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J. Phys. Chem. B 2006, 110, 10354-10357
Thermal Conductivity of Nanostructured Boron Nitride Materials Chengchun Tang,†,* Yoshio Bando,† Changhong Liu,‡ Shoushan Fan,‡ Jun Zhang,§ Xiaoxia Ding,§ and Dmitri Golberg† AdVanced Materials Laboratory, National Institute for Materials Science, 1-1 Namiki, Tskuba, Ibaraki 305-0044, Japan, Tsinghua-Foxconn Nanotechnology Research Center and Department of Physics, Tsinghua UniVersity, Beijing 100084, People’s Republic of China, and Department of Physics, Central China Normal UniVersity, Wuhan 430079, People’s Republic of China ReceiVed: February 2, 2006; In Final Form: April 7, 2006
We have measured the thermal conductivity of bulky pellets made of various boron nitride (BN)-based nanomaterials, including spherical nanoparticles, perfectly structured, bamboo-like nanotubes, and collapsed nanotubes. The thermal conductivity strongly depends on the morphology of the BN nanomaterials, especially on the surface structure. Spherical BN particles have the lowest thermal conductivity while the collapsed BN nanotubes possess the best thermoconductive properties. A model was proposed to explain the experimental observations based on the heat percolation passage considerations.
Introduction Rapid developments in microelectronic integrated circuits and micro-electro-mechanical systems (MEMS) have increased the demands for higher-power heat dissipation in operating devices. Thermal properties of interface materials used in these systems have become the crucial issue for the operation reliability of the ultimately downsized parts. Significant interest has been focused on carbon nanotubes due to their extremely high thermal conductivity of 6600 W/mK, as theoretically predicted by molecular dynamics simulations,1 although the measured experimental values display much lower values along with matched deviations.2 High thermal conductivity makes carbon nanotubes promising fillers for highly effective thermal interface materials, which are fabricated through adding carbon nanotubes into polymer matrixes.3 Thermal conductivity enhancement has been observed in carbon-nanotube-incorporated epoxy and silicon elastomer matrixes,4 albeit the enhancement is still not satisfying. It is worth noting that only 0.2 wt % of carbon nanotube incorporation into a matrix sharply increases the electrical conductivity by a factor of nearly 105, possibly due to the easy formation of a percolation nanotube network.5 This is not a positive sign for the thermal interface materials because a high electrical resistivity is needed in applications when materials come into contact with the working electrical components. In this context, searching for new thermally conducting but electrically insulating materials with superb rheological properties would be a vital issue for the heat dissipation problem. Boron nitride (BN) is a widely used ceramic material with attractive properties such as high thermal conductivity, low coefficient of thermal expansion, and high electrical resistivity in a wide temperature range. In addition, it is also chemically stable with respect to most molten metals and glasses, organic solvents, and polymers, even at high temperature. Therefore, BN can be used to improve the thermal conductivity of polymer * Author whom correspondence should be addressed. E-mail:
[email protected]. † National Institute for Materials Science. ‡ Tsinghua University. § Central China Normal University.
matrixes while preserving their electrically insulating properties.6,7 The extremely high thermal conductivity of carbon nanotubes has led researchers to speculate that one-dimensional nanostructured BN may possess even higher conductivities since its parent material, hexagonal BN, has exceptionally high inplane thermal conductivity.8 Very recent measurements 9 on BN nanotubes converted from BCN nanotubes suggest that although the room-temperature thermal conductivity of BN nanotubes is lower than that of carbon nanotubes, it is notably higher compared to other nanoscaled structures. Therefore, considering the remarkable insulating and mechanical properties,10 BN nanotubes are a highly promising filler candidate for future application in thermal interface materials. There have been several works emerged from our groups on the preparation, property explorations, and applications of BN nanostructures. For applications in thermal interface materials the surface scattering, defects, particle size, and shape of the nanostructured BN fillers play an important role in affecting both thermal and mechanical behaviors.11 Therefore, in this paper, we report on the systematic investigation of thermal conduction in several BN nanostructure types prepared in our laboratories. The goal is to reveal the relationship between the structure geometry/morphology and its thermal conductivity and to understand the intrinsic thermal properties of BN nanostructures. Experimental Section Four types of nanostructured BN were used in this study, as shown in Figures 1a-d. All BN nanomaterials were carbonfree. Spherical BN nanoparticles with uniform diameters ranging from 50 to 400 nm were synthesized via a two-step process. The spherical B-N-O precursors were first synthesized at 700 °C by a chemical vapor deposition (CVD) reaction of trimethoxyborane dissolved in a methanol solution and ammonia, and the oxygen was removed from the pyrolysis precursor via heating under ammonia atmosphere at 1100 °C. High-purity multiwalled BN nanotubes with a diameter of 10-80 nm and several micrometers long were synthesized through a catalytic reaction of Mg and B2O2 vapors under an ammonia atmosphere.
