Role of Interface on the Thermal Conductivity of Highly Filled Dielectric

Jun 5, 2012 - Journal of Chemical Education · Journal of Chemical Information and Modeling ..... Composites Science and Technology 2018 164, 153-159 ...
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Role of Interface on the Thermal Conductivity of Highly Filled Dielectric Epoxy/AlN Composites Xingyi Huang,*,†,‡ Tomonori Iizuka,† Pingkai Jiang,‡ Yoshimichi Ohki,§ and Toshikatsu Tanaka† †

IPS Research Center, Waseda University, Kitakyushu, Fukuoka, Japan Department of Polymer Science and Engineering and Shanghai Key Lab of Electrical Insulation and Thermal Aging, Shanghai Jiao Tong University, 200240 Shanghai, China § Department of Electrical Engineering and Bioscience, Waseda University, Tokyo, Japan ‡

S Supporting Information *

ABSTRACT: The interface between filler and matrix has long been a critical problem that affects the thermal conductivity of polymer composites. The effects of the interface on the thermal conductivity of the composite with low filler loading are well documented, whereas the role of the interface in highly filled polymer composites is not clear. Here we report on a systematic study of the effects of interface on the thermal conductivity of highly filled epoxy composites. Six kinds of surface treated and as received AlN particles are used as fillers. Three kinds of treated AlN are functionalized by silanes, i.e., amino, epoxy, and mercapto group terminated silanes. Others are functionalized by three kinds of materials, i.e., polyhedral oligomeric silsesquioxane (POSS), hyperbranched polymer, and graphene oxide (GO). An intensive study was made to clarify how the variation of the modifier would affect the microstructure, density, interfacial adhesion, and thus the final thermal conductivity of the composites. It was found that the thermal conductivity enhancement of the composites is not only dependent on the type and physicochemical nature of the modifiers but also dependent on the filler loading. In addition, some unexpected results were found in the composites with particle loading higher than the percolation threshold. For instance, the composites with AlN treated by the silane uncapable of reacting with the epoxy resin show the most effective enhancement of the thermal conductivity. Finally, dielectric spectroscopy was used to evaluate the insulating properties of the composites. This work sets the way toward the choice of a proper modifier for enhancing the thermal conductivity of highly filled dielectric polymer composites.

1. INTRODUCTION Recently, thermally conductive but electrically insulating materials have attracted increasing interest due to their wide applications in electronic devices and electrical equipments.1,2 Thermal management now is a great challenge in developing the next generation of integrated circuits and high power electronic devices.3 In many cases, a heat conductor but electrical insulator should be used to dissipate the heat (e.g., printed wiring boards, insulation for motor and generator coils, and sealants for light emitting diodes and solar cells). Although insulating polymers (e.g., epoxy resin) have been widely used for electronic packaging and electrical insulation applications because of easy processing, low cost and lightweight, most polymers are thermally insulating and have a thermal conductivity of only about 0.2 W/(m·K), which cannot meet the heat-dissipating requirements of modern electronic and electrical systems.4 Therefore, improvement of the thermal conductivity of the polymer becomes important. One efficient method to increase the thermal conductivity of a polymer is to introduce high-thermal-conductivity fillers. Electrically conductive fillers such as metal particles,5 graphite platelet,6,7 carbon nanotubes,8 and graphene sheets9,10 have a high intrinsic thermal conductivity and have been widely used © 2012 American Chemical Society

to increase the thermal conductivity of polymers. However, such fillers can change the intrinsic insulating properties of the polymer materials (e.g., causing extremely high electrical conductivity, dielectric loss, and dielectric constant) and thus cannot be used to prepare high-thermal-conductivity composites for dielectric applications. Therefore, much attention has been paid to the high-thermal-conductivity fillers with excellent insulating properties.1,2 Up to now, many kinds of ceramics fillers including alumina (Al2O3),11 aluminum nitride (AlN),12 and boron nitride (BN)1,12 have been used to prepare highthermal-conductivity polymer composites. It has been widely believed that composites with optimal properties can be achieved by tuning the interface interactions between the filler and the polymer matrix. In most cases, achieving desirable properties requires the development of effective methods to functionalize the filler or to tailor the surface chemistry of the fillers. For example, Chung and her coworkers clearly showed that silane treatment of BN and AlN resulted in effective enhancement in conductivity of epoxy Received: March 20, 2012 Revised: May 31, 2012 Published: June 5, 2012 13629

