Study On Preparation,Characterization Of Fe₃O₄ Nanoparticles And Dispersibility In Transformer Oil

Transformer oil as an important liquid insulating medium,it is necessary to improve its insulation performance for a better stable operation of the transformer and the entire power grid security.Fe3O4 nanoparticles have been proved to have great potential in improving the insulation performance of the transformer oil.At present,research at home and abroad mainly focuses on the macroscopic insulating properties and modification mechanism of Fe3O4 nano-transformer oil.Rare researches on the conductivity and dispersion stability of the transformer oil has been reported.Therefore,the preparation and size control on Fe3O4 nanoparticles were studied in this thesis.The breakdown strength of nanoparticle modified transformer oil(nanofluid)was also studied.The effects of the concentration and testing temperature on the conductivity of Fe3O4 nanofliuds were studied.The dispersion stability of Fe3O4 nano flu id was tested and analyzed.The nanoparticles have been prepared by both solvothermal and pyrolysis method.The crystal stucture and morphology of the as-prepared products were characterized by X-ray diffraction(XRD)and transmission electron microscopy(TEM).The effects of amount of modifying agent,the reaction time,ratio of reactants and other factors on the morphology were studied.Cubic and spherical nanoparticles with an average size among from 6 nm to 40 nm were prepared.The results show that the addition of oleic acid modifier turns Fe3O4 NPs from spherical to tetragonal structure,and the increase of the amount of Fe2O3 promotes the particle size of Fe3O4 nano particles.In the thermal decomposition process,spherical nanoparticles with the average particle sizes ranging from 20 to 40nm were prepared with the increase of reaction time.The breakdown strength of nano fluid was tested and analyzed,and the electrical conductivity of nanofluid with good breakdown strength was studied.The results show that the temperature and the concentration of nanoparticles have a significant effect on the electrical conductivity of nanofluid.When the volume concentration of nanoparticles is 0.40‰,the room temperature electrical conductivity of the nanofluid increases to 289 times of that of pure oil.In order to explain this phenomenon,we used the existing nanofluid conductivity model to analyze the measured values and found that the measured values are consistent at low concentrations(0.05 ‰ and 0.10 ‰).While the deviation between the calculated value and the measured value increases with the increase of nanoparticle concentration and temperature.The change of the particle size of the nanoparticles in the transformer oil was tested and the original size of solid nanoparticles was replaced by its size in the oil.The calculation formula of the electrophoretic conductance was changed and the prediction of the conductivity of the nanofluid was improved.Finally,the dispersion stability of Fe3O4 nano-transformer oil was analyzed under AC electric field.The results showed that the particle size of Fe3O4 nanoparticles under AC stress increases up to 1.8 times of that without electric field after aging for 45 hours,which may be attributed to the aggregation induced by the applied electric field,which increases the collision probability and surface charge of nanoparticles during the frequent migrations between two electrodes.