Synthesis and thermal transport properties of high-surface area hexagonal boron nitride foam structures

Abstract

Continuous and porous foam structures of thermally conducting and electrically insulating hexagonal boron nitride (h-BN) have been synthesized for heat dissipation and other applications. However, the volume fraction and effective thermal conductivity of existing h-BN foam structures are still limited by the large pore size and small specific surface area of the sacrificial reticular nickel foam templates used for chemical vapor deposition (CVD) of h-BN. Here we report experiments for increasing the volume fraction and effective thermal conductivity of CVD h-BN foams with the use of sintered nickel powder templates with reduced pore sizes. The volume fraction of the obtained high-surface area h-BN foam sample HS1 reaches 0.34 ± 0.06 %, which is an increase by a factor of 2.8 compared to a baseline h-BN foam grown on a reticulated nickel foam with the same growth time. Increasing the growth time by a factor of 3 results in a further increase of the volume fraction by an additional factor of 2 for sample HS2. With poly(methyl methacrylate) (PMMA) filled into the pore space of the h-BN foam, the room-temperature effective thermal conductivity of the composite increases from 0.31 ± 0.02 W m−1 K−1 for the baseline structure to 0.70 ± 0.05 W m−1 K−1 for HS2. Based on numerical analysis of the measured effective thermal conductivity, the solid thermal conductivity of the h-BN struts of the baseline structure is 580 ± 150 W m−1 K−1, which is comparable to a prior first principles calculation. The increases in the volume fraction and effective thermal conductivity are accompanied by a reduction of the h-BN solid thermal conductivity to 410 ± 80 W m−1 K−1 for HS1 and 250 ± 30 W m−1 K−1 for HS2 due to increased defect concentrations and surface curvature. The numerical simulation reveals that the thermal interface resistance between h-BN and PMMA plays only a small role on the effective thermal conductivity due to the dominant thermal transport contribution from the continuous high-thermal conductivity struts.