Influence of the dopants on the electrical resistance of hematite-based humidity sensors

Humidity sensing properties of α-Fe2O3 have been studied after doping with alkali and alkaline earth oxides precursors. Sensors were screen-printed and three different firing temperatures were investigated: 850, 900 and 950 °C. Formation of secondary phases has been investigated by means of 57Fe Mössbauer spectroscopy for K-doped hematite samples and by means of TG–DTA and XRD for all the compositions. Alkali and alkaline earth additions to hematite, except sodium ones, increased the opened porosity as evidenced by mercury porosimetry measurements onto pressed pellets. All the sensors exhibited a strong decrease in resistance as a function of relative humidity, except those doped with barium. Resistances died down over a limited RH range for lithium additions and over a wider range for all the other dopants. However, all the compositions evidenced a scarce sensitivity below 40 and 50% RH.