Abstract
Various compositions of barium-doped hematite between pure hematite (α-Fe2O3) and pure barium hexaferrite (BaFe12O19) were synthesized by solid state reaction. The XRD analyses confirmed the progressive evolution of the two crystalline phases. Tests as humidity sensors show that the electrical resistance of samples containing high proportions of hexaferrite phase is strongly influenced. Electrochemical impedance spectroscopy (EIS) analyses under air or argon revealed an intrinsic semiconducting behavior for hematite and samples doped with 3 and 4 wt % equivalent BaO. The samples containing higher proportions of barium exhibited an extrinsic semiconducting behavior characterized by a variation of the conductivity with the oxygen partial pressure. This study allowed us to define the percolation threshold of the barium hexaferrite crystalline phase in the hematite matrix. The value was estimated to hematite doped with 5 wt % BaO, i.e., 36 wt % of barium hexaferrite phase. EIS analyses under various NO2 partial pressures confirmed the sensitivity of these materials. The linearity of the response was particularly evident for the 5, 10 and 14 wt % samples.
Highlights
Nitrogen oxides (NO and NO2: NOx), released from combustion facilities and automobiles, are a main cause of air pollution
We observe (Figure 1) the intensity of the barium hexaferrite peaks increasing with BaO additions from the pure hematite sample (H) to the pure barium hexaferrite sample (14% BA i.e., α-Fe2O3 + 14 wt % BaO)
Theoretical densities were calculated considering that in all the mixtures the barium oxide was completely transformed into barium hexaferrite crystalline phase by reaction with hematite
Summary
Nitrogen oxides (NO and NO2: NOx), released from combustion facilities and automobiles, are a main cause of air pollution. They are responsible for acid rains, photochemical smog and are potentially eutrophying agents, i.e., can cause an oversupply of nutrient in soils and water bodies. They are known to be harmful to the environment, to people, and to historical monuments and buildings. Reliable, simple, effective and low-cost methods to monitor them have been highly demanded for atmospheric environmental measurements and controls. The main drawbacks associated to these techniques are represented by the use of expensive bench scale laboratory equipment including calibrating facilities
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