Tuned Local Surface potential of Epoxy Resin Composites by

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Tuned Local Surface potential of Epoxy Resin Composites by Inorganic Core-Shell Microspheres: The key roles of interface Jun Zhou, Yongfei Li, Yang Wu, Beibei Jia, Lingjie Zhu, Yingye Jiang, Zhihui Li, and Kai Wu Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.9b01216 • Publication Date (Web): 26 Aug 2019 Downloaded from pubs.acs.org on August 28, 2019

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Tuned Local Surface potential of Epoxy Resin Composites by Inorganic Core-Shell Microspheres: The key roles of interface Jun Zhou,* Yongfei Li, Yang Wu, Beibei Jia, Lingjie Zhu, Yingye Jiang, Zhihui Li, Kai Wu* State Key Laboratory of Electrical Insulation and Power Equipment, Xi’an Jiaotong University, Xi’an 710049, People’s Republic of China

Abstract: Designing and controlling the interface interaction between polymer and filler is a challenge for nanocomposite insulation materials with the enhanced insulating and thermal conductive properties simultaneously. Meanwhile, the roles of the interface on the charge distribution of composite at the macro scale are well studied. However, the effects of the interface in the nanoscale are not clear. In this work, firstly, we have demonstrated a method to modify the dielectric constant of composites by introduced air in the core-shell structured M-SiO2@Al2O3 particles. To clarify the electric interfacial region, we use a Kelvin probe force microscopy (KPFM) to image with high spatial resolution on surface charge distribution around an individual M-SiO2@Al2O3 particle embedded in the epoxy matrix. We find that the KPFM results of the distinct electric interfacial region are consistent with the finite element simulation. Moreover, the charge accumulation is much easier after the presence of the M-SiO2@Al2O3 particles due to an increasing concentration of traps. This work provides a significant insight to understand the intrinsic interfacial behavior in insulating polymeric composites.

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KEYWORDS: epoxy nanocomposites, core-shell particles, surface potential, KPFM

*Address correspondence to: [email protected] (J. Zhou)；[email protected] (K. Wu)

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Introduction Dielectric polymeric composites with high thermal conductivity but good electrical insulating have attracted increasing scientific and industrial interest due to their wide applications in microelectronic devices and electrical equipment.1-3 For example, polymeric materials used in insulating cables or electronic packaging materials are expected to have excellent electrical strength, low dielectric constant, and high thermal conductivity to avoid electrical or thermal damage. It is difficult to acquire these properties simultaneously by only developing the novel polymeric matrix. One of the alternative methods is that the inorganic fillers were embedded in polymeric composites to obtain unique performances by the combined advantages of inorganic fillers and polymer matrix.4 Recently, one-dimensional (1D) materials such as oxide (SiO2, Al2O3) nanoparticles have attracted significant attention because they can be used as fillers to embed in insulating polymers (e.g., epoxy resin, polyethylene) for the device with a wide range of applications.5-10 Generally, it is significant for using a filler to improve discharge resistance and thermal conductivity. However, the effect of adding micron-scale or nano-scale ﬁllers to the polymer on electrical strength and thermal conductivity is determined by the type, size, shape, and content of the filler. 11,12 Under certain circumstances, the introduction of conventional inorganic fillers may also give rise to space charge accumulation and the associated Maxwell-Wagner polarization due to the implanted interfaces.13 Moreover, it is difficult to control and develop the electric strength and thermal conductivity simultaneously via changing the structure of the polymer materials.14 In this case, it is considered that the fundamental strategy to optimize the electrical strength and thermal conductivity is designing new inorganic fillers for advanced insulating polymers. Thus, much attention has been paid to the high thermal conductive fillers with excellent insulating properties as well. Recently, 3


