Letter www.acsami.org
Thermal Conductivity Enhancement of Coaxial Carbon@Boron Nitride Nanotube Arrays Lin Jing,†,‡,§ Majid Kabiri Samani,†,⊥ Bo Liu,†,∥ Hongling Li,# Roland Yingjie Tay,# Siu Hon Tsang,△ Olivier Cometto,# Andreas Nylander,⊥ Johan Liu,⊥ Edwin Hang Tong Teo,*,‡,# and Alfred Iing Yoong Tok*,‡,§ ‡
School of Materials Science and Engineering, §Institute for Sports Research, ∥Environmental Process Modelling Centre, Nanyang Environment and Water Research Institute, #School of Electrical and Electronic Engineering, and △Temasek Laboratories@NTU, Nanyang Technological University, 50 Nanyang Avenue, Singapore 639798, Singapore ⊥ Electronics Material and Systems Laboratory, Department of Microtechnology and Nanoscience, Chalmers University of Technology, SE-412 96 Gothenburg, Sweden S Supporting Information *
ABSTRACT: We demonstrate the thermal conductivity enhancement of the vertically aligned carbon nanotube (CNT) arrays (from ∼15.5 to 29.5 W/mK, ∼90% increase) by encapsulating outer boron nitride nanotube (BNNT, 0.97 nm-thick with ∼3−4 walls). The heat transfer enhancement mechanism of the coaxial C@BNNT was further revealed by molecular dynamics simulations. Because of their highly coherent lattice structures, the outer BNNT serves as additional heat conducting path without impairing the thermal conductance of inner CNT. This work provides deep insights into tailoring the heat transfer of arbitrary CNT arrays and will enable their broader applications as thermal interface material. KEYWORDS: thermal conductivity, carbon nanotube arrays, boron nitride nanotube, coaxial structure, molecular dynamics simulation thermal stability (endure up to ∼800 °C in air, ∼2800 °C in inert gas)4 has attracted wide research interest. Recently, coaxial carbon@boron nitride nanotube (C@BNNT) structure has been reported to perform both enhanced mechanical stiffness and thermal stability.21 Moreover, instead of the massive infiltration of polymer/metal into CNT arrays which dominates the thermal property of the resulting composites, the introduction of thin-walled outer BNNT is expected to not only preserve the intrinsic thermal conductivity of the inner CNT but also provide additional heat transfer channel, which has not been investigated yet. In this work, outer BNNT is encapsulated onto the asprepared CNT arrays following previously reported thermal chemical vapor deposition (TCVD) protocols with slight modifications (see Methods in Supporting Information).21,22 Figure 1a, b presents the cross-sectional scanning electron microscopy (SEM) images of the as-prepared CNT and C@ BNNT arrays, respectively. The C@BNNT arrays retain the vertically aligned morphology of the starting CNT arrays with no observable change in tube areal density. It is also noted that the crystalline structure of the CNT is well preserved after the BNNT introduction as indicated by the corresponding Raman
T
hermal interface material (TIM), a key component of thermal management for efficient heat removal, has attracted particular attention due to the rapidly increasing power density of modern electronic devices.1 Among the various candidates, vertically aligned carbon nanotube (CNT) arrays exhibit promising potential due to the superior thermal conductivity of the individual CNT (∼3000 W/mK for multiwalled CNT2) and the absence of intertube phonon scattering.3 In addition, the outstanding thermal stability (withstand up to ∼400 °C in air, ∼2000 °C in inert gas)4 and mechanical property5,6 of the CNT arrays enable their applications in harsh environments. However, it is still not yet successful for the CNT arrays (∼0.1−220 W/mK)7,8 to inherit the excellent thermal conductivity of ideal CNT due to the obstacles such as low tube volume fraction7,9 and limited quality of the individual CNT,10 which are determined by the specific synthesis process. Nevertheless, few work has been done to improve the heat transfer of the CNT arrays. This is because most infiltrating materials such as polymer3,11−13 and metal14,15 have much lower thermal conductivities, and their structural incompatibility with CNT may impair the intrinsic heat transfer of CNT as well.12 On the other hand, boron nitride nanotube (BNNT) with natural advantages such as highly coherent structure with CNT, outstanding thermal conductivity (∼350 W/mK)16−18 and mechanical property,19 superb chemical resistance20 and © 2017 American Chemical Society
Received: February 14, 2017 Accepted: April 21, 2017 Published: April 21, 2017 14555
DOI: 10.1021/acsami.7b02154 ACS Appl. Mater. Interfaces 2017, 9, 14555−14560
Letter
ACS Applied Materials & Interfaces
Figure 1. Cross-sectional SEM and HR-TEM images of (a, c) CNT and (b, d) C@BNNT, respectively. BNNT with wall thickness of 0.97 nm (∼3− 4 walls) is encapsulated onto the CNT. (e) FT-IR and (f) EDX spectra of the NTs further confirm and quantify the introduction of BNNT for C@ BNNT arrays.