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composites.12 Although much experimental work showed that the filler surface treatment can result in improved interfacial adhesion and thus increased thermal conductivity, the understanding of the mechanisms behind the observed results are still limited.13 So far, many modifiers (e.g., silane coupling agent) have been used to functionalize the fillers, but few reports address the effects of the intrinsic property of the modifiers on the thermal conductivity of the composites, and there has been limited comparative investigation on the thermal conductivity of the composites among different modifiers. In addition, there is no explanation on the question of “why the same surface treatment shows different effects at different filler loading levels”. Herein, we try to understand these issues by investigating the effects of filler surface treatments on the thermal conductivity of the epoxy composites. AlN particles were used as fillers because of its high thermal conductivity and good insulating properties. Considering that the composites with high thermal conductivity are of great interest for practical applications, only composites with high filler loading are investigated. The particles were surface treated by a variety of surface modifiers including silane coupling agents with different terminal groups, polyhedral oligomeric silsesquioxane (POSS), hyperbranched aromatic polyamide and graphene oxide (GO). The resulting epoxy composites were prepared by using a method that the authors had developed by themselves. GO is chosen rather than graphene is because of its insulating nature.14 The aim of the work is to prepare dielectric polymer composites with high thermal conductivity, and thus the use of electrically conductive filler (e.g., graphene) should be avoided.4

methyl-2-pyrrolidone (NMP), N,N-dimethylformamide (DMF), triphenyl phosphite (TPP), pyridine, ninhydrin, toluene, and ethanol were purchased from Wako Pure Chemical Industries, Ltd. Octaglycidyldimethylsilyl polyhedral oligometric silsesquioxanes (epoxy-POSS) was purchased from Hybrid Plastics, Inc. All the chemicals were used as received. 2.2. Surface Modification of AlN by Silane Coupling Agents. Prior to surface modification, the AlN particles were dried in a vacuum oven at 105 °C for 12 h. After cooling to room temperature, AlN and a silane couple agent were added in a 500 mL three-necked flask containing 300 mL of dry toluene, which was equipped with a mechanical stirrer and a nitrogen inlet, and then the resulting mixture was stirred at 135 °C for 12 h under a N2 atmosphere. After cooling to room temperature, the mixture was filtered, and the wet cake was quickly washed with fresh ethanol. The product was dried in a vacuum oven for 24 h and then was stored in a desiccator. For the sake of understanding, the AlN treated by KBM-303, KBM803 and KBM-903 are named for epoxy-functionalized AlN, mercapto-functionalized AlN, and amino-functionalized AlN, respectively. 2.3. Preparation of GO-Encapsulated AlN Hybrid Fillers. GO was synthesized from the graphite powder through a modified Hummers method.15 For preparing GO/AlN hybrid filler, GO (0.4 g) was first dispersed in DMF (800 mL) by ultrasonic treatment. At the same time, amino-functionalized AlN particles (20 g) were dispersed in DMF (250 mL). Then the GO solution was added into the AlN/DMF suspension under mild stirring. After 2 h, the GO-encapsulated AlN hybrid filler was obtained by filtration. The product was dried at the vacuum oven for 24 h and then was stored in the desiccator. Figure 2 presents the assembly process of AlN and GO. 2.4. Preparation of Epoxy-POSS Treated AlN. In a 500 mL three-necked flask equipped with a mechanical stirrer and a nitrogen inlet, 60 g of amino-functionalized AlN particles were dispersed in 250 mL of THF. Then POSS (3 g) in 50 mL of THF was slowly injected into the flask. The mixture was heated to 60 °C, and the reaction was allowed to proceed for 4 h. After cooling to room temperature, the mixture was filtered and washed with THF. The product was dried in a vacuum oven for 24 h and then was stored in the desiccator. The preparation process for POSS treated AlN is shown in Figure 2 (route III). 2.5. Preparation of Hyperbranched Aromatic Polyamide Grafted AlN Particles. The polymerization of hyperbranched aromatic polyamide on the surface of AlN was done based on previous work.16 First, amino-functionalized AlN particles (30 g) were dispersed in 300 mL NMP and then DABA (15 g) was charged in the AlN/NMP mixture. After the DABA was dissolved completely, 75 mL of pyridine and 75 mL of TPP were added into the AlN/NMP/DABA mixture as a condenser. The mixture was heated to 100 °C and stirred under nitrogen for 6 h. After cooling to room temperature, the mixture was filtered and washed with DMF three times and then dried in a vacuum at 105 °C to a constant weight. The preparation process is shown in Figure 2 (route IV). In 1H NMR (400 Hz, DMSO-d6) of the hyperbranched aromatic polyamide, chemical shift δ = 8.3, 7.9 ppm (aromatic protons from the dendritic structure), δ = 7.3, 6.8 ppm (aromatic protons from the linear structure), δ = 6.3, 6.0 ppm (aromatic protons from the terminal structure), and δ = 10.4− 9.6 ppm (amide protons).