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these fillers are core-shell structured particles that show specific physical and chemical properties have received considerable attention due to the flexible controlled or expected shell surface.15,16 For example, Awais et al. introduced a filler consisting of TiO2 nanoparticles encapsulated in SiO2 coreshell to control the dielectric properties of the material. The lowest dielectric constant (4.1) at 1000 Hz at a filler mass fraction of 0.6% was obtained.17 Pan et al. obtained HGM@hBN hybrid particles by coating the surface of HGM with HBN platelets via an electrostatic-assembly process, and obtained HGM@HBN/PTFE composites with low dielectric constant and low hygroscopicity.18 Generally, numerous works were investigated the interfacial properties by predicting the obtained results which tested from the macro phenomenon of the bulk composite. However, it is complicated to obtain the information of morphology, structure, composition, and molecular-bonding mechanisms in the interfacial region between the filler and polymeric matrix. Only the average properties of the interfacial region are obtained by conventional testing methods. Moreover, it’s hard to acquire the information around the local physicochemical properties, bringing difficulties and challenges for understanding the effect of interfacial properties on composite materials. Therefore, how to understand the interfacial properties thoroughly between filler and matrix is the key to improve the bulk properties of composite materials. Kelvin probe force microscopy (KPFM) is a powerful tool to measure the local surface potential (VSP) of the composite samples. KPFM also allows for nanoscale mapping the VSP in the vicinity of inorganic fillers under different DC bias conditions. Several works studied the local surface potential in composites. For instance, Borgani et al. revealed the charge injection and extraction around Al2O3 in low-density polyethylene (LDPE) using a home-build scanning probe technique.19 Peng et al. reported that the electrostatic force microscopy (EFM) is used to study the local dielectric property of the interface of LDPE/TiO2 4


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nanocomposites. The results showed that the addition of TiO2 fillers leads to a decrease in local permittivity.20 They also used a modified KPFM method with the nanoscale spatial resolution to detect the local polarization property at the matrix/particle interface in ferroelectric nanocomposites.21 However, the electric interfacial region between filler and matrix is still needed to be identified further in epoxy composites. In this work, the core-shell structured mesoporous-SiO2@Al2O3 (M-SiO2@Al2O3) particles were synthesized by an electrostatic deposition method, which deposited the ultrathin Al2O3 layer on the surface of M-SiO2 spheres. Thus, air will be filled in between M-SiO2 core and Al2O3 shell. As novel inorganic fillers for insulating polymers, the core-shell structured M-SiO2@Al2O3 particles was introduced in the epoxy matrix to improve the thermal conductivity but retain excellent insulating properties. In addition, the various surfaces of filler in insulating polymers may significantly influence the distribution of local charge. The core-shell structured particles can be suitable candidates for changing the local charge distribution in epoxy composites. To study the viability of this strategy, M-SiO2 and M-SiO2@Al2O3 particles with large pore volume and high surface area were synthesized as inorganic fillers for insulating polymers.

Experimental Section Materials.

Tetraethoxysilane

(TEOS,

AR,

Shantou

Xiwan

Chemical

Co.,

Ltd.),

cetyltrimethylammonium bromide (CTAB, AR, Tianjin Kemiou Chemical Reagent Co., Ltd.), ammonia solution (25%, Tianjin Damao Chemical Reagent Co., Ltd.), ethanol (AR, Tianjin Damao Chemical Reagent Co., Ltd.), aluminium isopropoxide (AIP, AR, Tianjin Damao Chemical Reagent Co., Ltd.), concentrated nitric acid (AR, Tianjin Damao Chemical Reagent Co., Ltd.), acetone (AR, Tianjin Damao Chemical Reagent Co., Ltd.) were obtained as raw materials. Diglycidyl ether of 5


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bisphenol A (DGEBA) epoxy resin (ER) was purchased from Laizhou Baichen insulation material Co. Bisphenol A (BADCy) cyanate ester (CE) were purchased from Zhejiang Shangyu Chemical Reagent Co. All the chemical reagents were used as received. Synthesis. The M-SiO2 particles were prepared by sol-gel method in alkaline solution using TEOS as silicon source and CTAB as a template. 2.5 g of CTAB was dissolved in 50 ml deionized water at room temperature until clarifying after strong magnetic stirring. Then, 75 mL ethanol was added to the solution and 13 mL ammonia solution was added after 15 min magnetic stirring 5 mL. TEOS was added into the mixed solution which gradually became milky white turbid liquid after strong magnetic stirring for 2 h. The product was separated by centrifugation, washed with alcohol and deionized water for several times and dried in an oven for 6 h. Then, the final powders were calcined at 600 oC in air to remove CTAB and the final M-SiO2 particles were obtained. The core-shell structured M-SiO2@Al2O3 particles were synthesized via Electrostatic deposition method which deposited Al2O3 on the surface of M-SiO2 particles. Firstly, 11.3g AIP powders were dissolved in 100 ml deionized water with strong magnetic stirring. The pH was adjusted to 4 by adding Nitric acid and milky white Al2O3 sol formed. Secondly, 1 g as-prepared M-SiO2 powders were dispersed into 40 ml deionized water by ultrasonic treatment. The pH of the suspension was also adjusted to 4 by adding Nitric acid. At the same time, 2 ml Al2O3 sol was added into the MSiO2 suspension under mild stirring. Then, the pH of the mixed suspension was changed to 6 via adding ammonia water. The solid sample was separated by centrifugation, washed with alcohol and deionized water for several times and dried. Finally, the powders were calcined at 600 oC in air and the core-shell structured M-SiO2@Al2O3 particles were obtained. In general, the preparation process of epoxy/fillers composite could be described as following. 6


