Anisotropically Alignable Magnetic Boron Nitride Platelets Decorated

Jul 22, 2013 - Chem. Mater. , 2013, 25 (16), pp 3315–3319 ... The Fe3O4-decorated h-BN platelets will be promising candidates for significantly impr...
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Anisotropically Alignable Magnetic Boron Nitride Platelets Decorated with Iron Oxide Nanoparticles Ho Sun Lim,* Jin Woo Oh, So Yeon Kim, Myong-Jae Yoo, Seong-Dae Park, and Woo Sung Lee* Electronic Materials & Device Research Center, Korea Electronics Technology Institute, Seongnam-si, Gyeonggi-do 463-816, Republic of Korea S Supporting Information *

ABSTRACT: To effectively utilize the anisotropic characteristics of hexagonal boron nitride (h-BN), we have developed magnetic h-BN hybrid platelets decorated with iron oxide (Fe3O4) nanoparticles, which are used as magnetic carriers for tailoring the anisotropy of h-BN. The as-synthesized Fe3O4-coated hBN powders can easily move under a relatively low magnetic field. With the aid of iron oxide nanoparticles, h-BN platelets randomly dispersed in an epoxy matrix are successfully reoriented in a direction vertical to the film plane. Moreover, by utilizing the anisotropic characteristics of h-BN platelets, Fe3O4-coated h-BN/ epoxy composites exhibit exceptional performance in terms of in-plane thermal conductivity. This result is attributed to an improvement in the heat-transport pathways in composite films due to the anisotropic ordering of thermally conductive h-BN sheets. The Fe3O4-decorated h-BN platelets will be promising candidates for significantly improving the performances of advanced electronic devices that require excellent thermal conductivity and electrical insulation. KEYWORDS: iron oxide nanoparticles, hexagonal boron nitride, anisotropic alignment, thermal conductivity, composites

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conductor packaging.14,15 Nevertheless, the development of realistically practical technologies is still making slow progress owing to a lack of ways for deriving the maximum device performance by utilizing the anisotropic properties of h-BN. In this study, we synthesized a novel type of magnetic h-BN hybrid platelets decorated with iron oxide nanoparticles; these platelets are anisotropically operative under a magnetic field. Magnetite (Fe3O4) nanoparticles are used as magnetic carriers for tailoring the anisotropy of h-BN platelets. By taking advantage of the anisotropic characteristics of h-BN, the resulting Fe3O4-coated h-BN composites show exceptional performance in terms of the thermal conductivity in the vertical direction through magnetic alignment of the platelets. Magnetically actuatable h-BN platelets will offer a useful tool to manipulate the properties of anisotropic materials by applying a magnetic field to change the alignment of these platelets.

INTRODUCTION Boron nitride (BN) with a layered structure similar to that of graphite has a great potential for use in nanotechnology applications owing to its excellent thermal and chemical stability.1−8 An individual BN sheet (basal plane) comprises boron and nitrogen atoms bonded by strong covalent bonds; moreover, different layers in a BN sheet are held together by weak van der Waals forces.9 On the basis of their special lamellar structure, BN nanosheets can be used as electrical insulators with a wide band gap ranging from 5.5 to 6.4 eV depending on the polymorph.10,11 Among the types of BN with various crystalline structures, hexagonal-type BN (h-BN) exhibits unique anisotropic properties, appearing in platelike morphology, because of its disparate bonding character of both strong binding within basal planes and weak interactions between BN layers.12 As a result, the hardness and thermal conductivity of h-BN are much higher along the planes than perpendicular to them. In particular, the thermal conductivity of h-BN in the horizontal direction is the highest, compared with other electrically insulating materials such as alumina, silica, and so on.13 Moreover, composite films obtained by filling h-BN into a polymer matrix have low thermal expansion, high thermal conductivity and electrical resistivity. In particular, anisotropic alignment of h-BN platelets leads to a superior performance of composite materials for electronic applications even at low filler loading. Such composite materials are lightweight and inexpensive because of a significantly reduced use of fillers. Because of its excellent dielectric and thermal properties, h-BN can be potentially used for electronic applications such as thermal interface materials for semi© 2013 American Chemical Society

