α-alumina nanocomposite with high electrical insulation performance

α-alumina nanocomposite with high electrical insulation performance

Progress in Natural Science: Materials International xxx (xxxx) xxx–xxx HOSTED BY Contents lists available at ScienceDirect Progress in Natural Sci...

2MB Sizes 2 Downloads 11 Views

Progress in Natural Science: Materials International xxx (xxxx) xxx–xxx

HOSTED BY

Contents lists available at ScienceDirect

Progress in Natural Science: Materials International journal homepage: www.elsevier.com/locate/pnsmi

Original Research

Epoxy/α-alumina nanocomposite with high electrical insulation performance Yun Chena,b, Donghai Zhanga, Xiaofeng Wua, Haosheng Wanga, Chong Zhangb, Wei Yangb, ⁎ Yunfa Chena, a b

State Key Laboratory of Multi-phase Complex Systems, Institute of Process Engineering, Chinese Academy of Sciences, Beijing 100190, China State Key Laboratory of Advanced Transmission Technology, Global Energy Interconnection Research Institute, Beijing 102211, China

A R T I C L E I N F O

A B S T R A C T

Keywords: Nanocomposite Epoxy resin Insulation α-alumina

An experimental study was conducted to improve the electrical insulation of epoxy resin. The effects of boehmite, γ-alumina and α-alumina nanoparticles on the volume resistivity, dielectric strength and glass transition temperature of epoxy nanocomposites were investigated. The results showed that α-alumina nanoparticles displayed obvious advantages in enhancing electrical insulation performance of epoxy nanocomposites, compared to boehmite and γ-alumina nanoparticles. The direct current volume resistivity and breakdown strength of epoxy nanocomposite with 2.0 wt% α-alumina nanoparticles was improved to 2.2 × 1018 Ω cm and 76.1 kV mm−1 respectively. And these improved values of electrical insulation properties are much higher than these of epoxy nanocomposites reported in previous studies. The main reason of these improvements may be that the epoxy/α-alumina interaction zone was enhanced by crosslink.

1. Introduction Epoxy composites are preferred insulating materials and widely used for their high cohesiveness, superior dielectric property, small shrinkage, good chemical stability and easy processing [1,2]. However, conventional epoxy based materials fail for electrical insulation application in ultra-high voltage direct current transmission such as gas-insulated metal-enclosed transmission line (GIL) or gas-insulated metalenclosed switchgear (GIS), since many defects have also been lead into epoxy composite system while introducing microfillers. Recently, much research focuses on polymer nanocomposite applications in electrical insulation, and the incorporation of nanoparticles demonstrates combined progressive electrical, mechanical and thermal improvements over conventional microfiller systems. Polymer nanocomposites based on nanoclay [3–6], nano-fibers [7–10], metal particles [11–13] and metal oxide nanoparticles [14–16] with even a small fraction of inorganic nanofillers may improve their performance, such as mechanical properties, electromechanical effect [17], magneto elasticity [18], gas impermeability, thermal stability and flame retardancy. The improvement of the insulating performances such as breakdown strength [1,19–21], volume resistivity [22,23] and electric erosion resistance [24] has also been reported. Tanaka [25] proposed a multi-core model closely related to an “interaction zone” with a

thickness of 10–30 nm between polymer matrix and nanoparticles. The volume of interaction zone of the polymer nanocomposite may be even larger than that of nanoparticles, which affects the electrical and mechanical properties of composites directly. Li and coworkers [26] also believed that there is a potential barrier in the interaction zone which restrains carriers and improves the insulation performance of nanocomposites. Alumina nanoparticles have been widely used as nanofiller in epoxy nanocomposite. However, the recent research mostly focused on boehmite [16] or γ-alumina [1,15,27] which may not stable in high electric field or other tough environments. And most of these particles need to be surface-modified to improve the volume resistivity, breakdown strength or other electric performance of epoxy nanocomposite. Even though, the highest volume resistivity of these epoxy nanocomposite reported in the previous studies (see in Table 1) is still lower than 1017 Ω cm, and this volume resistivity is even lower than neat epoxy manufactured by VORATRON™ ER 113 Epoxy Resin and VORATRON™ EH 314 hardener. Moreover the α-alumina nanofiller, which is well known as chemical and mechanical stable, has not been mentioned in previous studies. For this reason, we prepared α-alumina nanofiller without surface treatment as a new type of nanofiller in epoxy nanocomposite. In this study, the epoxy composites modified by boehmite, γ-

