UC Berkeley

Direct usage of thermal energy, instead of electricity, is intriguing to diminish total energy consumption with a benefit of reducing the number of energy conversions by directly utilizing such solar power and waste heat. Although various thermally driven systems have been suggested to overcome the problem of electricity-dependent systems, materials for the mechanisms limit the realization and effective operation of the systems. A more complete theoretical understanding of materials for thermal applications can suggest guidance to determine the bottlenecks of current materials and then ways to develop new materials. Among many computational methods, molecular dynamics (MD) simulations especially provide a molecular insight into structures of materials and phononic heat transfer at nanoscale.

In this dissertation, I discuss unconventional phase transition materials for thermal applications. Here, I focus on two types of phase transition materials. The lower critical solution temperature (LCST) liquid-liquid transition of aqueous ionic liquids is a thermally responsive phenomenon with phase separation. The phase separation can be used for forward osmosis desalination which directly utilizes a low-grade of thermal energy. By using MD simulations, I describe the local molecular environment during the transition and the driving force. The unusual trend of osmotic strength of the ionic liquids is explained with a suggested definition of an apparent free ion ratio. MD simulations are also used to study the amorphous phases and their phase transition of sugar alcohols in nanopores. The nano-encapsulation is proposed to shift the melting temperature and here I suggest a methodology to determine the phase transition temperature of encapsulated materials. This thesis also uses a theoretical approach to explore the thermal conductivity of the nanoconfined composites. Phonon frequency analysis verifies the enhancement of heat transfer by the infusion of phase change materials to the frameworks due to additional heat pathways between the nanoporous structures. Finally, I propose a practical design for a thermal rectifier that can be realized by phase change materials and derive a general theory for the design. The general theory indicates the possibility to obtain a high thermal rectification from the design and introduces the critical parameters for theoretical maximal performance.