Cathodoluminescence scanning electron microscopy observations of (SrBaCa)TiO3 ceramic varistors

(Sr0.35Ba0.35Ca0.30)(1.026)TiO3 ceramic varistors were produced with and without an additional reducing heat-treatment (PO2 = 10(-12) Pa, 1100 degrees C, 2 h) in the sintering process (PO2 = 10(-8) Pa, 1400 degrees C, 2 h) followed by reoxidizing process (900 degrees C, 4 h) in air. The CL spectra of the varistors exhibit a broad luminescence, having a maximum at 465 nm, which correlates to the transition between single oxygen vacancy let el and valence band. CL images reveal that the grain, particularly near the varistor surface, has a high-conductive grain zone rich in oxygen vacancies and a less-conductive grain boundary zone poor in them. The boundary zones may work as the double Shottky barriers in the varistor. The additional heat-treatment produces thick boundary zones and consequently gives rise to high varistor coefficient (>4), favourable for practical use. It has also been found that a glassy phase (SiO2BaO-MgO) is segregated at boundary enclosed by three grains.