10.1021/jp0607014 CCC: $33.50 © 2006 American Chemical Society Published on Web 05/05/2006
Thermal Conductivity of Nanostructured BN
J. Phys. Chem. B, Vol. 110, No. 21, 2006 10355
Figure 2. Thermal conductivities of bulk pellet samples made of BNbased nanomaterials at different heat flows. The room-temperature conductivity κ (25 °C) was calculated by a linear fitting of the observed heat flow dependence.
Figure 1. Transmission electron microscopy images of the nanostructured BN materials used for thermal conductivity measurements: (a) spherical nanoparticles; (b) structurally perfect nanotubes; (c) bamboostructured nanotubes; (d) collapsed nanotubes. The scale bar is 200 nm.
The reactant vapors were in-situ-generated by a solid-state reaction between B and MgO at 1450 °C. Bamboo-structured BN nanotubes were prepared by reacting barium metaborate with ammonia at 1300 °C. Collapsed BN nanotubes were obtained by a high-temperature heat treatment of as-prepared multiwalled BN nanotubes in the presence of a platinum metal. The synthetic details were reported elsewhere.12-15 Loosely packed BN nanomaterials mentioned above were pressed into pellets under a 40 MPa static pressure and then were cut into rectangular samples with a surface area of (A) 8 × 10 mm2. For the measurements of thermal conductivity we employed the ASTM D5470 standard method. To increase the sample density and reduce the contact thermal resistance, a constant pressure of ∼3 MPa was applied to a specimen during the measurements. The thermal conductivity (κ) was calculated as κ ) Qd/∆TA, where Q is the heat flow generated from an electrical heater, ∆T is the temperature difference at the two ends of a specimen, and d is the specimen thickness. The measurement temperature was adjusted from 20 to 50 °C in all experimental runs. The room-temperature thermal conductivity was calculated by assuming linear temperature dependence within the utilized temperature range and then extrapolated to room temperature. The morphology of BN nanomaterials was verified by scanning electron microscopy (SEM, JSM-6700F) and transmission electron microscopy (TEM, JEOL 3010F). Results and Discussion The thermal conductivities of BN nanomaterial samples as a function of heat flow are shown in Figure 2. The obtained room-
temperature thermal conductivities are: κ ) 0.43 ( 0.01 W/mK for BN nanospheres; κ ) 0.93 ( 0.04 W/mK for pure BN nanotubes; κ ) 0.88 ( 0.03 W/mK for BN nanobamboo; and κ ) 2.23 ( 0.06 W/mK for collapsed BN nanotubes. With the correction of the low densities of the pressed pellets to the normal crystalline density (2.25 g/cm3), we estimated thermal conductivities of ∼14 W/mK for the nanospheres, ∼18 W/mK for the nanotubes. ∼17 W/mK for the nanobamboo, and ∼46 W/mK for the collapsed nanotubes. The derived thermal conductivities of the bulky BN sample pellets are noticeably high and comparable to the thermal conductivity of 36 W/mK obtained in a densely packed singlewalled carbon nanotube mat.16 However, these values are far lower than the conductivity of the sintered bulk hexagonal BN particles at room temperature (>200 W/mK).17 As considered in the study of the single-walled carbon nanotube mats,16 the lower conductivities should be attributed to the highly disordered arrangement of BN nanomaterials within a bulky sample, which results in a high contact thermal resistance between nanotubes. Most importantly, the thermal conductivity of the BN-based nanomaterials discussed here remarkably depends on their morphology. The spherical BN particles with a lower density and surface area have the lowest thermal conductivity, compared to the one-dimensional BN of a higher density and surface area. It is quite puzzling that a BN nanomaterial with the highest surface area (∼700 m2/g for collapsed BN nanotubes15) possesses the highest thermal conductivity, if one considers the common belief that the higher surface fraction usually