Formation Of La2Zr2O7 Research Articles

Fused LSM: A Novel Powder Limiting the Formation of La2Zr2O7 and SrZrO3 Resistive Phases at the Interface with YSZ in SOFC Cathodes

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Solid oxide fuel cell cathodes prepared by infiltration of LaNi0.6Fe0.4O3 and La0.91Sr0.09Ni0.6Fe0.4O3 in porous yttria-stabilized zirconia

SOFC composite electrodes of yttria-stabilized zirconia (YSZ) and either LaNi0.6Fe0.4O3 (LNF) or La0.91Sr0.09Ni0.6Fe0.4O3 (LSNF) were prepared by infiltration to a loading of 40 wt% of the perovskite into porous YSZ using aqueous solutions of the nitrate salts. XRD measurements indicated that the perovskite structures were formed following calcination at 850 °C, at which temperature the LNF and LSNF form small particles that coat the YSZ pores. Heating to 1100 °C causes the particles to form a dense film over the YSZ but caused no solid-state reaction. Calcination of an LNF–YSZ composite to 1200 °C led to an expansion of the LNF lattice, suggesting introduction of Zr(IV) into the perovskite; further heating to 1300 °C caused the formation of La2Zr2O7. For 850 °C calcination, the electrode performance of both LNF–YSZ and LSNF–YSZ composites was similar to that reported for composites of YSZ and La0.8Sr0.2FeO3 (LSF), with a current-independent impedance of approximately 0.1 Ω cm2 at 700 °C in air. For 1100 °C calcination, both LNF–YSZ and LSNF–YSZ composites exhibited impedances that decreased strongly under both anodic and cathodic polarization. The implications of these results for preparing electrodes based on LNF and LSNF are discussed.

Avoidance of La2Zr2O7 Formation on Co-sintering the (La0.8 Sr0.2)1-x MnO3 / YSZ System

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Avoidance of La2Zr2O7 Formation on Co-Sintering the (La0.8Sr0.2)1−xMnO3/YSZ System

The chemical compatibility of 8 mol% YSZ and (La o.8 Sr 0.2 ) 1-x MnO 3 (LSM) has been investigated at temperatures in excess of 1300°C. Stoichiometric LSM (x=0) reacted readily with YSZ producing the undesired La 2 Zr 2 O 7 (LZO) pyrochlore phase at 1300°C. An A-site deficiency of x=0.03 reduced the amount of pyrochlore present, and increasing the LSM particle size from 0.3 μm to 2.5 pm removed any visible LZO peaks in the XRD analysis. An A-site deficiency of x=0.05 was found to be adequate to prevent the formation of the La 2 Zr 2 O 7 (LZO) pyrochlore at temperatures up to 1400°C within the detection limits of X-ray diffraction. Increased A-site deficiency in the LSM resulted in improved adhesion between cathode and the electrolyte.

Understanding Materials Compatibility

▪ Abstract The use of chemical potential diagrams to examine interface chemistry is discussed in terms of the chemical reactions among oxides and associated interdiffusion across the interface. The driving force for both processes can be determined from the chemical potential values. The geometrical features of the chemical potential diagrams can be related to the valence stability of binary oxides and the stabilization energy of double oxides from the constituent oxides. The materials compatibility in solid oxide fuel cell materials is discussed with a focus on a lanthanum manganite cathode and a yttria-stabilized zirconia (YSZ) electrolyte. Emphasis is placed on the valence numbers of manganese in the fluorite solid solution and the perovskite oxides, which have been derived by thermodynamic analysis of the magnitude of the stabilization energy/interaction parameters as a function of ionic size for respective valence numbers. The change of manganese valence on La2Zr2O7 formation and Mn dissolution in YSZ are discussed in relation with the oxygen evolution/adsorption process. Oxygen flow associated with electrochemical reactions exhibits markedly different features depending on the direction of the polarization, which can lead to drastic changes in the interface chemistry (precipitation or interdiffusion).

Hot Isostatic Pressing of Submicron La2Zr2O7 Powder Prepared by the Hydrazine Method.

In the ZrO2-La2O3 system, metastable c-ZrO2 solid solutions (ss) containing up to ≈27 mo1% La2O3 crystallize at low temperatures from amorphous materials prepared by the hydrazine method The formation of La2Zr2O7 proceeds via two reaction processes; (i) decomposition of c-ZrO2 (ss) (c-ZrO2 (ss) → La2Zr2O7 + t-ZrO2) above 1100°C and (ii) asolid-state reaction of t-ZrO2 and La2O3 at 1200° to 1300°C. Submicron La2Zr2O7 powder is producedat 1300°C. Dense La2Zr2O7 ceramics (99. 5% of theoretical) with an average grain size of 10 μm have been fabricated by hot isostatic pressing (1500°C/2h/196MPa). Their fracture toughness and bending strength are 1. 9 MPa-m1/2 and 172 MPa, respectively. They exhibit an electrical conductivity of 1. 5x10-1 S-m-1 at 1000°C.

Air Electrode Property of La-Based Perovskite-Type Oxides (Part 1)

The sinterability, electric conductivity, expansion coefficient and chemical stability were examined for the perovskite-type oxides which are expressed as (La1-XAX)1-ZMO3-δ (A=Sr, Ca, M=Mn, Co, Cr). The sinterability depended on the A site composition X and the deficiency Z. The electric conductivity and the thermal expansion coefficient also depended strongly on the composition of the oxides. The electric conductivity was sensitive to the amount of Sr-substitution and the Co to Mn ratio and high conductivity could be attained without accompanying thermal expansion coefficient mismatch with YSZ. The chemical reaction between the perovskite-type oxides and YSZ resulted in the formation of La2Zr2O7 and SrZrO3 depending on the composition.

Chemical Compatibility between Strontium‐Doped Lanthanum Manganite and Yttria‐Stabilized Zirconia

Equimolar powder mixtures and multilayer pellets of single‐phase Sr‐doped lanthanum manganite perovskite materials Lay‐xSrxMnO3 with La content y = 1 and 0.95 and Sr content 0 ≤ x ≤ 0.5 were annealed in air with 8 mol% Y2O3‐ZrO2 at 1470 K, up to 400 h and at 1670 K. up to 200 h. X‐ray diffraction and electron probe microanalysis confirmed the formation of La2Zr2O7 or SrZrO3 depending on the composition of the perovskites. No reaction products could be detected for La0.95‐xSr xMnO3 with 0.2 ≤ x ≤ 0.4 after annealing for 400 h at 1470 K, and for the perovskite La0.65Sr0.3MnO3 even after annealing for 200 h at 1670 K. The results demonstrate the improved chemical compatibility of La‐deficient perovskites against reaction with zirconia and can provide a basis for the selection of a sufficiently chemically stable material for the air electrode of solid oxide fuel cells.