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EXPERIMENTS

Materials. Hexagonal boron nitride (h-BN; GP, 99%) with a particle size of 8 μm was supplied by Denka. Ferrous chloride (FeCl2· 4H2O), ferric chloride (FeCl3·6H2O), and 1,6-hexanediamine [H2N(CH2)6NH2] were purchased from Sigma-Aldrich Co. and used without further purification. Poly(sodium 4-styrenesulfonate) (PSS; mol wt = 70000) was purchased from Aldrich. 4,4′-Diglycidyl(3,3′,5,5′-tetramethylbiphenyl) epoxy resin used as a polymer matrix was obtained from Mitsubishi Chemical Co., and 4,4′-dihydroxybiphenyl (DHBP) used as a hardener was received by Sigma-Aldrich Co. Received: May 6, 2013 Revised: July 14, 2013 Published: July 22, 2013 3315

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Table 1. Synthesis of Fe3O4-Coated h-BN Platelets and Their Characteristics FeCl2 (mol/L) GP0 GP10 GP30 GP50 GP100

0.023 0.069 0.118 0.236

FeCl3 (mol/L)

density (g/cm3)

specific surface area (m2/g)

saturation magnetization Ms (emu/g)

coercive field Hc (G)

0.043 0.129 0.215 0.431

2.2 2.31 2.37 2.44 2.53

4.34 5.08 11.21 15.17 20.40

2.12 5.00 9.10 17.28

37.17 36.29 41.87 46.54

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Deionized (DI) water was of Millipore Milli-Q grade with a resistivity of about 18 MΩ/cm and was obtained from a Milli-Q purification system (Millipore, USA). All chemicals and solvents were of reagent grade and were used directly without further purification. Synthesis of Fe3O4-Coated h-BN Platelets. The iron oxide nanoparticles were synthesized on h-BN surfaces by hydrolysis of an aqueous solution containing iron salts and a base at room temperature in an ambient atmosphere. First, a PSS solution with a concentration of 0.1 mol/L (with respect to monomer units) was mixed with a 1 L aqueous solution containing 100 g of h-BN powder by stirring. This was followed by the addition of a 1 N HCl solution, bringing the pH down to 3. This concentration was high enough to ensure excess PSS in order to obtain complete adsorption on the surfaces of h-BN platelets.16 PSS was allowed to sufficiently adsorb for 2 h with periodic stirring, followed by filtering and washing three times with water/ ethanol. After drying at 70 °C, the preliminary PSS-coated h-BN sheets were redispersed in DI water with a concentration of 10 wt %. An iron salt solution containing predetermined amounts of FeCl3 and FeCl2 was added slowly to the PSS-coated h-BN dispersion. Table 1 shows each feeding amount of FeCl3 and FeCl2. 1,6-Hexanediamine was then quickly added to the mixed solution as a base, resulting in the immediate formation of a precipitate. After vigorous stirring for 24 h, the resulting dark-brown suspension was obtained; this suspension was then filtered, washed three times with water to remove excess amine molecules, and dried in a vacuum at 70 °C. Preparation of Fe3O4-Coated h-BN/Epoxy Composites. Assynthesized h-BN platelets were incorporated into a precured methyl ethyl ketone solution containing a tetramethylbiphenyl epoxy resin (YX-4000) and a curing agent (DHBP). The mixed solution was then ball-milled for 1 h to homogeneously disperse the thermally conductive fillers into the polymer resins. The powder content was maintained at 20 vol % in the solid mixtures, the molar ratio of YX4000 and DHBP was 1:0.5, and the viscosity of the mixed solution was around 1000 cPs. The prepared slurries were cast on Teflon films with a thickness of 1 mm and then were thermally cured in a convection oven for 12 h at a temperature of 150 °C. To embed vertically assembled h-BN platelets in the polymer matrix, the mixture-coated composite films were sandwiched between two magnets and kept overnight, followed by subsequent heat treatment at 150 °C for 12 h (Figure S2 in the Supporting Information). Characterization. The surface morphologies of the as-synthesized Fe3O4-coated h-BN platelets were imaged by scanning electron microscopy (SEM; JEOL JSM-6701F) and transmission electron microscopy (TEM; Hitachi-7600). The crystalline structures of the platelets were recorded using an powder X-ray diffractometer (Empyrean) equipped with monochromated Cu Kα radiation (λ = 1.54056 Å). Surface analysis of the platelets was performed using X-ray photoelectron spectroscopy (XPS; VG Microtech ESCA2000). The densities of powders were measured by a gas pycnometer (AccuPyc II 1340). Their magnetic properties were carried out on a vibrating scanning magnetometer (VSM; Lakeshore 7400). Their thermal conductivity (W/m·K) was calculated by the product of thermal diffusivity (mm2/s), specific heat capacity (J/g·K), and density (g/ cm3). The in-plane thermal diffusivity of epoxy/h-BN composites was characterized using a laser-flash apparatus (LFA; NETZSCH LFA447 Nanoflash) at 25 °C, and their specific heat capacity was monitored using a differential scanning calorimeter (DSC; NETZSCH DSC200 F3) with a heating rate of 10 °C/min. The density of the composite films was determined using a water displacement technique (ASTMD792-91).

