Fabrication and characterization of MgB₂ spherical shells with reduced thickness on 1 mm diameter Si₃N₄ spheres

Magnetic levitation of the fusion target by coating a thin MgB₂ superconducting shell on its outer surface has recently been proposed in inertial confinement fusion (ICF) to realize a noncontact support of the target at ∼ 20 K to boost the implosion performance and fusion yield. To avoid possible effects on target ablation, the coated MgB₂ shell is anticipated to be as thin as possible while fulfilling the target levitation requirements. Under this circumstance, the fabrication of an MgB₂ shell with reduced thickness has been explored using a hybrid physical-chemical vapour deposition method. By gradually decreasing the deposition time, a set of MgB₂ shells were grown on 1 mm diameter Si₃N₄ spheres with the thickness reducing from 720 nm to 200 nm. The spherical shells all have a polycrystalline structure characterized by closely packed hexagonal grains, with both the grain size and thickness diminishing as the shell thickness decreases. The superconducting transition temperature T_c of the shells, as determined by both resistance and magnetization measurements, is in the range of 38–40 K and all shells exhibit ideal diamagnetism at low temperatures. For the thinnest shell of 200 nm, the superconducting critical current density J_c at 20 K reaches 8.0 × 10⁶ A/cm² and 2.1 × 10⁵ A/cm² under zero and 2 T applied field, respectively. The results indicate that it is experimentally feasible to fabricate MgB₂ spherical shells with a thickness as low as 200 nm while maintaining the high T_c and J_c, thereby taking a further step towards the application of the shell in superconducting magnetic levitation for ICF.