into depth of 150μm. The diffusion depth of oxygen in V-4Cr-4Ti is limited because of Ti-O net precipitates formed as oxygen trap. Ti-O is expected to dissolve supplying free oxygen for forming the coating during the subsequent exposure to liquid Li. The study indicated feasibility to control the level and distribution of oxygen in the surface region of V-4Cr-4Ti. The optimized parameters of pre-charging oxygen into V-4Cr-4Ti for forming in-situ oxide coating are oxidation in Ar (700℃, 6h) followed by annealing in vacuum (700℃, 16h).
　　The second step is chemical formation of Er₂O₃ coating on V-4Cr-4Ti during exposure in liquid Li doped with Er. In the present study, V-4Cr-4Ti were oxidized at 700℃ for 1-8h, annealed at 700℃ for 16h, and finally exposed for 100 h at 600℃. By the exposure, the Er₂O₃ layer was formed on V-4Cr-4Ti oxidized plus annealed, but was not formed on those either as-received or oxidized only. The resistivity of V-4Cr-4Ti with Er₂O₃ coating is ~10⁶Ωm² which is much larger than the minimum requirement (~10^-2Ωm²). The results indicate that the in-situ formation of Er₂O₃ coating is viable, the oxygen source to form coating is the pre-charged oxygen in V-4Cr-4Ti, and that the annealing after oxidation to homogenize oxygen distribution in substrate is necessary for the formation of the coating.
　　The third step is to characterize and investigate the potential long-term stability of the Er₂O₃ coating. V-4Cr-4Ti substrate was oxidized in flowing Ar at 700℃, annealed in vacuum at 700℃, and finally exposed to liquid Li doped with Er from 500℃ to 700℃. The oxygen charged and homogenized in limited depth of the substrate was stored as Ti-O net precipitates. It was verified that Ti-O dissolved to release oxygen at high temperature acting as an oxygen source. The surface layer formed on V-4Cr-4Ti consists of double sub-layers, namely an insulating coating of Er₂O₃ and an intermediate layer of mixed ErN and V-compounds. The Er₂O₃ coating was found to be stable up to 750h of exposure time in liquid lithium doped with erbium at 600℃ with its saturated thickness of ~0.1μm, stable up to 300h at 650℃ with saturated thickness of ~0.6μm, and stable up to 300h at 700℃ with saturated thickness of ~1.3μm. Cracking of coating can be avoided during the Li cleanout process after exposure by using weak lotion (liquid NH₃) at low temperature (-33.5℃). Thus it is inferred that no cracks were formed during the exposure in liquid Li. By re-exposure of intentionally cracked coating in liquid Li doped with erbium, the potentiality of self-healing to cracks was shown. The coating once formed in Li doped with erbium at 700℃ was stable in pure Li at 700℃, thus demonstrating the stability of Er₂O₃ coating in liquid Li. The lower limits of the Er doping level to form Er₂O₃ coating were experimentally gotten to be in the range of 1-2.5wt%(0.04-1at%) and at exposure temperature higher than 500℃. The limit is expected to be much lower with reduced level of the dissolving oxygen in liquid Li. The in-situ measurement of resistivity during heating in vacuum showed that the coating had resistivity over minimum requirement (10^-2Ωm²) up to 550℃. The resistivity satisfying the requirement to higher temperature is expected to show with improved measurement apparatus.
　　Finally, the mechanisms for nucleation and growth of the coating were investigated. By oxidation and annealing, the oxygen was charged into substrate to form Ti-O net phase as oxygen source to the coating. The surface layer formed on V-4Cr-4Ti consists of two sub-layers, insulating Er₂O₃ coating and intermediate layer of mixed ErN and Er-V-O. The durations of nucleation at 600, 650 and 700℃ are experimentally proved to be quite short. The measured growth rate showed the kinetics of logarithmic law with high exponent (n〓3 or 4) at 600℃ and 650℃, suggesting that the rate of growth to Er₂O₃ coating should be very low. The growth was expedited suddenly at 700℃ resulting in low exponent (n〓2) that almost obeys parabolic law. A phenomenological model was proposed for mechanism of the coating kinetics, which showed growth of Er₂O₃ coating is controlled by diffusion of oxygen and delivery of erbium to interface between the vanadium alloy substrate and the liquid lithium. The model also showed impurity transport across the interface and formation mechanism of the resulting intermediate layer.
　　In summary, this study has demonstrated that in-situ Er₂O₃ coating on V-4Cr-4Ti in liquid Li doped with erbium is a viable technology. Coating was stable at 600℃ to 750h and at 700℃ to 300h. Coating showed satisfactory resistivity to minimum requirement for reducing MHD pressure drop. Coating showed self-healing potentiality. The kinetics of the coating showed quick nucleation followed by growth with logarithmic law. The phenomenological modeling showed formation mechanism of the observed intermediate layer., 総研大甲第896号}, title = {Development of Insulating Coating on Vanadium Alloys for Liquid Lithium Blanket of Fusion Reactors}, year = {} }