Study On Sol-gel Process For Thin Film Synthesis And Oxygen-sensing Properties Of Perovskite-like Type Complex Oxide La₂NiO₄

The environment pollution caused by the exhaust gas from automobiles becomes more and more serious. Due to the impetus on environmental consciousness and emission control regulations the need for highly efficient combustion engines has led to a quest for an efficient solid state oxygen sensor. For this reason, the sensing materials for automobile exhaust gas are widely studied and exploited. In this paper, rare-earth perovskite-like complex oxide La₂NiO₄ has been studies as a newly gas-sensing material used for automotive oxygen sensor. The synthesis, gas-sensing properties and the influence of doping on La₂NiO₄ thin film with nanocrystalline structure have been researched systematically.First, a newly prepare method for thin film - inorganic complex sol-gel process has been studied. The major results are as follows: (1) Complexing action is fundamental for this method, while the interaction of molecules in gel precursor is important for uniformity. The ideal structure of complex gel is that complex molecules in the gel connecting with each other mainly by chemical bonds. (2) The kind and proportion of chelating agent have great influence on quality of thin film and chelating agent with small molecule is preferably for thin film preparing. Urea is fit for preparing thin film because of its small molecular weight and good complex ability. The thin film of La₂NiO₄ prepared from urea complex precursor gel have good appearance.Conductivity and oxygen sensitivity of La₂NiO₄ thin film have been investigated. The major conclusions include: (1) La₂NiO₄ thin film exists metal-semiconductor transition. Its very low resistance-temperature coefficient from 400℃ to 800℃ is deal to the metallic conductivity at this temperature range. (2) La₂NiO₄ thin film has good oxygen-sensing properties within the temperature range 600℃⁸00℃. Its oxygen-sensitivity is related to its oxidation and reduction process and non-stoichiometric ratio. In La₂NiO₄+ā system with excess oxygen, the conductivity is in proportion to O21/6.The effect of doping on A and B site have been studied in this paper. The main results are as follows: (1) The doping of Sr on La site is in favor of the phase formation of La₂NiO₄ with K2NiF4 structure. Trifle doping of Sr can promote response speed but depress sensitivity. (2) While Fe and Li be used for doping on B site, oxygen-sensing properties of La2Ni1-xLixO4 and La2Ni1-xFexO4 are enhanced, and the work temperatures are decreased promotly.