Highlights Nanocrystalline pseudbrookite thick films  High humidity sensing  Fast response  Short time delay

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Zinc ferrite nanocrystalline powder was obtained by solid state synthesis of starting zinc oxide and hematite nanopowders. Field emission scanning electron microscopy and transmission electron microscopy, X‐ray diffraction, X‐ray photoelectron spectroscopy and Raman spectroscopy confirmed the formation of nanocrystalline zinc‐ferrite powder with a mixed spinel structure with small amounts of remaining zinc oxide and hematite as impurities. Thick film paste was formed and screen printed on test interdigitated PdAg electrodes on alumina substrate. Formation of a porous nanocrystalline structure was confirmed by scanning electron microscopy and Hg porosimetry. Humidity sensing properties of zinc ferrite thick films were investigated by monitoring the change in impedance in the relative humidity interval 30%‐90% in the frequency range 42 Hz‐1 MHz at room temperature (25°C) and 50°C. At 42 Hz at both analyzed temperatures the impedance reduced ~46 times in the humidity range 30%‐90%. The dominant influence of grain boundaries was confirmed by analysis of complex impedance with an equivalent circuit.

Iron manganite (FeMnO 3 ) powder with a cubic (bixbyite, ̅ ) crystal structure was obtained by a solid state reaction. Thick film paste (powder + organic vehicles) was screen printed on alumina substrate with test interdigitated PdAg electrodes. Significant porosity (60.6%) composed of macropores (larger than 100 nm) was determined by Hg porosimetry, changing only slightly from the first extrusion run indicating a stable pore system. Hg porosimetry evaluation of thick film samples enabled estimation of true textural parameters of the thick film compared to powder. Impedance response of the thick film sensor was monitored in a humidity chamber in the relative humidity range 30-90%, at room temperature (25 o C) and frequency range from 42 Hz to 1 MHz. At 100 Hz the impedance reduced from 10.41 MΩ to 0.68 MΩ for relative humidity of 30 and 90%, respectively. Analysis of complex impedance using an equivalent circuit showed the dominant influence of grain boundaries. The sensor response and recovery was fast (several seconds) and a relatively low hysteresis value of 2.8% was obtained.