Anodic alumina membrane capacitive sensors for detection of vapors

Here we report membrane capacitive sensors based on anodic aluminum oxide (AAO) Au/AAO/Au structures fabricated by aluminum anodization, followed by gold electrodes sputtering on the countersides of porous ceramic membrane. Electrochemical impedance spectroscopy with AC amplitude 5-100 mV in the frequency range of 1-1000 Hz was utilized for sensor characterization in the presence of water and organic vapors in a full range of P/P₀. The sensors illustrate ultimate sensitivity to ambient environment with exponential-scale capacitance relation to vapors content resulting in typical 4-6 orders of magnitude response signal change for 15-85% P/P₀ range at a single AC frequency, and up to 7 orders of magnitude response range for 0-100% P/P₀ pressure range with using two different AC frequencies. In case of water vapors, the sensitivity increases from ~0.5 nF/RH% at ~20 RH% to over ~1.0 μF/RH% at ~80 RH%. The sensors are capable for highly accurate sensing of gas humidity as well as any dissociative vapors with pKa <30. They are also sensible to polar components with high enough dipole moment or polarizability. The capacitance is affected by any adsorbed molecules, including those having zero dipole moment. The data for sensor response to CH₃OH, C₂H₅OH, CH₂ClCHF₂, i-C₄H₁₀ depending on partial pressures is provided. Due to high porosity (10-30%) and gaseous permeance (up to 200 m³(STP) m^-2 bar^-1 h^-1) the sensors offer fast response rate and a possibility for flow-through measurements, providing also a mass-flow response option, which was tested with SF₆, CO₂, N₂ and He. The basic principles of dielectric loss sensor and the equivalent scheme were proposed for sensor operation in different environment, allowing estimating sensor response.