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Title: Development of advanced spectrally selective coating for the application of energy-efficient windows
Authors: Shen, Boxu
Degree: Ph.D.
Issue Date: 2022
Abstract: The building sector makes up 36 % of the terminal energy use and accounts for 28 % of energy-related carbon emission in 2019. Heating, ventilation, and air conditioning (HVAC) systems in buildings contribute to about half of the energy used in buildings. Buildings in Hong Kong receive abundant of sunlight throughout the year, and sunlight flowing through windows is the primary source of heat gain in buildings. The transmittance of traditional soda-lime window glass for visible and near-infrared light is about 75 % to 90 % for the popular 4 mm thickness, indicating that the glazing lacks the spectrally selective property. Accordingly, strong solar radiation increases the indoor air temperature of buildings, which leads to the increase of air-conditioning energy consumption, especially for regions with cooling-demand climate. Thus, the development of spectrally selective material for energy-efficient glazing has been one of the effective means to reduce electricity consumption of air-conditioning and corresponding CO2 emission. This thesis is to present my new contribution in this academic area. Although Low-E glass exhibits quite good spectrally selective performance, the extra cost of one more glass pane and encapsulating configuration, complex fabrication procedure with slow sputter rate and the use of noble metals make it hard to conduct window retrofit for existing buildings. Antimony-doped tin oxide nanocrystals (ATO) have been regarded as a promising alternative due to its low cost and good stability, which can effectively block the solar radiation of wavelength from 1500 nm to 2500 nm. One general trouble for the synthesis of ATO by the co-precipitation way is that nanoparticles often agglomerate micron-sized secondary particles during annealing process. In addition, the coating with the fillers of cesium doped hexagonal tungsten bronze nanocrystals (Cs-HTBs) has been proved that the Cs-HTBs-based film exhibited excellent near-infrared shielding property in the wavelength below 1500 nm. Specifically, the Cs-HTBs, which exhibit good spectral selectivity without the need for post-heating in a N2/H2 atmosphere, were frequently prepared via a one-pot approach employing WCl6 and CsOH as raw materials. However, due to their greater price, inconvenient storage, and environmental pollution, WCl6 and CsOH are not suitable for scaling up manufacturing. This thesis aims to develop a new type of spectrally selective coating with superior near-infrared shielding performance by in-situ polymerization spraying process based on the functional nanofillers of ATO and Cs-HTBs. Firstly, a novel dual-titration co-precipitation method was developed for the synthesis of the scalable preparation of low-agglomerated ATO with high near-infrared shielding property. Specifically, two separating funnels were employed to load the Sn4+/Sb3+ solution and NH3┬ĚH2O precipitant respectively. After that, the liquid in the two separatory funnels was simultaneously dropped into the flask, while the blades of the mixer in the flask were stirred. The method is conducive to synthesize weakly agglomerated ATO nanoparticles to prevent optical distortion. The effects of pH value of the reaction system on the antimony doping level and optical performance of the ATO nanocrystals were investigated. The size distribution and mean crystalline size were conducted by counting enough nanoparticles in the TEM images and using the Gaussian fitting function. The ATO nanoparticles prepared by the dual-titration method demonstrated lower agglomeration than the nanoparticles prepared by the traditional method, which is beneficial to reduce the optical scattering.
Secondly, a novel one-pot synthesis without post heat-treatment was developed for the scalable production of Cs-HTBs, using ammonium metatungstate hydrate (AMT) as the W source and cesium carbonate (Cs2CO3) as Cs source. The CsxWO3 nanocrystals were prepared by ethanol with tartaric acid of 1 mol/L and a certain amount of chloroplatinic acid through one-pot method. The reducing mechanism of the tartaric acid and the tartaric acid with the assistance of the chloroplatinic acid for the synthesis of CsxWO3 were studied. The relationships between the different reducing agents used in the synthetic process and the crystalline composition of the as-prepared nanocrystals were discussed and the possible synthesis mechanism was subsequently proposed. Besides, the impacts of charge carrier density and free carrier mobility on LSPR and small polaron transfer have been studied in length, as well as the method for boosting near-infrared shielding performance. The results indicate that tartaric acid with the presence of chloroplatinic acid could promote most W5+ ions in the hexagonal CsxWO3 nanocrystals. Chloroplatinic acid has been shown to improve tartaric acid reducibility. In addition, Li, F-codoped CsxWO3 nanocrystals have been successfully produced using a controlled one-pot approach for use in energy-efficient windows. which expands the spectral tunability of LSPR and small polaron transfer. The effects of Li and F on the phase composition, valence state distribution of W, optical properties and microstructure of the as-prepared products have been investigated. Finally, the new type of spectrally selective coating composed of ATO and Li, F-codoped CsxWO3 nanocrystals as functional additives was developed via in-situ polymerization. Compared with the sputtering process of traditional Low-E coating, the spraying preparation is flexible and easily performed by manual operation which can prevent the complicated conditions like vacuum environment, sophisticated pre-process for glass surface and limited glazing size for depositing. The employment of spraying fabrication provides a more effective way to modify the window glazing that has been installed. The established synthetic strategy is critical for increasing the scale of manufacturing of antimony-doped tin oxide and cesium tungsten bronze with superior near-infrared shielding performance and promoting its practical application in the field of energy-efficient glazing, which plays an important role for developing carbon neutral buildings in Hong Kong and other cities.
Subjects: Windows -- Thermal properties
Insulation (Heat)
Dwellings -- Energy conservation
Hong Kong Polytechnic University -- Dissertations
Pages: xxii, 161 pages : color illustrations
Appears in Collections:Thesis

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