The Study Of Element Substitution Effects On Structure And Superconductivity Of MgB₂

The intermetallic compound of MgB₂ with six square crystal structure occurs superconducting transition at the temperature approaching 40K?-233??and becomes a new class of superconductor.So far,great progress has been made in the basic research and application development of MgB₂.In basic research,people not only have a clear understanding of the basic properties of the MgB₂ superconductor,but also have a deep understanding of its superconducting mechanism and characteristics,thus laying an important foundation for developing the practical application of MgB₂ superconductor.In practical applications,the practical wire based on MgB₂ superconducting material has been commercialized,and the related strong electric applications have also made significant progress.Many kinds of applications and components including motor,magnet et.al.have been developed.In conclusion,the discovery and related research of MgB₂ superconductors have greatly promoted the development and progress in the field of superconductivity.However,as other superconductors have been discovered,MgB₂ superconductor still remains some puzzling theoretical problems,imperfect performance in practical applications,and technical difficulties in its mass production,and so on.The solution of these problems depends on the more thorough and meticulous research and more elaborate development of the researchers on the basis of the predecessors.This paper is in such a context,on the sense of responsibility and mission for a researcher of superconducting materials,it is urgent to make our own contribution on focusing on the fundamental problems of application of MgB2 superconducting materials to study,striving in the related physical mechanism,deepening the insight into the MgB₂ superconductor,and promoting its preparation technology.In the third chapter,the doping of third main elements Al and In in MgB₂ is studied in this paper.Firstly,three series of Mg_1-x-x Al_xB₂?Mg_1-xIn_xB₂ and Mg_1-x(Al_0.5Li_0.5)_xB₂ samples were prepared through the optimized preparation method,and the doped samples with high purity and good uniformity were obtained,which laid a good foundation for subsequent structural characterization and physical property measurement.The phase structure and crystal structure of the samples were characterized by XRD.The information about phase structure and crystal structure was obtained.The superconducting transition temperature of these three series of samples with the doping amount was studied by DC magnetization measurement.In the Al doped system,significant crystal structure shrinkage and superconducting critical temperature suppression phenomenon induced by Al doping were observed.In the In doped system,slight c axis extension and cell volume expansion phenomenon were observed,while the critical temperature of superconductivity was slightly inhibited.The concept and characterization method of chemical doping were established,and combined with McMillan's Tc theory of BCS strong coupling superconductor and MgB2's dual band model,theoretical analysis was carried out.It is found that chemical pressure effect plays an important role in the inhibition of Tc.In addition,it is found that the Tc inhibition caused by the doping effect of Al and In can be explained by the interaction of the band filling effect and the chemical pressure effect.In order to verify the effectiveness of the chemical pressure model,we further studied the co-doping of Al and Li.It is found that the electron doping of Li and Al electron to some extent can be interacted with each other,which leads to the cancellation or weakening of Tc inhibition effect caused by Al doping.However,because the main role of Li is in the?energy band and Al mainly in the?band,the balance of the two kinds of doping is limited.The inhibition of the co-doping of Al and Li on Tc is mainly due to the chemical pressure effect,while the unbalanced or counterbalanced band filling effect is secondary.In the fourth chapter,we studied the influence of nonmagnetic transition metal doping on the crystal structure,electronic structure and superconductivity of MgB₂,and compared with the results of that of Al doping.First of all,Mg1-xAgxB₂ series samples were prepared by liquid-solid reaction,and the high purity and uniform doped samples were the most important step to study the doping effect.On this basis,the structure and superconductivity of a series of Ag doped samples were studied by means of XRD characterization and magnetic measurement.The results show that Ag doped in MgB₂ crystal structure leads to the occurrence of MgB₂ shrinkage,and the shrinkage degree of it is similar to that of Al doping.But the absolute value of the chemical pressure coefficient in the direction of a axis is greater than that of Al doping,and the chemical pressure on c axis direction is smaller than of the Al doped.The inhibition rate of superconductivity is greater than that of Al.The anomalous effect for Ag as a nonmagnetic transition metal can be understood in accordance with the characteristics of transition metals.The results of the measurement of DC magnetization show that the critical temperature of MgB₂ is reduced by Ag doping.Based on the strong coupling theory of BCS,the electronic band filling model and the chemical pressure effect,we have studied the mechanism of Ag doping on the MgB₂ superconducting critical temperature.The theoretical analysis is in good agreement with the experimental results.After studying and comparing the results of crystal structure change and Al doping effect on the Ag doping of non-magnetic transition metal,we found some anomalous characteristics.The influence of Al,In and Ag doping on the crystal structure is further compared.Aiming at the regularity and characteristics of crystal structure change of AlB2compounds with two boride crystal structure,we build the"distortion model of doped atoms"in MgB₂.Based on the strong covalently characteristic of B-B bond,this model put forward the hypothesis that the graphite like layer composed of B atoms is quasi rigid.Meanwhile,according to the minimum energy principle,it is assumed that the volume invariance requirement is maintained during the process of atomic distortion.Using this model,we not only explain the experimental phenomena observed in this study,but also can better understand the crystal structure distortion caused by doping different elements in the MgB2 to further change the superconductivity.This may be helpful for optimization design and subsequently improving the way to dope in MgB₂ superconducting material.In the fifth chapter,we studied the influence of 3D magnetic transition metal doping on the crystal structure,electronic structure and superconductivity of MgB₂,and compared with the results of Al doping samples.This chapter firstly made Cr doped MgB₂ series samples through two step solid phase with high energy ball milling,and the phase structure and crystal structure change with the doping amount are study with x-ray diffraction.The relationship of DC magnetization intensity versus temperature is measured with SQUID magnetometer.The results show that the doping of Cr in MgB₂ leads to a severe contraction effect of MgB₂ crystal structure,and the degree of shrinkage is far more than that of Ag and Al doping.Cr doped MgB₂ leads Tc to reduce significantly,inhibition rate of Tc calculated with the band filling theory and chemical pressure effect is far lower than that of the experimental observations.Furthermore,the sudden quenching of Tc with the increase of doping concentration cannot be explained by conventional band filling and chemical pressure effect.In addition to Cr doping,Fe doping and Mn doping also exhibit similar behavior.By comparing and fitting the theory of magnetic impurity removal with Abrikosov-Gor'KOV,it is found that 3D magnetic transition metal doping is a typical magnetic impurity pair effect.It is shown that MgB₂ belongs to the strong coupling BCS superconductor,and its electronic pairing is s wave pairing.In the sixth chapter,in order to reveal the unique behavior of 3D magnetic transition metal in MgB₂,specially prepared Ba2Ti2?Fe1-xCrx?2As4O the Cr doped Fe based superconductors were studied,which can further reveal the basic characteristics?such as MgB₂ in the superconducting symmetry distribution of superconducting electronic mechanism,energy gap etc.?and the similarities and differences of the characteristics of the corresponding magnetic impurities.Cr doped Ba2Ti2Fe2As4O is a new field that has not yet been reclaimed by other research groups.Because Cr doping Ba Fe As2 is much studied,and there is not much research about Ba2Ti2Fe2As4O.The strong anisotropy characteristics for Ba2Ti2Fe2As4O is more inclined to two-dimensional system to form a planar multilayer structure similar to MgB₂.Therefore,it can be used as an ideal system for comparative study.Finally,this paper makes a summary and prospect of the research work,and makes some thinking for the future research work.