identified, and the weight fraction of each element is listed in the inset table. It is noted that the weight ratio of BN:C is ∼40:50 for the C@BNNT arrays, which is consistent with the TEM results. In addition, the corresponding elemental maps show the homogeneous distribution of B, C, N within the aligned C@BNNT arrays (Figure S2). To study the effects of outer BNNT on the thermal conductivity of the coaxial C@BNNT arrays, pulsed photothermal reflectance (PPR) measurement was employed (see Methods in Supporting Information),24 as schematically illustrated in Figure 2a. To enhance the surface uniformity and reflectivity, a ∼500 nm-thick titanium nitride (TiN) layer was deposited by sputtering prior to the measurement. As shown in the cross-sectional SEM image (Figure 2b), the uniform and dense TiN layer adheres well to the top ends of NTs without affecting their aligned morphology, which is believed to facilitate the capture of photothermal signal.25 During the PPR measurement, the TiN surface was struck by a Nd:YAG laser (pump beam), resulting in the sudden rise in the surface temperature and a decay to room temperature was
spectra (Figure S1). To clearly investigate the nanostructures of the NT before and after the BNNT encapsulation, high resolution transmission electron microscopy (HR-TEM) study of the individual CNT and C@BNNT was further carried out. The starting CNT exhibits an inner diameter of 4.83 nm with an average wall thickness of 1.98 nm (corresponding to ∼4−5 C walls, Figure 1c), and an increase of 0.97 nm in wall thickness can be observed for the C@BNNT (corresponding to the ∼3− 4 outer BN layers, Figure 1d). Figure 1e shows the Fourier transform infrared (FT-IR) spectra of CNT and C@BNNT. Besides the characteristic CC stretching vibrations located at 1651 and 1441 cm−1 for CNT,22 additional in-plane B−N stretching and out-of-plane B−N−B bending at 1384 and 799 cm−1 are observed for the C@BNNT,21 respectively, further confirming the BNNT introduction. To better quantify the BNNT introduced, the energy dispersive X-ray spectroscopy (EDX) was collected for the resulting C@BNNT arrays.23 Figure 1f shows the EDX spectroscopy of the C@BNNT arrays (collected from cross-sectional view), where B (0.183 keV), C (0.277 keV), N (0.392 keV) and O (0.525 keV) can be 14556
DOI: 10.1021/acsami.7b02154 ACS Appl. Mater. Interfaces 2017, 9, 14555−14560
Letter
ACS Applied Materials & Interfaces
Figure 2. (a) Schematic illustration of the PPR technique and sample structure. (b) Cross-sectional SEM image of the TiN/NT/Si shows that the aligned morphology of the NTs was well-preserved after the deposition of ∼500 nm-thick TiN layer on top. (c) Normalized surface temperature− time curves of the NT arrays measured by PPR indicate that the C@BNNT arrays dissipate heat faster than the bare CNT arrays. (d) Theoretically fitted curves of the C@BNNT arrays match reasonably well with the experimental data.
