Ch. Ftikos

Properties of A-site-deficient La0.6Sr0.4Co0.2Fe0.8O3-δ-based perovskite oxides

Abstract A-site-deficient La0.6Sr0.4Co0.2Fe0.8O3−δ-based oxides of the compositions La0.6−zSr0.4Co0.2Fe0.8O3−δ, La0.6Sr0.4−zCo0.2Fe0.8O3−δ and (La0.6Sr0.4)1−zCo0.2Fe0.8O3−δ (0≤z≤0.2) were prepared and characterized. The crystal structure, electrical conductivity and thermal expansion of these oxides were studied using X-ray diffraction, four-point DC and dilatometry, respectively. All oxides had a rhombohedral perovskite structure. The pseudo-cubic lattice parameter increased with increasing z. The electrical conductivity increased with temperature up to about 600°C, and then decreased due to the loss of lattice oxygen. The charge compensation mechanism in these A-site-deficient perovskites was probably the formation of oxygen vacancies rather than the oxidation B3+→B4+. The conductivity decrease and the activation energy increase became more significant in the order La0.6Sr0.4−z>(La0.6Sr0.4)1−z>La0.6−zSr0.4. The TEC was generally lower in the A-site-deficient oxides. The lowest TEC values at 700°C were 14.2, 14.1 and 13.8×10−6 cm (cm°C)−1 for La1−zSr0.4Co0.2Fe0.8O3−δ, with z=0.05, and La0.6Sr0.4−zCo0.2Fe0.8O3−δ, with z=0.1 and 0.2, respectively.

Crystal structure, thermal and electrical properties of Pr1−xSrxCoO3−δ (x=0, 0.15, 0.3, 0.4, 0.5) perovskite oxides

The crystal structure at room temperature, and the thermal expansion and electrical conductivity, from room temperature up to 800°C, of the perovskite-type oxides in the system Pr1−xSrxCoO3−δ (x=0, 0.15, 0.3, 0.4, 0.5) were studied. All compounds have the orthorhombic perovskite GdFeO3-type structure (Pbnm space group). The lattice parameters were determined by X-ray powder diffraction. The introduction of Sr2+ into the lattice is compensated by the oxidation of Co3+ to form Co4+ (holes) for low x, and at reduced temperatures on higher x values, or by the formation of oxygen vacancies as x increases and at high temperatures. Above x=0.4 oxygen vacancies are formed even at room temperature. The thermal expansion is almost linear for x≥0.15. The thermal expansion coefficient (TEC), determined in the temperature range from room temperature to 500°C, decreases with x, but shows an increase above x=0.4. The electrical conductivity increases with x, but when x≥0.4 it decreases. The conductivity of the undoped sample in the examined temperature range, and that of x=0.15 up to 500°C is p-type semiconducting, and can be described by the small polaron hopping mechanism. Semi-metallic behavior was observed for x=0.15 above 500°C and for x≥0.3 in the whole examined temperature range. The metal–insulator (M–I) transition at room temperature occurs at approximately x=0.25.

Chemical reactivity of perovskite oxide SOFC cathodes and yttria stabilized zirconia

Abstract The chemical reactivity of perovskite oxide SOFC cathodes and yttria stabilized zirconia (YSZ) solid electrolyte was studied. The perovskite oxides that were examined were La 1− x Sr x Co 0.2 Mn 0.8 O 3− d , La 1− x Sr x Co 0.2 Fe 0.8 O 3− d and La 1− x Ca x Co 0.2 Fe 0.8 O 3− d (0≤ x ≤0.5), as well as, the A-site deficient oxides La 0.6− z Sr 0.4 Co 0.2 Fe 0.8 O 3− d , La 0.6 Sr 0.4− z Co 0.2 Fe 0.8 O 3− d and (La 0.6 Sr 0.4 ) 1− z Co 0.2 Fe 0.8 O 3− d (0≤ z ≤0.2). Equimolar perovskite/YSZ powder mixtures were prepared and annealed at 1100°C for 120 h. X-ray diffraction analysis was conducted to identify any reaction products. Si-standard was used as an internal standard for d -value calibration, during the determination of the lattice parameter of cubic YSZ. The main reaction products were La 2 Zr 2 O 7 for the undoped and lightly doped Fe-containing compositions and (Sr/Ca)ZrO 3 for the Sr/Ca doped compositions. Also, CoFe 2 O 4 was formed in the case of the Fe-containing compositions and monoclinic zirconia for La 1− x Ca x Co 0.2 Fe 0.8 O 3− d with x =0.4, 0.5. A-site deficiency in La 0.6 Sr 0.4 Co 0.2 Fe 0.8 O 3− d -based perovskites reduced the amount of SrZrO 3 formed, only in the case of La 0.6 Sr 0.4− z Co 0.2 Fe 0.8 O 3− d with 0≤ z ≤0.2 and (La 0.6 Sr 0.4 ) 1− z Co 0.2 Fe 0.8 O 3− d with z =0.2. The lattice parameter of YSZ exhibited a shift, corresponding to a lattice contraction, due to the diffusion of the transition metal cations from the perovskite into YSZ.

