ISSN 0862-5468 (Print), ISSN 1804-5847 (online) 

Ceramics-Silikáty 59, (2) 176 - 180 (2015)

Ondruška Ján 1, Trnovcová Viera 1, Štubňa Igor 1, Podoba Rudolf 2
1 Department of Physics, Constantine the Philosopher University, A. Hlinku 1, 949 74 Nitra, Slovakia
2 Institute of Materials Research, Slovak Academy of Sciences, Watsonova 47, 040 01 Košice, Slovakia

Keywords: Clay, Kaolinite, Illite, Calcite, DC conductivity, Phase transitions

The temperature dependences of the electrical DC conductivity of calcite waste, kaolinite and illite based ceramics were measured in the temperature range of 20 - 1050oC. The ceramic mass that was used was a mixture of 60 wt. % kaolinitic-illitic clay, 20 - 40 wt. % of this clay was fired at 1000oC for 90 min and 0, 10 and 20 wt. % of calcite waste. During heating, several processes take place - the release of the physically bound water, the burning of organic impurities, the dehydroxylation of kaolinite and illite, the decomposition of calcite, and the creation of anorthite and mullite. All of these processes were checked by means of differential thermal analysis (DTA), derivative thermogravimetry (DTG) and thermodilatometry (TDA). At low temperatures (20 - 200oC), due to the release and decomposition of physically bound water, H+ and OH- are dominant charge carriers. After completion of release of physically bound water, up to the start of dehydroxylation at the temperature of ~ 450oC, the DC conductivity is dominated by a transport of Na+, K+, and Ca2+ ions. During dehydroxylation, H+ and OH- ions, which are released from kaolinite and illite lattices, contribute to the DC conductivity. Decomposition of calcite runs between ~ 700oC and 900oC. The glassy phase has a dominant influence on the DC conductivity in the fired ceramics. Its high conductivity is determined by the high mobility of Na+, K+, and Ca2+ ions.

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