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论文题目： Effect of He content on the evolution of He bubbles and He thermal desorption behaviors in FeCr based nanocrystalline film

发表时间：Available online 15 June 2021

刊源：Journal of Nuclear Materials 555 (2021) 153136 [ 点击下载PDF ]

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As shown in Fig. 2 (a-d), all the FeCr films exhibit a completely dense microstructure, in which many spherical grains are observed. The addition of He induced the change of surface morphology of FeCr films. With He content increasing, the grain size was reduced. In a word, He atoms, which was introduced into FeCr films, can refine the grains of these films and improve the roundness and uniformity of globular grains. The cross-section morphologies of FeCr films were presented in Fig. 2(e-h). These films have the typical columnar crystal structure. The film thickness ranges from 1.2 μm to 2.8 μm. The differences in grain sizes of these films were not easy to be distinguished from the cross-section morphologies of films, because the grain sizes are very small and the columnar crystal structures are very dense.

Fig. 2. Surface and cross-sectional morphology and of FeCr based films deposited on Si substrates at RT under various He/Ar ratios: (a) and (e) 0; (b) and (f) 1:1; (c) and (g) 2:1; (d) and (h) 4:1.

As presented in Fig. 3 , the integrated intensity of desorption peaks increases with the He/Ar ratio increasing, which is consistent with the positive correlation between the He concentration and the He/Ar ratio of the mixed gas. It is obvious that there is no He releasing peak in the films at He/Ar = 0 due to no He atoms. For the FeCr films prepared at He/Ar = 1, 2 and 4, the released signal can be divided into four groups. In general, Group I is located at 250℃~400℃, the second desorption peaks (Group II) occur around 600℃, Group III is shifted to a temperature range (800℃-850℃) and Group IV is obtained at the higher temperature ( >1000℃).

Fig. 3. Thermal desorption spectrum of FeCr films deposited on Si substrates at RT under different He/Ar ratios of 0, 1:1, 2:1 and 4:1, respectively.

Fig. 4 (a) shows that a large number of bubbles are homogeneously dispersed in the deposited films. However, a significantly inhomogeneous size distribution was observed in the TEM images of samples irradiated to 2.25×10²¹ He m^-2. This is because that the high dose He+ ions may induce some bubble agglomeration. This is because that the high dose He⁺ ions may induce some bubble agglomeration. Therefore, the distribution of He bubbles introduced by magnetron sputtering is similar to the result acquired by a low dose of He⁺ ion implantation.As shown in Fig. 4 (a) and (b), grain boundaries and dislocations are decorated by He bubbles. The preferential nucleation sites for He bubbles may be attributed to misfit dislocation, grain boundary orientation and free volume regions at the grain boundaries. Specifically, vacancies were produced during magnetron sputtering which could easily accumulation at grain GBs. These vacancies trapped by GBs may drag He atoms to GBs as a result of the He-vacancy affinity. Generally, the migration energy of He in the metal lattice is very small, and the binding energy of He to vacancies along GBs is strong. Thus, the introduced He atoms would easily diffuse to precipitation into He-vacancy clusters and then grow into bubbles.

Fig. 4. TEM images of FeCr films prepared at RT under different He/Ar ratios of (a) 0, (b) 1:1, (c) 2:1, and (d) 4:1, respectively.