Properties of Modified Anodic‐Spark‐Deposited Alumina Porous Ceramic Films as Humidity Sensors

α‐Alumina films formed by anodic spark deposition in the melt of NaHSO₄‐KHSO₄ mixture (200°C) exhibited a porous structure. It has been found that the porosity and amount of α‐phase varied with anodic current density. By electrical measurements, films were found to exhibit a continuous open‐pore structure, causing failure of humidity sensors. This problem can be overcome by reanodization of the anodicspark‐deposited alumina films in either borax aqueous solution or sulfuric acid solution for a short time to form an effective barrier layer at the pore base. The modified anodic‐spark‐deposited alumina films showed highly sensitive response to water vapor change in the range of −76° to +20°C dew point with much improved long‐term stability. The humidity characteristics of the α‐alumina films were qualitatively explained in terms of the BET theory and condensation theory.