The Effect of Partial Substitution of Alkaline Earth Ion for La Site in LaNi0.2Fe0.8O3-δ on Crystal Structure and Electrical Conductivity

Abstract

It was reported that electrical conductivity increased with increasing Ni content in LaNi1-x Fe x O3-δ for 0.4≤x≤1.0 and that LaNi0.6Fe0.4O3-δ showed superior property as cathode material for solid oxide fuel cells (SOFCs). However, deterioration of stability at high temperature and sintering property with increasing Ni content was also reported as problem for practical application. As a method to produce high electrical conducting material with high stability, partial substitution of divalent alkaline earth ion for trivalent La in LaNi1-x Fe x O3-δ with low Ni content can be proposed. Although investigation of La1-x Sr x Ni0.6Fe0.4O3-δ as cathode material for SOFCs was reported, report on effect of partial substitution of alkaline earth ion for La site in LaNi1-x Fe x O3-δ with low Ni content has been limited. In this study, preparation of La2-x Ae x Ni0.2Fe0.8O3-δ (Ae: Ca, Sr and Ba) was examined. The crystal structure, structural phase transition behavior, sintering property and electrical property of the prepared specimens were evaluated and effect of kinds and composition of Ae on material property was investigated.La1-x Ae x Ni0.2Fe0.8O3-δ was prepared by Pechini method with starting materials of La2O3, AeCO3, Ni(NO3)2·6H2O and Fe(NO3)3·9H2O. Before dissolution, AeCO3 was dried at 150 ºC and La2O3 was heated at 1000 °C in air. Purity of Ni(NO3)2·6H2O and Fe(NO3)3·9H2O was verified by ignition loss method. The solutions were mixed together with nominal ratio of the metal ions. After addition of citric acid and ethylene glycol, the mixed solution was heated at 450 °C until the residual materials was fired, resulting in precursor. The molar ratio of metal ions, citric acid and ethylene glycol was approximately 1:2:8. The obtained precursor was crushed into powder and calcined at 1000 °C for 10-12 h in air or O2, followed by heating at 1200 °C for 10 h in air or O2. For the specimens in which impurity was observed, additional heating at 900 °C for 12 h in O2 was carried out.X-ray diffraction measurement (CuKα: 50 kV, 250 mA, RINT-2500, Rigaku Co., Ltd.) was performed at room temperature to distinguish whether single phase was obtained or not. Crystal system, lattice constants and molar volumes were also evaluated from X-ray diffraction patterns. The structural phase transition behavior of La1-x Ae x Ni0.2Fe0.8O3-δ was investigated with high temperature X-ray diffraction measurements in air. Measurement temperature range was 100-1000 °C. The structural phase transition behavior was evaluated also with dilatometer (TMA8310, Rigaku Co., Ltd.). The single phase powder was pressed into pellet, followed by sintering at 1000-1200 °C for 10 h in air. The sintered pellet was subjected to dilatometry. The electrical conductivity of cylindrical La1-x Ae x Ni0.8Fe0.2O3-δ sintered specimens was measured with DC four probe method with Pt electrodes and wires. The measurement temperature range was 300-800 °C in air. The cylindrical specimens were prepared by pressing the single phase powder followed by sintering at 1000-1200 °C for 10 h in air.So far, no second phase is observed in X-ray diffraction patterns of La1-x Ca x Ni0.2Fe0.8O3-δ and La1-x Sr x Ni0.2Fe0.8O3-δ with x=0.2 or less. For La1-x Ba x Ni0.2Fe0.8O3-δ , single phase is obtained for the specimens with x=0.3 or less. The diffraction patterns of La1-x Ca x Ni0.2Fe0.8O3-δ (x=0.1-0.2) could be indexed as orthorhombic distorted perovskite with space group of Pnma (No. 62). Also, the diffraction patterns of La0.9Sr0.1Ni0.2Fe0.8O3-δ and La0.9Ba0.1Ni0.2Fe0.8O3-δ could be indexed as orthorhombic distorted perovskite; however, crystal system varied with increasing Sr or Ba content. The pattern of La0.8Sr0.2Ni0.2Fe0.8O3-δ could be indexed as mixture of orthorhombic Pnma (No. 62) and rhombohedral R3barc (No. 167). The patterns of La1-x Ba x Ni0.2Fe0.8O3-δ with x=0.2 and 0.3 could be indexed as rhombohedral distorted perovskite with space group of R3barc(No. 167). This structural variation shows fair correspondence with tolerance factor approaching 1 with increasing Ba and Sr content. Figure attached shows temperature dependence of electrical conductivity of La0.9Ae0.1Ni0.2Fe0.8O3-δ in air. In the figure, the electrical conductivity of LaNi0.2Fe0.8O3-δ is presented for comparison. With partial substitution of Ae, electrical conductivity increased by more than one order regardless of kinds of Ae. The increase can be attributed to increase of hole concentration by substitution of divalent Ae for trivalent La. Among La0.9Ae0.1Ni0.2Fe0.8O3-δ , La0.9Ba0.1Ni0.2Fe0.8O3-δ showed the highest electrical conductivity. Dependence of logarithm of product of electrical conductivity, σ, and temperature, T, on reciprocal of temperature showed almost linear relationship, which indicated hopping conduction model could be applicable. From the slope of the relationship between logσT and 103/T, activation energy was estimated. The activation energy of La0.9Sr0.1Ni0.2Fe0.8O3-δ and La0.9Ca0.1Ni0.2Fe0.8O3-δ was 0.22 eV, whereas that of La0.9Ba0.1Ni0.2Fe0.8O3-δ was 0.19 eV, which was lower than those of La0.9Sr0.1Ni0.2Fe0.8O3-δ and La0.9Ca0.1Ni0.2Fe0.8O3-δ . Figure 1