Thermomagnetic instability of type-II superconducting bulks with pores based on fractal theory

The magnetic flux jump phenomenon induced by thermal disturbance easily occurs in the magnetization process of type-II superconducting bulk materials, which seriously affects their macroscopic electromagnetic properties and application stability. Traditional research studies assume that materials are homogeneous and dense, ignoring the influence of the complex pore structure formed in the actual preparation process on magnetocaloric behavior. In this paper, based on the fractal theory, we establish a magnetic–thermal coupling model considering a complex pore structure to study the thermomagnetic instability of BiSrCaCuO bulk samples under different magnetization processes. The effects of fractal dimension, iteration times, porosity and magnetization mode (zero-field cooling and field cooling) on magnetization curve, temperature evolution and flux jump behavior are studied. The results show that the pore structure significantly affects the effective specific heat and the induced current of superconductors, and changes the frequency and initial field of magnetic flux jump. When the porosity exceeds a certain threshold, the Joule heat decreases significantly and the magnetic flux jump disappears. Different fractal structures exhibit similar magnetization correspondence at the same volume fraction, which proves that volume fraction is a key variable affecting magnetization behavior. In addition, the effects of the parameters such as sample size, ambient temperature, and external magnetic field sweep rate on magnetocaloric behavior are also discussed. This study provides a new theoretical method for understanding and optimizing the thermomagnetic stability of porous superconducting materials.