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Electrical dc conductivity of kaolin and ceramic material consisting of 50 wt.% of kaolin and clay, 25 wt.% of quartz and 25 wt.% of feldspar was measured in the temperature range of 20–600 °C. At low temperatures (20–200 °C), charge carriers are H+ and OHions released from defects located on crystal surfaces and polar water molecules in the pores. In this temperature region, the conduction activation energy is 0.3−0.35 eV. Upon releasing physically bound water, up to start of dehydroxylation at temperature ∼450 °C, dc conductivity is determined by transport of Na+, Ca2+ and K+ ions. The conduction activation energy in this region is 1.03 eV. During dehydroxylation, the charge carriers associate into neutral complexes with OHions, which are released from kaolinite lattice. The result is a temporary decrease in the dc conductivity. Temperature dependences of the dc conductivity show a slight anisotropy. Conductivity is higher in the direction parallel with the basic faces of kaolinite crystals. In fired ceramics, the glassy phase has a dominant influence on the dc conductivity, which is determined by the concentration and mobility of Na+ and K+ ions. The conduction activation energy is 0.78 eV. Above 300 °C, fired kaolin-based ceramics can be considered fast ionic conductors. Viera TRNOVCOVÁ Graduated from Faculty of Electrical Engineering (1958). Then she worked at the Institute of Physics of the Slovak Academy of Sciences (1959-2000) where she obtained a PhD (CSc) degree (1969) from research on defect interaction, diffusion mechanisms, electrical and dielectrical properties of ionic crystals. Later she works as a leading researcher at Faculty of Materials Science and Technology of Slovak University of Technology, and, in the present, for Catholic University in Ružomberok. A field of research: structure and electrical and dielectrical properties of ionic crystals, ceramics, composites, and glasses.

DC conductivity of kaolin-based ceramics in the temperature range 20–600°C