Silicone Elastomer

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Mussel inspired modification for aluminium oxide/ silicone elastomer composites with largely improved thermal conductivity and low dielectric constant Dan Yang, Shuo Huang, Mengnan Ruan, Shuxin Li, Jinwei Yang, Yibo Wu, Wenli Guo, and Liqun Zhang Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.7b04970 • Publication Date (Web): 14 Feb 2018 Downloaded from http://pubs.acs.org on February 22, 2018

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Industrial & Engineering Chemistry Research

Mussel inspired modification for aluminium oxide/silicone elastomer composites with largely improved thermal conductivity and low dielectric constant

Dan Yang, ab Shuo Huang, ab Mengnan Ruan, ac Shuxin Li,ab Jinwei Yang,a Yibo Wu,ab Wenli Guo,ab* Liqun Zhang c*

a

Department of Materials Science and Engineering, Beijing Institute of Petrochemical

Technology, Beijing 102617, China b

c

Beijing Key Lab of Special Elastomeric Composite Materials, Beijing 102617, China

Department of Materials Science and Engineering, Beijing University of Chemical

Technology, Beijing 100029, China

*To whom correspondence should be addressed. E-mail: W. Guo ([email protected]) or L. Zhang ([email protected])

ACS Paragon Plus Environment

Industrial & Engineering Chemistry Research 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Abstract: Inspired by unparalleled adhesion of mussels, bio-inspired poly(dopamine) (PDA) has been used to functionalize the surface of aluminium oxide (Al2O3) nano-particles aiming at improving thermal conductivity of silicone rubber (SR). The successful and effective preparation of PDA modified Al2O3 nano-particles (Al2O3-PDA) was confirmed by XPS, HR-TEM, and XRD. The PDA coating on the Al2O3 nano-particles improved its interfacial interaction between polymeric matrix and facilitated the uniform-dispersion of filler, leading to the 30 vol% Al2O3-PDA/SR composite exhibited a high thermal conductivity (0.585 W/(mK)), which was almost 400% of pure silicone rubber (0.147 W/(mK)). In addition, the 30 vol% Al2O3-PDA/SR composite displayed a relatively low dielectric constant (4.06 at 1 kHz), which was a bit higher than that of pure SR (2.59 at 1 kHz), a big advantage for the electronic or electrical engineering application. With advantages of efficient, easy handling, controllable, and eco-friendly, this modification method provides a new universal route to improve the thermal conductivity of composites.

Keywords: thermal conductivity, silicone elastomer, dopamine, dielectric, aluminium oxide


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1. Introduction With increasing power densities in modern microelectronic device, the growing demand for dissipation of generated heat has prompted great interest in dielectric materials [1-3]. Polymer composites consisting of a polymeric matrix and thermal conductive fillers are usually applied in preparing high thermal conductive materials because of easy process ability and low cost. Silicone rubber (SR) is a typical kind of engineering polymeric material to be used in cable accessories, gate dielectrics, and electronic devices, because it has excellent electrical insulation, relatice low dielectric constant, high elasticity, small mechanical hystersis, high breakdown strength, easy film formation, and can be used in a wide range of temperature [4-6]. However, the thermal conductivity of SR is relatively low, limits its wide use as thermally conductive material in industry. During recent years, the most efficient strategy to improve thermal conductivity of silicone rubber is to introduce high thermal conductive filler, such as carbon materials [7], ceramic [8], metal particles [9], and metal oxide [10]. Though carbon materials have the advantage of high thermal conductive and lightweight, the increased cost and decreased electrical insulation of composites limit their wide applications, especially in electronic packaging needing high electric breakdown field. Therefore, various low-cost inorganic ceramic fillers (Al2O3 [10, 11], BN [3, 12], SiN [5], SiC [13], etc) are used as the heat conductive fillers in wide application. However, the weak interfacial interaction between fillers and matrix will result in inhomogeneous dispersion and agglomeration, which further leading to enhancement



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of interface thermal resistance. In order to improve the interfacial interaction, surface treatment and functionalization have been demonstrated to be an efficient method [14], such as hydroxylation [15], coupling agents [16], surfactants [17, 18], and phosphoric acids [19, 20]. Na Yang et al.[1] improved interface action between hexagonal boron nitride (h-BN) micro-particles and polyimide (PI) matrix by functionalizing h-BN with 3-glycidyloxypropyltrimethoxy silane (g-MPS), during preparation of thermally conductive composites. Yuan Wang et al.[21] modified aluminium nitride (AlN) microspheres by 3-aminopropyltriethoxysilane (KH550) and then added KH550 modified AlN into ultra-high molecular weight polyethylene (UHMWPE) matrix to prepare thermal conductive polymer composites. The results showed that the composites filled with modified AlN displayed much higher thermal conductivity than the commercial AlN/UHMWPE composites. Although these modification methods can improve the dispersion of particles to a certain extent, there still are some practical limitations, such as multi-step production, complex instruments, and environmental unfriendliness. Especially, these chemical reactions need additional heating equipment, resulting in high energy cost. Recently, marine mussel adhesive has attracted broad interest due to their marvelous

properties [22,

23].

