Date of Award
Master of Science in Mechanical Engineering (MSME)
Mechanical, Industrial and Systems Engineering
The horizon in material science has expanded drastically by the prospect of stacking two dimensional (2D) materials into vertical stacks. One such emerging class of materials are 2D covalent organic frameworks (COFs), which have already established themselves as game-changing porous polymeric crystals due to their exceptional physical properties such as high porosities and large surface areas with additional flexibility of precisely controlling the functional building blocks. These unique properties have made COFs ideal candidates for a plethora of applications, such as gas separation, adsorption and storage, drug delivery, and as solid-state electrolytes for upcoming energy storage technologies. However, the excessive heat generation due to the adsorption of guest species can severely limit their suitability in aforementioned applications. Therefore, it is crucial to find innovative ways to modulate their thermal properties by fully comprehending the essential characteristics that control the effectiveness of energy transfer efficacies in these materials for successful implementation of COFs in practical devices.
In this work, we carry out systematic atomistic simulations to study the influence of pore geometry of the 2D COFs on their anisotropic heat transfer mechanisms and to answer the long-standing debate on whether the guest molecules increase or decrease their overall thermal conductivity. We show that the thermal conductivity can either increase due to additional heat channels introduced by the gas adsorbates or decrease from enhanced phonon scattering resulting from the interactions between the gas and the solid pore walls of the COFs. More specifically, we conduct atomistic simulations on pristine and gas adsorbed COF structures with different pore sizes and reveal that for COFs with relatively larger pore sizes (≳2 nm), despite the reduction of the thermal conductivity of the solid framework due to solid-gas scattering, the increase in contribution to the overall thermal conductivity by the gas adsorbates (methane) in the 1D channel can dramatically enhance the cross-plane thermal conductivity. On the other hand, for COFs with= relatively smaller pores (≲2 nm), the increase in solid-gas scattering results in a monotonic reduction in the cross-plane and in-plane thermal conductivity. We at- tribute the increase in cross-plane thermal conductivity of the larger-pore COFs to the elevated heat conduction by the lower-frequency vibrations (≲0.5 THz) compared to the total frequency range of the gas molecules confined in the larger pore. However, due to substantial broadening of the vibrational modes to higher frequencies, these low frequency vibrations of the gas adsorbates do not contribute to the heat conduction in small-pore COFs. Through our results we reveal the complex relationship between the pore geometry, diffusivity of confined gas molecules and solid-gas interactions, which dictate the tunable thermal conductivities of these porous crystals. Additionally, our results shed light on the inherent nature of gas diffusion and heat transfer in porous organic frameworks, and providing a new path forward to tune the thermal conductivity of 2D COFs by modulating the gas diffusion inside the pores. Our findings can have major implications in designing 2D polymeric crystals with desired thermal conductivities for efficient gas storage, separation, and catalysis applications.
Rahman, Muhammad Akif, "STUDY OF ANISOTROPIC THERMAL CONDUCTIVITY OF GAS ADSORBED TWO DIMENSIONAL COVALENT ORGANIC FRAMEWORKS" (2023). Open Access Master's Theses. Paper 2352.