Peer review under responsibility of Chinese Materials Research Society. ⁎ Corresponding author. E-mail address: [email protected] (Y. Chen). http://dx.doi.org/10.1016/j.pnsc.2017.09.003 Received 16 February 2017; Received in revised form 12 September 2017; Accepted 13 September 2017 1002-0071/ © 2017 Chinese Materials Research Society. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/BY-NC-ND/4.0/).

Please cite this article as: Chen, Y., Progress in Natural Science: Materials International (2017), http://dx.doi.org/10.1016/j.pnsc.2017.09.003

Progress in Natural Science: Materials International xxx (xxxx) xxx–xxx

Y. Chen et al.

2.2. Manufacturing of neat epoxy and epoxy nanocomposites

Table 1 The electrical insulation performances of different epoxy nanocomposites reported in previous studies. Nanoparticles

Highest resistivity/ Ω cm

Surface-treated γ-Al2O3 [1] Boehmite [16] Surface-treated SiO2 [20] Polyhedral oligosilsesquioxane functionalized BN nanotubes [28] 1-pyrenebutyric acid functionalized BN nanotubes and BN nanosheets [29] ZrW2O8 [30] Surface-treated ZnO [31] Surface-treated AlN [32] Carbon black [33] Ag [11] Al [34]

~ ~ ~ ~

3 × 1016 3 × 1010 3 × 1015 1013

9.3 × 1016

~ ~ ~ ~ ~

1010 2 × 1016 1013 108 5 × 1010

The weight ratio of mresin:mhardener = 100:52.5 has been used for this study. For manufacturing epoxy nanocomposites, the epoxy resin was mixed with nanofillers and stirred at the rate of 1000 rpm in a flask for 2 h at 135 °C in vacuum, and then the hardener was added in the epoxy/nanofillers mixture and stirred at the rate of 600 rpm for 15 min at 135 °C in vacuum. The concentration of nanofillers in the epoxy/ hardener mixture is 2.0 wt%. (As a result of our former research, the reason of choosing 2.0 wt% is that the epoxy/α-alumina nanocomposite with this concentration of nanofillers shows the highest DC volume resistivity at room temperature.) The neat epoxy/hardener mixture without nanofillers being added was also made in the same process as a comparison. The test specimens were cast by pouring the epoxy/hardener mixture or epoxy/hardener/nanofillers mixture into stainless steel molds. The specimens were then pre-cured at 115 °C for 15 h and finally post-cured at 150 °C for 18 h.

Highest breakdown strength/kV mm−1 32.83 – 50.8 –



~ 27 – – – – –

2.3. Characterization of nanoparticles The particle size distribution was characterized using a Beckerman Coulter LS 13 320 laser diffraction particle size analyzer. Field emission scanning electron microscope (FE-SEM, Hitachi SU8000) was used to observe the morphology and microstructure of the alumina nanoparticles. X-ray diffraction (XRD) patterns of the different nanoparticles were obtained using Rigaku D/Max-RB automatic diffractometer with nickelfiltered CuKα (λ = 1.5418 Å) radiation at 40 kV and 100 mA. The solid-state Al27 nuclear magnetic resonance (NMR) experiments of different nanoparticles were performed with Bruker Avance III 400 spectrometer operating at a frequency of 130.4 MHz for Al27 using a 4 mm o.d. Bruker cross polarization/magic angle spinning (CPMAS) probe with a spinning frequency of 12.5 kHz. The chemical shifts of Al27 were referenced using 1 M AlCl3 solution as an external reference (0 ppm). The X-ray photoelectron spectroscopy (XPS) measurements are made using a SHIMADZU/KRATOS X-ray photoelectron spectrometer with a monochromatic Al Ka source (1486.6 eV) to detect the O 1s signal of different nanoparticles.

