Thermal conductivity of anodized aluminum oxide layer: The effect of electrolyte and temperature

Thermal conductivity of aluminum anodic oxide layers were measured using a steady-state method. The effects of chemical composition and oxide layer porosity, varied by electrolysis conditions, on thermal conductivity were analyzed. Measured thermal conductivities of oxide layers produced using different electrolytes were as follows; 1.31–1.62 W m−1 K−1 for oxalic acid, 0.53–1.01 W m−1 K−1 for sulfuric acid and 0.82–1.12 W m−1 K−1 for mixed acid; values depended on electrolyte temperature. Oxide layers with higher porosities and O/Al ratios (a measured value of the amount of hydrated Al2O3 present) had lower thermal conductivities. Increasing electrolyte temperatures decreased O/Al ratios but the oxide layer became more porous. A reduction of the hydrated Al2O3 is believed to restrict a considerable decrease of thermal conductivity of oxide layer with increase of the porosity by increase of electrolyte temperature. The presence of sulfate chemicals in the oxide layer from the sulfuric acid containing electrolyte reduced thermal conductivity as compared with oxalic acid.