Yufei Gao — Page 2 | Search arXiv

arXiv2021

Narrow-gap Semiconducting Superhard Amorphous Carbon with Superior Toughness

New carbon forms exhibiting extraordinary physico-chemical properties can be generated from nanostructured precursors under extreme pressure. Nevertheless, synthesis of such fascinating materials is often not well understood that results, as is the case of C60 precursor, in irreproducibility of the results and impeding further progress in the materials design. Here the semiconducting amorphous carbon having bandgaps of 0.1-0.3 eV and the advantages of isotropic superhardness and superior toughness over single-crystal diamond and inorganic glasses are produced from transformation of fullerene at high pressure and moderate temperatures. A systematic investigation of the structure and bonding evolution was carried out by using rich arsenal of complimentary characterization methods, which helps to build a model of the transformation that can be used in further high p,T synthesis of novel nanocarbon systems for advanced applications. The produced amorphous carbon materials have the potential of demanding optoelectronic applications that diamond and graphene cannot achieve

cond-mat.mtrl-sci↗

arXiv2020

The Origin of the Glass-like Thermal Conductivity in Crystalline Metal-Organic Frameworks

It is textbookly regarded that phonons, i.e., an energy quantum of propagating lattice waves, are the main heat carriers in perfect crystals. As a result, in many crystals, e.g., bulk silicon, the temperature-dependent thermal conductivity shows the classical 1/T relationship because of the dominant Umklapp phonon-phonon scattering in the systems. However, the thermal conductivity of many crystalline metal-organic frameworks is very low and shows no, a weakly negative and even a weakly positive temperature dependence (glass-like thermal conductivity). It has been in debate whether the thermal transport can be still described by phonons in metal-organic frameworks. Here, by studying two typical systems, i.e., crystal zeolitic imidazolate framework-4 (cZIF-4) and crystal zeolitic imidazolate framework-62 (c-ZIF62), we prove that the ultralow thermal conductivity in metal-organic frameworks is resulting from the strong phonon intrinsic structure scattering due to the large mass difference and the large cavity between Zn and N atoms. Our mean free path spectrum analysis shows that both propagating and non-propagating anharmonic vibrational modes exist in the systems, and contribute largely to the thermal conductivity. The corresponding weakly negative or positive temperature dependence of the thermal conductivity is stemming from the competition between the propagating and non-propagating anharmonic vibrational modes. Our study here provides a fundamental understanding of thermal transport in metal-organic frameworks and will guide the design of the thermal-related applications using metal-organic frameworks, e.g., inflammable gas storage, chemical catalysis, solar thermal conversion and so on.

cond-mat.mtrl-sci↗