DOI: 10.1002/cphc.201402814
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Boron Nitride Nanomaterials for Thermal Management Applications Mohammed J. Meziani,*[a, b] Wei-Li Song,*[c] Ping Wang,[a] Fushen Lu,*[d] Zhiling Hou,[a] Ankoma Anderson,[a] Halidan Maimaiti,[a] and Ya-Ping Sun*[a] Hexagonal boron nitride nanosheets (BNNs) are analogous to their two-dimensional carbon counterparts in many materials properties, in particular, ultrahigh thermal conductivity, but also offer some unique attributes, including being electrically insulating, high thermal stability, chemical and oxidation resistance, low color, and high mechanical strength. Significant
recent advances in the production of BNNs, understanding of their properties, and the development of polymeric nanocomposites with BNNs for thermally conductive yet electrically insulating materials and systems are highlighted herein. Major opportunities and challenges for further studies in this rapidly advancing field are also discussed.
1. Introduction Hexagonal boron nitride (h-BN) isomorphs of widely investigated carbon nanostructures (graphenes in particular) have generated much recent interest for their unique characteristics, especially with the decoupling of thermal and electrical transport properties in these materials. BNs are known for being highly thermally conductive yet electrically insulating, and therefore, have traditionally been considered as a material of choice in thermal management applications. For nanoscale BNs, oneand two-dimensional structures with a high aspect ratio have been used as fillers in polymeric matrices for nanocomposite materials of enhanced thermal conductivity.[1–9] The similarity between graphenes/graphite and BNs is such that both have a layered structure, which could be considered as being constructed from sp2-bonded hexagonally packed sheets. In BNs, however, the boron and nitrogen atoms are alternatively bonded and positioned to form planar conjugated layers in such a configuration that the two in the neighboring layers are eclipsed on top of one another because of the polar-
ity mismatch. Slight ionic bonding both in plane and out of the plane (“lip–lip” interactions) in BNs also make them different from graphenes/graphite in their structural configuration, and consequently, in some of the nanomaterial properties. Specifically, stronger interlayer interactions make the exfoliation of h-BN into BN nanosheets (BNNs; Figure 1) more difficult
Figure 1. Structures of a graphene sheet (left), a boron nitride sheet (middle), and a boron nitride nanotube (right).
[a] Dr. M. J. Meziani, P. Wang, Dr. Z. Hou, A. Anderson, Dr. H. Maimaiti, Prof. Y.-P. Sun Department of Chemistry and Laboratory for Emerging Materials and Technology, Clemson University Clemson, SC 29634 (USA) E-mail: [email protected] [email protected]
than peeling off graphene from graphite.[10–13] Similar to the conceptual understanding of carbon nanotubes,[14, 15] a BN nanotube (BNNT; Figure 1) may also be viewed as being geometrically derived by rolling a hexagonal single-layer BNN.[16, 17] Both BNNs and BNNTs (Figure 1) have been attracting increasing attention for thermal transport and other uses, although the former is the primary focus of this article. Growing interest in BN nanomaterials in general is due to their distinct and advantageous properties in comparison to those of their carbon counterparts. First, nanoscale BNs are electrically insulating with a wide band gap of 5.6 eV, yet are extremely thermally conductive, and therefore, are amenable to many unique applications. For thermal management in electronics as an example, the miniaturization of devices demands more effective
[b] Dr. M. J. Meziani Department of Natural Sciences Northwest Missouri State University Maryville, MO 64468 (USA) [c] Dr. W.-L. Song Institute of Advanced Materials and Technology University of Science and Technology Beijing Beijing 100083 (P.R. China) E-mail: [email protected] [d] Dr. F. Lu Department of Chemistry, Shantou University Shantou, Guangdong 515063 (P.R. China) E-mail: [email protected]
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Minireviews heat dissipation, which has been identified as a major bottleneck in terms of performance and reliability improvements.[18, 19] Similar issues are encountered in electric motors and generators, heat exchangers in power generation, automotives, and so forth. The BN-based materials of decoupled thermal and electrical transport properties offer potentially breakthrough solutions for these challenging issues. BNs are of superb thermal and chemical stabilities with high oxidation resistance and passivity to reactions with acids and melts,[20] and are particularly suitable for those applications under extreme conditions, such as devices operating in an oxidative environment at high temperatures.[20, 21] More specifically, graphite oxidation in air starts at 400–450 8C, yet h-BN is stable at temperatures up to 1000 8C in air and 1400 8C in vacuum.[20] BNs are insoluble in commonly used acids, and there are only a few substances, such as molten alkalis and alkaline solutions, that could attack or dissolve BNs at high temperatures.[20] These excellent properties have made nanoscale BNs and their derived materials increasingly popular candidates for a variety of technological needs. Figure 2. a) A TEM image of a h-BN nanosheet functionalized with amine-terminated polyethylene glycol. [Reprinted with permission from Ref. [30], copyright (2010) American Chemical Society.] b) A high-resolution TEM image of a h-BN nanosheet folded along the [120] axis (with the fringe contrast at the edge). Inset I: The diffractogram from the circled area I, showing the hexagonal symmetry of the nanosheet. Inset II: The diffractogram from area II, with the streaks lying perpendicular to the edge, similar to those seen in the electron diffraction of nanotubes. [Reprinted with permission from Ref. [28], copyright (2008) American Institute of Physics.] c) AFM image of the h-BN nanosheets, the sharp edges of which are indicated by the white arrows. Inset: the height distribution along the yellow line [Reprinted with permission from Ref. [49], copyright (2012) American Chemical Society]. d) AFM topographic image showing an area populated with h-BN nanosheets with feature heights less than 1 nm (the height profile plot corresponding to the dotted line). Scale bar = 200 nm. [Reprinted with permission from Ref. [33], copyright (2011) American Chemical Society].
2. BNNs—Synthesis and Structures Compared with ever-popular single- and few-layer graphene nanosheets, BNNs are understudied, despite their many important and/or unique advantages. One excuse might be such that the synthesis and exfoliation of BNNs with a controlled number of layers and in reasonable yields have proven difficult. Nevertheless, both bottom-up[22–26] and top-down[16, 17, 27–46] approaches have been explored for the preparation of BNNs. Single- or few-layer BNNs thus obtained have lateral sizes in the range of a few hundred nanometers to as large as a few centimeters (Figure 2). In the bottom-up approach for BNNs, earlier syntheses involved reactions of boron oxides or boric acid with urea or melamine.[31, 47] Recently, few-layer BNNs were also prepared from the chemical reactions of boric acid and urea at 900 8C under N2, in which some variations in the number of layers were achieved by changing the reactant concentrations.[31] A more popular method in the bottom-up approach has been the use of chemical vapor deposition (CVD).[25, 26] Thus, highquality thin BNNs of large lateral sizes have been synthesized from the thermal decomposition of various nitrogen-containing organoboron precursors (amine–boranes, ammonia– borane, and borazine) on metal or graphene substrates,[48–51] or the plasma-induced reactions of BF3 in a H2/N2 atmosphere.[26] However, more complicated procedures and high cost may limit the applicability of these syntheses in the larger-scale production of BNNs and for some important applications, such as in thermally conductive polymeric composites, in which substrate-free BNNs are required. The top-down approach with h-BN as the starting material has been considered to be relatively simple and more economical, although again the partially ionic character of B N bonding in h-BN makes exfoliation into BNNs more difficult than peeling off graphene from graphite. In this regard, Pacil et al. did try the micromechanical cleavage method by using adhe-
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sive tape to peel off BNNs from h-BN powder, but the yield was too scarce for real applications.[27] More productive has been the sonication-assisted liquid-phase exfoliation of h-BN for few-layer BNNs. This method and the choice of solvents in relevant studies have benefited from recent advances and extensive results on similar exfoliation of graphite.[52] In their early work, Zhi et al. sonicated h-BN in DMF for 10 h to yield a dilute suspension of BNNs at a concentration of 0.01–0.03 mg mL 1.[29] Warner et al. changed the solvent to 1,2dichloroethane for sonication and obtained BNNs of larger lateral dimensions.[36] Other strong polar organic solvents, such as N-methylpyrrolidone (NMP); N,N-dimethylacetamide (DMAc); 1,2-dichlorobenzene; and ethylene glycol, are useful or effective in the exfoliation of h-BN with sonication, from which relatively stable dispersions of BNNs are obtained.[35] Because of the clearly important role of solvent in the sonication-assisted processing, Coleman and co-workers attempted to use Hansen solubility parameter theory to explain and guide solvent selection for the exfoliation and dispersion of BNNs and other inorganic nanosheets.[35] For BNNs, isopropanol was identified as a favorable solvent, with which BNNs of 2
