Peak thermal conductivity measurements of bulk boron arsenide crystals and individual carbon nanotubes

High-thermal conductivity materials are useful for thermal management applications and fundamental studies of phonon transport. Conventional criteria suggests high thermal conductivity only exists in strongly bonded simple crystal structures of light elements, such as diamond, graphite, graphene, and carbon nanotubes (CNTs). In comparison, recent theories and experiments have shown zincblende boron Arsenide (BAs) as the first known semiconductor with a room-temperature thermal conductivity close to 1000 W m⁻¹ K⁻¹. The unusual high thermal conductivity is achieved via an unconventional route based on isotopically pure heavy atom and a large mass ratio of constituent atoms, the latter of which results in a large energy gap between the acoustic and option phonon polarizations and bunching of the acoustic phonon dispersions. These features in the phonon band structure limit three-phonon scattering and scattering by isotopic impurities. Past measurements of several ultrahigh thermal conductivity materials, including BAs bulk crystals, were not able to obtain the peak thermal conductivity, which is expected to appear at a low temperature and contains insight into the competition between extrinsic phonon-defect and phonon-boundary scattering with intrinsic phonon-phonon processes. Meanwhile, past thermal conductivity measurements of CNTs are subjected to errors caused by contact thermal resistance. The observed peak temperatures are much higher than those reported for bulk graphite. The results suggest that extrinsic phonon scattering mechanisms dominate intrinsic phonon-phonon scattering that is predicted to give rise to non-diffusive phonon transport phenomena including hydrodynamic, ballistic, and quantized phonon transport regimes. Here we report a peak thermal conductivity measurement method based on differential Wheatstone bridge measurements of the small temperature drop between two thin film resistance thermometers patterned directly on a bulk sample. With the use of a mesoscale silicon bar sample as the calibration standard, this method is able to obtain results that agree with past measurements of large bulk silicon crystals at high temperatures and first principles calculation results that accounts for additional phonon-boundary scattering in the sample. The agreement demonstrates the accuracy of this measurement method for peak thermal conductivity measurements of high-thermal conductivity materials. This method was employed to measure the peak thermal conductivity of several BAs crystals. In addition, a multi-probe thermal transport measurement method was used to determine both the contact thermal resistance and the intrinsic thermal conductance of different segments of the same individual multi-walled CNT samples simultaneously and directly. The differential thin film resistance thermometry method is expected to address the need of accurate peak thermal conductivity measurement methods and find use in the ongoing search of high-thermal conductivity materials for thermal management. The obtained peak thermal conductivity measurements of BAs can help to advance the understanding of phonon scatterings by phonons, boundaries, and defects in ultrahigh thermal conductivity materials. The thermal transport measurement of CNTs validates the multi-probe method for probing intrinsic thermal conductivity of nanostructures, and can provide an essential tool for further studying hydrodynamic, ballistic, and quantized phonon transport phenomena in high-quality CNTs and other low-dimensional structures.Mechanical Engineerin