Experimental Investigation On Singularity Of Current Distribution At Crack Tip Of YBCO Superconducting Films

Compared with traditional materials,high temperature superconducting materials have important application prospects in transportation,medical treatment,communication,energy and other fields because of their high critical transition temperature,no obstruction current and complete diamagnetism.For thin-film high-temperature superconducting materials,filters and quantum interferometers have been widely used in information,military,scientific research and other frontier fields,and become a new technological highland that countries around the world are competing to seize.YBa₂Cu₃O_7-x?YBCO?,the most representative high-temperature superconductor,has a transition temperature of about 90 K and excellent current-carrying density under high magnetic field,thus becoming the preferred material for preparing high-field magnets of about 20 T.However,this material is oxide ceramics,and some defects such as micro-cracks and holes will inevitably occur in the preparation process,which affect the critical current of the material and change the electromagnetic force distribution.Therefore,the distribution characteristics of the critical current at the crack tip of superconducting material become one of the basic problems to understand its mechanical behavior in the electromagnetic field.In recent years,through theoretical modeling,Lanzhou University predicted that the distribution of critical current at crack tip of superconducting materials is singular,and the order of singularity is"-1".In order to predict the theoretical results,Lanzhou University used YBCO superconducting blocks to fabricate artificial cracks.The singularity of the critical current distribution at the crack tip was deduced by measuring the magnetic field distribution around the crack,and the order of singularity was'-1.37'.Although the results confirm the singularity of the current distribution at the crack tip,the order is quite different from the theoretical prediction.The main reason is that the thickness of the superconducting block is large,the width of the artificial crack is more than 1 mm,and only a few magnetic field probes are arranged around the crack,which makes the measurement accuracy not high.In this paper,the magnetic field distribution near the crack of YBCO thin film in micron scale is measured by magneto-optical microscopy.The current density distribution near the crack is obtained by two-dimensional Fourier transform inversion.The singularity of current density near the crack tip is verified.On the basis of this,the singularity results closer to'-1'times are obtained,and the reliability of early theoretical prediction is proved.The main work of this paper is as follows:Firstly,an extremely low temperature magneto-optic experimental platform was set up to measure the magnetic field distribution near the crack of YBCO thin film based on Faraday effect.The current density distribution characteristics were given by two-dimensional Fourier transform.In the process of calculation,Hanning window low-pass filter is used to eliminate the high-frequency noise generated by CCD camera in the process of experiment,which improves the accuracy of experiment data.Then,an adaptive calibration method of magnetic field gray level is proposed,which solves the problem of non-uniform illumination and enlarges the available field of view of experiment.The reliability of the calculation program is verified.In addition,the dual-wavelength color magneto-optic method is used to obtain the conclusion that the magnetic field around the crack is consistent with the direction of the external magnetic field during the process of increasing the magnetic field around the crack,and there is no reverse magnetic flux distribution.A method of pre-deflection angle is proposed to increase the sensitivity of magneto-optic observation method to small magnetic field measurement.Secondly,based on the self-built experimental platform,the magneto-optical image near the crack of YBCO thin film was obtained,and the magnetic field and current density distribution near the crack were calculated by the program.Through the analysis of current density,it is found that the logarithm of current density near the crack tip of YBCO thin film is a function of slope close to the logarithm of'-1',which proves that the current density is singular and the theoretical prediction of its order is'-1'.In addition,it is found that the current densities in different directions near the crack tip also exhibit a singularity of'-1'.Finally,the magneto-optical observation experiments near the cracks of YBCO thin films under different temperature and magnetic field conditions were carried out.The distribution of magnetic field and current density near the crack is obtained.The results show that the current density at the crack tip still has a'-1'subsingularity at different temperatures and magnetic fields.These results show that the singularity and singularity order of the current density at the crack tip of superconducting materials do not change with the change of external conditions.