RESULTS AND DISCUSSION Magnetic h-BN microplatelets were synthesized by the coprecipitation of Fe3+ and Fe2+ precursors on the surfaces of h-BN particles (Denka, GP). The molar ratio of Fe2+/Fe3+ was controlled as 1:2 to obtain Fe3O4 nanoparticles.17,18 Scheme 1 Scheme 1. Schematic Representation of Fe3O4-Coated h-BN Platelet Synthesis

illustrates the procedure for the production of magnetite-coated h-BN hybrid flakes. The h-BN platelets and Fe3O4 nanoparticles were used as a core and a shell, respectively, for preparing magnetic hybrid composites. First, h-BN surfaces were modified with PSS as a strong polyelectrolyte to maintain a negative electric charge over the whole pH range.19 PSS plays an important role in improving the water solubility of h-BN powders as well as the interactions between h-BN surfaces and magnetite nanoparticles. In the second step, a mixed aqueous precursor solution of FeCl3 and FeCl2 was slowly added to the preliminary PSS-coated h-BN solution. This was followed by the quick addition of hexamethylenediamine as a base to precipitate Fe2+/Fe3+ ions for the synthesis of magnetite nanoparticles.18 The as-synthesized magnetite nanoparticles were predominantly absorbed onto and the uniformly covered surface of PSS-coated h-BN, as shown in Figure 1. The magnetic properties of Fe3O4-coated h-BN platelets were controlled by feeding certain amounts of iron salt sources. Figure 1 shows the surface morphologies of Fe3O4-coated hBN hybrid particles with different magnetite concentrations. As shown in Figure 1a, pristine h-BN powders, which consisted of micrometer-scale platy particles with sizes of around 8 μm, exhibited a smooth surface morphology. However, when Fe3O4 nanoparticles were coated on the surface of h-BN platelets, their surface microstructures gradually became rough, depending on the degree of adsorbed magnetic particles. Dispersions for the synthesis of magnetic hybrid particles contained 10 wt % PSS-coated h-BN. Moreover, on the basis of the feeding ratio of Fe2+/Fe3+ ions, the concentrations of reactant mixtures were controlled from 0.023 to 0.236 mol/L for FeCl2 and from 0.043 to 0.431 mol/L for FeCl3 (Table 1). When a total weight ratio of iron salt sources to PSS-coated h-BN is changed from 0 to 3316

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enhancement slowed down with an increase in the amount of absorbed magnetite. To confirm the successful synthesis of Fe3O4 nanoparticles, powder X-ray diffraction (XRD) patterns were obtained and analyzed for bare h-BN and as-synthesized magnetite-coated hBN hybrid composites (Figure 2a). The diffraction peaks at 2θ

Figure 1. SEM images of Fe3O4-coated h-BN platelets synthesized in iron salt solutions with different concentrations: (a) GP0; (b) GP10; (c) GP30; (d) GP50; (e) GP100. Panels f and g indicate magnified SEM and TEM images, respectively, of panel e. Figure 2. (a) XRD patterns for Fe3O4-coated h-BN platelets. Filled triangles and unfilled diamonds indicate diffraction peaks corresponding to the main phases of Fe3O4 and h-BN, respectively. (b and c) XPS spectra for the Fe 2p and O 1s levels from the outermost surface of Fe3O4-coated h-BN platelets.