Table 1. Thermal Properties of the CNT and C@BNNT Arrays Measured by the PPR Technique NT arrays
k (W/mK)
α (× 10−5 m2/s)
TBRTiN−NT (× 10−6 m2K/W)
TBRNT−Si (× 10−6 m2K/W)
CNT C@BNNT
15.50 ± 3.50 29.50 ± 4.50
4.80 ± 2.20 4.75 ± 0.85
0.95 ± 0.05 0.80 ± 0.10
6.20 ± 2.50 5.75 ± 3.25
Table 1 shows the thermal conductivity (k), thermal diffusivity (α) of the NT arrays, as well as TBRTiN‑NT and TBRNT‑Si extracted from the corresponding normalized surface temperature vs time curves. Impressively, a ∼90% increase in the thermal conductivity is achieved for the C@BNNT arrays (∼29.5 W/mK) as compared to the bare CNT arrays (∼15.5 W/mK, similar to most TCVD-grown CNT arrays3). This is expectable according to the thermal conductance model26 k = δkI + (1 − δ)kair (δ is tube volume filling fraction, kI and kair are thermal conductivities of individual NT and air, respectively), as the thermally conductive outer BNNT (∼350 W/mK)16 squeezes the volume of thermally resistant air (∼0.026 W/ mK)9 between the NTs. The thermal diffusivity is generally constant for the CNT arrays before and after the BNNT encapsulation (from ∼4.80 × 10−5 to 4.75 × 10−5 m2/s), which is comparable with most TCVD-grown CNT arrays.8,26,27 The TBRs in the magnitudes of ∼1 × 10−7 to 1 × 10−6 m2K/W are consistent for the CNT and C@BNNT arrays, which are similar to those of most sandwiched metal/NT/substrate structures.26,28,29 Molecular dynamics (MD) simulation has been found to be a very useful technique to study the thermal characteristic of CNT and BNNT.30 To gain deeper insights into the mechanism of heat transfer enhancement of C@BNNT arrays,
followed. Based on the temperature dependence of the TiN’s reflectivity, the surface temperature of the TiN layer was recorded by measuring the intensity of the He−Ne laser (probe beam) reflected from the center of the pump beam. Figure 2c shows the obtained normalized surface temperature−time curves of the NT arrays. Obviously, the surface temperature decays faster for the C@BNNT arrays as compared to the bare CNT arrays, indicating that the heat transports more efficiently through the C@BNNT arrays. To extract the thermal properties of the NT arrays, we used a three-layer heat conduction model to fit the obtained temperature excursion files (see Methods in the Supporting Information).24 In addition, the fitting parameters including thermal conductivity and diffusivity of NT arrays, thermal boundary resistances at TiN-NT and NT-Si interfaces (in short, k, α, TBRTiN‑NT and TBRNT‑Si, respectively) were varied ± 20% and a least-squares optimization method was applied to minimize the fitting errors.24 As a representative, the resulting theoretical fitted curve of the C@BNNT arrays matches well with the experimental curve (Figure 2d), and the latter lies in between the two fitted curves with ±20% variations in the best fitted values, indicating that the experimental data was reasonably fitted. 14557
DOI: 10.1021/acsami.7b02154 ACS Appl. Mater. Interfaces 2017, 9, 14555−14560
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ACS Applied Materials & Interfaces
Figure 3. (a) Computational model (front/side view) of the coaxial C@BNNT structure. The middle region (heat source, red) and the two ending regions (heat sink, dark green) are controlled at temperatures of T + ΔT (350 K) and T (300 K), respectively. (b) Temperature distribution along the heat flow direction and (c) the cumulative energy changes vs simulation time for the coaxial C@BNNT, inner CNT, and outer BNNT, respectively.
(A) change on the simulation of individual NT, the thermal conductance (σ, σ = h/∇T) instead of thermal conductivity (k, k = h/∇TA) of the individual NT was calculated for comparison. The thermal conductance of the inner CNT, outer BNNT and C@BNNT was then obtained as 1.89 × 105, 0.65 × 105 and 2.26 × 105 Wm/K, respectively (Table 2). To
corresponding MD simulation was conducted (see Methods in the Supporting Information). Figure 3a shows the computational model of coaxial C@BNNT structure with axial length of 200 Å constructed by encapsulating three outer coaxial BNNT walls onto four inner CNT walls. Other parameters such as diameters, interwall distance were all set based on the TEM observations. To extract the thermal properties of the coaxial construct, reverse nonequilibrium MD simulation was further carried out, during which the middle region (heat source, red) and two ending regions (heat sink, dark green) of the NT were set at temperatures of T + ΔT (350 K) and T (300 K), respectively. A steady temperature gradient was reached in 100 ps under constant volume and energy (NVE) ensemble for the C@BNNT with slight ripples formed along the tangential and axial directions (snapshot is shown in Figure S3), whereas the coaxial NT structure was well kept without observable bending. The temperature distributions along the axial direction of the C@BNNT, inner CNT, and outer BNNT