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Minireviews 3. Thermal Transport Properties of BNNs
less than 8 layers in dispersions with concentrations as high as 0.06 mg mL 1 were produced.[35] Similarly, Zhou et al. suggested a mixed-solvent strategy by using ethanol/water mixtures to improve the exfoliation and dispersion of BNNs.[53] There have been more choices for solvents in the exfoliation method, such as the recent use of methanesulfonic acid by Wang et al.,[34] and essentially unlimited combinations in terms of solvent mixtures. However, the solvent dispersion-based method is generally associated with low yields, typically for BNNs of 2–10 nm thick at milligram levels, which may be attributed to only weak interactions between BNNs and the solvent molecules. The use of chemical functionalization has been more effective to overcome the lip–lip interaction between the layers in h-BN for improved exfoliation.[17, 54] Amino molecules and other Lewis bases, such as phosphine molecules, were used to complex with boron atoms on the h-BN surface for exfoliation and the solubilization of the resulting BNNs;[30, 31] a strategy borrowed from the previous functionalization and solubilization of BNNTs.[54, 55] For example, Lin et al. functionalized h-BN with amino molecules, including octadecylamine (ODA) and amineterminated polyethylene glycol, coupled with exfoliation through ball-milling to yield mostly BNNs of 3–20 layers and lateral sizes up to about a micron.[30] In a further improvement, the same group increased the ball-milling time to enhance the exfoliation efficiency, with milling for 60 min resulting in the exfoliation of up to 40 % of the starting h-BN into BNNs of less than 50 nm thick with lateral dimensions up to 5 mm.[32] Separately, the ability for amino and phosphine molecules to complex with the few-layer BNNs produced in a bottom-up method was demonstrated by Nag et al.,[31] again conceptually similar to the functionalization and solubilization of BNNTs.[54, 55] Beyond ball-milling exfoliation, other shear-force-based mechanical processing methods, which are to some extent analogous to the infamous micromechanical cleavage, have been applied to the preparation of BNNs at a significant scale. For example, in the processing by Yurdakul et al. based on highpressure microfluidization, the starting h-BN powder in a DMF/ chloroform mixture was pumped through a microfluidic processor at a pressure of up to 207 MPa with multiple circulations.[44] It was suggested that the large shear force in the microfluidic channel was responsible for the exfoliation of h-BN into BNNs (8–12 nm thick with micrometer lateral dimensions), with a relatively high production efficiency in gram quantities. Similarly, the exfoliation of h-BN was accomplished by shearing vortex fluidic films in the solvent NMP at 8000 rpm in a glass tube with a fixed angle.[45] In a more recent use of the functionalization approach, a “soup” of h-BN with hydrazine, 30 % H2O2, and HNO3/H2SO4 (or oleum) was heated in an autoclave to 100 8C for functionalized h-BN.[46] The sample formed stable colloid solutions in water and DMF, which were found to contain few-layer BNNs with lateral dimensions in the order of several hundred nanometers.
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Thermal conductivity (k) is a property of materials to transport heat best described by the Fourier law, q = k !T, in which q is the heat flux or heat flow per unit area, !T is the temperature gradient through the conducting medium, and the negative sign indicates heat flow from high to low temperature.[56] Generally, in solids heat is transported by lattice/atomic vibrations (phonons) and/or free electrons, so that k is the sum of kp and ke, which represent the phonon and electron contributions, respectively. The ke term dominates in metals or heavily doped semiconductors due to the large numbers of free electrons. In nonmetallic materials such as h-BN, phonons are the dominant heat carriers, since free movement of electrons is impossible. Another commonly used parameter is thermal diffusivity (a), which defines how fast the material conducts heat. It is related to k by a = k/Cp1m, in which Cp is the specific heat and 1m is the mass density of the material. Many experimental techniques measure a rather than k directly. BNs are generally known as excellent thermal conductors,[43, 57–59] so are expected for BNNs.[43] However, investigations into the thermal transport properties of single- and fewlayer BNNs have been limited, with mostly estimated thermal conductivity values ranging from 300 to 2000 W mK 1. In an early study, Kumar and Pal found that high thermal conductivity could only be obtained inside the (002) planes (up to hundreds to thousands of W mK 1 units), whereas the thermal conductivity was only several W mK 1 units in other lattice planes.[60] Recent theoretical calculations have suggested that the thermal conductivity of suspended few-layer BNNs should increase as the number of layers decreases due to reduced interlayer phonon scattering.[61] Lindsay and Broido used an exact numerical solution of the phonon Boltzmann transport equation, in which both phonon–phonon scattering and the isotope effect were incorporated simultaneously, and they found that the thermal conductivity in monolayer BNNs was significantly higher than that in their bulk counterpart, again due to the reduction in interlayer phonon scattering (Figure 3).[61] Experimentally, however, recent results appeared that were at odds with the theoretical predictions for the BNN thickness effect, and showed that the measured thermal conductivity in the 5-layer BNN sample was actually lower than that in the 11layer sample (Figure 4).[62] In that experimental investigation, thermal conductivity values of suspended few-layer BNNs of different lengths and layer thicknesses were measured by using a microbridge device with built-in resistance thermometers (Figure 4). In the device fabrication process, a poly(methyl methacrylate) (PMMA) layer was coated on the sample and patterned with the use of electron-beam lithography, and then selectively removed in acetone. As shown in Figure 4, the room-temperature thermal conductivity thus measured for the 11-layer sample with lengths ranging from 3 to 7.5 mm was about 360 W mK 1, which approached the basal plane value reported for bulk h-BN.[62] As the temperature decreased, the thermal conductivity of this sample initially increased due to reduced Umklapp scattering, reaching the maximum value at 3
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Minireviews conductivity in BNNRs to four to six times lower than that in their graphene counterparts.[63] The nanoribbons may have a zigzag (z-BNNR) or armchair edge structure (a-BNNR), and the results also suggest that for both edge structures the ribbon width has a strong influence on the thermal conductivity (Figure 5).[63]
Figure 3. Left: The solid red curve shows the calculated kL of h-BN as a function of temperature, T, compared with measured values (black diamonds). The dashed red curve shows the kL for isotopically pure h-BN. The solid green curve gives the calculated kL for naturally occurring single-layer h-BN, whereas the dashed green curve shows calculated kL for isotopically pure single-layer h-BN. Right: The percentage enhancement, P, of kL in isotopically pure single-layer h-BN compared with naturally occurring single-layer h-BN as a function of T for different values of L (a measure of the length between boundaries in the transport direction). Inset: a comparison of P for singlelayer h-BN and bulk h-BN for L = 2 mm. [Reprinted with permission from Ref. [61], copyright (2011) American Physical Society].
Figure 5. The width dependence of room-temperature lattice thermal conductivity in a- and z- BNNRs. [Reprinted with permission from Ref. [63], copyright (2011) American Physical Society].
The theoretical predictions and experimental results discussed above have generally established that BNNs are highly thermally conductive, yet electrically insulating, and thermal transport is due to the phonon conduction mechanism. However, when compared with their carbon analogues, BNNs are much less explored for their superior and unique thermal transport properties. More studies, especially experimental investigations, on various parameters affecting thermal conductivity and for improved mechanistic understanding are still in demand.
Figure 4. Left: A microbridge device for the thermal transport measurements of suspended few-layer h-BN samples. Inset: An 11-layer h-BN sample suspended on the device. Right: Thermal conductivity values of the two 7.5 mm long, 11- and 5-layer thick suspended h-BN samples as a function of temperature. [Reprinted with permission from Ref. [62], copyright (2013) American Chemical Society].
