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Alignment of Boron Nitride Nanotubes in Polymeric Composite Films for Thermal Conductivity Improvement Takeshi Terao,*,†,‡ Chunyi Zhi,‡ Yoshio Bando,‡ Masanori Mitome,‡ Chengchun Tang,‡ and Dmitri Golberg*,†,‡ Graduate School of Pure and Applied Sciences, UniVersity of Tsukuba, Tennodai 1, Tsukuba, 305-8571 Japan, and International Center for Materials Nanoarchitectonics (MANA), National Institute for Materials Science (NIMS), Tsukuba, Namiki 1-1, Ibaraki 3050044, Japan ReceiVed: December 2, 2009; ReVised Manuscript ReceiVed: January 27, 2010
Boron nitride nanotube (BNNT)/polyvinyl alcohol (PVA) composite fibers (10 vol %) alignment within the polymeric composites is proposed to be a promising way to further increase the polymeric film thermal conductivities toward wide practical applications. Introduction In recent years, there has been a growing interest in nanoparticle, nanotube, and nanosheet research. The most popular “nanosystem” so far has been a carbon nanotube (CNT).1 In fact, CNTs possess a high Young’s modulus and superb electrical and thermal conductivities.2-5 As a result, they have been proposed for various applications in composite materials,6 field emission electron sources,7 hydrogen absorbers,8 and many other fields. However, the real practical applications of nanotubes have not been notably advanced as yet. Regarding the most plausible CNT applications, a particular focus has been put on the development of polymeric composite materials having high thermal conductivities for utilization in IC packaging, heatsinks, etc. However, the electrical insulation of a material is an important concern for such manufacturing. Taking this point into account, we suggest that the usage of dielectric boron nitride nanotubes (BNNTs), rather than conducting or semiconducting CNTs, as fillers for thermoconductive polymers looks more logical and realistic. In fact, a BNNT has a tubular structure similar to that of a CNT and equally excellent mechanical properties, but, additionally, it has several specific advanced features; for example, while exhibiting analogously high thermal conductivity, it also possesses perfect electrical insulation9 and superb oxidation resistance.10 To date, the present authors have made several important steps toward achieving high thermal conductivity in polymeric composite films loaded with BNNTs. One method was to load a polymer with a high-BNNT fraction (>10 wt %).11 In another * To whom correspondence should be addressed. E-mail:
[email protected] (T.T.),
[email protected] (D.G.). † University of Tsukuba. ‡ International Center for Materials Nanoarchitectonics (MANA), National Institute for Materials Science (NIMS).
work, the affinity between BNNTs and a polymer at the matrix/ tube interfaces was improved by the BNNT surface modification.12 The present study makes one crucial step further. Herein, we focused on a precise control of BNNT orientation in a polymeric composite. It has been known that a BNNT has the high thermal conductivity along the tube axis direction (e.g., experimental values, 200-300 W/mK; theoretical estimates, 6600 W/mK or more).13,14 On the other hand, that value in the transverse direction is of the order of 20-30 W/mK only. Thus, one can acquire the thermal conductivity of a polymer fiber/sheet/film if one can adjust the BNNT orientation within the polymer matrix. Herein, to achieve this goal, first, we fabricated the polymer composite fibers containing BNNTs by means of an electrospinning method. BNNTs in such fibers were spontaneously aligned along the fiber casting direction. A several-fibers-thick sheet composed of parallel fibers became a basic constituent for further making multilayer bulky polymer films. To do so, 18 sheets were stacked in two different fashions: (i) all fibers/ BNNTs in adjacent sheets were oriented in parallel, and (ii) the adjacent sheets were alternately rotated 90° so that the fibers/ tubes in the adjacent layers became perpendicularly oriented. Such stacked “sandwiches” were then hot-pressed. The latter led to the complete dissolution of fibers and formation of bulky dense films whose thermal conductivities were measured in different orientations. Experimental Section Making Polymer Films with BNNTs. BNNTs were synthesized using a high-frequency induction furnace through a carbon-free CVD method.15 The tubes were purified by heating in N2 atmosphere at 1500-1600 °C and washing with HNO3
10.1021/jp911431f 2010 American Chemical Society Published on Web 02/22/2010
Alignment of BNNTs in Polymeric Composite Films
Figure 1. Sketch showing the process of PVA/BNNT fiber electrospinning.