Preparation and characterization of Pr1-xSrxMnO3 ± δ (x = 0, 0.15, 0.3, 0.4, 0.5) as a potential SOFC cathode material operating at intermediate temperatures (500–700 °C)

Abstract The crystal structure, thermal expansion and electrical conductivity of Sr-doped praseodymium manganites (Pr1-xSrxMnO3 ± δ where x = 0, 0.15, 0.3, 0.4, 0.5) were studied in air, and their potential use as cathodes in intermediate temperature SOFCs was evaluated. All compositions have an orthorhombic perovskite-type structure (Pbnm space group). The lattice parameters were determined at room temperature by X-ray powder diffraction. The thermal expansion is almost linear for compositions with x≥0.15. The electrical conductivity can be described by the small polaron hopping conductivity model. The conductivity increases with temperature for all compositions. Both the thermal expansion coefficient (TEC) and the electrical conductivity increase with increasing Sr content, while the activation energy decreases. The low values of the calculated activation energies are in agreement with the small polaron hopping mechanism. Among the compositions studied, Pr0.5Sr0.5MnO3±δ, with a TEC value of 12.2 × 10−6 cm (cm °C)−1 and a conductivity value of 226 S cm−1 at 500 ° C, seems to be the most promising material that can function as cathode in the intermediate temperature SOFC.

Preparation and characterization of Ln0.8Sr0.2Fe0.8Co0.2O3−x (Ln=La, Pr, Nd, Sm, Eu, Gd)

Abstract In the present study the oxides Ln 0.8 Sr 0.2 Fe 0.8 Co 0.2 O 3− x (Ln=La, Pr, Nd, Sm, Eu, Gd) were prepared and characterized to study the influence of the host rare earth cation on their properties and their potential application as intermediate temperature solid oxide fuel cell cathodes with higher ionic conductivity, lower linear thermal expansion coefficient and better chemical compatibility with the electrolyte. The oxides were prepared by the amorphous citrate synthesis. From X-ray powder diffraction measurements it was deduced that all oxides were single-phased with the sensitivity of the method. However, irregularities during thermal expansion measurements indicated that some structural changes might occur during heating. Only after examination of the powders in a scanning electron microscope, the presence of a second phase (cobalt oxide) in most compositions became evident.

Chemical compatibility of alternative perovskite oxide SOFC cathodes with doped lanthanum gallate solid electrolyte

This paper reports on the investigations of the chemical compatibility between SOFC cathode materials with compositions Pr0.8Sr0.2Co0.2Mn0.8O3−δ, Pr0.8Sr0.2Co0.2Fe0.8O3−δ, Pr0.8Sr0.2Co0.3Mn0.7O3−δ and Pr0.75Sr0.2Co0.2Mn0.8O3−δ and the electrolyte materials with compositions La0.8Sr0.2Ga0.9Mg0.1O3−δ, and La0.9Sr0.1Ga0.8Mg0.2O3−δ. The lanthanum gallate electrolyte with 20 mol.% Sr contained two additional phases, namely, LaSrGa3O7 and LaSrGaO4, while that with 10 mol.% Sr was formed in nearly single phase. Two types of experiments were performed: (a) reactivity experiments of powder mixtures and (b) diffusion experiments in cathode/electrolyte double-layer pellets. No reaction products were detected by XRD. High Co diffusion into the electrolyte was identified with SEM/EDX in all diffusion experiments examined. The transition metals diffuse in the order Mn<Fe<Co. La, Pr and Ga show a smaller tendency for diffusion. The diffusion of transition metal cations into the electrolyte La0.8Sr0.2Ga0.9Mg0.1O3−δ caused the destabilisation and disappearance of the second phases in the interdiffusion zone. In the case of the A-site deficient cathode, the formation of LaSrGa3O7 second phase was identified on the electrolyte side, near the interdiffusion zone.

