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Ultrastable Natural Ester-Based Nanofluids for High Voltage Insulation Applications Georgios D. Peppas,*,† Aristides Bakandritsos,*,‡ Vasilis P. Charalampakos,§ Eleftheria C. Pyrgioti,† Jiri Tucek,‡ Radek Zboril,‡ and Ioannis F. Gonos# †
Department of Electrical and Computer Engineering, University of Patras, 26504 Rio, Greece Regional Centre for Advanced Technologies and Materials, Department of Physical Chemistry, Faculty of Science, Palacky University in Olomouc, 17. listopadu 1192/12, 771 46 Olomouc, Czech Republic § Department of Electrical Engineering Technological Educational Institute of Western Greece, Patras, Greece # School of Electrical and Computer Engineering, National Technical University of Athens, Athens, Greece ‡
S Supporting Information *
ABSTRACT: Nanofluids for high voltage insulation systems have emerged as a potential substitute for liquid dielectrics in industrial applications. Nevertheless, the sedimentation of nanoparticles has been so far a serious barrier for their wide and effective exploitation. The present work reports on the development and indepth characterization of colloidally ultrastable natural ester oil insulation systems containing iron oxide nanocrystals which lift the problem of sedimentation and phase separation. Compared to state-of-the-art systems, the final product is endowed with increased dielectric strength, faster thermal response, lower dielectric losses (decreased dissipation factor: tan δ), and very high endurance during discharge stressing. The developed nanofluid was studied and compared with a similar system containing commercial iron oxide nanoparticles, the latter demonstrating extensive sedimentation. Herein, the dielectric properties of the nanofluids are analyzed at various concentrations by means of breakdown voltage and dissipation factor measurements. The characterization techniques unequivocally demonstrate the high performance reliability of the reported nanofluid, which constitutes a significant breakthrough in the field of high voltage insulation technologies. KEYWORDS: nanofluids, insulation systems, dielectrics, nanocrystals, colloids
1. INTRODUCTION Nanofluids have been intensively studied as candidates for replacement of mineral oils in high voltage engineering because they demonstrate outstanding (increased) dielectric insulation properties and thermal response compared to those of matrix oils.1−10 In particular, these two fundamental properties of nanofluids are vital in the transformer and insulator industry by decreasing their size and weight. The main drawback to research advancement on such nanofluids in the high voltage industry is the stability problem of the nanofillers. The term nanofluid in the field of insulation fluids was introduced by Choi et al.,1,11 which refers to the possibility of increasing the heat transfer coefficient by the addition of various metal oxide nanoparticles into the oil matrix. However, the idea of using metallic particles to enhance the thermal conductivity of composite materials has been well-known since Maxwell in 1873. The dominating model, among others,12,13 which takes into consideration both the electrical properties and breakdown mechanism in nanoparticle-containing fluids, is Tanaka’s model,14 which possibly explains electrical discharge phenomena (electrical treeing discharge) into solid-state (polymer) nanocomposites. A critical addition to the latter model has been proposed by Hwang et al.,15 who investigated the nanoparticles’ © 2016 American Chemical Society
influence on the streamer development during a breakdown (or discharge) event in a voltage stressed nanofluid. According to the aforementioned model, nanoparticles (NPs) act as “electron (shallow) traps” inside a nanofluid; NPs are charged negatively, thereby generating a local electrical field opposite to the external field. This reduces the field strength near the cathode;16 hence, the dielectric strength of the nanofluid is increased compared to that of typical insulating liquids (i.e., mineral oil, natural ester oil, and synthetic ester oil). The problem is that particles on the micro- or nanoscale lead to sedimentation.5,17 The march of nanotechnology and insights in synthesis and characterization of nanoparticles resulted in the emergence of nanofluids as a potential solution toward thermally conductive liquids. Apart from the increase in thermal conductivity, increased breakdown voltage (BDV) was also recorded for TiO2,18,19 SiO2,20 iron oxide,21,22 and nanodiamond Ni nanoparticles,23 thus manifesting even more advantages emerging from nanofluids. In another report, TahaTijerina et al.10 achieved enhanced thermal and electrical Received: May 21, 2016 Accepted: September 1, 2016 Published: September 1, 2016 25202
