Abnormally high thermal conductivity in fivefold twinned diamond nanowires

Fivefold twins (5FTs), discovered nearly 200 years ago, are a common multiply twinned structure that usually dramatically deteriorate the thermal transport properties of nanomaterials. Here, we report the anomalous thermal conductivity (kappa) in a novel fivefold twinned diamond nanowires (5FT-DNWs). The kappa of 5FT-DNWs is effectively enhanced by the defects of 5FT boundaries, and non-monotonically changes with the cross-sectional area (S). Above the critical S = 7.1 nm(2), 5FT-DNWs show a constant value of kappa, whereas below it, there appears a sharp increase in kappa with decreasing S. More importantly, 5FT-DNWs with minimal S show a superior kappa over the bulk diamond. By confirming the Normal-process-dominated scattering event, it is demonstrated that the phonon hydrodynamic behavior plays a determinative role in abnormally high kappa of 5FT-DNWs with small S. The super-transported phonon hydrodynamic phenomenon unveiled in the twinned diamond nanowires may provide a new route for pursuing highly thermally conductive nanomaterials.