Vibrational modes with long mean free path and large volumetric heat capacity drive higher thermal conductivity in amorphous zeolitic imidazolate Framework-4

Abstract

The thermal conductivity of amorphous structure is generally thought to be lower than that of its crystalline counterpart. However, recent experiments show the thermal conductivity of amorphous metal-organic frameworks can be higher than that of their crystalline counterparts, which is explained by the higher atomic number density in the amorphous metal-organic frameworks. To tune the thermal transport properties of the metal-organic frameworks through the amorphous structures, it is necessary to understand the scattering picture among the heat carriers in metal-organic frameworks. Here, by quantitively characterizing the heat carriers based on the mean free path in the example systems, i.e., amorphous ZIF-4 and crystalline ZIF-4, we find that there are longer mean free path vibrations existing in amorphous ZIF-4 comparing to that of crystalline ZIF-4. At the same time, the density or equivalently the volumetric heat capacity of amorphous ZIF-4 is found to be increased around 50 % comparing to that of the crystalline ZIF-4. As a result, the thermal conductivity of amorphous ZIF-4 can be 1.7 times higher than that of its crystalline counterpart. Meanwhile, both amorphous and crystalline ZIF-4s show weak temperature dependent thermal conductivity, which is resulting from the local structural discontinuity for crystalline ZIF-4 and intrinsic harmonicity of the heat carriers in amorphous ZIF-4. Our study provides a fundamental understanding of thermal transport in ZIF-4, and suggests the amorphous ZIF-4 may have advantages in its thermal-related applications, e.g., inflammable gas storage, chemical catalysis and solar thermal conversion and so on.