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Article

Highly Improved Dielectric and Thermal Performance of Polyalphaolefin Oil-Based Fluids Using MgO Nanoparticles

by
Nguyen Van Thanh
1,*,
Nguyen Thi Hong Ngoc
1,
Dang Minh Thuy
1,
Luu Van Tuynh
2,
Ha Huu Son
1,* and
Nguyen Phi Long
1,*
1
Joint Vietnam—Russia Tropical Science and Technology Research Center, Ha Noi 11300, Vietnam
2
Department of Physical and Chemical Engineering, Le Quy Don Technical University, Ha Noi 11900, Vietnam
*
Authors to whom correspondence should be addressed.
Coatings 2023, 13(5), 931; https://doi.org/10.3390/coatings13050931
Submission received: 10 April 2023 / Revised: 30 April 2023 / Accepted: 12 May 2023 / Published: 16 May 2023
(This article belongs to the Special Issue Advances in Oxide Thin Films and Nanostructures)
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Abstract

:
Polyalphaolefin (PAO) oil is widely used as a dielectric liquid due to its outstanding dielectric strength, high flash point, good oxidation resistance, and stability. The dispersion of MgO nanoparticles in PAO yields nanofluids with many properties superior to base oils. This study clarifies the influence of MgO nanoparticles on the dielectric properties (breakdown voltage, volume resistivity, and relative permittivity) and heat transfer properties of PAO/MgO nanofluids. Changes in the concentration and size and the modification of MgO nanoparticles with surfactants change the dielectric and thermal performance of PAO/MgO nanofluids. Using PAO/MgO nanofluids as raw material to prepare dielectric fluid obtains a product with higher dielectric strength and thermal conductivity than those using PAO. The results show that PAO/MgO nanofluid-based dielectric fluid has the potential to be applied as a soft coating to protect electronic equipment in industries.

1. Introduction

Nowadays, dielectric lubricants and greases play a particularly important role in the manufacture of electronic devices, automobiles, wind power, and aerospace. They are used to minimize leakage and discharge [1], avoid heat accumulation, anticorrosion [2], and reduce friction and wear [3,4]. The mandatory specifications of these dielectric lubricants and greases include high dielectric strength, good corrosion resistance, and heat dissipation [1]. Dielectric lubricants and greases are mainly composed of highly-refined base oils and additives. However, the use of anti-corrosion and antioxidant additives significantly reduces the dielectric strength of products because these additives are often polar compounds with high electrical conductivity. Therefore, improving the dielectric and thermal performance of dielectric lubricants and greases is practical and urgent.
Dielectric lubricants and greases are mainly composed of naphthenic and paraffinic base oils, with low viscosity, high dielectric strength, and oxidation stability [1]. Polyalphaolefin (PAO) is preferred as a raw material for obtaining dielectric liquids due to its high purity and medium molecular weight in a narrow range [5]. Therefore, PAO oil ensures dielectric strength, long-term stability, and oxidation resistance during use. In addition, PAO oil is a popular oil with a high heat transfer coefficient and flash point. It is also safe for human health.
The research and application of dielectric liquids have gained important achievements in the last few decades thanks to the breakthroughs in improving the quality of base oils and producing nanofluids [6,7,8]. Nanofluids, especially metal oxide nanofluids, have received great attention from scientists, aiming to increase the heat transfer efficiency of coolants [9,10], improve the dielectric strength of transformer oil [11,12], and increase the wear resistance [13,14] of engine oil. Specifically, metal oxide nanoparticles, when dispersed in base oils, can increase the breakdown voltage and thermal conductivity of the oil [13,15]. It can be said that nanofluids have the potential to become new-generation dielectric liquids.
Among metal oxide nanoparticles, MgO nanoparticles have many outstanding advantages, such as their relatively low cost, non-toxicity, high thermal conductivity, dielectric strength [16,17], corrosion resistance [18], antibacterial activity, outstanding physio-mechanical strength, and heat resistance [19]. Therefore, MgO is widely applied in many fields, such as the manufacture of insulators, wastewater treatment, and manufacture of protective coatings.
MgO nanofluids based on different base fluids, such as water [20,21], ethylene glycol [22], kerosene [23], engine oil [10], and insulating oil [24,25,26], have been studied. MgO nanofluids in ethylene glycol and water are applied to produce coolants [20,27,28]; the dispersion of MgO nanoparticles in engine oil improves thermal conductivity [10]; dispersing MgO nanoparticles in ester oil increases the electrical insulation and heat dissipation of oil [24,25,26]. The influence of MgO nanoparticles on the dielectric and thermal performance of PAO oil and PAO/MgO nanofluid-based dielectric fluid has not been elucidated up to now.
In this work, we studied the influence of MgO nanoparticles on the dielectric properties (breakdown voltage, volume resistivity, and relative permittivity) and heat transfer properties of PAO oil and PAO/MgO nanofluid-based dielectric fluid, aiming to manufacture dielectric lubricant and grease for application as soft coatings to protect electronic devices of the electric automobile, marine, and aviation industries.

