Oxygen-permeable membrane materials based on solid or liquid Bi 2 O 3
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Oxygen-permeable membrane materials based on solid or liquid Bi2O3 Valery V. Belousov, A.A. Baikov Institute of Metallurgy and Materials, Russian Academy of Sciences, 49 Leninskii Pr., 119991 Moscow, Russia Address all correspondence to Valery V. Belousov at [email protected] (Received 5 August 2013; accepted 27 September 2013)
Abstract Most important advances of the last years in research and development of oxygen ion transport membrane (ITM) materials based on solid or liquid Bi2O3 are briefly given. Special attention is paid to the transport properties of novel NiO/δ-Bi2O3 and In2O3/δ-Bi2O3 ceramic and ZnO/ Bi2O3 solid/liquid composites. These composites show promise for use as ITM with the oxygen permeation rate comparable with that of the state-of-the-art membrane materials. The in situ Bi2O3 melt crystallization and grain boundary wetting methods of formation of the gas-tight composites are considered.
Introduction Gas-tight ceramic or solid/liquid metal oxide composites exhibiting high mixed conductivity could be used as ion transport membranes, ITM, for oxygen separation from air.[1–5] The advantages of oxygen separation by ITM over technologies such as pressure- and temperature-swing adsorption and porous membrane separation methods[6–9] is their infinite separation factor. The composite ITM are usually made from electronand ion-conducting chemically compatible and thermodynamically stable metal oxides. In an electrochemical field, the high ambipolar conductivity of oxygen ions and electrons results in the high oxygen permeation rate (Fig. 1). The oxygen permeation flux through ITM depends on three mechanisms: the first is the dissociation of oxygen molecules into oxygen anions at the membrane surface, known as surface exchange kinetics.[10] The second is the migration of oxygen anions through the membrane,[11] while the third is the recombination of oxygen anions back to oxygen molecules on the other side of the membrane. Each mechanism has the potential to become the rate-limiting step. Bismuth oxide exists as four crystallographic polymorphs.[12–18] The room temperature α-Bi2O3 is monoclinic. It transforms at 730 °C into the face-centered cubic (fcc) δ-Bi2O3, which is stable up to the melting point at 820 °C.[13,14] On cooling, large thermal hysteresis occurs and the metastable tetragonal β or body-centered cubic (bcc) γ form can be obtained, depending on the cooling conditions, transforming to the β-phase at 650 °C or the γ-phase at 639 °C.[15–17] The β-phase transforms to the α-phase at 303 °C, and the γ-phase at 500 °C, although the γ-phase may persist to room temperature with slow cooling rates.[12,18] The α-monoclinic and γ-bcc forms are semiconductors, whereas the β-tetragonal and δ-fcc forms are oxide ion conductors. The δ-phase has the highest known oxygen ionic conductivity. The high ionic
conductivity is caused by the large number of oxygen vacancies and high anion mobility. The high concentration of oxygen vacancies is a result of obtaining the fluorite structure with a Bi3+ ca
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