Thermal conductivity of carbon nanotubes and graphene in epoxy nanofluids and nanocomposites
© Martin-Gallego et al; licensee Springer. 2011
Received: 19 August 2011
Accepted: 1 December 2011
Published: 1 December 2011
We employed an easy and direct method to measure the thermal conductivity of epoxy in the liquid (nanofluid) and solid (nanocomposite) states using both rodlike and platelet-like carbon-based nanostructures. Comparing the experimental results with the theoretical model, an anomalous enhancement was obtained with multiwall carbon nanotubes, probably due to their layered structure and lowest surface resistance. Puzzling results for functionalized graphene sheet nanocomposites suggest that phonon coupling of the vibrational modes of the graphene and of the polymeric matrix plays a dominant role on the thermal conductivities of the liquid and solid states.
PACS: 74.25.fc; 81.05.Qk; 81.07.Pr.
Due to the increasing importance of energy dissipation in the electronic industry, thermal conductivity of cured epoxy resins has been widely investigated over the years. One strategy to improve the thermal transport of epoxy resins has been the addition of highly conductive fillers, such as carbon-based or metallic fillers . However, the effect of such additions on either the uncured system or the cure reaction of resins has not yet been fully established. The thermal conductivity of the uncured liquid resin plays an important role to define the variables involved in the transformation process, such as time, applied heat, or cooling time, which will then have a profound effect on the cross-link density and hence, on the final properties of the system. Thus, we aimed at studying the effect of two types of carbon-based nanofillers, in particular, nanotubes and graphene sheets, on the thermal conductivity of an uncured liquid epoxy resin.
There have been considerable interest and effort in the transport properties of carbon nanotube [CNT] filled polymer nanocomposites [2, 3]. Electrical conductivity of epoxy nanocomposites increases by several orders of magnitude with CNT concentration ; this effect can be explained by the established percolation theory  with the shift from an insulator into a conductive material when a critical concentration of the conductive filler is reached, commonly known as percolation threshold. However, the thermal conductivity has shown at best linear enhancements with nanotube content with a lack of thermal percolation. The main reason for this fact is the relatively small thermal conductivity ratio (K cnt /K matrix) by comparison with the corresponding ratio of electrical conductivities .
Graphene is a two-dimensional carbon nanofiller with a one-atom-thick sheet of sp 2 bonded carbon atoms that are densely packed in a honeycomb crystal lattice [7, 8]. Single layer graphene is predicted to have a remarkable performance, such as high thermal conductivity of 5,000 W/mK, which corresponds to the upper bound of the highest values reported for single-walled carbon nanotube bundles , high electrical conductivity of up to 6,000 S/cm , and superior mechanical properties with Young's modulus of 1 TPa and ultimate strength of 130 GPa . In addition to these outstanding properties, the recent developments on graphene synthesis routes and on the understanding of their unique properties have prompted the development and study of graphene filled nanocomposites [12, 13].
This communication analyzes the epoxy-nanofiller blend in the liquid state as a nanofluid and takes into consideration the current theories to explain its transport properties, particularly, the thermal conductivity. Some studies report large thermal enhancements by adding a small percentage of nanoparticles to a fluid . This anomalous behavior, far away from the predicted data by standard theoretical models, is explained by several physical mechanisms like the Brownian motion of the particles or changes in the distribution of the molecules in the liquid state at the particle/liquid interface .
Diglycidyl ether of bisphenol-A epoxy resin (product number: 405493), diethylene triamine curing agent (D93856), and single-walled nanotubes [SWNTs] (519308; diameter 1.2 to 1.5 nm; length 2.5 μm; and specific surface area 1,300 m2/g) used in this study were purchased from Sigma-Aldrich (St. Louis, MO, USA), while multiwalled nanotubes [MWNTs] (diameter 40 nm; length 120 μm, and specific surface area 250 to 300 m2/g) were synthesized in-house by a chemical vapor deposition technique . These MWNTs were then functionalized [f-MWNT] with a 3:1 concentrated H2SO4/HNO3 mixture refluxed at 120°C for 30 min and thoroughly washed with distilled water until neutral. Functionalized graphene sheets [FGS] were also synthesized in-house by the rapid thermal expansion of graphite oxide [GO] at 1,000°C under an inert atmosphere. This results in a high surface area carbon material consisting of graphene layers with residual hydroxyl, carbonyl, and epoxy groups. GO was synthesized from natural graphite flakes obtained from Sigma-Aldrich (St. Louis, MO, USA; universal grade, purum powder ≤ 0.1 mm, 200 mesh, 99.9995%), according to the Brödie method. Full characterization of the FGS used in this work is described elsewhere .