2. EXPERIMENTAL SECTION 2.1. Raw Materials. High purity aluminum nitride particles with an average size of 1.1 μm were obtained from Tokuyama Corp. Diglycidyl ether of bisphenol A (DGEBA) epoxy resin (JER 828) and amine hardener (JER 113) were purchased from Japan Epoxy Resin Co. All silane coupling agents (2-(3,4epoxycyclohexyl) ethyltrimethoxysilane, KBM 303; 3-glycidoxypropyltrimethoxysilane, KBM 803; 3-aminopropyltrimethoxysilane, KBM 903) were purchased from ShinEtsu Chemical Co. Figure 1 shows the chemical structures of the epoxy resin, curing agent, and the silane coupling agents. Gaphite powder, lithium chloride (LiCl), 3,5-diaminobenzoic acid (DABA), N-

Figure 1. Chemical structure of the silane coupling agents, epoxy resin, and curing agent used in this work. 13630

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Figure 2. Surface treatment and assembly procedures of AlN particles. Route I: preparation process of amino-functionalized AlN. Route II: assembly of amino-functionalized AlN and GO. Route III: reaction between amino-functionalized AlN and POSS. Route IV: preparation of hyperbranched polymer modified AlN.

Figure 3. Preparation procedures for uncured epoxy/AlN mixture: (a) surface treatment of AlN, (b) addition of AlN into MEK/epoxy/curing-agent solution, (c) tip-sonication, (d) moving the AlN mixture into a Thinky Mixer, and (e) mixing and removing MEK by rotation and revolution.

2.6. Preparation of Uncured Epoxy/AlN Mixture. The preparation process of uncured epoxy/AlN mixture can be divided into the following two steps, as shown in Figure 3. Step 1: The AlN filler was dispersed in an epoxy/curingagent/MEK solution by 5 min (25% amplitude for 2.5 min and 50% amplitude for 2.5 min) tip-sonication (Hielscher UP200S, Germany), and the temperature of the mixture was kept to be below 20 °C by an ice bath. Step 2: The epoxy/curing agent/AlN/MEK mixture was stirred with a rotation speed of 2000 rpm and a revolution speed of 1000 rpm by using a Thinky Mixer (ARE250, Thinky Co.). The purpose of this step is to remove most of the solvent (MEK) during the mixing process of epoxy, curing agent, and AlN particles. 2.7. Sample Preparation for Thermal Conductivity Measurements. The preparation process of the composite samples for thermal conductivity measurements was shown in Figure 4, including (i) injecting the uncured epoxy/AlN mixture into a mold, (ii) placing the mold into an oven for 60 min degassing in vacuum, (iii) applying a pressure of 40

MPa on the mixture for 10 min, (iv) releasing the pressure and removing the unnecessary mixture, (v) reapplying a pressure of 40 MPa on the mixture and heating to 70 °C for 3 h precuring and then heating to 150 °C for 3 h postcuring, and (vi) cooling to room temperature and then taking the samples out of the mold. 2.8. Measurements. The fractured surface of epoxy composites was observed using a Keyence VE-7800 scanning electron microscope (SEM). The composite sheets were first cracked, and then the resulting fractured surface was sputtered with thin layers of gold to avoid accumulation of charges. Cross section polishing (CP) via argon beam milling allows the precise observation of the particle dispersion. A cross section is prepared by polishing the fractured surface via argon beam milling in the through-thickness direction.17 During the CP processing, the argon ion beam is irradiated parallel to the surface of cross section and thus the irradiation damage because of the ion beam is minimized. Thermal diffusivity (δ) was measured on disk samples (diameter 12.6 mm, thickness 1 mm) using an LFA447 light 13631

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(δ, mm2/s), specific heat (C, J/(g·K)), and bulk density (ρ, g/ cm3): λ = δCρ

(1)

Thermogravimetric analysis (TGA; NETZSCH TG209 F3) was carried out to investigate the weight loss of as received and surface treated AlN particles. The samples were approximately 10 mg, and all of the measurements were carried out under N2 flow. Dynamic runs were carried out from 50 to 800 °C at the heating rate of 20 °C/min. The broadband frequency dielectric characteristics of the composites were measured using a Solartron SI 1260 impedance analyzer (Advanced Measurement Technology, Inc., U.K.) in a frequency range of 10−1 to105 Hz. All of the samples with diameter of 12.6 mm have a layer of gold evaporated on both surfaces to serve as electrodes. The samples are considered as plane capacitors and described by parallel resistor−capacitor (RC) circuit systems. The complex dielectric constant (ε*) is calculated as follows: ε* = ε′ − jε″

(2)

where ε′ and ε″ correspond to the real and imaginary parts of the complex dielectric constant, respectively. ω = 2πf is the angular frequency, and j = (−1)1/2. The dielectric loss tangent (tan δ) is defined as tan δ =

ε″ ε′

(3)

