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

本文研究了氢同位素在面向等离子体材料W和Mo上的吸收、保留和释放特性。采用浮区熔炼法制备了W、Mo单晶片。在300~900K温度范围内，用射频磁控溅射W靶在一定的Ar/O2混合气体中制备了钨薄膜作为再淀积层的模拟。薄膜的微观结构强烈依赖于衬底温度和氧气浓度。在800K以上的衬底温度下制备的高取向薄膜具有良好的气致变色性能。然而，氧化物的组成和沉积层的微观结构对氢的保持率没有显著影响。研究了稀有气体离子辐照下钨表面的氢积累与表面形貌和成分变化的关系。在产生气泡和剥落的He和Xe离子注入层中发现了显著的氢保持。像WO2这样的钨氧化物是沿着注入离子的投影范围形成的。结果表明，多孔氧化物表面层对氢的解离起着重要作用，氢捕获与注入惰性气体原子形成气泡和团簇的微观结构变化有关。当样品暴露在主要含有D_2和D_2O气体的大气中时，H原子在氦饱和层中优先吸收，在D_2气体暴露过程中，H原子的吸收速率比D高一个数量级。观察到的同位素效应可能是由于在He辐照的W和Mo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.