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Firstly, the desired inorganic fillers (M-SiO2, and M-SiO2@Al2O3 particles) were dispersed in acetone with a high-shear mixer and ultrasonic treatment. Amount of epoxy resins (ER) was heated at 120 oC and added into the calculated inorganic fillers composite microspheres with ultrasonic treatment for 30 min. Cyanate ester (CE) were heated at 120 °C for 10 min with vigorous stirring. Then, epoxy/fillers composites were added into CE until the mixture was stirred to form homogeneous liquid for 0.5 h. The mixed liquid was heated in a vacuum oven at about 60 °C to remove any residual bubble. A preheated mold with silicon coating on the inner surface was heated at 120 °C for 1 h. The mixed liquid was poured into the preheated mold with silicon coating on the inner surface and degassed at 120 °C for 0.5 h in a vacuum oven. Characterization. The particle size and morphology were visualized by using a FEI Quanta 600 FEG scanning electronic microscope (SEM) and an FEI Tecnai G2 F20 S-TWIN transmission electron microscope (TEM). Tests of surfaces and cross-section microscopic morphology of epoxy composites films were also elucidated by SEM under an acceleration voltage of 20 kV. Specific surface areas of fully grinding powder were determined by N2 adsorption-desorption measurements at -196 °C by employing the Brunauer–Emmet–Teller (BET) method (Gold App Vsorb 2008p), which was performed via Micrometeritics 2020M analyzer and the pore volumes were calculated by Barrett–Jioner–Halenda (BJH) method. The FT-IR spectra were recorded ranging from 400 to 4000 cm-1 on a RENISHAW inVia (UK) infrared spectrometer. The phase composition of M-SiO2 and M-SiO2@Al2O3 particles were determined by X-ray diffraction (XRD, Bruker D2 PHASER, Germany) operated at 30 kV and 10 mA using Cu Kα radiation with a wavelength of 1.54 Å at room temperature, the scans were measured in the 2θ ranging from 10° to 90° at a scan rate of 0.05°/s. Small angle X-ray scattering diffraction were 7


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carried out by RigakuD/Max 2500 operated at 15 kV in the ranging from 1.5° to 8.5° at a scan rate of 0.1 °/s. All epoxy composites samples were prepared by fracturing the composites at liquid nitrogen temperature and then sputter-coated with a homogeneous gold layer to avoid accumulation of charges. The broadband frequency dielectric properties of the composites were measured using a Concept 80 impedance analyzer (Novo control, Germany) over the frequency range of 10-1 to 106 Hz.The thermal diffusivity (δ) and specific heat (C) were measured on disk samples by using a LFA447 light flash system (NETZSCH, Selb, Germany) at 25oC. The bulk density (ρ)of the specimen was measured by water displacement. The thermal conductivity (λ, W ∙ m -1 ∙ K-1 ) was given by the product of the thermal diffusivity (δ, mm2 ∙ s-1 ), specific heat (C, J ∙ g-1 ∙ K -1), and bulk density (ρ, g ∙ cm-3 ): λ=δ∙ C∙ρ The volume resistance (RV) of the samples was measured by an Agilent 6517B high resistance meter after obtaining the thickness of these composites pellets. The voltage and current limits were set at 1.0 V and 5 mA for all samples. Broadband dielectric spectrum test system (concept 80, Novocontrol technologies, Germany) was used to collect the dielectric data at room temperature. Thermally stimulated current (TSC) was collected at various temperatures by the Novo control system (Germany). Environmental and AFM Experiment. The KPFM and AFM measurements were carried out with a Dimension Icon (Bruker) in air and in an electromagnetic shielding room. The temperature was around 25.5 oC, and the relative humidity was 33% during the experiment. Platinum–iridium-coated conductive probes (SCM-PIT, Bruker) were used in the KPFM and AFM measurements. The 8