Fig. 1. Particle size distribution of alumina nanoparticles.

alumina and α-alumina nanoparticles were manufactured respectively. The electrical insulation properties of the composites were characterized to evaluate the effectiveness of nanoparticles in these applications.

2.4. Performances of epoxy nanocomposites The DC volume resistivity was measured using a direct-reading electrometer (Keithley 6517B) with a test fixture (Keithley 8009) under 1000 V DC voltage at ambient temperature. The specimens were round shape with diameter of 70 mm and limited less than 1 mm for thickness. DC breakdown strength at ambient temperature was measured using a dielectric strength tester (BTF Science and technology, China) with a increased voltage at a speed of 1 kV/s. The specimens were tested between two copper rod electrodes with a diameter of 25 mm, and the whole system was carried out in a pure silicone oil to prevent the surface flashover. The dynamic modulus and dynamic loss tangent values of nanocomposites were measured using the dynamic mechanical thermal analysis (Netzsch® DMA 242 D) in a tensile mode from 25 °C to 200 °C at the heating rate of 2 °C/min under the frequency of 1 Hz.

2. Experimental 2.1. Materials VORATRON™ ER 113 Epoxy Resin and VORATRON™ EH 314 hardener were both obtained from Dow Chemical Company. The boehmite nanoparticles were prepared by the microwave reaction method at 2450 MHz for 30 min using surface-activated aluminum and deionized water with a resistivity over 18 MΩ(see Eq. (1)). The suspension was centrifuged and washed through Avanti® J-26 XP Centrifuge (Beckman Coulter, Inc.) at 4000 rpm for 15 min, and the resulted slurry was dried at 125 °C for 10 h. 2450MHzmicrowave ↑ 2Al + 4H2 O⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯→2AlOOH + 3H2 ⏐ ⏐ ⏐

3. Results and discussion

(1)

3.1. Characterization of nanoparticles

Then, the boehmite nanoparticles produced above were reacted with isooctanoic acid in dimethylbenzene at 120 °C to prepare the alumina precursor. The γ-alumina and α-alumina nanoparticles were finally manufactured by annealing the alumina precursor at the rate of 5 °C/min to 1000 °C and 1150 °C for 1 h, respectively.

The particle size distribution of three types of alumina nanoparticles are shown in Fig. 1. The distribution curve shows that the average sizes of these species of nanoparticles were at the same order of magnitude (~ 100 nm). The average size of α-alumina nanoparticles was smaller than that of boehmite and γ-alumina nanoparticles as these 2

Progress in Natural Science: Materials International xxx (xxxx) xxx–xxx

Y. Chen et al.

Fig. 2. SEM image of (a) boehmite, (b) γ-alumina and (c) α-alumina nanoparticles.

species of α-alumina exhibits the highest content of surface lattice oxygen, which may be due to its high crystallinity and compact crystal structure (all Al atoms are octa-coordinated).

nanoparticles shrank after annealing. The Scanning Electron Microscope (SEM) images of alumina nanoparticles are shown in Fig. 2. It can be observed that all these alumina nanoparticles are irregular particles. XRD spectra of alumina nanoparticles are shown in Fig. 3(a). Patterns of boehmite, γ-alumina and α-alumina are corresponding to JCPDS reference No. 00-049-0133 No. 00-010-0425 and No. 00-0100173, respectively. The wide peaks of boehmite/γ-alumina patterns and sharp peaks of α-alumina patterns indicate that the crystallinity of αalumina nanoparticles is higher than that of boehmite/γ-alumina nanoparticles. Al27 NMR spectra of nanoparticles are presented in Fig. 3(b). All spectra in Fig. 3(b) show resonance peaks (chemical shift ≈ 6 ppm) corresponding to aluminum atoms in coordination 6 (octa-coordinated) [35]. On the other hand, the resonance peak (chemical shift ≈ 63 ppm) corresponding to aluminum atoms in coordination 4 (tetra-coordinated) [35] can only be found from spectra of γ-alumina. The Al27 NMR spectrum of α-alumina indicates that Al in this kind of alumina only occurred in octahedral environments, which matches the report of structure of α-alumina (corundum) [36]. And we may attribute the tetra-coordinated resonance peak (25.05% of total integrated peak area, matches the theoretical proportion) to tetra-Al in the γ-alumina structure [37,38]. The O 1s XPS spectra of alumina nanoparticles are displayed in Fig. 3(c). As shown in Fig. 3(c), the spectra of O 1s XPS signal consists of three different chemical shifted components which can be de-convoluted into the surface lattice oxygen (Olatt), the surface adsorbed oxygen (Oads) and the surface OH groups or adsorbed molecular water (O-OH), respectively [39–41]. The values of their binding energy obey the order of Olatt < Oads < O-OH. The surface oxygen compositions of these alumina nanoparticles are shown in Table 2. The OH groups in boehmite (AlOOH) structure may increase both the content of adsorbed oxygen (as a result of hydrogen bonding) and surface OH groups. The