3. RESULTS AND DISCUSSION 3.1. Characterization of Functionalized AlN Particles. The surface modification of inorganic filler by silane, either under a wet condition or under an anhydrous condition, has long been studied.18 AlN can react with water to form lowthermal-conductivity aluminum hydroxide, which is undesirable, and thus a dry method was chosen for the surface treatment of AlN by silanes.19 Methoxysilanes are effective for this method without catalysis and thus were used in this study. Recently two publications not only described the possible reaction process between silane and inorganic substance but also provided the surface structure of silane modified inorganic substance.20,21 In order to rapidly verify the presence of a silane on the treated surface of AlN, KBM-903 treated particles were washed by ethanol three times and then dispersed in a 2 wt % ninhydrin/ethanol solution. It can be seen from the photograph (see Figure S1 in the Supporting Information) that the color of the solution containing surface treated AlN turned blue within several minutes whereas the solution with as received filler did not show any change of color as the time went on, indicating the existence of amine groups on the AlN surface.22 The POSS

Figure 4. (a) Scheme of the composite preparation process. (b) Mold and samples.

flash system (NETZSCH, Germany) at 25 °C. Bulk density (ρ) of a specimen was measured by water displacement. Specific heat was measured by using a SHIMADZU differential scanning calorimeter (DSC60). The samples were crimped in nonhermetic aluminum pans. Thermal conductivity (λ, W·m−1·K−1) was given by the product of thermal diffusivity

Figure 5. TEM images of as received AlN (a) and hyperbranched polymer grafted AlN particles (b and c). (c) An enlarged image from panel b. 13632

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Figure 6. (a) TGA curves of as received and surface treated AlN. (b) TGA curves of pure POSS and hyperbranched polymer.

functionalized AlN particles were washed by THF three times and then characterized by energy-dispersive X-ray spectroscopy (EDX). As shown in Table S1 (see the Supporting Information), the existence of silicon was detected for AlN treated by silane and POSS, whereas POSS treated AlN contains a larger percentage of silicon. Thus, a comparison of silicon percentage provides evidence that the introduction of POSS onto the amino-functionalized AlN is successful. The AlN particles treated by hyperbranched polymer were directly characterized by TEM. As one can see from Figure 5, a thin polymer shell is clearly observed on the surface of AlN, and its thickness is from several to 20 nm. The as received and surface treated AlN particles were further characterized by TGA. One can see from Figure 6 that the weight of as received AlN decreased linearly with the temperature under a N2 atmosphere and that the maximum weight loss is around 2%. However, after the surface treatment, all of the particles show the decrease in weight loss. These results indicate that there exist some thermally unstable compounds (e.g., hydroxide hydrate19) on the surface of AlN and that these compounds are partly removed during the surface treatment by silanes. The surface treatment of AlN was performed in an anhydrous condition with 12-h stirring at 135 °C, which may be enough to cause the decomposition of the unstable compounds. The low weight loss of hyperbranched polymer treated AlN and POSS treated AlN is attributed to their intrinsic thermal properties. The degradation of the hyperbranched polymer and POSS at high temperature gives monomers, water and residue (carbon for hyperbranched polymer, SiO2 for POSS). The monomers and water can be removed through the flow of nitrogen whereas the residue would remain after the thermolysis, resulting in a low weight loss. As can be seen from Figure 6b, the weight loss of the pure POSS and the hyperbranched polymer is about 57 and 32%, respectively, indicating that the residue occupies a large proportion among the degradation products. GO and amino-functionalized AlN can be homogeneously dispersed in DMF by a tip-sonication (see Figure S2 in the Supporting Information). However, once the GO solution were added into the AlN/DMF suspension with mechanical stirring, a stable suspension cannot be observed any more and gray precipitation become more and more in the bottom of the mixture. The mechanism behind this phenomenon should be attributed to electrostatic interaction between positively charged amino-functionalized AlN and negatively charged GO in DMF solution.23 The precipitation was further characterized by TEM and SEM and the results are shown in Figure 7. One

Figure 7. TEM (a) and SEM (b) images of GO-encapsulated AlN particles.