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PeakForce KPFM-HV mode, combining the tapping mode AFM with frequency modulation KPFM can measure the topographic and surface potential signals of the same area. The lift height for KPFM measurements was 80 nm for all samples. Local surface potential were measured in fixed locations on the film as a function of a DC bias applied on the probe. Finite element simulation. The simulating model was built using the finite element simulation software Comsol Multiphysics. The nano-alumina particle is embedded on the surface of the epoxy body, and the horizontal size of the epoxy board is much larger than the size of the nano-alumina particle. The lower surface of the epoxy group is grounded, and the upper electrode is a platinum electrode. The distance between the upper electrode and the nano-alumina particle is nanometer. The entire system is placed in the air. The whole model has meshed, and the shape of the mesh is tetrahedron, then a finite element simulation model is established. A voltage of +2 V was applied to the upper electrode to obtain a simulation result.

RESULTS AND DISCUSSION Figure 1a shows a schematic diagram that illustrates the preparation of M-SiO2 and M-SiO2@Al2O3 particles. Firstly, M-SiO2 microspheres were synthesized using tetraethyl orthosilicate (TEOS) and cetyltrimethylammonium bromide (CTAB) as the silica source and template respectively, via the sol-gel method in the condition of alkaline. Then, the core-shell structured M-SiO2@Al2O3 microspheres were synthesized through electrostatic deposition method which deposited Al2O3 on the surface of M-SiO2 particles. Figure 1b illustrates the scanning electron microscopy (SEM) and Figure 1c-e show the transmission electron microscopy (TEM) images of synthesized M-SiO2 particles. The diameter of M-SiO2 particles is approximately 400～500 nm, and it was observed from SEM and TEM images that particles have a well symmetric structure with sphere shape. The 9


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appearance of the lighter (grey) color of SiO2 is due to its mesoporous nature (Figure 1e). According to the calculation from small-angle X-ray scattering (SAXS) pattern of M-SiO2 (Figure S1a and b, Supporting Information), the diameter of pores is about 3.6 nm, which is close to the result calculated from the Barrett-Joyner-Halenda (BJH) method (Figure S2, Supporting Information). Meanwhile, Al2O3 was deposited on the surface of M-SiO2 spheres and the SEM image is showed in Figure 1f. As shown in the figure, the shape of the spheres maintained, and the diameter of MSiO2@Al2O3 spheres increased slightly to around ～550 nm. Moreover, Al2O3 is thoroughly coated on the surface of M-SiO2 particles after electrostatic deposition and this is confirmed by TEM images shown in Figure 1g-i. The Al2O3 shell thicknesses of the M-SiO2 particles are approximately 50 nm. FT-IR curves were also elucidated and suggested that Al2O3 shells were coated on the surface of M-SiO2 cores successfully (Figure S3, Supporting Information). In this case, air will be introduced into the inside of the pores of M-SiO2@Al2O3 spheres as these spheres are used as fillers into the polymer matrix. To investigate the local physical properties of the composites, light loading of M-SiO2 and MSiO2@Al2O3 particles as fillers were embedded into the epoxy matrix, respectively. In this work, PeakForce QNM at the ambient environment is used to investigate the surface properties of epoxy composites simultaneously, such as topography and mechanical properties. The pristine epoxy film shows fine flat surface and no obvious bulges were observed in Figure S4a (Supporting Information). After added fillers into epoxy, the inorganic particles were vividly observed on the surface of composites (Figure S4b and 4f, Supporting Information). SEM images also show that the cured pristine epoxy illustrates a smooth fracture surface, while the composites show a rough fracture surface due to the matrix shear yielding or the polymer deformation between the inorganic oxide 10


