Temperature Effect on the Structure of Transformer Oil Based Magnetic Fluids Using Acoustic Spectroscopy

Abstract

The changes in structural arrangement in transformer oil based magnetic uids upon the eect of an external magnetic eld and temperature were studied by acoustic spectroscopy. The attenuation of acoustic waves was measured as a function of an external magnetic eld in the range of 0300 mT, parallel to the direction of the eld and as a function of temperature in the range of 1535• C for various magnetic nanoparticles concentrations. The strong inuence of the steeped magnetic eld on the acoustic wave attenuation was detected and its hysteresis was observed, too. When a magnetic eld is swept at a constant rate, the dominant interactions between the external magnetic eld and the magnetic moment of the nanoparticles occur, leading to the aggregation of magnetic nanoparticles and clusters formation. However, the temperature of magnetic uids has very important inuence on the obtained dependences, where the mechanism of thermal motion acts against the cluster creation. The observed inuences of magnetic eld and temperature on the investigated magnetic liquids structure are discussed.

Measurement of Complex Permittivity of Oil-Based Ferrofluid in Magnetic Field

“…2, 3). The same effect was measured also by acoustic spectroscopy [6]. The magnetic field is linearly increased from 0 mT to 200 mT for 1 h and decreases again with the same rate.…”

“…The permittivity of the ferrofluid is increased for parallel and decreased for perpendicular orientation only for frequencies lower than 4 kHz. The variations of the dielectric permittivity with the applied magnetic (931) field are a result of rearrangement of nanoparticles into new structures -chains aligned in direction of magnetic field [6]. The induced anisotropy in real permittivity of a ferrofluid subjected to an external magnetic field can be seen.…”

“…These simple structures have no additional effect on monotonic decrease of real permittivity because they have only translational motion in the direction of magnetic field. If these structures would be bigger -clusters, their rotational motion must be taken in account and the monotonic decrease of ε r with angle could be changed, a local maximum will appear at angle around 45 • [6]. For perpendicular orientation of the electric field to magnetic field Lorentz force has maximum value.…”

“…This effect is known as magneto-dielectric anisotropy effect [2,4]. The acoustic spectroscopy is also very useful tool for the study of structure changes of ferrofluids in the magnetic fields [5,6].…”

“…This effect is known as magneto-dielectric anisotropy effect [2,4]. The acoustic spectroscopy is also very useful tool for the study of structure changes of ferrofluids in the magnetic fields [5,6].The complex relative permittivity is one of important dielectric parameter, which is a measure of how much energy from an external electric field is stored in a material and how dissipative a material is to an external field. It can be modeled by the Cole-Cole model plus the addition of dc losses [7] in the form…”