Sol-gel synthesis, X-ray, infrared spectra and electrical conductivity of lanthanum/barium-cobaltite systems

摘要

Cubic perovskite-type pure lanthanum cobaltite, LaCoO_3, LC and barium-substituted lanthanum of the chemical formula La_(1-x)Ba_xCoO_3, LBC, x = 0.0 (LC), 0.1, 0.3, 0.5, 0.8, 1.0, powders are prepared using the polymerized complex technique at relatively low temperature. The complex solution, with molar ratios La (Ba) / Co/ CA (citric acid) / EG (ethylene glycol) = 1 : 1 : 10 : 40, is heated at 130 deg C to produce a dark Prussian transparent polymeric gel without any precipitation. Pyrolysis of the el at 350 deg C gave a powder precursor of LC or LBC. Thermal analyses, X-ray diffraction and IR spectroscopy indicate the formation of single phase cubic LC or LBC when their precursors are heat-treated in air at temperatures > 500 deg/2h. For comparison, a solid-state reaction sample, LC, has been prepared, where traces of the parent oxides appear even at higher sintering temperature (900 deg C). The lattice vibrations in LC precursor are heat-treated at different temperatures and LBC as function of Ba-additions indicate the structural changes with temperature or barium effect respectively. The amounts of metal determined by AAS, in the samples after annealing confirm the composition to be LaCoO_3 at 900 deg C. The electrical conductivity #sigma#, of the ceramic LBC formed at 900 deg C/2h, as a function of Ba-content (x), has been measured at temperatures, 20 (room temp.), 100, 200 and 300 deg C.From the results, it is found that #sigma#increases with the measuring temperature, while substitution of La- for Ba- is effective for enhancing the oxide ion conductivity in the range 0 < x < 0.5 where it decreases with further increase of x.