4. Thermally Conductive Nanocomposites BNNs are logically considered as unique nanofillers for thermally conductive, but electrically insulating polymeric composites.[1, 9, 43, 57, 64–69] Again for thermal management in electronics as an example, these nanocomposite materials enable an excellent thermal interface for uneven or rough surface topography in the electronic devices. They may also serve as precursors for fabrication into various device forms, such as tubing and ribbons, to be incorporated into thermal management systems. Polymers are typically poor thermal conductors. A variety of experimental strategies have been designed and practiced for the incorporation of BNNs into polymeric matrices of low thermal conductivity for substantially enhanced thermal transport performance, although the results have been mixed at best. Generally speaking, an important task for high-performance nanocomposites is to disperse BNNs into polymeric matrices in a homogeneous fashion, for which solution-based processing represents a more favorable option. However, fabrication based on simple blending of BNNs with polymers has not been so effective in achieving the desired dispersion. Among more successful attempts have been the use of some specifically selected polar aprotic solvents in combination with vigo-
a temperature between 100 and 200 K, similar to bulk samples. At lower temperatures, the thermal conductivity of the 11-layer sample became lower than the bulk values. The apparent trend of decreasing thermal conductivity with decreasing BNN thickness was attributed to phonon scattering by the polymer residue from the fabrication process, which was consistently observed in the TEM images.[62] Nevertheless, despite the clear intent of the authors’ explanations to be consistent with the common belief established almost exclusively on the basis of theoretical computational predictions, further experimental investigations on this important issue are needed. There have also been theoretical studies on thermal transport in BN nanoribbons (BNNRs), which are essentially BNNs of a narrower width with significant edge effects, although the outcomes were clearly dependent on the method employed.[63] For example, a study that used nonequilibrium Green’s functions and neglected phonon–phonon scattering predicted a much higher thermal conductivity in the BNNR, approaching 1700–3000 W mK 1, which was similar to that in graphene nanoribbons.[63] In a study based on the classical molecular dynamics method, strong phonon scattering reduced thermal
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Minireviews rous sonication and the chemical functionalization of BNNs with polymers or surfactants. These dispersion and fabrication approaches also have different technical issues and/or shortcomings, from low product yields in some methods to the “contamination” of unwanted dispersion agents, causing microscopic phase separation within the nanocomposite structure in others. Representative studies on relevant topics in the literature, along with a discussion on the challenges and opportunities in this important technological field, are highlighted below. A simple dispersion of BNNs into various polymers through wet processing has typically increased the thermal conductivity of the resulting nanocomposite over that of the corresponding neat polymer (commonly considered as a thermal insulator), sometimes by a large percentage, but in an absolute sense the thermal transport performance of the composite is still low or unimpressive. For example, Li and Hsu dispersed surface-modified large (average size about 1 mm) and small (average size about 70 nm) BNNs into polyimide; composites with a filler loading of 30 wt % exhibited thermal conductivity of up to 1.2 W mK 1.[70, 71] Wong and co-workers prepared BNNs by ballmilling and sonication and then dispersed the fillers into epoxy resin.[72] The thermal conductivity in the resulting composite at 30 wt % was 3 times higher than that in neat epoxy (0.15 W mK 1), although still relatively poor in terms of thermal transport performance. Song et al. used isopropanol for exfoliation to obtain BNNs with thicknesses in the order of 10 nm and lateral sizes in the order of 1 mm or larger.[2] In the dispersion of BNNs in poly(vinyl alcohol) (PVA) and epoxy matrices, keeping the fillers from aggregating or restacking was emphasized. Nanocomposite films of around 50 mm in thickness were fabricated by wet casting on an etched glass slide and then made to be freestanding (Figure 6) for subsequent characterization and measurements. According to TEM results (Figure 6), BNNs embedded in the films were largely the same as those in the isopropanol suspension, with sheet thicknesses remaining generally 10 nm or less (thus, there is no significant aggregation and/or restacking of BNNs in the composites). These nanocomposite films exhibited superior thermal transport performance, with an observed in-plane thermal diffusivity as high as 19 mm2 s 1 in the epoxy film with a filler loading of 50 vol % (equivalent to a thermal conductivity of 30 W mK 1; Figure 6).[2] BNNs as fillers in polymeric nanocomposites are anisotropic, so that their alignment has a significant effect on the thermal transport properties of the nanocomposites. Nakayama and co-workers dispersed BNNs (surface modified with iron oxide nanoparticles) into polysiloxane, and during film fabrication aligned the fillers in a magnetic field.[73] Their results suggested that the alignment improved the thermal conductivity of the resulting nanocomposite film. Similarly, Tanimoto et al. examined the effect of the filler orientation and structural anisotropy on the thermal transport properties in flexible and rigid polyimide matrices (Figure 7).[3] In a comparison between BN sheets and particles as fillers, the former exhibited significantly larger ratios of in-plane to out-of-plane thermal diffusivity values. ChemPhysChem 0000, 00, 0 – 0
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Figure 6. Top: TEM images of cross-sectional microtomed slices of PVA/BNNs (a, b; inset: photograph of a piece of film) and epoxy/BNNs films (c, d). Bottom: Thermal diffusivity values of epoxy/BN nanocomposite films at different BNN loadings (&) compared with those for PVA/BNNs films (*). Inset: Photograph of a piece of film with 50 % BNN loading. [Reprinted with permission from Ref. [2], copyright (2012) Wiley-VCH].
Figure 7. Schematic representation of a study on correlations between the orientation function of h-BN nanofillers in composite films and anisotropy in thermal diffusivity. [Reprinted with permission from Ref. [3], copyright (2013) American Chemical Society].
Song et al. took advantage of the known properties of PVA to mechanically stretch the PVA/BNN composite films for the alignment of the embedded BNNs, resulting in a dramatic enhancement in the in-plane thermal diffusivity (Figure 8).[2] More 5
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Minireviews gation, with their thicknesses in the range of several nanometers to tens of nanometers and the length and width dimensions mostly in thousands of nanometers. The filler mass fraction was 5 wt % in all three composites. The composite with HBP-functionalized BNNs exhibited better mechanical and thermal properties than those of the composite with ODA-functionalized BNNs, whereas the composite containing BNNs without functionalization (from DMF dispersion) performed more poorly. The difference was attributed to the beneficial effect from the homogeneous dispersion of the chemically modified BNNs in the polymer matrix and their strong interfacial adhesion to epoxy. The thermal conductivity of the epoxy composites exhibited temperature dependences similar to that of the neat epoxy, increasing with temperature in the 25–200 8C range. The best thermal conductivity was found in the composite of epoxy with the HBP-functionalized BNNs (5 wt % loading), 0.329 W mK 1 at 100 8C;[76] a significant improvement from that of neat epoxy, but still not so thermally conductive. The performance improvement from the benefit of functionalization was apparently only incremental. Tseng et al. used titanate coupling agent (KR-44) to functionalize BNNs (4 or 15 mm in size) for their more homogeneous dispersion in polyimide composites.[77] These composites at a filler loading of 50 wt % exhibited a thermal conductivity of about 0.86 W mK 1 (in comparison with 0.13 W mK 1 in neat polyimide). For further improvement, the same group introduced two-phase fillers into polyimide, with the aim of establishing a thermally conductive network in the polymer matrix.[4] Polyimide composites with 50 wt % of the functionalized BNNs and 1 wt % of glycidyl methacrylate grafted graphene (g-TrG) exhibited improved thermal conductivity of 2.1 W mK 1. It was believed that the tiny amount of g-TrG might fill the gaps between BNNs and the polyimide matrix, and contribute to phonon transfer at the polymer–filler interfaces.[4] As reported recently,[78] a solvent-free process was used in the fabrication of epoxy composites with BNNs at various filler loadings. The BN powder was surface treated with silane coupling agent and then mixed mechanically with epoxy resin in the solid state. The resulting composite with 70 wt % BNNs exhibited a thermal conductivity of up to 5.24 W mK 1.[78] Among other uses of BNNs was the fabrication of optically transparent or semitransparent polymeric nanocomposites with low loadings of BNNs.[5, 79, 80] For example, in the work by Wang et al.,[79] various amounts of few-layer BNNs were dispersed in PMMA films. The composite film at about 10 wt % loading of BNNs was still optically semitransparent, with an observed thermal conductivity of about 0.4 W mK 1, which was still higher than that of blank PMMA (0.15 W mK 1). Similarly, Zhang et al. used BNNs with an average thickness of around 6 nm to fabricate PVA composite films.[5] At 3 wt % loading of BNNs, the film appeared to be almost transparent, with an observed thermal conductivity close to 0.4 W mK 1, which was three times higher than that in a neat PVA film (0.115 W mK 1). In a recent study by Ajayan and co-workers,[81] BNNs were also used as additives in mineral oil for stable Newtonian nanofluids with improved thermal conductivity.