2. Materials and Methods

2.1. Materials

The base oil in this study is Synfluid PAO 6 cSt, which was purchased from Chevron Phillips Chemical, The Woodlands, TX, USA. The MgO nanoparticles were obtained from US Research Nanomaterials, Houston, TX, USA. The particle size was between 20 and 50 nm. The specifications of the MgO nanoparticles are given in Table 1. Span 80 was purchased from Sigma-Aldrich (St. Louis, MO, USA) to be used as a surfactant.
The materials used to produce oil-based dielectric fluids included polyisobutylene (PIB), aluminum stearate, corrosion inhibitor (barium sulfonate and benzotriazol), anti-foam additive, and antioxidant additive.

2.2. Preparation of PAO/MgO Nanofluids

MgO nanoparticles modified with Span 80 surfactant (MgO/Sp) are described in the literature [29]. The nanofluid was prepared by dispersing MgO nanopowder at the concentrations of 0, 0.001, 0.002, 0.0025, 0.003, and 0.005 by weight percent in polyalphaolefin oil (PAO). To obtain a nanofluid, the mixtures were stirred for 20 min, then ultra-sonicated using a probe ultrasonicator (VCX 500, Sonics, Newton, CT, USA) with a power of 500 W at a frequency of 20 kHz for 90 min. To remove air bubbles and moisture content, the nanofluids were dried at 80 °C, with a pressure of 100 mbar for 4 h. Then, the next tests were conducted. A schematic diagram of the nanofluid preparation process is shown in Figure 1.

2.3. Preparation of PAO/MgO Nanofluid-based Dielectric Fluid

The PAO-based dielectric fluid with and without MgO nanoparticles had the following components: base oil 52 ÷ 58%; thickener (PIB) 33 ÷ 47%; aluminum stearate 1 ÷ 2%; anti-corrosion additive (barium sulfonate and benzotriazol compound) 4 ÷ 7%; antioxidant (BHT) 0.2 ÷ 0.5%; anti-foam additive (HiTEC 2030) 1 ÷ 2%. The dielectric fluid preparation process (Figure 2), with a scale of 500 g/batch, was as follows: The PAO/MgO nanofluid, thickener, and antioxidant were weighed, placed in a beaker, and then magnetic stirred at 400 rpm and heated to 130 °C. The mixture was kept at 130 °C for 10 min to form a homogeneous mixture. Then the mixture was cooled to 70 °C, anti-corrosion additive and anti-foam additive were added to the mixture, and it was stirred for 2 h until homogeneous. The mixture was then cooled to room temperature. PAO/MgO nanofluids with different contents of nanoparticles were prepared in accordance with Section 2.2.

