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 °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 °C. Significant grain growth of hematite grains was noted at 1100 °C. The influence of relative humidity in the range 30–90% was monitored at room temperature (25 °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 SnO2 and sintering at 1000 °C resulted in the highest sensitivity at 105 Hz of 0.391 MΩ/%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 ~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%).
Highlights
A porous microstructure has made different ceramic oxides good candidates for humidity and gas sensing [1]
Cubic spinel ferrites are a class of oxide semiconducting materials especially interesting for gas sensing applications as a change in the chemical composition and structure reflected in the cation distribution on the tetrahedral or octahedral sites of the cubic spinel structure has a profound influence on gas sensing properties such as sensitivity, selectivity, response and recovery and long term stability [7]
M110) MgFe2 O4 formed with a spinel structure (JCPDS 71-1232) and no other phases were noted shown in Figure 1, with magnesium ferrite peaks marked as M)
Summary
A porous microstructure has made different ceramic oxides good candidates for humidity and gas sensing [1]. Many oxides have been investigated and applied for gas sensing. They have included metal oxides such as α-Fe2 O3 [2], SnO2 [3,4,5] and complex oxides such as spinel ferrites [6]. Tin oxide (SnO2 ) is a wide band semiconductor (~3.6 eV) widely applied in gas sensing as a morphologically and chemically stable and low cost material that is highly sensitive to different gases but suffers from a lack of selectivity [6]. Research still continues in the search for a more effective gas sensing material that will have high sensitivity, a fast response and recovery time and good selectivity to detect low concentrations of gases [7]. Many mixed and cation substituted spinel ferrites have been investigated [8,9,10,11,12]
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