Sample preparation and characterization
Results and discussion
The thermal conductivity [K] of CNTs depends on several factors such as the morphology, the chirality, the diameter and length of the tubes, the number of structural defects, and the specific surface area [19, 20]. Thus, a description of the thermal conduction mechanisms is nontrivial. Liu et al.  reported a K for a SWNT and a MWNT of 2,400 W/mK and 1,400 W/mK, respectively, measured using the non contact Raman spectra shift method. The lower intrinsic conductivity of MWNTs was assigned to the fact that thermal transport mainly occurs by the outermost wall and by the existence of intertube Umklapp scattering processes. In addition, SWNTs exhibit a higher number of phonon vibrational modes and a lower defect density in relation to MWNTs, leading to a higher intrinsic K[22, 23]. CNTs are characterized by a large aspect ratio and a huge surface area. It is assumed that the K of CNTs will be higher for CNTs with a greater aspect ratio . In this study, both CNTs exhibit a similar aspect ratio, so this issue does not seem to affect the K of the nanofluid. Another factor that determines the K of CNTs is the presence of structural defects. Che et al.  revealed that the K of CNTs decreased with increasing defect concentration. Finally, the heat transfer mechanism of CNTs takes place with phonons and electrons and depends on their chirality . However, to simplify the discussion, we assumed that the thermal conductance mainly occurs via a phonon conduction mechanism since the aim of this article is to provide a general description of the experimentally determined K.
Thermal conductivity of epoxy nanofluids and nanocomposites
0.150 ± 0.001
0.22 ± 0.07
0.2 wt.% MWNT
0.162 ± 0.004
0.4 wt.% MWNT
0.176 ± 0.009
0.6 wt.% MWNT
0.202 ± 0.004
0.29 ± 0.05
0.8 wt.% MWNT
0.220 ± 0.001
1 wt.% MWNT
0.250 ± 0.001
0.38 ± 0.07
0.6 wt.% f-MWNT
0.180 ± 0.001
0.6 wt.% SWNT
0.180 ± 0.001
1 wt.% FGS
0.150 ± 0.001
0.36 ± 0.04
1 wt.% Graphite
0.176 ± 0.005
1 wt.% GO
0.150 ± 0.001
A lower improvement was obtained with the acid-treated carbon nanotubes (f-MWNTs), even though the functionalization decreases the SSA of the nanotubes. This result can be explained by the presence of the functional groups, hydroxyl and carbonyl, that act as scattering points on the surface where phonons can be transferred from the nanotube crystalline structure into the insulating polymer matrix. This behavior has already been observed in cured resins [27, 28], but not in the pre-cured state.
We observe an anomalous enhancement of the experimental K. This behavior could be related to two effects. The first effect is the presence of an organized structure of the molecules in the liquid state at the solid/liquid interface that facilities the coupling between the solid particles and the fluid . The second effect could be contributions from the Brownian motions of the particles that modify the heat transfer in the fluid .
We employed an easy and direct method based on the hot wire technique to measure the thermal conductivity of epoxy nanofluids. We also studied the differences in heat conduction mechanisms using graphene sheets and different types of CNTs analyzing the role of surface functionalization and resistance to heat flow at the interface in the thermal conductivity. The results show that the layered structure of MWNTs enables an efficient phonon transport through the inner layers, while SWNTs present a higher resistance to heat flow at the interface due to its higher SSA, and f-MWNTs have functional groups on their surface acting as scattering points for the phonon transport. The dominant role of coupling vibrational modes between the matrix and the filler is evident in the case of FGS which induces a transition from a non thermal conductive nanofluid into a thermal-conductive nanocomposite in the solid state.
The work was supported by the Spanish Ministry of Science and Innovation (MICINN) under project MAT 2010-18749. MMG thanks the CSIC for a JAE-Pre grant.
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