Synthesis Of 11-type Iron-based Superconductors With High Critical Current Density And The Exploration Of Low Dimensional Materials

Iron-based superconductors?FeSCs?are new high-temperature superconducting systems discovered in 2008.Similar to the cuprate superconductors,they have a layered tetragonal structure,and their superconducting Fe₂X₂?X=As,Se,etc.?layers are mainly composed of Fe and pnictogen or chalcogen.The parent materials of FeSCs are bad metals,and the anisotropy of the FeSCs is smaller than that of the cuprate superconductors,and their upper critical field(H_c2)and irreversible field(H_irr)are much higher than the conventional alloy superconductors,which gives them unique advantages in high field applications.In this dissertation,11-type FeSCs with no toxicity and no active elements were studied.The effects of various synthesis methods and trace element addition on Fe?Te,Se?phase formation,microstructures and superconducting properties were studied.Furthermore,some new superconducting materials have been explored in several low dimensional systems.In the Fe?Te,Se?polycrystals synthesized by solid-state reaction,interstitial iron and impurities such as FeTe₂ usually appears,which results in very low critical current density?J_c?in superconducting wires.In order to improve the superconducting properties of Fe?Te,Se?polycrystals synthesized by solid-state reaction,we investigated the effects of trace element addition on their phase formation,microstructures and superconducting properties at first.Among more than ten elements studied,trace addition of Ca and Bi significantly improved the polycrystalline superconductivity of Fe?Te,Se?.Polycrystalline X-ray diffraction?XRD?analysis showed that Ca could enter the crystal lattice of FeTe_0.5Se_0.5,but the addition of Ca would lead to the reduction of the content of FeTe₂ impurities,and at the same time improve the inter-grain connectivity and to form uniform and dense grains,which are conducive to reducing the weak connection effect in the samples.Moreover,the magnetic background of the M-H curve of Ca-added samples was significantly weakened,which may be due to the reduction of interstitial Fe.As a result,an appropriate amount of Ca addition can increase the J_c of the polycrystalline FeTe_0.5Se_0.5 samples by 2³ times compared with the unadded samples.For the samples added with trace Bi,polycrystalline XRD analysis showed that Bi did not enter the lattice of FeTe_0.5Se_0.5,and their FeTe₂ impurity was significantly reduced compared with the sample without addition.Furthermore,due to the liquid-assisted growth effect of Bi,the Bi-added samples have a large grain size,tight connection and a layered feature,which significantly reduce the weak-link effect in FeTe_0.5Se_0.5 polycrystalline samples.The?-T curves indicate that the trace addition of Bi can improve the T_c^zero of FeTe_0.5Se_0.5 polycrystals.For the majority of Bi added samples,the H_c2 at 2 K exceeds 50 T.Due to the reduction of FeTe₂ impurities and the weakening of weak-link effect,the addition of Bi can increase the J_c of FeTe_0.5Se_0.5 by dozens of times,and its J_c under zero field can reach 28000 A/cm²?2 K?.We also synthesized Fe?Te,Se?polycrystals by various methods,in which vapor transport reaction synthesis and microwave synthesis can significantly improve the superconducting properties of Fe?Te,Se?polycrystals.The vapor transport reaction synthesized samples have large crystal grains and are tightly connected,but XRD analysis indicates that there is always a problem of uneven composition.The samples synthesized by vapor transport reaction have higher T_c on the M-T curve than the samples synthesized by solid-state reaction,but the non-uniform composition makes them easy to exhibit two superconducting transitions at low temperatures.Due to the good interconnection between grains of Fe?Te,Se?synthesized by vapor transport reaction,we obtained Fe_0.95Te_0.5Se_0.5 polycrystal with high field J_c as more than 10times of the samples synthesized by the solid-state reaction synthesis,but the compositional unevenness limits the further improvement of J_c.The FeFe_0.6Se_0.4polycrystal synthesized by microwave are dense and have uniform grain orientation,which is beneficial to the reduction of weak-link effect.Compared with the solid-state reaction synthesized FeTe_0.6Se_0.4 polycrystal,the microwave synthesized sample has a smaller c-axis,a higher T_c on the M-T curve,and no magnetic background on the M-H curve,which give strong indications that there are possibly no interstitial Fe atoms existing in the crystal lattice,which is beneficial to the improvement of J_c.In addition,microwave-synthesized samples require annealing to improve their superconducting properties.Compared with solid-state reaction synthesis,microwave synthesis can increase the high field J_c of FeTe_0.6Se_0.4 polycrystal by about 30 times to 12000 A/cm²?7 T,2 K?.K₂Cr₃As₃ is a recently reported superconductor with quasi-one-dimensional crystal structure,and it is found that superconductivity is also present in the K₂Mo₃As₃ which has the same structure.In order to understand the relationship between these two superconductors,we synthesized and studied a series of doping samples of K₂(Cr_1-xMo_x)₃As₃.The resistivity and susceptibility data show that Mo doping can suppress the superconductivity of K₂Cr₃As₃,and its superconductivity is completely suppressed when the doping amount is 0.2.For K₂Mo₃As₃,superconductivity can be completely suppressed by Cr doping with a ratio of only 0.05.After the superconductivity of K₂Cr₃As₃ and K₂Mo₃As₃ is suppressed,their normal resistance changes from the metallic behavior of the superconducting samples to the semiconductor behavior.Moreover,K(Cr_1-xMo_x)₃As₃ polycrystals of x?0.6 can be obtained by deintercalation of K treatment of K₂(Cr_1-xMox)₃As₃,whose?-T curve shows semiconductor behavior.Furthermore,we found superconductivity in the LaRe₂Si₂ samples with a T_c of 3.2K and an H_c2 of 1.35 T.LaSSb was found,and its space group is P4/nmm?No.129?,which is the same as LiFeAs,and its?-T curve shows a semiconductor behavior.We also found the SrFRuAs with the ZrCuSiAs structure,and the?-T curve shows metal behavior.In summary,We studied the effects of trace addition of elements and different synthesis methods on the superconductivity of Fe?Te,Se?polycrystals.The superconductivity of Fe?Te,Se?polycrystals was significantly improved by the addition of Ca and Bi.Compare with solid-state reaction synthesized samples,Fe?Te,Se?polycrystals synthesized by vapor transport reaction and microwave have better superconducting properties.Moreover,we studied the effects of Mo doping on the superconductivity of K₂Cr₃As₃.During the exploration of new superconductors,it was found that LaRe₂Si₂ samples had superconductivity,and two new materials,LaSSb and SrFRuAs,were synthesized.