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Title:Thermal conductivity switching of polymers and lithium-ion battery electrode materials in response to external stimuli
Author(s):Shin, Jungwoo
Director of Research:Braun, Paul V.
Doctoral Committee Chair(s):Cahill, David G.
Doctoral Committee Member(s):Chen, Qian; Ewoldt, Randy H.
Department / Program:Materials Science & Engineerng
Discipline:Materials Science & Engr
Degree Granting Institution:University of Illinois at Urbana-Champaign
Subject(s):Thermal conductivity
Thermal switch
Thermal switching material
Time-domain thermoreflectance
Li-ion battery
Liquid crystal
Abstract:Advances in technologies have led to a utilization of a wide range of transport, storage and release of energy as a form of heat. Researchers in electronics, thermoelectrics, energy science and biomaterials seek for materials with either high or low thermal conductivity as they help to manage heat in the device and the system. However, traditional materials can only provide thermal functionality in one way—thermal conductor or insulator—to prevent or facilitate heat transport. Therefore, a discovery of materials that have a bifunctional thermal transport property in which materials can switch between more than one thermal conductivity state (high and low states) can provide a desirable thermal transport property on-demand. Although there have been attempts to control thermal conductivity of materials using external stimuli, many have failed to show a high and fast thermal property switching of material. In this dissertation, I present novel stimuli-responsive polymers and Li-ion battery electrode materials that involve a wide range of thermal conductivity modulation in response to external stimuli. I describe thermal conductivity switching between high and low states of these thermal switching materials (TSMs) with structural changes in electronic, atomic and/or molecular levels that are associated with the fundamental thermal transport property. First, stimuli-responsive liquid crystal networks (LCNs) and light-sensitive azobenzene polymers (azopolymers) were designed and synthesized. Thermal conductivity and macromolecular structure transitions of LCNs and azopolymers were studied by in-situ time-domain thermoreflectance (TDTR) and in-situ synchrotron X-ray scattering techniques under external stimuli. I observed thermal conductivity switching contrast r = Λhigh/Λlow ~ 1.5–3.5 on a switching time of τ ~ 10 s – 10 min for LCNs and azopolymers. This thermal conductivity switching resulted from an alignment of the backbone and side-chain mesogen groups with applied magnetic and electromagnetic fields in LCNs and azopolymers. Then, thermal conductivity and elastic modulus changes of metal and metal oxides Li-ion electrode materials were studied during electrochemical reaction with Li+ ions. Five electrode materials with three electrochemical phase transition mechanisms with Li+ ions were chosen: Fe2O3 and NiO as conversion reaction systems, V2O5 and TiO2 as intercalation systems and Sb as an alloying reaction system. These electrode materials involve characteristic lattice and electronic band structure transitions with respect to their phase transition mechanisms. For conversion materials, 200~300% volume changes were observed due to the large amount of Li+ ion intake. This large volume changes were associated with an irreversible decrease in thermal conductivity r ~ 2.5–7.5 due to the lattice disordering by Li+ ion. Intercalating V2O5 and TiO2 showed reversible r ~ 1.6–1.8 along with small volume changes and stable crystal structure up to 1 mole of Li+ (x ≤ 1) intake. Alloying Sb showed the largest volume change of ~300% up to 3 moles of Li+ intake in the lithiated state with the semimetal (Sb) to semiconductor (Li3Sb) transition. The large lattice and electronic transition resulted in the largest value r ~ 30.
Issue Date:2019-03-07
Rights Information:Copyright 2019 Jungwoo Shin
Date Available in IDEALS:2019-08-23
Date Deposited:2019-05

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