| Author: | Li, Ziqi |
| Title: | Electrically responsive emissivity regulating materials |
| Advisors: | Chen, Wei (SFT) |
| Degree: | Ph.D. |
| Year: | 2023 |
| Subject: | Emissivity Materials Hong Kong Polytechnic University -- Dissertations |
| Department: | School of Fashion and Textiles |
| Pages: | xxviii, 164 pages : color illustrations |
| Language: | English |
| Abstract: | Thermal regulating materials are attracting more and more attention because of their numerous applications in thermal camouflage, radiative cooling, personal thermal management, invisible communications, and food packaging. According to Stefan Boltzmann's law, thermal regulating materials primarily consist of temperature regulating materials and emissivity controlling materials. Considering the inconvenience and inapplicability of temperature-regulated materials, emissivity-regulated materials offer an intrinsic solution for thermal regulation. Among multiple stimulus-responsive materials, electrically responsive emissivity regulating materials are of great potential because of the possibility of controllable modulation, fast response, lightweight, and flexibility. However, challenges still remain in in-depth studying the mechanism, realizing precise control, scalable production, enhanced regulating performance, and finding new materials. To address the challenges above, the thesis concentrates on researching the electrically responsive emissivity regulating materials through structural design, mechanism investigation, production technology, and materials development. The whole research was studied through three experiments. Firstly, dynamic emissivity under an electrical field was realized based on graphene with a multilayer structure. Parameter optimization was conducted, and an emissivity modulator was fabricated with the optimized parameters of a graphene working electrode film with ~100 layers, a nanoPE separator containing ionic liquid 1-ethyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide, and Au counter electrode with a thickness of 60 nm. A linear emissivity regulation and a thickness variation of the device were found. By analyzing the electrical, electro-optical, electromechanical, and electrochemical properties, the nature of the device was identified as a capacitor. In addition, the correlation of dynamic emissivity, thickness variation, and ions' motion was determined, with three stages of ions' movements including dispersion, intercalation, and double layer capacitance formation. The multilayer graphene-based electrochemical device was utilized as a smart surface with dual functions of thermal regulation and actuation. Subsequently, a scalable production strategy of van der Waals graphene film without binder and solution assistance at room temperature was proposed. The film is highly electrically conductive, flexible, and highly deformed. It was further assembled into a thermal modulator with a modulation depth of 0.5, an ultra-low limit of 0.1, and 75% radiative heat flux suppression. Significant thermal regulation performance was determined through an in-situ XRD characterization, finding that the ions intercalated into graphene interlayers. Furthermore, the thermal regulator exhibited a fast response time of ~7s and excellent stability with 90% modulation retention in 300 cycles. The new preparation strategy of van der Waals graphene film proposed a solution for facile, clean, low-cost, and scalable production of large-area graphene films and a candidate for thermal regulators. Lastly, graphdiyne, the allotrope of graphene, was synthesized with controllable morphologies and utilized as an emissivity regulating material. The intrinsically integrated emissivity of graphdiyne was determined, and graphdiyne composited with graphene was utilized as the electrode to fabricate an electrochemical emissivity modulator. Interestingly, the graphdiyne-based modulator demonstrated a unique capacity to modulate emissivity under negative voltage, unlike pure graphene and other materials, which can only regulate emissivity under positive voltage. To explore the dynamic control mechanism of graphdiyne, an in-situ Raman characterization setup with a simulative cell was designed, which proposed a facile mechanism investigation method and verified the alkyne-to-alkene transition in graphdiyne. Alkyne bonds transform into alkene bonds by coordinating with 1-Ethyl-3-methylimidazolium cations in the presence of an electrical field, resulting in an energy level change and a change in the optical properties of graphdiyne. The discoveries may open the way for applying graphdiyne in thermal radiation control and studying molecular-scale active materials in electro-optical modulation. In summary, this thesis studied the device structure, working principle, fabrication techniques, and materials innovations for electrically responsive emissivity regulation. The precise, fast, and bi-directional control of emissivity is realized. Besides, fabrications techniques of facile, clean, large-area and high-performance emissivity regulating films are obtained. Also, new material and mechanism with molecular scale electroactivity are found to be applicable in emissivity regulation. This study paves the way for electrochemical devices and electroactive materials to be used for emissivity regulation. In principle, the structure design, materials' selecting rule, assembling techniques, and regulating method of the electrically responsive emissivity devices are applicable to other emissivity regulators. Thus, the research presented is expected to play an impact on the electrical responsive thermal regulators. |
| Rights: | All rights reserved |
| Access: | open access |
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