Figure 8. Thermal diffusivity values at different BN loadings in PVA/BNNs films as-fabricated (*) and mechanically stretched ( 2: ^; and 3: ~). Inset: Photographs of as-fabricated and stretched films with 10 % BNN loading. [Reprinted with permission from Ref. [2], copyright (2012) Wiley-VCH].
specifically, upon the mechanical stretching of the film with 15 vol % BNNs, the observed in-plane thermal diffusivity values were up to 9 mm2 s 1 (equivalent to a thermal conductivity of 13 W mK 1), which were many times higher than those recorded before stretching (Figure 8).[2] The effect of polymeric matrices and their orientations on the thermal transport performance of the composites with BNNs have also been considered.[74, 75] For example, in the work by Shoji et al.,[74] oriented cross-linked liquid-crystalline polyimide was used as a matrix for thermally conductive composites with BNNs. The observed thermal diffusivity in these composites was in the order of 0.68 mm2 s 1 at a 30 vol % h-BN loading, which was somewhat higher (about 10 %) than that in the composites made with amorphous polyimide.[74] The increasing amount of BNNs in the composites apparently had no significant impact on the orientation order of the cross-linked liquid-crystalline chains. Yoshihara and co-workers investigated the effect of the molecular orientations (perpendicular, random, and parallel to the h-BN plane) of three liquid-crystalline polyesters on the in-plane thermal conductivity of injection-molded polymer/h-BN (about 45 mm in size) composites.[75] The thermal conductivity in these composites varied from 1 to 22 W mK 1 at 50 vol % loading, depending on the polymer matrices and their orientations. The observed difference was attributed to the difference of the heat conductive functions of the matrices between the h-BN platelets closely stacked. The composites in which the polymers were perpendicular to the BNN plane exhibited the highest in-plane thermal conductivity, serving as more effective heat paths than those parallel to the BNN plane. Chemically functionalized BNNs were used in polymeric composites to study the effect of their interfacial compatibility with the polymer matrix on the thermal transport properties of the composites.[76] For example, Yu et al. fabricated a series of epoxy composites by incorporating three kinds of fillers into an epoxy matrix through a sonication–centrifugation technique: BNNs, BNNs noncovalently functionalized with ODA, and BNNs covalently functionalized with hyperbranched aromatic polyamide (HBP).[76] The BNNs were prepared from the exfoliation of h-BN powders in DMF by sonication and centrifu-
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Minireviews The development of high-performance nanocomposite materials based on BNNs is still at an early stage, especially with respect to the preparation of high-quality BNNs in a more efficient and controllable fashion. More homogeneous filler dispersion in polymeric matrices is also an important issue because there is an intrinsic mismatch between BNNs as a hard ceramic material and the relatively soft matrix polymer. However, the studies available have provided valuable proof of concept results on the desired thermally conductive yet electrically insulating nanocomposite materials, some of which are already competitive in performance. An important and challenging issue is on the interfacial thermal resistance caused by the phonon mismatch at the interface between the polymer matrix and the embedded BNNs, and also the gaps between adjacent BNNs, for which an improved theoretical understanding complemented by well-designed and executed experimental evaluations is required.
Keywords: boron · conducting materials · graphene · materials science · nanostructures [1] K. Sato, H. Horibe, T. Shirai, Y. Hotta, H. Nakano, H. Nagai, K. Mitsuishi, K. Watari, J. Mater. Chem. 2010, 20, 2749 – 2752. [2] W. L. Song, P. Wang, L. Cao, A. Anderson, M. J. Meziani, A. J. Farr, Y.-P. Sun, Angew. Chem. Int. Ed. 2012, 51, 6498 – 6501; Angew. Chem. 2012, 124, 6604 – 6607. [3] M. Tanimoto, T. Yamagata, K. Miyata, S. Ando, ACS Appl. Mater. Interfaces 2013, 5, 4374 – 4382. [4] M. H. Tsai, I. H. Tseng, J. C. Chiang, J. J. Li, ACS Appl. Mater. Interfaces 2014, 6, 8639 – 8645. [5] S. Zhang, G. Lian, H. Si, J. Wang, X. Zhang, Q. Wang, D. Cui, J. Mater. Chem. A 2013, 1, 5105 – 5112. [6] C. Y. Zhi, Y. Bando, T. Terao, C. C. Tang, H. Kuwahara, D. Golberg, Adv. Funct. Mater. 2009, 19, 1857 – 1862. [7] C. Y. Zhi, Y. B. Xu, Y. Bando, D. Golberg, ACS Nano 2011, 5, 6571 – 6577. [8] X. Y. Huang, C. Y. Zhi, P. K. Jiang, D. Golberg, Y. Bando, T. Tanaka, Adv. Funct. Mater. 2013, 23, 1824 – 1831. [9] W. J. Meng, Y. Huang, Y. Q. Fu, Z. Wang, C. Y. Zhi, J. Mater. Chem. C 2014, 2, 10049 – 10061. [10] B. Q. Dai, G. L. Zhang, Mater. Chem. Phys. 2003, 78, 304 – 307. [11] R. S. Pease, Nature 1950, 165, 722 – 723. [12] N. G. Chopra, R. J. Luyken, K. Cherrey, V. H. Crespi, M. L. Cohen, S. G. Louie, A. Zettl, Science 1995, 269, 966 – 967. [13] A. Rubio, J. L. Corkill, M. L. Cohen, Phys. Rev. B 1994, 49, 5081 – 5084. [14] R. Saito, M. Fujita, G. Dresselhaus, M. S. Dresselhaus, Appl. Phys. Lett. 1992, 60, 2204 – 2206. [15] R. H. Baughman, A. A. Zakhidov, W. A. D. Heer, Science 2002, 297, 787 – 792. [16] H. B. Zeng, C. Y. Zhi, Z. H. Zhang, X. L. Wei, X. B. Wang, W. L. Guo, Y. Bando, D. Golberg, Nano Lett. 2010, 10, 5049 – 5055. [17] K. J. Erickson, A. L. Gibb, A. Sinitskii, M. Rousseas, N. Alem, J. M. Tour, A. K. Zettl, Nano Lett. 2011, 11, 3221 – 3226. [18] A. S. Luyt, J. A. Molefi, H. Krump, Polym. Degrad. Stab. 2006, 91, 1629 – 1636. [19] R. Prasher, Proc. IEEE 2006, 94, 1571 – 1586. [20] G. E. Spriggs in Group VIII Advanced Materials and Technologies: Powder Metallurgy Data, Refractory, Hard and Intermetallic Materials, Vol. 2A2 (Eds.: R. Ruthardt, H. W. Landolt-Bçrnstein), Springer, Berlin, 2002, pp. 118 – 139. [21] Y. Lin, J. W. Connell, Nanoscale 2012, 4, 6908 – 6939. [22] M. Corso, W. Auwarter, M. Muntwiler, A. Tamai, T. Greber, J. Osterwalder, Science 2004, 303, 217 – 220. [23] Y. Zhu, Y. Bando, L. Yin, D. Golberg, Nano Lett. 2006, 6, 2982 – 2986. [24] Z. G. Chen, J. Zou, G. Liu, F. Li, Y. Wang, L. Wang, Z. L. Yuan, T. Sekiguchi, H. M. Cheng, G. Q. Lu, ACS Nano 2008, 2, 2183 – 2191. [25] L. Song, L. J. Ci, H. Lu, P. B. Sorokin, C. H. Jin, J. Ni, A. G. Kvashnin, D. G. Kvashnin, J. Lou, B. I. Yakobson, P. M. Ajayan, Nano Lett. 2010, 10, 3209 – 3215. [26] J. Yu, L. Qin, Y. Hao, S. Kuang, X. Bai, Y. Chong, W. Zhang, E. Wang, ACS Nano 2010, 4, 414 – 422. [27] D. Pacil, J. C. Meyer, C. O. Girit, A. Zettl, Appl. Phys. Lett. 2008, 92, 133107 – 133107. [28] W. Q. Han, L. Wu, Y. Zhu, K. Watanabe, T. Taniguchi, Appl. Phys. Lett. 2008, 93, 223103. [29] C. Y. Zhi, Y. Bando, C. Tang, H. Kuwahara, D. Golberg, Adv. Mater. 2009, 21, 2889 – 2893. [30] Y. Lin, T. V. Williams, J. W. Connell, J. Phys. Chem. Lett. 2010, 1, 277 – 283. [31] A. Nag, K. Raindogia, K. P. S. S. Hembram, R. Datta, U. V. Wangmare, C. N. R. Rao, ACS Nano 2010, 4, 1539 – 1544. [32] Y. Lin, T. V. Williams, W. Cao, H. E. Elsayed-Ali, J. W. Connell, J. Phys. Chem. C 2010, 114, 17434 – 17439. [33] Y. Lin, T. V. Williams, T. B. Xu, W. Cao, H. E. Elsayed-Ali, J. W. Connell, J. Phys. Chem. C 2011, 115, 2679 – 2685. [34] Y. Wang, Z. Shi, J. Yin, J. Mater. Chem. 2011, 21, 11371 – 11377. [35] J. N. Coleman, M. Lotya, A. O’Neill, S. D. Bergin, P. J. King, U. Khan, K. Young, A. Gaucher, S. De, R. J. Smith, I. V. Shvets, S. K. Arora, G. Stanton, H. Y. Kim, K. Lee, G. T. Kim, G. S. Duesberg, T. Hallam, J. J. Boland, J. J.
