High thermal conductivity epoxy-silver composites based on self-constructed nanostructured metallic networks

We demonstrate epoxy-silver nanoparticle composites with high thermal conductivity kappa enabled by self-constructed nanostructured networks (SCNN) forming during the curing process at relatively low temperatures (150 degrees C). The networks formation mechanism involves agglomeration of the polyvinylpyrrolidone (PVP) coated nanoparticles, PVP removal, and sintering of the nanoparticles at suppressed temperatures induced by their small diameters (20-80 nm). Sintering and the SCNN formation are supported by differential scanning calorimetry and electron microscopy investigations. The formation of SCNN with high aspect ratio structures leads to enhancements in the measured thermal conductivity kappa of the composite by more than two orders of magnitude versus the pure epoxy. However, kappa enhancements are modest if microparticles (1.8-4.2 mu m) are employed instead of PVP coated nanoparticles. The kappa trends are qualitatively explained using a percolating threshold thermal conductivity model for the microcomposites. For the nanocomposites the measured kappa is similar to 14% of the upper limit value predicted by the Hashin and Shtrikman (H-S) theory for an ideally connected network, a measure of the non-ideal network inside the nanocomposites. (C) 2012 American Institute of Physics. [http://dx.doi.org/10.1063/1.4716179]