J. Phys. Chem. C, Vol. 114, No. 10, 2010 4341 pressed at 100 °C. As a result, the bulk composite films with controlled and different BNNT alignments were prepared. For comparison, we also prepared a PVA/BNNT film by another method. In this method, a BNNT/PVA aqueous solution was spread over a Petri dish. After water evaporation, the film was simply peeled off. As a result, a composite film with randomly dispersed BNNTs was formed. Sample Analysis. The morphologies and fine structures of the composite fibers, sheets, and films were investigated in an optical microscope (VHX-900, KEYENCE), a scanning electron microscope (SEM, JSM-6700FSEM, JEOL), and a highresolution field emission transmission electron microscope (HRTEM, JEM-3100FEF, Omega Filter, JEOL). The specific heat capacity was measured with a differential scanning calorimeter (DSC, Thermo plus EVO II DSC8230, Rigaku). The thermal diffusivity was analyzed with a thermowave analyzer (TA, Bethel) under periodic laser heating. The latter equipment can measure the thermal diffusivity of a thin film in the inplane direction. In the case of a thickness direction, we used a hot-disk thermal constants analyzer TPA-501. Results and Discussion
Figure 2. Control of the BNNT alignment in a resultant bulky polymer film: scheme A, BNNTs are aligned in parallel; scheme B, BNNTs form a 90° cross-linked network.
and high-purity water. We used polyvinyl alcohol (PVA) as a polymer matrix and water as a solvent. Therefore, preliminarily, we needed to functionalize BNNTs to make them soluble in water before preparing the composite fibers. To do so, the surface of the BNNTs was first modified with H2O2 using the method reported by us previously.16 After this pretreatment, the BNNTs were added into a PVA aqueous solution, and a composite fiber was fabricated by electrospinning,17 as illustrated in Figure 1. In this method, the polymer solution casts into the polymer fiber when a high voltage is applied between the nozzle and the cathode (Al plate). The fibers were rolled up into the fiber sheet on an Al wire drum. We then cut away one portion of a wrapped material, and a Teflon platform was inserted between the wrapped sheet and the drum and gently lifted. All sides of the wrapped material were cut and analogously removed from the drum, giving numerous flat units for further bulk film stacking. The fiber sheets were stacked in line with the two schemes illustrated in Figure 2. In scheme A, BNNTs in all adjacent sheets were aligned along one direction. By contrast, for scheme B, two alternate sets were rotated 90° with respect to each other, creating a cross-linked BNNT network. For each scheme, 18 sheet layers were stacked. The stacked layers were then hot-
Composite Fiber/Sheet/Film Analysis. The morphology of an obtained electrospun composite fiber is shown in Figure 3a. BNNTs introduced into a PVA fiber are aligned along the long axis direction of the PVA fiber (casting direction), as we expected. It is noted that BNNTs have straight shapes compared with normally curled CNTs. BNNTs do not have a tendency to form entangled and curved structures. This makes it impossible for air bubbles to be embedded into the macroscopic voids formed at the interfaces between the nanotubes and the polymer matrix and thus significantly improves the resultant composite material quality. In fact, there was a report that the thermal conductivity had been influenced by the waviness of CNTs in a CNT composite.18 Therefore, in general, a composite material, including BNNTs, can be expected to possess higher thermal conductivity than a counterpart CNT composite. A BNNT/PVA fiber sheet peeled-off from the drum and made of numerous in-parallel oriented composite fibers is shown in Figure 3b. Figure 3c,d shows that PVA fibers are indeed wellaligned within the sheets. Importantly, BNNTs took the fiber orientation and are similarly aligned. The images of sheets/films after stacking and hot-pressing are shown in the bottom row (Figure 3e-h). After hot-pressing, the fibers are entirely dissolved to form a bulky homogeneous polymer matrix with aligned BNNTs left behind. Note that the sheets became fully transparent, Figure 3f (they used to be white-colored, Figure 3b), and their surfaces became perfectly smooth, as shown in Figure 3g,h, without any visible structural details. Specific Thermal Capacity Measurements. The thermal conductivity, k, is denoted by the following equation (eq 1):
