SEM image of (a) the microcomposite and (b) the nanocomposite. While in the microcomposite the silver fillers are not connected, a network structure is revealed in the nanocomposite.
TEM micrographs of (a) 20 nm and (b) 80 nm diameter 35 vol. % silver-epoxy nanocomposites after processing showing the self-constructed networks. Necking due to sintering (shown with blue arrows) appeared between nanoparticles after 150 °C composite processing. Pathways of connected nanorods are shown by white dashed lines.
DSC of silver nanoparticles and microparticles reveals low temperatures sintering peaks only for nanoparticles. The melting point of the nanoparticles is reduced to around ∼410 °C and ∼450 °C for 20 nm diameter 80 nm silver particles, respectively. The microparticles did not show sintering/meltingpeaks in the tested range of temperatures.
The mechanism for the formation of the self-constructed metallic network in the nanocomposite and its synergy with its flow and thermal properties: (a) high κ nanoparticles functionalized with PVP and dispersed in uncured epoxy form a high solids volume fraction slurry able to flow and fill the roughness of two mating surfaces; (b) the molecular coating is removed by low temperature processing and the metallic nanoparticles start to agglomerate and sinter into high κ networks of nanorods; (c) the high κ network at the end of the process with the cured polymer matrix providing compliant mechanical support.
Total thermal resistivity vs. sample thickness for 35 vol. % silver/epoxy composites: (a) with 4.2 μm silver fillers and (b) with 20 nm PVP coated silver fillers. The thermal conductivity is the inverse slope of the curve. The intersection of these lines with the ordinate axis (zero thickness) yields rint for the setup for the specific sample type.
Measured and predicted thermal conductivity vs. particle volume fraction for: (a) microcomposites; (b) nanocomposites. The solid lines represent the effective κ predictions of percolation threshold model for microcomposites and ∼14% of the maximum L-H theoretical limit for the ideal network structure. The dots are the experimentally measured thermal conductivities (For the nanocomposite the error bars are typically smaller than the dots). The nanocomposites have up to 50 fold larger thermal conductivities than the microcomposites.
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