Up take and release characteristics of hydrogen isotopes in re-deposited layers on the plasma facing wall

在本研究中，研究了氢同位素在W和Mo上的吸收、保留和释放特性，这两种材料有望成为等离子体表面材料。采用浮区熔炼法制备了钨、钼单晶片。作为再沉积层的模拟，在300 ~ 900 K的温度范围内，用常规的射频磁控溅射法制备了钨薄膜。沉积膜的微观结构与衬底温度和氧浓度密切相关。在800 K以上的衬底温度下制备的高取向薄膜在氢气暴露下表现出良好的气致变色性能。然而，氢潴留不受氧化物组成和沉积层微观结构的显著影响。研究了稀有气体离子辐照后W表面氢积累与形貌和成分变化的关系。在He和Xe离子注入层中发现了显著的氢潴留，在这些层中产生了水泡和去角质。钨氧化物如WO2沿着注入离子的投影范围形成。结果表明，多孔氧化物表面层对氢的解离起着重要的作用，并且由于注入惰性气体原子形成气泡和团簇，氢捕获与微观结构变化有关。当样品暴露于主要含D_2和D_2O气体的大气中时，在He饱和层中发现了H原子的优先吸收；在D_2气体暴露过程中，H的吸收速率比D的吸收速率高一个数量级。所观察到的同位素对吸收行为的影响可能是由于在W和Mo的He辐照表面形成了质子导电氧化物。

In the present study, up-take, retention and release characteristics of hydrogen isotopes were studied on the W and Mo, which are promising candidates for plasma facing materials. W and Mo single crystalline disks were made by the floating zone melting method. As a simulation of the re-deposition layers, tungsten films were prepared by a conventional rf magnetron sputtering with a W target in defined Ar/O2 mixture, at a temperature range between 300 and 900 K. The micro structure of the deposited films strongly depended on the substrate temperature and oxygen concentration. The highly oriented films prepared at the substrate temperature above 800 K exhibit good gasochromic properties for hydrogen gas exposure. However, hydrogen retention was not significantly affected by the composition of oxides and the micro structure of the deposited layer. Hydrogen accumulation in W surface irradiated by noble gas ions was examined in connection with morphological and compositional changes. Remarkable hydrogen retention was found for the He and Xe ion implanted layers where blisters and exfoliation were created. Tungsten oxides such as WO2 were formed along projected ranges of the implanted ions. The results indicated that the porous oxide surface layers play an important role for dissociation of hydrogen, and hydrogen trapping is correlated with microstructural changes owing to the formation of bubbles and clusters of implanted noble gas atoms. Preferential up-take of H atoms was found in the He saturated layer, when the specimen was exposed to the atmosphere mainly containing D_2 and D_2O gas ; the up-take rate for H was an order of magnitude higher than that for D during D_2 gas exposure. The observed isotope effects on the up-take behavior might be due to the proton conductive oxides formed on the He irradiated surface of the W and Mo.