Formation and modulation mechanisms of Fe-As phases in melting-processed Ba(Fe1-xCox)2As2 superconductor

The Fe -As phases play a crucial role in determining the superconducting properties of iron arsenic-based superconductors. In this study, we employed high-temperature confocal laser scanning microscopy (HT-CLSM) to investigate the formation mechanism of Fe -As phases in the melting processed Ba(Fe0.92Co0.08)2As2 superconductor through in situ observations. By combining these observations with microstructural and magnetic characterizations, we have confirmed that the high-temperature Fe -As phases primarily originate from the partial incongruent melting of the Ba122 superconducting phase. Moreover, we have elucidated the transition process of the Fe -As liquid and identified two dominant eutectic reactions in normal cooling sample: "L -> Fe2As + FeAs" and "L -> Fe2As + alpha-Fe". In case of extremely slow cooling rate, a further eutectic reaction between the Fe -As and superconducting phases ("L -> Ba122 + Fe2As + FeAs") occurs, leading to the elimination of alpha-Fe and an increased volume of non-superconducting phases. We have also analyzed the effects of the Fe -As phases on the magnetic behavior, critical current density, and vortex pinning behavior. This study provides valuable insights for designing the microstructure of new type iron arsenic-based superconductors through the melting process.