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Nb is a very popular material in superconductive electronic devices such as superconducting single-photon detectors and radio-frequency (RF) cavities for particle accelerators. Before MgB2 can be used as a substitute material, the stability of its properties over time must be confirmed. We studied how ageing over a period of 514 weeks affected the microwave surface resistance (RS) and the normal-state resistivity (ρ) of epitaxially-grown MgB2 films. MgB2 films with thicknesses of 1.0 μm and 1.7 μm were prepared using the hybrid physical CVD method, and exhibited a critical temperature (TC) of 39 K. They were stored in low-vacuum to prevent reactions with humidity, except when the films were taken out of storage for measurement purposes. Variation in RS with the ageing period appeared to be significant at very low temperatures. The RS values of 1.0 μm-thick and 1.7 μm-thick MgB2 films increased by ~ 8 times and ~ 3 times, respectively, over ~ 10 years. The aged MgB2 films appeared to have reduced π-band gap energy values. The pristine MgB2 films value of 2.3 meV was reduced to 1.3–1.7 meV over ~ 10 years. The TC of the MgB2 films remained almost the same regardless of the ageing time. The enhanced RS and ρ in the aged MgB2 films were attributed to increased intraband scattering within the π-band and the σ-band. [ABSTRACT FROM AUTHOR]

Abstract: High-TC GdBa2Cu3O7−δ (GdBCO) superconductor has been popular for making superconductive tapes that have much potential for various fields of large-scale applications. We investigated aging effects on the microwave surface resistance (RS) of Ag-coated GdBCO layer on Hastelloy substrate, so called GdBCO coated conductors (CCs), and Ag-coated GdBCO films on LaAlO3 (LAO) single-crystal substrates at cryogenic temperatures and compared them with each other. Unlike the RS of Ag-coated GdBCO films showing significant degradation in 4 weeks, no significant aging effects were found in our Ag-coated GdBCO CCs aged 85 weeks. The reactive co-evaporation deposition and reaction (RCE-DR) method was used for preparing the Ag-coated GdBCO CCs. Such durability of the Ag-coated GdBCO CCs in terms of the RS could be explained by existence of a protective impurity phase, i.e., Gd-free Ba-Cu-O phase as confirmed by transmission electron microscopy study combined with the energy-dispersive X-ray spectroscopy measurements. Although the scope of this study is limited to the Ag-coated GdBCO CCs prepared by using the RCE-DR method, our results suggest that a solution for preventing the aging effects on transport properties of other kinds of Ag-coated GdBCO CCs could be realized by means of an artificially-grown protective impurity layer.Graphical Abstract: [ABSTRACT FROM AUTHOR]