Synthesis and thermal transport properties of tantalum nitride phases and composites

The metallic theta-phase tantalum nitride (θ-TaN) has been predicted to exhibit ultrahigh thermal conductivity (κ) due to a large energy gap between acoustic and optical phonon branches and a low electron density of states at the Fermi level. However, the reported room-temperature thermal conductivity (κ_RT) of polycrystalline θ-TaN synthesized via high-pressure and high-temperature (HPHT) phase transition from the commercial epsilon-phase tantalum nitride (ε-TaN) has so far remained below 100 W m^-1 K^-1. Increasing grain size can reduce the grain-boundary scattering, thereby enhancing κ_RT. Here, we report a new synthesis route that yields micrometer-sized θ-TaN grains and a bulk κ_RT exceeding 100 W m^-1 K^-1. Moreover, by tuning solvents and synthesis temperatures, ε-TaN, θ-TaN, and their composites can be directly obtained at ambient pressure, which enables scalable production of highly thermally conductive nitrides.