Understanding thermal transport across filler-polymer interfaces and filler-filler contacts within polymer composites is of great importance for better thermal design of the composites that are widely used in high-performance heat exchangers, energy storage devices, and flexible electronics.  Over the past several decades, considerable progress has been made in improving the thermal conductivity of polymer composites, but several key questions concerning the influence of interfacial thermal resistance, or Kapitza resistance, still remain.  Firstly, the thermal properties of these composites are highly dependent on thermal transport through the filler network and its contacts.  For metallic nanofillers, the thermal conductivity is often estimated using the Wiedemann-Franz law based on the electrical conductivity; however, the question remains as to whether the Wiedemann-Franz law still holds at nanoscale contacts.  Through investigation of silver nanowires of varying diameters, we were able to demonstrate that the Lorenz number for silver nanowire increases with decreasing nanowire diameter.  Examination of the corresponding electrical and thermal conductivities indicates that these changes are due to that the relative contribution of phonons becomes more significant as a result of elastic stiffening.  Furthermore, we show that for silver nanowires, the contact thermal resistance is as much as 8 times lower than that of multi-walled carbon nanotube (MWCNT) of similar diameters.      

            Additionally, through systematic studies of electrospun polymer-silver nanowire composite nanofibers, we investigate the impact of interface morphology on the thermal conductivity enhancement of the composite system and probe the value of the Kapitza resistance for individual polymer-filler interfaces.  For polymer nanofibers containing continuous, single silver nanowires, the thermal conductivity increases linearly with increasing volume fraction of silver, which is consistent with the prediction of percolation theory for samples above the percolation limit.  By comparing this linear trend to the measured thermal conductivities of composite nanofibers with more complex structures, we were able to determine the resistance associated with any additional polymer-filler boundaries.  In doing so we find that the thermal boundary resistance for polyvinylpyrrolidone (PVP)–silver interfaces is as much as an order of magnitude lower than that of comparable polymer-MWCNT composite systems.  Additionally, by comparing the vibrational density of states (VDOS) of silver to that of PVP, we find that there is significant overlap of available phonon modes below frequencies of 10 THz which support van der Waals modulated thermal transport.  Together our studies provide new insights into thermal transport in polymer nanocomposites and should help facilitate the design of high performance polymeric thermal interface materials. 


Understanding Thermal Transport in Polymer \u2013 Silver Nanowire Composites

Division / Sponsors: 
Aerospace, Bioengineering, Applied Mechanics, Design Engineering Division, Advanced Energy Systems, Manufacturing Engineering Division, Noise Control and Acoustics, Nondestructive Evaluation, Diagnosis, and Prognosis (NDPD), Nuclear Engineering Division, Process Industries Division, Fluids Engineering, Heat Transfer, Materials, Microelectromechanical Systems, Safety Engineering and Risk Analysis, Computers & Information in Engineering


Paper Proceedings: https://asmedigitalcollection.asme.org/IMECE/IMECE2020/volume/84478

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