Enhanced Thermal Conductivity of Polyimide Composites with Boron Nitride Nanosheets

www.nature.com/scientificreports OPEN Received: 2 November 2017 Accepted: 10 January 2018 Published: xx xx xxxx Enhanced Thermal Conductivity of Polyimide Composites with Boron Nitride Nanosheets Ting Wang1, Mengjie Wang1, Li Fu2, Zehui Duan3, Yapeng Chen1, Xiao Hou1, Yuming Wu1, Shuangyi Li1, Liangchao Guo1, Ruiyang Kang1, Nan Jiang1 & Jinhong Yu1 A strategy was reported to prepare boron nitride nanosheets (BNNSs) by a molten hydroxide assisted liquid exfoliation from hexagonal boron nitride (h-BN) powder. BNNSs with an average thickness of 3 nm were obtained by a facile, low-cost, and scalable exfoliation method. Highly thermally conductive polyimide (PI) composite films with BNNSs filler were prepared by solution-casting process. The in-plane thermal conductivity of PI composite films with 7 wt% BNNSs is up to 2.95 W/mK, which increased by 1,080% compared to the neat PI. In contrast, the out-of plane thermal conductivity of the composites is 0.44 W/mK, with an increase by only 76%. The high anisotropy of thermal conductivity was verified to be due to the high alignment of the BNNSs. The PI/BNNSs composite films are attractive for the thermal management applications in the field of next-generation electronic devices. With the rapid development of electronics industry, there is an increasing demand for electrically insulating polymer-based materials with enhanced capability of heat dissipation1–5. Furthermore, low cost and light weight polymer-based materials for next-generation electronic device, power systems, and communication equipment are needed. Polymers such as polyimide (PI) has been widely used as an electronic packaging material due to good thermal and mechanical properties6,7. In particular, it possesses a low dielectric constant, low loss tangent, high thermal stability and high storage modulus8. However, PI exhibit a poor thermal conductivity in the order of 0.1 W/mK9–11, which cannot meet the requirement of fast heat conduction for the advanced electronic devices. The general strategy to improve the thermal transport performance is using thermally conductive fillers such as carbon materials12–15, metal or ceramic materials16–19 are added to the polymer matrix20. However, the carbon and metal materials are highly electrically conductive and small additions of these fillers into polymers result in high electrical conductivity of the composites, which restricts the application. Meanwhile, hexagonal boron nitride (h-BN) is a typical ceramic filler21 has attracted much attention due to its excellent electrical insulation and high thermal conductivity22. As the low aspect ratio of h-BN filler, conventional PI composite achieve the thermal conductivity of 1–5 W/mK by utilizing large loading volume fraction h-BN filler (of up to ~50%). Large loading volume fraction in polymer composites leads to many problems such as mechanical property deterioration and processing difficulty as well as cost enhancement. In the contrast, large aspect ratio boron nitride nanosheets (BNNSs) exhibit many potential applications including ultraviolet light emitter, field emitters, and a superior substrate for graphene-based electrical devices23–28 and a superior thermal conductivity ranges from 1,700–2,000 W/mK29. Furthermore, BNNSs are an electrical insulation material with dielectric constant of 2–430. Therefore, BNNSs can be an ideal thermal conductive filler for polymer composites. Recently, many efforts have been used to prepare BNNSs including micromechanical cleavage31, ultrasonication32, and high energy electron beam irradiation33, chemical vapor deposition34, and liquid exfoliation35. However, BNNSs still suffer from a low cost, high yield, and facile exfoliation method. Micromechanical cleavage and electron beam irradiation technique are inefficient and unscalable. Furthermore, chemical vapor deposition method usually involved expensive templates and complicated fabrication processes, which appears to be tedious and expensive for large-scale production. Though the liquid-exfoliation method is popular method, still 1 Key Laboratory of Marine Materials and Related Technologies, Zhejiang Key Laboratory of Marine Materials and Protective Technologies, Ningbo Institute of Materials Technology and Engineering, Chinese Academy of Sciences, Ningbo, 315201, China. 2College of Materials and Environmental Engineering, Hangzhou Dianzi University, Hangzhou, 310018, China. 3Chemical Engineering and Biotechnology, National Taipei University of Technology, Taipei, 10608, China. Ting Wang and Mengjie Wang contributed equally to this work. Correspondence and requests for materials should be addressed to N.J. (email: ) or J.Y. (email: ) SCIENTIfIC REPOrTS | (2018) 8:1557 | DOI:10.1038/s41598-018-19945-3 1 www.nature.com/scientificreports/ Figure 1. Schematic diagram of the exfoliation process. suffers from low product yield and sometimes the toxic reagents have to be employed36,37. The high yielding and high-quality of BNNSs by exfoliation of h-BN powder remains a tough challenge. Herein, BNNSs were prepared by a molten hydroxide assisted liquid exfoliation from h-BN powder. BNNSs with an average thickness of 3 nm were obtained in a high product yield of 19%38. This method has several advantages, such as cheap precursors, high yields, and without the use of organic solvents, catalysts and vacuum systems. More importantly, we developed a PI composite film incorporated with BNNSs. As a result, the incorporation of low loading BNNSs into the PI matrix shows a significant enhancement of thermal conductivity, especially along the in-plane direction. The composites are promising for using as a heat dissipation material in next-generation electronic device. Materials and Methods Materials. The h-BN powders were purchased from ESK Ceramics GmbH & Co. (Germany) with lateral size of 7 µm. Sodium hydroxide and potassium hydroxide were purchased from Sinopharm Chemical Reagent Co., Ltd (China). Poly(amic acid) synthesized by pyromellitic dianhydride and 4,4-oxydianiline was obtained from Ningbo Cen Electrical Material Co., Ltd (China). All chemicals were of analytical reagent grade and used without further purification. Preparation of BNNSs. The molten alkali-assisted exfoliation of h-BN was following by two steps. Firstly, NaOH (2.84 g) and KOH (2.16 g) were finely ground, and then h-BN micropowder (1.0 g) was added. The mixture was further ground into a homogeneous form and transferred to a 100 mL Teflon-lined stainless steel autoclave. The sealed autoclave was heated and kept at 180 °C for 2 h. After cooling down to room temperature, the solid product was collected from the autoclave and dispersed into 300 mL deionized water. The dispersion was sonicated for 1 h using a tip sonicator (SJIA-650, Ningbo Yinzhou Sjia Co., China). Subsequently the sample was filtered, re-dispersed in deionized water, and centrifuged to remove hydroxides and other unreacted materials. After centrifugation, the (...truncated)