Development of Refractory Metal Coatings on Graphite Substrate for High Heat Flux Applications /

Abstract

The world is striving hard to fulfil the growing energy demands. The problems faced in meeting the demands are twofold. One, depletion of fossils and the constraints in the availability of non-conventional resources. Secondly, environmental concerns due to excessive burning of fossil fuels. Mankind is looking for clean, sustainable alternatives. The solution is employing hydrogen as a fuel. Being used in different advanced energy systems, fusion reactor is the most appealing option. It is considered as a future of clean energy. In order to endure a high temperature plasma environment, it is needed to make plasma facing components (PFCs) of high heat flux tolerance and erosion resistance. Initially, graphite was considered as a suitable choice, but it encounters high chemical erosion due to hydrogen atmosphere. This has put the question mark on the credibility of graphite based PFCs. On the other hand, high atomic number (Z) materials having high melting point, low sputter yield, and little chemical erosion properties have emerged as more suitable candidates for PFC applications. One such materials is molybdenum (Mo). It is a high Z refractory metal. It has high melting temperature 2617°C and low coefficient of thermal expansion 4.8 μm/m°C. Combination of robust chemical and thermomechanical properties make it suitable for various high heat flux applications. This study is about the development of Mo films on graphite substrate and its subsequent irradiation analysis and high heat flux loading. The work is dividable into three sections. We begin with the development of Mo films over graphite substrate through DC magnetron sputtering. The effects of process parameters like sputter power and substrate temperature in the range from 50 to 200 watts and 25°C to 300°C respectively, were investigated. The objective was to develop high quality films with strong adhesion properties. SEM analysis revealed the development of wheat grain like crystalline structures for the films deposited at 200W power and 300°C temperature. Optimum adhesion was observed for films deposited at 100 W sputter power. AFM results demonstrate that the root mean square roughness is directly proportional to sputter power and inversely related to substrate temperature. From X-ray diffraction it can be inferred that films deposited at 300°C are of the highest crystalline nature. Electrical resistivity was estimated through Hall effect measurements using Van der Pauw arrangement and was found to increase with decreasing substrate temperature and sputter power. In the second portion, ion beam irradiation experiments were conducted to simulate the IV irradiation effects of energetic light and heavy ions. Silicon and helium ions of 2.2 MeV and 490 KeV energy respectively were irradiated on the coated coupons. Post irradiation analysis was carried out through SEM, XRD, and electrical resistivity measurements. Lastly, ANSYS finite elemental modeling was used to simulate thermal effects produced by high heat flux of 5-20 MWm-2. Our results demonstrate that by adopting suitable deposition parameters and proper heat transfer mechanisms, Mo coated graphite configuration is applicable for PFCs.