Preparation, Electrical Properties And Applications Of La₂(Mo_1-xV_x)₂O_9-α

Solid oxide-ion conductor has received much attention for their potential applications in fuel cell, oxygen sensor, temperature sensor, oxygen pump etc. La₂Mo₂O₉ was reported as a new oxide-ion conductor by Lacorre in 2000. It has a considerable number of inherent oxygen vacancies in crystal lattice even without doping low-valence metal cations, and the oxide-ion conductivity of La₂Mo₂O₉ is higher than YSZ under the same conditions. Therefore, La₂Mo₂O₉-based oxides have attracted great interest. However, La₂Mo₂O₉ undergoes a phase transition from low temperature monoclinicα-phase to high temperature cubicβ-phase around 853 K, associated with a sharp jump in ionic conductivity, which makes a great limitation on its practical applications.In order to stabilize the high temperature phase to room temperature, adequate substitutions on La site and/or on Mo site have been investigated in the latest few years. To our best knowledge, there have been no sufficient studies on ionic conductivity and the influence factors, especially no reports on its protonic conductivity, though there are a few reports on V-doped La₂Mo₂O₉ on Mo site.In this paper, the ceramic samples La₂(Mo_1-xV_x)₂O_9-α (x=0, 0.02, 0.04, 0.06, 0.08) were prepared by a solid-state reaction and sol-gel methods. The ionic conductive properties of ceramic samples were investigated using gas concentration cells and electrochemical impedance spectroscopy at 823-1123 K in different atmospheres.Main works and results in this paper are as follows:1,A ceramic sample of La₂Mo₂O₉ phase was prepared by a solid-state reaction method. The ceramic sample La₂Mo₂O₉ is almost a pure oxide-ionic conductor and doesn't possess the protonic conduction in dry and wet oxygen atmospheres. It has the oxide ionic conductivity of 0.045 S cm^-1 at 1123 K.2,A series of ceramic samples La₂(Mo_1-xV_x)₂O_9-α (x=0.02, 0.04, 0.06, 0.08) were prepared via solid-state reaction and sol-gel methods.The following results were found in this study:(1) All the samples have completely suppressed the phase transition which occurs in La₂Mo₂O₉ around 853 K.(2) The ceramic samples are almost pure oxide-ionic conductors and don't possess the protonic conduction in dry and wet oxygen atmospheres.(3) The dependance of conductivity on oxygen partial pressure reveals that the samples are almost pure oxide-ionic conductors at high oxygen partial pressure range, whereas exhibit mixed ionic and electronic conduction at low oxygen partial pressure range.(4) The oxide-ionic conductivity (σ) is affected by the doping level, and increases in the order:σ(x = 0.02) <σ(x = 0.08) <σ(x = 0.06) <σ(x = 0.04). La₂(Mo_0.96V_0.04)₂O_9-α exhibits the highest oxide-ionic conductivity of 0.051 S cm-1.(5) The changing order of oxygen ionic conductivity is well consistent with that of the free volume of crystal lattice, indicating that larger free volume may be conducive to oxygen ion transmission and enhancement of oxygen ionic conductivity.(6) The oxygen ionic conductivities of La₂(Mo_0.96V_0.04)₂O_9-α prepared by a solid- state reaction method and sol-gel method are almost the same, indicating that sol-gel method doesn't have a positive effect on improving oxygen ionic conductivity.(7) In addition, the application of the oxide ion conductor was also studied in this paper. Using La₂(Mo_0.96V_0.04)₂O_9-α as a solid electrolyte, a compact and mobile oxygen sensor was fabricated. The oxygen sensor can detect the content of oxygen in the high purity Ar atmosphere. This work provides an important reference for making intermediate-temperature oxygen sensor based on La₂Mo₂O₉ solid electrolyte.