Vortex Pinning in Complex Heterogeneous Microstructures in REBCO Thin Films Formed by High-Density Artificial Pinning Centers

Development of REBCO coated conductors is in progress, and it is desired to improve the superconducting current-carrying capability for equipment applications at operating temperatures near 20 K and 65 K. For this purpose, appropriate introduction of artificial pinning centers (APCs) into REBCO thin films is effective, and higher concentrations of APCs are needed. For example, the main pinning center in NbTi metallic wires is the α-Ti normal-conducting precipitate, with an upper limit of about 25-30% in volume fraction. However, the upper limit for the volume fraction of APCs in REBCO thin films is much lower. If it is possible to introduce APC in REBCO thin films up to the same level as the volume fraction of α-Ti, higher Jc performance is expected to be achieved. At present, it is known experimentally that the upper limit of APC concentration in REBCO thin films is about 5-10%. The reason for this is thought to be the increase in epitaxial distortion, formation of various lattice defects, and oxygen deficiency at the interface due to the introduction of APC, as well as the reduction of the superconducting current path due to the complex structure. Under such a complex and inhomogeneous structure, it is very difficult to find the optimal APC distribution for high magnetic field applications by a conventional screening method. We first investigate the microstructure of REBCO thin films near the upper limit of APC concentration and the degradation of superconducting performance caused by epitaxial distortion, in order to obtain clues for solving the upper limit problem of APC volume fraction. In this study, GdBCO thin films with BHO nanorods were fabricated on IBAD-CeO2 substrates by PLD method. The PLD targets were 4.0, 4.5, 5.0, and 5.5 wt% BHO-doped GdBCO, and the PLD conditions were as follows: deposition temperature 760-840°C, oxygen partial pressure 300 mTorr, distance between substrates 60 mm, laser frequency 10 and 100 Hz, and laser energy 300 mJ/pulse. The sample was subjected to structural analysis by XRD, evaluation of superconducting properties such as Jc and Tc by PPMS, and microstructural observation by TEM and STEM. Degradation of Jc properties was observed along with a decrease in Tc at higher BHO concentration. Interestingly, in the films fabricated at 10 Hz, the BHO nanorods collapsed and a BHO layered structure appeared. Elemental mapping by STEM confirmed that these were BHO additives. The collapse of the nanorod structure clearly causes degradation of the c-axis Jc property. This structural transition from nanorods to layers is considered to depend on the competition between the release of strain energy stored in the thin film and the kinetic process of thin film nucleation and growth. To quantitatively evaluate the origin of the above structural transition, epitaxial strain energy was analyzed based on micromechanics theory. Furthermore, vortex pinning simulation using TDGL (time-dependent GL) was performed to evaluate the elemental pinning forces at the complex APC interface, and the optimized APC structures to be considered in the future were discussed.