It is believed

that dopamine

containing

3,4-dihydroxy-L-phenylalanine (L-DOPA) and lysine amino acids (lysine), is the first molecule to display material-independent surface modification because the poly(dopamine) (PDA) film has the ability of attachment to various inorganic and organic substrates, owing to the strong hydrogen bonding from its abundant catechol


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groups [23, 24]. In addition, the PDA provides secondary reactivity with various functional groups, including thiol, amine, and quinone itself via Michael addition or Schiff base reactions, serving as a versatile platform for secondary reactions with further specific functionalization [25, 26]. Recently, PDA has been demonstrated in literatures to provide as a popular modifier to enhance the interfacial adhesion between inorganic filler and polymeric matrix [27-30]. Here, we report an efficient and easy approach to improve thermally conductivity of silicone elastomer using bio-inspired PDA to modify aluminium oxide (Al2O3) nano-particles. The Al2O3 nano-particles were used in our work due to their high thermal conductivity (32 W/m⋅K), low cost, and stable chemical performance [31]. The successful deposition of PDA on Al2O3 surface were demonstrated by XPS, HR-TEM, and XRD. The effects of PDA layer on the thermal conductivity and dielectric properties of silicone elastomer composites were studied. Indeed, the cost of dopamine monomers is relatively high for extensive industry production. Fortunately, researches showed that low cost catechol and polyamine could polymerize and deposit on the surface of substance with similar adhesive property of PDA [32, 33]. In our future, we will use low cost catechol and polyamine for the replacement of dopamine to improve interfacial interaction between the inorganic nano-particles and polymeric matrix. 2. Experimental Section 2.1 Materials



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Silicone rubber (110-2) was supplied by Chenguang Research Institute of Chemical Industry, Sichuan Province, China. The α-Al2O3 nano-particles with type of AKP-30 (with an average diameter of 300 nm) were provided by Sumitomo Chemical. The crosslinking agent of dicumyl peroxide (DCP) was provided by from Beijing Chemical

Plant,

China.

Dopamine

hydrochloride

(Dopamine,

99%)

and

tris(hydroxymethyl)aminomethane (Tris, 99%) were received from Beijing Chemicals Co., Ltd. 2.2 Coating of PDA on Al2O3 nano-particles First, 0.2 g dopamine was immersed in 100 ml distilled water to prepare dopamine solution (2.0 g/L), followed by addition of Tris which can form a Tris-HCl buffer solution and its pH was adjusted to 8.5. Then, 15 g Al2O3 nano-particles were immersed into above solution and under magnetic stirring for 24 h at 25 oC. Last, the modified Al2O3 particles were collected via filtration and washed for three times with deionized (denoted as Al2O3-PDA). The filter material is commercial medium-speed qualitative filter paper with 100% cotton cellulose and type of 102, which is supplied by Hangzhou Special Paper Industry Co., Ltd. Figure 1(a) and Figure 1(b) present the procedure for surface functionalization of Al2O3-PDA nano-particles and possible reaction mechanism of dopamine self-polymerization. 2.3 Preparation of SR composites filled with Al2O3 and Al2O3-PDA nano-particles First, 80 g silicone elastomer was put into a 6 inch double-roll open mill. Then, 0 vol%, 10 vol%, 20 vol%, and 30 vol% of Al2O3 or Al2O3-PDA particles were mixed.


