HUMIDITY-SENSITIVE ELECTRICAL RESISTIVITY OF MgFe_2O_4 AND Mg_(0.9)Sn_(0.1)Fe_2O_4 POROUS CERAMICS

摘要

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_2O_4 ferrite. The average grain size changes from 500 nm to 100 nm by partial substituting Mg with Sn ions in MgFe_2O_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.