Thermal conductivity of nanostructured boron nitride materials

Article abstract of DOI:10.1021/jp0607014

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.

Full text of DOI:10.1021/jp0607014

10354
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
05-0044, Japan, Tsinghua-Foxconn Nanotechnology Research Center and Department of Physics, Tsinghua
3
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
matrixes while preserving their electrically insulating proper-
6
,7
ties. 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 in-
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, although the measured ex-
perimental values display much lower values along with
matched deviations. High thermal conductivity makes carbon
nanotubes promising fillers for highly effective thermal interface
materials, which are fabricated through adding carbon nanotubes
8
9
plane thermal conductivity. Very recent measurements 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
1
10
the remarkable insulating and mechanical properties, BN
nanotubes are a highly promising filler candidate for future
application in thermal interface materials.
2
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
3
into polymer matrixes. Thermal conductivity enhancement has
been observed in carbon-nanotube-incorporated epoxy and
silicon elastomer matrixes, 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
4
1
1
both thermal and mechanical behaviors. 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 nanostruc-
tures.
5
electrical conductivity by a factor of nearly 10 , possibly due
to the easy formation of a percolation nanotube network. This
5
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 com-
ponents. 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
Experimental Section
Four types of nanostructured BN were used in this study, as
shown in Figures 1a-d. All BN nanomaterials were carbon-
free. 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 tri-
methoxyborane 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.
*
Author whom correspondence should be addressed. E-mail:
tang.chengchun@nims.go.jp.
†
National Institute for Materials Science.
Tsinghua University.
Central China Normal University.
‡
§
1
0.1021/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 BN-
based nanomaterials at different heat flows. The room-temperature
conductivity κ (25 °C) was calculated by a linear fitting of the observed
heat flow dependence.
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
3
normal crystalline density (2.25 g/cm ), 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 single-
1
6
walled carbon nanotube mat. However, these values are far
lower than the conductivity of the sintered bulk hexagonal BN
Figure 1. Transmission electron microscopy images of the nanostruc-
tured BN materials used for thermal conductivity measurements: (a)
spherical nanoparticles; (b) structurally perfect nanotubes; (c) bamboo-
structured nanotubes; (d) collapsed nanotubes. The scale bar is 200
nm.
17
particles at room temperature (>200 W/mK). As considered
16
in the study of the single-walled carbon nanotube mats, 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
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.
12-15
The synthetic details were reported elsewhere.
2
15
surface area (∼700 m /g for collapsed BN nanotubes ) pos-
sesses the highest thermal conductivity, if one considers the
common belief that the higher surface fraction usually increases
the thermal resistance.
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
2
×
10 mm . For the measurements of thermal conductivity we
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
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).
18
of phonons and phonon scattering. 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
19
that perpendicular to the plane. The second aspect involves
the percolation phenomenon, where conductive paths for
20
randomly distributed BN nanocomponents are formed. 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
Results and Discussion
The thermal conductivities of BN nanomaterial samples as a
function of heat flow are shown in Figure 2. The obtained room-

10356 J. Phys. Chem. B, Vol. 110, No. 21, 2006
Tang et al.
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
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 compart-
ments, 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-to-
layer 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.
1
/κpellet ) F /κ + F /κ + F /κ
(1)
ab ab
c
c
s
s
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-of-
plane, 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 contribu-
tions of Fab and Fc (Fs) in the percolation passages. The thermal
conductivity might be expressed in the two extreme cases as
follows
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 ultrasonic-
dispersion 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 neighbor-
ing nanotubes, forming a tight contact between them. The tight
contact ensures the formation of a percolation passage with very
few surface heat resistances.
κpellet ) κ /(F + F ) when F + F . F
(2)
c
s
c
c
s
ab
κpellet ) κ (1 - κ /κ ‚(F + F )/F )/F
when
ab
ab
s
s
c
ab
ab
F + F , F (3)
c
s
ab
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 predomi-
nantly on the interlayer number for heat to pass from one
hexagonal plane to another during percolation passage. Figure
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 con-
ductivity 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.
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-dimen-
sional materials. Therefore, although the spherical structures
possess a higher volume contact fraction and excellent rheo-
logical behavior than wirelike morphologies, the Fab value of