2.4. Determination of Breakdown Voltage

The AC breakdown voltage was measured with a breakdown voltage oil tester (OLS-90, DTE, DneproEnergoTechnologiya, Dnipro, Ukraine) according to the IEC60156 standard. The electrode configuration consisted of two mushroom-shaped electrodes with a 2.5 mm gap distance. The measurement was carried out under the conditions of ambient humidity of less than 55% RH and a temperature of 22 ÷ 25 °C.

2.5. Measurements of Volume Resistivity, Dielectric Loss, and Relative Permittivity

Measurements of the volume resistivity, dielectric loss, and relative permittivity were performed on an insulating oil dielectric loss tester (HTYJS-H, Huatian, Wuhan, China) according to the IEC 60247:2014 standard. The volume resistivity was measured at a 50 °C, with a DC voltage of 600 V.

2.6. Measurement of Thermal Conductivity of Liquids

Measurements of the thermal conductivity of the PAO base oil, PAO/MgO nanofluids, and dielectric fluids were performed on a Linseis transient hot bridge (THB 500, Linseis, Selb, Germany).

3. Results and Discussion

3.1. Dielectric Properties of PAO/MgO Nanofluids

The results in Figure 3 show that the breakdown voltage of the nano PAO/MgO-20/Sp and PAO/MgO-20 liquids increased when the MgO nanoparticle content reached 0.0025%, and then it decreased. The enhancement of the breakdown voltage of the nanofluid is explained by the rapid increase in the electron shallow trap density [30]. The maximum breakdown voltage value indicates the maximum density of the electron shallow traps formed in the PAO oil. However, when the MgO nanoparticle concentration was higher than 0.0025%, the breakdown voltage of the nano-liquid decreased. This is explained by the surface effect, where the tendency of nanoparticle coagulation increased when reaching the saturation concentration, leading to a decrease in the density of electron shallow traps, or a decrease in the breakdown voltage [31]. The change in the number of electron shallow traps as the MgO nanoparticle content increased in the PAO oil led to the graph of the insulation property characteristics (breakdown voltage, dielectric constant), with the nanoparticle concentration shown as a curve above. When the MgO nanoparticle content in the PAO oil reached the optimum concentration of 0.0025%, the breakdown voltage of the nano PAO/MgO-20 liquid increased by 19.33% (MgO-20/Sp) and 14.66% (MgO-20), and the dielectric constant decreased by 1.67 times (MgO-20/Sp) and 1.31 times (MgO-20) compared to the initial PAO oil.
The increase in the breakdown voltage of the PAO/MgO-20 nanofluid is explained by the fact that the free electrons in the PAO oil near the positive electrode were captured by nanoparticles, forming negative ions, and thereby creating an additional electric field near the positive electrode and preventing discharge through this electrode [32]. In addition, the improvement of the breakdown voltage of the PAO oil in the presence of the MgO-20 nanoparticles is explained by the effect of the electron shallow trap density [30]. According to the electron shallow trap density effect, when there are nanoparticles present in the oil, the ability to capture free electrons increases compared to the base oil. This process will gradually lose the kinetic energy of the free electrons and reduce their mobility. As the nanoparticle concentration increases, the effect of the electron shallow trap density also increases, causing the breakdown voltage of the nanofluid to increase. However, when the concentration of the MgO nanoparticles increases to a certain value, the collision between the nanoparticles increases, increasing the agglomeration and sedimentation of the dispersion system, thereby reducing the breakdown voltage of the system. The results show that the decrease in the relative permittivity of the PAO/MgO-20 nanofluids when increasing the concentration of MgO-20 nanoparticles further confirms the effect of the electron shallow trap density, which reduced the movement of the free electrons, thereby decreasing the electrical conductivity of the liquids.
