The dielectric properties (permittivity, loss factor, dielectric breakdown strength) of magnetic liquids were investigated. The magnetic liquids were composed of magnetite particles coated with oleic acid as surfactant and dispersed in transformer oil. To determine their dielectric properties they were subjected to a uniform magnetic field at high alternating electric fields up to 14 MV m(-1). Nearly constant permittivity of magnetic liquid with particle volume concentration Φ = 0.0019 as a function of electric field was observed. Magnetic liquids with concentrations Φ = 0.019 and 0.032 showed significant changes of permittivity and loss factor dependent on electric and magnetic fields. The best concentration of magnetic fluid was found at which partial current impulse magnitudes were the lowest. The breakdown strength distribution of the magnetic liquid with Φ = 0.0025 was fitted with the Duxbury-Leath, Weibull and Gauss distribution functions.
This work is devoted to the study of heating of a biocompatible magnetic fluid due to time-varying magnetic induction. The adsorption of dextran on magnetite particles was confirmed by IR spectroscopy. A considerable thickness of the surfactant layer (oleate sodium + dextran) of about 4.3 nm prevents the formation of clusters made of nanomagnetic particles as evidenced by the fact that no maxima of the ultrasound wave absorption coefficient corresponding to cluster formation have been detected. The results show that the observed heating effect may be applied in hyperthermia treatments especially in the preferable region of 500 -800 kHz. An "H 2 -law" observed for the dependence of the SAR on the square of the amplitude of the magnetic field demonstrates the presence of superparamagnetic particles in the ferrofluid.
Magnetic fluids have been shown to provide both thermal and dielectric benefits to the power transformers. They can improve the cooling by enhancing the fluid circulation within transformer windings, as well as they can increase the transformer capacity to withstand lightning impulses [1]- [2], while also minimizing the efect of moisture on typical insulating fluids. The benefits of magnetic fluids may be utilized to design smaller, more effcient new transformers, or to extend the life or loading capability of existing units. Since magnetic fluid experiences a magnetically driven flow unlike oil, the results of effcient heat removal with such fluid could be expected. However, the results showing the increased dielectric strength of magnetic fluid, compared with pure transformer oil, are surprising. The presence of foreign particles has a profound effect on the dielectric breakdown strength of liquid insulators. Polarizable magnetic (e.g. magnetite Fe 3 O 4 ) particles, which are of higher permittivity than the surrounding liquid, experience an electrical force directed towards the place of maximum stress. With uniform field electrodes the movement of particles is presumed to be initiated by the surface irregularities on the electrodes, which give rise to local field gradients. The accumulation of particles continues and tends to form a bridge across the gap, which leads to the initiation of the breakdown. The aggregation of magnetic particles in external magnetic field produced by the transformer windings influences the dielectric breakdown strength of a transformer oil based magnetic fluid. However, Segal et al. [2] found out, that the presence of magnetic particles in transformer oil improved its dielectric properties by increasing the DC impulse breakdown voltage from 78 to 108 kV. In recent time the DC dielectric breakdown strength of transformer oil-based magnetic fluids was studied [3]. Now the motivation was to investigate the AC dielectric breakdown in magnetic fluids and to compare it with the previous observations. Magnetic fluid used in experiments consisted of magnetite particles (mean magnetic diameter D m =8.6 nm), surfacted by oleic acid, dispersed in transformer oil TECHNOL US 4000 (ε r = 2.15). The volume concentration of magnetic particles was Φ=0.0025. The dielectric breakdown strength was measured by properly shaped electrodes of a uniform gap of electric field-Rogowski profile [4]. Two permanent NdFeB magnets produced the external magnetic field up to 50 mT, approximately uniform in the measured gap of electric field. The time development of the breakdown was measured with help of an inductive probe and a programmable oscilloscope with its own memory. The development of the AC dielectric breakdown in magnetic fluid was compared with the development of the DC dielectric breakdovn. In the case of the DC dielectric breakdown the onset of measurable ionisation leads to complete breakdown of the gap. In the case of the AC dielectric breakdown various manifestations of luminous and audible dischar...
In this work, dielectric properties of magnetic fluids composed of magnetite particles dispersed in transformer oil and subjected to a uniform magnetic field were investigated at the high alternating electric field intensities in the range of 0.5-2.5 MV/m. Dielectric stability of the magnetic fluid with the particle volume concentration Φ = 0.0019 was observed. The magnetic fluid with the concentration Φ = 0.019 showed the significant changes of permittivity and dielectric losses, too.
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