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Afterwards, 2 g DCP and 20 g silicone oil mingled with rubber matrix. The obtained uncured composites rested for 24 hours to assure good dispersion of ingredients. Last, the uncured composites were vulcanized at 160oC and 25 MPa for T90 (optimum cure time). A curemeter (GT-M2000-FA Goteah Limted Co., Taiwan) was used to determined the T90. 2.4 Characterization methods High resolution transmission electron microscope (HR-TEM) studies were conducted on a H 9000 (Hitachi, Japan) to observe surface morphologies of the Al2O3 and Al2O3-PDA nano-particles. X-ray photoelectron spectrometry (XPS) was recorded on an ESCALAB 250 spectrometry (Thermo Electron Corporation, USA) with an Al Kɑ X-ray source (1486.6 eV) to determine chemical compositions of the samples. Powder X-ray diffraction (XRD) patterns were taken on D8 Focus X-ray diffractometer (Bruker, Germany) to investigate the crystalline structure of the Al2O3 and Al2O3-PDA nano-particles. Thermogracimetric analysis (TGA) was performed in the temperature range of 30-540 oC with heating rate of 20 oC/min by using a TA Q500 thermogravimetric analyzer. The TGA experiments were carried out under a nitrogen atmosphere. Scanning electron microscopy (SEM) images of SR composites filled with Al2O3 and Al2O3-PDA nano-particles were taken on a FEI NanoSEM 430 field-emission Scanning electron microscopy. Prior to observation, a layer of gold was sputtered on the fractured surfaces of samples to avoid charge accumulation.



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The elemental composition of the samples was observed by using a scanning electron microscopy (SEM, SSX-550) equipped with an Energy Dispersive X-Ray Spectroscopy (EDX) detector. Prior to observation, a layer of gold was sputtered on the fractured surfaces of samples to avoid charge accumulation. The dielectric properties were measured using a Novochtrol Alpha-A impedance analyzer over the frequency range of 101-106 Hz at room temperature. The volume resistivity of SR composites was measured using an EST 121 resistivity meter (Beijing Huajinghui Technology Co. Ltd, Beijing, China). The volume resistivity (ρν) of samples can be calculated according to following equation:

ρv = 4L /( Rv ⋅ π ⋅ d 2 )

(1)

where L is the thickness of the specimen, Rν is the resistance of the specimen, and d is the diameter of the electrode. The thermal conductivity of samples was measured using a HC-110 thermal conductivity meter (Laser Comp. Inc., USA). The temperature of the hot and cold plate of the thermal conductivity meter was 20°C and 40°C, respectively. The pressure is 414 kPa. 3. Results and Discussion

3.1 Coating of PDA on Al2O3 nano-particles The surface chemical compositions of Al2O3 nano-particles before and after PDA deposition were investigated by recording the XPS wide scan, N 1s core-level spectra, and C 1s core-level spectra, as shown in Figure 2. As we all known, the samples will contact with air inevitably during save, transport, and analyze, therefore, the C 1s


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signal in pristine Al2O3 particles mainly come from the contaminant [34]. It can be noted that in Figure 2(a) and Figure 2(b), Al2O3 and Al2O3-PDA nano-particles exhibit both peaks of C 1s and O 1s but a new peak component of N 1s species was only observed in Al2O3-PDA nano-particles at binding energy (BE) of about 400 eV. Figure 2(d) shows the N 1s core level spectrum of Al2O3-PDA nano-particles was deconvoluted into two peaks at BE of 398.5 eV and 399.5 eV, attributable to =Nspecies and N-H species from dopamine and dopamine self-polymerization. The C1s XPS spectra of pristine Al2O3 particles can be deconvoluted into two peak components at binding energies of about 284.6 and 286.4, attributed to the C-C and C-O species, respectively. On the other hand, two additional peaks of C-N and O-C=O species at 285.5 eV and 288.5 eV are appeared in Al2O3-PDA nano-particles, which can be another fingerprint of PDA coating on Al2O3-PDA nano-particles. In addition, determined by XPS spectra of pristine Al2O3 nano-particles, the C, N, and O concentration is 35.5%, 41.6%, and 0.76%, respectively. After PDA was coated on Al2O3 nano-particles, the O/C ratio is increased from 0.85 to 1.03, attributed to higher O/C ratio of PDA than that of pristine Al2O3 nano-particles. These above results demonstrated the successfully self-polymerization of dopamine on the Al2O3 nano-particles. The surface topographies of Al2O3 nano-particles before and after treatment by PDA was observed using HR-TEM. Figure 3(a) shows that the pristine Al2O3 nano-particles display smooth surface. By contrast, the surface of Al2O3-PDA nano particles exhibit a distinct and continuous deposition layer with thickness of ≈2 nm



