Mechanosynthesis of complex oxides with fluorite and perovskite-related structures and their sintering into nanocomposites with mixed ionic–electronic conductivity
Introduction
Dense oxide ceramic membranes with mixed ionic and electronic conductivity have been extensively studied because of their promising applications for oxygen separation, partial oxidation of hydrocarbons, syngas production and oxidative coupling of methane [1], [2], [3]. The main advantages of such membranes are related to a perm-selectivity with respect to oxygen. A promising feature is also the presence of reactive lattice oxygen species at the membrane surface, which may improve selectivity of the oxidation reactions.
However, the application of mixed-conducting membranes is limited by some disadvantages of existing membrane materials. Thus, La0.7Sr0.3Ga0.6Fe0.4O3 − δ perovskite possessing the highest oxygen mobility at elevated (750∼1000 °C) temperatures [4] is rather expensive and exhibits a high reactivity towards CO2 leading to degradation of its performance with time. Fluorite-related Pr-doped CeO2 as well as SrFeO3-based perovskites possess poor mechanical properties.
Another group of membrane materials called the dual-phase composites is comprised of the mixture of an oxide ionic conductor and an electronically conducting phase [4], [5], [6], [7]. For the composite mixed conductor, it is possible to choose good ionic and electronic conductors as components, and to control the mixed conductivity by adjusting the fraction of constituents. Composites based on CeO2 are considered to be the most promising. Gd-doped CeO2 is a well-known oxygen-conducting electrolyte recently receiving much attention as a model material to investigate the nano-scale effects in conductivity [8], [9]. Preparation of composites from commercially available fine powders usually requires long time ball milling for better mixing of components. Direct preparation of nanocomposites is possible by the Pechini process [10], but this technique possesses some disadvantages: (1) facilities for complex systems are limited; (2) adjustment of interface is rather difficult; (3) too large shrinkage of thick films and bulk ceramics due to lowered green density.
The mechanosynthesis of mixed oxides is a well known approach [11], but technical problems related to its application, such as products contamination by milling bodies, agglomeration of ceramic powders and technology up-scaling were solved only recently [12], [13]. Binary oxides obtained by mechanosynthesis are frequently metastable disordered solid solutions undergoing phase transformations at moderate temperatures [14]. Kinetic stabilization of the mixed oxides is achieved by their complex doping thus forming multicomponent solid solutions [15]. Such a complex doping is required as well for obtaining compatible phases in composites. Dense ceramics comprised of complex fluorites and perovskites obtained by mechanosynthesis were shown to sinter at relatively low temperatures [15]. In our opinion, the promising approach for design of multilayer membranes should combine both mechanochemical and Pechini routes, each offering some advantages. Thus, the mechanochemical method provides compatible nanocomposites with a high green density and sintering activity, while Pechini process produces a binding polymer and a catalyst precursor [7]. This work is focused on the synthesis of mixed conducting ceramic nanocomposites, which comprise of complex fluorites and perovskites prepared via mechanochemical route. Sintering of the composites, characterization of their oxygen mobility and conductivity are studied. Potential application of studied composites as membrane materials is estimated.
Section snippets
Experimental
For preparation of single phase powders with fluorite or perovskite structure, compositions of oxides were milled in a high energy planetary mill AGO-2 for 20–30 min. The mass of chromium steel balls (10 mm diameter) loaded into a drum was 220 g, while starting mass of powder mixtures was about 15 g. For monitoring the process of mechanosynthesis by X-ray diffraction (XRD) analysis, 2–3 probes of a product were taken during milling. A special procedure of the mechanical treatment was used to
Results and discussion
The parameters of the complex oxides' crystal structure are presented in Table 1. XRD patterns for individual phases and dual-phase composites fluorite + perovskite (F + P) and perovskites + perovskite (P + P) are shown in Fig. 1, Fig. 2. Some parameters of composites after sintering are presented in Table 2. The procedure used in nanocomposite preparation in micro-mill is efficient enough because the X-ray particle sizes of components in composites are smaller than those of individual phases sintered
Summary and conclusion
Mechanochemical approach was shown to be a promising technique for synthesis of nano-size powders of complex oxides having a high green density and a good sintering activity, thus allowing preparation of their nanocomposites with mixed ionic–electronic conductivity. The main advantages of the mechanochemical approach are opportunities of the fast optimization of compositions for selection of the compatible components in composites and adjustment of the microstructure of multilayered membranes.
Acknowledgements
This work was supported by INTAS under grant 01-2162.
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