can see that the AlN particles were encapsulated by flexible and ultrathin GO sheets, which showed crinkled and rough textures (Figure 7a). It also appears that the GO sheets link the neighboring particles together (Figure 7b). 3.2. Microstructure of the Composites. Figure 8 shows the fractured surface morphology of the epoxy composites. One can see from Figure 8a that the as received AlN particles show such a weak filler−polymer interface at which the interfacial debonding can be seen from place to place. In addition, some particles were pulled out of the matrix during the fracture process, resulting in exposing holes. On the contrary, the AlN particles treated by KBM-903, KBM303, hyperbranched polymer, and POSS are covered by a layer of polymer (Figure 8b,d−f), indicating strong interfacial adhesion between AlN and the epoxy matrix. The common characteristic of these particles is that all of them have functional groups capable of reaction with epoxy resin or curing agents. It is believed that the coupling reactions between the functional groups of organic molecules and epoxy/curing agent are the main reason for the excellent interfacial adhesion between AlN and the epoxy matrix. KBM 903 and the hyperbranched polymer treated AlN particles have amino terminated groups capable of reacting with epoxy groups of the epoxy resin, while KBM 303 and POSS treated AlN particles have epoxy groups capable of reacting with amino groups of the curing agents. Although the interfacial adhesion between KBM-803 treated AlN and the epoxy matrix is improved in comparison with the composites with as received AlN (Figure 8c), the interfacial adhesion in the composites with AlN treated by KBM-803 is not as good as those in other composites (e.g., composites with KBM-903 treated AlN), and filler−matrix debonding can still be observed. For amine-cured epoxy resin, the curing mechanisms have been fully investigated (see Figure S3 in the Supporting Information).24 The addition reaction between the epoxy group and a primary amine is the main reaction and proceeds in two 13633

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Figure 8. SEM images of epoxy composites with as received AlN (a), KBM-303 treated AlN (b), KBM-803 treated AlN (c), KBM-903 treated AlN (d), POSS treated AlN (e), and hyperbranched polymer treated AlN (f). All of the samples have 60 vol % AlN.

steps involving successive reactions of the primary and secondary amino group. In addition to these reactions etherification also takes place, which is initiated by the −OH group formed in the epoxy−amine reaction. After the surface treatment by KBM-303, KBM-903, POSS and hyperbranched polymer, the surface of the AlN particles is functionalized by epoxy or amino groups capable of reacting with the hardener and epoxy resin, resulting in interfacial bonding between the particles and epoxy matrix and thus good interfacial adhesion. Although the reactions between mercapto groups (from KBM803) with epoxy can take place and form covalent bonding between the AlN and epoxy matrix, such a reaction generally needs accelerator or catalysis which was not used in this work. In the absence of accelerator or catalysis, the reactions of mercapto groups with epoxy group might not take place, at

least not completely, in comparison with those between amino groups and epoxy groups. This is a possible reason for the weak interfacial adhesion in the composites containing KBM-803 treated AlN. Concerning the composites with GO capsulated AlN, the SEM image in Figure 9 reveals that there exist some larger GO layers and GO-encapsulated AlN particle clusters in the composites. Although the GO-encapsulated AlN was prepared by assembling GO and amino-functionalized AlN, the composites with GO-encapsulated AlN have more large voids and pores when compared with the composites containing amino-functionalized AlN. This is first because of the large size of GO-encapsulated AlN particle clusters, which cannot be closely packed together and thus result in large empty space between them. Another reason is that the introduction of GO 13634

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the density even decreases as the filler loading increases to 65 vol % from 60 vol %. 3.3. Thermal Conductivity of the Composites. Figure 11 shows the thermal conductivity of the epoxy composites with 50−65 vol % AlN. It is clear that the effect of AlN surface treatment on the thermal conductivity is not only dependent on the modifier but also dependent on the filler loading. For the composites with relatively low AlN loading (50 and 55 vol %), a significant enhancement of the thermal conductivity is seen in the composites with GO-encapsulated AlN and hyperbranched polymer treated AlN, and the composite with GO-encapsulated AlN has the highest values of thermal conductivity. When compared with as received AlN filler, KBM-303 and KBM-903 treated AlN can clearly increase the thermal conductivity of the composites, whereas KBM-803 and POSS treated AlN do not show any increase in thermal conductivity. Concerning the composites with 60 and 65 vol % AlN, the three kinds of silane surface treatments results in significant enhancement of thermal conductivity, and the composite with KBM-803 treated AlN has the highest values of thermal conductivity. Regarding the filler loading dependence of thermal conductivity, we can see that the thermal conductivity always increases with increasing the filler loading in the composites with silane and POSS treated AlN. However, the thermal conductivity of other composites shows significant increase only when the filler loading increases to 55 from 50 vol %. When the filler loading further increases to 65 from 55 vol %, the thermal conductivity shows almost no variation in the composites with as received and hyperbranched polymer treated AlN, and the thermal conductivity shows a tendency to decrease in the composites with GO-encapsulated AlN. For an insulating polymer filled with high-thermalconductivity fillers, high thermal conductivity can be achieved only when the composites possess many thermally conductive pathways4 or thermal percolating network. According to Figure 11, all of the composites show an increase in thermal conductivity as the AlN loading increases from 50 to 55 vol %, indicating that much more thermally conductive pathways have formed inside the composites with 55 vol % filler loading. Clearly, such an observation means that the thermal percolation threshold is much higher than the electrical percolation threshold of about 16−30 vol % in the conductive spherical particle filled insulating matrix system. It is believed that the small thermal conductivity ratio of the filler to the matrix and the high contact thermal resistance between fillers are the main reasons.3,25 From the viewpoint of particle dispersion, the formation of thermally conductively pathways means that the particles should contact each other to form continuous chains of particles over the whole samples. For verifying this speculation, cross-section views of four composites with 60 vol % AlN were provided in Figure 12. Clearly, the percolating networks are seen in all the four samples. Taking the aforementioned results into account, we suggest a composite microstructure model shown in Figure 13 to explain our experimental results. In the epoxy/AlN composites, there exist two types of interfaces. One is the filler−matrix interface, and the other is the filler−filler interfaces. When the filler loading is relatively low, the particles in the polymer matrix are isolated and the filler−matrix interface is dominant, while the filler−filler interface is negligible. The filler−filler interface

