Some Spinel Oxide Compounds as Reducing Gas Sensors

Four spinel ferrites, MFe2O4 (M = Cu, Cd, Zn and Ni), having various grain sizes (100 – 700 nm) were prepared by sol-gel-selfcombustion and their sensing properties to reducing gases were investigated. The gas sensing characteristics were obtained by measuring the sensitivity as a function of various controlling factors, like operating temperature, composition and concentration of the gas, and finally the response time. The sensitivity of four ferrites to reducing gases like acetone, ethanol and LPG was been compared. It was revealed that CuFe2O4 is the most sensitive to LPG and ZnFe2O4 can be used as a sensor to selectively detect ethanol vapors in air. The strong interaction between ethanol and porous ZnFe2O4 can explain the selective sensitivity to ethanol and negligible sensitivity to the other reducing gases

HUMIDITY-SENSITIVE ELECTRICAL RESISTIVITY OF MgFe 2 O 4 AND Mg 0.9 Sn 0.1 Fe 2 O 4 POROUS CERAMICS

Pure and Sn substituted Mg ferrites have been prepared by sol-gel autocombustion method. This method offers the advantage to prepare ultra-fine, homogeneous and reproducible ferrite powders using aqueous solutions of constituent ions (metal nitrates) salts. Sn ion partially substituted Mg ion to improve the humidity sensitivity of Mg polycrystalline ferrite. The phase composition and lattice parameters were determined by X-ray diffraction (XRD), and the effect of Sn substitution on the granular structure was investigated by scanning electron microscopy (SEM). We have found that Sn ions affect crystallite size, surface area and porosity. Sn ions assure a nanocrystalline structure of Mg 0.9 Sn 0.1 Fe 2 O 4 ferrite. The average grain size changes from 500 nm to 100 nm by partial substituting Mg with Sn ions in MgFe 2 O 4 . Also, Sn ions enhance the humidity sensitivity of Mg ferrite. For Sn substituted ferrite, the electrical resistivity decreases by about three orders of magnitude, from 10 9 Ω·cm to 10 6 Ω·cm when the relative humidity increases from 11% to 85%. These results show that the promoting effect of Sn ions on humidity sensitivity of Mg ferrite can be related with the changes in porosity and specific surface area. Further investigations are necessary to shorten response time to humidity variations