Effects of branching and polydispersity on thermal conductivity of paraffin waxes

M.W. Boomstra (Corresponding author), M.W.J. van Asseldonk, B.J. Geurts, V.M. Nazarychev, A.V. Lyulin

Research output: Contribution to journalArticleAcademicpeer-review

11 Citations (Scopus)
162 Downloads (Pure)

Abstract

Paraffin waxes are promising phase change materials, abundantly available at very low cost. Having large latent heat, these materials can be used for thermal energy storage. However, when used in heat batteries, paraffin's low thermal conductivity prevents fast charging and discharging. This calls for the design of tailored hybrid materials with improved properties, the present study concentrates on properties of pure paraffin wax. Using fully atomistic molecular-dynamics (MD) simulations, we study the effects of polydispersity and branching on the thermal conductivity of paraffin waxes, in molten (450 K) and solid (250 K) state. Both branching and polydispersity affect the density and especially the crystallinity of the solid. Branching has a pronounced effect on crystallisation caused by inhibited alignment of the polymer backbones while the effect of polydispersity is less pronounced. The thermal conductivity (TC) has been simulated using the reverse non-equilibrium molecular-dynamics method, as well as the equilibrium Green-Kubo approach. Increased branching, added to backbones comprised of twenty monomers, results in decreasing TC of up to 30%, polydispersity only has an effect in the semi-crystalline state. Comparison to available experiments shows good agreement which validates the model details, applied force field and the calculation methods. We show that at comparable computational costs, the reverse non-equilibrium MD approach produces more reliable results for TC, as compared to the equilibrium Green-Kubo method. The major contribution to TC by acoustic phonon transport along the backbone was shown by analysing extreme cases. The phonon density of states (PDOS) of samples with high branching or with small chain length displayed diminished peaks in the acoustic range as compared to the PDOS of samples with low branching or larger chain length, respectively. The suggested MD approach can definitely be used to investigate specific material modifications aimed at increasing the overall TC.

Original languageEnglish
Article number123192
Number of pages11
JournalInternational Journal of Heat and Mass Transfer
Volume195
DOIs
Publication statusPublished - Oct 2022

Bibliographical note

Funding Information:
This study is part of the research project “Nanofiller-enhanced wax for heat storage (Wax+)” with project number 18052 of the Open Technology Programme which is (partly) financed by the Dutch Research Council (NWO), domain Applied and Engineering Sciences (TTW).

Funding

This study is part of the research project “Nanofiller-enhanced wax for heat storage (Wax+)” with project number 18052 of the Open Technology Programme which is (partly) financed by the Dutch Research Council (NWO), domain Applied and Engineering Sciences (TTW). This study is part of the research project “Nanofiller-enhanced wax for heat storage (Wax+)” with project number 18052 of the Open Technology Programme which is (partly) financed by the Dutch Research Council (NWO), domain Applied and Engineering Sciences (TTW). This work was carried out on the Dutch national e-infrastructure with the support of SURF Cooperative. The authors would like to thank prof. V. Koelman, prof. P. Bobbert and dr. S. Gireesan from the Centre for Computional Energy Research (CCER) in Eindhoven for the fruitful discussions. Chain length values of experimentally used paraffin wax were kindly provided by Lisette Wijkhuijs.

Keywords

  • Branching
  • Fully atomistic molecular dynamics
  • Paraffin wax
  • Phase change material
  • Polydispersity
  • Thermal conductivity

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