Capacitance spectroscopy of alumina sol-gel capacitors with Al top contacts

We measured the complex capacitance C( f ) of ITO/sol-gel alumina/Al capacitors deposited on glass (and some on stainless steel foil) in the frequency range 15 Hz–10 MHz. The sol-gel films were deposited by dip-coating and following a two step process. The capacitance C( f ) found was much higher than that of a pure Al2O3-film due to the remaining porosity of the film and the uptake of H2O from the environment. The C( f ) curves are useful as a sensitive probe for the porosity of the sol-gel film. In particular the evolution of the capacitance curve with time after drying has been measured. The curves can qualitatively be understood by modelling the capacitor as a (nearly) percolating random insulator/conductor network, using the effective medium approximation. However a detailed understanding of the relation between the C( f , t) curves and the structure of the films requires a more elaborate model. Films sintered at 500oC-550oC for 1 hour initially behave exactly as not sintered films but in contrast with the latter they improve slowly over time. Fast sintering yields better films.