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Influence of flexure and crowding on nanoscale thermal transport Bhardwaj, Aashish


Nanoscale thermal transport has been studied by scientists for decades. Low dimensional materials have shown two significant characteristics - (1) Thermal conductivity (κ) can be dependent on the size of the system, (2) A significant reduction in κ has been observed in an array-like arrangement. Thus, it is essential to understand the mechanism to tailor material properties for different applications. Fourier’s law is an empirical relation between average thermal flux and temperature gradient. It indicates κ is an intrinsic material property, but studies have shown that it breaks down in low-dimensional systems. The heat flux (J) depends on the size of the system (N) by the relation J ∝ N α−1. Traditionally, 1D studies have mostly focused on the effect of two-body interactions on κ. In this thesis, we study the effect of multibody interactions in the presence of two-body interactions on thermal transport. We use Nℓ (number of persistence lengths) to define system size and study the asymptotic limit of NJ. The transition from ballistic to superdiffusive behaviour was observed near 100 Nℓ in the ordered systems. In contrast, disordered systems showed only superdiffusive transport. Coherent wave patterns emerged as thermal carriers in superdiffusive regimes. Further, modelling crowding as transverse pinning, we observe a non-monotonous transition from superdiffusive to ballistic behaviour as we increased the crowding. While the single chain models have been extensively studied to understand the length dependence of κ, simulation studies on their bundles and forests are very few. One such example is the experimentally observed reduced heat conduction in carbon nanotube (CNT) forests compared to an isolated CNT. Here, the all-atom simulations require a significant computational expense. Therefore, we have used a coarse-grained model to study the heat flow in molecular forests by incorporating the concepts known from polymer physics and thermal transport to propose a generic picture of the reduction of κ. We show that a delicate balance between the bond orientations, the persistence length of an isolated Q1DM (Quasi-one dimensional material), and the non-bonded inter-chain interactions govern the reduction of κ.

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