Electric properties of anorthite ceramics prepared from illitic clay and oil shale ash

Abstract

Electrical properties during firing of anorthite ceramic prepared from mixtures of illitic clay (from Northeastern Hungary) and fly ashes (obtained from circulating fluidized bed combustion – IA4 and pulverized firing – ID4) with high content of CaO were investigated. Mixtures were prepared with 60 wt% of illitic clay and 40 wt% of oil shale ash. The ashes were hydrated prior to mixing, to prevent the reaction of free lime with water during sample preparation. The mixtures were subjected to thermal and structural analyses to describe the processes induced by thermal treatment. The contraction of the IA4 and ID4 samples reached 1.8% and 2.5%, respectively, at 1100 °C, which is significantly lower than that of the illitic clay (7%). X-ray diffraction (XRD) analysis revealed a presence of diopside, gehlenite, cristobalite, and leucite minerals as additional mineral phases in the prepared anorthite ceramics. Measurement of AC conductivity showed that after the decomposition of the CaCO3 at ∼880 °C, Ca2+ ions became the dominant charge carriers, and the electrical current was carried by a hopping mechanism throughout the studied temperature region. The electrical conductivity and the relative dielectric constant of the ID4 sample was higher compared to that of the IA4 sample. The relative dielectric constant reached values of 6 (sample IA4) and 8 (sample ID4) at 140 °C (after drying).