5. Summary and Outlook BNNs are analogous to their two-dimensional carbon counterparts in many materials properties, especially the ultrahigh thermal conductivity, but also offer some unique attributes, including being electrically insulating, high thermal stability, chemical and oxidation resistance, low color, and high mechanical strength. According to the studies highlighted herein, major progress has been made in the development of polymeric nanocomposites with BNNs for thermally conductive yet electrically insulating materials and devices. Significant challenges for further efforts in this rapidly advancing field include more controlled production of high-quality BNNs in sufficient quantities and their improved dispersion in polymeric matrices, both of which are affected by the ceramic characteristics of BNs. Strategies such as chemical modification through functionalization of BNNs and their precursors for exfoliation and/ or dispersion purposes have found major success, and more may be expected in further investigations. With the recent proliferation of studies on graphene and related materials and technologies, some of the widely identified and pursued potential applications are probably better served by BNNs and composites, in addition to their uniquely qualified uses in modern high-speed electronics, next-generation coating technologies, and space exploration.
Acknowledgements Financial support of research efforts on the subject from the Air Force Office of Scientific Research (AFOSR) through the program of Dr. Charles Lee and the South Carolina Space Grant Consortium (Y.-P.S., including a Graduate Research Fellowship to A.A.) is gratefully acknowledged. The preparation of this article was also made possible by the support of U.S. NSF (Y.-P.S.), NSF China [51302011 (W.-L.S.) and 51272152 (F.L.)], and Guangdong NSF [S2013010014171 (F.L.)] and DoE [2012KJCX0053 (F.L.)]. ChemPhysChem 0000, 00, 0 – 0
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Received: November 17, 2014 Revised: December 26, 2014 Published online on && &&, 2015
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MINIREVIEWS M. J. Meziani,* W.-L. Song,* P. Wang, F. Lu,* Z. Hou, A. Anderson, H. Maimaiti, Y.-P. Sun* && – && Boron Nitride Nanomaterials for Thermal Management Applications Carbon copy? Significant recent advances in boron nitride nanosheets, including their production, properties, and dispersion into polymeric matrices for
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Boron Nitride Nanomaterials for Thermal Management Applications Mohammed J. Meziani,*[a, b] Wei-Li Song,*[c] Ping Wang,[a] Fushen Lu,*[d] Zhiling Hou,[a] Ankoma Anderson,[a] Halidan Maimaiti,[a] and Ya-Ping Sun*[a] Hexagonal boron nitride nanosheets (BNNs) are analogous to their two-dimensional carbon counterparts in many materials properties, in particular, ultrahigh thermal conductivity, but also offer some unique attributes, including being electrically insulating, high thermal stability, chemical and oxidation resistance, low color, and high mechanical strength. Significant
recent advances in the production of BNNs, understanding of their properties, and the development of polymeric nanocomposites with BNNs for thermally conductive yet electrically insulating materials and systems are highlighted herein. Major opportunities and challenges for further studies in this rapidly advancing field are also discussed.
1. Introduction Hexagonal boron nitride (h-BN) isomorphs of widely investigated carbon nanostructures (graphenes in particular) have generated much recent interest for their unique characteristics, especially with the decoupling of thermal and electrical transport properties in these materials. BNs are known for being highly thermally conductive yet electrically insulating, and therefore, have traditionally been considered as a material of choice in thermal management applications. For nanoscale BNs, oneand two-dimensional structures with a high aspect ratio have been used as fillers in polymeric matrices for nanocomposite materials of enhanced thermal conductivity.[1–9] The similarity between graphenes/graphite and BNs is such that both have a layered structure, which could be considered as being constructed from sp2-bonded hexagonally packed sheets. In BNs, however, the boron and nitrogen atoms are alternatively bonded and positioned to form planar conjugated layers in such a configuration that the two in the neighboring layers are eclipsed on top of one another because of the polar-
ity mismatch. Slight ionic bonding both in plane and out of the plane (“lip–lip” interactions) in BNs also make them different from graphenes/graphite in their structural configuration, and consequently, in some of the nanomaterial properties. Specifically, stronger interlayer interactions make the exfoliation of h-BN into BN nanosheets (BNNs; Figure 1) more difficult
Figure 1. Structures of a graphene sheet (left), a boron nitride sheet (middle), and a boron nitride nanotube (right).
[a] Dr. M. J. Meziani, P. Wang, Dr. Z. Hou, A. Anderson, Dr. H. Maimaiti, Prof. Y.-P. Sun Department of Chemistry and Laboratory for Emerging Materials and Technology, Clemson University Clemson, SC 29634 (USA) E-mail: [email protected] [email protected]
than peeling off graphene from graphite.[10–13] Similar to the conceptual understanding of carbon nanotubes,[14, 15] a BN nanotube (BNNT; Figure 1) may also be viewed as being geometrically derived by rolling a hexagonal single-layer BNN.[16, 17] Both BNNs and BNNTs (Figure 1) have been attracting increasing attention for thermal transport and other uses, although the former is the primary focus of this article. Growing interest in BN nanomaterials in general is due to their distinct and advantageous properties in comparison to those of their carbon counterparts. First, nanoscale BNs are electrically insulating with a wide band gap of 5.6 eV, yet are extremely thermally conductive, and therefore, are amenable to many unique applications. For thermal management in electronics as an example, the miniaturization of devices demands more effective
[b] Dr. M. J. Meziani Department of Natural Sciences Northwest Missouri State University Maryville, MO 64468 (USA) [c] Dr. W.-L. Song Institute of Advanced Materials and Technology University of Science and Technology Beijing Beijing 100083 (P.R. China) E-mail: [email protected] [d] Dr. F. Lu Department of Chemistry, Shantou University Shantou, Guangdong 515063 (P.R. China) E-mail: [email protected]
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Minireviews heat dissipation, which has been identified as a major bottleneck in terms of performance and reliability improvements.[18, 19] Similar issues are encountered in electric motors and generators, heat exchangers in power generation, automotives, and so forth. The BN-based materials of decoupled thermal and electrical transport properties offer potentially breakthrough solutions for these challenging issues. BNs are of superb thermal and chemical stabilities with high oxidation resistance and passivity to reactions with acids and melts,[20] and are particularly suitable for those applications under extreme conditions, such as devices operating in an oxidative environment at high temperatures.[20, 21] More specifically, graphite oxidation in air starts at 400–450 8C, yet h-BN is stable at temperatures up to 1000 8C in air and 1400 8C in vacuum.[20] BNs are insoluble in commonly used acids, and there are only a few substances, such as molten alkalis and alkaline solutions, that could attack or dissolve BNs at high temperatures.[20] These excellent properties have made nanoscale BNs and their derived materials increasingly popular candidates for a variety of technological needs. Figure 2. a) A TEM image of a h-BN nanosheet functionalized with amine-terminated polyethylene glycol. [Reprinted with permission from Ref. [30], copyright (2010) American Chemical Society.] b) A high-resolution TEM image of a h-BN nanosheet folded along the [120] axis (with the fringe contrast at the edge). Inset I: The diffractogram from the circled area I, showing the hexagonal symmetry of the nanosheet. Inset II: The diffractogram from area II, with the streaks lying perpendicular to the edge, similar to those seen in the electron diffraction of nanotubes. [Reprinted with permission from Ref. [28], copyright (2008) American Institute of Physics.] c) AFM image of the h-BN nanosheets, the sharp edges of which are indicated by the white arrows. Inset: the height distribution along the yellow line [Reprinted with permission from Ref. [49], copyright (2012) American Chemical Society]. d) AFM topographic image showing an area populated with h-BN nanosheets with feature heights less than 1 nm (the height profile plot corresponding to the dotted line). Scale bar = 200 nm. [Reprinted with permission from Ref. [33], copyright (2011) American Chemical Society].
