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RESEARCH PAPERS: Micro/Nanoscale Heat Transfer

Computational Model for Transport in Nanotube-Based Composites With Applications to Flexible Electronics

[+] Author and Article Information
Satish Kumar, Muhammad A. Alam

School of Mechanical Engineering,  Purdue University, West Lafayette, IN 47907School of Electrical and Computer Engineering,  Purdue University, West Lafayette, IN 47907

Jayathi Y. Murthy1

Schoo of Mechanical Engineering,  Purdue University, 585 Purdue Mall, West Lafayette, IN 47907jmurthy@ecn.purdue.edu

1

Corresponding author.

J. Heat Transfer 129(4), 500-508 (Aug 25, 2006) (9 pages) doi:10.1115/1.2709969 History: Received March 28, 2006; Revised August 25, 2006

Thermal and electrical transport in a new class of nanocomposites composed of random isotropic two-dimensional ensembles of nanotubes or nanowires in a substrate (host matrix) is considered for use in the channel region of thin-film transistors (TFTs). The random ensemble of nanotubes is generated numerically and each nanotube is discretized using a finite volume scheme. To simulate transport in composites, the network is embedded in a background substrate mesh, which is also discretized using a finite volume scheme. Energy and charge exchange between nanotubes at the points of contact and between the network and the substrate are accounted for. A variety of test problems are computed for both network transport in the absence of a substrate, as well as for determination of lateral thermal and electrical conductivity in composites. For nanotube networks in the absence of a substrate, the conductance exponent relating the network conductance to the channel length is computed and found to match experimental electrical measurements. The effective thermal conductivity of a nanotube network embedded in a thin substrate is computed for a range of substrate-to-tube conductivity ratios. It is observed that the effective thermal conductivity of the composite saturates to a size-independent value for large enough samples, establishing the limits beyond which bulk behavior obtains. The effective electrical conductivity of carbon nanotube-organic thin films used in organic TFTs is computed and is observed to be in good agreement with the experimental results.

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Copyright © 2007 by American Society of Mechanical Engineers
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Figures

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Figure 1

(a) Schematic of thin-film transistor showing source (S), drain (D), and channel (C). The channel region is composed of a network of CNTs; (b) geometric parameters.

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Figure 2

(a) Tube segment nomenclature, and (b) substrate control volume nomenclature. The displacement vector Δξ⃗ from substrate cell centroid to tube segment centroid is shown.

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Figure 3

Comparison of numerically computed dimensionless temperature distribution in tube and substrate with analytical results (θ definition corresponds to that in Eq. 11). The schematic of the tube embedded in the substrate and the boundary conditions are shown in the inset.

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Figure 4

Comparison of heat transfer rate in a nanotube network with analytical results for the case of zero tube–tube contact

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Figure 5

(a) Computed conductance dependence on channel length for different densities (ρ) in the strong coupling limit (cij=50) compared with experimental results from Ref. 11. For ρ=10.0μm−2, Go=1.0 (simulation), and Go=1.0 (experiment). For ρ=1.35μm−2, Go=1.0 (simulation) and Go=2.50 (experiment). The number after each curve corresponds to the value of ρ used in the simulation. The number in [] corresponds to ρ in experiments from Ref. 11; (b) dependence of conductance exponent (n) on channel length for different densities (ρ) based on (a).

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Figure 6

Increase in composite effective thermal conductivity (keff) over the substrate value for two different grid sizes: LC∕Lt=2.0; H∕Lt=2, Bic=10.0, Bis=10−5, ks∕kt=0.001, and ρ*=10.0

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Figure 7

Nondimensional temperature distribution in (a) substrate (b) tube network: LC∕Lt=2.0, H∕Lt=2, Bic=10.0, Bis=10−5, ks∕kt=0.001, and ρ*=14.0

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Figure 8

Effect of substrate–tube conductivity ratio on keff for varying channel length: LC∕Lt=0.25–7.0, Bic=10.0, Bis=10−5, and ρ*=10.0

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Figure 9

Comparison of computed conductivity (normalized by the conductivity at ρ*=0.05) of organic transistor against the experimental conductivity (13)

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