Preparation And Gas Sensing Properties Of Nano-ZnFe₂O₄ And Doped-ZnFe₂O₄

ZnFe₂O₄ nanoparticles have been prepared by sol-gel method, and the effect of process parameters on the composition and structure of ZnFe₂O₄ was investigated using X-ray diffraction (XRD) and FSEM. Gas sensors have been produced using ZnFe₂O₄ nanoparticles as gas sensitive materials, and their sensitivities to volatile organic compounds (VOCs) have been tested. Doped ZnFe₂O₄ nanoparticles have been synthesized by sol-gel method above and the effect of doping on the compositions, resistance-temperature behaviors and gas sensing properties was studied in detail. ZnFe₂O₄ nanoparticles were obtained by sol-gel method at lower temperature. The as-received ZnFe₂O₄ powders were pure spinel-type structure and the average size of the spherical particles was below 50nm with narrow distribution. ZnFe₂O₄ powders were synthesized directly. The zinc-iron citrate precursor was thermally decomposed, and then non-crystalline decomposition formed ZnFe₂O₄ with spinel-type structure immediately. There were two growth stages during the crystal growth of ZnFe₂O₄, I stage of quick growth and II stage of normal growth, as calcining temperature increases. Gas-sensing properties of ZnFe₂O₄ gas sensors were tested. The results demonstrated that the sensitivities to VOCs were ranked: acetone > ethanol > xylene > toluene > benzene and they were related to functional groups of VOCs. The optimal operating temperatures of ZnFe₂O₄ gas sensors were 300℃for acetone, 320℃for ethanol, 360℃for benzene, toluene and xylene. In order to improve gas-sensing properties of ZnFe₂O₄ gas sensors, doping was used in gas sensitive materials. It was found that Mg and Sb doping can increase the sensitivities, and V doping can improve the sensitivities and selectivities to benzene and its derivatives at higher operating temperature, and Bi and Mn doping poisons gas-sensing properties of ZnFe₂O₄ and decrease the sensitivities dramatically. The paper has four parts. In the first part, detecting techniques of VOCs, the preparation and applications in gas sensors at home and abroad of nanosized ZnFe₂O₄ powders have been summarized, and then the purpose and importance of our research have been presented. In the second part, ZnFe₂O₄ nanoparticles were prepared by sol-gel method and its phase constituents and microstructure were investigated. In the third part, introducing different process parameters and their effects on gas sensitivities of ZnFe₂O₄ to VOCs were discussed. In the fourth part, the influences of doping on phase constituents, relation between the conductances and operating temperature, gas-sensing properties were studied.