Synthesis and characterization of high-surface area hexagonal boron nitride foam structures

Hexagonal boron nitride (h-BN) shares a similar layered crystal structure as graphite. Graphite is a semimetal with an ultrahigh thermal conductivity. In comparison, h-BN is a high-thermal conductivity electrical insulator, which is more suitable than metals for a number of applications in thermal management of electronic devices. Continuous and porous foam structures of both graphitic carbon and h-BN have been synthesized for thermal management and other applications. High-thermal conductivity and light-weight graphite foams have demonstrated superior thermal performance than both metal foams and van der Waals bonded networks of carbon nanotubes and graphitic flakes. However, the volume fraction and effective thermal conductivity of existing h-BN foam structures are still limited by the large pore size and limited specific surface area of the sacrificial reticular nickel foam templates used for chemical vapor deposition (CVD) of h-BN. This thesis reports on an investigation of the use of sacrificial sintered nickel powder templates with reduced pore sizes to increase the surface area of h-BN foam grown by atmospheric pressure CVD (APCVD) on the template. The volume fraction of the obtained h-BN foam is increased by a factor of 2.8 compared to a baseline h-BN foam grown on a commercial reticular nickel foam template. 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 Wm⁻¹K⁻¹ for the baseline structure to 0.51 ± 0.04 Wm⁻¹K⁻¹ for the structure with increased h-BN volume fraction. The room-temperature solid thermal conductivity of the h-BN strut is determined from the effective thermal conductivity via the Lemlich model to be 380 ± 80 Wm⁻¹K⁻¹, which is comparable to literature basal-plane values for h-BN crystals and compressed pellets. For a h-BN foam structure annealed at 700°C upon growth for 48 hrs, the solid thermal conductivity is 490 ± 120Wm⁻¹K⁻¹Mechanical Engineerin