3.2. Electrical performance 3.2.1. DC volume resistivity The DC volume resistivity (ρv) of neat epoxy and epoxy nanocomposites are shown in Fig. 4. It can be seen that the α-alumina nanofiller is able to increase ρv of epoxy nanocomposite at the temperatures in the range of 30 °C to 80 °C. The highest values of ρv at 30 °C and 80 °C are 2.2 × 1018 Ω cm and 4.6 × 1017 Ω cm, respectively. The boehmite nanofiller may also increase the ρv of epoxy resin material slightly at the temperatures below 40 °C, the trend for ρv at 30 and 40 °C is ργ-alumina/epoxy < ρneat epoxy < ρboehmite/epoxy < ρα-alumina/epoxy. However, when the temperature grows up to 50 °C or higher, ρv of epoxy/ boehmite nanocomposite decreases sharply. On the other hand, ρv of epoxy/α-alumina nanocomposite is almost unaffected by temperature. The trend for resistivity at 80 °C becomes ρboehmite/epoxy < ργ-alumina/ epoxy < ρneat epoxy < ρα-alumina/epoxy. To understand this phenomenon, the Arrhenius plots (lnσ ~ 1/T) of neat epoxy and epoxy nanocomposites with 2 wt% of nanoparticles are shown in Fig. 5. It can be found that the temperature dependence of DC conductivities (σDC as the reciprocal of ρv) of neat epoxy, epoxy/γalumina and epoxy/α-alumina nanocomposites follow the Arrhenius equation (Eq. (2)): ΔEa

σDC = σ0 e− RT

(2)

where σ0 is a constant relating to the nature of material, ΔEa is the charge carrier activation energy and R is the ideal gas constant. However, the temperature dependence of σDC of epoxy/boehmite 3

Progress in Natural Science: Materials International xxx (xxxx) xxx–xxx

Y. Chen et al.

Fig. 3. (a) XRD, (b) Al27 NMR and (c) O 1s XPS spectra of alumina nanoparticles.

Table 2 The surface oxygen compositions of these alumina nanoparticles. Species of nanoparticles

Olatt

Oads

O-OH

Boehmite γ-alumina α-alumina

20.4% 69.1% 74.1%

65.7% 30.9% 25.9%

13.9% – –

Fig. 5. Arrhenius plots of ΔEa of neat epoxy and epoxy nanocomposites with 2 wt% of nanoparticles.

The –OH groups on the surface of boehmite nanoparticles may generate a dielectric double layer in the epoxy/boehmite interaction zone which traps charge carriers and increases the volume resistivity of epoxy nanocomposite at temperatures below 40 °C. With the increasing temperature, the charge carriers may hop between neighbor dielectric double layers around neighbor nanoparticles, and the ρv of epoxy/ boehmite nanocomposite decreases sharply above 50 °C [42]. The changing trend for the volume resistivity of neat epoxy and epoxy nanocomposites is very similar to the trend of the glass transition temperatures, especially at 80 °C. This similarity indicates that the variation of volume resistivity may be influenced by the interaction between nanofillers and epoxy matrix [1]. The movement of polymer chains in

Fig. 4. DC volume resistivity (ρv) of neat epoxy and epoxy nanocomposites with 2 wt% of nanoparticles at different temperatures.