The results also indicate that the PAO/MgO-20/Sp nanofluids obtained by surface modification of the MgO-20 nanoparticles by the Span 80 surfactant had higher dielectric strength than the PAO/MgO-20 nanofluids, which were not surface-modified (Figure 3). It is easy to see that at the 0.002% and 0.0025% concentrations of MgO nanoparticles, the breakdown voltage of the PAO/MgO-20/Sp nanofluids was higher than that of PAO/MgO-20, by 5.26% and 4.07%, respectively. The increase in the breakdown voltage of the MgO-20 nanofluids when surface-modified with Span 80 is explained by the fact that the surfactant modified the charge on the nanoparticles, increased their ability to capture free electrons, and increased the effect of the electron shallow trap density in the PAO oil [33].
The results of measuring the volume resistivity of the PAO/MgO-20 nanofluids (Figure 4) reflect the resistance to the movement of charge carriers in the nanofluids. When the MgO-20 nanoparticles were dispersed in the PAO oil, the effect of the electron shallow trap density made it more difficult for the electrons to move and increased the volume resistivity of the liquids. For example, when the concentration of MgO-20 nanoparticles was 0.0025%, the volume resistivity was 3.76 times (MgO-20/Sp) and 1.598 times (MgO-20) higher than the original PAO oil. At the same concentration of 0.0025% of MgO-20 nanoparticles, the volume resistivity of the PAO/MgO-20/Sp nanofluids was 2.37 times higher than that of the PAO/MgO-20 nanofluids.
The results show the influence of the MgO nanoparticle size on the breakdown voltage of the PAO/MgO nanofluids (Figure 5). For the modified or unmodified MgO nanoparticles, at a small concentration of 0–0.002%, the size of the MgO nanoparticles was directly proportional to the breakdown voltage of the PAO/MgO nanofluids. When the nanoparticle concentration increased to 0.002–0.005%, the nanoparticle size was inversely proportional to the breakdown voltage. This phenomenon is explained as follows: For the small concentration of MgO nanoparticles, the larger the particle size was, the greater the ability to capture free electrons in the nanofluids because the large particles created an electron shallow traps density with higher energy, thereby increasing the breakdown voltage [34]. However, when the nanoparticle concentration increased to a certain value (0.002–0.0025%), the collision of larger nanoparticles accelerated the deposition of the MgO nanoparticles, breaking the dispersion stability of the system, and thereby reducing the breakdown voltage of nanofluids.
For the modified and unmodified PAO/MgO-50 nanofluids (Figure 5), the peak range displacement of the breakdown voltage value was recorded. The displacement of the peak range of the breakdown voltage of the PAO/MgO-50/Sp nanofluids with surface modification (0.002% concentration range) and the PAO/MgO-50 nanofluids without surface modification (0.001% concentration range) shows that the surfactants kept the nanoparticles dispersed better in the PAO oil, and at the same time, created enclosures to limit the collision of the nanoparticles, reducing the deposition of the dispersion system.
The results of previous research show that PAO/MgO-20/Sp nanofluid is obtained by dispersing MgO-20/Sp nanoparticles in PAO oil at a concentration of 0.0025%. The obtained PAO/MgO-20/Sp nanofluid has the best dielectric properties, with a breakdown voltage up to 71.6 kV, relative permittivity of 1.251, and volume resistivity of 4.313.1013 Ωm.

