Oxygen Transport Property And Low-Frequency Internal Friction Behavior Of Oxygen Separation Membranes Materials

Dense ceramic membranes with mixed oxygen ionic and electronic conductivity hold promise of finding applications in oxygen production and oxygen-based industrial processes such as POM. Currently, the most investigated membranes are the single-phase perovskite oxide such as La_1-xSr_xCo_1-yFe_yO_3-δ. These materials possess high oxygen permeability at elevated temperature, usually at temperature above 850℃. Membranes operated at the temperature above 850℃often bring many problems such as the long-term instability and incompatibility in contact with other components. It has been a focus to find some new oxygen permeation materials for intermediate temperature application. Our research work is described in chapter 3-5, which is gained by bring forward the concept of dual-phase composite material and seeking new suitable materials. In addition, the results of the internal friction study of a series of RBaCo₂O_5+δ and La_1-xSr_xFeO_3-δ samples were shown in the Chapters 6 and 7 respectively. The internal friction technique has proved to give valuable information about defects and phase transitions in ceramic samples.Chapter 1 introduces the concepts and theories of oxygen permeation for dense ceramic membranes, presents a brief overview of membrane materials, and describes the scope of this thesis.Chapter 2 deals with the theory of anelasticity, the measurement methods, the standard anelastic solid and its dynamic properties as function of temperature, the theory of point-defect relaxations and the theory of the first-order phase transition.In Chapter 3, the effects of Sn doping on the microstructure, electric and oxygen transport properties of Bi₂V_0.9Cu_0.1-xSn_xO_5.35-δ(x=0, 0.025, 0.05, 0.075) are investigated. The measurement results show that doped-Sn benefits to the control of microstructure of the sample and favor their mechanic property though it doesn't further improve the electrical conductivity of the system. The oxygen permeation measurement result (externally short-circuited by painting Pt paste) show that Bi₂V_0.9CU_0.1-xSn_xO_5.35-δ possess very low oxygen permeation fluxes, 10^-8～10^-9 mol·cm^-2·s^-1 at the temperature range of 500～750℃, indicateing oxygen transport in Bi₂V_0.9Cu_0.1-xSn_xO_5.5-δ system is controlled mainly by the process of surface oxygen exchange.Chapter 4, three kinds of dual-phase composites consisting of Bi_1.5Y_0.3Sm_0.2O₃ /La_0.8Sr_0.2MnO₃, Bi_1.5Y_0.3Sm_0.2O₃/La_0.8Sr_0.2CrO_3-δ, and Ce_0.8Sm_0.2O_2-δ (SDC)/Sm_0.5Sr_0.5CoO_3-δ(SSC) are investigated. BYS/LSM dual-phase composites with different volume ratio (70/30;65/35;60/40) all show large permeability at intermediate temperatures. Among them the oxygen permeability of BYSLSM 35% is the best, in which oxygen permeation fluxes are 6.3×10^-8mol·cm^-2·s^-1 at 700℃and 2.5×10^-7mol·cm^-2·s^-1 at 827℃under a small oxygen partial pressure gradient of 0.21atm/0.0055atm. However BYS/LSC dual-phase composites (60/40; 55/45) possess the relative low oxygen permeation fluxes, which are smaller one order than that of BYS/LSM dual-phase composites at the same temperanges. For SDC/SSC dual-phase composites, it was found that the doped-SDC improves the oxygen transport property, especially at low temperatures. But over-doped SDC will decrease the oxygen permeation fluxes of SDC/SSC dual-phase composites. The SSC/SDC (65:35) composite membrane possesses reasonably high oxygen permeability and a flux of 1.3×10^-7mol·cm²Â·s^-1 is observed for a 1.0 mm thick membrane at 810℃and under a small oxygen partial pressure gradient of 0.21atm/0.0052atm.Chapter 5 presents the oxygen transport and electrical properties of Pr_1-xGd_xBaCo₂O_5+δ. Appreciable oxygen permeation fluxes through a dense membrane were observed at the moderate temperature. The membrane of 1.0 mm thickness allows oxygen to be transported at a rate of 2.6×10^-8mol.cm^-2·s^-1 for Gd_xBaCo₂O_5+δ and 8.0×10^-8 mol.cm^-2.min^-1 for PrBaCo₂O_5+δ respectively at 800℃under a gradient of air/helium. Pr_1-xGd_xBaCo₂O_5+δ(x=0.0,0.4,0.6) samples exhibit a big increase in electrical Conductivity about at 75℃. The compound is rather semiconducting than metallic as usually accepted, which is agreement with the thermally activated hole (polaronic) hopping mechanism.An investigation of RBaCo₂O_5+δ by low-frequency internal friction was shown in Chapter 6. RBaCo₂O_5+δsystem exist a relaxation internal friction peak originating from the jump of excess oxygen .The excess oxygenδ, which dependent strongly on Rare earth R and annealing conditions, affects greatly the height, shape and position of the peaks, which reflects the different states of the excess oxygen with the differentδ. In addition, another internal friction peak (around 350K) with the features of phase transition may be attributed to metal -insulator transition. For GdBaCo₂O_5.005, at 225K an internal friction peak and the corresponding hardening of shear modulus were observed, showing the existence of lattice distortion. This lattice distortion indicates the present of charge ordering phenomena, which is conformed by the slight upturn on the electrical resistivity cure. The anomalies below Tco were suggested to be correlated with the structure change caused by the Jahn-Teller effect of Co³⁺ (HS state) located in square pyramidal sites. The large softening of shear modulusobserved in GdBaCo₂O_5+δ(5=0.499 and 0.515) at low temperature is related toAFM-FM phase transition, and can be explained by the orbital ordering of IS-Co³⁺located in octahedral sites.Chapter 7 presents the results of internal friction measurements on La_1-xSr_xFeO_3-δ. The internal friction peak with a feature of phase transition is observed in the La_2/3Sr_1/3FeO_3-δ. This intemal friction peak is related to the first-order phase transition from orthorhombic-to-rhombohedral. Samples with x=2/3, exhibited rapid stiffening on the shear modulus curves which was related to a phase transition from ferroelastic rhombohedral to paraelastic cubic during heating. This phase transition is accompanied by the formation of transformation twins. If the temperature is high enough, thermal energy is sufficient to allow domain walls to move in response to applid stress, which result in an abrupt increase in Q^-1, forming an internal friction peak. Domain walls are strongly pinned by oxygen vancanies. Combined with the electrical and magnetization measurements, the local miminum in modulus about 190K in oxygen annealed La_1/3Sr_2/3FeO_3-δ samples is believed to be related to Charge ordering phase transition (CO) which is very sensitive to oxygen content. The change in modulus indicates that the CO is accompanied by a lattice distortion.