increases the thermal resistance. We now attempt to explain the morphological dependence of measured thermal conductivities of various BN nanomaterials. Heat is transported through nanostructured BN by the motion of phonons and phonon scattering.18 Therefore, there are at least two factors affecting thermal conduction of the BN-based nanocomposite bulky samples. The first factor is anisotropic heat transport. The basic BN hexagonal crystal structure results in two principal conductivity modes: in-plane conductivity and out-of-plane conductivity. The conduction parallel to the basal plane was found to be approximately 100 times greater than that perpendicular to the plane.19 The second aspect involves the percolation phenomenon, where conductive paths for randomly distributed BN nanocomponents are formed.20 The thickness of BN pellets used in this study is 1-2 mm; thus the heat percolation must be at least considered for several thousand surfaces of nanostructured BN, which consists of an assembly of poorly oriented nanocrystallites. Therefore, the caused contact heat resistance is far higher compared to their bulk sintered counterparts. In fact, for thermal conductivity enhancement it
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Figure 3. Schematic illustration of the heat percolation passages for BN nanospheres, BN nanobamboo, multiwalled BN nanotubes, and collapsed multiwalled BN nanotubes. The coarse gray lines denote the heat percolation passages.
is most important to maximize the conductive paths with the minimum contact and interfacial surface resistance. Some strategies have been reported toward reduction of thermal interface resistances by aligning carbon nanotube arrays in polymer matrixes or by surface chemical decoration of randomly distributed carbon nanotubes.21,22 On the basis of the analyses and assuming that the pressed pellets are free from pores, a rough relationship between the thermal conductivity and the factors mentioned above can be given as follows
1/κpellet ) Fab/κab + Fc/κc + Fs/κs
(1)
The κab is the in-plane (ab plane) thermal conductivity for BN nanospheres or the longitudinal conductivity for one-dimensional BN tubes; κc is the out-of-plane conductivity for nanospheres and transverse conductivity for tubes; κs is the surface thermal conductivity for the heat to cross over the nanospheres or nanotubes through the percolation passages. Fa, Fc, and Fs are the geometrical fractions in the passage for in-plane, out-ofplane, and surface, respectively. Further considering the fact that κab is very much larger than κs (κc), the measured thermal conductivity must depend consequently on the relative contributions of Fab and Fc (Fs) in the percolation passages. The thermal conductivity might be expressed in the two extreme cases as follows
κpellet ) κc/(Fs + Fc) when Fc + Fs . Fab
(2)
κpellet ) κab(1 - κab/κs‚(Fs + Fc)/Fab)/Fab when Fc + Fs , Fab (3) Here we assume that the surface heat resistance is equal to the out-of-plane resistance. This simple relationship can be used to explain the measured thermal conductivities in this study. For the two cases, the thermal conductivity depends predominantly on the interlayer number for heat to pass from one hexagonal plane to another during percolation passage. Figure 3 shows the schematic illustration for the heat percolation passages in the bulky specimens made of four BN nanomaterial types. With a shorter length (approximately diameter), spherical BN particles need more surface and out-of-plane transports to complete a percolation passage, compared to the one-dimensional materials. Therefore, although the spherical structures possess a higher volume contact fraction and excellent rheological behavior than wirelike morphologies, the Fab value of
Figure 4. Low-magnification TEM image of collapsed BN nanotubes pressed under 40 MPa (a). The contact area (framed) between two nanotubes is enlarged in part b.
