Effect of Mg infiltration time in the gas-solid method on the microstructure and superconducting properties of MgB2 bulk

Magnesium diboride (MgB2) bulks, have shown great potential for applications in various fields. Among the available fabrication methods, the gas-solid method has attracted considerable attention due to its ability to significantly reduce the MgO impurity content in the samples. However, cracking is prone to occur during the gas-solid reaction process, which adversely affects the structural integrity of the bulks. The introduction of molds can effectively suppress crack formation and improve the shaping quality of MgB2 bulks. Nevertheless, the use of molds also reduces the diffusion rate of Mg vapor, resulting in a slower reaction process and consequently lower production efficiency. In this work, the optimal sintering time for MgB2 bulk fabrication via the mold-assisted gas-solid method is optimized, thereby reducing experimental cost. A series of MgB2 bulk samples with sintering times ranging from 12 h to 72 h were fabricated using the mold-assisted gas-solid method. The results show that all samples are dominated by the MgB2 phase with a small amount of MgO impurity. With increasing sintering time, the grain size gradually increases, mechanical properties such as microhardness are significantly enhanced, and electrical resistivity decreases. Meanwhile, the Af, Jc, Fp, and Hirr are all improved. Nano-CT results reveal that Mg vapor diffuses from the exterior to the interior of the boron compact and reacts progressively. The internal defects evolve from dispersed pores at the early stage of sintering to planar cracks at later stages. Among all samples, the one sintered for 60 h exhibits the optimal comprehensive performance, indicating that 60 h is the optimal sintering time for the mold-assisted gas-solid fabrication of MgB2 bulks. This work provides important experimental evidence and theoretical guidance for further optimization of the fabrication process, microstructural control, and performance enhancement of MgB2 bulk materials