(shown in Figure 3(b)), providing further evidence for deposition of PDA on Al2O3 nano-particles. In addition, the PDA amount of on the surface of Al2O3 particles is 1.89%, according to calculation of TGA and gravimetric method (shown in Figure S1). Until now, the mechanism of dopamine self-polymerization is still under some debate. It is widely think that the catechol groups firstly oxidized into quinone at weak alkaline PH, which then forms a key intermediate 5,6-dihydroxyindole (DHI) through rearrangement and intramolecular cyclization via 1, 4-Michael addition. The DHI further oxidized into 5,6-indolequinone, which can further undergo intermolecular cross-linking to form PDA layer, amelanin-like polymer (shown in Figure 1(b)) [25, 35-37]. The XRD diffraction patterns of the Al2O3 and Al2O3-PDA nano-particles are presented in Figure 4. Comparing with XRD patterns of Al2O3 and Al2O3-PDA nano-particles, we can see that the characteristic peaks of Al2O3 and Al2O3-PDA nano-particles show the same diffraction peaks, suggesting that the coating process of PDA does not affect the crystal structure of Al2O3 nano-particles and the thin PDA layer is amorphous. 3.2 Microstructure of SR composites filled with Al2O3 and Al2O3-PDA nano-particles To directly observe the filler dispersion in SR composites, we took SEM to investigate, which is shown in Figure 5. In order to determine the elemental composition of white spots in SEM images, the EDX spectrum of white spots in 30 vol% Al2O3/SR composite was tested and shown in Figure 5(e). From Figure 5(e), we


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can find the elemental content ratio of Al to O is 3/2, demonstrating the particle is Al2O3. It can be seen that the Al2O3 nano-particles are exposed on the fractured surfaces in Figure 5(a), indicating a poor interfacial adhesion between the Al2O3 nano-particles and SR matrix. When the content of Al2O3 nano-particles increases to 30 vol%, there are some serious agglomerates in the SR composite. On the contrary, the Al2O3-PDA/SR composites exhibit better dispersion and smoother fractured surfaces comparing with the Al2O3/SR composites. Especially in the SR composite filled with 10 vol% Al2O3-PDA nano-particles, almost every individual Al2O3-PDA nano-particles are being surrounded by a layer of polymeric matrix, attributed to compatibility between filler and matrix was improved by dopamine modification. In addition, FTIR measurements were used to analysis the interaction between Al2O3-PDA and SR matrix, which is shown in Figure S2. The absorption peaks at 1085 cm-1, which are attributed to Si–O–Si symmetric stretch shifts to higher wavenumber in the Al2O3-PDA/SR composites, attributed by the formation of hydrogen bonding between the dopamine and SR [38, 39]. The illustration of hydrogen bonding formed in Al2O3-PDA/SR composites is shown in Figure 1(c). These results demonstrated that PDA can provide strong adhesion to surface of Al2O3 nano-particles and SR macromolecules. 3.3 Mechanical properties of SR composites filled with Al2O3 and Al2O3-PDA nano-particles In order to evaluate the mechanical properties of SR composites filled with different volume fraction of Al2O3 and Al2O3-PDA nano-particles, tensile tests were



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performed. The stress-strain curves are presented in Figure 6. By comparing with pure SR, an increase of tensile strength and elongation at break are easily observed by SR composites. The tensile strength of 30 vol% Al2O3/SR composite and 30 vol% Al2O3-PDA/SR composite are increased from 0.17 MPa for pure SR to 1.77 MPa and 1.29 MPa, respectively. The much increased tensile strength can be explained by enhancement of inorganic fillers. In addition, comparing with Al2O3/SR composites, the Al2O3-PDA/SR composites display a lower tensile strength at the same content of filler. This phenomenon might be responsible by two competing factors. One is the strong interfacial interaction between Al2O3-PDA nano-particles and SR matrix will improve filler dispersion and suppressed the mobility of polymer chains of polymer chains, thus increasing the tensile strength. Second is the softening effect as a result of the rigid Al2O3 nano-particle coated by a PDA layer with much lower modulus than that of pristine Al2O3 nano-particles. The PDA layer will increase the volume fraction of whole polymeric phase in SR composites [30]. Assuming that the Al2O3 nano-particle was spherical and 171 g Al2O3 nano-particles were incorporated into 100 g SR matrix (with volume fraction of 30%), we can calculate the volume of PDA layer (VPDA) coated on Al2O3 nano-particles roughly according to the following equation: VPDA =n Al2O3 × 4π r 2 × h =

V Al O

2 3

VsAl O

× 4π r 2 × h

2 3

=

m Al O

ρ

Al2 O3

m Al O 2 3 × 4π r × h = × 4π r 2 × h 4 3 ρ Al2O3 × π r 3

(2)

