Mechanical and Structural Stability of Perovskites Membranes in Reducing Environments
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Mechanical and Structural Stability of Perovskites Membranes in Reducing Environments Nagendra Nagabhushana1, William F. Haslebacher2, Venkat K. Venkataraman2 and Sukumar Bandopadhyay1 1 School of Mineral Engineering, University of Alaska Fairbanks Fairbanks, AK 99775-5800 USA 2 US Department of Energy/National Energy Technology Laboratory, 3610 Collins Ferry Road, Morgantown, WV 26507-0880 ABSTRACT Mixed ionic electronic conducting perovskite type oxides are promising materials for potential use in various applications such as in fuel cells and membranes for air separation. An important issue in the development of the perovskites is the structural, chemical and mechanical stability of these materials at high temperatures and reducing environments (oxygen partial pressure from 0.21 to 10-17 atm) encountered in membrane reactors. SrFeO3 oxides doped with La on the A-site and Cr on the B-site showed high strength at room temperature in air. The strength degrades rapidly with an increase in temperature in air as compared to in N2 and CO2/CO environment. Fracture in the material is characterized by non-equilibrium segregation of elements within the grains. The observations provide valuable structure-property correlation as applicable to the long-term behavior of the material in advanced catalytic membrane reactors.
INTRODUCTION In recent years, mixed conducting perovskites of the general composition LaxSr1-xMxM´1yO3-δ (where M = Co, Mn and Cr and M’ = Fe and Al) have attracted considerable attention for possible applications as high temperature electrochemical oxygen membranes – devices that serve to separate oxygen containing gas mixtures due to difference in oxygen partial pressures on the two sides of the membrane. This concept is utilized in membrane reactors that can produce synthesis gas by direct conversion of hydrocarbons [1,2]. For the above-mentioned applications, the ABO3 type oxides have to be dense and importantly show high ionic and/or electronic conductivity. To achieve this, the A-site cations are often substituted by a lower valence state, effectively forming oxygen vacancies and causing a change of valence state in B-site cation to maintain charge neutrality. The concentration of oxygen vacancies can also be tailored by substituting ions of smaller size but lower valence B sites. It has also been observed that the oxygen vacancies in many of these materials can be created by exposure to an environment having a sufficiently low thermodynamic oxygen activity. The formation of oxygen vacancies so essential for the functional properties in the material often leads to dimensional instability and strength degradation under reducing conditions. Thus, one of the important issues that need to be addressed is the structural, chemical and mechanical stability and reliability of these materials in application environments (the P(O2) in reactor conditions may vary from ≈ 0.2 to10-16 atm). Much of the available literature has focused on membrane synthesis, characterization [3] and functional properties suc
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