Figure 9. SEM image of an epoxy composite with GO-encapsulated AlN. The arrow indicates a GO-encapsulated AlN cluster.

significantly reduces the flowability of the uncured epoxy/AlN mixture. As can be seen in Figure 10, the composites with GO-

Figure 10. Density of the epoxy/AlN composites.

encapsulated AlN loading higher than 60 vol % have a lower density in comparison with the composites containing the same loading of other treated AlN. The density of the epoxy/AlN composites is associated with filler-to-polymer ratio and the microstructure of the composites. At a relatively low filler loading (50 or 55 vol %), the effect of filler surface treatment on the density is not significant because of the high flowability of the uncured epoxy/curing agent mixture. In this case, the uncured epoxy/curing agent mixture can fully penetrate into the space among the AlN particles, thus producing composites with comparable densities regardless the surface treatment. At a relatively high filler loading (60 vol %), the flow of the uncured epoxy/curing agent mixture becomes difficult. In such a case, the surface modification can wet the filler surface and reduce the viscosity of the uncured epoxy/filler mixture, resulting in a closely packed filler structure and low fraction of voids and holes within the composites. Therefore, the composites with AlN treated by silanes, POSS, and hyperbranched polymer have higher densities in comparison with the composites with as received AlN. At a very high filler loading (65 vol %), the flow of the uncured epoxy/filler mixture becomes so difficult that the surface treatment of the AlN fillers cannot avoid the presence of a large number of voids in the composites. In such a case, the density of the composites does not increase with further increase of filler loading. For several composites with 65 vol % filler (e.g., the composite with GO-encapsulated AlN), 13635

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Figure 11. Thermal conductivity of the epoxy/AlN composites. H-polymer indicates the hyperbranched polymer.

loading. Random close packing of spheres in three dimensions gives maximum packing fraction about 64 vol %.26 Therefore it is not difficult to understand why the composites with 65 vol % have high defect fraction of defects (voids and pores). Both the epoxy matrix and the AlN filler are insulating materials, and thus the transport of thermal energy inside the epoxy/AlN composites should be a phonon conduction mechanism. The phonon transport in the composite systems is typically dominated by scatter effects from interface and defects (voids and pores).27 When the filler loading is relatively low (50 vol %), the uncured epoxy/curing agent mixture can easily penetrate into the space among the AlN particles thus producing defect-free composites. Therefore the thermal conductivity of the composites is closely associated with the filler−matrix interface strength.13,27 The phonon scattering in the filler−polymer interface is closely related to interfacial strength. Interfacial bonding can cause significant reduction of phonon scattering and thus is beneficial to increase the thermal conductivity of the composites.28 According to Figure 8, the composites with as received AlN exhibit filler−matrix debonding, namely weak interfacial adhesion, and thus showing lower thermal conductivity. After the surface treatment by KBM-303 or KBM-903, the surface of the AlN particles is functionalized by epoxy and amino groups capable of reacting with the curing agents and epoxy resin, respectively, resulting in interfacial bonding between the particles and epoxy matrix and thus higher thermal conductivity. Similarly to KBM-303 treated AlN, POSS treated AlN can react with the curing agents and led to interfacial bonding (Figure 8). However, any enhancement of thermal conductivity was not observed in the composites with POSS treated AlN. This result can be explained by the intrinsic low thermal conductivity of the POSS itself, which has a closed-

Figure 12. Cross-section views of composites with 60% as received AlN (a), KBM-303 treated AlN (b), KBM-903 treated AlN (c), and hyperbranched polymer treated AlN (d).