2. BNNs—Synthesis and Structures Compared with ever-popular single- and few-layer graphene nanosheets, BNNs are understudied, despite their many important and/or unique advantages. One excuse might be such that the synthesis and exfoliation of BNNs with a controlled number of layers and in reasonable yields have proven difficult. Nevertheless, both bottom-up[22–26] and top-down[16, 17, 27–46] approaches have been explored for the preparation of BNNs. Single- or few-layer BNNs thus obtained have lateral sizes in the range of a few hundred nanometers to as large as a few centimeters (Figure 2). In the bottom-up approach for BNNs, earlier syntheses involved reactions of boron oxides or boric acid with urea or melamine.[31, 47] Recently, few-layer BNNs were also prepared from the chemical reactions of boric acid and urea at 900 8C under N2, in which some variations in the number of layers were achieved by changing the reactant concentrations.[31] A more popular method in the bottom-up approach has been the use of chemical vapor deposition (CVD).[25, 26] Thus, highquality thin BNNs of large lateral sizes have been synthesized from the thermal decomposition of various nitrogen-containing organoboron precursors (amine–boranes, ammonia– borane, and borazine) on metal or graphene substrates,[48–51] or the plasma-induced reactions of BF3 in a H2/N2 atmosphere.[26] However, more complicated procedures and high cost may limit the applicability of these syntheses in the larger-scale production of BNNs and for some important applications, such as in thermally conductive polymeric composites, in which substrate-free BNNs are required. The top-down approach with h-BN as the starting material has been considered to be relatively simple and more economical, although again the partially ionic character of B N bonding in h-BN makes exfoliation into BNNs more difficult than peeling off graphene from graphite. In this regard, Pacil et al. did try the micromechanical cleavage method by using adhe-
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sive tape to peel off BNNs from h-BN powder, but the yield was too scarce for real applications.[27] More productive has been the sonication-assisted liquid-phase exfoliation of h-BN for few-layer BNNs. This method and the choice of solvents in relevant studies have benefited from recent advances and extensive results on similar exfoliation of graphite.[52] In their early work, Zhi et al. sonicated h-BN in DMF for 10 h to yield a dilute suspension of BNNs at a concentration of 0.01–0.03 mg mL 1.[29] Warner et al. changed the solvent to 1,2dichloroethane for sonication and obtained BNNs of larger lateral dimensions.[36] Other strong polar organic solvents, such as N-methylpyrrolidone (NMP); N,N-dimethylacetamide (DMAc); 1,2-dichlorobenzene; and ethylene glycol, are useful or effective in the exfoliation of h-BN with sonication, from which relatively stable dispersions of BNNs are obtained.[35] Because of the clearly important role of solvent in the sonication-assisted processing, Coleman and co-workers attempted to use Hansen solubility parameter theory to explain and guide solvent selection for the exfoliation and dispersion of BNNs and other inorganic nanosheets.[35] For BNNs, isopropanol was identified as a favorable solvent, with which BNNs of 2
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Minireviews 3. Thermal Transport Properties of BNNs
less than 8 layers in dispersions with concentrations as high as 0.06 mg mL 1 were produced.[35] Similarly, Zhou et al. suggested a mixed-solvent strategy by using ethanol/water mixtures to improve the exfoliation and dispersion of BNNs.[53] There have been more choices for solvents in the exfoliation method, such as the recent use of methanesulfonic acid by Wang et al.,[34] and essentially unlimited combinations in terms of solvent mixtures. However, the solvent dispersion-based method is generally associated with low yields, typically for BNNs of 2–10 nm thick at milligram levels, which may be attributed to only weak interactions between BNNs and the solvent molecules. The use of chemical functionalization has been more effective to overcome the lip–lip interaction between the layers in h-BN for improved exfoliation.[17, 54] Amino molecules and other Lewis bases, such as phosphine molecules, were used to complex with boron atoms on the h-BN surface for exfoliation and the solubilization of the resulting BNNs;[30, 31] a strategy borrowed from the previous functionalization and solubilization of BNNTs.[54, 55] For example, Lin et al. functionalized h-BN with amino molecules, including octadecylamine (ODA) and amineterminated polyethylene glycol, coupled with exfoliation through ball-milling to yield mostly BNNs of 3–20 layers and lateral sizes up to about a micron.[30] In a further improvement, the same group increased the ball-milling time to enhance the exfoliation efficiency, with milling for 60 min resulting in the exfoliation of up to 40 % of the starting h-BN into BNNs of less than 50 nm thick with lateral dimensions up to 5 mm.[32] Separately, the ability for amino and phosphine molecules to complex with the few-layer BNNs produced in a bottom-up method was demonstrated by Nag et al.,[31] again conceptually similar to the functionalization and solubilization of BNNTs.[54, 55] Beyond ball-milling exfoliation, other shear-force-based mechanical processing methods, which are to some extent analogous to the infamous micromechanical cleavage, have been applied to the preparation of BNNs at a significant scale. For example, in the processing by Yurdakul et al. based on highpressure microfluidization, the starting h-BN powder in a DMF/ chloroform mixture was pumped through a microfluidic processor at a pressure of up to 207 MPa with multiple circulations.[44] It was suggested that the large shear force in the microfluidic channel was responsible for the exfoliation of h-BN into BNNs (8–12 nm thick with micrometer lateral dimensions), with a relatively high production efficiency in gram quantities. Similarly, the exfoliation of h-BN was accomplished by shearing vortex fluidic films in the solvent NMP at 8000 rpm in a glass tube with a fixed angle.[45] In a more recent use of the functionalization approach, a “soup” of h-BN with hydrazine, 30 % H2O2, and HNO3/H2SO4 (or oleum) was heated in an autoclave to 100 8C for functionalized h-BN.[46] The sample formed stable colloid solutions in water and DMF, which were found to contain few-layer BNNs with lateral dimensions in the order of several hundred nanometers.
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Thermal conductivity (k) is a property of materials to transport heat best described by the Fourier law, q = k !T, in which q is the heat flux or heat flow per unit area, !T is the temperature gradient through the conducting medium, and the negative sign indicates heat flow from high to low temperature.[56] Generally, in solids heat is transported by lattice/atomic vibrations (phonons) and/or free electrons, so that k is the sum of kp and ke, which represent the phonon and electron contributions, respectively. The ke term dominates in metals or heavily doped semiconductors due to the large numbers of free electrons. In nonmetallic materials such as h-BN, phonons are the dominant heat carriers, since free movement of electrons is impossible. Another commonly used parameter is thermal diffusivity (a), which defines how fast the material conducts heat. It is related to k by a = k/Cp1m, in which Cp is the specific heat and 1m is the mass density of the material. Many experimental techniques measure a rather than k directly. BNs are generally known as excellent thermal conductors,[43, 57–59] so are expected for BNNs.[43] However, investigations into the thermal transport properties of single- and fewlayer BNNs have been limited, with mostly estimated thermal conductivity values ranging from 300 to 2000 W mK 1. In an early study, Kumar and Pal found that high thermal conductivity could only be obtained inside the (002) planes (up to hundreds to thousands of W mK 1 units), whereas the thermal conductivity was only several W mK 1 units in other lattice planes.[60] Recent theoretical calculations have suggested that the thermal conductivity of suspended few-layer BNNs should increase as the number of layers decreases due to reduced interlayer phonon scattering.[61] Lindsay and Broido used an exact numerical solution of the phonon Boltzmann transport equation, in which both phonon–phonon scattering and the isotope effect were incorporated simultaneously, and they found that the thermal conductivity in monolayer BNNs was significantly higher than that in their bulk counterpart, again due to the reduction in interlayer phonon scattering (Figure 3).[61] Experimentally, however, recent results appeared that were at odds with the theoretical predictions for the BNN thickness effect, and showed that the measured thermal conductivity in the 5-layer BNN sample was actually lower than that in the 11layer sample (Figure 4).[62] In that experimental investigation, thermal conductivity values of suspended few-layer BNNs of different lengths and layer thicknesses were measured by using a microbridge device with built-in resistance thermometers (Figure 4). In the device fabrication process, a poly(methyl methacrylate) (PMMA) layer was coated on the sample and patterned with the use of electron-beam lithography, and then selectively removed in acetone. As shown in Figure 4, the room-temperature thermal conductivity thus measured for the 11-layer sample with lengths ranging from 3 to 7.5 mm was about 360 W mK 1, which approached the basal plane value reported for bulk h-BN.[62] As the temperature decreased, the thermal conductivity of this sample initially increased due to reduced Umklapp scattering, reaching the maximum value at 3
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Minireviews conductivity in BNNRs to four to six times lower than that in their graphene counterparts.[63] The nanoribbons may have a zigzag (z-BNNR) or armchair edge structure (a-BNNR), and the results also suggest that for both edge structures the ribbon width has a strong influence on the thermal conductivity (Figure 5).[63]
Figure 3. Left: The solid red curve shows the calculated kL of h-BN as a function of temperature, T, compared with measured values (black diamonds). The dashed red curve shows the kL for isotopically pure h-BN. The solid green curve gives the calculated kL for naturally occurring single-layer h-BN, whereas the dashed green curve shows calculated kL for isotopically pure single-layer h-BN. Right: The percentage enhancement, P, of kL in isotopically pure single-layer h-BN compared with naturally occurring single-layer h-BN as a function of T for different values of L (a measure of the length between boundaries in the transport direction). Inset: a comparison of P for singlelayer h-BN and bulk h-BN for L = 2 mm. [Reprinted with permission from Ref. [61], copyright (2011) American Physical Society].