2

2 3

× VsAl O

2 3

where r and VsAl O are the radius and volume of signal Al2O3 nano-particle, 2 3


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respectively, n Al2O3 is the number of Al2O3 nano-particles in the matrix, m Al O is 2 3

weight of all the Al2O3 nano-particles in the matrix (i.e. 171 g), h is the thickness of PDA (i.e. 2 nm), and ρ Al O is the density of Al2O3 nano-particles (i.e. 4.0 g/cm3). It 2 3

was found that the polymeric volume introduced by PDA coating is about 1.71 cm3, while the volume of Al2O3 nano-particles ( VAl O ) is about 42.75 cm3 in the ca.100 cm3 2 3

SR. The softening effect of Al2O3-PDA nano-particles is the dominant factor and the interfacial interaction has a small impact on tensile strength, leading to the tensile strength of Al2O3-PDA/SR composites is lower than that of Al2O3/SR composites. In addition, the elongation at break of SR composite filled with 30 vol% Al2O3 or Al2O3-PDA nano-particles is almost three times of pure SR. This phenomenon can be explained by the decreased crosslink density of SR composites after filled with inorganic filler. The interference of inorganic nano-particles on the crosslinking process of elastomer also can be found in associated literature [30]. Usually, a low crosslink density will lead to a large elongation at break. Moreover, comparing with Al2O3/SR composites, Al2O3-PDA/SR composites display a larger elongation at break at the same content of filler, ascribe to strong interfacial interaction and uniform dispersion with fewer defects.

3.4 Thermal properties and dielectric properties of SR composites Figure 7 shows the in-plain thermal conductivity of Al2O3/SR composites and Al2O3-PDA/SR composites. The results demonstrate that thermal conductivity of SR composites increases with the fraction of Al2O3 nano-particles whether they are modificated by PDA or not. This can be explained by compact thermal conductive



chains or pathways can be easily formed in polymeric composites with the increase of the volume fraction of fillers. In addition, the higher thermal conductive filler were introduced into SR matrix will cause the thermal conductivity of SR composites increased. Meanwhile, we can find the thermal conductivity difference between Al2O3/SR composites and Al2O3-PDA/SR composites is not considerable at low content. With increasing filler content, the increase of Al2O3-PDA/SR composites is faster than that of Al2O3/SR composites. It may be due to that the thermally conductive particles surrounded by SR matrix cannot touch each other at the low loading. As the volume fraction of the filler increases, the Al2O3-PDA/SR composites display better dispersion and the thermal conductive chains can be formed easier comparing with Al2O3/SR composites. The schematic diagram of dispersion of Al2O3 and Al2O3-PDA nano-particles in SR composites is shown in Figure 8. Therefore, the largest thermal conductive of 0.585 W/(mK) is obtained by SR composite filled with 30 vol% Al2O3-PDA, which is almost 400% of SR without filler. The room temperature dielectric properties of samples were investigated by dielectric relaxation spectroscopy. Figure 7 displays dielectric constant of pure SR and SR composites at frequency of 1 kHz. From Figure 7, we can find the dielectric constant of all composites increase with increasing volume fraction of filler. There are two reasons can be explained for this result. First, the dielectric constant of Al2O3 nano-particles is higher than that of SR matrix. Second, increase in dipole polarization of Al2O3 particles and interface polarization between the SR matrix and Al2O3-PDA particle [5]. In addition, the dielectric constant of Al2O3-PDA/SR composites is higher


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than that of Al2O3/SR composites at the same filler content, ascribe to uniform dispersion of Al2O3-PDA particles in the SR matrix [16]. Poor dispersion and poor interfacial adhesion between nano-particles and polymeric matrix is a key factor for low dielectric constant [40]. However, the 30 vol% Al2O3-PDA/SR composite displayed a relatively low dielectric constant (4.06 at 1 kHz), which is a bit higher than that of pure SR (2.59 at 1 kHz), a big advantage for the electronic or electrical engineering application. The frequency dependence of dielectric properties of SR composites filled with different volume fraction of Al2O3-PDA nano-particles is shown in Figure 9. From Figure 9(a), we can observe the spectra of dielectric constant of all samples are almost flat over the entire range of frequency, indicating the polarization in composites can catch up with the frequency change of electric field [41]. On the contrary, the dielectric loss tangent exhibits a dramatically decrease at the low frequency range of 101-103 Hz, as shown in Figure 9(b). However, the dielectric loss of the composites is also increased with increasing volume fraction of Al2O3-PDA particles. For example, the dielectric loss tangent of 30 vol% Al2O3-PDA/SR composite at 100 Hz is increased from 0.001 for pure SR to 0.004. In short, the dielectric loss tangent of all composites is lower than 0.01 over wide frequency range, a big advantage for dielectric materials in wide application.

