Oxidation of carbon on nickel-based metallic substrates: Implications for high-temperature superconductor coated conduct
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Adhesion of thin films of epitaxial oxides to nickel-based metallic substrates is important for the successful development of high-temperature superconductor coated conductors. Detachment of epitaxial oxide buffer layers at the oxide/metal interface during either oxide growth or subsequent processing renders the conductor useless. In this study, thermal desorption spectroscopy (TDS) has been used to identify and understand one of the causes of buffer layer detachment, oxidation of carbon at the oxide–metal interface to form carbon monoxide. Results of TDS indicate that on the surface of a bare nickel-based alloy substrate, the rate of carbon oxidation depends on both the supply of carbon from the substrate and the supply of oxygen from the vapor. Sulfur at the surface of the alloy substrate reduces the rate of carbon oxidation. The effectiveness of various treatments of the bare substrate to eliminate CO formation and epitaxial oxide detachment has been demonstrated. TDS provides both a means to evaluate the kinetics of the oxidation reaction and a tool to assess the need and effectiveness of a substrate oxidation treatment.
I. INTRODUCTION
For the past decade, the development of hightemperature superconductor (HTS) coated conductors has relied predominantly on nominally pure nickel and nickel-based alloy substrates. These substrates (generally ∼50–100 m thick) provide a strong, flexible support for an epitaxial multilayer structure consisting of a variety of oxide buffers (∼0.1–0.5 m thick) and superconductors (∼0.3–3 m thick).1–4 Nickel and nickel-based alloy substrates can also provide a potentially significant and unintended source of carbon: carbon initially in the form of a substrate impurity. The role this carbon can play in conductor fabrication is addressed in this paper. One of the challenges for successful fabrication of HTS-coated conductors is to maintain mechanical integrity of the multilayer structure throughout all processing steps. Detachment of the buffer layers from the substrate is occasionally observed during processing and leads to a catastrophic degradation of conductor performance. To
a)
Address all correspondence to this author. e-mail: [email protected] b) This author was an editor of this journal during the review and decision stage. For the JMR policy on review and publication of manuscripts authored by editors, please refer to http:// www.mrs.org/publications/jmr/policy.html. DOI: 10.1557/JMR.2005.0103
reduce the tendency for buffer layers to detach, the factors contributing to detachment must be identified, understood, and controlled. Clearly, many factors can contribute to buffer layer detachment including (i) film stress (both thermally and growth induced), (ii) film porosity and morphology (leading to enhanced diffusion and interface reactions), and (iii) interfacial segregation of impurities. In this study, we attempt to demonstrate that the oxidation of carbon impurities to form CO can be a significant factor contributing to buffer layer detachment and that a simple heat treatm
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