Research Of Fabricating(Cu,C)Ba₂Ca₃Cu₄O_y Film By Pulsed Laser Deposition Method And Its Superconducting Properties

Ever since Heike Kamerlingh Onnes discovered the phenomenon of superconductivity in mercury in 1911,superconductivity has always been the focus of scientists'attention due to its unique and extraordinary characteristics.The potential applications inspire generations of researchers to make unremitting efforts to hunt for new and even room temperature superconductors.Among all the superconductors that have been discovered so far,only the critical temperature of cuprate superconductors exceeds the liquid nitrogen temperature under normal pressure.And from the perspective of application,superconducting materials working at liquid nitrogen(boiling point,77K)rather than liquid helium(boiling point,4.2K)are more economical,which makes cuprate superconductors be the most feasible candidates for large-scale industrial ap-plication.At present,the commercially available and non-toxic copper oxide supercon-ducting materials are Bi-based superconductors and REBCO systems(YBa₂Cu₃O_7-?,GdBa₂Cu₃O_7-?,etc.).Bi-based superconducting wires are not suitable for application-s at the liquid nitrogen temperature under high field,while REBCO superconduct-ing tapes in contrast,are suitable for large-scale applications at higher temperature and have better superconducting performance.Recently,compared to REBCO series,polycrystalline(Cu,C)Ba₂Ca₃Cu₄O_y[(Cu,C)-1234]was identified with higher critical temperature and irreversible field.However,polycrystalline(Cu,C)-1234 can only be prepared in small sizes by high-pressure synthesis technology and thus,difficult to be industrialized.If the preparation process of REBCO superconducting tapes can be transferred to realize the application of(Cu,C)-1234 thin film,it will be of great value.This is also the reason why(Cu,C)-1234 thin film is chosen to be deposited and investigated in our experiments.The thesis contains six chapters as follows:The first chapter gives a brief introduction to the development of superconductivity,including superconducting phenomenological theory and microscopic BCS theory.Also,different superconducting material systems,such as heavy fermion superconductors and iron-based superconductors are introduced,with cuprate superconductors highlighted.Then we review the application progress of Cu-based and iron-based hightemperature superconducting wires or tapes,and analyze the advantages of REBCO series in application,which are usually referred to as the second-generation hightemperature superconducting tapes.In the end,we present the typical characteristics of(Cu,C)-1234 superconductor in detail,including its crystal structure,doping insensitivity to T_c and rather high irreversible field.The second chapter is mainly about preparation techniques and characterization methods of films.Firstly,the conventional film preparation techniques are briefly introduced,such as sputtering deposition,molecular beam epitaxy method and so on.Secondly,pulsed laser deposition(PLD)system used in our experiments,with a upgraded multichannel gas-intake accessory is described in detail.Finally,the involved characterization methods of films in our experiments are presented.In the third chapter,we give several results related with growth conditions during(Cu,C)-1234 film deposition process by PLD method.It is found that the temperature range for the film phase is within 70K,which means it is very important to choose an appropriate deposition temperature for the substrate.Also,to obtain films with(Cu,C)-1234 phase,one must choose a substrate with smaller in-plane lattice constants than one of the material.And a relatively long growth time reduces flatness of the film surface and affects Cu content in the film.Moreover,we find the annealing temperature has little influence on the film phase and the film can survive at a really high annealing temperature.In addition,we also summarize other interesting conclusions obtained in the experiments,such as some results involved in ex-situ annealing process.In Chapter 4,by utilizing different CO₂ gas flow rates during film deposition,we obtained a series of a-axis oriented(Cu,C)-1234 superconducting films.We performed transport,magnetization and mid-infrared transmittance measurements on these films.It is found that the flow rate of CO₂directly affects the critical temperature of the films.The absorption peak in the mid-infrared spectrum reveals that there exists CO₃^2-clusters in the films,and the CO₃^2-clusters in the films are positively correlated with the CO₂flow rate.Therefore,the content of the CO₃^2-clusters significantly modulate the zero resistance temperature of the films.In the fifth chapter,we carried out transport and magnetization measurements on the a-axis oriented(Cu,C)-1234 superconducting film with the highest zero resistance temperature of 96K up to now.The data show that both of the critical current density and the irreversible field of the film are higher than the results reported in the previous literatures,and the film exhibits better superconducting performance than before.We give the possible reasons for the improvement of the superconducting properties of the film,and propose a possible way to further prepare the c-axis oriented(Cu,C)-1234 superconducting film by PLD method.At the end of the thesis,there is a summary of the whole thesis.