The Thickness Dependence of the Superconducting Properties of MgB2 Spherical Shells Deposited on 1 mm Diameter Si3N4 Spheres

Superconducting spherical shells have important application value or potential in various fields, such as inertial navigation, gravity measurement, and nuclear fusion. Depending on the needs of the applications, the thickness of the superconducting shell may vary from micrometers to nanometers. We report an investigation into the superconducting properties of MgB2 spherical shells deposited on 1 mm diameter Si3N4 spheres, with the shell thickness varying from approximately 200 nm to 720 nm. The upper critical field Hc2, the lower critical field Hc1, and the superconducting critical current density Jc were determined from electrical resistance, initial magnetization, and magnetization hysteresis loop measurements, respectively. As the shell thickness decreased, it was observed that Hc2 was enhanced, Hc1 declined, and Jc rose. This systematic evolution of the superconducting properties is suggested to have arisen from a reduction in the grain size with a decrease in the shell thickness, as shown by surface morphology images of the shells. A reduction in the grain size leads to increased grain boundaries and scattering strength, which decrease the electron diffusivities, thereby enhancing Hc2 and suppressing Hc1 as the shell thickness diminishes. The proliferation of grain boundaries would also provide more effective flux pinning in the shell, giving rise to an improvement in Jc. Our study demonstrated a close correlation between the microstructure, specifically, the grain size, and the superconducting properties in MgB2 spherical shells and the effectiveness of tuning them by varying the thickness of the shell. Moreover, it also revealed that the superconducting properties of the fabricated spherical shells, particularly their evolutions with the shell thickness, were critically influenced by the multiband superconductivity of MgB2. These findings would provide a helpful reference for fabricating MgB2 superconducting shells with desired thicknesses and properties, enabling them to better meet the requirements of applications in various fields.