| The chemically-induced phase transformation of Fe5O7(OH)·4H2O was investigated. Based on the phase transformation, a binary nanoparticle system (γ-Fe2O3-α-FeO(OH)) and a single nanoparticle system (γ-Fe2O3) were prepared, respectively. Experimental results show that there was no phase transformation occurred when the precursor was treated with either FeCl2 or NaOH solution. However, when the precursor was treated with mixed FeCl2/NaOH solution, the ferromagnetic response of the products increased significantly, and the chemical compositions and morphology were closely associated with the molar ratio of FeCl2 to NaOH.The sequence of phase transformation was determined to be:ferrihydriteâ†' α-FeO(OH)â†' γ-Fe2O3. Unless specified otherwise, all the concentration of FeCl2 solution used in this thesis was 0.01mol/L. The morphology of as-prepared γ-Fe2O3 nanoparticles was flake and sphere when the molar ratios of FeCl2 to NaOH reached 0.4 and 0.6, respectively. Characterization indicated that:(a) the content of Fe in the flake-like γ-Fe2O3 nanoparticles was lower than that in the spherical nanoparticles; (b) the coercivity of the flake-like γ-Fe2O3 nanoparticles is larger than that of the spherical nanoparticles; and (c) both have similar magnetization. The discrepancy of morphology and magnetization, between flake-like and spherical γ-Fe2O3 nanoparticles, was attributed to the surface effect.Using chemically-induced phase transformation, the γ-Fe2O3/Ni2O3 magnetic composite nanoparticles were prepared. The water-glyceryl carrier γ-Fe2O3/Ni2O3 ferrofluids were synthesized by similar self-formed ionic ferrofluids. Then, both density (p) and viscosity (η) of ferrofluids were measured using densities and viscometer (SV-10), respectively. The vibrating sample magnetometer (VSM, HH-15) was applied to measure the magnetic properties (σ-H curve) of γ-Fe2O3/Ni2O3 composite magnetic nanoparticles and that of the water-glycerol ferrofluids.The results show that the degree of saturation magnetizations of γ-Fe2O3/Ni2O3 ferrofluids increased when reducing the amount of water-glyceryl carrier. The degree of the saturation magnetizations varied with different carriers, even though the volume fraction of the water-glyceryl γ-Fe2O3/Ni2O3 ferrofluids was identical. Compared with glyceryl ferrofluids, the water-based ferrofluids matched much better with the magnetic nanoparticles. The magnetization of ferrofluids decreased gradually with increasing glycerol content. The loss in magnetization was due to the formation of the "ring-aggregates" with a closed magnetic flux. Such ring-aggregates, existing in the zero magnetic field, made no impact on the magnitude of magnetization in either low or high field regime; therefore, the effective volume fraction of the magnetic in ferrofluids was reduced. An increase in the viscosity of the carrier liquid weakened the thermal effect of nanoparticles, which further increased the number of ring-aggregates. Finally, the degree of the magnetization of ferrofluids was reduced. This relationship, between the magnetization and the viscosity of the ferrofluid carrier, was termed as "viscomagnetic effect".The binary water-glycerol based ferrofluids with different proportion were synthesized using the composite γ-Fe2O3/Ni2O3 nanoparticles and the ferrihydrite (Fe5O7(OH)-4H2O) nanoparticles. Then, the magnetization behaviors, density and viscosity of such binary ferrofluids were investigated. Experimental results show that the magnetization properties of the binary ferrofluids with different ratio of water-glyceryl were in accord with the ferrofluids containing only γ-Fe2O3/Ni2O3 composite. This implies that the magnetization of the binary ferrofluids resulted from the γ-Fe2O3/Ni2O3 composite nanoparticle, and the magnetization decreased along increasing the viscosity of solution.The relaxation behavior of the transmitted light of the water-glycerol based γ-Fe2O3/Ni2O3 ferrofluids with different ratio between water and glycerol were studied. When increasing the content of glycerol in the magnetic field, the viscosity of the ferrofluids solution increased gradually, leading to increase in the relaxation behavior of the transmitted light of ferrofluids. In summary, the content of glycerol in solution does not only affect the magnetization of ferrofluids, but also the value of the relaxation behavior of the transmitted light. |
PDF Full Text Request