Solvent-free fabrication of thermally conductive insulating epoxy composites with boron nitride nanoplatelets as fillers
© Wang et al.; licensee Springer. 2014
Received: 3 November 2014
Accepted: 14 November 2014
Published: 29 November 2014
A solvent-free method for the fabrication of thermally conductive epoxy-boron nitride (BN) nanoplatelet composite material is developed in this study. By this method, polymer composites with nearly any filler fractions can be easily fabricated. The maximum thermal conductivity reaches 5.24 W/mK, which is 1,600% improvement in comparison with that of pristine epoxy material. In addition, the as-fabricated samples exhibit excellent overall performances with great mechanical property and thermal stability well preserved.
With the miniaturization and integration trend of transistors in the microelectronic devices, heat management has become an important issue in manufacturing more powerful and reliable devices [1, 2]. In order to realize this purpose, thermally conductive but electrically insulating materials are selected as thermal interface materials for heat dissipation. Polymer-based composite materials with inorganic fillers incorporated are potential candidates and have attracted increasing attention for their processability and low density [3–7]. As is known to all, commonly used polymers, such as epoxy resin, polyacrylic resin, and polyurethane have a thermal conductivity in the range of 0.1 to 0.4 W/mK, which is far below the required levels. Therefore, to improve thermal conductivity, high fraction of inorganic fillers should be added into the matrix [8, 9]. Kim et al. reported a thermal conductivity of 2.85 W/mK of epoxy-boron nitride (BN)-filled composite at a filler fraction of 70 wt.% . Huang et al. reported a maximum thermal conductivity around 6 W/mK with a KBM303-treated AlN particles at a filler fraction of 65 vol.% . Other types of fillers like Al2O3, with a thermal conductivity of 4.3 W/mK at a filler fraction of 60 vol.%, have also been reported .
In spite of the progresses achieved, however, problems still exist. In order to disperse high loading fraction of fillers in the matrix, toxic organic solvents, such as dimethylformamide (DMF), tetrahydrofuran (THF), and methyl ethyl ketone (MEK), are sometimes inevitable for composite materials’ fabrication, which is not environmentally friendly enough [7, 11, 13–15]. In addition, effective removal of solvents remains a difficulty when bulk sample instead of films is fabricated. These problems are especially stubborn for fabrication of epoxy-based composite materials.
Boron nitride (BN) with a band gap of 6.0 eV possesses a high intrinsic in-plane thermal conductivity of 30 to 300 W/mK and the theoretical thermal conductivity of a BN nanotube may even high up to approximately 3,000 W/mK [12, 16, 17]. By virtue of its unique thermal conductivity, BN has been used intensively as ideal filler for thermally conductive composite materials. In terms of BN-filled polymer composite, appropriate highly effective mixing methods are necessary to produce material with high thermal conductivity [18, 19]. On the one hand, it was found that the thermal conductivity of polyethylene-graphite composite materials with different mixing methods can be ranked as: powder mixing > solution mixing > roll-mill mixing > melt mixing . On the other hand, Jonathan et al. demonstrated that mechanical shear is a facile and effective method to exfoliate materials with layered structure . Therefore, it is believed that the combination of these two methods, i.e. utilizing powder mixing mechanically exfoliated BN nanoplatelets (BNNPs), is a promising approach for fabricating highly thermally conductive polymeric composite filled with exfoliated BNNPs.
In this article, we present a facile solvent-free method for fabrication of BNNPs/epoxy resin composite materials. By utilizing the solid-state epoxy resin, the composite can be fabricated at almost any BNNP loading fraction without the help of organic solvent. The morphology, thermal conductivity, mechanical property, and density of the fabricated composite materials are studied systematically.
Solid-state epoxy resin (E13) and hardener (TP41) were purchased from TECH-POWER (HUANGSHAN) LTD, China. Raw BN powder was purchased from Zibo Jonye Ceramic Technologies Co., Ltd, Shandong province, China. KH550 silane coupling agent was purchased from Wancheng Chemical, China. All chemicals and materials were used as received without further purification. In order to form bulk-sized samples, a properly designed mold was used in the fabrication.
