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Research Papers

Enhanced Specific Heat Capacity of Nanomaterials Synthesized by Dispersing Silica Nanoparticles in Eutectic Mixtures

[+] Author and Article Information
Donghyun Shin

Mechanical and Aerospace Engineering Department,
The University of Texas at Arlington,
Arlington, TX 79019

Debjyoti Banerjee

Mechanical Engineering Department,
Texas A&M University,
College Station, TX 77843-3123
e-mail: dbanerjee@tamu.edu

1Corresponding author.

Contributed by the Heat Transfer Division of ASME for publication in the Journal of Heat Transfer. Manuscript received September 26, 2010; final manuscript received September 20, 2011; published online February 8, 2013. Assoc. Editor: Patrick E. Phelan.

J. Heat Transfer 135(3), 032801 (Feb 08, 2013) (8 pages) Paper No: HT-10-1443; doi: 10.1115/1.4005163 History: Received September 26, 2010; Revised September 20, 2011

Anomalous enhancements in the specific heat capacity values of nanomaterials were measured in this study. Silica nanoparticles (∼2–20 nm) were dispersed into eutectic of lithium carbonate and potassium carbonate (62:38 by molar ratio) at 1.5% mass concentration. The specific heat capacity measurements were performed using a differential scanning calorimeter (DSC). The specific heat capacity of the silica nanocomposite (solid phase) was enhanced by 38–54% and the specific heat of the silica nanofluid (liquid phase) was enhanced by 118–124% over that of the pure eutectic. Electron microscopy of the samples shows that the nanoparticles induce phase change (forms a higher density “compressed phase”) within the solvent material. Hence, a new model is proposed to account for the contribution of the compressed phase to the total specific heat capacity of the nanomaterials. The proposed model is found to be in good agreement with the experimental data. These results have wide ranging implications, such as for the development of efficient thermal storage systems that can enable significant reduction in the cost of solar thermal power.

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Figures

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Fig. 1

(a) TEM image of type-A samples after repeated thermocycling experiments involving multiple freezing and melting cycles in the DSC. It was observed that the nanoparticles were not agglomerated and the nominal size of the nanoparticles varied from ∼2 to 20 nm in the TEM image. (b) Image of the petridish obtained after complete evaporation of water. Image shows two distinct regions—composed of coarse amorphous powder (type-B) and fine amorphous powder (type-A).

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Fig. 2

Variation of the specific heat capacity with temperature for type-A and type-B nanocomposite samples (solid phase; 350 °C–500 °C). The specific heat capacity of type-A nanocomposite samples were enhanced by 38–54% compared to that of the pure eutectic, while no enhancement of specific heat capacity was observed for type-B nanocomposite. The melting point of both nanomaterials was observed to be depressed by ∼2 °C.

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Fig. 3

Variation of the specific heat capacity with temperature for type-A and type-B nanofluid samples (liquid phase; 525 °C–555 °C). The specific heat capacity of type-A nanocomposite samples were enhanced by 118–124% compared to that of the pure eutectic, while no enhancement of specific heat capacity was observed for type-B nanofluids.

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Fig. 4

Scanning electron micrographs (SEM) of type-B nanomaterials. No distinctive structures were observed due to the addition of nanoparticles (compared to that of the pure eutectic samples).

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Fig. 5

SEM of type-A nanomaterials. The molten salt eutectic formed a substructure resembling weave pattern in the bulk phase (transformed phase nucleated by nanoparticles that grows to form “large-scale” microstructures). The transformed phase nucleated by the nanoparticles is expected to contribute significant proportion of the specific heat capacity enhancements observed for the type-A nanofluid samples.

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Fig. 6

Comparison of the predictions for the specific heat capacity by the analytical models (models 1 and 2) with the experimental data for the pure eutectic, type-A and type-B nanocomposites (solid phase of the nanomaterials)

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Fig. 7

(a) SEM image of type-A nanomaterial (same as Fig. 5). (b) Histogram plot of the pixel intensities obtained from the SEM image (a). (c) Binary image of the SEM image after setting a threshold intensity of 128. (d) Histogram plot of the binary image after image processing in (c). Based on the image histogram plot, 40.5% of the image area is covered by the brighter pixels of the woven structures.

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Fig. 8

Comparison of the predictions for the specific heat capacity by the analytical models (models 1 and 2) with the experimental data for the pure eutectic, type-A and type-B nanofluids (liquid phase of the nanomaterials)

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