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particles (Figure S5, Supporting Information). The introduction of particles and the effect of the particles on the morphology of the surrounding epoxy groups may form more thermal conduction paths in the matrix to enhance thermal conductivity. Meanwhile, the thermal conductivity the composites increased 143% by embedding only 3.0 wt.% of M-SiO2@Al2O3 microspheres in pure epoxy (Figure S6, Supporting Information). Figure 2a-d show the frequency dependence of the dielectric constant and the dielectric loss of M-SiO2/epoxy and M-SiO2@Al2O3/epoxy composites with low content (0～3 wt.%) of inorganic fillers, respectively. However, an exceptional feature of our work shows that the dielectric constant of M-SiO2/epoxy composites decreased dramatically with increasing content of M-SiO2 fillers at the same frequency. The dielectric constant is only 3.2 at 1×106 Hz with the 3.0 wt.% M-SiO2/epoxy composite, and this value is much smaller than that of the pristine epoxy. In this case, a lot of air filled in the mesoporous SiO2 when M-SiO2 particles were embedded in the epoxy. This could be considered that an exceptional substance (air) with lower dielectric constant (εr=1.00053) as a filler introduced into the epoxy matrix, resulting in a lower dielectric constant of the composites. On the other hand, the dielectric constant of polymeric composites could also be tuned by the change of the gas-solid interface of fillers. However, the dielectric constant of M-SiO2@Al2O3/epoxy composites increased when compared with that of the M-SiO2/epoxy composites, which is ascribed to Al2O3 shell with the higher dielectric constant. As we all know, Al2O3 shell has a high dielectric constant (about 10), which in turn leads to a phenomenon in which the dielectric of the composite is often higher than that of pure epoxy. Figure 2c and 2d show that the dielectric loss of all composites increases initially at the range of 1～100 Hz and then increases dramatically (100～1×106 Hz). Figure 2e shows the volume resistivity of composites as a function of the mass fraction of fillers. It 11


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is noticed that the data of resistivity is around 2.1×1016 Ω m and that a gently decrease in volume resistivity is observed as the increase of fillers mass fraction. Moreover, the electrical breakdown strength enhanced after increasing the content of M-SiO2 in epoxy (Figure 2f). The MSiO2@Al2O3/epoxy composites also maintain excellent insulating properties even though a slight decrease in electrical breakdown strength as a function of the mass fraction of M-SiO2@Al2O3 was observed. These results are comparable to other works. For instance, we successfully synthesized a core-shell polystyrene@SiO2 (PS@SiO2) microspheres and introduced them into epoxy resin at a mass fraction of 1%. The thermal conductivity of the material was increased by 131% under the premise that the insulation performance did not decrease significantly.22 On the other hand, the application of spherical core-shell particles as fillers, thus, would help in a more holistic way to achieve better performance of the composite insulators.23 In order to acquire the detailed information of charge distribution around a single particle, the PeakForce KPFM measurements were conducted in a small area of epoxy composites with the size of 1 um. Figure 3a shows a typical height image of an uncovered M-SiO2@Al2O3 core-shell structural particle. The size of the exposed part is ~180 nm. As mentioned above, the as-prepared M-SiO2@Al2O3 particles are ~500 nm in diameter, suggesting that most of the particle is embedded underneath the surface of epoxy. The modulus and adhesion images confirmed the height particle is M-SiO2@Al2O3 microsphere (Figure S7, Supporting Information). Figure 3b-d show the surface potential (△Vsp) images around a M-SiO2@Al2O3 single particle as the probe was given various voltages (0, +5V, and -5 V respectively). Obviously, the area around the single particle demonstrates a darker ring due to a lower △Vsp. Figure 3e shows the curves of △Vsp evolution around the single particle under different voltages. The difference of charge distribution in the interface is expressed 12


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by the gap between the highest and lowest mean values of potential (△Vg). Without an additional voltage on the probe, the △Vg1 shows the value of 0.347 V. It is noted that there are still lots of charges even Cu foil was covered partially on the surface of composites to diminish the effect of surface electrostatic (Figure 4a). We can see that the charges prefer to accumulate around the MSiO2@Al2O3 single particle. The electrical interface between filler and matrix was observed clearly and the size of the interfacial region is ~50 nm. This value is consistent with the previous theoretical model proposed by Tanaka et. al.22 Moreover, the value of △Vg2 increased to 0.545 V after a voltage of +5 V applied on the probe. It is noted that the surface potential of epoxy matrix and the MSiO2@Al2O3 single particle also increased simultaneously. However, the value of △Vg3 decreased to 0.219 V since the probe was applied a voltage of -5V. The surface potential images of a M-SiO2 particle embedded in epoxy were also obtained via KPFM at same condition (Figure S8, Supporting Information). It is found that the charge can also accumulate around the interfacial region of the uncovered M-SiO2 particle. In our KPFM testing system, the tip of the probe was considered as an electrode, which did not directly inject charges into the surface of the sample (Figure 4a). However, a strong electric field was added if the probe was applied a voltage, resulting in the redistribution of the inherent charge and the introduction of surface polarization charge (SPC) on the sample’s surface. Borgani et al. suggested that an additional localized energy level is located close to the conduction band, which introduces new shallow traps in the epoxy composites with the embedded M-SiO2@Al2O3 particles.19 Figure 4b exhibits a schematic diagram which demonstrates a hole-transfer-behavior under different applied voltages of the probe. If there is a positive voltage (VDC=+5V) applied between probe and the Au film, holes will transfer to the surface of the sample, leading to the filled 13


