Effect of oxalic acid concentration on the magnetically enhanced capacitance and resistance of AAO humidity sensor

The enhanced capacitance and resistance behavior of anodic aluminum oxide (AAO) humidity sensor by both the high concentration of oxalic acid and applied magnetism has been investigated. At low relative humidity (RH, ≤45%), the enhanced capacitance response from high concentration of oxalic acid was attributed to phonon-assisted electron tunneling mechanism that was promoted by more acid anions within the AAO pore wall and barrier layer. The other enhancement from magnetic addition was due to the magnetic field induced more orderly arrangement of water molecules for higher capacitance. At high RH (>45%), the capacitance of AAO sensors at different acid concentrations (0.3–0.7 M) with magnetic field was larger than that without magnetic field under the same mechanisms at low RH. Moreover, the medium-concentration AAO sensor exhibited the highest capacitance at RH over 60% due to the enhanced sensing surface area and pore conductance. The short and reliable capacitance response/recovery time was also obtained from the high-concentration AAO sensor under presence or absence of a magnetic field. In addition, the applied magnetism can significantly enhance the ordinary magnetoresistance response of humidity sensors at different acid concentrations at low-to-high RH for more linear-like relationship between the resistance and RH.