3.2. Thermal Conductivity of PAO/MgO Nanofluids

The results in Figure 6 show that as the ambient temperature increased, the thermal conductivity of the PAO oil and nano PAO/MgO-20/Sp liquid increased, and the thermal conductivity of the nano PAO/MgO-20/Sp liquid was higher than that of the PAO oil at all temperature points within a range of 30–50 °C. Compared to the PAO oil, the thermal conductivity of the nanofluid increased by 1.58 times at 30 °C and 1.38 times at 50 °C. The thermal conductivity of the nano MgO-20 was significantly higher at 48.4 W/m.K [35] compared to the experimental thermal conductivity of the PAO oil at 0.327 W/m.K. Therefore, increasing the concentration of nano MgO-20 in the PAO oil increased the thermal conductivity of the nanofluid. In addition, the surface effect of the 20 nm average-sized MgO nanoparticles also contributed to increasing the thermal conductivity of the nano PAO/MgO-20/Sp liquid. As the temperature increased, the collision density between the MgO nanoparticles in the PAO oil increased, increasing the heat transfer ability due to enhanced Brownian motion between the particles and reducing the aggregation of the nanoparticles caused by a decrease in the surface energy [36,37,38].
The results in Figure 6 show that the thermal conductivity of the PAO oil and nano PAO/MgO-20/Sp liquid increased rapidly when the temperature reached 40 °C. This can be explained by the fact that the PAO oil studied has a relatively large average molecular weight, with a dynamic viscosity of 30.8 cSt at 40 °C. The rapid increase in the thermal conductivity at 40 °C can be explained by the nature of the hydrocarbon molecules in the PAO oil. At temperatures between 30 and 40 °C, the hydrocarbon molecules in the PAO oil began to move limitedly, but the free volume did not increase significantly. When the temperature reached 40–50 °C, the hydrocarbon molecular chains began to move faster, the distance between the molecules increased, the interaction force between the molecules decreased, and the viscosity decreases; in other words, the atomic flexibility increased [38]. Therefore, the thermal conductivity started to increase rapidly at this temperature.

3.3. Dielectric Properties of PAO/MgO Nanofluid-based Dielectric Fluid

PAO/MgO nanofluid is used to improve the dielectric properties of PAO-based dielectric fluids to manufacture dielectric lubricants, greases, and protective coatings for electronic devices (MIL-PRF-81309G standard). PAO/MgO-20/Sp nanofluid was added to the composition of dielectric fluids in accordance with Section 2.3.
The research results of the breakdown voltage (BDV) and relative permittivity (RP) of dielectric fluids based on PAO/MgO-20/Sp nanofluid (with the MgO-20/Sp concentration varying from 0 to 0.005 wt% compared to PAO) are presented in Figure 7. The results show that when dispersing PAO/MgO-20/Sp nanofluid into the mixture of many components to make dielectric fluid, the nanofluid effect increased the dielectric properties (breakdown voltage and relative permittivity) of the fluids. The breakdown voltage of the fluid increased by 15.44% when the concentration of MgO-20/Sp nanoparticles in the PAO oil reached 0.0025% compared to the fluid using only PAO oil. The relative permittivity of the dielectric fluid decreased from 3.339 to 2.133 when the concentration of MgO-20/Sp reached 0.0025%. This result opens a new direction for the application of nanofluids in improving the dielectric properties of materials, especially the dielectric lubricants, greases, and protective coatings for electronic devices in electric vehicle manufacturing and aerospace.

3.4. Thermal Conductivity of PAO/MgO Nanofluid-Based Dielectric Fluid

For the dielectric fluids to protect electrical equipment and electronic components, the heat transfer performance of protective materials is very important, directly affecting the lifespan of electronic devices. The research results of the influence of MgO-20/Sp nanoparticles on the thermal conductivity of dielectric fluids (DF) (Figure 8) show that the thermal conductivity of the PAO/MgO-20/Sp nanofluid-based dielectric fluid (DF(MgO-20/Sp)) was higher than that of the dielectric fluids that did not contain MgO nanoparticles (DF). Thus, using MgO-20/Sp nanoparticles in dielectric fluids increased the thermal conductivity of materials.

4. Conclusions

The results show that the concentration, size, and surface modification of MgO nanoparticles significantly impacted the dielectric strength and thermal conductivity of PAO-based dielectric fluids. The dielectric fluids prepared by using nanofluid with 0.0025% MgO-20 nanoparticles surface-modified with Span 80 (MgO-20/Sp) in PAO had the best dielectric and thermal performance. Specifically, the breakdown voltage (78.5 kV) increased by 15.44%, the relative permittivity (2.133) decreased by 1.57 times, and the thermal conductivity (0.301 W/mK) increased by 1.32 times at 50 °C, compared to the dielectric fluids without MgO nanoparticles. The aforementioned results are the basis for orienting the application of PAO/MgO-20/Sp nanofluid-based dielectric fluids as soft coatings to protect electrical and electronic equipment as required by the MIL-PRF-81309G standard.