Figure 13. Schematic of microstructure evolution in polymer/filler composites.

becomes more and more important as the filler loading increases. However, defects such as voids and pores will appear and become a technical problem with the increase of filler 13636

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cage siloxane (SiO) structure with a very low thermal conductivity.29 The composites with 50 vol % KBM-803 treated AlN also did not show any enhancement of thermal conductivity when compared with the composites containing as received AlN. This is because of the weak interfacial adhesion between filler and matrix, as shown in Figure 8. Of interest is that the hyperbranched polymer treated AlN resulted in stronger enhancement of the thermal conductivity in comparison with the silane (e.g., KBM 903) treated AlN. This observation can be easily understood by the fact that the hyperbranched polymer has much more amino terminal groups than the KBM 903 molecules,16,17 which can form much stronger interface with the epoxy matrix, resulting in a much higher thermal conductivity. The highest thermal conductivity of the composites with 50 vol % GO-encapsulated AlN should be attributed to the bridge effect of the GO sheets among AlN particles. Since GO sheet has a higher thermal conductivity than the epoxy matrix,30 such a bridge effect is much stronger than that of the covalent interaction between AlN and the polymer matrix. When the AlN loading increases to 55 vol %, the thermal conductivities become much higher in the composites. However, the variation of the thermal conductivity caused by the AlN surface treatment shows a similar tend in comparison with the composites with 50 vol % AlN, suggesting that these variations are still associated with the filler−matrix interface. As the AlN loading further increase from 55 vol %, there will exist more and more thermally conductive pathways in the composites. However, the defects will also appear because that the flow of the uncured epoxy/curing agent become more difficult. In such a case, the thermal conductivity variation of the composites is mainly determined by three factors: filler− filler interface, filler−matrix interface and defects. According to Figure 10, the composites with 60 and 65 vol % as received AlN and GO-encapsulated AlN have lower density in comparison to other composites with the same AlN loading, indicating much more defects inside the materials. Therefore, the lower thermal conductivity of the composites with 60 and 65 vol % as received AlN should be not only attributed to the weak interfacial adhesion but also to the high defect content. In contrast, the low thermal conductivity enhancement in the composites with GO-encapsulated AlN should be mainly attributed to the high defect content. According to Figure 10, the composites with silane, POSS and hyperbrached polymer treated 60 vol % AlN have the comparable density, therefore, the variation of the thermal conductivity for these composites should be attributed to the defects content in the materials. Unlike the composite with 50 and 55 vol % AlN, unexpected thermal conductivity enhancement in the composites with 60 and 65 vol % AlN is observed. First, the most effective enhancement in thermal conductivity was observed in the composites with KBM-803 treated AlN. Second, the composites with hyperbranched polymer treated AlN show lower (Figure 11c) or comparable (Figure 11d) thermal conductivity with the composites with KBM-903 treated AlN. This result indicates that the thermal conductivity of the composites was mainly determined by the filler−filler interfaces. For illustrating the thermal conductivity variation in the composites with 60 and 65 vol % AlN, two types of thermally conductive pathways were suggested. As shown in Figure 14, an ideal thermally conductive pathway in the composites should be

Figure 14. Schematic of two types of thermally conductive pathways in the epoxy/AlN composites.

formed by direct contact of particles. If the particles are covered by a layer of polymer, the filler−polymer interfacial scattering would not be avoided and thus the heat would be transferred with a low efficiency. According to Figure 8, the KBM-303, KBM-903, POSS and hyperbranched polymer treated AlN are covered by a layer of polymer, which resulted in low thermal conductivities in their composites. The hyperbranched polymer on the surface can also react with the epoxy molecules. Therefore, the thermally conductive pathways in the composites with hyperbranched polymer treated AlN might have a thick layer of thermal barrier. This is why we observed a lower thermal conductivity in the composite with 60 vol % hyperbranched polymer treated AlN in comparison with the composites with 60 vol % KBM-903 treated AlN. The much lower thermal conductivity of the composites with POSS treated AlN should be mainly attributed to the very low thermal conductivity of POSS. On the other hand, the POSS molecules on the surface of AlN can react with the curing agents, which would increase the thickness of thermal barrier between paticles. The existence of naked AlN particles in the composites with KBM-803 treated AlN means there might exist thermally conductive pathway formed by direct contact of AlN particles, which led to the largest enhancement of the thermal conductivity in the composites. According to such explanation, as received AlN particles should form thermally conductive pathway with high thermal transfer efficiency and should show the largest enhancement of thermal conductivity in the composites. However, the high defect content inside the composites offsets the positive effect of the filler−filler interface, resulting in lower thermal conductivity. When we increase the AlN loading up to 65 vol %, the density of composites will not continue to increase in comparison with the composites with 60 vol % (Figure 10), indicating that much defects exist inside of the composites. That is, the defects in composites with 65 vol % AlN play a more important role in affecting the thermal conductivity of the composites. This is the main reason why the composites containing 65 vol % as received AlN and GO-encapsulated AlN show decreased values of thermal conductivity in comparison with the corresponding composites with 60 vol % filler. 3.4. Dielectric Properties of the Composites. Apart from understanding the effect of interface on thermal conductivity of highly filled epoxy composites, another interest to us is to evaluate the influence of AlN surface modification on the dielectric property of the composites. Figure 15 shows the frequency dependences of ε′ and tan δ for the composites. 13637

