| Inorganic dense oxygen permeating membrane is one of the important inorganic membranes used in gas separation and membrane catalytic reactions at high temperature. It is of capability to separate oxygen selectively from gas mixture. It can also be applied to many reactions, such as oxidative coupling of methane and partial oxidation of methane to syngas, solid-state oxide fuel cells, oxygen sensors, and oxygen pumps. The research progress will promote the development of many basic industry areas including chemical, energy generation, and metallurgy. Some new type mixed conducting oxides, which possess both ionic and electronic conductivities, and with high catalytic activities, have been developed to be oxygen-permeating materials recent years. Among which La2NiO4+δ is noticed to be the novel one. These new materials have been widely paid attentions to.The thickness of membranes is one of the key problems in the application of inorganic dense oxygen permeating membranes. The migration of oxygen ions in materials needs higher activation energy. The decrease of membrane thickness is one of the effective ways for obtaining higher oxygen permeating flux. Most of present reports are concerned with bulk membranes. It is considered to be difficult to reach a high oxygen permeation flux with significant mechanical strength. So the investigation of La2NiO4+δ series supported dense membranes preparation to improve its oxygen-permeating properties will be of interest for the industrial application of oxygen permeating membranes.In the present work the La2NiO4+δ series supported dense oxygen-permeating membranes are fabricated by sol-gel method and characterized by XRD, SEM, TG-DSC, FT-IR, iodimetry titration, and GC. The influence of partial substitution of B-site Ni by Fe is systematically studied in detail. A new method for ionic-electronic conductivities measurement is also established.The main research work and results are as follows:The La2NiO4+δ supported dense oxygen permeating membranes are prepared by complex sol-gel method. A series of preparation parameters is determined and the essential properties of obtained membranes are characterized.The possibility of La2NiO4+δused as oxygen permeating material is discussed according to the structure stability, oxygen ion migration, and ionic-electronic conductivities. The results show that for La2NiO4+δthe structure tolerance factor t-= 0.890.899, and the M-0 bond average bonding energy (ABE) forLa2NiO4+δ is 298.27 kJ/mol, which means the structure is stable. Furthermore, because of the valence changeability of nickel ions, La2NiO4+δ should possess a better electronic conductivity.La2NiO4+δ supported dense oxygen permeating membrane of thickness less than 40 U m was prepared on a porous a -AI2O3 substrate with pore size of 0.2 p m by complex sol-gel method, on which there has been no report till now. The optimized conditions for stable (La, Ni) -EDTA complex sols are ￡Mjn+: EDTA (mol/mol) =1:1.5 and pH=56. The gel-to-oxide transition temperature is 850 "C. The product so obtained is La2NiO4+δ with K2NiF4-type structure, which is proved by XRD. The supported membrane is perfect by the evidence of SEM, EDS, and GC, and it performs a high oxygen permeating flux of 3.33 X 10"7mol ? cm'2 ? s"1 at 550 °C, which is at least one magnitude greater than that of lmm-thick bulk membranes made of same material at 650°C. This shows a promising prospect for the supported membranes. A reversible weight lose-recovery between 600700°C was found forLa2NiO4+δ, which may cause the leakage of thick supported membranes at higher temperature.Fe is introduced into La2NiO4+δ as substitute species to partially replace B-site Ni. This is favorable to oxygen ions migration in material. A significant effect was found. A theoretical analysis was given for the influence of Fe introduction on the stability of La2NiO4+δ and the relationship between substitution ratio and tolerance factor. Results show that La2Nii.xFex04+6 will keep a stable K.2NiF4-type structure when x is relatively small. A formulae for calculating the M-0 bond average bondingenergy (ABE) for K2NiF4-type complex oxide A2Bi-xB'xO4+6 is given as fallows: ABE =305.55 - 0.503* (kJ/mol), which is not reported previously. It is concluded that a reasonable value of x is less than 0.3.A series of La2Nii.xFexC>4+ s supported dense membranes were primarily prepared via complex sol-gel process and the oxygen permeating flux was measured. Results show a significant increase of flux due to the introduction of Fe. The oxygen permeating flux, when x=0.20, is greater than 1.0X lO^mol-cm'V1 at 477°C. As a comparison, the flux for Fe-free bulk membrane is only 33 X 10'8 mol-cm'2-s'' at 550 °C, with a difference of at least 2 magnitude lower.XRD results reveal that La2Nii.xFex04+6 will keep a perfect K2NiF4 structure when x is less than 0.30. The addition of Fe leads to an increase of excess oxygen ions concentration 8 . A large 8 is good to obtaining higher oxygen flux. The vibration of Ni-0 bond strengthens with the increase of Fe amount added, which might cause oxygen ions to move more easily. A lower apparent activation energy of oxygen permeation is observed after Fe is added into La2NiO4+6.The ionic mass transport mechanism through solid electrolyte was discussed based on the comparision of conductance properties for different types of electrolyte. As a result, an electrochemical model for oxygen ions passing through the dense oxygen permeating mixed conductors was set up.A new approach for conductivity measurement of mixed conductors has been established. The ionic conductivity of mixed conductors is identified from total conductivity of the material based on that ions will tend to be ceased when a high frequency alternative current passing through the material if the frequency is high enough. The conductivity of La2NiO4+g was experientially measured and the ionic conductivity was identified successfully. The relationship between ionic conductivity and temperature is established as well. For La2NiO4+s oxygen permeating material, the conductivity under very low frequency is equal to total one, and the conductivity will be electronic one when the frequency is higher than 80kHz. Results show that when temperature is higher than 300 °C the dependence of conductivity of La2NiO4+s on temperature will become weaker. This implies someequilibrium between ionic and electronic conductance above this temperature. The ionic conductivity of La2NiO4+s increases with elevated temperature and small activation energy for oxygen ion migration is observed. The results coincide with that measured by conventional direct current four-terminate method.The oxygen permeating flux was calculated based on Wagner's theory with the experimental conductivity data mentioned above and compared with the experimental results measured by gas chromatography. The calculation results are significantly comparative with experimental results of bulk membrane and match that of supported membrane in magnitude. This implies the validity of the new technique for conductivity measurement of mixed conductor.
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