# Nb\({}_{3}\)Sn Cavities Coated by Tin Vapor Diffusion Method at Peking University
## 3 Results Analysis
### Tin Segregation Effect and Principles of Coating Stage  
Discussions in this section are focused on how different coating stage parameters affect the coatings. Samples from the 12 th to the 19 th experiments were SEM/EDS-tested to measure the surface Sn% and the grain sizes. The tin segregation effect was observed on  
Figure 14: The curve of AGS versus height from single tin source (6 th and 7 th) and three tin source (13 th–16 th) coatings.  
Figure 13: SEM pictures of samples at the top and bottom of the cavity; (**a**) is from a single tin source and (**b**) is from three tin source coatings.  
the 12 th, 13 th, and 14 th witness samples. The tin content of the surface is determined by the reaction rate and the evaporation rate of the surface, which can be controlled by coating temperature and time for a certain amount of tin usage. As the evaporation continues, the pressure decreases and the reaction rate decreases to less than the evaporation rate in the surface, and the tin begins to segregate from the Nb\({}_{3}\)Sn surface.  
Thus, an insufficient tin supply or too long a coating time will cause a tin segregation effect. In the 12 th experiment, Nb\({}_{3}\)Sn grains were incomplete, and the grain boundaries were inconspicuous, due to only 1.2 g of tin being utilized.  
In the 14 th-16 th coatings, the tin sources were consumed completely, and the coating temperature was the same at 1200 \({}^{\circ}\)C, but the coating times were 300 min, 150 min and 120 min, respectively. The samples were TEM/EDS-tested to obtain the Nb\({}_{3}\)Sn thickness and cross-sectional Sn%. From the test results, surface Sn%, cross-sectional Sn%, grain size and thickness all decrease with the coating time, as shown in Figure 15. The sample of 300 min coating gives a morphology feature of holes inside the grain, while the samples of 150 min and 120 min coating have no tin segregation effect and show complete grains.  
Comparing 1150 \({}^{\circ}\)C 300 min and 1200 \({}^{\circ}\)C 300 min samples with tin segregation, the tin segregation effect became worse with increases in temperature. When the coating temperature changed from 1150 \({}^{\circ}\)C to 1200 \({}^{\circ}\)C, the film thickness decreased from 1796.6  
Figure 15: Comparisons of 1200 \({}^{\circ}\)C 300 min, 1200 \({}^{\circ}\)C 150 min, and 1200 \({}^{\circ}\)C 120 min coatings: (**a**) surface Sn%, AGS, and average depth of the Nb\({}_{3}\)Sn films; (**b**) SEM photographs of Nb\({}_{3}\)Sn grains with and without tin segregation effect; (**c**) TEM photographs of Nb\({}_{3}\)Sn films; cross-sectional Sn% is annotated.  
\(\pm\) 0.24 nm to 1624.1 \(\pm\) 0.37 nm, and the cross-sectional Sn\(\%\) also decreased from 24.4\(\%\) to 20.3\(\%\).  
The normal Nb\({}_{3}\)Sn coatings in the 16 th, 17 th, 19 th, 22 nd, and 23 rd experiments have similar topography; the Sn\(\%\) and AGS comparisons are given in Table 4. It can be found that the growth of Nb\({}_{3}\)Sn at 1200 \({}^{\circ}\)C is faster than that of 1150 \({}^{\circ}\)C compared with the 16 th and 17 th coatings. The tin contents are generally larger than 25\(\%\), even achieving 26\(\%\) in our coatings, which exceeds the levels of 24-25\(\%\) reported by other institutions [7; 13; 14; 15; 16]. Subsequently, we attempted to add an annealing stage at 1100 \({}^{\circ}\)C and reduce the coating time to 80 min in 19 th, 22 nd, and 23 rd experiments in order to reduce the temperature and vapor pressure of the tin vapor and adjust the tin content in the film to 25\(\%\). It can be seen that annealing can lightly reduce the grain size and help lower the surface Sn\(\%\).