Temperature dependence of DC electrical conductivity of kaolin

The samples from kaolin Sedlec were investigated by the help of DTA, TG, and temperature dependences of DC conductivity using Pt wire electrodes and linear heating up to 1,050 °C. After drying, the samples contained ~1.5 mass% of the physically bound water. DTA and TG reflected generally known facts about a release of the physically bound water, dehydroxylation, and metakaolinite → Si–Al spinel transformation. The results of electrical measurements showed the electric current passed over the maximum at 60 °C. The self-ionization of water results in the process H2O → H+ + OH− in the water layers on the crystal surfaces; consequently, OH− and H+ are the main charge carriers in the low-temperature region. The water molecules simultaneously evaporate from the sample which decreases the number of the charge carriers. When the physically bound water evaporates, the current is carried mostly by K+ and Na+ ions. During dehydroxylation, the hydroxyls OH− split into H+ and O2−. The ions H+ jump to the neighboring OH− groups creating the water molecules. The ions O2−remain bounded to the newly created metakaolinite lattice. Therefore, mobile protons contribute to the electric current. At the same time, this contribution gradually decreases because of the escape of H2O from the sample. The sharp current peak and DTA peak at 970 °C imply relatively fast metakaolinite → Si–Al spinel transformation. This DC current peak results from the shift of Al3+ and O2− ions into new positions.