were obtained, respectively (Figure 3b). It is found that dramatic temperature drops occur near the heat source and sink regions because of the intensive phonon scattering, while for the rest areas ranging from ± 25 to ± 75 Å, the temperatures decrease almost linearly from the heat source to the sink. The temperature of the BNNT is higher than that of the CNT along the tube axial direction, whereas the overall temperature of the C@BNNT lies in between. The cumulative energy changes in the heat source and sink regions were recorded as well (Figure 3c), which increase linearly with the simulation time. By fitting the slopes of the linear regions of temperature distribution (Figure 3b) and the cumulative energy changes (Figure 3c), temperature gradient (∇T), and heat flow (h) of the inner CNT, outer BNNT and C@BNNT were obtained as 0.265, 0.425, 0.345 K/Å, and 31.10, 17.35, 48.45 eV/ps, respectively. To eliminate the influence of cross-section area
Table 2. Thermal Conductance of Various NT Structures Extracted from MD Simulation NT
inner CNT
outer BNNT
thermal conductance (× 105 Wm/K)
1.89
0.65
C@BNNT
isolated CNT
CNT within cluster
2.26
1.88
1.97
more comprehensively study the effect of the tube−tube interactions on the heat transfer of individual NT, isolated CNT and aligned CNT cluster were further constructed (Figure S3). The thermal conductance of the isolated CNT and individual CNT within aligned CNT cluster was extracted to be 1.88 × 105 and 1.97 × 105 Wm/K, respectively (Table 2). Their similar thermal conductance demonstrates that the effect of tube−tube interactions on the heat transfer of NT can be neglected considering the weak van der Waals forces.10 In addition, the inner CNT (within C@BNNT) exhibits almost same thermal conductance with that of the isolated CNT, indicating that the heat transfer of CNT is not impaired by BNNT encapsulation. On the other hand, with the additional contribution of outer BNNT, C@BNNT exhibits a 20.21% higher thermal conductance than the isolated CNT, which shows a similar enhancement tendency with that of the experimental result. It is noted that the enhancement value obtained from simulation is lower than that of experimental result, attributing to the limited quality of the CNT arrays used 14558
DOI: 10.1021/acsami.7b02154 ACS Appl. Mater. Interfaces 2017, 9, 14555−14560
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in the experiment. Furthermore, the phonon power spectra of inner CNT and outer BNNT are highly overlapped (Figure S4) due to their similar lattice structures, which indicates the weak interfacial phonon scattering between them. Considering the negligible tube−tube interactions and the weak interfacial phonon scattering between the inner CNT and outer BNNT, it is reasonable to conclude that the improved heat transfer of asprepared C@BNNT arrays is attributed to the outer BNNT, which serves as an additional heat conducting channel while not affecting the thermal conductivity of the inner CNT. In summary, coaxial C@BNNT arrays were fabricated and their thermal conductivity was measured using PPR technique and compared with that of the bare CNT arrays. Impressively, a tremendous ∼90% improvement (from ∼15.5 to 29.5 W/mK) in the thermal conductivity was achieved for the C@BNNT arrays with the introduction of 0.97 nm-thick (∼3−4 walls) outer BNNT. This enhancement is due to that the outer BNNT provides additional thermal conductance without affecting that of the inner CNT attributing to their coherent structures, as supported by the corresponding reverse nonequilibrium MD simulation. This work sheds new lights on improving the thermal conductivity of arbitrary CNT arrays from both experimental and theoretical perspectives.
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.7b02154. Raman spectra of the CNT and C@BNNT; EDX elemental maps of the C@BNNT arrays; snapshots of the MD simulations, temperature distribution along the heat flow direction and the cumulative energy changes vs simulation time for the inner CNT within C@BNNT, isolated CNT, individual CNT within aligned CNT cluster, respectively; phonon power spectra of the inner CNT and outer BNNT within C@BNNT structure (PDF)
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AUTHOR INFORMATION
Corresponding Authors
*E-mail:
[email protected]. *E-mail:
[email protected]. ORCID
Lin Jing: 0000-0003-2282-5129 Hongling Li: 0000-0002-2292-1949 Olivier Cometto: 0000-0003-0269-3624 Author Contributions †
L.J., M.K.S., and B.L. contributed equally to this work.
Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The authors would like to acknowledge funding support from Institute for Sports Research at Nanyang Technological University (ISR/NTU) and Singapore Ministry of Education Academic Research Fund Tier 2 No. MOE2013-T2-2-050.
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REFERENCES
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