Synthesis, Electrical Properties And Applications Of (La_1-xSr_x)₂(Mo_1-yGa_y)₂O_9-α

In 2000, Lacorre reported a new oxide-ion conductor La₂Mo₂O₉. This material has a substantial quantity of intrinsic oxygen vacancies in crystal lattice even without doping low-valence metal cations, and exhibits higher ionic conductivity than YSZ under same conditions. Therefore, La₂Mo₂O₉-based oxides have attracted much interest.However, La₂Mo₂O₉ undergoes a phase transition between low temperature monoclinicα-phase with lower conductivity and high temperature cubicβ-phase with higher conductivity at about 580℃, resulting in a sharp change in ionic conductivity. In order to stabilize the high temperature phase to room temperature, adequate substitutions have been investigated. To our best knowledge, Sr²⁺ and Ga³⁺ double-doped La₂Mo₂O₉ oxides haven't been reported till now.Therefore, a novel series of (La_0.95Sr_0.05)₂(Mo_1-yGa_y)₂O_9-α (y = 0, 0.01, 0.03, 0.05) ceramic samples were synthesized via a solid-state reaction method. XRD,SEM were used to character the samples. Using the samples as solid electrolytes and porous platinum as electrodes, the electrical properties of these samples were investigated by means of AC impedance spectroscopy and gas concentration cells at 550～1000℃in different atmospheres. The results are as follows. The ceramic samples are of a single cubic phase of La₂Mo₂O₉ except (La0.95Sr0.05)2(Mo0.95Ga0.05)2O9-α. The solubility limit of Ga³⁺ in (La_0.95Sr_0.05)₂(Mo_1-yGa_y)₂O_9-α was between 0.03 and 0.05. The observed EMF values of dry oxygen concentration cell coincided with the theoretical values perfectly, indicating that the ceramic samples were pure oxide-ionic conductors with oxygen ion transference number of unity under dry oxygen-containing atmospheres over the whole temperature range. In wet oxygen-containing atmosphere, the ceramic samples were also almost pure oxide-ionic conductors, but didn't possess the protonic conduction. The oxide-ionic conductivities were affected by the doping levels, increased in the order:σ(y = 0.03) <σ(y = 0) <σ(y = 0.01). The phase transition has been suppressed in the series of samples above, however,the ionic conductivities have not been improved effectively. So the doping level of Sr was reduced to 0.03. A novel series of double-doped (La_0.97Sr_0.03)₂(Mo_1-yGa_y)₂O_9-α (y = 0, 0.01, 0.03, 0.05, 0.08) ceramic samples were prepared by a solid-state reaction method. The samples were characterized by XRD,SEM and DSC. Using the samples as solid electrolytes and porous platinum as electrodes, the electrical properties of these samples were investigated at 550～1000℃in various atmospheres. The following results were found in this study. The solubility limit of Ga³⁺ in (La_0.97Sr_0.03)₂(Mo_1-yGa_y)₂O_9-α was between 0.05 and 0.08. All the double-doped ceramic samples with y≤0.05 have completely suppressed the phase transition. In dry oxygen-containing atmosphere, the ceramic samples were almost pure oxide-ionic conductors. In wet oxygen-containing atmosphere, the ceramic samples were also almost pure oxide-ionic conductors, but didn't possess the protonic conduction. The oxide-ionic conductivities were affected by the doping levels, and increased in the order:σ(y = 0.05) <σ(y = 0) <σ(y = 0.03) <σ(y = 0.01). (La_0.97Sr_0.03)₂(Mo_0.99Ga_0.01)₂O_9-α exhibited the highest oxide-ionic conductivity of 0.103 S·cm-1 at 1000℃,which is comparable with pure La₂Mo₂O₉.In this paper, an oxygen sensor was fabricated by using the oxide-ion conductor exhibited the highest oxide-ionic conductivity, (La_0.97Sr_0.03)₂(Mo_0.99Ga_0.01)₂O_9-α, as an electrolyte. The oxygen sensor was used to detect the content of oxygen in high purity Ar gas in steel cylinder. The performance of this sensor was closely similar to that of the sensor using YSZ as an electrolyte. This work provides important reference for using the new oxide-ion conductor based on La₂Mo₂O₉ as an electrolyte in the intermediate-temperature oxygen sensor.