Electronic conductivity in the Pr1−xSrxCo1−yMnyO3−δ system

The effect of Mn substitution for Co and of Sr doping for Pr on the electronic conductivity of Pr1−xSrxCo1−yMnyO3−δ (x=0.3, 0.5, 0≤y≤1) was investigated. All oxides have an orthorhombic GdFeO3-type structure (space group Pbnm), as determined by XRD. The pseudo-cubic lattice constant increases with increasing Mn content. The electrical conductivity was measured in the temperature range from 100 to 800°C in air. The substitutionally-mixed compositions exhibit a semiconducting behavior, where the small polaron hopping conductivity model applies. The normalized conductivity shows a sharp decrease when y increases in the range 0<y<0.4, and it reaches a minimum in the range 0.4<y<0.6. The small polaron is trapped at the energetically lower Mn sites, and the slow transition rate to a neighboring Co site dominates the hopping process. At higher y values, the hopping proceeds among adjacent Mn sites, and as a result, an increase of conductivity is observed. The calculated activation energy values agree with the above mechanism.

Electrical conductivity and thermal expansion of ceria doped with Pr, Nb and Sn

Abstract The effect of Pr, Nb and Sn substitution on the electrical conductivity, thermal expansion and the structure of the phases formed was investigated in a series of samples based on ceria solid solutions. X-Ray diffraction (XRD) analysis was performed in order to study the phase content and to measure the cell lattice parameters. Electrical conductivity was measured in air by means of the four-point DC technique, within the temperature range 400–1000°C. Also, thermal expansion coefficient measurements were carried out using the dilatometer technique. The results show that doping of ceria with 8–10 mol% praseodimia increases the electrical conductivity, while the addition of Nb 2 O 5 leads to the formation of a second, monoclinic, phase which decreases the conductivity. The thermal expansion coefficient in Pr-doped ceria is 8 × 10 −6 /K at temperatures lower than 500°C, then it increases to 10−11 × 10 −6 /K at 900°C. Doping with Nb 2 O 5 eliminates the increase of thermal expansion at high temperatures. Sn was found to be a possible substitute for Ce in ceria solid solutions.

Mixed electronic/ionic conductivity of the solid solution La(1 − x)SrxCo(1 − y)NiyO3 − δ (x: 0·4, 0·5, 0·6 and y: 0·2, 0·4, 0·6)

The effect of Sr and Ni substitution on the mixed electronic/ionic conductivity was investigated in a series of the perovskite-type oxides mLa(1 − x)SrxCo(1 − y)NiyO3 − δ (x: 0·4, 0·5, 0·6 and y: 0·2, 0·4, 0·6) prepared. Electrical conductivity was measured, in air, by a four-point probe technique, within the temperature range 500–1000°C. Oxygen self-diffusion coefficient (D∗), activation energy (Ea), and surface exchange (k) data were obtained, by combining the isotopic 18O/16O exchange diffusion profile (IEDP) technique, at 700, 800 and 900°C, with the secondary ion mass spectroscopy (SIMS) technique for the 18O depth profile measurement. The results derived suggest that the present perovskite-type oxides are excellent mixed conductors, having high electronic conductivity in addition to their high O2− self-diffusivity. Oxygen flux through the material was principally controlled by the surface exchange kinetics. As far as the effect of Sr and Ni substitution on the electrical properties is concerned, a strong interdependence was found between the dopants content.

Crystal structure, thermal expansion and electrical conductivity of Pr1−xSrxCo0.2Fe0.8O3−δ (0≤x≤0.5)

Perovskite oxides in the system Pr1−xSrxCo0.2Fe0.8O3−δ (0≤x≤0.5) were prepared and characterized. The crystal structure, thermal expansion and electrical conductivity of these oxides were studied by X-ray diffraction, dilatometry and 4-point DC, respectively. The structure of the oxides in the range 0≤x≤0.4 was orthorhombic (Pbnm space group) and became cubic for x=0.5. The lattice parameters were determined. The thermal expansion coefficient (TEC) decreased for x=0.1, and increased for higher x values. The slope of the thermal expansion curves increased and the electrical conductivity decreased at high temperatures, as a result of the loss of lattice oxygen and the formation of oxygen vacancies, a process which is enhanced as the Sr-doping level is increased. The composition Pr0.8Sr0.2Co0.2Fe0.8O3−δ showed the best performance, since its TEC and conductivity values at 700°C (14×10−6 cm (cm °C)−1 and 159 S cm−1, respectively), meet the requirements for application in SOFC cathodes.