Insulating oil modified by nanoparticles (often called nanofluids) has recently drawn considerable attention, especially concerning the improvement of electrical breakdown and thermal conductivity of the nanofluids. In this paper, three sized monodisperse Fe3O4 nanoparticles were prepared and subsequently dispersed into insulating vegetable oil to achieve nanofluids. The dispersion stability of nanoparticles in nanofluids was examined by natural sedimentation and zeta potential measurement. The electrical breakdown strength, space charge distribution, and several dielectric characteristics, for example, permittivity, dielectric loss, and volume resistivity of these nanofluids, were comparatively investigated. Experimental results show that the monodisperse Fe3O4 nanoparticles not only enhance the dielectric strength but also uniform the electric field of the nanofluids. The depth of electrical potential well of insulating vegetable oils and nanofluids were analyzed to clarify the influence of nanoparticles on electron trapping and on insulation improvement of the vegetable oil.
Recent research works have shown that conductive and semiconductive nanoparticles can be dispersed in mineral and vegetable insulating oils to enhance dielectric strength or thermal conductivity of the insulating oils. Magnetic Fe3O4 nanoparticles have been proved of increasing the AC breakdown voltage of insulating oils for the case that the nanoparticles were well dispersed in oil [
It is already clarified that the dielectric performance of nanofluids is critically determined by nanoparticles’ size [
Our work aims to explore how the size of monodisperse nanoparticles generates various dielectric performances of nanofluids. The insulating vegetable oil-based nanofluids were prepared by adding three sized monodisperse Fe3O4 nanoparticles. Their basic physical and chemical properties are first presented briefly. The dispersion stability of nanofluids was determined by comparison of zeta potential measurement. Next, the different dielectric properties of nanofluids are presented and discussed. The space charge distribution and electrical potential well depth were analyzed for the explanation of different breakdown voltages between the nanofluids and insulating vegetable oils.
All reagents used in the experiment were of analytical grade without further purification.
The insulating vegetable oil-based nanofluids are obtained via three main procedures: preparation of the iron oleate precursor, preparation of the monodisperse Fe3O4 nanoparticle, and synthesis of the nanofluids.
6.48 g of iron (III) chloride hexahydrate was dissolved in the mixture of 48 mL ethanol and 84 mL N-hexane. The obtained solution was slowly added by 21.9 g sodium-oleate-vigorous with magnetic string at 60°C for 12 h. The precipitated iron oleate was washed twice with methanol and was redissolved in hexane afterwards. The solution was additionally washed three times with warm (~60°C) deionized water in a separatory funnel and subsequently dried in vacuum at 80°C for 24 h.
2.1 g iron oleate precursor and 0.64 mL oleic acid were mixed in 10 mL octadecene followed by transferring into a three-neck-round-bottom flask and drying at 120°C for 30 min under nitrogen protection to remove water and oxygen. Then, the resulting mixture was heated to 320°C with 12 h, 24 h, and 48 h, respectively, to realize Fe3O4 nanoparticles with varied sizes. After cooling down to room temperature, the nanoparticles were subsequently centrifuged and washed several times with ethanol and cyclohexane before drying in air at 70°C.
The three Fe3O4 nanoparticles obtained by different reaction time were dispersed in the insulating vegetable oil through ultrasonic treatment. They were tagged by samples A, B, and C, respectively. The FR3 natural ester was used as received [
Basic physical and chemical properties of the FR3 and the insulating vegetable oil-based nanofluids.
Parameter | FR3 | Nanofluids |
---|---|---|
Appearance | Light yellow | Dark yellow |
Density (kg·m−3, 20°C) | 0.90 | 0.90 |
Kinematic viscosity (mm2·s−1, 40°C) | 43.0 | 44.0 |
Pour point (°C) | −18 | −18 |
Flash point (°C) | 325 | 325 |
Acid value (mg·KOH·g−1) | 0.03 | 0.03 |
Interfacial tension (mN/m) | 30 | 30 |
The X-ray diffraction (XRD) pattern was obtained by using a powder X-ray diffraction meter equipped with a rotating anode and a Cu-K
XRD patterns of pure Fe3O4 and Fe3O4 nanoparticle at different reaction time: (A) 12 hours, (B) 24 hours, and (C) 48 hours.
The morphologies of the three sized nanoparticles were observed by transmission electron microscopy (TEM), as shown in Figure
TEM images of monodisperse Fe3O4 nanoparticles at different reaction times (a) 12 hours, (b) 24 hours, and (c) 48 hours and the high resolution TEM image of monodisperse Fe3O4 (top right).
The average size of the Fe3O4 crystal in Figure
Natural sedimentation is an indicator of the dispersion stability for nanoparticles in nanofluids. Three sized Fe3O4 nanoparticles were dispersed in the vegetable oil, to realize nanofluids that are recognized as 1, 2, and 3 to examine their storage-time dependent dispersion stability. As shown in Figure
(a) FR3 and (b) vegetable oil-based nanofluids.