DOI: 10.1021/acsami.6b06084 ACS Appl. Mater. Interfaces 2016, 8, 25202−25209
Research Article
ACS Applied Materials & Interfaces
2. EXPERIMENTAL SECTION
properties in nanofluids according to thermal conductivity, dissipation factor (DF), and electrical resistivity measurements using two-dimensional nanomaterials (BN and graphene). The particular mineral oil-based nanofluid was presented as a nextgeneration thermal nanofluid for power transformers. Nevertheless, nanofluids with long-term stability and performance reliability documented through electrical discharge endurance tests have not been reported. This property is fundamental for understanding the long-term performance of materials for high voltage applications, and currently, it is the most critical issue toward the establishment of oil-based nanofluids in this direction. To achieve stable response over time, the filler (i.e., NPs) must be properly dispersed with no agglomeration and sedimentation during the commercial life of the nanofluid. Simple addition of NPs into the matrix oil has been reported by several groups.16,19,20,24 Surface modification by adding coating agents on commercial nanopowders has been also studied,7−10,18,25−27 and in some cases agglomeration is recognized as a critical performance barrier.18,20 Currently, various in situ surface modification techniques have been explored to increase the dispersibility and miscibility of the inorganic NPs with solvents and matrixes. Several molecules have been tested as coating agents such as dodecyl-benzenesulfonic acid, lauric acid, myristic acid, trioctylphosphine oxide, triphenylphosphine, dodecanethiol, tetraoctylammonium bromide, and oleic acid (OA). 28−33 Taking advantage of such well-established procedures, we applied them herein by synthesizing oleic acid-coated magnetic iron oxide nanocrystals (MIONs) through the typical thermal decomposition of molecular iron precursors. The reason that we selected MIONs as the nanoparticulate fillers for the transformer oil over other metal oxide materials is this availability of well-explored and controlled wet chemical processes, which allow for the synthesis of very stable organophilic colloids of monodispersed iron oxide nanocrystallites. We hypothesized that the superfine colloidal nature of the dispersions of such nanocrystals in organic solvents would render them excellent fillers toward ultrastable oil-based nanofluids. The present results verified this hypothesis with the dispersed nanoparticles displaying unprecedented colloidal stability in the oil matrix even after one year monitoring. In addition, the nanofluid is endowed with augmented dielectric strength, faster thermal response, and lower dielectric losses (decreased dissipation factor: tan δ). More importantly, it is for the first time documented that during discharge stressing the nanofluid displayed increased endurance of its dielectric strength for 200 continuous AC high voltage discharge events. Nanofluids employed in high voltage applications are based on mineral or natural ester oil.19,24,34−36 Another advantage of the product pertains to the use of a natural ester oil as a matrix instead of mineral oil because of its environmentally friendly properties such as high biodegradability according to IEC 6103937 and nontoxic and aquatic life-friendly nature according to Umweltbundesamt (UBA),38 unlike the mineral oils which are toxic for the environment. The first study with natural ester (vegetable) oil-based nanofluids was performed by Li et al.22 by the addition of commercial Fe3O4 nanoparticles after their surface modification with oleic acid. The latter method was also adopted by Lü et al.39 and Andritsch et al.40 However, these techniques did not ameliorate the agglomeration and phase separation matter. After a short period of a week to one month (depending on the type of NPs and concentration), sedimentation was observed due to aggregation.
2.1. Preparation of Nanofluid with Commercial MIONs Powder (pNF) and Oleic Acid Surface Modification. A standardized technique for surface modification of the commercial Fe2O3 NPs was adopted according to Li et al.22 Commercial MIONs (10 g,
