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High-resolution electron microscopy and cathodoluminescence scanning electron microscopy of SrTiO3-based ceramics elucidating their varistor property.

Shiojiri M., Isshiki T., Nishio K., Saijo H., Nomura T., Hitomi A., Sato S.

Inżynieria Materiałowa

1998

R. XIX, nr 4

779-784

Semiconductive SrTiO3-based ceramics practically used as varistors and capacitors have been investigated using HRTEM, EDX and CLSEM. They were produced with and without an additonal reducing treatment at Po2=10 raising to a 12th power Pa in the sintering process at Po2=10 raising to a -8 power Pa, followed by reoxidizing process in the air. (Sr0.35Ba0.35Ca0.30)1.026TiO3 varistors produced without the additional heat-treatment are composed of well-developed crystalline grains with facet boundaries, having a low varistor coefficient (alpha<4). CL revealed that the grain, particularly near varistor surface, has high conductive inside containing oxygen vacancies and less-conductive boundary layer without the oxygen vacancies. The boundary layers may work as the double Shottky barriers. The additional reducing treatment forms a lot of oxygen vacancies in the grains of the varistors and damaged the crystallinity near the grain boundaries. The damaged regions easily introduce oxygen atoms into the grains during the reoxydizing process. The additional treatment, hence, produces thick boundary layers and consequently gives rise to high varistor coefficient (alpha>4), which is favourable for practical use. (Sr0.94Ba0.01Ca0.05)0.99TiO3 capacitors were also produced without the additional reducing treatment. Their grains have the facet boundary structure. The dielectric boundary layer and semiconductive inside of the grain are also observed. The boundary layer is as thin as a few ten nanometers, which cause the ceramic a high capacitance.

High-resolution transmission electron microscopy and cathodoluminescence scanning electron microscopu observations of SrTiO₃-based varistors

Shiojiri M., Isshiki T., Saijo H., Sato S., Hitomi A.

Electron Technology

1998

Vol. 31, No. 2

170-175

(Sr₀.₃₅Ba₀.₃₅Ca₀.₃₀)₁.₀₂₆TiO₃ ceramic varistors, which were produced by sintering in different reducing ambiences and by reoxidizing, have been investigated by high-resolution electron microscopy, electron probe microanalysis, and cathodoluminescence (CL) scanning electron microscopy. Varistors produced by ordinary sintering Po₂=10⁻⁸ Pa, 1400°C, 2 h) have low varistor coefficient are compos4ed of well-developed crystalline grains with facet boundaries. A severe reducting treatment (Po₂=10⁻¹² Pa, 1100°C, 2 h), added in the sintering process, damages the crystallinity in the grains by introducing many oxygen vacancies and changes the boundary zone to amorphous-like microcrystallites, which causes high varistor coefficient favourable for practical use. CL images vizualize the oxygen vacancies in the grains and glassy segregate phase (SiO₂-BaO-MgO) at boundary among three grains. It is found that the surface layer, which is composed of bright enclosed by dark boundary zones in CL images, is responsible for the varistor property.