Relationship between Microstructure and Vacuum Leak Characteristics of SiC Coating Layer

Transcription

1 , pp Relationship between Microstructure and Vacuum Leak Characteristics of SiC Coating Layer Yootaek Kim 1 and Junwon Choi 2 1 Dept. of Materials Engineering, Kyonggi University, Suwon, Korea, 16227, ytkim@kgu.ac.kr 2 Dept. of Materials Engineering, Kyonggi University, Suwon, Korea, 16227, jwchoi0619@naver.com Abstract. Silicon carbide is one of prospective coating material for very high temperature gas cooled reactors (VHTR) because of their high strength and oxidation resistance. SiC is coated on a graphite substrate using chemical vapor reaction, physical vapor transport, and chemical vapor deposition. The surface microstructure of the coating layers were compared and analyzed. A device for gas leak or vacuum leak tests was fabricated to compare the leak characteristics of coating layers by three different methods. SiC coated specimens synthesized by the chemical vapor reaction method showed the best vacuum leak characteristics. It was concluded that the vacuum leak characteristics depend on the defects in the SiC coating layer rather than other parameters. Keywords: Silicon carbide coating, Gas leak, Surface coating 1 Introduction Nuclear reactors are dangerous due to the potential for hydrogen explosion. Hydrogen leakage must be prevented or minimized by coating a leak resistant material on a graphite substrate for containing nuclear fuels in the event of a nuclear reactor-related accident. Silicon carbide (SiC) is a candidate coating material for heat exchangers for very high temperature gas cooled reactors (VHTR) owing to its high hardness, strength, and oxidation resistance, among other properties. [1]. Chemical vapor deposition (CVD) is commonly used to manufacture high density SiC layers with high purity. SiC layers deposited by CVD also have good thermal and mechanical properties [2-4]. The chemical vapor reaction (CVR) method is better suited for forming SiC coating layers on components with complex shapes such as spherical pebbles. In addition, the SiC conversion layer formed via the CVR method adheres well to graphite matrices [5]. The physical vapor transport (PVT) method has been a most successful and common method for the growth of bulk SiC crystals [6]. Therefore, the PVT method was also studied for comparison. The relationship ISSN: ASTL Copyright 2015 SERSC

2 between microstructure and vacuum leak characteristics of SiC coating layers was studied for three coating methods. 2 Methodology 2.1 Coating methods For CVD coating, a tetrachlorosilane (SiCl 4 ) solution was used as the source material for silicon. Methane gas was used as a carbon source. Hydrogen was used for both controlling the atmospheric gas in the reactor and as a carrier gas for the vaporization of SiCl 4 by bubbling. The SiCl 4 solution was maintained at 0 C when bubbling. The reaction temperature for the SiC deposition was fixed at 1300 C. The deposition time was fixed at 100 min. The length of the stainless steel input gas line was fixed at 30 cm. The reaction pressure was maintained to less than 1 Torr. The flow ratio for methane gas was fixed at 0.33 and that for the hydrogen gas was fixed at 20. A dense SiC coating was formed over the surface of the graphite substrate by a CVR process for the solid-solid and vapor-solid reaction in the SiO 2 (s)-c(s)-sio(v)- CO(v) system. From the reduction of SiO 2 powder with a graphite substrate, SiO vapor was created and infiltrated into the graphite substrate. Finally, a SiC conversion layer formed from the vapor-solid reaction of SiO and graphite [7]. The SiC coating was deposited by CVR at a temperature of 1600 C for 160 min at a pressure of 10-2 Torr. In the PVT process, source material is evaporated or dissociated at the bottom of the crucible and crystal growth occurs in colder regions of the crucible. The source material is evaporated or dissociated under proper conditions and re-condenses in the colder regions of the crucible to promote crystal growth. Graphite crucibles are used because they are inexpensive and relatively stable at these temperatures. PVT was conducted at a temperature of C for 10 h at a pressure of 200 Torr. 2.2 Leak test device A laboratory fabricated device for gas leak tests was designed to compare the gas leak characteristics of specimens before and after coating. The diameter and height of the graphite and stainless steel substrates (reference specimens) for gas leak tests were 1 inch and 1.5 mm, respectively. The vacuum level profile of the test chamber with the standard specimen during pumping is shown in Fig. 1. The vacuum decreased drastically in 6 h and then stabilized near Torr for up to 24 h. Vacuum leak characteristics were measured with SiC specimens deposited by CVR, PVT, and CVD methods. Tests were conducted after 6 h of pumping by a rotary pump. The pump was then turned off and left in a completely closed state for 24 h. No gas was provided in this study as the vacuum leak results were adequate for comparing the gas leak characteristics of the three different types of specimens. 48 Copyright 2015 SERSC