10, 30, 50, and 100 wt %, the Fe3O4-coated h-BN particles are hereafter referred to as GP0, GP10, GP30, GP50, and GP100, respectively. Further, iron oxide nanoparticles were randomly distributed on the outer surface of h-BN platelets and existed as patches of aggregated clusters. With an increase in the amount of added iron salt sources, the density of the Fe3O4 magnetic particles covering the surface of h-BN cores increased (Figure 1e). In particular, some Fe3O4 nanoparticles aggregated themselves because of strong interactions between them, as shown in Figure 1f. The diameter of the coated Fe3O4 nanoparticles ranged from 53.7 to 71.1, averaging 60.7 nm over 50 individual Fe3O4 nanoparticles, and their size distribution was quite homogeneous. The microstructures of Fe3O4-coated h-BN hybrid composites were further investigated using TEM. Figure S1 in the Supporting Information shows a typical TEM image of an individual PSS-coated h-BN platelet, indicating a uniform coating of a strong negative polyelectrolyte with a thickness below 10 nm on the surface of h-BN. The TEM image shown in Figure 1g clearly reveals that the coating layer on the h-BN surface is composed of Fe3O4 nanoparticles with sizes on the scale of tens of nanometers. These results are in good agreement with the SEM images shown in Figure 1. Moreover, the attachment of Fe3O4 nanoparticles leads to greatly improved surface-to-volume ratios for magnetic h-BN platelets. As a result of measurement by Brunauer−Emmett−Teller (BET), the specific surface area of the bare h-BN powder was found to be 4.34 m2/g, while those of the Fe3O4-coated h-BN showed significant enhancements from 5.08 (GP10) to 11.21 (GP30), 15.17 (GP50), and 20.40 m2/g (GP100) (Table 1). It should be noted that the increase in the surface area of hybrid magnetic h-BN particles was nonlinear, where the rate of

= 26.81°, 41.63°, 43.91°, 50.20°, and 55.19° correspond to hBN platelets. It was seen that the other peaks could be exactly indexed to the pure cubic phase of magnetite (JCPDS no. 19629).17 The reflection peaks appearing at 2θ = 30.22°, 35.64°, 43.32°, 53.69°, 57.27°, and 62.81° are indexed as (220), (311), (400), (422), (511), and (440) planes of Fe3O4, respectively, indicating that the coating layer consisted of Fe3O4 nanoparticles. Using the Bragg equation for the diffraction peak observed at 2θ = 35.64°, the d value of the lattice spacing for the (311) plane was calculated to be about 2.52 Å; this calculated value matched well with the corresponding standard value of 2.532 Å. As can be observed in Figure 1, the intensities of the diffraction peaks were enhanced with an increase in the amount of absorbed Fe3O4 magnetic particles. As an additional evidence to verify the presence of magnetite, XPS was performed to examine the chemical composition of the resulting magnetic h-BN powders. Parts b and c of Figures 2 show the XPS patterns for the Fe 2p and O 1s regions. Two peaks with binding energies of 710.6 and 724.0 eV corresponded to Fe 2p1/2 and Fe 2p3/2, respectively, and no satellite peak situated at 718.8 eV, which is a major characteristic of γ-Fe2O3, was observed.20 In the O core level spectrum, the main peak at 529.9 eV was attributed to the oxygen atoms in Fe3O4 and a shoulder centered at 531.3 eV was ascribed to surface traps (Figure 2c). These results are in good agreement with the values reported for magnetite in the 3317

