Abstract

Vertically-aligned metal nanowire (NW) arrays can be effective thermal interface materials owing to the combination of high thermal conductivity and mechanical compliance. In the present work, copper NW arrays are synthesized via template-assisted electrodeposition, where a sacrificial porous membrane is used to mask a substrate while copper is electrochemically deposited into the pores. Both polycarbonate track-etched membranes and anodized aluminum oxide membranes are commonly used as templates due to their large areas and high degree of aligned pores with tunable properties without the need for lithographic processing. Using this method, NWs are fabricated with diameters ranging from 50-1000 nm and lengths up to 30 microns. The constituent NWs are synthesized to be nominally vertically-aligned and uniform over centimeter-sized length scales. The effective thermal conductivity of the NW array is measured using the 3-omega method, which is implemented in the present work by synthesizing the NW array directly above an electrically-isolated, microfabricated metal heater/thermometer. We report a measured cross-plane (i.e. parallel to the NWs) effective thermal conductivity of the array to be keff = 37 W m-1 K-1 for 12% dense copper NWs. These arrays are highly anisotropic due to the preferential vertical-alignment of the NWs, where cross-plane conduction is primarily facilitated by axial conduction along the NWs while in-plane conduction is predominately limited by interactions between adjacent NWs. Consequently, the lateral (i.e. perpendicular to the NWs) thermal conductivity is found to be nearly an order of magnitude smaller than the cross-plane conductivity. The effective thermal conductivity of a NW array is then correlated to both the morphology of the array and the thermal conductivity of the individual NWs, which includes contributions due to grain boundaries and size effects. This becomes particularly interesting as the NW diameters approach the room temperature mean free path of the conduction electrons in the bulk material (~40 nm in copper). Further consideration is then given to optimizing the transport properties of metal NW arrays for thermal interface applications, including the effects of array geometry, bulk metal conductivity, and interstitial matrix material. These results are compared to effective medium theory models to predict effective transport properties in composite media based on the individual properties of the constituent phases.