ИСТИНА

Today, electric energy is one of the most basic, but also the most resource-demanding needs of the modern society. It is from the generation of electric energy that we trace such problems as global warming and greenhouse effect, yet without it, our society would be unable to run. With the ever-increasing population, the needs for electricity continue to rise every day, and the current technologies would soon be unable to support them. For that reason, it is vital to perfect more effective means of electric energy generation, such as the SOFC (solid oxide fuel cell). The SOFC, like any other electrochemical cell, is composed of a cathode, an anode and an electrolyte. While all the parts of the cell are not yet perfected, in this work, the emphasis is made on the electrolyte materials. Currently, fluorite-structured YSZ (yttria-stabilised zirconia), GDC (gadolinia-doped ceria) and perovskite-structured LSGM (lanthanum strontium gallium magnesium oxide) are considered best suited for this purpose. However, each of them has its own issues, such as their high cost of production and low availability of resources, and it is imperative to research new and better materials to serve as the electrolyte for the SOFC. In earlier works, Fe-doped SrSnO3 was shown[1-2] to display high conductivity; however, the addition of Fe introduced electronic conductivity, which would hinder its performance as an electrolyte. For that reason, doping of SrSnO3 by Ga was proposed as a potentially effective way of improving the material’s conductivity while keeping its electronic component sufficiently low. SrSn1-xGaxO3-δ powders were prepared from Ga2O3, La2O3 and SrCO3 powders via solid-state synthesis at temperatures of 1250 to 1350oC. A shift in peaks’ positions is visible on the XRD patterns of the powders, with the a parameter of the unit cell ranging from 4.0317(3) Å for SrSnO3 to 4.0234(4) Å for SrSn0.5Ga0.5O3-δ. The oxides’ coefficients of thermal expansion were measured via high-temperature X-ray diffraction, the resulting values ranging from 1.09∙10-5 for SrSnO3 to 1.28∙10-5 for SrSn0.625Ga0.375O3-δ. Dense SrSnO3 and SrSn0.75Ga0.25O3-δ membranes were consequently prepared from the resulting mixed oxide powders. The highest densities (95% and 87% correspondingly) were achieved via SPS (Spark Plasma Sintering) of the resulting powders. Other methods of achieving high densities (sintering with the addition of PVA or 5 mol. % Co(NO3)2 and Cu(NO3¬)2) were tried, however those methods yielded considerably lower densities (up to 65%). The materials’ conductivities were measured using impedance-spectroscopy. SrSnO3, obtained via SPS sintering, displayed conductivities of 2.68∙10-3 S∙cm-1 at 1073 K and 8.45∙10-5 S∙cm-1¬ at 873 K, while the one obtained via sintering with addition of 5 mol. % Co(NO3)2 displayed corresponding values of 5.56∙10-4 S∙cm-1 and 1.39∙10-6 S∙cm-1, which may be attributed to the higher density of the one synthesised using SPS. SrSn0.75Ga0.25O3-δ, too, displayed lower conductivities of 1.02∙10-4 S∙cm-1 at 1073 K and 1.13∙10-5 S∙cm-1¬ at 873 K. This can be attributed to the molten admixtures, which, although are invisible on the XRD pattern, can be seen on the SEM image. Currently, we are leading attempts to prepare admixture-pure membranes of SrSn0.75Ga0.25O3-δ and SrSn0.125Ga0.875O3-δ. References: [1] V. Thangadurai, Robert A. Huggins, W. Weppner, J. Power Sources, 108 (2002) 64-69. [2] V. Thangadurai, P. Schmid Beurmann, W. Weppner, Mater. Sci. Eng. B, 100 (2003) 18-22.