Figure 5. The width dependence of room-temperature lattice thermal conductivity in a- and z- BNNRs. [Reprinted with permission from Ref. [63], copyright (2011) American Physical Society].
The theoretical predictions and experimental results discussed above have generally established that BNNs are highly thermally conductive, yet electrically insulating, and thermal transport is due to the phonon conduction mechanism. However, when compared with their carbon analogues, BNNs are much less explored for their superior and unique thermal transport properties. More studies, especially experimental investigations, on various parameters affecting thermal conductivity and for improved mechanistic understanding are still in demand.
Figure 4. Left: A microbridge device for the thermal transport measurements of suspended few-layer h-BN samples. Inset: An 11-layer h-BN sample suspended on the device. Right: Thermal conductivity values of the two 7.5 mm long, 11- and 5-layer thick suspended h-BN samples as a function of temperature. [Reprinted with permission from Ref. [62], copyright (2013) American Chemical Society].
4. Thermally Conductive Nanocomposites BNNs are logically considered as unique nanofillers for thermally conductive, but electrically insulating polymeric composites.[1, 9, 43, 57, 64–69] Again for thermal management in electronics as an example, these nanocomposite materials enable an excellent thermal interface for uneven or rough surface topography in the electronic devices. They may also serve as precursors for fabrication into various device forms, such as tubing and ribbons, to be incorporated into thermal management systems. Polymers are typically poor thermal conductors. A variety of experimental strategies have been designed and practiced for the incorporation of BNNs into polymeric matrices of low thermal conductivity for substantially enhanced thermal transport performance, although the results have been mixed at best. Generally speaking, an important task for high-performance nanocomposites is to disperse BNNs into polymeric matrices in a homogeneous fashion, for which solution-based processing represents a more favorable option. However, fabrication based on simple blending of BNNs with polymers has not been so effective in achieving the desired dispersion. Among more successful attempts have been the use of some specifically selected polar aprotic solvents in combination with vigo-
a temperature between 100 and 200 K, similar to bulk samples. At lower temperatures, the thermal conductivity of the 11-layer sample became lower than the bulk values. The apparent trend of decreasing thermal conductivity with decreasing BNN thickness was attributed to phonon scattering by the polymer residue from the fabrication process, which was consistently observed in the TEM images.[62] Nevertheless, despite the clear intent of the authors’ explanations to be consistent with the common belief established almost exclusively on the basis of theoretical computational predictions, further experimental investigations on this important issue are needed. There have also been theoretical studies on thermal transport in BN nanoribbons (BNNRs), which are essentially BNNs of a narrower width with significant edge effects, although the outcomes were clearly dependent on the method employed.[63] For example, a study that used nonequilibrium Green’s functions and neglected phonon–phonon scattering predicted a much higher thermal conductivity in the BNNR, approaching 1700–3000 W mK 1, which was similar to that in graphene nanoribbons.[63] In a study based on the classical molecular dynamics method, strong phonon scattering reduced thermal
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Minireviews rous sonication and the chemical functionalization of BNNs with polymers or surfactants. These dispersion and fabrication approaches also have different technical issues and/or shortcomings, from low product yields in some methods to the “contamination” of unwanted dispersion agents, causing microscopic phase separation within the nanocomposite structure in others. Representative studies on relevant topics in the literature, along with a discussion on the challenges and opportunities in this important technological field, are highlighted below. A simple dispersion of BNNs into various polymers through wet processing has typically increased the thermal conductivity of the resulting nanocomposite over that of the corresponding neat polymer (commonly considered as a thermal insulator), sometimes by a large percentage, but in an absolute sense the thermal transport performance of the composite is still low or unimpressive. For example, Li and Hsu dispersed surface-modified large (average size about 1 mm) and small (average size about 70 nm) BNNs into polyimide; composites with a filler loading of 30 wt % exhibited thermal conductivity of up to 1.2 W mK 1.[70, 71] Wong and co-workers prepared BNNs by ballmilling and sonication and then dispersed the fillers into epoxy resin.[72] The thermal conductivity in the resulting composite at 30 wt % was 3 times higher than that in neat epoxy (0.15 W mK 1), although still relatively poor in terms of thermal transport performance. Song et al. used isopropanol for exfoliation to obtain BNNs with thicknesses in the order of 10 nm and lateral sizes in the order of 1 mm or larger.[2] In the dispersion of BNNs in poly(vinyl alcohol) (PVA) and epoxy matrices, keeping the fillers from aggregating or restacking was emphasized. Nanocomposite films of around 50 mm in thickness were fabricated by wet casting on an etched glass slide and then made to be freestanding (Figure 6) for subsequent characterization and measurements. According to TEM results (Figure 6), BNNs embedded in the films were largely the same as those in the isopropanol suspension, with sheet thicknesses remaining generally 10 nm or less (thus, there is no significant aggregation and/or restacking of BNNs in the composites). These nanocomposite films exhibited superior thermal transport performance, with an observed in-plane thermal diffusivity as high as 19 mm2 s 1 in the epoxy film with a filler loading of 50 vol % (equivalent to a thermal conductivity of 30 W mK 1; Figure 6).[2] BNNs as fillers in polymeric nanocomposites are anisotropic, so that their alignment has a significant effect on the thermal transport properties of the nanocomposites. Nakayama and co-workers dispersed BNNs (surface modified with iron oxide nanoparticles) into polysiloxane, and during film fabrication aligned the fillers in a magnetic field.[73] Their results suggested that the alignment improved the thermal conductivity of the resulting nanocomposite film. Similarly, Tanimoto et al. examined the effect of the filler orientation and structural anisotropy on the thermal transport properties in flexible and rigid polyimide matrices (Figure 7).[3] In a comparison between BN sheets and particles as fillers, the former exhibited significantly larger ratios of in-plane to out-of-plane thermal diffusivity values. ChemPhysChem 0000, 00, 0 – 0
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Figure 6. Top: TEM images of cross-sectional microtomed slices of PVA/BNNs (a, b; inset: photograph of a piece of film) and epoxy/BNNs films (c, d). Bottom: Thermal diffusivity values of epoxy/BN nanocomposite films at different BNN loadings (&) compared with those for PVA/BNNs films (*). Inset: Photograph of a piece of film with 50 % BNN loading. [Reprinted with permission from Ref. [2], copyright (2012) Wiley-VCH].
Figure 7. Schematic representation of a study on correlations between the orientation function of h-BN nanofillers in composite films and anisotropy in thermal diffusivity. [Reprinted with permission from Ref. [3], copyright (2013) American Chemical Society].