the BN nanospheres will be smaller than that of BN nanotubes, consequently resulting in a low κpellet. Perfectly structured BN multiwalled nanotubes have a slightly higher κpellet value than bamboo-structured BN nanotubes, because the heat transport in the latter case must cross over some bamboo-like compartments, resulting in the increase of the Fc fraction. It is most interesting to understand the highest thermal conductivity measured in the collapsed BN nanotubes. Due to the highly defective and disordered walls, the layers of the neighboring nanotubes can easily contact each other. Heat thus can be easily transported by automatically selecting the percolation passage with the lowest thermal resistance; that is, just a few layer-tolayer channels could complete the heat transport from one tube to another. Therefore, the fractions of Fc and Fs decrease, which consequently results in the increasing thermal conductivity of the bulky sample. Detailed TEM structural analysis of the collapsed BN nanotubes, which were pressed under 40 MPa and then used for the thermal conductivity measurements, further supports the conjecture of heat percolation passage. Although the ultrasonicdispersion method was used to prepare a TEM sample, the observed BN nanotubes usually appear as a networklike structure, as revealed by the extensive TEM examinations. A typical morphology is shown in Figure 4a, and a high-resolution TEM image of the jointed area between two nanotubes is shown in Figure 4b. The broken BN layers protrude into the neighboring nanotubes, forming a tight contact between them. The tight contact ensures the formation of a percolation passage with very few surface heat resistances. The formation and structure of the percolation passages existing in BN nanomaterials of different morphologies, of course, is more complex than in the frame of the simple model proposed in this study. Further experiments focused on the passage formation in various specimens subjected to the different treatments, e.g., high pressure or temperature, are needed. However, our study on morphology-dependent thermal conductivity in various BN nanomaterials clearly indicates that the predicted high thermal conductivity of a BN nanotube could be reached only when the interface thermal resistance could be effectively suppressed. This would pave the way for the smart integration of BN nanotubes into thermal conductor technology.
Thermal Conductivity of Nanostructured BN Summary In conclusion, we have performed an experimental study of the thermal conduction properties of various BN-based nanomaterials. A simple morphological approach has been proposed to explain the experimental results, based on the considerations of the heat percolation passages. Surface-collapsed BN nanotubes easily contact each other and form the heat transport passage with the low surface numbers, supporting the experimental observation that the collapsed BN nanotubes possess the highest thermal conductivity. The morphology-dependent thermal conductivity first observed in this study will be useful for further real applications of BN nanotubes for thermal interface materials. Acknowledgment. This study was financially supported by the State Key Project of Fundamental Research, the Fok Ying Tong Education Foundation (Grant No. 91050), and the National Natural Science Foundation of China (Grant No. 50202007). References and Notes (1) (a) Berber, S.; Kwon, Y. K. Tomanek, D. Phys. ReV. Lett. 2000, 84, 4613. (b) Che, J. W.; Cagin, T.; Goddard, W. A. Nanotechnology 2000, 11, 65. (2) (a) Hone, J.; Whitney, M.; Piskoti, C.; Zettl, A. Phys. ReV. B 1999, 59, R2514. (b) Yi, W.; Lu, L.; Zhang, D. L.; Pan Z. W.; Xie, S. S. Phys. ReV. B 1999, 59, R9015. (c) Hone, J.; Liaguno M. C.; Nemes, N. M.; Johnson, A. T.; Fischer, J. E.; Walters, D. A.; Casavant, M. J.; Schmidt, J.; Smalley R. E. Appl. Phys. Lett. 2000, 77, 666. (d) Kim, P.; Shi, L.; Majumdar, A.; McEuen, P. L. Phys. ReV. Lett. 2001, 87, 215502. (e) Yang D. J.; Zhang, Q.; Chen, G.; Yoon, S. F.; Ahn, J.; Wang, S. G.; Zhou, Q.; Wang, Q.; Li, J. Q. Phys. ReV. B 2002, 66, 165440. (3) Gao, X. Q.; Liu, L.; Guo, Q. G.; Shi, J. L.; Zhai, G. T. Mater. Lett. 2005, 59, 3062.
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