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

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. (C) 2021 Elsevier Ltd. All rights reserved.

Automated summary

Recent experiments indicate that the thermal conductivity of amorphous metal-organic frameworks can be higher than their crystalline counterparts due to higher atomic number density. The study shows that amorphous ZIF-4 has longer mean free path vibrations and increased volumetric heat capacity compared to crystalline ZIF-4, leading to a potentially 1.7 times higher thermal conductivity. Both amorphous and crystalline ZIF-4 exhibit weak temperature-dependent thermal conductivity, attributed to different structural characteristics and heat carrier properties.