High-precision thermal conductivity measurements as a probe of polymer/nanoparticle interfaces

We use the 3ω method to study the thermal conductivity of composites of nanoscale alumina particles in polymethylmethacrylate (PMMA) matrices. Effective medium theory and data for the changes in conductivity produced by low volume fractions of particle fillers are used to estimate the thermal conductance G of PMMA/alumina interfaces in the temperature range of 40<T<280 K. Near room temperature, G≈30±10 MW m−2 K−1 and the critical particle radius, r0=Λ0/G, is extremely small, r0≈7.5±2.5 nm. (Λ0=0.205 W m−1 K−1 is the conductivity of the PMMA matrix.) Therefore, high volume fractions of typical ceramic nanoparticles with r≫r0 can be used in thermal interface materials such as adhesives, pastes, and pads.