74 research outputs found
Heat transfer from nanoparticles: a corresponding state analysis
In this contribution, we study situations in which nanoparticles in a fluid
are strongly heated, generating high heat fluxes. This situation is relevant to
experiments in which a fluid is locally heated using selective absorption of
radiation by solid particles. We first study this situation for different types
of molecular interactions, using models for gold particles suspended in octane
and in water. As already reported in experiments, very high heat fluxes and
temperature elevations (leading eventually to particle destruction) can be
observed in such situations. We show that a very simple modeling based on
Lennard-Jones interactions captures the essential features of such experiments,
and that the results for various liquids can be mapped onto the Lennard-Jones
case, provided a physically justified (corresponding state) choice of
parameters is made. Physically, the possibility of sustaining very high heat
fluxes is related to the strong curvature of the interface that inhibits the
formation of an insulating vapor film
Enhancing surface heat transfer by carbon nanofins: towards an alternative to nanofluids?
Background: Nanofluids are suspensions of nanoparticles and fibers which have recently attracted much attention because of their superior thermal properties. Nevertheless, it was proven that, due to modest dispersion of nanoparticles, such high expectations often remain unmet. In this article, by introducing the notion of nanofin, a possible solution is envisioned, where nanostructures with high aspect-ratio are sparsely attached to a solid surface (to avoid a significant disturbance on the fluid dynamic structures), and act as efficient thermal bridges within the boundary layer. As a result, particles are only needed in a small region of the fluid, while dispersion can be controlled in advance through design and manufacturing processes. Results: Toward the end of implementing the above idea, we focus on single carbon nanotubes to enhance heat transfer between a surface and a fluid in contact with it. First, we investigate the thermal conductivity of the latter nanostructures by means of classical non-equilibrium molecular dynamics simulations. Next, thermal conductance at the interface between a single wall carbon nanotube (nanofin) and water molecules is assessed by means of both steady-state and transient numerical experiments. Conclusions: Numerical evidences suggest a pretty favorable thermal boundary conductance (order of 107 W·m-2·K-1) which makes carbon nanotubes potential candidates for constructing nanofinned surface
Radius and chirality dependent conformation of polymer molecule at nanotube interface
Temperature dependent conformations of linear polymer molecules adsorbed at
carbon nanotube (CNT) interfaces are investigated through molecule dynamics
simulations. Model polyethylene (PE) molecules are shown to have selective
conformations on CNT surface, controlled by atomic structures of CNT lattice
and geometric coiling energy. PE molecules form entropy driven assembly
domains, and their preferred wrapping angles around large radius CNT (40, 40)
reflect the molecule configurations with energy minimums on a graphite plane.
While PE molecules prefer wrapping on small radius armchair CNT (5, 5)
predominantly at low temperatures, their configurations are shifted to larger
wrapping angle ones on a similar radius zigzag CNT (10, 0). A nematic
transformation around 280 K is identified through Landau-deGennes theory, with
molecule aligning along tube axis in extended conformationsComment: 19 pages, 7 figure2, submitted to journa
Effect of Covalent Functionalisation on Thermal Transport Across Graphene-Polymer Interfaces
This paper is concerned with the interfacial thermal resistance for polymer
composites reinforced by various covalently functionalised graphene. By using
molecular dynamics simulations, the obtained results show that the covalent
functionalisation in graphene plays a significant role in reducing the
graphene-paraffin interfacial thermal resistance. This reduction is dependent
on the coverage and type of functional groups. Among the various functional
groups, butyl is found to be the most effective in reducing the interfacial
thermal resistance, followed by methyl, phenyl and formyl. The other functional
groups under consideration such as carboxyl, hydroxyl and amines are found to
produce negligible reduction in the interfacial thermal resistance. For
multilayer graphene with a layer number up to four, the interfacial thermal
resistance is insensitive to the layer number. The effects of the different
functional groups and the layer number on the interfacial thermal resistance
are also elaborated using the vibrational density of states of the graphene and
the paraffin matrix. The present findings provide useful guidelines in the
application of functionalised graphene for practical thermal management.Comment: 8 figure
Graphene -- Based Nanocomposites as Highly Efficient Thermal Interface Materials
We found that an optimized mixture of graphene and multilayer graphene -
produced by the high-yield inexpensive liquid-phase-exfoliation technique - can
lead to an extremely strong enhancement of the cross-plane thermal conductivity
K of the composite. The "laser flash" measurements revealed a record-high
enhancement of K by 2300 % in the graphene-based polymer at the filler loading
fraction f =10 vol. %. It was determined that a relatively high concentration
of single-layer and bilayer graphene flakes (~10-15%) present simultaneously
with thicker multilayers of large lateral size (~ 1 micrometer) were essential
for the observed unusual K enhancement. The thermal conductivity of a
commercial thermal grease was increased from an initial value of ~5.8 W/mK to
K=14 W/mK at the small loading f=2%, which preserved all mechanical properties
of the hybrid. Our modeling results suggest that graphene - multilayer graphene
nanocomposite used as the thermal interface material outperforms those with
carbon nanotubes or metal nanoparticles owing to graphene's aspect ratio and
lower Kapitza resistance at the graphene - matrix interface.Comment: 4 figure
Revealing the nature of morphological changes in carbon nanotube-polymer saturable absorber under high-power laser irradiation
Composites of single-walled carbon nanotubes (SWNTs) and water-soluble polymers (WSP) are the focus of significant worldwide research due to a number of applications in biotechnology and photonics, particularly for ultrashort pulse generation. Despite the unique possibility of constructing non-linear optical SWNT-WSP composites with controlled optical properties, their thermal degradation threshold and limit of operational power remain unexplored. In this study, we discover the nature of the SWNT-polyvinyl alcohol (PVA) film thermal degradation and evaluate the modification of the composite properties under continuous high-power ultrashort pulse laser operation. Using high-precision optical microscopy and micro-Raman spectroscopy, we have examined SWNT-PVA films before and after continuous laser radiation exposure (up to 40 hours) with a maximum optical fluence of 2.3 mJ·cm−2. We demonstrate that high-intensity laser radiation results in measurable changes in the composition and morphology of the SWNT-PVA film due to efficient heat transfer from SWNTs to the polymer matrix. The saturable absorber modification does not affect the laser operational performance. We anticipate our work to be a starting point for more sophisticated research aimed at the enhancement of SWNT-PVA films fabrication for their operation as reliable saturable absorbers in high-power ultrafast lasers
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