Effect of Dispersion Method on Stability and Dielectric Strength of Transformer Oil-Based TiO2 Nanofluids
© The Author(s). 2016
Received: 4 July 2016
Accepted: 17 November 2016
Published: 24 November 2016
Dispersion stability of nanoparticles in the liquid media is of great importance to the utilization in practice. This study aims to investigate the effects of mechanical dispersion method on the dispersibility of functionalized TiO2 nanoparticles in the transformer oil. Dispersion methods, including stirring, ultrasonic bath, and probe processes, were systematically tested to verify their versatility for preparing stable nanofluid. The test results reveal that the combination of ultrasonic bath process and stirring method has the best dispersion efficiency and the obtained nanofluid possesses the highest AC breakdown strength. Specifically, after aging for 168 h, the size of nanoparticles in the nanofluid prepared by the combination method has no obvious change, while those obtained by the other three paths are increased obviously.
Nanofluids are a new type of engineering materials by dispersing nanoparticles into the base fluid, which have received considerable attention for many years due to their superior thermal and dielectric properties [1–4]. Transformer oil, as a cooling and insulating medium, is a major part of the electrical insulation system in many types of electrical equipment, such as transformers, cables, and bushings. The dielectric strength and thermal conductivity of transformer oil are of great importance to keep power transformers operating safely and optimize their structure design. Recently, it has been found that the presence of nanoparticles can greatly improve thermal conductivity and breakdown strength of transformer oil [5–12]. However, the nanoparticles tend to aggregate into bigger particles mainly by the attractive forces and external stresses [11–13], leading to the performance degradation of nanofluids. So, the long-term stability of nanoparticles dispersion in the host oil is still a key challenge in this field.
Much work has been done to improve the dispersion stability of the nanoparticles in the base fluid [14–16]. In comparison with mechanical method, surface functionalization has been proved to be a more useful approach to control the balance of Van der Waals attraction and electrostatic repulsion between nanoparticles through surface modification of nanoparticles [17, 18]. Dispersion stability of iron oxide nanoparticles in mineral oil was greatly improved by optimizing their surface functionalization state and the nanofluid had no visible sedimentation after aging for 24 months at room temperature . The dispersion of TiO2 nanoparticles in the mineral oil can also be improved by adjusting the usage of modifying agents to functionalize the nanoparticles . Meanwhile, the adsorption of functional groups on the surface of nanoparticles could be influenced by many factors, including temperature, the type of base liquid, and the interaction between functional group and nanoparticle. Although the mechanical dispersion process can provide energy to overcome the adhesion force between nanoparticles, it may affect the interaction between functional group and nanoparticle at the same time. However, no other studies have been found directly point out the effect of the mechanical dispersion method on the stability and breakdown strength of transformer oil-based nanofluids modified by functionalized nanoparticles.
In this paper, TiO2 nanoparticles functionalized by oleic acid were synthesized by a solvothermal method. Three kinds of mechanical dispersion methods were employed to prepare transformer oil-based TiO2 nanofluids. The dispersion stability, AC breakdown strength, and thermos-physical property of obtained nanofluids were measured and compared.
Nanoparticle Synthesis and Functionalization
TiO2 nanoparticles were prepared and functionalized by using titanium n-butoxide and DI water as reactants by a solvothermal method. In a typical procedure, reactants were first introduced into a mixed solution of cyclohexane and triethylamine under stirring. After stirring for 5 min, oleic acid was added into the above solution at room temperature with vigorous agitation. The resulting mixture was subsequently heated to the temperature of 150 °C. After heating for 24 h, the resulting product was cooled down naturally and washed with distilled water and absolute ethanol for several times to remove the ions possibly remaining in the product and finally dried in the vacuum at 70 °C.
The morphology of the as-prepared nanoparticles was characterized by high-resolution transmission electron microscopy (HRTEM: JEM-2100F). Fourier transform infrared spectra (FT-IR) was used to analyze the surface functionalization state of TiO2 nanoparticles scanned from 400 to 4000 cm-1 with a resolution of 4 cm-1. A dynamic light scattering device (Malvern Nano ZS90) was used to determine the average size of nanoparticles in the fresh and aged nanofluids. The polydispersity index (PdI) describes the width of the particle size distribution. The viscosity of pure oil and nanofluid was measured at the temperature of 29 °C with the rotational viscometer Brookfield DVII, and their thermal conductivity was characterized by a Netzsch LFA447 tester. A portable Jian-tong Oil Tester 6801 was used to measure AC breakdown voltages of pure oil and nanofluids according to IEC standard 60156 using brass spherically capped electrodes set at 2 mm gap.
Results and Discussion
Morphology and Surface Functionalization of Nanoparticles
The C=O stretch from oleic acid is replaced by the appearance of two new peaks at 1554 and 1431 cm-1, which correspond to the asymmetric and symmetric carboxylate (–COO–) stretching modes [23, 24]. Studies have shown that these two peaks could be utilized to predict the types of binding interaction between the carboxylate head and metal oxide surface . Depending on the wave number separation between asymmetric and symmetric peaks, it is indicated that the functional group of oleic acid is covalently bonded with the titanium sites at the nanoparticles surface mainly by bidentate linkages [17, 26]. These results confirm that the carboxylate group is chemically bonded with the surface titanium ion, and the as-prepared TiO2 nanoparticles are well functionalized by oleic acid.
Dispersion Stability of Nanofluids
For the nanofluid prepared by stirring for 10 min, the nanoparticle size is abruptly enlarged from 30.63 to 72.52 nm with the prolonging of aging time and then achieves a constant value of 81.24 nm after aging for 192 h. With the increasing of stirring time from 30 to 180 min, the stability of as-prepared nanofluids is greatly improved. The size of nanoparticles in nanofluid stirring for 180 min is smaller than those in other nanofluids, which is in the range from 18.3 to 20.0 nm during the aging time of 336 h. In addition, the PdI of particles in this nanofluid is 0.16, indicating the uniform size distribution of nanoparticles. These test results demonstrate that for the functionalized TiO2 nanoparticles, the shear force at a high agitation speed can decrease the tendency of particle agglomeration and the nanofluid with a good dispersion stability can be observed by stirring for 180 min.
Thermal conductivity and viscosity for the transformer oil and nanofluid
Viscosity (29 °C)
Enhancement ratio (%)
This study investigated the dispersion stability of functionalized TiO2 nanoparticles in the transformer oil-based nanofluids, their AC breakdown strength and thermos-physical property, which were prepared through stirring, ultrasonic bath, and probe processes. The test results show that the dispersion stability of functionalized nanoparticles is clearly dependent on the dispersion method. The stirring and ultrasonic bath processes exhibit better dispersion efficiency than the ultrasonic probe process, which may disrupt the adsorption balance of functional group on the surface of nanoparticles due to the limitation of the high-intensity sonication energy around the probe tip. The combination method of stirring and ultrasonic bath can effectively reduce the tendency for nanoparticles to agglomerate and prepare the nanofluid with the best dispersion stability and breakdown performance.
The authors would like to thank the National Natural Science Foundation of China for supporting this research under Contract Nos. 51337003, 51472084, and 51477052 and the Fundamental Research Funds for the Central Universities (JB2015019).
The experiments were guided by YL. CL and QS prepared the TiO2 nanofluids. CL and MH characterized the nanofluids. RL and BQ participated in the discussion and gave valuable suggestions. The manuscript was composed by YL. All authors approved the final manuscript.
The authors declare that they have no competing interests.
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