ИСТИНА

Perovskites containing transition metals are studied as possible cathode materials for solid oxide fuel cells (SOFC). One of their disasvantages is a low chemical stability to electrolyte materials (Zr,Y)O2-x (YSZ) or (Ce,Gd)O2-x (GDC) related to the presence of rare-earth and alkaline earth metals in the cathode material. It is interesting to synthesize transition metal perovskites containing small cations as Ca and Y in the A-position. They are expected to be more stable to electrolyte materials based on acid oxides than cathode materials containing strontium,barium and lanthanum. The aim of this work was the synthesis of solid solutions Y1-хСахFe1-yCoyO3-δ (0.1 ≤ х ≤ 0.2, 0.1 ≤ y ≤ 0.3), study of their homogeneity range and of the temperature dependence of specific conductivity. As standard solid state reaction doesn’t lead to pure samples, one of the soft chemistry methods – the citrate method – was used for the synthesis. The reagents were dissolved in citric acid melt, then a high temperature annealing (1150-1200 о С for 24-40h) was made after combustion of the organic components. One-phase samples were synthesized for the compositions Y1-хСахFe1-yCoyO3-δ (0.1 ≤ х ≤ 0.2, y = 0.1 ≤ y ≤ 0.3). In the case of larger content of yttrium or calcium admixtures of Ca2Fe2O5 and Y2O3 were formed. Replacement of iron with cobalt leads to the decrease of the unit cell parameters, for example, the cell paramaters are a = 5.5807(4)Å, b = 7.5859(6)Å, c = 5.2767(3)Å for Y0.9Са0.1Fe0.9Co0.1O3-δ and a = 5.5474(3)Å, b = 7.5391(5)Å, c = 5.2515(3)Å for Y0.9Са0.1Fe0.7Co0.3O3-δ. It may be related to the smaller ionic radius of Co3+-cation (r(HS Co3+) = 0.61 Å; r(LS Co3+) = 0.525Å) compared to the Fe3+ cation radius (r(HS Fe3+) = 0.645 Å). When increasing the calcium content one can expect the cell parameters to decrease but this decrease is small. The specific conductivity measurements for the samples of Y1-хСахFe1-yCoyO3, (x=0.1,0.2, y = 0.2, 0.3 ) was made by the four probe method with permanent current. Round ceramic tablets with the diameter 17-18 mm were used for the measurements. The temperature dependence of the specific conductivity has the semiconductor character. The specific conductivity at 900о С is in the range from 22.7 Sm/cm for Y0.8Са0.2Fe0.7Co0.3O3-δ,Y0.9Са0.1Fe0.8Co0.2O3-δ to 27.2 Sm/cm for Y0.9Са0.1Fe0.7Co0.3O3-δ. So the specific conductivity at 900оС is higher for solid state solutions Y1-хСахFe1-yCoyO3 then for described in the literature [1] compounds Y1-xCaxFeO3-δ (0 ≤ х ≤ 0.1) (for example, specific conductivity is 9 Sm/cm for Y0.9Ca0.1FeO3). Variation of cobalt content in the solid solutions does not significantly influence the specific conductivity. It may be related to high degree of distortion of the perovskite structure in this compound that results in the constriction of the conduction band.