k ) RFCp
(1)
Here, R and F are the thermal diffusivity and density, respectively, and Cp is the specific thermal capacity. Experimentally, we measured the thermal diffusivity. Therefore, we needed to evaluate the regarded parameters before conversion to thermal conductivity data. The specific heat capacity of BNNTs was measured to be 1431 J/kg · K from the conducted differential scanning calorimetry (DSC) measurements. In the case of PVA, the literature values were used for the specific heat capacity (1532 J/kg · K)19 and density (980 kg/m3).20 For
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Figure 3. Comparison of the structure/morphology of the composite fibers/sheets/films before (upper row, a-d) and after (lower row, e-h) hotpressing. (a, e) TEM images of an individual BNNT within a fiber/film. The inset in (e) shows a high-resolution TEM image of the interface between the tube wall and the polymer matrix. (b, f) Digital camera PVA sheet/film images. (c) Optical microscopy image of a polymer sheet and (g) of a polymer film. SEM images of a sheet (d) and a film (h). No fiber contrast is seen in the lower row images (g, h) due to complete fiber dissolution and forming a homogeneous bulky film after hot-pressing with no structural details.
TABLE 1: Thermal Conductivity Data for Various BNNT/Polymer Composite Filmsa
a
The numbers in brackets are theoretical values deduced by computations in the framework of the Nielsen’s model, as described in the text.
BNNTs, as the upper limit for density, a value of 2180 kg/m3 was taken, in line with the density of a hexagonal BN.21 Thermal Conductivity Values. Table 1 shows the results of thermal conductivity measurements. The data were converted from thermal diffusivity data, as described above. In this table, the Z direction means the thickness direction and X and Y directions indicate the in-plane directions, as shown in the corresponding panels in the left-hand side of Table 1. It is noted that the thermal conductivities of the composite films prepared by both methods (i.e., by electrospinning and by peeling-off from a Petri dish) were generally improved as the volume fractions of BNNTs increased. This improvement is due to the contribution of the high thermal diffusivity by BNNTs, rather than other parameters (specific thermal capacity or density). In fact, the thermal capacity of BNNTs is smaller than that of PVA, as mentioned in the preceding section (Specific Thermal Capacity Measurements), and the influence of density is negligible, provided rather low BNNT loading fractions are used.
The thermal conductivity for the in-plane directions has generally more profound improvement than along the thickness direction. This originates from the in-plane positioning of BNNTs within the individual composite sheets. Interestingly, some improvement is even noticed for the simple laying method (assembling on a Petri dish). It should be admitted that BNNTs could be partially and naturally aligned (at least in-plane) even in the latter technique. The height of a polymer solution changed to several micrometers from several tens of millimeters during water evaporation on a Petri dish. The evaporation process led to the partial in-plane BNNT self-assembling. The thermal conductivities between X and Y directions are identical, except for a composite film in which BNNTs are oriented in a single direction. Also, a big difference is apparent between the X and Z directions in the latter case. We now compare an alternately stacked composite film with a composite film possessing BNNTs aligned in one direction. In the latter film, the thermal conductivity along the long axis
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Figure 4. Computed thermal conductivity versus BNNT loading fractions; black and red lines show theoretical plots for a composite with parallel-oriented BNNTs and for randomly dispersed BNNTs, respectively.