3.5 Volume resistivity of SR composites To

investigate

the

insulating

property

of

Al2O3/SR

composites

and

Al2O3-PDA/SR composites, the volume resistivity was measured. As shown in Figure 10, the volume resistivity of SR composites decreases with the increasing volume



fraction of Al2O3 or Al2O3-PDA nano-particles, ascribe to the electrical conductivity of Al2O3 nano-particles is higher than that of SR matrix. However, the volume resistivity Al2O3-PDA/SR composites are a little higher than that of Al2O3/SR composites at the same filler content. This pehemenon can be explained that the insulating PDA hinders the electron transition of the composites, thus improving the volume resistivity [16, 42]. However, volume resistivity of all of samples is higher than 10E13, indicating the composites are insulate, which can be used in electrical insulation field.

4. Conclusion The Al2O3 nano-particles were successfully coated an organic PDA layer through biomimetic method. The organic PDA layer coating on the surface of Al2O3 nano-particles improved homogeneous dispersion of Al2O3-PDA/SR composites and increased interface interaction between inorganic nano-particles and polymeric matrix. Therefore, the 30 vol% Al2O3-PDA/SR composite exhibited a high thermal conductivity (0.585 W/(mK)), which was almost 400% of pure silicone rubber (0.147 W/(mK)). However, the dielectric constant of 30 vol% Al2O3-PDA/SR composite is relatively low (4.06 at 1 kHz), which was a bit higher than that of pure SR (2.59 at 1 kHz), a big advantage for the electronic or electrical engineering application of SR composite. This biomimetic method is efficient, easy handling, controllable, and eco-friendly, which can be used to modify other thermal conductive filler to improve thermal conductivity of composites. ASSOCIATED CONTENT


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The Supporting Information is available free of charge on the ACS Publications website at DOI: ***********. Figure S1 shows TG curves of Al2O3, Al2O3-PDA particles, and pure PDA. Figure S2 shows FTIR spectra of Al2O3/SR composites and Al2O3-PDA/SR composites (PDF).

Acknowledgements This work was supported by the National Natural Science Foundation of China (No. 51503019), Beijing Natural Science Foundation (No. 2162014), Beijing Science and Technology

Project

of

Beijing

Municipal

Education

Commission

(KM201710017005).

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Figure 1 Illustration of (a) procedure for preparation of Al2O3-PDA nano-particles, (b) possible mechanism of dopamine oxidative self-polymerization, and (c) hydrogen bonding formed in Al2O3-PDA/SR composites.


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Figure 2 XPS wide-scan, N 1s core-level spectra, and C 1s core-level spectra of (a, c, e) Al2O3 and (b, d, f) Al2O3-PDA nano-particles.



Figure 3 HR-TEM images of (a) Al2O3 and (b) Al2O3-PDA nano-particles.


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Figure 4 XRD patterns of (a) Al2O3 and (b) Al2O3-PDA nano-particles.



Figure 5 SEM micrographs of SR composites filled with (a) 10 vol% Al2O3, (b) 10 vol% Al2O3-PDA, (c) 30 vol% Al2O3, and (d) 30 vol% Al2O3-PDA nano-particles. (e) EDX spectrum of white spots in 30 vol% Al2O3/SR composite.


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Figure 6 Stress-strain curves of SR composites filled with different contents of Al2O3 and Al2O3-PDA nano-particles.



Figure 7 Thermal conductivity and dielectric constant (1 kHz) of SR composites filled with different contents of Al2O3 and Al2O3-PDA nano-particles.


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Figure 8 The schematic diagram of dispersion of Al2O3 and Al2O3-PDA nano-particles in SR matix.



Figure 9 Dependence of (a) dielectric constant and (b) dielectric loss tangent of SR composites on frequency at different contents of Al2O3-PDA nano-particles.


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Figure 10 Volume resistivity of SR composites filled with different contents of Al2O3 and Al2O3-PDA nano-particles.



Table of Contents

Largely improved thermal conductivity and low dielectric constant of aluminium oxide/silicone elastomer composites were prepared by mussel inspired modification method.


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Silicone Elastomer

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