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Figure 15. Frequency dependences of ε′ tan δ for the epoxy/AlN composites. H-polymer indicates the hyperbranched polymer.

conductivity enhancement. Below the critical concentration the covalent bonding tends to result in higher enhancement of thermal conductivity. Above the critical concentration, however, covalent bonding results in less effective improvement of thermal conductivity in the composites. We interpret this new finding as a result of the percolating process of AlN particles in the epoxy matrix. The covalent bonding between the particles and the matrix are important to increase the thermal conductivity of the composites with filler loading lower than the critical concentration. Direct contacts among neighboring particles, however, give better solutions to result in higher thermal conductivity enhancement in the composites with particle loading higher than the critical concentration, if there are no voids or few voids formed in the composites. The functionalization of AlN by mercapto-terminated silane is considered not only to cause direct contact of AlN in the composites but also to reduce the formation of voids during the curing process above the critical filler concentration. Actually, the highest thermal conductivity enhancement was observed in the composite with 65 vol % mercapto-functionalized AlN in our experiments. Their thermal conductivity is about 6 W/ (m·K) that is 60% higher than that of the composites containing the same content of as received AlN. It was elucidated that the thermal conductivity enhancement originated from the interface improvement is significantly offset by the defects (e.g., voids and pores). Voids and pores can be reduced by a certain surface treatment technique, i.e., functionalization by mercapto-terminated silane mentioned above. As voids and pores are absent below the critical concentration, the composites loaded by the highly conductive GO treated AlN exhibit the most effective thermal conductivity enhancement, and the composites with low-thermal-conductivity POSS treated AlN show no enhancement of the thermal conductivity. It is thus expected that the composites with GO treated AlN might have high thermal conductivity above the critical concentration, if defect formation was avoided by a certain technique. Dielectric properties are also influenced by interfaces formed around the fillers. It was clarified that the covalent bonding in

There are three noteworthy features in the frequency dependences of dielectric properties of the composites. First, the composites with as received AlN and GO treated AlN not only show clear frequency dependence of dielectric properties but also have relatively high tan δ and ε′ at low frequencies. Second, the composites with KBM-803 treated AlN also show clear frequency dependence of ε′ and the highest tan δ in the whole frequency range. Third, the composites with covalent bonding interfaces, namely those with KBM-303, KBM-903, POSS and hyperbranched polymer treated AlN, show weak frequency dependence of dielectric properties and low tan δ throughout the whole frequency range. The strong frequency dependence behaviors should be related to the interfacial polarization.31,32 The GO sheets may have higher electrical conductivity than the epoxy resin and AlN particles, which can lead to a charge buildup at the GOmatrix and GO-AlN interfaces when a voltage is applied to the samples in series.33 The as received AlN fillers usually have some impurities on their surfaces, as shown in Figure 6, which can cause strong interfacial polarization and should be the origin of the strong frequency dependence of ε′ and high tan δ.31,32 Concerning the composites with KBM-803 treated AlN, their electrical properties should be closely associated with the surface chemical groups of the AlN filler. After the surface treatment by KBM-803, the mercapto groups were introduced onto the surface of AlN. However, these groups cannot fully react with the epoxy matrix under the present curing conditions, and their hydrophilic polar nature can lead to interfacial polarization under ac electric field, resulting in high tan δ. Other composites with KBM-303, KBM-903, POSS and hyperbranched polymer treated AlN can form covalent bonds with the epoxy matrix, and thus showing a weak frequency dependence of dielectric property and low tan δ.

4. CONCLUSIONS Highly filled epoxy composites were prepared by using six kinds of surface treated AlN particles as well as untreated AlN. This work leads to some new insights into the effects of interface on the thermal conductivity of highly filled composites. It was found that there exists a critical concentration for the thermal 13638

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filler−matrix interface is beneficial to suppress the dielectric loss and to make dielectric properties less dependent on frequency.

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ASSOCIATED CONTENT

S Supporting Information *

The ninhydrin test for confirming the existence of amine groups at the surface of KBM-903 modified AlN, the results of EDX analyses of as received and modified AlN, the photos of GO/DMF suspension, AlN/DMF suspension, GO/AlN/DMF mixture, and the schematic illustration showing the networkforming reactions in amine-cured epoxy resins. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported in part by the Ministry of Education, Culture, Sports, Science, and Technology of Japan on the second Stage Knowledge Cluster Initiative No. 23, to which the authors are indebted. X.Y.H. is also thankful for the support from the National Nature Science Foundation of China (No. 51107081). The authors would like to thank Liyuan Xie, Chao Wu, and Mi Li for their help with thermal diffusivity, TEM, and TGA measurements.

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