Study On The Improvement Of Current-carrying Properties And Mechanism Of YBCO And MgB₂ Superconducting Films By Heavy Ions Irradiation

In the present work,two kinds of practical superconductors,i.e.YBCO and Mg B₂ thin films,were irradiated by using swift heavy ions of Institute of Modern Physics（IMP）and low-energy heavy ions of Korea Atomic Energy Research Institute（KAERI）,respectively.The variation of microstructures and superconducting properties with ion fluence were systematically studied by using a variety of analytical methods.The research work presented here shows that in-field current carrying capability of superconducting materials can be obviously enhanced by ions irradiation methods,providing a reference for the application of ion irradiation technology to the batch preparation of superconducting materials in the future.The main conclusions are shown as follows:（1）The in-field current carrying capability of YBCO can be significantly enhanced by one-dimensional continuous and amorphous latent tracks produced by swift heavy ions irradiation,without degenerating critical temperature.Gd-YBCO(YGd_0.2Ba₂Cu₃O_7-x)commercial coated conductors were irradiated by using 1.9-Ge V Ta ions,and the latent tracks produced by heavy ions have been observed by TEM（transmission electron microscopy）.Mixed-pinning landscapes were formed by irradiation-induced latent tracks and oxide particles induced during film manufacture.Critical temperature T_c is almost unchanged（the decrease of T_c is smaller than 0.5 K）when irradiation fluence is smaller than 5×10¹⁰ ions/cm².Whereas,in-field critical current density J_c increasing obviously,withαvalue,i.e.the slope of linear part in double logarithmic J_c-H curve,decreased from 0.72 to 0.31,which indicating that the current transmission capacity of Gd-YBCO commercial coated conductors can be enhanced by Ta ions irradiation.（2）Stress and Ba-Cu-O non-superconducting phase were induced in YBCO thin films after swift heavy ions irradiation.The microstructure changes of Ta ions irradiated YBCO films were studied by using Raman spectroscopy.Raman spectra showed no obvious changes when fluence is smaller than 8×10¹⁰ ions/cm².Whereas,a new peak near 600 cm^-1 gradually appeared and its intensity increased with the continuously increasing fluence,indicating that new phase was generated in YBCO after irradiation.Moreover,a red shift occurred in O（2,3）-B_1g peak.The peak gradually shifts from 338.1 cm^-1 to 335.4 cm^-1 when irradiated at a fluence of1.6×10¹² ions/cm²,which indicating that tensile stress was induced in irradiated YBCO.（3）The influence of temperature,magnetic field and diameter of latent tracks on pinning efficiency of YBCO were studied.YBCO films were irradiated with three kinds of ions with different electronic energy loss,i.e.43.7 ke V/nm Bi ions,35.8ke V/nm Ta ions and 17.7 ke V/nm Kr ions,with a fluence of 5×10¹⁰ions/cm².TEM results show that one-dimensional latent tracks were formed in both Bi ions and Ta ions irradiated samples.However,no visible latent tracks in Kr ions irradiated sample.The pinning efficiencyηwas represented as ratio between pinning force density of irradiated sample and that of pristine sample.In both Ta ions and Bi ions irradiated samples,ηincreases firstly and then decreases with increasing magnetic field,totally different from that of Kr ions irradiated sample.In addition,the larger diameters of latent tracks correspond to a higherηand J_c value when B<1 T,contrary to high field,indicating that the practical operating environment needs to be considered when determining the diameters of one-dimensional columnar defects in YBCO superconductors.（4）The pinning strength of grain boundaries in small-grained Mg B₂ thin films can be significantly enhanced by low energy ions irradiation.The high field current carrying capability of Mg B₂ was obviously enhanced after irradiated by 120-ke V Mn ions.For instance,the J_c value of 250 nm-thick sample increased more than 10-fold from approximately 0.02 MA/cm² for the pristine sample to 0.26 MA/cm² at 5 K and2.4 T when irradiated with 1×10¹³ ions/cm².Normalized pinning force density curve of irradiated samples were fitted by using Dew-Hughes model.Result shows that surface pinning was the dominant pinning mechanism in irradiated samples,indicating that the pinning effects of grain boundaries in small-grained Mg B₂ can be enhanced by low energy ions irradiation,which is in contrast to the single-crystalline or coarse-grained Mg B₂ films where normal point pinning was dominant after low-energy ions irradiation.