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traps. This is the reason why the interfacial region blurred in the image of surface potential (Figure 3c), though the measured surface potential of bulk increases after applied a positive voltage. On the contrary, the interfacial region of △Vsp appeared distinctly again since a negative voltage (VDC=5V) was applied to the probe. The phenomenon reveals that more shallow traps present due to traps were extracted by the probe. Generally, the accumulation of charges around the interfacial regions is ascribed to the chemical defects and different dielectric properties in the filler-embedded composites, resulting in lots of interfacial traps. Figure 4c shows the thermally stimulated current (TSC) spectra of without and with filler-embedded epoxy. One distinct peak around to 225oC is observed in both samples and this peak is ascribed to the interfacial polarization in the composite.24 The TSC peak of the filler-embedded epoxy shifts toward lower temperature and the relative intensity of the current is higher than that of pure epoxy. We also revealed the charge distribution around a single Al2O3 particle in epoxy using finite element simulation (FES) method. An obvious electric interfacial region was vividly showed around the Al2O3 particle after applied a positive voltage (Figure 4d). These results demonstrate that the charge accumulation enhances after the presence of the M-SiO2@Al2O3 particles, as well a rising concentration of traps, which are consistent with the previous analysis of surface potential.25 Li et al. also reported that chemicallybonded surface layers for silica nanoparticles were prepared by grafting epoxy molecules.26 Their results also illustrated that the chemically-bonded interface can hinder the transport of charge carriers and generates deep traps. With regard to the content of fillers, when the much more fillers are embedded in the epoxy, several particles are observed with the visualized surface potential (Figure S4g and S4k, Supporting Information). However, the interfacial region of surface potential for every single particle is indiscernible due to the overlapped particles and the limited resolution 14


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of KPFM. In summary, the core-shell structured M-SiO2@Al2O3 particles were synthesized by modified electrostatic deposition method and the particles were embedded in epoxy as fillers. The improved thermal conductivity and retained excellent insulating properties of the M-SiO2@Al2O3/epoxy composites are ascribed to be the unique core-shell structure of fillers. A vividly electric interfacial region was observed for the first time around a M-SiO2@Al2O3 particle in epoxy by KPFM. On the basis of surface potential, TSC curves, and the simulation, we can conclude that the charge accumulation is much easier after the presence of the single inorganic particle, as well a rising concentration of traps. Our work demonstrates that the electric interfacial region between filler and matrix is significantly determined the insulating properties in nanocomposites.

Conflict of Interest The authors declare no competing financial interest.

Acknowledgment Financial supports from the National Key Research and Development Program of China (Grant No. 2017YFB0903803) is gratefully acknowledged.

ASSOCIATED CONTENT Supporting Information Supporting Information is available free of charge on the Internet at http://pubs.acs.org. XRD patterns, BET results, FTIR curves of M-SiO2 and M-SiO2@Al2O3 powders. Thermal conductivity curves of 15


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the composites. AFM and SEM images of the composites.

