Thermal conductivity enhancement of paraffins by increasing the alignment of molecules through adding CNT/graphene☆
Introduction
Introduction of nanoscale highly-conductive fillers into phase change materials (PCM) in order to enhance their thermal conductivity has attracted significant attention in recent years [1], [2], [3], [4], [5], [6], [7], [8]. For well-dispersed suspensions of spherical nanoparticles, the thermal conductivity is described adequately by the effective medium theory and significant enhancement is not exhibited [9]. However, through introduction of carbon nanotubes (CNT), carbon nanofibers, graphite nanoplatelets and graphene sheets into materials, considerable augmentation of thermal conductivity for both liquid and solid phases has been observed [3], [4], [5], [6], [7], [8], [10], [11], [12], [13]. Considering the high aspect ratio of these fillers, such enhancement is also predicted from the effective medium theory [14]. However, the magnitude of the enhancement can be greatly reduced by high interfacial thermal resistance between CNT/graphene and matrix material [15], [16], [17], [18].
The filler–matrix interface can also have a potentially positive effect on thermal conductivity due to the ordering of the atomic structure at the solid–liquid interface [19]. This idea is motivated by the fact that crystalline solids exhibit much greater thermal conductivity when compared to their amorphous counterparts [20], [21] due to lack of polarized thermal waves (phonons) in amorphous materials as well as in liquids. However, molecular simulations of simple, small molecule liquids at solid interfaces indicate that interfacial order in the liquid has little effect on thermal transport [22].
In this work, we use molecular dynamics (MD) simulation to study bigger molecules in their liquid and solid phases, specifically n-alkane (n-paraffins) molecules [23], [24], [25], [26], [27], [28], in order to investigate the effect of solid fillers-induced structures on thermal conductivity of such materials. The simulation methodology and thermal conductivity calculation method are described in Section 2. Model structures and their preparation are described in Section 3. We report the results of calculated thermal conductivity in Section 4. Finally, a summary and conclusions are presented in Section 5.
Section snippets
Simulation methodology
The direct method for the determination of thermal conductivity was utilized in combination with the non-equilibrium molecular dynamics (NEMD) simulation method [29]. In the direct method, a heat flux is imposed through the simulation box by adding heat to molecules inside a planar slab (heat source) in one region of the box and extracting the same amount of heat from molecules inside another slab (heat sink) in another region. Upon reaching the steady state, based on the Fourier’s law, the
Bulk structures
The reference pure n-octadecane structure contains 600 CH3(CH2)16CH3 molecules in a cubic box. The system was initially equilibrated at 300 K and 1 atm for 4,000,000 time steps under isothermal–isobaric conditions (NPT) leading to an equilibrium liquid structure. To obtain the solid phase of the paraffin, the system was first heated to 320 K and then cooled down to 190 K at the rate of 2 K/ns. Fig. 1(a) and (b) show snapshots of the solid and liquid structures at 190 K and 300 K, respectively. The
Thermal conductivity
The temperature profile within the bulk liquid in response to application of the direct method is shown in Fig. 5. Based on the heat current and temperature gradient, we determine the thermal conductivity of the liquid n-octadecane to be 0.164 W/m K, which is in good agreement with the experimental value of 0.153 W/m K [41]. The alignment parameter for the liquid system is 0.02, which is close to the expected value of zero for randomly-distributed molecular end-to-end vectors.
Thermal conductivity
Summary and conclusions
We performed molecular dynamics simulations to investigate the effect of alignment of n-octadecane molecules on its thermal conductivity. We also studied the influence of adding CNT and graphene on the alignment of molecules and consequently, on the thermal conductivity in the direction along which the molecules are aligned.
A summary of the thermal conductivity values obtained using the direct method and alignment parameter values for all different systems were provided. The predicted thermal
Acknowledgments
This material is based upon work partially supported by the US Department of Energy under Award Number DE-SC0002470. The first author also acknowledges financial support provided by the Alabama EPSCoR Program under the Graduate Research Scholars Program (Round 6). He is also grateful to the Samuel Ginn College of Engineering and the Department of Mechanical Engineering at Auburn University for providing support for his Dean’s Fellowship since Fall 2009. He also acknowledges the Alabama
References (43)
- et al.
