Effect of the gravitational force on electrical alignment of diamond filler particles in polydimethylsiloxane-based heat-conduction sheets

Heat-conduction sheets, a type of thermal interface material consisting of an elastomer matrix and thermally conducting fillers, require high thermal conductivity. Diamond particles with high thermal conductivity are a candidate for the filler in heat-conduction sheets. Electrical alignment is an effective method for filler alignment to achieve high thermal conductivity. In the alignment of filler particles in an elastomer matrix, the effect of the gravitational force is crucial and filler particles sedimented during polymerization lower the thermal conductivity. In this study, we investigated the effect of the gravitational force on the electrical alignment of diamond particles with diameters of tens of micrometers in a liquid polydimethylsiloxane matrix and their through-plane thermal conductivity. A system to alternate the gravitational direction to the samples was introduced, in which high-voltage electrodes were rotated during electrical alignment. We found that the gravitational force was related not only to particle sedimentation, but also to the tilt and thickness of the particle chains. By canceling the gravitational force, the particle chains formed ideally and the thermal conductivity of the heat-conduction sheets was enhanced. The thermal conductivity of the 60 wt% sample increased by a factor of ∼2.5 to 1.05 W m⁻¹ K⁻¹ by electrical alignment with rotating the electrodes. This gravity-cancellation method may expand the possibilities of the particle-alignment technique for the fabrication of composite materials.