Characterisation and humidity-sensing properties of aluminium (oxy)-hydroxide films prepared by cathodically induced precipitation

Cathodically-induced precipitated aluminium (oxy)-hydroxide films, prepared using hydrogen-sorbing palladium electrodes, have been characterised and their humidity-sensing electrical properties studied. Infrared (IR) spectroscopy demonstrated that the films are hygroscopic, and this property allows their use as the active component in a humidity sensor. Films formed as bridging precipitates between twin palladium electrodes were tested for electrical parameter response to relative humidity changes. A linear correlation with impedance is obtained on application of a 100 mV applied ac signal amplitude at 1 kHz. The linearity of response is comparable to commercial sensors. Cathodically-induced precipitate films contain 24 wt% aluminium and 10 wt% sulfate, the balance being oxygen and hydrogen. As-deposited films are amorphous, but change to a crystalline form after heat treatment at high temperature. XRD patterns taken as films are heated reveal transformation into γ- Al2O3, followed by production of α-Al2O3 at temperatures greater than 1100°C. An electrochemical quartz crystal nanobalance (EQCN) study demonstrated that dissolution of the precipitate is possible at positive potentials, cycling to which allows film compaction.