Preparation Of He-Charged Tungsten Based Film And The Growth Mechanism Of Helium Bubbles

Tungsten has a high melting point,low sputtering yield,good mechanical properties and low tritium retention,so that it has been considered as one of the most important material as a plasma facing component(PFC)in the divertor and baffle regions of ITER and Domo.Plasma facing materials(PFMs)not only are exposed to unprecedented steady-state and transient power fluxes(?10MW/m2),but also are bombarded with unprecedented neutron fluxes(?14 MeV).In working conditions,Owing to(n,a)reaction,hydrogen isotope decay or bombardment of a plasma particles,He atoms can be easily generated and accumulated in tungsten structural materials.These accumulated He atoms are prone to be trapped by various defects because of their insolubility and high mobility in metals,leading to the formation of helium-vacancy aggregations.With the increase of accumulation amount,helium atoms trapped by defects have a strong tendency to precipitate into bubbles,which will consequently cause performance deterioration of structural material,such as swelling,surface blistering,crack and flaking.On the other hand,nanocrystalline structure materials shows unique properties of increasing irradiation resistance,with the high density of dislocation and the large volume fraction of grain boundaries which can act as an important sink for radiation-induced point defects.Therefore,a detailed investigation about the growth mechanism of He bubble in tungsten nanocrystalline films is very necessary.In this study,tungsten films and tungsten films containing various helium concentration were prepared by RF magnetron sputtering at a He/Ar mixed atmosphere,power 90 W and substrate unheated conditions.The microstructure and properties of W films were studied by X ray diffraction analysis(XRD),field emission scanning electron microscopy(FESEM)and nanoindentation Technology.The results showed that the tungsten films prepared in different mixing atmospheres showed a typical columnar crystal structure,and as the He/Ar ratio increased,the half peak width of the XRD diffraction peak became wide which indicated the smaller grain size.These results showed that the introduction of helium could refine the grain of tungsten film.At the same time,the lower the content of helium,the greater the nano-hardness,indicating that the introduction of helium could improve the mechanical properties of tungsten film.The desorption behavior of helium and the growth mechanism of helium bubble were studied by means of thermal desorption spectroscopy(TDS)and transmission electron microscopy(TEM).For the TDS spectra of the helium-charged tungsten films in the range of RT?1000 ?,there were four He desorption peaks,and the whole TDS spectrum could be divided into three temperature regions,which are low temperature zone(200?400 ?),middle temperature zone(500?800 ?)and high temperature zone(900?1000 ?).The desorption peaks in the low temperature region were mainly corresponding to the desorption of the helium atoms trapped by the interstitial position or surface vacancies,and the desorption peak in the middle temperature region owed to the release of the helium space complex in the thin film,while the release peak of the high temperature region were mainly caused by the release of small He bubbles.The results showed that different kinds of He atoms(interstitial helium,helium vacancy clusters and helium bubbles etc.)could be introduced into W film by the magnetron sputtering method.In order to characterize and understand the growth mechanism of He bubbles,the film samples annealed at different annealing temperatures were observed by transmission electron microscopy(TEM).The results showed that the helium bubbles in the as-deposited tungsten films were uniformly distributed in the grain interiors and grain boundaries,and the bubble size was mostly around 1 nm.And as the annealing temperature increased,the size of the helium bubble increased.This indicates that during the annealing process,the interstitial helium atoms,the small He clusters and the He bubbles were migrated,and accumulate to form larger helium bubbles,which would affect the structure and mechanical properties of tungsten film.Based on the study of the growth mechanism of the He bubble and the related literatures,it could be found that the addition of dispersed particles in the material can act as the nucleation site of helium and inhibit the growth of helium bubbles.To this end,we made W-Y2O3 films,using XRD,FESEM and other instruments to characterize the crystal structure and morphology.The results showed that,compared with pure W film,the grains of W-Y2O3 thin film were very fine.With the increase of Y2O3 content,the crystallinity became worse,and the hardness increased obviously.Helium-charged W-Y2O3 film was prepared in a mixed atmosphere of He/Ar using a magnetron sputtering method.By observing the W-Y2O3 film containing helium with TEM,it can be seen that a large number of helium bubbles around 1 nm are distributed in the grain interiors and on the grain boundaries of the film.In order to study the effect of the addition of Y2O3 on the growth of He bubbles,TEM observations of the samples annealed at 500 ?,700 ? and 1000 ? were made in the W-Y2O3 film samples containing helium.There is no significant increase in the size of helium bubbles compared with helium-charged W films annealed at different temperatures.This indicted that the addition of Y2O3 can inhibit the growth of helium bubbles.