Preparation,Microstructure And Properties Of Oxygen Permeable Membrane Materials With K₂NiF₄ Structure

The mixed ionic electronic conductor（MIECs）ceramic oxygen permeation membrane has the advantages of easy operation,high efficiency and low energy consumption in oxygen related production processes.So far,inorganic dense MIEC oxygen permeable membrane materials are mainly focused on perovskite oxides（ABO₃）with high oxygen permeability.The perovskite oxygen permeable membrane,represented by Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3-δwith high oxygen permeability,has poor structural stability and is easy to decompose due to reaction with carbon dioxide in air.In recent years,MIECs materials with K₂NiF₄ structure have attracted much attention in the field of oxygen separation membranes and solid oxide fuel cell（SOFC）cathode materials because of their high electrochemical properties,thermal stability and oxygen and reduction catalytic activity.However,due to the poor oxygen permeability compared with perovskite materials,It is necessary to improve the oxygen permeability for current researches.So far,the research reports on the oxygen separation of Ln₂NiO4+δ（Ln=La,Nd,Pr）materials are mostly concentrated on(Pr_0.9La_0.1)₂(Ni_0.74Cu_0.21Ga_0.05)O4+δmaterials,and it is still needed to explore new materials to improve the oxygen permeability,and element doping and surface modification are considered to be approaches.In this dissertation,Pr_1.3Sr_0.7Ni_0.7Cu_0.3O4+δ（PSNC）was chosen as the basement to investigate the effects of Fe³⁺doping on its properties,and PrO_x and Pr_1.3Sr_0.7Ni_0.7Cu_0.3O4+δwere chosen to modify La₂Ni_0.8Cu_0.2O4+δmembranes.Also,the effects of Zr⁴⁺doping on the properties of LNC materials was investigated.In the third chapter of this dissertation,Pr_1.3Sr_0.7Ni_0.7-xCu_0.3Fe_xO4+δ（x=0,0.025,0.05,0.1）（PSNCF）precursor powders were synthesized by the glycine nitrate method（GNP）,and PSNFC powders with tetragonal structure were obtained by calcining the precursors at 1000℃ for 5 h.Total electrical conductivity of the PSNFC samples reaches above 200 Scm^-1 in 300-950℃,and decreases gradually with the increase of Fe doping amount.At 950℃,oxygen ion conductivity of the Pr_1.3Sr_0.7Ni_0.65Cu_0.3Fe_0.05O₄₊dsamples is 0.194 Scm^-1,and the oxygen permeability increases with the increase of Fe doping amount.At 950℃ under（40%O₂-60%N₂）/He mixed gases,oxygen permeability of the Pr_1.3Sr_0.7Ni_0.65Cu_0.3Fe_0.05O₄₊dsamples reaches 0.034 mLcm^-2min^-1,which is 3 times as high as that of the undoped samples.In the fourth chapter,PrO_x oxide and PSNC materials were deposited onto the surface of La₂Ni_0.8Cu_0.2O₄₊δmembranes by a dip-coating process.Phase structure,microstructure and oxygen permeability of the modified materials were characterized by XRD,SEM,EDS and oxygen permeation test.The results show that cubic Pr₄O₇ and tetragonal PSNC compound form on the surfaces of LNC samples by heating at1000℃ for 2h.The Pr₄O₇ catalytic layer is a homogeneous porous structure with a thickness of 5¹0μm.The PSNC catalysts adhere to the membrane surface with a coverage less than 50%.The Pr₄O₇ catalysts significantly enhanced the oxygen permeation rate of LNC at 850⁹00℃,and the oxygen permeability(0.26 mLcm^-2min^-1)is 3.5 times higher than the unmodified LNC film(0.058 mLcm^-2min^-1)at 850℃;and the LNC membranes modified by PSNC catalysts have an oxygen permeability of 0.27mLcm^-2min^-1 at 1000℃.In the range of 850⁹00℃,the activation energy of LNC membrane modified by Pr₄O₇ and PSNC catalysts decreases 53%and 64%respectively.In the fifth chapter,La₂Ni_0.8-xCu_0.2Zr_xO4+δ（x=0,0.025,0.05,0.1）were successfully synthesized by solid-state method.The effects of Zr doping on the phase structure,microstructure and oxygen permeation of materials were studied by XRD,SEM,conductivity and oxygen permeation test.The results show that the introduction of Zr elements is beneficial to the formation of the tetragonal structure,while the La₂NiZrO₆impurities exist when the Zr⁴⁺doping ratio is>0.025.The grain size of the LNCZ samples decreases with the increase of Zr doping ratio.The total conductivity is in the range of 60¹15 Scm^-1,and decreases gradually with the increase of Zr doping ratio.The oxygen permeability of the LNCZ materials increases with a small amount of Zr doping,and the oxygen activation energy decreases slightly.The oxygen permeability of La₂Ni_0.775Cu_0.2Zr_0.025O_4+δreaches 0.262 mLcm^-2min^-1（1000℃,0.8mm thick）,which is1.45 times higher than LNC.