Abstract:
Asymmetric potentials are produced on heating a kaolinite sample held between platinum electrodes. Under vacuum the potentials rise to a peak of about 500 mV at about 500°C and under atmospheric pressure a peak of about 120 mV at about 600°C. The potential peak at 500°C is reproduced at about 575°C on cooling and reheating under vacuum conditions. The potential peak is dependent on heating rate to a marked degree; a slower heating rate of around 5°C/min produced no peak at 500 - 700°C but did produce a peak that is stable for a week at 800°C of about 800 mV. while a faster rate of around 30° C/min introduces further peaks.
The potentials under vacuum at a heating rate of 10°C/min are shown to agree well with a calculated curve based on dehydroxylation of material at one electrode in preference to the material at the other electrode, such that where E is the potential produced, U is the heat of transport for the conducting species plus the heat of formation of a defect, F is the Faraday constant,ρ and σ are the density and thermal conductivity of the material, ΔH is the enthalpy of the reaction,Ap and Ep are the Arrhenius parameters R is the gas constant and α is the fraction of material which has reacted. The other potential curves are interpreted in terms of this equation. A study of oxygen loss from a metakaolinite sample was negative.
