Synthesis And Electrical Properties Of (La_1-xSr_x)₂(Mo_1-yM_y)₂O_9-α(M=Al³⁺,Sc³⁺) And YSZ

Solid state oxide-ion conductor has received much attention for their potential applications in fuel cells, oxygen sensor, temperature sensor, etc. La2Mo2O9 was reported as a new oxide-ion conductor by Lacorre in 2000. It has quite a few inherent oxygen vacancies in crystal lattice even without doping low-valence metal cations, and the oxide-ion conductivity of La2Mo2O9 is higher than YSZ in the same conditions.A series of double doped (La_1-xSr_x)₂(Mo_1-yAl_y)₂O₉ (x = 0.05,y = 0,0.01,0.03,0.05;x = 0.025,y = 0,0.01,0.03) samples were prepared by high-temperature solid-state reaction. The samples were characterized by XRD and SEM. Using the samples as solid electrolytes and porous platinum as electrodes, the ion conduction properties of these samples were investigated by means of ac impedance spectroscopy and oxygen concentration cell at 550 1000℃in various atmospheres. The results are as follows: The samples are of a single cubic phase of La2Mo2O9. The observed EMF values of oxygen concentration cell coincided with the theoretical values perfectly, indicating that the ceramics were pure oxide-ionic conductors with oxygen ion transference number of 1 under oxygen-containing atmospheres over the whole temperature range. The conductivity of the samples doped with 2.5% Sr²⁺ was much higher than the samples doped with 5% Sr²⁺. Among the double doped samples, the highest conductivity was observed for the sample doped with 2.5% Sr²⁺ and 1% Al³⁺, and the conductivity of the sample was 0.067 S·cm^-1 at 1000℃.A series of La₂(Mo_1-zSc_z)₂O₉ (z = 0,0.01,0.03,0.05,0.07) samples were synthesized via high-temperature solid-state reaction. And XRD and SEM were used to character the samples. Using the samples as solid electrolytes and porous platinum as electrodes, the ion conduction properties of these samples were investigated by means of ac impedance spectroscopy and oxygen concentration cell at 450 850℃in different atmospheres. The results were shown as follows: The samples are of a single cubic phase of La₂Mo₂O₉. The observed EMF values of oxygen concentration cell coincided with the theoretical values perfectly, indicating that the ceramics were purely oxide-ionic conductors with oxygen ion transference number of 1 under oxygen atmospheres over the whole temperature range. The conduction of the samples is different in the different oxygen partial pressure ranges. In high oxygen partial pressure range (pO₂ = 10-13 1atm) at 800℃, the total conductivity is independent of oxygen partial pressure, indicating that the samples were almost pure ionic conductors, which was consistent with the result by means of oxygen concentration cell. Whereas in low oxygen partial pressure range (pO2 = 10-14 10-20 atm) at 800℃, the total conductivity increased with the decreasing oxygen partial pressure, indicating that it is a mixed conductor of ion and electron. The phase transition in La2Mo2O9 is completely suppressed with z≥5 % Sc³⁺ at Mo⁶⁺ sites and theβ-phase (high-temperature phase) was kept up to room temperature. The doping level distinctly affects the conductivities of the samples. The conductivities first increase and then reduce with the increasing level of dopant, the highest oxide ionic conductivity is shown for the sample of z = 0.05 to be 0.04 S·cm^-1 at 850℃.Based on the basic research, the application of the oxide ion conductor was also studied in this article. The conventional oxide ion conductor YSZ (8%Y₂O₃·ZrO₂) was prepared by a hydrothermal route. And using the YSZ as an electrolyte, a compact mobile oxygen sensor was fabricated. The oxygen sensor can detect the content of oxygen in the high purity Ar atmosphere. This work is so important that it's bedding for La2Mo2O9 used as oxygen sensor.