Research articles Electrical conduction in a transformer oil-based magnetic nanofluid under a DC electric field Michal Rajnak a,b,⇑ , Milan Timko a , Juraj Kurimsky b , Bystrik Dolnik b , Roman Cimbala b , Tomas Tobias a , Katarina Paulovicova a , José Fernando Morais Lopes Mariano c , Peter Kopcansky a a Institute of Experimental Physics SAS, Watsonova 47, 04001 Košice, Slovakia b Faculty of Electrical Engineering and Informatics, Technical University of Košice, Letná 9, 04200 Košice, Slovakia c Department of Physics and CeFEMA, Faculty of Science and Technology, University of Algarve, Campus de Gambelas, Faro 8005-139, Portugal article info Article history: Received 5 July 2017 Received in revised form 26 October 2017 Accepted 6 November 2017 Available online xxxx Keywords: Magnetic nanofluid Electric field Magnetic nanoparticles Electric charge Current-voltage characteristics abstract In this paper, we report on the experimental study of the direct current electrical conduction in a mag- netic nanofluid based on transformer oil and iron oxide nanoparticles stabilized with oleic acid. We pre- sent the current–voltage characteristics of the pure transformer oil and the magnetic nanofluid with various particle volume fractions. From the Ohmic region, we calculate the electrical conductivity values and confirm the effect of increasing conductivity with increasing particle volume fraction. A few sources of space charge have been taken into account, among which the ion impurities play the key role. It was found that the current–voltage characteristics exhibit the inverse hysteresis-like behavior. Then, we focus on the magnetic field influence on the hysteresis behavior of a selected sample. The external magnetic field was applied in both, parallel and perpendicular configuration in regard to the electric field direction. It is shown that the magnetic field acting on the magnetic nanofluid in the perpendicular configuration results in the remarkable thinning of the current–voltage inverse hysteresis loop. Ó 2017 Elsevier B.V. All rights reserved. 1. Introduction Recent progress in high voltage engineering and power trans- mission industry has stimulated development of alternative cool- ing and electrical insulating nanofluids [1,2]. Among the various types of novel nanofluids, the magnetic nanofluids (ferrofluids) are of special interest due to their remarkably enhanced heat trans- fer properties and resulting cooling effectiveness [3]. It is well known that an external magnetic field is a vital factor affecting the convective heat transfer performances of the magnetic nanofluids and the control of heat transfer processes of a magnetic nanofluid flow can be possible by applying an external magnetic field [4]. It was shown [5] that the enhancement of the ferrofluids’ heat transfer coefficient is caused due to remarkable changes in thermophysical properties of ferrofluids under the influence of the applied magnetic field. Moreover, under strong magnetic field and temperature gradients inherently present around electrical devices like power transformers, the magnetic nanofluid could undergo the thermomagnetic convection [6–9]. Thus, the magnetic nanofluid based on transformer oils have a potential to provide more effective cooling of the transformer core and windings as compared to pure transformer oils. Besides the investigation of heat transfer properties of trans- former oil-based magnetic nanofluids, much effort has been put into studying their dielectric and insulating properties, too. The increased dielectric permittivity due to the presence of nanoparti- cles was demonstrated in several papers [10,11]. Furthermore, from broad dielectric spectra, polarization and relaxation pro- cesses, as electric double layer and Maxwell-Wagner polarization, or space charge migration were deduced and analyzed [12–15]. Recently, electrode polarization and unusual magneto-dielectric effect in a magnetic nanofluid exposed to a magnetic field were reported in [16]. From electrical insulation point of view, the most intriguing phenomenon experimentally confirmed on various mag- netic nanofluids is the increased electrical breakdown field strength as compared to pure oils [17,18]. A theoretical model explaining the phenomenon suggests that the dispersed nanoparti- cles act as electron scavengers, which slow down the streamer propagation leading to the electrical breakdown in the oil [19,20]. However, less attention has been paid to conduction cur- rents in this type of magnetic nanofluids. In general, suspensions containing nanoparticles in nonpolar liquids may exhibit the intrinsic r i and extrinsic conductivity r e https://doi.org/10.1016/j.jmmm.2017.11.023 0304-8853/Ó 2017 Elsevier B.V. All rights reserved. ⇑ Corresponding author. E-mail address: rajnak@saske.sk (M. Rajnak). Journal of Magnetism and Magnetic Materials xxx (2017) xxx–xxx Contents lists available at ScienceDirect Journal of Magnetism and Magnetic Materials journal homepage: www.elsevier.com/locate/jmmm Please cite this article in press as: M. Rajnak et al., Electrical conduction in a transformer oil-based magnetic nanofluid under a DC electric field, Journal of Magnetism and Magnetic Materials (2017), https://doi.org/10.1016/j.jmmm.2017.11.023