of the BNNTs has the highest value; in contrast, it lowers for the thickness direction. In the composite film prepared from alternately stacked fiber sheets, the thermal conductivity has a value intermediate between the two values for the composite film possessing BNNTs aligned in one direction. From these results, it is evident that we indeed succeeded in the control of the BNNTs’ orientation by the electrospinning method, followed by physical sheet stacking and hot-pressing. Calculations in the Framework of the Nielsen’s Model. Nielsen proposed a relatively simple model for the evaluation of thermal conductivities of a composite material.22 In his approach, the thermal conductivity of a composite material, Kc, is related to the thermal conductivity of a matrix, Km, and a filler, Kf, according to the following equation
Kc ) Km[(1 + ABVf)/|1 - BβVf |]
(2)
where the parameters B and β are given by
B ) [(Kf /Km - 1)/(Kf /Km + A)]
(3)
2 β ) [(1 - VM)/VM ]Vf + 1
(4)
Here, Vf means the volume fraction of the filler, A is a constant related to the generalized Einstein coefficient reported for most of the materials,22 and VM is the maximum packing fraction. The theoretical plots drawn using the Nielsen’s model and adopted to our experiments are shown in Figure 4. A black line indicates the thermal conductivity of a BNNT/PVA composite having oriented and parallel BNNTs; the red line shows that for the composite with randomly dispersed BNNTs. The experimental thermal conductivity values for several obtained composite films matched very well the calculations within the Nielsen’s model (see Table 1). Moreover, the tendency for differences in thermal conductivity depending on the BNNT orientation is also easily noticed. It is worth noting that Pezzotti et al. have reported on the AlN/polystyrene composites.23 In that report, the authors also considered the thermal conductivity behavior by using the Nielsen’s model.22 The AlN particles were taken as the fillers in the Pezzotti et al. research. In the latter case, the theoretical and experimental results were drastically changed when the AlN fraction became
over 50 vol %. At this stage, the AlN particles start to contact each other. As a result, a significant improvement of thermal conductivity took place. The higher thermal conductivity is expected for the presently created nanotube composites while using fewer filler fractions due to the higher aspect ratio of the tubes compared with particles. In fact, the immediate rise in thermal conductivity could be computed based on the estimations in the framework of the Nielsen’s model when the BNNT fraction exceeded 10 vol % (for the aligned BNNT case). This value became 30 vol % of a BNNT fraction in a composite with randomly dispersed BNNTs. These calculations indicate that there is still room for drastic improvement of the thermal conductivities of BNNT-containing polymeric films if one manages to load higher fractions of BNNTs (more than 10 vol %) into a polymer while maintaining good alignment of the fillers. Such experiments are underway. Another prospective way may be to substitute multiwalled structures with single-walled BN nanotubes. In fact, it has recently been computed by Shen24 that the thinner BNNTs, for example, single-walled ones, have the higher thermal conductivity values compared with thick multiwalled tubes. Conclusion We succeeded in the preparation of BNNT/PVA composite fibers by electrospinning. The individual sheets made of parallelarranged ensembles of these fibers were then utilized