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literature, validating the successful formation of Fe3O4 on h-BN surfaces. The powder densities of the iron oxide-coated h-BN platelets changed from 2.2 (GP0) to 2.31 (GP10), 2.37 (GP30), 2.44 (GP50), and 2.53 g/cm3 (GP100) (Table 1). The density of pure magnetite particles was taken as 5.1 g/cm3; on the basis of this value, their volume fractions in hybrid h-BN platelets were estimated to be 3.8, 5.9, 8.3, and 11.4 vol % for GP10, GP30, GP50, and GP100, respectively.21 The magnetic properties of iron oxide-coated h-BN composites were investigated using a vibrating sample magnetometer at room temperature, which is desirable for many practical applications, as shown in Figure 3a. Although

Figure 4. Cross-sectional SEM images of Fe3O4-coated h-BN/epoxy composites aligned (a) randomly and (b) vertically to the film plane. Figure 3. (a) Magnetic hysteresis loops for Fe3O4-coated h-BN platelets synthesized in iron salt solutions with different concentrations. The top inset shows a close view of the hysteresis loops. (b) Fe3O4-coated h-BN platelets dispersed in a water solution (left) and their magnetic separation (right).

h-BN sheets in the epoxy composites were further evaluated by measuring their in-plane thermal diffusivity using an LFA. The composite films containing randomly oriented h-BN particles did not exhibit a remarkable increase in their thermal conductivities owing to the poor thermal conductivity of pure epoxy (0.2 W/m·K) and severe phonon scattering at the interface between fillers and resins.14,23,24 However, although the thermal conductivity of the composite films containing randomly oriented h-BN particles was below 1.0 W/m·K, the composite films containing vertically aligned magnetic h-BN particles revealed a dramatic increase in the thermal conductivity, with the average value being 4.3 ± 0.5 W/m·K at the same filler loading. The highest thermal conductivity obtained was 4.7 W/m·K at 20 vol % Fe3O4-coated h-BN, which is 23.5 times higher than that of pure epoxy resins. The heat capacity, density, and thermal diffusivity of the composite film were 1.11 J/g·K, 1.15 g/cm3, and 3.68 mm2/s, respectively. This result is consistent with the direction-dependent thermal transport properties of h-BN platelets, where the thermal conductivity of h-BN sheets is 20 times higher within the planes than perpendicular to them.25 Therefore, these results obviously demonstrate an effective way of achieving an outstanding thermal transfer performance even at relatively low h-BN loadings, resulting from efficient thermal transfer pathways attributed to anisotropic alignments of h-BN sheets in an epoxy matrix obtained using a magnet. In general, the vertical ordering of magnetic h-BN powders is complexly determined from the balance between the viscosity of mixed resins, force, and direction of the applied magnetic field and magnetic characteristics of the hybrid particles. Our future studies will be focused on maximizing the heat-transporting performance of composite films through anisotropy optimization of Fe3O4-decorated h-BN platelets. Moreover, we expect a more drastic improvement in the magnetic h-BN fraction higher than 30 vol %.

the magnetic properties of materials are generally influenced by many factors such as the size, structure, surface disorder, and morphology, the saturation magnetization (Ms) values of assynthesized h-BN hybrid particles were found to be dependent on the content of the absorbed magnetic particles.17 The Ms values for Fe3O4-coated h-BN platelets were 2.12, 5.00, 9.10, and 17.28 emu/g for GP10, GP30, GP50, and GP100, respectively (Table 1). In addition, as-synthesized magnetic h-BN platelets exhibited a typical ferromagnetic behavior, which was attributed to a large coercive force (Hc) under a small applied magnetic field. It was found that these coercive forces ranged from 36.29 to 46.54 G, irrespective of the feeding rate of iron salt sources (Figure 3a, inset). Figure 3b shows that Fe3O4-coated h-BN platelets dispersed in a water solution can be magnetically separated from water. To utilize the anisotropy of such hybrid fillers for realizing heat transmitters, we embedded the resulting Fe3O4-coated hBN platelets (GP100) within an epoxy resin, which has been widely used as a fundamental material for electronic packaging and thermal management applications.22 The h-BN powder content was maintained at 20 vol % in the solid mixtures. Figure 4 shows the SEM images of magnetic h-BN/epoxy composite films under a vertically applied magnetic field of 150 mT. We observed that the Fe3O4-coated h-BN sheets showing a random distribution within the polymer matrix were successfully reoriented in a direction vertical to the substrates. This implies that the alignment of the h-BN platelets in any matrix can be effectively controlled with the aid of the coated iron oxide nanoparticles under an applied magnetic field. In particular, it should be noted that our Fe3O4-coated h-BN sheets sensitively responded even to a low magnetic field of 150 mT. The anisotropic thermal transport properties of magnetic 3318

