Solution-processable thermally conductive polymer composite adhesives of benzyl-alcohol-modified boron nitride two-dimensional nanoplates
Graphical abstract
Introduction
Epoxy resins have been extensively used to bond parts in electronic devices and machinery [1], [2]. Recently, owing to the miniaturization, integration, and weight-reduction of products, the resin has been required to dissipate the generated heat in such structures to ensure good performance, lifetime, and reliability [3], [4], [5], [6]. However, the low thermal conductivity (∼0.2 W/m·K) of the polymer cannot meet the performance criteria [7]. Therefore, it is required to develop thermally conductive polymer-based composites containing high-performance thermal conductors such as alumina, silica, silicon carbide, aluminum nitride, silicon nitride, and boron nitride (BN) [8], [9], [10], [11], [12], [13]. Among them, the hexagonal BN used as a filler has superior properties including a high thermal conductivity (150–225 W/m·K at 300 K) and thermal and chemical stabilities, owing to its unique crystal structure (layered structure similar to that of graphite, but with a reduced electron delocalization) [14], [15], [16], [17]. However, the hydrophobicity of BN hinders its dispersion in a polymer matrix and limits the loading amount in the hydrophilic epoxy matrix, which restricts the heat dissipation [18]. Therefore, it is very important to improve the interaction of BN with a polar substance. The most popular process for this purpose utilizes the covalent bonding to modify the BN surface. Kim et al. reported a BN–TiO2 structure with a thermal conductivity (κ) of 1.54 W/m·K at a filler loading of 20 vol%, which could be vertically oriented by an external electric field [19]. Two surface curing agents, 3-glycidoxypropyltrimethoxysilane and 3-chloropropyltrimethoxysilane, were doped onto the surfaces of a hydroxyl-functionalized BN using a simple sol–gel process, acting as fillers in the thermally conducting composite with a thermal conductivity of 4.11 W/m·K at 70 wt% [20]. Hou et al. reported that grafting of silane molecules onto the surfaces of BN particles improved the wettability and homogeneous dispersion of BN in the epoxy matrix with a strong interface interaction [21]. Additionally, Yu et al. presented a method to synthesize a cellulosic fiber introduced onto the BN surface modified by sodium hydroxide and 3-aminopropyl triethoxysilane (APTES) providing a thermal conductivity of 0.682 W/m·K [22]. Muratov et al. reported that a surface treatment with a silane coupling agent of ceramic fillers was effective for a thermal conductivity enhancement in BN/polypropylene composites [23].
In the above studies, the fillers commonly required hydroxyl groups covalently bonded to the BN surface, acting as a linker for desirable chemicals. However, although the hydroxyl-functionalized BN has been extensively utilized for polymer composites, several critical problems were reported. For example, the B–N bonds may be weakened by the covalent bonding in the surface modification; in addition, byproducts such as sodium salt are difficult to completely remove [24], [25]. These problems might deteriorate the intrinsic phonon transfer in BN. In addition, for the introduction of functional groups, the modification process has proceeded with a NaOH solution, plasma, and oxidation process, which were not environment-friendly and cost-effective for mass production [26], [27], [28]. Therefore, it is still challenging to develop a large-scale, cost-effective, and environment-friendly method for the surface modification of BN nanoplates.
We present a simple large-scale synthesis method for preparation of a noncovalently functionalized BN using benzyl alcohol (B-BN), which is further utilized to prepare solution-processable epoxy-based adhesives with high thermal conductivities for an efficient heat dissipation. Their thermal conductivities were compared with those of other epoxy-based composites with bare BN and chemically modified BN (S-BN). Bare BN can be mixed with epoxy resin by a simple mechanical mixing; however, the maximum loading for further processing was 40 wt%, above which the mixture became too viscous to be utilized as an adhesive. At the filler concentration of 40 wt%, the thermal conductivities of the composites with bare BN and B-BN were comparable to and significantly higher than that of the S-BN composite, respectively. However, we could not further increase the concentration of bulk BN in the composite as the viscosity abruptly increased so that it was impossible to utilize it as an adhesive for heat dissipation. In addition, among the considered structures, the B-BN-based epoxy composite exhibited superior characteristics including better coefficient of linear thermal expansion and effective modulus. In the case of B-BN, we further increased its concentration in the epoxy resin up to 46 wt%, yielding a thermal conductivity of 2.11 W/m·K.
Section snippets
Materials
BN (purity > 98.5%, 3M™), benzyl alcohol (99.0%, Aldrich), ethanol (99.0%, Aldrich), sodium hydroxide (99.9%, Aldrich), and epoxy resin (Stycast 1266 A/B, Loctite) were used without further purification.
Surface modification of BN by benzyl alcohol (B-BN)
For the introduction of benzyl alcohol on the BN surface, 100 g of BN powders were dispersed into 500 mL of benzyl alcohol in a glass bottle. The solution was irradiated by a 60-Hz ultrasound (Power Sonic 405, Hwashin Tech) for 10 min. Then, it was washed three times with ethanol by
Results
In order to achieve a considerable processing scale, benzyl alcohol was chosen for the surface modification of BN plates, as it could be attached by a noncovalent bonding in a simple mixing process; in addition, its toxicity and cost are relatively low [30]. As shown in Fig. 1, its structural and chemical characteristics are favorable to provide π-stacking on the BN surface owing to the hexagonal planar ring with π-electrons [31]. Further, the sp3-hybridized carbon of the methylene group can
Conclusions
We demonstrated a sonochemically driven synthesis of noncovalently modified BN. Benzyl alcohol was deposited on the BN surface through π-electron sharing. The large numbers of hydroxyl groups as well as π-electrons of B-BN could provide the functional sites for hydrogen bonding and van der Waals attraction with the epoxy resin, leading to the higher affinity between the polymer and filler. Consequently, owing to the smaller deterioration in the intrinsic phonon transfer of the free BN, the
Acknowledgements
This research was partly supported by the Korea Basic Science Institute (No. D38614) and Fundamental R&D program (10083640), funded by the Ministry of Trade, Industry, and Energy (MOTIE), Korea. G.-R.Y. acknowledges the partial support from the Korea Institute of Industrial Technology (Kitech JA-17-0045).
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