Nanoparticle-enhanced phase change materials (NEPCM) with great potential for improved thermal energy storage
Int. Commun. Heat Mass Transfer
(2007) - et al.
Effect of carbon nanofiber additives on thermal behavior of phase change materials
Carbon
(2005) - et al.
High latent heat storage and high thermal conductive phase change materials using exfoliated graphite nanoplatelets
Sol. Energy Mater. Sol. Cells
(2009) - et al.
Thermal properties of paraffin based composites containing multi-walled carbon nanotubes
Thermochim. Acta
(2009) - et al.
Investigation of exfoliated graphite nanoplatelets (xGnP) in improving thermal conductivity of paraffin wax-based phase change material
Sol. Energy Mater. Sol. Cells
(2011) - et al.
Significant thermal conductivity enhancement for nanofluids containing graphene nanosheets
Phys. Lett. A
(2011) - et al.
Adjustable thermal conductivity in carbon nanotube nanofluids
Phys. Lett. A
(2009) - et al.
Mechanisms of heat flow in suspensions of nano-sized particles (nanofluids)
Int. J. Heat Mass Transfer
(2002) - et al.
Heat flow and lattice vibrations in glasses
Solid State Commun.
(1989) - et al.
Effect of liquid layering at the liquid–solid interface on thermal transport
Int. J. Heat Mass Transfer
(2004)
Fast parallel algorithms for short-range molecular dynamics
J. Comput. Phys.
Enhanced thermal conductivity in a nanostructured phase change composite due to low concentration graphene additives
J. Phys. Chem. C
Carbon nanoadditives to enhance latent energy storage of phase change materials
J. Appl. Phys.
A benchmark study on the thermal conductivity of nanofluids
J. Appl. Phys.
Graphite nanoplatelet–epoxy composite thermal interface materials
J. Phys. Chem. C
Enhanced thermal conductivity in a hybrid graphite nanoplatelet – carbon nanotube filler for epoxy composites
Adv. Mater.
Effective thermal conductivity of particulate composites with interfacial thermal resistance
J. Appl. Phys.
Interfacial heat flow in carbon nanotube suspensions
Nat. Mater.
Role of thermal boundary resistance on the heat flow in carbon-nanotube composites
J. Appl. Phys.
Determination of interfacial thermal resistance at the nanoscale
Phys. Rev. B
Cited by (200)
Nanoscale study on enhanced interfacial heat transfer in confined space with spontaneously formed diol molecular bridge
2024, International Journal of Thermal SciencesMolecular dynamics study of phase change properties of paraffin-based phase change materials with carbon nano-additives
2024, Journal of Energy StorageDirectional enhancement and potential reduction of thermal conductivity induced by one-dimensional nanoparticle addition in pure PCMs
2023, International Journal of Heat and Mass TransferTribological properties of an epoxy polymer containing a magnetically oriented graphene oxide/iron oxide nanoparticle composite
2023, Diamond and Related MaterialsThe size effect on filling and phase change behavior of paraffin within carbon nanotube
2023, International Journal of Heat and Mass Transfer
- ☆
Disclaimer: This report was prepared as an account of work sponsored by an agency of the United States Government. Neither the United States Government nor any agency thereof, nor any of their employees, makes any warranty, express or implied, or assumes any legal liability or responsibility for the accuracy, completeness, or usefulness of any information, apparatus, product, or process disclosed, or represents that its use would not infringe privately owned rights. References herein to any specific commercial product, process, or service by trade name, trademark, manufacturer, or otherwise does not necessarily constitute or imply its endorsement, recommendation, or favoring by the United States Government or any agency thereof. The views and opinions of authors expressed herein do not necessarily state or reflect those of the United States Government or any agency thereof.