Dielectric breakdown strength in magnetic fluids

Abstract

The application of an external magnetic field on the magnetic fluids induces needle like structuralization of magnetic particles. The influence of this aggregation of magnetic particles in an external magnetic field H on the magneto-dielectric behaviour and dielectric breakdown strength of a transformer oil based magnetic fluids was measured. The observed decrease of dielectric breakdown strength (for H = 0 and H ⊥ E) vs. distance between electrodes is found to be in agreement with a theory of semi-classical bond percolation for a non-linear composite material. In situation when E  H oscillations of dielectric breakdown strength were observed that are associated with a size effect probably. IntroductionIt is well known that as a consequence of dipol-dipol interaction between magnetic particles in magnetic fluids, magnetic particles tend to attract the neighboring particles in the direction of the magnetic moment. It is expected, therefore, that the magnetic particles will form chains and chain like elongated clusters in which the particles are connected magnetically. Such structural configurations of particles result in many physical properties of magnetic fluids i.e. magnetomechanical effects, magnetooptical effects, magneto-dielectric behaviour and so on. The long-chain and cluster models of the magneto-dielectric effect have been analyzed in papers [1,2] for example. It has long been recognized [3] that the presence of foreign particles in liquid insulators has a profound effect on the dielectric breakdown strength of liquids. In magnetite based magnetic fluids (transformer insulation oil as carrier base for example) the suspended particles are polarizable and are of higher permittivity than the liquid. As a result they 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 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 breakdown [3]. In this sense the motivation of this work was to study the effect of magnetic field induced structuralization of magnetic particles on the dielectric breakdown strength of magnetic fluids.

The experimental study of the DC dielectric breakdown strength in magnetic fluids

“…[4]. No oscillations of the dielectric breakdown strength were observed, what proves, that the oscillations observed in Ref.…”

“…No oscillations of the dielectric breakdown strength were observed, what proves, that the oscillations observed in Ref. [4] were caused by the presence of the needle-like structures.…”

“…The observed decrease of the dielectric breakdown strength (for H ¼ 0 and H>E ) vs. distance between the electrodes was found to be in agreement with a theory of semi-classical bond percolation in a non-linear composite material. In case of EjjH; the oscillations of the dielectric breakdown strength were observed, that are associated with the aggregation and size effects, probably [4]. The aim of this work was to study the effect of an external magnetic field and magnetite particles concentration upon the DC dielectric breakdown strength in magnetic fluids, and to verify the improvement of dielectric properties, found by Segal [3] for DC voltage (not impulse DC voltage).…”

“…Since only the lower value of the impulse voltage can be relied upon for the safe operation of the transformer, magnetic fluid increased the limit from 78 to 108 kV [3]. In our previous work the DC dielectric breakdown effect in magnetic fluids based on transformer oil [4] was studied. The observed decrease of the dielectric breakdown strength (for H ¼ 0 and H>E ) vs. distance between the electrodes was found to be in agreement with a theory of semi-classical bond percolation in a non-linear composite material.…”

Dielectric characterisation of Fe3P nanoparticles based ester oil

“…Magnetic nanofluids, also termed as ferrofluids, are solutions prepared from magnetic nanoparticles such as Fe 3 O 4 , Co 3 O 4 , iron phosphide (Fe 3 P), γ-Fe 2 O 3 , cobalt (Co) ferrite, Co, Fe, or Fe-C evenly distributed in an insulating liquid [9]. Magnetic nanofluids on examination have shown thermal and dielectric advantages [6,[10][11][12][13]. Stability is the significant consideration for preparation of nanofluid to gain longer lifespan.…”