Flux pinning study of ZrO₂-doped YBa₂Cu₃O_7- superconducting films

The critical current density (J(c)) of the YBa2Cu3O7-delta (YBCO) superconducting film will decrease with the increase of the external magnetic field due to the movement of flux vortex. The existence of the magnetic flux movement will cause energy consumption similar to the resistance, which usually can be pinned by different kinds of defects located in the YBCO film. However, the defects formatted during the YBCO film growth alone is not enough for the increasing application requirements. Thus, it is necessary to introduce pinning centers artificially to enhance the in-field critical current capacity. Element doping is one of the most commonly used methods. The metal-organic deposition method is used to generate nano-heterogeneous BaMO3 (M is metal, such as Zr, Zn, Hf, etc.) in YBCO films. These heterogeneous phases generally do not produce large texture distortion and can be used as effective flux pinning centers. However, doped heterogeneous phases still have problems such as large doped phase size and uneven spatial distribution of nanoparticles. Although there are some research methods that are conducive to solving the above problems, the ability to control element doping is limited. The addition of the preformed nanocrystals is a very efficient means to introduce the artificial pinning centers, which has the advantage of highly controlling of the shape and size and solving the problem of agglomeration in the traditional elemental doping route. In the present work, two kinds of dot-like and rod-like ZrO2 nanocrystals with sizes smaller than 10 nm were controllably prepared by the hydrothermal method. In order to achieve the monodispersive state of the ZrO2 nanocrystals in the acidic YBCO precursor solution, their surfaces were modified with gluconic acid. The transmission electron microscope tests revealed that the ZrO2 nanocrystals were randomly mono-dispersed in the gel film after coating. Compared with undoped YBCO films, zirconia nanocrystals doping reduces the superconducting critical transition temperature of the films, and the superconducting critical transition temperature decreases with the increase of doping amount, which may be caused by the volume proportion of nanocrystals in the films and the increase of Ba source consumption. The doping of ZrO2 nanocrystals significantly improves the low-temperature magnetic flux pinning and the in-field critical current densities of YBCO films. At 65 K, the maximum pinning force density of YBCO films doped with 5% ZrO2-C nanocrystals is about 15.2 GN/m(3), which is almost twice the pinning force density (7.3 GN/m(3)) of YBCO films without nanocrystals in the same magnetic field. Even at 77 K, the pinning force density of YBCO films increased by 66.6% compared with undoped films after doping with 5% dot-like nanocrystals. However, doping too much will lead to the increasing competition with the growth of the YBCO film due to the consumption of more Ba sources to form the BaZrO3 (BZO), resulting in a decline in the quality and superconducting performance of the ZrO2-doped YBCO thin films. In addition, it is revealed that the doping with 5% of the dot-like ZrO2 nanocrystals results in a better flux pinning property and in-field performance of the YBCO film than that of the rod-like nanocrystals.