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Title: Investigation on NIR-shielding multifunctional heat insulation coating materials based on ATO for energy saving of buildings
Authors: Zhong, Hong
Degree: Ph.D.
Issue Date: 2019
Abstract: In Hong Kong, the extensive use of building glazing materials is further leading to high heat gains through building envelop and high cooling demand. Therefore, over 50% of building energy consumption in Hong Kong arises from the air-conditioning systems for temperature cooling. Glazing windows are typically the weakest part of a building's thermal envelope. Onto the glazing windows applying transparent thermal insulation coatings, which can strongly shield the near infrared (NIR) and UV radiation and thus largely reduce the solar heat gain through windows, has been widely explored and applied in buildings in the aspect of energy conservation. For example, low-E coating, which possesses high reflection of light, has been widely applied in buildings in the past years. However, the low-E coating tends to cause light pollution because of its visible light reflection, which nowadays has been prohibited in many countries. Therefore, substitutes with selective bandwidth of light wavelength have attracted a large amount of attention. This thesis newly presents a novel black TiO₂/ATO/CsxWO₃ triple-structured thin film for producing IR-blocking multifunctional heat insulation coating on glass with high transparency and excellent thermal blocking performance.
Firstly, a novel mixture of antimony doped tin oxide (ATO) and CsxWO₃ aqueous dispersions was obtained with controllable morphology, excellent optical properties and stability, through a controlled hydrothermal reaction and a series of optimization experiments. Generally, ATO, as a kind of transparent conductive oxides (TCO), shows optical property of high transmittance in visible light wavelength range and NIR absorption in the wavelength range of 1500 to 2500 nm. Meanwhile, the CsxWO₃ shows a high shielding performance in the NIR wavelength range of 800 to 1300 nm. Hence, a brand-new combination of ATO and CsxWO₃ coating in this thesis exhibits an excellent blocking characteristic in the whole NIR light wavelength range. However, the primary size of the as-prepared nanoparticles is more than 100 nm when using the traditional ceramic method, thus hindering the uniform dispersion of the nanoparticles in water and producing poor NIR shielding performance and visible light transmittance. In this study, nanoparticles of ATO and CsxWO₃ were successfully synthesized by controllable hydrothermal reaction with primary particle size of 5-10 nm. In addition, highly dispersive aqueous suspension was achieved by physical method with secondary particle size measured less than 60 nm. The blue transparent aqueous liquid was steady with no sedimentation after standing for 7 days. After mixed with film-forming resins, the final coating was obtained onto glass samples. The transparent and NIR shielding coating consisting of binary components of ATO and CsxWO₃ was synthesized by using the waterborne polyurethane (WPU) as the matrix. The transmittances of WTNS coated glass samples in visible and NIR regions are 70.6% and 14.2%, respectively. As the samples show excellent thermal insulation property and see-through capability, the nanomaterials can be applied in building glazing facades for reducing solar heat gain through building envelop and saving energy for cooling. Secondly, to furtherly enhance the ability of NIR light shielding, the black TiO₂ film mainly composed of self-doped TiO₂, was developed by controllable solvothermal technique and optimized electroreduction experiment. A portion of Ti⁴⁺ ions is reduced to Ti³⁺, and then the oxygen vacancies are passively protected. Next, the electroreduction conditions have been investigated and optimized. The black-TiO₂ film exhibits an outstanding super-hydrophilicity with water contact angle lower than 5° and organic-matter degradation under illumination of natural light. Additionally, the prepared black TiO₂ effectively blocks more than 38% of the NIR light. Finally, a novel TiO₂/ATO/CsxWO₃ tri-structured thin film was successfully fabricated in this study. Excitingly, it shows excellent NIR shielding performance and high visible transparency. The TiO₂ film acts as the external layer in the substrate glass with optimal thickness of 4 μm. Meanwhile, the ATO/CsxWO₃ film with an optimized thickness of 4.63 μm serves as the internal layer in the substrate glass. The visible transmittance and NIR absorbance of the entire thin films are 58.6% and 93.4%, respectively. Moreover, the tri-structured thin films also demonstrate prominent photocatalytic property, mirrored by a degradation test of 29.4% of methylene blue within 30 minutes. Hence, the tri-structured film shows a great potential in energy-efficient glazing windows.
Subjects: Hong Kong Polytechnic University -- Dissertations
Glass coatings
Buildings -- Energy conservation
Pages: xxvi, 198 pages : color illustrations
Appears in Collections:Thesis

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