nanocomposites shows nonlinear characterization with two liner regions below and above 40 °C. This phenomenon may be attributed to the changing of conducting mechanism. 4

Progress in Natural Science: Materials International xxx (xxxx) xxx–xxx

Y. Chen et al.

Table 3 Weibull breakdown strength (E0) and shape parameter (β) of different Nanocomposites. Nanocomposite

E0/kV mm−1

β

Neat epoxy 2% AlOOH 2% γ-alumina 2% α-alumina

71.3 52.9 55.6 76.1

4.35 7.51 6.72 16.43

Table 4 Tg extracted from DMA and crosslink density at 195 °C calculated from rubber elasticity theory.

Fig. 6. Weibull plots of DC breakdown strength of neat epoxy and nanocomposites with 2 wt% of nanoparticles.

Nanocomposite

Tg

E'r/MPa at 195 °C

υe (mmol/cm3)

Neat epoxy 2% Boehmite 2% γ-alumina 2% α-alumina

132.0 ± 4.5 93.9 ± 2.1 119.4 ± 1.9 147.3 ± 3.7

18.5 ± 1.6 10.8 ± 1.8 15.5 ± 0.3 25.0 ± 0.9

1.58 ± 0.14 0.93 ± 0.16 1.33 ± 0.02 2.14 ± 0.08

the epoxy/alumina interaction zone may be restricted of by nanofillers [25]. On the contrary, the polymer chains in the epoxy/boehmite and epoxy/γ-alumina interaction zone may be loosened.

alumina/epoxy nanocomposite increased. This may also be due to the restriction of molecular chain movements in the α-alumina/epoxy interaction zone.

3.2.2. Breakdown strength Weibull plots were widely used in analyzing the breakdown strength of insulating materials. The two-parameter Weibull distribution formula of breakdown strength is [19,30]:

3.3. Mechanism discussion

PE = 1 − e

−( E ) β E0

Fig. 7 shows the storage modulus (E′) and loss tangent (tanδ) results of epoxy nanocomposites with 2 wt% nanofillers analyzed by dynamic mechanical thermal analysis (DMA). The storage modulus shows sharply decline, and loss tangent reaches the peak when glass transition happens. Tg value in Table 3 was determined by tanδ result, from the peak of tanδ-temperature (T) curve in Fig. 7(b). To give a better insight into the role of boehmite, γ-alumina and αalumina on the network structure of epoxy matrix, the crosslink density in Table 4 was calculated using rubber elasticity theory based on the elastic modulus of specimens at rubbery state obtained from DMA. The model is described as follows [43,44]:

(3)

where E is the electric field when breakdown happens, PE is the cumulative breakdown probability at the electric field of E, E0 is the breakdown strength when PE = (1−e−1) ≈ 63.2% which may also be called Weibull breakdown strength, and β is a parameter that reflects the distribution of experimental breakdown strength data. A higher β value indicates a narrower breakdown strength distribution, which indicates the electric stability of tested sample [1].

lg [− ln(1 − PE )] = β lg E − β lg E0

υe =

(4)

E ′r 3RTr

(5)

where υe is the crosslink density of the epoxy matrix which indicates the quantity of crosslink bonds between neighbor epoxy chains (see in Fig. 8(a)) in a specific volume of epoxy matrix, R is the ideal gas constant, E′r is the storage modulus and Tr is absolute temperature in rubbery region. In this study, Tr is considered to be 195 °C where all samples stands above their Tg. The ρv at 80 °C, the E0 in Table 3 and Tg in Table 4 show almost the same trend of decreasing and increasing, which reveals that there is a correlation between dielectric and dynamics behavior of epoxy

Eq. (3) was transformed into Eq. (4) for liner regression plot. Thus, the slopes of lines in Fig. 6 are equal to β, and the intercepts of lines in Fig. 7 are equal to −βE0. Table 3 shows the DC breakdown strength and β of neat epoxy and nanocomposites with 2 wt% nanofiller. The deviation of the data in Table 3 is not needed as the value of β actually reflects the distribution of breakdown strength data. It can be seen that boehmite and γ-alumina fillers show negative effect on breakdown strength of epoxy resin. At the same time, both breakdown strength and electric stability of α-

Fig. 7. (a) Storage modulus (E') and (b) loss tangent (tanδ) results of epoxy nanocomposites with 2 wt% nanofillers at the testing frequency of 1 Hz.

