Preparation And Properties Of Doped Cobalt Ferrite And Its Composites

For many applications such as defense, home-land security, environment-monitoring and food industry, materials of chemical vapor detection plays an important role. In recent years, the novel gas sensitive materials including ABO3 or AB2O4 type oxides and CNTs-based materials gain great interest due to their better selectivity and chemical stability compared with traditional n-type semiconducting oxides such as SnO2 and In2O3. The previous research indicated that AB2O4 type ferrites such as cobalt ferrite, nickel ferrite and their mixed ferrite are promising gas sensitive materials. However, the low gas sensitivity (Sr) of ferrites limits its applications in gas sensors (the Sr only range from 0.1 to 5 under 100-1000 ppm gas concentrations). In this thesis, A-site and B-site doped CoFe2O4 nanoparticles (NPs) and its CNTs nanocomposites were synthesized by wet chemical methods. The as-synthesized samples were characterized systemically, the gas sensing properties and magnetic properties of the products were investigated deeply.Co1-xNixFe2O4 (x=0,0.05,0.1,0.2,0.4,0.5,0.6,0.8) NPs have been successfully synthesized using a solvothermal method. X-ray diffraction analysis (XRD), transmission electron microscopy (TEM), field emission scanning electron microscope (FESEM) and vibrating sample magnetometer (VSM) were used to characterize the Ni2+ doped cobalt ferrites. Results indicated that the sample shows very high phase purity and crystallinity, and the M5 of the Co1-xNixFe204 NPs goes on decreasing with increasing amount of Ni2+. When tested against ammonia, ethanol, toluene and benzene vapors, Co1-xNixFe2O4 NPs show high selectivity towards ammonia vapors and the materials exhibit the best response when Ni2+ doping amounts is 0.2. The optimum working voltage of the Co1-xNixFe2O4NPs toward ammonia vapors is 8 V. The gas sensitivity (Sr) is up to-11 for 4000 ppm ammonia concentration at working voltage of 8 V.La3+ doped Co0.8Nio.2Fe2O4(Co0.sNi0.2LaxFe2-xO4, x=0.02,0.05,0.1,0.2,0.5) NPs have been successfully synthesized using a solvothermal method. XRD, TEM, FESEM and VSM were used to characterize the La3+ doped Co-Ni mixed ferrites. The sample shows very high phase purity and crystallinity, and the Ms of the Co0.8Ni0.2LaxFe2-xO4 NPs goes on increasing when the La3+ content is less than 0.02 and decreasing when the La3+ content is larger than 0.05. When tested against ammonia, ethanol, toluene and benzene vapors, Co0.8Ni0.2LaxFe2-xO4 NPs show high selectivity towards ammonia vapors. The optimum working voltage of the Co0.8Ni0.2LaxFe2-xO4 NPs toward ammonia vapors is 8 V. The gas sensitivity (S1-) is up to～13.8 for 4000 ppm ammonia concentration at working voltage of 8 V.Co1-xNixFe2O4/MWCNTs nanocomposites have been successfully synthesized using an in situ solvothermal route. XRD, TEM, FESEM and VSM were used to characterize the nanocomposites. Results indicated that the Co1-xNixFe2O4 nanocrystallites were compactly deposited onto the surface of MWCNTs through the solvothennal reaction. Particles show very high phase purity and crystallinity in powder XRD analysis. The maximum Ms of the Co1-xNixFe2O4/MWCNTs nanocomposites (x=0.2) is 48.5 emu/g. When tested against ammonia, ethanol, toluene and benzene vapors, nanocomposites showed a high selectivity towards ammonia vapors and the Co0.8Ni0.2Fe2O4/MWCNTs have the best response at working voltage of 8 V. Compared with the bare CNTs and ferrite NPs, the gas sensing properties of nanocomposites were been improved clearly. Co1-xNixFe2O4/MWCNTs nanocomposite is a novel and promising gas sensitive material to detect ammonia vapors.Composite magnetic nanofibers with Co0.8Nio.2Fe2O4 nano-particle contents up to 75 wt% were prepared by direct-dispersed electrospinning. Composite nanofibers containing modified nano-particle have a narrow diameter distribution of about 100～500 nm and the NPs distribution is relatively even in polymer matrix. The as-spun nonwovens show a superparamagnetic behavior. We conclude that combining surface modification and direct-dispersed electrospinning is an effective way to fabricate composite nano-fibers with high solid contents.