Flux Pinning Research Progress of ‘11’ System Iron-Based Superconducting Materials AITranslate

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Basic Information:

DOI：10.13373/j.cnki.cjrm.XY23080006

Chinese Library Classification Number:TF803.21

Citation Information:

With the gradual advancement of the practicality of superconducting materials in more and more fields such as superconducting motors，superconducting cables，levitated superconducting magnet trains，and superconducting magnets，the demand for the scale and industrialization of superconducting materials has become increasingly urgent. Developing more advanced superconducting materials and studying the mechanism of their superconducting properties are the foundation for achieving superconducting practicality. For Class Ⅱ superconductors such as iron based materials，a mixed state of normal and superconducting states will occur under a certain external magnetic field and temperature. This mixed state has characteristics that high upper critical magnetic field and large critical current，which making it possible for superconducting materials to be practical. Relatively speaking，in order to expand the application range of superconductivity，it is necessary to improve related superconducting properties like the upper critical magnetic field and critical current of superconductors. The "11" series material，which is easily prepared and non-toxic in iron-based superconductors has rich superconducting properties while being stable for impurities，and the simplest layered tetragonal crystal makes it a premium platform for studying and improving superconducting properties. Currently，considerable progress has been made in the study of the mechanisms for improving the performance of these superconducting properties. Based on Dew-Hughes theory standard，materials can be roughly divided into two categories according to their pinning properties：δT_c pinning is generated by the fluctuation of the transition temperature T_c in the sample and the other type that δl pinning is caused by spatial changes in the average free path of charge carriers. Based on this theory，the type and morphology of pinning were determined，and on the temperature range from 4.25 to 14 K，the transformation of the pinning mechanism was also observed. The influence of different magnetic field strengths from 0 to 20 kOe and two magnetic field directions about perpendicular and paralleled to the c-axis of the material on the pinning mechanism was also observed. And some factors unrelated to the pinning force density curve were attempted to be excluded and discussed the standard for obtaining the normalized magnetic field h value source in the pinning curve. The conclusion was that a more suitable and unified upper critical magnetic field （H_c2）should be chosen instead of an irreversible field （H_irr）to reduce the impact of data errors on the experimental results. By analyzing the behavior curves of the critical current and magnetic field of materials and comparing them with the performance of other superconducting materials，more information could be obtained. Under the consideration of thermal fluctuations （Ginzburg number-1×10⁻³），Griessen theory model under collective pinning theory was used to determine the type of pinning and verified with the conclusions under Dew-Hughes theory standards. Also considering the strong and weak pinning theories under different magnetic fields to fit and analyze the data，as well as considering the influence of magnetic relaxation on pinning transformation and compared and analyzing it using multiple theories. It was found that the pinning mechanism might be dominated by multiple mechanisms rather than one. It was attempted to integrate several possible pinning types into the same formula and tested them under different magnetic field intensities to obtain the transformation process and degree of influence of the pinning mechanism in different environments，and also attempted to combine the weak pinning theory with the strong pinning theory，using the weak pinning theory at low fields，the strong pinning theory at high fields，and the combination of the two theories at mid field to describe the critical current curves under different magnetic fields. It was undeniable that Dew Hughes model could provide with a very powerful basis for analyzing the superconducting pinning mechanism，but more and more studies had found that relying solely on Dew-Hughes model was no longer sufficient to explain the increasingly in-depth pinning mechanism research. Currently，attempts were being made to combine different and diverse theoretical models to compensate for the limitations of a single theory，achieve a more comprehensive and accurate description of pinning behavior，understand the mechanism of superconducting flux pinning，and seek methods to improve superconducting performance，ultimately achieving the goal of promoting the practicality of superconducting materials. Although there might be differences or conflicts among the various viewpoints introduced，each viewpoint achieved logical self-consistency and provided an angle and method for analyze the pinning mechanism of superconducting materials. Obviously，in the process of continuously exploring its pinning essence，the current theoretical models had become increasingly difficult to achieve expectations，which might also be one of the reasons for the current disagreement. Therefore，a more appropriate theoretical model might be one of the most effective methods to solve the current problem，and it was also an important work to further achieve practicality.