5

Progress in Natural Science: Materials International xxx (xxxx) xxx–xxx

Y. Chen et al.

Fig. 8. Estimated curing mechanism of (a) neat epoxy, (b) epoxy/boehmite nanocomposite and (c) epoxy/α-alumina nanocomposite.

nanocomposite. A higher υe means that there are more crosslink bonds between neighbor epoxy chains in the epoxy matrix. The highest value of υe indicates that the restriction movement of epoxy chains around αalumina nanoparticles actually results from these extra crosslink bonds. The decrease of the crosslink density may due to the reaction between -OH groups on the surface of boehmite and the anhydride curing agent (Eq. (6), and the anhydride curing agent is actually cyclic). This

nanocomposites. This kind of correlation is also observed by Kyritsis and coworkers [16]. A higher Tg means that molecule chains of epoxy resin need more energy to reform from glass state to rubber state, which indicates that the existence of α-alumina restricts the movement of epoxy molecule chains in the α-alumina/epoxy interaction zone. And this restriction also may block the transporting path of electric charges and make a contribution to the insulation performance of epoxy 6

Progress in Natural Science: Materials International xxx (xxxx) xxx–xxx

Y. Chen et al.

Oads adsorbed on the surface of γ-alumina nanoparticles may also block the crosslink of epoxy matrix in the interaction zone. On the other hand, the α-alumina nanoparticles have the least Oads on their surface and the hydrogen bonding between -OH groups in the epoxy chains and Olatt may enhance the epoxy matrix in the epoxy/α-alumina interaction zone [1,31,43]. This enhancement also brings about more crosslink bonds between neighbor epoxy chains in the interaction zone after the curing process. The α-alumina/epoxy interaction zone becomes stable to avoid water absorption or other negative effect on electric insulation performance. Fig. 8 shows the estimated curing mechanism of neat epoxy and epoxy nanocomposites. Fig. 9 shows the estimated models of two different types of interaction zone. When carriers travel through the epoxy/boehmite nanocomposite, they may be trapped by the dielectric double layer around the boehmite nanoparticles. This trapping mechanism could be changed by the increasing of voltage or temperature (see in Fig. 10). On the other hand, the carriers travelling through the epoxy/α-alumina nanocomposite may always be delayed by the compact polymer zone around the α-alumina nanoparticles.

Fig. 9. Estimated model of interaction zone around (a) boehmite and (b) α-alumina nanoparticles.

reaction may also have built the dielectric double layer in the epoxy/ boehmite interaction zone as the reaction created many ions on the surface of boehmite.

2AlOOH + RCOOOCR′ → (RCOO)AlO + (R′COO)AlO + H2 O

4. Conclusions In order to study the effects of the phase of alumina on the dielectric properties of epoxy nanocomposites, epoxy composites modified by boehmite, γ-alumina and α-alumina nanoparticles were manufactured

(6)

The hydrogen bonding between -OH groups in the epoxy chains and

Fig. 10. Estimated carriers travelling mechanism through epoxy/boehmite nanocomposite.