Song et al. took advantage of the known properties of PVA to mechanically stretch the PVA/BNN composite films for the alignment of the embedded BNNs, resulting in a dramatic enhancement in the in-plane thermal diffusivity (Figure 8).[2] More 5
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Minireviews gation, with their thicknesses in the range of several nanometers to tens of nanometers and the length and width dimensions mostly in thousands of nanometers. The filler mass fraction was 5 wt % in all three composites. The composite with HBP-functionalized BNNs exhibited better mechanical and thermal properties than those of the composite with ODA-functionalized BNNs, whereas the composite containing BNNs without functionalization (from DMF dispersion) performed more poorly. The difference was attributed to the beneficial effect from the homogeneous dispersion of the chemically modified BNNs in the polymer matrix and their strong interfacial adhesion to epoxy. The thermal conductivity of the epoxy composites exhibited temperature dependences similar to that of the neat epoxy, increasing with temperature in the 25–200 8C range. The best thermal conductivity was found in the composite of epoxy with the HBP-functionalized BNNs (5 wt % loading), 0.329 W mK 1 at 100 8C;[76] a significant improvement from that of neat epoxy, but still not so thermally conductive. The performance improvement from the benefit of functionalization was apparently only incremental. Tseng et al. used titanate coupling agent (KR-44) to functionalize BNNs (4 or 15 mm in size) for their more homogeneous dispersion in polyimide composites.[77] These composites at a filler loading of 50 wt % exhibited a thermal conductivity of about 0.86 W mK 1 (in comparison with 0.13 W mK 1 in neat polyimide). For further improvement, the same group introduced two-phase fillers into polyimide, with the aim of establishing a thermally conductive network in the polymer matrix.[4] Polyimide composites with 50 wt % of the functionalized BNNs and 1 wt % of glycidyl methacrylate grafted graphene (g-TrG) exhibited improved thermal conductivity of 2.1 W mK 1. It was believed that the tiny amount of g-TrG might fill the gaps between BNNs and the polyimide matrix, and contribute to phonon transfer at the polymer–filler interfaces.[4] As reported recently,[78] a solvent-free process was used in the fabrication of epoxy composites with BNNs at various filler loadings. The BN powder was surface treated with silane coupling agent and then mixed mechanically with epoxy resin in the solid state. The resulting composite with 70 wt % BNNs exhibited a thermal conductivity of up to 5.24 W mK 1.[78] Among other uses of BNNs was the fabrication of optically transparent or semitransparent polymeric nanocomposites with low loadings of BNNs.[5, 79, 80] For example, in the work by Wang et al.,[79] various amounts of few-layer BNNs were dispersed in PMMA films. The composite film at about 10 wt % loading of BNNs was still optically semitransparent, with an observed thermal conductivity of about 0.4 W mK 1, which was still higher than that of blank PMMA (0.15 W mK 1). Similarly, Zhang et al. used BNNs with an average thickness of around 6 nm to fabricate PVA composite films.[5] At 3 wt % loading of BNNs, the film appeared to be almost transparent, with an observed thermal conductivity close to 0.4 W mK 1, which was three times higher than that in a neat PVA film (0.115 W mK 1). In a recent study by Ajayan and co-workers,[81] BNNs were also used as additives in mineral oil for stable Newtonian nanofluids with improved thermal conductivity.
Figure 8. Thermal diffusivity values at different BN loadings in PVA/BNNs films as-fabricated (*) and mechanically stretched ( 2: ^; and 3: ~). Inset: Photographs of as-fabricated and stretched films with 10 % BNN loading. [Reprinted with permission from Ref. [2], copyright (2012) Wiley-VCH].
specifically, upon the mechanical stretching of the film with 15 vol % BNNs, the observed in-plane thermal diffusivity values were up to 9 mm2 s 1 (equivalent to a thermal conductivity of 13 W mK 1), which were many times higher than those recorded before stretching (Figure 8).[2] The effect of polymeric matrices and their orientations on the thermal transport performance of the composites with BNNs have also been considered.[74, 75] For example, in the work by Shoji et al.,[74] oriented cross-linked liquid-crystalline polyimide was used as a matrix for thermally conductive composites with BNNs. The observed thermal diffusivity in these composites was in the order of 0.68 mm2 s 1 at a 30 vol % h-BN loading, which was somewhat higher (about 10 %) than that in the composites made with amorphous polyimide.[74] The increasing amount of BNNs in the composites apparently had no significant impact on the orientation order of the cross-linked liquid-crystalline chains. Yoshihara and co-workers investigated the effect of the molecular orientations (perpendicular, random, and parallel to the h-BN plane) of three liquid-crystalline polyesters on the in-plane thermal conductivity of injection-molded polymer/h-BN (about 45 mm in size) composites.[75] The thermal conductivity in these composites varied from 1 to 22 W mK 1 at 50 vol % loading, depending on the polymer matrices and their orientations. The observed difference was attributed to the difference of the heat conductive functions of the matrices between the h-BN platelets closely stacked. The composites in which the polymers were perpendicular to the BNN plane exhibited the highest in-plane thermal conductivity, serving as more effective heat paths than those parallel to the BNN plane. Chemically functionalized BNNs were used in polymeric composites to study the effect of their interfacial compatibility with the polymer matrix on the thermal transport properties of the composites.[76] For example, Yu et al. fabricated a series of epoxy composites by incorporating three kinds of fillers into an epoxy matrix through a sonication–centrifugation technique: BNNs, BNNs noncovalently functionalized with ODA, and BNNs covalently functionalized with hyperbranched aromatic polyamide (HBP).[76] The BNNs were prepared from the exfoliation of h-BN powders in DMF by sonication and centrifu-
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Minireviews The development of high-performance nanocomposite materials based on BNNs is still at an early stage, especially with respect to the preparation of high-quality BNNs in a more efficient and controllable fashion. More homogeneous filler dispersion in polymeric matrices is also an important issue because there is an intrinsic mismatch between BNNs as a hard ceramic material and the relatively soft matrix polymer. However, the studies available have provided valuable proof of concept results on the desired thermally conductive yet electrically insulating nanocomposite materials, some of which are already competitive in performance. An important and challenging issue is on the interfacial thermal resistance caused by the phonon mismatch at the interface between the polymer matrix and the embedded BNNs, and also the gaps between adjacent BNNs, for which an improved theoretical understanding complemented by well-designed and executed experimental evaluations is required.
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5. Summary and Outlook BNNs are analogous to their two-dimensional carbon counterparts in many materials properties, especially the ultrahigh thermal conductivity, but also offer some unique attributes, including being electrically insulating, high thermal stability, chemical and oxidation resistance, low color, and high mechanical strength. According to the studies highlighted herein, major progress has been made in the development of polymeric nanocomposites with BNNs for thermally conductive yet electrically insulating materials and devices. Significant challenges for further efforts in this rapidly advancing field include more controlled production of high-quality BNNs in sufficient quantities and their improved dispersion in polymeric matrices, both of which are affected by the ceramic characteristics of BNs. Strategies such as chemical modification through functionalization of BNNs and their precursors for exfoliation and/ or dispersion purposes have found major success, and more may be expected in further investigations. With the recent proliferation of studies on graphene and related materials and technologies, some of the widely identified and pursued potential applications are probably better served by BNNs and composites, in addition to their uniquely qualified uses in modern high-speed electronics, next-generation coating technologies, and space exploration.
Acknowledgements Financial support of research efforts on the subject from the Air Force Office of Scientific Research (AFOSR) through the program of Dr. Charles Lee and the South Carolina Space Grant Consortium (Y.-P.S., including a Graduate Research Fellowship to A.A.) is gratefully acknowledged. The preparation of this article was also made possible by the support of U.S. NSF (Y.-P.S.), NSF China [51302011 (W.-L.S.) and 51272152 (F.L.)], and Guangdong NSF [S2013010014171 (F.L.)] and DoE [2012KJCX0053 (F.L.)]. ChemPhysChem 0000, 00, 0 – 0
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Received: November 17, 2014 Revised: December 26, 2014 Published online on && &&, 2015
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MINIREVIEWS M. J. Meziani,* W.-L. Song,* P. Wang, F. Lu,* Z. Hou, A. Anderson, H. Maimaiti, Y.-P. Sun* && – && Boron Nitride Nanomaterials for Thermal Management Applications Carbon copy? Significant recent advances in boron nitride nanosheets, including their production, properties, and dispersion into polymeric matrices for
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thermally conductive yet electrically insulating nanocomposite materials and systems, are highlighted.
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