Multilayer Graphene: A Potential Anti-oxidation Barrier in Simulated Primary Water
Introduction
In pressurized water reactors (PWRs), nickel-based alloys are now widely used to fabricate tubes in steam generators (SGs) due to their good corrosion resistance[1]. However, oxide films which play a crucial role in stress corrosion cracking and crack propagation can be formed on the tube surfaces during service[2]. In addition, the oxide films on the surface can decrease the heat transfer efficiency of the tubes, which correspondingly increases the cost of management and operation of the nuclear power plants[3]. In order to protect metal surfaces from oxidation, many approaches, such as organic layers[4], [5], [6], inorganic coatings[7], paints[8], [9], and polymers[10], [11] have been developed. However, these coatings are not suitable to be used in PWRs as protective barriers. Is there any effective barrier to protect the materials in PWRs?
Graphene, a monolayer of carbon atoms tightly packed into a honeycomb lattice, is being widely studied across the whole world due to its unique structure and properties[12], [13]. Four properties have determined graphene to have the potential as a good anti-oxidation barrier in PWRs. Firstly, graphene on the substrate can act as a barrier to provide a physical separation between the substrate and reactants[14]. By creating an air-tight ‘‘balloon’’, Bunch et al.[15] have demonstrated that a monolayer graphene is impermeable to standard gases including helium. Secondly, graphene is chemically inert and is stable in high temperature air[14], [16], [17] or under conditions where other substrates will undergo rapid chemical reactions[18], [19], [20], [21], [22], [23], [24]. Chen et al.[14] have proved that graphene can not only be stable when heated to 200 °C in laboratory air for 4 h or immersed into a solution of 30% (weight/weight) hydrogen peroxide up to 45 min but also act well to prevent the substrates from oxidation. Thirdly, the wettability of the substrate is independent of the number of layers of graphene sheets and the interaction between the substrate and overlying water can be in the same way as in the absence of graphene, indicating graphene barrier is especially suitable for SG heat transfer tubes[25]. Fourthly, the room temperature value of the thermal conductivity of a single-layer graphene is ∼5 × 103 W/(m K), about ten times higher than that of copper[26], indicating the graphene barrier is advantageous for improving the heat transfer efficiency of SG tubes.
In addition, many methods[27], [28], [29], [30] have been developed to produce high quality graphene in mass production. To date, by using chemical vapor deposition (CVD), large area, monolayer or multilayer graphene can be synthesized on different substrates including Pt[31], Ni[32], [33], [34], Cu[27], Pd[35], Ru[36], Ir[37] and even stainless steels[38]. Graphene layers can also be transferred to any arbitrary substrate by using various kinds of transfer methods[39], [40], [41], [42], [43], [44], which makes graphene being protective barriers possible.
Section snippets
Materials and Experimental Methods
Since the methods of synthesizing graphene on nickel have been technologically matured[32], [33], [34], in this work, nickel foils were selected as substrate. Graphene coated nickel foils (labeled as “G/Ni”) and uncoated nickel foils (99.99% Ni, labeled as “Ni”) were purchased from XIAMEN G-CVD MATERIAL TECHNOLOGY CO., LTD (China). The graphene was synthesized on nickel foils by CVD. The general CVD process was: argon was introduced into the quartz tube before and after the nickel foils were
Results and Discussion
The surface morphology of the Ni sample is shown in Fig. 1(a). Grain boundaries, dotted inclusions along the grain boundaries and rolling indentations are obvious. Although it is very difficult to observe the monolayer graphene by SEM as the ultra-low secondary electron emission of graphene[45], SEM graph of G/Ni sample in Fig. 1(b) clearly shows the morphology of the multilayer graphene. The nickel foil is thoroughly covered by multilayer graphene and this multilayer graphene is obviously
Conclusion
The structure of the multilayer graphene on nickel foils remains unchanged after a 1000 h immersion in simulated primary water. No oxidation is found on the multilayer graphene coated nickel foils, indicating multilayer graphene can effectively act as the protection barrier to protect the substrate from oxidation and hence improve the heat transfer efficiency of the substrate in the simulated primary water of PWRs.
Acknowledgments
This work is supported by the National Basic Research Program of China (No. 2011CB610502) and the National Natural Science Foundation of China for Distinguished Young Scholars (No. 51025104).
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