Practical high-temperature superconducting materials possess excellent unobstructed current-carrying capacity under high-temperature (liquid nitrogen temperature range) or high-magnetic-field conditions, leading to their wide applications in large-scale power transmission and distribution, ultra-strong magnets, superconducting energy storage devices, superconducting generators, transformers, and maglev trains. The fabrication of high-temperature superconducting wires and tapes is a high-tech field of strategic economic significance in the 21st century. In 2008, Japan discovered a novel iron-based superconductor, LaFeAsO1-xFx, with a superconducting critical transition temperature (Tc) reaching 26 K, attracting immense interest from the global superconducting community.
In practical applications, such as winding superconducting magnets and manufacturing superconducting cables, superconducting wire strips are essential. Therefore, to realize the application of novel iron-based superconducting materials, it is necessary to develop practical methods for preparing long superconducting wire strips. However, due to the relatively hard and brittle mechanical properties of iron-based superconducting materials, they are difficult to plastically deform and process.
Based on current research, the preferred method for preparing iron-based superconducting wires and strips is the powder-in-tube (PIT) method. The specific process of the PIT method involves first uniformly mixing starting powders under an Ar atmosphere and then filling them into a metal tube. The metal tube is then shaped into wires or strips through cold working processes such as forging, drawing, and rolling. Finally, the formed wires and strips are heat-treated under a protective atmosphere to form superconducting wires and strips with good bonding properties. The PIT method is generally divided into in-situ and ex-situ methods, the biggest difference being the starting powder used. In the in-situ method, the reactants are mixed uniformly in a suitable stoichiometric ratio and then loaded into a metal tube, while in the ex-situ method, pre-sintered superconducting bulk materials are ground and mixed uniformly as filler powder, i.e., precursor powder. The advantages of the in-situ method are its simple preparation process and low risk of introducing impurities. However, the in-situ method only allows for a single grinding and mixing operation, which can easily lead to uneven sample composition and affect the final performance of the wire and strip material. Furthermore, this method can only optimize the superconducting properties of the material by adjusting the temperature and time of the final heat treatment. In contrast, the pre-in-situ method can obtain a more fully reacted and denser superconducting phase through multiple mixing and sintering processes. At the same time, the pre-in-situ method allows for the selection of appropriate cladding materials and heat treatment processes during multiple sintering processes, further improving the transmission performance of the wire and strip material. Currently, the PIT pre-in-situ method has become a key focus of research on the practical application of iron-based superconductors. The prepared wire and strip material has a critical transport current density (Jc) exceeding 10⁵ A/cm² at 4.2 K and zero field, and exceeding 10⁴ A/cm² at a high field of 10 T.
From an industrial application perspective, the PIT method is also very attractive. It uses low-cost materials and has a simple plastic deformation process, making it easy to achieve large-scale production. Currently, the PIT method has been widely used in the fabrication of superconducting wires and tapes, enabling the fabrication of commercial superconducting long wires on the order of kilometers.
In the powder-in-tube process, the preparation of iron-based superconducting powder is a crucial step. Generally, high-purity starting materials are mixed in a specific stoichiometric ratio and then directly sintered in a solid-state environment. Because iron-based superconducting materials are prone to oxidation in air, the proportioning, grinding, mixing, and sintering of the starting materials must be carried out in a glove box or under an Ar protective atmosphere to reduce the oxygen content in the powder. High-energy ball milling is typically used to crush and mix the starting materials of the iron-based superconductor, with repeated grinding steps possible. The homogenized starting materials are then placed in a sealed quartz or metal tube, and argon gas is introduced under a specific atmosphere. Sintering at a high temperature of 800–1200℃ yields a high-performance superconducting powder. Several key issues need to be considered during the synthesis of iron-based superconducting powder, such as elemental proportions, oxygen content control, and heat treatment optimization.