The zeta potential measurement is another method to evaluate stability of the nanofluids. As the stabilization theory [
Zeta potential and associated suspension stability.
|
Stability |
---|---|
0 | Little or no stability |
15 | Some stability but settling lightly |
30 | Moderate stability |
45 | Good stability, possible settling |
60 | Very good stability, little settling likely |
The zeta potential for all the three nanofluids 1, 2, and 3 display values above 30 mV, that is, 74.0, 60.7, and 47.4 mV, respectively. It has been well accepted that a zeta potential which is great than 30 mV should mark a sufficiently good dispersion stability [
The absolute moisture content of all nanofluids was controlled at a value below 200 mg/kg. The AC breakdown voltages of each nanofluid were characterized in accordance with IEC 60156 [
The lightning-impulse breakdown voltages for nanofluids were obtained by means of a configuration consisting of a container and an electrode in Figure
Sketch of electrode setup and oil vessel for lightning-impulse breakdown experiments on insulating oils.
Figure
Influence of contents and sizes of Fe3O4 nanoparticles on the AC breakdown voltage of the vegetable oil-based nanofluids.
The lightning-impulse breakdown voltages of the three nanofluids and FR3 oil for both polarities are summarized in Figures
Influence of contents and size of nanoparticles on the positive lightning breakdown voltage of the vegetable oil-based nanofluids.
Influence of contents and size of nanoparticles on the negative lightning breakdown voltage of the vegetable oil-based nanofluids.
From the results in Figures
The frequency dependences of permittivity between 10−2 and 106 Hz for FR3 oil and the three nanofluids are summarized in Figure
Variation of the relative permittivity of vegetable oil modified by Fe3O4 nanoparticles with different sizes at different frequencies.
It is obvious that the permittivity values of nanofluids varied with the size of nanoparticles. Considerable results have confirmed that smaller sized nanoparticles usually possess higher permittivity than that of larger ones. The reason can be found from an internal-stress model which has been introduced by Buessem et al. [
The dielectric loss of nanofluids and FR3 oil shows a decrease with increasing frequency as the results shown in Figure
Variation of the dielectric loss of vegetable oil modified by Fe3O4 nanoparticles with different sizes at different frequencies.
The loss of a fluid is composed of two contributions, that is, conductance loss and polarization loss. Vegetable insulating oil (e.g., FR3 oil in this work) is a weak polar liquid dielectric, which implies that the conductance loss dominants at low frequency [
Figure
Variation of the volume resistivity of vegetable oil modified by Fe3O4 nanoparticles with different sizes at different frequencies.
Nanoparticles dispersed in nanofluids are polarized when the nanofluids are subjected to an externally applied electric field [
In order to investigate the influence of sizes of monodisperse Fe3O4 nanoparticle on charge carriers transport characteristics of the vegetable oil and vegetable oil-based nanofluids, the pulse electroacoustic (PEA) tests were carried out to investigate the space charge density of all samples, stressed 15 kV/mm for varying time (0.5, 1, 5, 10, and 30 min). As shown in Figure
Charge density of the samples vegetable oil modified by Fe3O4 nanoparticles with different sizes.
The similar results also can be found in [
To understand why the breakdown voltage of nanomodified insulating vegetable oil is higher than that of pure vegetable oil, a dipole model of nanoparticles in [
The spherical nanoparticles (diameter
Diagram depicting the dipole surface charge density,
As the direction of
Figure
Dependence on size of nanoparticle induced trapped depth.
In Figure
The nanofluids were developed by dispersing Fe3O4 nanoparticles with different sizes in insulating vegetable oil to enhance its breakdown strength. For negative lightning-impulse the nanofluids provide insignificant effects, but for the AC profile or for the positive lightning-impulse the attained improvement is significant. Meanwhile, the increasing nanoparticle size will improve the breakdown performance of nanofluids. With increasing the nanoparticle size, the volume resistivities of the nanofluids are almost equal and their dissipation factors increase at frequencies below 0.1 Hz. The relative permittivities of the nanofluids are greater than that of the FR3 oil between 10−2 and 107 Hz, probably because of the much higher relative permittivity of the Fe3O4 nanoparticles. The addition of monodisperse Fe3O4 nanoparticle into vegetable insulating oil will increase the electrical potential well depth and nanoparticle size could significantly influence the electrical potential well depth. The increased electrical potential well depth could enhance the capability of breakdown performance of nanofluids.
The authors declare that they have no financial or personal relationship with any people or any organization that may inappropriately influence their work and there is no professional or commercial interest of any kind in all of the commercial entities mentioned in our paper.
The authors acknowledge the National Science Foundation of China (nos. 51321063 and 51377176) and the 111 Project of the Ministry of Education, China (B08036). A visiting scholarship provided by the State Key Lab of Power Equipment & System Security and New Technology at Chongqing University (no. 2007DA10512713408) and the Doctoral Program of Higher Education of China (SRFDP) (20110191110017) are also appreciated for supporting this work.