3 Fig. 1. The vacuum leak profile of the test device. The vacuum level of the specimen chamber was measured after maintaining atmospheric pressure for 1 h. The abscissa indicates the hours spent pumping the device. 3 Results and Discussion 3.1 SEM analysis Fig. 2 shows SEM surface micrographs of SiC coating layers fabricated by CVR, PVT, and CVD methods. As shown in Fig. 2, the CVR coating layers appear smooth and densely coated without voids or pores on the surface. Moreover, well formed, small crystallites and crystal facets were observed at high magnification ( 10,000). Thus, it is expected that the vacuum leak characteristics of CVR specimens will be superior. Very large crystals were observed on the coating layers made by the PVT method. The size of some crystallites exceeded 50 micrometers in diameter. There were many pores and voids on the surface and the PVT film appears less dense than the CVR and CVD specimens. Therefore, the highest vacuum leaks were expected for the PVT specimens. Judging from the figures at the bottom of Fig. 2, the CVD formed surfaces were smoother and denser than any of the other specimens. However, a few voids and cracks were found at lower magnification. Well-formed crystals with clear facets and grain boundaries were also observed at higher magnification ( 10,000). 3.2 Vacuum leak tests Fig. 3 shows the vacuum leak test results for SiC coated specimens deposited by CVR, PVT, and CVD methods. Reference data were obtained before the main experiment by conducting the leak test with two standard specimens: a stainless steel plate and a graphite substrate before coating. Specimens coated with SiC using PVT and CVD Copyright 2015 SERSC 49

4 methods showed fast vacuum leaks. Only 13 min 14 s and 30 min 2 s elapsed to reach 760 Torr for the PVT and CVD films, respectively. The specimen coated by CVR showed the slowest vacuum leak compared to the other specimens. The vacuum level reached 3.5 Torr after 24 h. The results of the vacuum leak test show that the CVR coating layers have the best vacuum leak characteristics, in good agreement with the expectation from microscopic observations by SEM. By judging from the SEM observation, the degree of crystallinity of the coating layer is not the main factor for optimizing the leak characteristics of coating layers. Instead, defects such as pores, voids, and micro-cracks are more important factors for determining the vacuum leak resistance. X1,000 X3,000 X10,000 CVR PVT CVD Fig. 2. SEM surface micrographs of SiC coating layers deposited by CVR, PVT, and CVD methods. As seen in Fig. 3, a SiC coating layer proved to be effective in increasing leak resistance regardless of the particular coating method, although the degree of resistance is different. Therefore, it is speculated that the SiC coating on graphite substrates may offer one way to enhance hydrogen leak resistance for mitigating nuclear reactor-related accidents. 4 Conclusions The vacuum leak characteristics of SiC layers coated by three different methods were compared and explained using their microstructures. The specimens coated using CVR showed the smallest vacuum leaks, where the vacuum level was maintained at 3.5 Torr for 24 h after turning off the vacuum pump. Considering that a graphite specimen without a coating reached 760 Torr in 90 s, it is believed that the SiC 50 Copyright 2015 SERSC

5 coating deposited by CVR on graphite may be one way of minimizing hydrogen gas leaks. Hydrogen gas leak tests with the specimens studied here will be conducted in the near future. Fig. 3. Vacuum leak test results for SiC coated by CVR, PVT, and CVD methods. Acknowledgments. This work was supported by Kyonggi University Research Grant References 1. Seo, J.W., Choi, K.: Application of Computational Fluid Dynamic Simulation to SiC CVD Reactor for Mass Production. J. Kor. Ceram. Soc. 50(6), (2013) 2. Hirai, T., Sasaki, M.: Silicon Carbide Prepared by Chemical Vapor Deposition. Elsevier Science Publishers. Silicon Carbide Ceramics. New York 1, (1991) 3. Kim, J.H., Lee, H.K., Park, J.Y., Kim, W.J., Kim, D.K.: Mechanical Properties of Chemical- Vapor-Deposited Silicon Carbide using a Nanoindentation Technique. J. Kor. Ceram. Soc. 45(9), (2008) 4. Lespiaux, D., Langlais, F., Naslain, R.: Correlations between Gas Phase Supersaturation, Nucleation Process and Physico-Chemical Characteristics of Silicon Carbide Deposited from Si-C-H-Cl System on Silica Substrates. J. Mater. Sci. 30, (1995) 5. Yun, Y.H., Park, Y.H., Ahn, M.Y., Cho, S.: CVR-SiC coating of graphite pebbles for fusion blanket application. Ceram. Int. 40, (2014) 6. Shiramomo, T., Gao, B., Mercier, F., Nishizawa, S., Nakano, S., Kangawa, Y., Kakimoto, K.: Thermodynamical analysis of polytype stability during PVT growth of SiC using 2D nucleation theory. J. Cryst. Growth. 352, (2012) 7. Letts, E.R.: Physical Vapor Transport Growth of Aluminum Nitride. A Dissertation submitted in partial satisfaction of the requirements for the degree Doctor of Philosophy in Materials. University of California, Santa Barbara (2007) Copyright 2015 SERSC 51