AUTHOR INFORMATION ORCID

Jun Zhou: 0000-0001-5729-7261 REFERENCES (1) Lu, J. X.; Moon, K. S.; Kim, B. K.; Wong, C. P. High dielectric constant polyaniline/epoxy composites via in situ polymerization for embedded capacitor applications. Polymer 2007, 48, 1510−1516. (2) Zhou, T.; Zha, J. -W.; Hou, Y.; Wang, D. R.; Zhao, J.; Dang, Z. -M. Surface-Functionalized MWNTs with Emeraldine Base: Preparation and Improving Dielectric Properties of Polymer Nanocomposites. ACS Appl. Mater. Inter. 2011, 3, 4557−4560. (3) Bubenhofer, S. B.; Schumacher, C. M.; Koehler, F. M.; Luechinger, N. A.; Sotiriou, G. A.; Grass, R. N.; Stark, W. J. Electrical resistivity of assembled transparent inorganic oxide nanoparticle thin layers: influence of silica, insulating impurities, and surfactant layer thickness. ACS Appl. Mater. Inter. 2012, 4, 2664–2671. (4) Huang, X. Y.; Jiang, P. K.: Tanaka, T. A review of dielectric polymer composites with high thermal conductivity. IEEE Trans. Dielectr. Electr. Insul. 2011, 27, 8−16. (5) Liu, H. Y.; Shen, Y.; Song, Y.; Nan, C. -W.; Lin, Y. H.; Yang, X. P. Carbon nanotube array/polymer core/shell structured composites with high dielectric permittivity, low dielectric loss, and large energy density. Adv. Mater. 2011, 23, 5104−5108. (6) Pallon, L. K. H.; Hoang, A. T.; Pourrahimi, A. M.; Hedenqvist, M. S.; Nilsson, F.; Gubanski, S.; Gedde, U. W.; Olsson, R. T. The impact of MgO nanoparticle interface in ultra-insulating 16


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polyethylene nanocomposites for high voltage DC cables. J. Mater. Chem. A. 2016, 4, 8590−8601. (7) Roy, M.; Nelson, J. K.; MacCrone, R. K.; Schadler, L. S. Candidate mechanisms controlling the electrical characteristics of silica/XLPE nanodielectrics. J. Mater. Sci. 2007, 42, 3789−3799. (8) Murakami, Y.; Nemoto, M.; Okuzumi, S.; Masuda, S.; Nagao, M.; Hozumi, N.; Sekiguchi, Y.; Murata, Y. DC conduction and electrical breakdown of MgO/LDPE nanocomposite. IEEE Trans. Dielectr. Electr. Insul. 2008, 15, 33−39. (9) Xu, J. W.; Wong, C. P. Low-loss percolative dielectric composite. Appl. Phys. Lett. 2005, 87, 082907. (10) Fu, Y. X.; Wang, Y. X.; Wang, S.; Gao, Z. D. F.; Xiong, C. X. Enhanced breakdown strength and energy storage of PVDF ‐ based dielectric composites by incorporating exfoliated mica nanosheets. Polymer Composites. 2018, 40, 2088-2094. (11) Ding, S. J.; Yu, S. H.; Zhu, X. D.; Xie, S. H.; Sun, R.; Liao, W. H.; Wong, Q. P. Enhanced breakdown strength of polymer composites by low filler loading and its mechanisms. Appl. Phys. Lett. 2017, 111,153902. (12) Castellon, J.; Nguyen, H. N.; Agnel, S.; Toureille, A.; Frechette, M.; Savoie, S.; Krivda, A.; Schmidt, L.E. Electrical properties analysis of micro and nano composite epoxy resin materials. IEEE Trans. Dielectr. Electr. Insul. 2011, 18, 651-658. (13) Tanaka, T. Dielectric nanocomposites with insulating properties. IEEE Trans. Dielectr. Electr. Insul. 2005, 12, 914-28. (14) Yao, Y.; Zhu, X.; Zeng, X.; Sun, R.; Wong, C. H. Vertically Aligned and Interconnected SiC Nanowire Networks Leading to Significantly Enhanced Thermal Conductivity of Polymer Composites. ACS Appl. Mater. Inter. 2018, 10, 9669–9678. 17


