A graphitic structure was synthesized by catalyst-free chemical vapor deposition on anodized aluminum oxide (AAO) templates using acetylene as the carbon source at a temperature of 620 °C. The AAO template was removed by chemical etching, which yielded a three-dimensional structure featuring planar layers seamlessly joined together by nanotube pillars via continuous carbon-carbon bonding. Raman and transmission electron spectroscopy measurements reveal that the deposited carbon is nanocrystalline graphite with a thickness of about 10 nm. Carbon nanotubes were isolated from the three-dimensional nano-pillar graphitic structure and measured with a thermal four-probe method to obtain the intrinsic thermal conductance. Discrete modulated heating and Fourier transform analysis were used to improve the signal to noise ratio of the thermal measurement of the low-conductance nanostructure. The measured thermal conductivity of the nanotube wall increased with increasing temperature and was 3.9 ± 0.3 Wm−1K−1 at room temperature. Both the temperature dependence and the magnitude are consistent with the nanocrystalline graphitic structure.
Conventional theory predicts that ultrahigh lattice thermal conductivity can only occur in crystals composed of strongly bonded light elements, and that it is limited by anharmonic three-phonon processes. We report experimental evidence that departs from these long-held criteria. We measured a local room-temperature thermal conductivity exceeding 1000 watts per meter-kelvin and an average bulk value reaching 900 watts per meter-kelvin in bulk boron arsenide (BAs) crystals, where boron and arsenic are light and heavy elements, respectively. The high values are consistent with a proposal for phonon-band engineering and can only be explained by higher-order phonon processes. These findings yield insight into the physics of heat conduction in solids and show BAs to be the only known semiconductor with ultrahigh thermal conductivity.