Composition and Morphology Effects on the Thermal Transport Mechanisms of Low Thermal Conductivity Crystalline Nanomaterials and Frameworks

The research detailed in this dissertation investigates the roles of composition and morphology on thermal transport mechanisms in low thermal conductivity functional materials for applications towards energy conversion (thermoelectric systems) and gas storage technologies. Specifically, I experimentally investigated the thermal transport processes of chalcogenide nanocomposite thermoelectric (TE) thin films, and pristine and infiltrated porous crystalline metal-organic frameworks (MOFs), with time-domain thermoreflectance (TDTR). I find that the roles of compositional and structural disorder in the form of defects, grain boundaries, and compositional boundaries in PbTe-PbSe nanostructured ALD composites are effective in reducing the phonon thermal conductivity, and represent a pathway for further improvement of the figure of merit (ZT) for achieving higher efficiency TE devices. While it is well known that disorder in non-porous, crystalline nanomaterials generally acts to reduce thermal transport, the role of compositional disorder, in the form of an adsorbate, within porous crystalline material remains an open question; and it is unclear whether the introduction of guest molecules into the pores helps or hinders heat transfer. To study this, I investigated the effect of adsorbates spanning a range of energy landscapes by varying both the phase (gas, liquid, solid) and guest-host bonding environment, on the thermal processes of two compositionally different, but morphologically similar MOFs (HKUST-1 and ZIF-8). The results show that the pore morphology is critical in defining the thermal transport mechanisms in MOFs. When the pores are small and rigid (HKUST-1), adsorbates drastically reduce the thermal conductivity through both extrinsic adsorbate/MOF collisions that reduce vibrational mode lifetimes and intrinsic changes to the vibrational structure which change the mode character. The degree of the effect of each mechanism is dictated by the adsorbate bonding environment. Further, larger, mechanically flexible pores (ZIF-8) serve to dampen the thermally inhibiting effects of the adsorbates, so that the overall thermal conductivity is neither enhanced nor reduced. These results are critically important for designing future MOF functional materials towards both gas storage and TE applications, where efficiency is closely tied to how thermal properties behave in the presence of adsorbates and effective medium theory is not an adequate approximation for predicting the thermal conductivity of the infiltrated system.