Author Contributions

Conceptualization, N.V.T., H.H.S. and N.P.L.; methodology, H.H.S. and D.M.T.; formal analysis and investigation, N.T.H.N., L.V.T., H.H.S. and N.P.L.; writing—original draft preparation, N.V.T.; writing—review and editing, N.P.L., H.H.S. and N.V.T. All authors have read and agreed to the published version of the manuscript.

Funding

This article is supported by Ministry of Science and Technology of Vietnam, project number DTĐL.CN-73/21-C.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

The data presented in this study are available in the article.

Conflicts of Interest

The authors declare no conflict of interest.

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Figure 1. PAO/MgO nanofluid preparation process.
Figure 1. PAO/MgO nanofluid preparation process.
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Figure 2. The PAO/MgO nanofluid-based dielectric fluid preparation process.
Figure 2. The PAO/MgO nanofluid-based dielectric fluid preparation process.
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Figure 3. Influence of MgO-20 nanoparticles on breakdown voltage and relative permittivity of PAO oil.
Figure 3. Influence of MgO-20 nanoparticles on breakdown voltage and relative permittivity of PAO oil.
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Figure 4. Influence of MgO nanoparticles with a 20 nm size on volume resistivity of PAO oil.
Figure 4. Influence of MgO nanoparticles with a 20 nm size on volume resistivity of PAO oil.
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Figure 5. Influence of MgO nanoparticle size on the breakdown voltage of PAO oil.
Figure 5. Influence of MgO nanoparticle size on the breakdown voltage of PAO oil.
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Figure 6. Influence of MgO-20/Sp nanoparticles on thermal conductivity of PAO oil.
Figure 6. Influence of MgO-20/Sp nanoparticles on thermal conductivity of PAO oil.
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Figure 7. Influence of MgO nanoparticles on breakdown voltage (BDV) and relative permittivity (RP) of dielectric fluids.
Figure 7. Influence of MgO nanoparticles on breakdown voltage (BDV) and relative permittivity (RP) of dielectric fluids.
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Figure 8. Influence of MgO-20/Sp nanoparticles on thermal conductivity of dielectric fluids.
Figure 8. Influence of MgO-20/Sp nanoparticles on thermal conductivity of dielectric fluids.
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Table 1. Specifications of MgO nanoparticles.
Table 1. Specifications of MgO nanoparticles.
Nanoparticle TypePurity, %Surface Area, m2/gDensity, g/m3Ca, ppmK, ppmNa, ppm
MgO-20 (av. size 20 nm)>99>603.589602281600
MgO-50 (av. size 50 nm)>99.9520 ÷ 503.5816389228
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Thanh, N.V.; Ngoc, N.T.H.; Thuy, D.M.; Tuynh, L.V.; Son, H.H.; Long, N.P. Highly Improved Dielectric and Thermal Performance of Polyalphaolefin Oil-Based Fluids Using MgO Nanoparticles. Coatings 2023, 13, 931. https://doi.org/10.3390/coatings13050931

AMA Style

Thanh NV, Ngoc NTH, Thuy DM, Tuynh LV, Son HH, Long NP. Highly Improved Dielectric and Thermal Performance of Polyalphaolefin Oil-Based Fluids Using MgO Nanoparticles. Coatings. 2023; 13(5):931. https://doi.org/10.3390/coatings13050931

Chicago/Turabian Style

Thanh, Nguyen Van, Nguyen Thi Hong Ngoc, Dang Minh Thuy, Luu Van Tuynh, Ha Huu Son, and Nguyen Phi Long. 2023. "Highly Improved Dielectric and Thermal Performance of Polyalphaolefin Oil-Based Fluids Using MgO Nanoparticles" Coatings 13, no. 5: 931. https://doi.org/10.3390/coatings13050931

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