for making bulky polymer films with controlled BNNT orientation under various sheet stacking and subsequent hot-pressing. The thermal conductivity values of the composites were measured in various directions. The highest value (0.54 W/mK) was documented when the composite film thermal properties were elucidated along the long axis of the oriented BNNTs; on the other hand, the lower value (0.38 W/mK) was measured in films made of alternately stacked sheets with 90° cross-linked BNNT networks. The experimental results were compared with the theoretical computations in the framework of the model proposed by Nielsen.22 These show a rather good match. On the basis of the present results, we envisage that the right direction for further research development would be an increase of BNNT loading fractions (more than 10 vol %) in polymeric composites while keeping their decent alignment in a polymer. Acknowledgment. The authors thank Dr. A. Nukui and Mr. K. Kurashima for continuous technical support and Dr. U. Gautam and Mr. Y. Huang for useful suggestions. The authors acknowledge the financial support from the MANA project tenable at NIMS. References and Notes (1) Iijima, S. Nature 1991, 354, 56–58. (2) Treacy, M. M. J.; Ebbesen, T. W.; Gibson, J. M. Nature 1996, 381, 678–680. (3) Zettl, A. Presented at the Adsorbent Carbon Workshop and Science of Carbon Nanotubes Workshop, Lexington, KY, July 10-11, 1997. (4) Kim, P.; Shi, L.; Majumdar, A.; McEuen, P. L. Phys. ReV. Lett. 2001, 87, 215502. (5) Berber, S.; Kwon, Y. K.; Tomanek, D. Phys. ReV. Lett. 2000, 84, 4613–4616. (6) Ajayan, P. M.; Stephan, O.; Colliex, C.; Trauth, D. Science 1994, 265, 1212–1214. (7) de Heer, W. A.; Chatelain, A.; Ugarte, D. Science 1995, 270, 1179– 1180. (8) Dillon, A. C.; Haben, M. J. Appl. Phys. A: Mater. Sci. Process. 2001, 72, 133–142. (9) Chang, C. W.; Han, W. Q.; Zettl, A. Appl. Phys. Lett. 2005, 86, 173102.
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(10) (a) Golberg, D.; Bando, Y.; Kurashima, K.; Sato, T. Scr. Mater. 2001, 44, 1561–1565. (b) Golberg, D.; Bando, Y.; Tang, C. C.; Zhi, C. Y. AdV. Mater. 2007, 19, 2413–2432. (11) Zhi, C. Y.; Bando, Y.; Terao, T.; Tang, C. C.; Kuwahara, H.; Golberg, D. AdV. Funct. Mater. 2009, 19, 1857–1862. (12) Terao, T.; Bando, Y.; Mitome, M.; Zhi, C. Y.; Tang, C. C.; Golberg, D. J. Phys. Chem. C 2009, 113, 13605–13609. (13) Chang, C. W.; Fennimore, A. M.; Afanasiev, A.; Okawa, D.; Ikuno, T.; Garcia, H.; Li, D.; Majumdar, A.; Zettl, A. Phys. ReV. Lett. 2006, 97, 085901. (14) Chang, C. W.; Han, W. Q.; Zettl, A. J. Vac. Sci. Technol., B 2005, 23, 1883–1886. (15) Tang, C. C.; Bando, Y.; Sato, T.; Kurashima, K. Chem. Commun. 2002, 290. (16) Zhi, C. Y.; Bando, Y.; Terao, T.; Tang, C. C.; Kuwahara, H.; Golberg, D. Chem.sAsian J. 2009, 4, 1536–1540.
Terao et al. (17) Teo, W. E.; Ramakrishna, S. Nanotechnology 2006, 17, R89–R106. (18) Zhang, J.; Tanaka, M. Eng. Anal. Boundary Elem. 2007, 31, 388– 401. (19) Gaur, U.; Wunderlich, B. B.; Wunderlich, B. J. Phys. Chem. Ref. Data 1983, 12, 29–63. (20) Nishino, T.; Ohkubo, H.; Nakamae, K. J. Macromol. Sci., Part B: Phys. 1992, 31, 191–214. (21) Suryavanshi, A. P.; Yu, M. F.; Wen, J.; Tang, C. C.; Bando, Y. Appl. Phys. Lett. 2004, 84, 2527–2529. (22) Nielsen, L. E. Mechanical Properties of Polymers and Composites; Marcel Dekker: New York, 1974; Vol. 2. (23) Pezzotti, G.; Kamada, I.; Miki, S. J. Eur. Ceram. Soc. 2000, 20, 1197–1203. (24) Shen, H. J. Comput. Mater. Sci. 2009, 47, 220–224.
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