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(14) Huang, X. Y.; Jiang, P. K.; Tanaka, T. IEEE Electr. Insul. Mater. 2011, 27, 8. (15) Huang, X. Y.; Iizuka, T.; Jiang, P. K.; Ohki, Y.; Tanaka, T. J. Phys. Chem. C 2012, 116, 13629. (16) Han, J. T.; Zheng, Y.; Cho, J. H.; Xu, X.; Cho, K. J. Phys. Chem. B 2005, 109, 20773. (17) Iida, H.; Takayanagi, K.; Nakanishi, T.; Osaka, T. J. Colloid Interface Sci. 2007, 314, 274. (18) Huang, Z.; Tang, F. J. Colloid Interface Sci. 2004, 275, 142. (19) Boudou, T.; Crouzier, T.; Ren, K.; Blin, G.; Picart, C. Adv. Funct. Mater. 2009, 21, 1. (20) Yamashita, T.; Hayes, P. Appl. Surf. Sci. 2008, 254, 2441. (21) Zheng, X.; Zhou, S.; Xiao, Y.; Yu, X.; Li, X.; Wu, P. Colloids Surf., B 2009, 71, 67. (22) He, Y.; Moreira, B. E.; Overson, A.; Nakamura, S. H.; Bider, C.; Briscoe, J. F. Thermochim. Acta 2000, 14, 1. (23) Wattanakul, K.; Manuspiya, H.; Yanumet, N. Colloids Surf., A 2010, 369, 203. (24) Zhou, W.; Qi, S.; An, Q.; Zhao, H.; Liu, N. Mater. Res. Bull. 2007, 42, 1863. (25) Cho, H.-B.; Tokoi, Y.; Tanaka, S.; Suematsu, H.; Suzuki, T.; Jiang, W.; Niihara, K.; Nakayama, T. Compos. Sci. Technol. 2011, 71, 1046.

CONCLUSION The as-synthesized magnetic h-BN platelets hybridized with iron oxide nanoparticles offer a new strategy for manipulating the anisotropic properties of direction-dependent materials through magnetic alignments. As-synthesized magnetic h-BN particles randomly dispersed in an epoxy matrix are reoriented in a direction perpendicular to the substrate under a magnetic field. As a corollary, the thermal conductivity of epoxy composites with vertically aligned h-BN sheets is much higher than the values reported in the literature. Moreover, such composite materials are lightweight and cheap because of a significantly reduced use of expensive fillers. Thus, anisotropically operable magnetite-coated h-BN particles can be effectively utilized as thermal transport materials. In addition, owing to their excellent dielectric and thermal properties, Fe3O4-coated h-BN platelets can be potentially used for a wide range of microelectronic applications that require manipulation of the anisotropic material properties, such as semiconductors, chip packaging substrates, and printed circuit boards.

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ASSOCIATED CONTENT

S Supporting Information *

TEM image of PSS-coated h-BN platelets and scheme of the experimental setup for the preparation of magnetically aligned Fe3O4-coated h-BN/epoxy composites. 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] (H.S.L.), [email protected] (W.S.L.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by a grant from the Fundamental R&D Program (Grant 10038631) for Core Technology of Industry funded by the Ministry of Trade, Industry & Energy, Republic of Korea.

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