At short length scales phonon transport is ballistic: the thermal resistance of semiconductors and insulators is quantized and length independent. At long length scales, on the other hand, transport is diffusive and resistance arises as a result of the scattering processes experienced by phonons. In many cases of interest, however, these two transport regimes coexist. Here we propose a first-principles approach to treat quasiballistic phonon transport where diffusive and ballistic phonons receive separate theoretical treatments.
Partitioning the overall phonon population for a given transport length is performed examining the mean free paths obtained from the solution of the Boltzmann transport equation and allowing only diffusive phonons to participate in anharmonic phonon-phonon scattering processes. We present results for Si and diamond, discussing the crossover from ballistic to diffusive transport as the length scale and/or the temperature increases and compute the relative contribution of ballistic and diffusive phonons to the thermal conductance in each transport condition.
Sustainable energy conversion & storage systems
Quasiballistic phonon transport from first principles
Pol Torres, Miquel Royo, Miquel López-Suárez, Junichiro Shiomi, and Riccardo Rurali
The discovery of novel high-performing materials such as non-fullerene acceptors and low band gap donor polymers underlines the steady increase of record efficiencies in organic solar cells witnessed during the past years. Nowadays, the resulting catalogue of organic photovoltaic materials is becoming unaffordably vast to be evaluated following classical experimentation methodologies: their requirements in terms of human workforce time and resources are prohibitively high, which slows momentum to the evolution of the organic photovoltaic technology.
Major research efforts are being carried out for the technological advancement to an energetically sustainable society. However, for the full commercial integration of electrochemical energy storage devices, not only materials with higher performance should be designed and manufactured but also more competitive production techniques need to be developed.
Recently synthesized hexagonal group IV materials are a promising platform to realize efficient light emission that is closely integrated with electronics. A high crystal quality is essential to assess the intrinsic electronic and optical properties of these materials unaffected by structural defects. Here, we identify a previously unknown partial planar defect in materials with a type I3 basal stacking fault and investigate its structural and electronic properties.
The advanced materials industry is one of the leading technology sectors worldwide. The development of such materials is at the core of the technological innovations and has been possible in the last century thanks to the transition from “observational” science to “control” science.
Transition metal carbides have gathered increasing attention in energy and electrochemistry applications, mainly due to their high structural and physicochemical properties. Their high refractory properties have made them an ideal candidate coating technology and more recently their electronic similarity to the platinum group has expanded their use to energy and catalysis. Here, we demonstrate that the nanostructuring and stoichiometry control of the highest melting point material to this date (Ta-Hf-C) results in outstanding electrochemical stability.