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Research Papers: Micro/Nanoscale Heat Transfer

Array Volume Fraction-Dependent Thermal Transport Properties of Vertically Aligned Carbon Nanotube Arrays

[+] Author and Article Information
Yang Zhao

Department of Precision Machinery
and Precision Instrumentation,
University of Science and Technology of China,
Hefei, Anhui 230026, China;
Department of Mechanical Engineering,
University of California,
Berkeley, CA 94720

Rong-Shiuan Chu

Applied Science and Technology
Graduate Group,
University of California,
Berkeley, CA 94720

Costas P. Grigoropoulos

Department of Mechanical Engineering,
University of California,
Berkeley, CA 94720;
Applied Science and Technology
Graduate Group,
University of California,
Berkeley, CA 94720

Oscar D. Dubon

Applied Science and Technology
Graduate Group,
University of California,
Berkeley, CA 94720;
Department of Materials Science
and Engineering,
University of California,
Berkeley, CA 94720;
Materials Sciences Division,
Lawrence Berkeley National Laboratory,
Berkeley, CA 94720

Arun Majumdar

Department of Mechanical Engineering,
University of California,
Berkeley, CA 94720;
Applied Science and Technology
Graduate Group,
University of California,
Berkeley, CA 94720;
Department of Materials Science
and Engineering,
University of California,
Berkeley, CA 94720;
Materials Sciences Division,
Lawrence Berkeley National Laboratory,
Berkeley, CA 94720

1Corresponding author.

Contributed by the Heat Transfer Division of ASME for publication in the JOURNAL OF HEAT TRANSFER. Manuscript received October 10, 2015; final manuscript received April 25, 2016; published online May 17, 2016. Editor: Portonovo S. Ayyaswamy.

J. Heat Transfer 138(9), 092401 (May 17, 2016) (7 pages) Paper No: HT-15-1650; doi: 10.1115/1.4033538 History: Received October 10, 2015; Revised April 25, 2016

Vertically aligned carbon nanotube (CNT) arrays are promising candidates for advanced thermal interface materials (TIMs) since they possess high mechanical compliance and high intrinsic thermal conductivity. However, the overall thermal performance of CNT arrays often falls short of expectations when used as TIMs, and the underlying reasons have yet to be fully understood. In this work, the volume fraction of CNT arrays is demonstrated to be the key factor in determining the CNT array thermal transport properties. By increasing the array volume fraction, both the CNT array effective thermal conductivity and the CNT array–glass thermal contact conductance were experimentally found to increase monotonically. One interesting phenomenon is that the increasing rate of thermal conductivity is larger than that of array volume fraction. Compressive experiments verified that the CNT arrays with lower volume fractions suffer from severe buckling, which results in a further decreasing trend. By understanding the underlying reasons behind this trend, the overall thermal performance of vertically aligned CNT arrays can be further increased.

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Figures

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Fig. 1

SEM images of the catalyst nanoparticles exposed by removing the CNT array (top) and TEM images of the CNTs (bottom). Before CNT array growth, the catalysts were annealed in H2: 80 sccm for (a) 7 min, (b) 2 min, and (c) 0 min. The number of catalyst particles increased as the annealing duration decreased.

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Fig. 2

With H2 flow rate of 110 sccm, effect on the nanoparticle formation and CNT growth of different underlayers: (a) sputtered Al2O3 and (b) ALD Al2O3

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Fig. 3

Experimental setup of the phase-sensitive thermal reflectance thermometry

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Fig. 4

Schematic of the glass/In/CNT array three-layer structure

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Fig. 5

GGlass–CNT array and kCNT array versus volume fraction (%) of CNT arrays. Both GGlass–CNT array and kCNT array increase monotonically as the volume fraction increases.

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Fig. 6

Side views SEM images of the CNT array with (a) 1.84% and (b) 0.87% in volume fraction before compression. Both the CNT arrays are ∼130 μm in length. After compression, the CNT array with initially (c) 1.84% in volume fraction decreases 30 μm in length while the CNT array with initially (d) 0.87% in volume fraction decreases 100 μm in length. The scale bar in images (a)–(d) is 40 μm.

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Fig. 7

(a) In ideal case, vertically aligned CNT array can bridge two surfaces and each tube independently forms a thermal path and (b) when CNTs buckle under bonding pressure, some tubes are overshadowed by adjacent tubes and form thermal paths through tube-to-tube contact

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