Methodology for the Characterization of Surface Treated High Purity Niobium

High-purity niobium is the material of choice for making superconducting radio-frequency cavities for the modern particle accelerator. While making the cavities from the Nb sheet, it goes through a series of processes including chemical and thermal treatments. These treatments affect the microstructural as well as magnetic properties of niobium. In order to optimize the efficiency of Nb cavities, various treatments have been explored. During the last decade, surface modification through impurity diffusion has been extensively explored and found that the quality factor of Nb cavities can be enhanced by impurity diffusion to the niobium surface. The surface treatments that are found to enhance the quality factor include high-temperature nitrogen doping, low-temperature nitrogen infusion, and most recently mid-temperature furnace baking. Even though these treatments have been shown to improve cavity performance, the fundamental understanding of the phenomenon is very limited, and each treatment has its own drawbacks. During the operation of Nb cavities, the oscillating electromagnetic field penetrates the Nb surface only to the 40-200 nm depth. Hence, the cavity performance is dependent largely on the condition of this layer. Thus, a detailed understanding of the effect of different treatments on the Nb surface is essential. Cutting a Nb cavity for the characterization of its surface after different treatments is economically not viable and the characterizing tools are also limited for the cut-out samples. Based on the characterization techniques, the requirements for sample shape and size vary, for example, microscopy, x-ray photoelectron spectroscopy, ellipsometry etc. require flat samples whereas magnetization measurements are best made using cylindrical samples. Hence, using a coupon sample characterization turns out to be a cost-effective way of probing the effect of different processing steps. In this thesis, I have used surface-characterizing tools such as scanning electron microscopy (SEM), x-ray photoelectron spectroscopy (XPS), and magnetization to explore the effect of some of the recent cavity treatments on the microstructure, chemistry, and magnetic properties of high purity Nb surface. The goal is to construct a coupon-based methodology to explore the effect of different processing treatments.