ELECTRICAL AND MICROSTRUCTURAL STUDIES OF SNO2 CERAMICS OBTAINED BY TIN SULFATE PYROLYSIS

SnO2 powder showing small particle size has been synthesized by means of pyrolysis of an aerosol produced by ultrahigh-frequency spraying of an aqueous tin sulfate solution. Microstructural characterizations (S EM, TEM, x-ray diffraction) were performed on as-received samples, and on pressed and annealed samples. A microstructural hierarchy was obse rved: an agglomerate of polycrystalline grains (mean size: 100 nm) the mselves formed with clusters of crystallite (mean size: 9 nm). Hence t wo kinds of crystallite boundaries are to be considered. By annealing at 800, 1000, and 1100-degrees-C a change of the microstructural hiera rchy occurs. DC conductance measurements were carried out from 50-500- degrees-C under pure air (G = G0), and air mixed with 80 ppm ethanol o r with 300 ppm CO. In presence of reducing gases in air, a marked maxi mum occurs on G = f(T) curves. It has been established by impedance da ta that the total resistance of the samples is mainly due to the resis tance of the grain boundaries. In addition the capacitance value of th e ceramic in air is discussed as well as its large change (x60) which occurs under air/ethanol. This effect is discussed in terms of grain b oundary phenomena.