Influence of SnO2 Content on the Humidity Dependent Impedance of the MgFe2O4-Fe2O3-SnO2 Compound

Abstract

A porous MgFe2O4-Fe2O3-SnO2 bulk compound with varying SnO2 content was obtained by sintering an appropriate mixture of magnesium oxide, hematite and tin oxide nanopowders at 1000 and 1100 degrees C. The obtained structure was confirmed by X-ray diffraction analysis. Scanning electron microscopy was used to analyze sample morphology, showing that the addition of SnO2 resulted in an inhomogeneous microstructure with smaller grain size especially at 1000 degrees C. Significant grain growth of hematite grains was noted at 1100 degrees C. The influence of relative humidity in the range 30-90% was monitored at room temperature (25 degrees C) in the frequency range 42 Hz-1 MHz. The highest reduction of impedance with humidity was noted at lower frequency. Addition of low amounts of SnO(2)and sintering at 1000 degrees C resulted in the highest sensitivity at 105 Hz of 0.391 M Omega/%RH in the RH30-90% range, while the compound with the highest amount of SnO2 showed the largest decrease in impedance with increase in relative humidity similar to 26 times. All samples showed low hysteresis (below 2%). Complex impedance data was analyzed using equivalent circuits reflecting the dominant influence of the grain boundary in the lower relative humidity range (30-60%) and both grain boundary and grain components in the higher relative humidity range (60-90%).