7

Progress in Natural Science: Materials International xxx (xxxx) xxx–xxx

Y. Chen et al.

2415–2422. [5] R.P. Wang, T. Schuman, R.R. Vuppalapati, K. Chandrashekhara, Green Chem. 16 (2014) 1871–1882. [6] K.S. Triantafyllidis, P.C. LeBaron, T.J. Pinnavaia, Chem. Mater. 14 (2002) 4088–4095. [7] S. Gurusideswar, R. Velmurugan, Mater. Des. 60 (2014) 468–478. [8] D. Kim, I. Chung, G. Kim, Fibers Polym. 14 (2013) 2141–2147. [9] M.E. Hossain, M.K. Hossain, M. Hosur, S. Jeelani, J. Compos. Mater. 48 (2014) 879–896. [10] D.R. Bortz, C. Merino, I. Martin-Gullon, Compos. Sci. Technol. 71 (2011) 31–38. [11] M. Zulkarnain, M. Mariatti, I.A. Azid, J. Mater. Sci. Mater. Electron. 24 (2013) 1523–1529. [12] W.Y. Zhou, D.M. Yu, J. Mater. Sci. 48 (2013) 7960–7968. [13] Z. Wenying, Y. Demei, J. Compos. Mater. 45 (2011) 1981–1989. [14] M. Kobayashi, H. Saito, B. Boury, K. Matsukawa, Y. Sugahara, Appl. Organomet. Chem. 27 (2013) 673–677. [15] B.B. Johnsen, T.R. Fromyr, T. Thorvaldsen, T. Olsen, Compos. Interfaces 20 (2013) 721–740. [16] A. Kyritsis, G. Vikelis, P. Maroulas, P. Pissis, B. Milosheva, R. Kotsilkova, A. Toplijska, C. Silvestre, D. Duraccio, J. Appl. Polym. Sci. 121 (2011) 3613–3627. [17] S. Rudykh, A. Lewinstein, G. Uner, G. deBotton, Appl. Phys. Lett. 102 (2013). [18] E. Galipeau, S. Rudykh, G. deBotton, P.P. Castaneda, Int. J. Solids Struct. 51 (2014) 3012–3024. [19] J.K. Nelson, J.C. Fothergill, Nanotechnology 15 (2004) 586–595. [20] A. Krivda, T. Tanaka, M. Frechette, J. Castellon, D. Fabiani, G.C. Montanari, R. Gorur, P. Morshuis, S. Gubanski, J. Kindersberger, A. Vaughn, S. Pelissou, Y. Tanaka, L.E. Schmidt, G. Iyer, T. Andritsch, J. Seiler, M. Anglhuber, IEEE Electr. Insul. Mag. 28 (2012) 38–51. [21] L.J. Fang, C. Wu, R. Qian, L.Y. Xie, K. Yang, P.K. Jiang, RSC Adv. 4 (2014) 21010–21017. [22] M.G. Veena, N.M. Renukappa, J.M. Raj, C. Ranganathaiah, K.N. Shivakumar, J. Appl. Polym. Sci. 121 (2011) 2752–2760. [23] B. Mazurek, L. Moron, Mater. Sci. 25 (2007) 899–911. [24] P. Maity, S. Basu, V. Parameswaran, N. Gupta, IEEE Trans. Dielectr. Electr. Insul. 15 (2008) 52–61. [25] T. Tanaka, IEEE Trans. Dielectr. Electr. Insul. 12 (2005) 914–928. [26] L. Shengtao, Y. Guilai, B. Suna, L. Jianying, IEEE Trans. Dielectr. Electr. Insul. 18 (2011) 1535–1543. [27] P. Maity, P.K. Poovamma, S. Basu, V. Parameswaran, N. Gupta, IEEE Trans. Dielectr. Electr. Insul. 16 (2009) 1481–1488. [28] X.Y. Huang, C.Y. Zhi, P.K. Jiang, D. Golberg, Y. Bando, T. Tanaka, Adv. Funct. Mater. 23 (2013) 1824–1831. [29] J.L. Su, Y. Xiao, M. Ren, Phys. Status Solidi A Appl. Mater. 210 (2013) 2699–2705. [30] H.C. Wu, M. Rogalski, M.R. Kessler, ACS Appl. Mater. Interfaces 5 (2013) 9478–9487. [31] W. Yang, R. Yi, S. Hui, Y. Xu, X. Cao, J. Appl. Polym. Sci. 127 (2013) 3891–3897. [32] W.Y. Peng, X.Y. Huang, J.H. Yu, P.K. Jiang, W.H. Liu, Compos. Part A Appl. Sci. Manuf. 41 (2010) 1201–1209. [33] S. Rastegar, Z. Ranjbar, Prog. Org. Coat. 63 (2008) 1–4. [34] Z.M. Elimat, A.M. Zihlif, G. Ragosta, J. Phys. D Appl. Phys. 41 (2008) 7. [35] X. Pardal, F. Brunet, T. Charpentier, I. Pochard, A. Nonat, Inorg. Chem. 51 (2012) 1827–1836. [36] L. Pauling, S.B. Hendricks, J. Am. Chem. Soc. 47 (1925) 781–790. [37] X. Krokidis, P. Raybaud, A.E. Gobichon, B. Rebours, P. Euzen, H. Toulhoat, J. Phys. Chem. B 105 (2001) 5121–5130. [38] M. Digne, P. Sautet, P. Raybaud, P. Euzen, H. Toulhoat, J. Catal. 226 (2004) 54–68. [39] Z. Zhu, G. Lu, Z. Zhang, Y. Guo, Y. Guo, Y. Wang, ACS Catal. 3 (2013) 1154–1164. [40] K. Ji, H. Dai, J. Deng, L. Song, B. Gao, Y. Wang, X. Li, Appl. Catal. B Environ. 129 (2013) 539–548. [41] W. Tang, W. Li, D. Li, G. Liu, X. Wu, Y. Chen, Catal. Lett. 144 (2014) 1900–1910. [42] X. Wang, J.K. Nelson, L.S. Schadler, H. Hillborg, IEEE Trans. Dielectr. Electr. Insul. 17 (2010) 1687–1696. [43] H. Etemadi, A. Shojaei, Polym. Compos. 35 (2014) 1285–1293. [44] J.A. Schroeder, P.A. Madsen, R.T. Foister, Polymer 28 (1987) 929–940.

