| Author: | Liu, Jin |
| Title: | A systematic study on energy properties of ionic thermoelectric materials during phase transition |
| Advisors: | Tao, Xiao-ming (SFT) |
| Degree: | Ph.D. |
| Year: | 2023 |
| Subject: | Thermoelectric materials Thermoelectric generators Semiconductors Electronics -- Materials Phase transformations (Statistical physics) Hong Kong Polytechnic University -- Dissertations |
| Department: | School of Fashion and Textiles |
| Pages: | xv, 210 pages : color illustrations |
| Language: | English |
| Abstract: | With the expanding growth of wearable electronics and Internet of Things, the efficient and environmental-friendly power sources are urgently needed. Thermoelectric generators (TEGs), which can harvest energy from waste heat and directly convert it into electricity, can be one of the possible solutions. Different from conventional TEGs made by electronic thermoelectric (TE) materials, ionic thermoelectric (i-TE) materials have been explored as candidates for TEGs with superior TE performances comparing with electronic TE counterparts at room temperature. However, simple i-TE systems made by salt aqueous solutions still possess unsatisfied performances for applications and the enhancement of output thermopower needs either complicated mechanisms or uncommon ion species which are difficult to synthesis. This thesis, based on the analysis about the ionic transportations in i-TE systems, systematically studies the energy properties (including thermopower (or ionic Seebeck coefficient), electrical conductivity, thermal conductivity and ionic figure of merit (ZTi)) of i-TE materials during phase transition and establishes an analytical model that can describe the ionic transportations in phase-transitional i-TE systems. Starting with the analysis and investigation about the effects of phase transition in four different types of i-TE materials (non-phase-transition, thermal sol-to-gel, thermal gel-to-sol and UV-induced sol-to-gel phase-transition), this thesis, for the first time, reports a discovery of a 6.5-fold (from 2.5 mV/K to 15.4 mV/K) and 23-fold (from less than 0.03 to around 0.68) increment of the thermopower and ionic figure of merit during the thermal-induced sol-to-gel phase transition in a poloxamer/LiCl aqueous i-TE material. A large drop of thermopower during the gel-to-sol phase transition in an agarose/LiCl system is also observed, which indicates the possibility of this method to modify the TE performances of the i-TE materials. In order to reveal the operational mechanism, this thesis provides a semi-quantitative model based on Onsager relations and Eastman entropy theory, which can cover the pre-, post- and during-transition stages. Based on the analysis model, six dimensionless influencing factors that may affect the change of thermopower during phase transition are probed and derived. The analysis result confirms that gigantic increment (larger than 1000 times) of the thermopower can possibly be achieved in the i-TE systems during phase transition. In addition, the theoretical analysis also speculates the universal applications of phase transition in i-TE materials, which means that the modifications of TE performances in i-TE systems can be achieved by phase transition regardless the type of causes for the sol-gel phase transition and the prediction is further confirmed by the rise of thermopower in epoxy/LiCl system during the phase-transition caused by UV irradiation. The phase-transitional i-TE systems share similar drawbacks with other thermodiffusive i-TE systems, which are the decay of output current with time when connected to the external load. This thesis, further investigates these limitations, provides possible solutions by using thermogalvanic effects (adding redox couples) or metal electrodes, and observes a continuous current output in the phase-transitional i-TE systems by using these methods. This thesis, for the first time, systemically studies the energy properties of i-TE materials during phase transition, points out a possible, novel and universal-applicable method to modify the TE performances of the i-TE systems, which may lead to a new perspective for future tunable i-TE devices for low-heat energy harvesting applications. |
| Rights: | All rights reserved |
| Access: | open access |
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