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(15) Zheng, Y.; Wang, Y.; Deng, Y.; Bai, J. Enhanced dielectric properties of ferroelectric polymer composites induced by metal-semiconductor Zn-ZnO core–shell structure. ACS Appl. Mater. Inter. 2012, 4, 65-68. (16) He, D.; Wang, Y.; Song, S.; Liu, S.; Deng, Y. Significantly Enhanced Dielectric Performances and High Thermal Conductivity in Poly(vinylidene fluoride)-Based Composites Enabled by SiC@SiO2 Core–Shell Whiskers Alignment. ACS Appl. Mater. Inter. 2017, 9, 44839–44846. (17) Muhammad, A.; Raji, S.; Intisar, A, S.; Shaikh, S, H.; Salman, A.; Hammad, S.; Muhammad, A, N. Investigation on optimal filler loadings for dielectric strength enhancement of epoxy/TiO2@ SiO2 nanocomposite. Mater. Res. Express 2019, 6, 065709. (18) Pan, C.; Kou, K, C.; Zhang, Y.; Li, Z, Y.; Ji, T, Z.; Wu, G, L. Investigation of the dielectric and thermal conductive properties of core–shell structured HGM@hBN/PTFE composites. Materials Science and Engineer :B. 2018, 238, 61-70. (19) Borgani, R.; Pallon, L. K.; Hedenqvist, M. S.; Gedde, U. W.; Haviland, D. B. Local Charge Injection and Extraction on Surface-Modified Al2O3 Nanoparticles in LDPE. Nano Lett. 2016, 16, 5934–5937. (20) Peng, S.; Zeng, Q.; Yang, X.; Hu, J.; Qiu, X.; He, J. Local dielectric property detection of the interface between nanoparticle and polymer in nanocomposite dielectrics. Sci. Rep. 2016, 6, 38978. (21) Peng, S.; Yang, X.; Yang, Y.; Wang, S.; Zhou, Y.; Hu, J.; Li, Q.; He, J. Direct Detection of Local Electric Polarization in the Interfacial Region in Ferroelectric Polymer Nanocomposites. Adv. Mater. 2019, 1807722. (22) Zhou, J; Jiang, Y, Y.; Wu, G, Q.; Wu, W, J.; Wang, Y.; Wu, K.; Cheng, Y, H. Investigation of dielectric and thermal conductive properties of epoxy resins modified by core-shell structured PS@ 18


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SiO2. Compos. A: Appl. Sci. Manuf. 2017, 97, 76-82. (23) Mao, D.; Chen, J.; Ren, L.; Zhang, K.; Yuen, M.; Zeng, X.; Sun, R.; Xu, J.; Wong, C. Spherical core-shell Al@Al2O3 filled epoxy resin composites as high-performance thermal interface materials. Compos. A: Appl. Sci. Manuf. 2019, 123, 260-269. (24) Takada, T.; Hayase,Y.; Tanaka,Y. Space charge trapping in electrical potential well caused by permanent and induced dipoles for LDPE/MgO nanocomposite. IEEE Trans. Dielectr. Electr. Insul. 2008, 15, 152. (25) Nelson, J. K.; Fothergill, J. C. Internal charge behaviour of nanocomposites. Nanotechnology, 2004, 15, 586. (26) Li, H.; Liu, F. H.; Tian, H. D.; Wang, C.; Guo, Z. H.; Liu, P.; Peng, Z. R.; Wang, Q. Synergetic enhancement of mechanical and electrical strength in epoxy/silica nanocomposites via chemicallybonded interface. Compos. Sci. Technol. 2018, 167, 539–546.

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Figure captions:

Figure 1. Schematic of synthesis of M-SiO2 and M-SiO2@Al2O3 Microspheres (a). SEM (b) and TEM (c), (d) of M-SiO2 microspheres. SEM (e) and TEM (f), (g) of M-SiO2@Al2O3 microspheres.

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Figure 2. The dielectric and electrical properties of the composites. The frequency dependence of dielectric constant of M-SiO2/epoxy composites (a) and M-SiO2@Al2O3/epoxy composites (b). The dielectric loss of composites with mass fraction of M-SiO2 (c) and M-SiO2@Al2O3 (d). The volume resistivity (c) and brokendown strength (d) of the composites.

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Figure 3. PeakForece KPFM study of the M-SiO2@Al2O3 single particle embedded in epoxy. (a) Height image. (b), (c), (d) Surface potential under the value of applied voltage with 0, +5, and -5, respectively. (e)

Cross sections

of surface potential along the red lines shown in (b)-(d).

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Figure 4. (a) Schematic setup of PeakForce KPFM measurements. (b) The diagram of band evolution under KPFM tests. (c) TSC curves of pure epoxy and he M-SiO2@Al2O3/epoxy composite. (d) The simulating value of surface potential along an Al2O3 particle embedded in epoxy via Finite element simulation.

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Graphical Abastract:

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Tuned Local Surface potential of Epoxy Resin Composites by

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