respectively. The DC volume resistivity, breakdown strength and dynamic mechanical thermal analysis reveal that the α-alumina nanoparticles may improve the electrical insulation performance of epoxy nanocomposites significantly. The main conclusions may be summarized from this work: 1. The XRD, Al27 NMR and O 1s XPS results show that boehmite, γalumina and α-alumina nanoparticles with the average size ≈ 100 nm were well prepared. The α-alumina nanoparticles show the highest crystallinity. Almost all Al atoms in α-alumina nanoparticles only exist in octa-coordinated environments. And the species of αalumina exhibits the highest content of surface lattice oxygen among these three species of alumina. 2. The DC volume resistivity and DC breakdown strength analysis reveal that α-alumina particles may improve both the DC volume resistivity and DC breakdown performance of epoxy nanocomposites. On the contrary, boehmite and γ-alumina particles show negative effects on these properties. The DC volume resistivity at 30 °C may be improved from 9.4 × 1017 Ω cm (neat epoxy) to 2.2 × 1018 Ω cm (α-alumina/epoxy nanocomposite). The breakdown strength may improve from 71.3 kV mm−1 (neat epoxy) to 76.1 kV mm−1 (α-alumina/epoxy nanocomposite). And these improved values of electrical insulation properties are much higher than these of epoxy nanocomposites reported in previous studies (see in Table 1). 3. Tg and υe analysis reveals that the existence of α-alumina restricts the movement of epoxy molecule chains in the α-alumina/epoxy interaction zone and blocks the electric charge transporting path. The α-alumina/epoxy interaction zone becomes stable to avoid water absorption or other negative effect on electric insulation performance. On the contrary, -OH groups on the surface of boehmite nanoparticles may react with the anhydride curing agent and build a dielectric double layer in the epoxy/boehmite interaction zone and block the crosslink of epoxy matrix. Acknowledgement This work was supported by the National Key Research and Development Program of China (No. 2017YFB0903804) and the Science and Technology Program of the State Grid Corporation of China (No. 5455DW160007). References [1] J. Yu, R. Huo, C. Wu, X. Wu, G. Wang, P. Jiang, Macromol. Res. 20 (2012) 816–826. [2] X. YU, Z. YU, H. LU, Epoxy Resin Electric Insulating Materials, Chemical Industry Press, Beijing, 2007. [3] Y. Dong, D. Chaudhary, C. Ploumis, K.T. Lau, Compos. Part A Appl. Sci. Manuf. 42 (2011) 1483–1492. [4] A. Yasmin, J.J. Luo, J.L. Abot, I.M. Daniel, Compos. Sci. Technol. 66 (2006)

8