Abstract:
The chemical and physical compatibilities of the candidates, LaNi[subscript 1-x] Fe[subscript x] O[subscript 3] (LNF) perovskites and K[subscript 2] NiF[subscript 4]–typed La[subscript 2] Ni[subscript 1-x] F[subscript x] O[subscript 4] (L[SUBSCRIPT 2] NF), of a cathode material for solid oxide fuel cell with the commercial solid electrolyte, 8 mol % Yttria Stabilized Zirconia (8-YSZ), were- studied. Firstly, LNF and L[SUBSCRIPT 2] NF with x = 0-1.0 were prepared by Water Citrate (WC) and Modified Water Citrate (MWC) methods, and characterized by XRD. It was found that the single phased LNF perovskites were synthesized successfully by both methods at 700-900 ℃ for 5h. As x is 0-0.5, the crystal structure is cubic and rhombohedral at the calcination temperature of 700 ℃ and 900 ℃, respectively. The further investigation indicated that the cubic structure changed to the rhombohedral structure at 900 ℃, and was stable up to 1200 ℃. As x is 0.6-1.0, the crystal structure is in the orthorhombic phase at both calcination temperatures. This orthorhombic phase decomposed at 1200 ℃. The single-phased L[SUBSCRIPT 2] NF systems were achieved only that of x = 0 and 0.1 after calcination at 900 ℃. The crystal structure of L[SUBSCRIPT 2] NF is tetragonal. In all cases, the lattice constants and the volume per formula unit in all series generally increased as the Fe content increased. Following the reaction by XRD and SEM-EDX, the stoichiometric La(Ni[subscript 0.6] Fe[subscript 0.4])O[subscript 3] and La[subscript 2](Ni[subscript 0.9] Fe[subscript 0.1])O[subscript 4] have a good chemical compatibility with 8-YSZ from room temperature up to 900 ℃. In the case of the non-stoichiometric La(Ni[subscript 0.6] Fe[subscript 0.4])[subscript y] O[subscript 3] and La[subscript 2](Ni[subscript 0.9] Fe[subscript 0.1])[subscript y] O[subscript 4] (y = 0.95 and 1.05), the chemical compatibility with 8-YSZ is good up to 800 ℃. In addition the thermal expansion coefficients of La(Ni[subscript 0.6]Fe[subscript 0.4])O[subscript 3] and La[subscript 2](Ni[subscript 0.9]Fe[subscript 0.1])O4 are 13.2 x 10[superscript -6] K[superscript -1] and 12.7 x 10[superscript -6] K[superscript -1], respectively. These values are close to that of 8-YSZ; therefore, lanthanum nickel iron oxide of both structures have a good physical compatibility with 8-YSZ.