Thermal Conductivity of Nanostructured BN
J. Phys. Chem. B, Vol. 110, No. 21, 2006 10357
Summary
(4) (a) Bryning, M. B.; Milkie, D. E.; Islam, M. F.; Kikkawa, J. M.;
Yodh, A. G. Appl. Phys. Lett. 2005, 87, 161909. (b) Liu, C. H.; Huang, H.;
Wu, Y.; Fan, S. S. Appl. Phys. Lett. 2004, 84, 4248.
In conclusion, we have performed an experimental study of
the thermal conduction properties of various BN-based nano-
materials. A simple morphological approach has been proposed
to explain the experimental results, based on the considerations
of the heat percolation passages. Surface-collapsed BN nano-
tubes easily contact each other and form the heat transport
passage with the low surface numbers, supporting the experi-
mental observation that the collapsed BN nanotubes possess the
highest thermal conductivity. The morphology-dependent ther-
mal conductivity first observed in this study will be useful for
further real applications of BN nanotubes for thermal interface
materials.
(
5) Biercuk, M. J.; Liaguno, M. C.; Radosavijevic, M.; Hyun, J. K.;
Johnson, A. T.; Fischer, J. E. Appl. Phys. Lett. 2002, 80.
6) (a) Lu, X.; Xu, G.; Hofstra, P. G.; Bajcar, R. C. J. Polym. Sci.,
(
Part B: Polym. Phys. 1998, 36, 2259. (b) Tang, C. C.; Bando, Y. Appl.
Phys. Lett. 2003, 83, 659.
(
7) (a) Xu, Y. S.; Chung, D. D. L. Compos. Interfaces 2000, 7, 243-
56. (b) Lee, W. S.; Yu, J. Diamond Relat. Mater. 2005, 14, 1647.
8) (a) Sichel, E. K.; Miller, R. E.; Abrahams, M. S.; Buiocchi, C. J.
2
(
Phys. ReV. B 1976, 13, 4607. (b) Xiao, Y.; Yan, X. H.; Xiang, J.; Mao, Y.
L.; Zhang, Y.; Cao, J. X.; Ding, J. W. Appl. Phys. Lett. 2004, 84, 4626. (c)
Yamamoto, T.; Watanabe, S.; Watanabe, K. Phys. ReV. Lett. 2004, 92, 4613.
(
9) Chang, C. W.; Han, W. Q.; Zettl, A. Appl. Phys. Lett. 2005, 86,
73102.
(10) Suryavanshi, A. P.; Yu, M. F.; Wen, J. G.; Tang, C. C.; Bando, Y.
Appl. Phys. Lett. 2004, 84, 2527.
11) (a) Ng, H. Y.; Lu, X. H.; Lau, S. K. Polym. Compos. 2005, 26, 66.
b) Huang, M. T.; Ishida, H. J. Polym. Sci., Part B: Polym. Phys. 1999,
1
(
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).
(
37, 2360.
(12) Tang, C. C.; Bando, Y.; Golberg, D. Chem. Commun. 2002, 2826.
(
13) (a) Tang, C. C.; Bando, Y.; Sato T.; Kurashima, K. Chem. Commun.
2
002, 1290. (b) Tang, C. C.; Bando, Y.; Golberg, D. J. Solid State Chem.
2
004, 177, 2670.
(
References and Notes
14) Yuan, S. D.; Ding, X. X.; Huang, Z. X.; Huang, X. T.; Gan, Z.
W.; Tang, C.; Qi, S. R. J. Cryst. Growth 2003, 256, 67.
15) Tang, C. C.; Bando, Y.; Ding, X. X.; Qi, S. R.; Golberg, D. J. Am.
Chem. Soc. 2002, 124, 14550.
(
1) (a) Berber, S.; Kwon, Y. K. Tomanek, D. Phys. ReV. Lett. 2000,
(
8
1
4, 4613. (b) Che, J. W.; Cagin, T.; Goddard, W. A. Nanotechnology 2000,
1, 65.
(16) Hone, J.; Whitney, M.; Zettl, A. Synth. Met. 1999, 103, 2498.
(17) Simpson, A.; Stuckes, A. D. J. Phys. D: Appl. Phys. 1976, 9, 621.
(18) Morelli, D. T.; Heremans, J. P.; Slack, G. A. Phys. ReV. B 2002,
(
2) (a) Hone, J.; Whitney, M.; Piskoti, C.; Zettl, A. Phys. ReV. B 1999,
5
9, 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.
66, 195304
(19) Kelly, B. T. J. Nucl. Mater. 1977, 68, 9.
(20) Grimmett, G. Percolation, 2nd ed.; Springer: New York, 1999.
(21) Huang, H.; Liu, C. H.; Wu. Y.; Fan, S. S. AdV. Mater. 2005, 17,
1652.
(3) Gao, X. Q.; Liu, L.; Guo, Q. G.; Shi, J. L.; Zhai, G. T. Mater. Lett.
(22) (a) Padgett, C. W.; Brenner, D. W. Nano Lett. 2004, 4, 1051. (b)
2
005, 59, 3062.
Liu, C. H.; Fan, S. S. Appl. Phys. Lett. 2005, 86, 123106.