Fabrication And Optimization Of Fe(Se,Te)Superconducting Materials

In 2008,the first iron-based superconducting materials based on LaOFeAs compounds was reported by Hosono et al,with the critical temperature Tc reached 26 K by F-doping,which attracted great attentions from scientists and thus injected new vitality into superconducting science researches.Based on the different structures,iron-based superconductors can be distinguished into four major catalogs:"111"(represented by)"122"(represented by),"111"(represented by)and "11"(represented by).Among Fe based superconductors,FeSe exhibit the simplest lattice structures,with only superconducting layers composed by Fe-Se and no blocking layers.Therefore it is possible to tune the superconducting properties by simply introduction of lattice strain or carrier concentrations.Besides,the fabrication process of FeSesuperconductors can not only avoid the usage of toxic element As,but can decrease the cost of starting materials.Therefore,more and more researches focus there researches on FeSe based superconductors for both fundamental study and applications.However,it is found that in final materials,besides the target tetragonal FeSe phase,the existence of hexagonal non-superconducting FeSe phase can not be easily avoided,which is harmful to the superconducting related study.Thus it is very important to study the phase transition mechanism during sintering to find the key factors for the fabrication of FeSe superconducting bulks and powders with high superconducting phase volume for further study and wires/tapes fabrication.In this thesis,Fe(Se,Te)superconducting bulks have been prepared by solid state sintering process and high energy ball mill(HEBM)aided sintering process.The phase evolution mechanisms involved in different process have been systematically discussed.The better preparation technique of Fe(Se,Te)bulks with high content of superconducting phase and high bulk density have been obtained.The main conclusions obtained are listed as follows:1.The phase evolution mechanism studies showed that Se-rich(Se,Te)ss and Te-rich(Se,Te)ss appered in Fe-Se-Te phase system after the melting of Se at first.With the increasing temperature,the diffusion of Fe into Se-rich(Se,Te)ss lead to the formation of both Fe(Se,Te)2 and Fe(Se,Te)based on different local Fe content.Then,Fe(Se,Te)2 reacted with residual Fe to form more Fe(Se,Te)phase.And over the entire process,Se and Te diffused between each other and the uniformity of chemical composition kept improving.At last,the superconducting tetragonal β-Fe(Se,Te)have been obtained with better crystallization and high uniformity.Both the Fe content and sintering temperature is important to the final superconducting properties.Thus It can be concluded that Fe content of 1.10,sintering temperature of 800 ℃ for 12 h and slow cooling process with the cooling rate of>25 ℃/h was beneficial for the synthesis of FeSe superconductors.2.Based on above study,it is possible to notice that the diffusion between Fe and(Se,Te),as well as Se and Te,is very important to the formation of superconducting phase.In order to further promote the distribution process during sintering,high-energy ball milling has been adopted to obtain precursor powders with high chemical composition uniformity and small particle size.The phase composition change of precursor powders during ball milling was studied.Amorphous appeared first and with further increasing ball milling time,mechanical alloying process also began and bothβ-Fe(Se,Te)and δ-Fe(Se,Te)can be observed in the final powders.By comprehensive analysis,the optimal ball milling time is determined as 2-4 h.The phase evolution mechanism of HEBM aided sintering process was investigated.The precursor powders were composed of Fe and(Se,Te)ss.With increasing sintering temperature,the two phases began to diffuse into each other and Fe(Se,Te)2 and β-Fe(Se,Te)phases were formed at the temperature about 100 ℃ lower than that in traditional solid stater sintering process.Then,with the further increasing temperature,superconducting β-Fe(Se,Te)phase is obtained with the elemental diffusion between Fe、Fe(Se,Te)2.In addition,the density of sintered HEBM bulks were 2.5 times than non-HEBM bulks.Thus it showed great potential to be adopted for the fabrication of high quality precursor powders for wires/tapes fabrication.3.Based on the fabrication process of Fe(Se,Te)superconducting bulks,in-situ power in tube(PIT)process has been adopted to preparate Fe(Se,Te)superconducting wires and tapes.The influences of Fe content and sintering temperature on the superconducting properties of final wires and tapes were investigated detailly and the Tc of superconducting filaments reached 10.5～14.2 K.It can be concluded that the increase of Fe content led to the enhancement of Tc.And the increasing sintering temperature could effectively increase the superconducting phase content in final filaments.Finally,the influence of fabrication parameters on the transport properties of Fe(Se,Te)wires and tapes will be studied in my future study.