Research Papers: Micro/Nanoscale Heat Transfer

Particle Aspect-Ratio and Agglomeration-State Effects on the Effective Thermal Conductivity of Aqueous Suspensions of Multiwalled Carbon Nanotubes

[+] Author and Article Information
Anna S. Cherkasova

Department of Mechanical and Aerospace Engineering, Rutgers University, Piscataway, NJ 08854-8054

Jerry W. Shan

Department of Mechanical and Aerospace Engineering, Rutgers University, Piscataway, NJ 08854-8054jshan@jove.rutgers.edu

J. Heat Transfer 132(8), 082402 (Jun 09, 2010) (11 pages) doi:10.1115/1.4001364 History: Received September 22, 2009; Revised February 17, 2010; Published June 09, 2010; Online June 09, 2010

The effective thermal conductivities of aqueous nanofluids containing surfactant-stabilized multiwalled carbon nanotubes were measured and compared with the predictions of effective medium theory (Nan, C.-W., , 1997, “Effective Thermal Conductivity of Particulate Composites With Interfacial Thermal Resistance,” J. Appl. Phys., 81(10), pp. 6692–6699). Detailed characterization of nanotube morphology was carried out through electron microscopy, while the nanotube agglomeration state was monitored through optical microscopy and absorption measurements. An optimum surfactant-to-nanotube mass ratio was found for the particular surfactant, sodium dodecylbenzene sulfonate, which resulted in the greatest increase in thermal conductivity. Taking into consideration the volume-weighted aspect ratio of the nanotubes, the measured thermal conductivities of the suspensions were shown to be in good agreement with calculations for a reasonable choice of interfacial resistance on the particle/liquid interface. The effect of particle aspect ratio on the suspension’s thermal conductivity was further demonstrated and compared with theory by reducing the nanotube length through intense ultrasonication. The effect of particle aggregation on the thermal conductivity was also investigated by destabilizing previously stable suspensions with ethanol addition, which causes surfactant desorption and bundling of nanotubes. The measured thermal conductivities were correlated with absorption measurements and microscopic visualizations to show that particle aggregation decreases the thermal conductivity of the nanofluid by reducing the effective particle aspect ratio.

Copyright © 2010 by American Society of Mechanical Engineers
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Figure 1

TEM of dried sample originally containing 0.1% vol MWNTs. Such images were used to measure the length and diameter distribution of nanotubes in suspension.

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Figure 2

Suspensions with NaDDBS to CNT mass ratios (a) 5× and (b) 100× 1 week after preparation. From right to left, the MWNT volume fractions are 0.01%, 0.03%, 0.05%, 0.1%, and 1% vol. The unstable suspensions, which had NaDDBS concentrations exceeding 20–50 g/l, are indicated with black borders.

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Figure 3

Measured thermal conductivities of aqueous suspensions of MWNTs. Mass ratio of NaDDBS to MWNTs is indicated in legend.

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Figure 4

Reported axial thermal conductivities of MWNTs as a function of tube diameter

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Figure 5

Measured thermal conductivities of aqueous suspensions of MWNTs compared with EMT calculations for different Kapitza resistances. Different symbols indicate different surfactant-to-MWNT mass ratios.

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Figure 6

Aspect-ratio distributions for MWNTs before and after varying durations of intense tip ultrasonication

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Figure 7

Measured thermal conductivities of 0.1% vol MWNT suspensions as a function of volume-weighted particle aspect ratio

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Figure 8

Measured absorbance as function of time for nanofluids containing different fractions of ethanol

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Figure 9

Optical micrographs at different magnifications of suspensions containing 0.1% vol MWNTs. Aqueous suspensions without ethanol are shown in (a) and (c), while aqueous suspensions containing 50% ethanol by volume (S1) are shown in (b) and (d).

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Figure 10

Measured thermal conductivities of ethanol destabilized nanofluids. The lower axis is the real time for the thermal-conductivity measurement. The upper axis shows a time scaled to allow comparison with the optical absorption measurements of Fig. 8 (i.e., divided by 5 due to the smaller size of the absorption cuvette). For comparison, EMT and Maxwell (spherical-particle) predictions are shown assuming no interfacial resistance.




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