Nanocomposite Zn2SnO4/SnO2 Thick Films as a Humidity Sensing Material

Nanocomposite Zn2SnO4/SnO2 powder was obtained by solid state synthesis from homogenized starting nanopowders of ZnO and SnO2, mixed in the 1:1 molar ratio, structurally and morphologically characterized using X-ray diffraction (XRD) and Scanning Electron Microscopy (SEM). Thick film paste was made by adding organic vehicles to the obtained powder. Three to five layers (layer thickness approx. 12~\mu \text{m} ) were screen printed on alumina substrate with small test PdAg electrodes and fired at 600 °C for 30 minutes. SEM analysis confirmed formation of a porous structure suitable for humidity sensing. Impedance response was studied at the working temperatures of 25 and 50 °C in a humidity chamber where the relative humidity (RH) was 30-90% and measured frequency 42 Hz - 1 MHz. With increase in film thickness the overall sensor impedance increased. It reduced at 100 Hz from 36 to 0.25 \text{M}\Omega ( 60~\mu \text{m} ), from 23.4 to 0.25 \text{M}\Omega ( 48~\mu \text{m} ) and from 6.8 to 0.02 \text{M}\Omega ( 36~\mu \text{m} ) at 25 °C, while at 50 °C and also 100 Hz it reduced from 14 \text{M}\Omega to 0.72 \text{M}\Omega ( 48~\mu \text{m} ) for RH 30 and 90%, respectively. The response (8 s) and recovery (10 s) was fast, showing that this nanocomposite has potential for application in humidity sensing.