Silane treatment for BNNPs
BN powder was treated with silane coupling agent (KH550) following the method reported by Chung et al. . A silane-water solution was made at selected concentration in a flask. Then, BN powder was weighed at selected weight (BN: silane = 100:2.4) and added into the solution under magnetic stirring. After that, the flask was heated up to 65°C in a water bath with continued magnetic stirring for about 30 min. Finally, the treated powder was rinsed with water by filtration and the collected powder was dried in an oven at 110°C for 12 h.
Epoxy resin-BNNP composite fabrication
Characterization and measurements
The morphology and structural investigation were performed by scanning electron microscopy (SEM, JSM6335F FEG and JEOL JSM-820, Akishima-shi, Tokyo, Japan) and transmission electron microscopy (TEM, CM20). Thermal conductivities of as-fabricated samples were measured by DZDR-S thermal conductivity tester, which utilizes the transient plane source method. A plane sensor was sandwiched by two pieces of specimens with flat surface and thermal conductivity on the vertical direction perpendicular to the testing plane can be read from the screen directly. The density and hardness of the material were measured successively, by electronic densimeter (MD-300S) and Vickers hardness tester (FV-700), respectively.
Results and discussion
Ignoring the engineering and fabrication processes, the mechanisms of thermal conductivity improvement can be further phenomenologically discussed. At low filler fractions, long-range thermally conductive paths are not steadily formed, i.e. BNNPs are not effectively contacted with each other. As a result of silane treatment, although filler dispersion might be improved, thermal resistance actually exists at the interfaces between filler and matrix, which causes serious phonon scattering and deficiency. At high filler fractions, however, long-range thermally conductive paths are formed as there are enough BNNPs involved in the heat conduction. In this case, agglomeration of highly concentrated fillers and dispersion of them in the matrix become the other problems. These problems can be partially overcome by silane treatment to BNNPs. As is mentioned above, silane coupling agent can enhance interfacial adhesion between polymer matrixes and inorganic fillers thus improving the dispersion and preventing serious agglomeration of the fillers. Although interfacial thermal resistance still exists, which may cause deficiency in heat conduction, the improvement of thermal conductivity is greater due to the suppressed phonon scattering at interfaces. In our study, silane treatment works most effectively at 60% BNNP fraction for the thermal conductivity improvement.
Thermogravimetric analyses (TGAs) are conducted in nitrogen (N2) for pure epoxy, EP/BN_10, EP/BN_40, and EP/BN_70, as examples. As shown in Figure 8c, the thermal decomposition temperature of the four reflected by TGA spectra keeps almost the same at around 450°C. This implies that the addition of BNNPs does not change the decomposition temperature of the epoxy resin matrix. The epoxy resin used in our studies shows a thermal stability in nitrogen up to nearly 350°C without significant weight loss, which reveals an excellent thermal stability of the fabricated composite materials.
A facile solvent-free process is developed to fabricate thermally conductive electrically insulating epoxy-BNNP composites with various filler fractions. Benefiting from solid-state epoxy resin adopted, the epoxy resin and BNNPs are mixed uniformly only by mechanical mixing with a juice maker. Without the very difficult desolvation processes, the developed method can be used to fabricate bulk-sized composite samples with very high filler fractions. The fabricated epoxy-BNNPs exhibit steadily improved thermal conductivity up to 5.24 W/mK at 70 wt.% filler fraction, which is 1,600% better than that of pristine epoxy material. Surface treatment of BNNPs by silane coupling agent is demonstrated to be effective for further enhancement of the thermal conductivity of the composites, especially at high filler fractions. In addition, the as-fabricated composite materials exhibit excellent overall performance with high density, well preserved hardness, and great thermal stability. Further improvements of the fabrication method in the future can be realized in these aspects, such as: (1) modify the method for non-solid state material; (2) combine chemical modifications with our method to obtain better thermal conductivity values; (3) extend time of mixing to obtain a better dispersion. With those optimizations, the fabricated epoxy-BNNP composite materials and developed facile solvent-free fabrication method are promising for various heat dissipation-oriented applications.
This research was supported by the Early Career Scheme of the Research Grants Council of Hong Kong SAR, China, under the Project No. CityU 9041977, the Science Technology and Innovation Committee of Shenzhen Municipality (the Grant No. JCYJ20130401145617276), and a grant from the Innovation and Technology Commission of Hong Kong (ITS/331/13).
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