Thick films of MgFe₂O₄ for humidity sensors

In this paper the humidity-sensitive electrical properties of MgFe₂O₄ thick-films are studied. Thick films were prepared by screen printing on alumina substrates and subsequent firing. Pastes for screen printing were obtained by adding an organic vehicle and Bi₂O₃, as a sintering aid, to MgFe₂O₄ powders. The microstructure of the films was varied using MgFe₂O₄ powders with different grain sizes and by changing the firing temperatures. The microstructure of the films was studied by scanning electron microscopy (SEM), energy dispersion spectroscopy (EDS), and Hg porosimetry. The humidity-sensitive electrical properties of thick films were tested by electrochemical impedance spectroscopy (EIS). D.C. measurements were made to test reproducibility and time response, and to study the conduction mechanism. The electrical response of the films was correlated with their microstructure and the distribution of Bi₂O₃ in the films.