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论文题目： Effect of He content on microstructure, mechanical properties and He thermal desorption behavior of W film fabricated by RF magnetron sputtering

发表时间：Available online 11 April 2020

刊源：Journal of Nuclear Materials 534 (2020) 152151 [ 点击下载PDF ]

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Fig. 1 shows the effect of He atoms on the surface morphology of W films. As shown in Fig. 1, the structure of both pure W films and He-charged W films are very compacted. For W film deposited by magnetron sputtering, there may be some nano-scale grains in films, which can be hardly detected by SEM. Moreover, not all grain boundaries in the deposited film can be easily shown in SEM images, which may cause some the measurement errors. Therefore, the SEM images can’t assess accurately the average grain size. Nevertheless, the SEM images of surface morphology still can indicate roughly that the grain size becomes smaller with increasing He/Ar ratio. The trend of the change of grain size obtained from the surface SEM images corresponds to the following result of grain size calculated by the XRD peak profile analysis

Fig. 1. The surface SEM images of W films deposited on the Si substrate at RT under the different He/Ar ratios: (a) 0; (b) 1; (c) 2; (d) 3.

To observe the cross-section morphology of W films on the Si substrate, the cross-section morphology SEM images are shown in Fig. 2. It can be easily seen that allWfilms prepared at different He/ Ar ratios have the typical and dense columnar crystalline structure and the thickness of the films is about 5 mm. The grain size is so fine that we can hardly confirm the actual grain size by means of SEM images.

Fig. 2. Cross-sectional morphology of W films prepared under different He/Ar ratios: (a) He/Ar ¼ 0; (b) He/Ar ¼ 1; (c) He/Ar ¼ 2; (d) He/Ar ¼ 3.

Fig. 6(aec) shows the distribution of He bubbles in W films fabricated at different He/Ar ratios, and the mean diameter of these bubbles was estimated to be about 1.0 nm. As Fig. 6(a) shown, a large number of He bubbles distributed in the grain interiors and around the grain boundaries (GBs) ofWfilms. In particular, the size of He bubbles trapped by GBs was bigger than that located in grain interiors. Moreover, these He bubbles around GBs were accumulated and connected to each other to form a necklace-like line as shown in Fig. 6(a). The above results indicate that the nucleation and growth of He bubbles are preferential on the GBs for rapid diffusion than that in the grain interiors, which had been found by J.H. Evans et al. and O. El-Atwani et al. This is because a large number of vacancies exist on the GBs so that more He atoms can be trapped by vacancies to form bubbles at GBs. Meanwhile, it was easily found that the shape of He bubbles in grain interiors was spherical and the shape of He bubbles trapped by the GBs was ellipsoidal. For this difference of He bubble shape, a similar phenomenon had been studied by A. Ofan et al. In their work, they believed that the shape of He bubbles is closely related to the surface free energy and the elastic free energy. As a result, in our experiments, when the surface energy was dominated, the spherical He bubbles in grain interiors were formed. When the elastic energy played amajor role in this process, elongated He bubbles along the grain boundary were formed. For W films prepared at He/Ar=2 and He/Ar=3, He bubble size becomes a little larger than before, but the mean diameter of these bubbles was estimated to be still less than 2 nm, as shown in Fig. 6(b) and (c).

Fig. 6. TEM images of W films fabricated at different He/Ar ratios: (a) He/Ar = 1; (b) He/Ar = 2; (c) He/Ar =3.