Development of inorganic IR-blocking and heat-insulation self-cleaning materials based on hollow glass microspheres

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Development of inorganic IR-blocking and heat-insulation self-cleaning materials based on hollow glass microspheres


Author: Hu, Yan
Title: Development of inorganic IR-blocking and heat-insulation self-cleaning materials based on hollow glass microspheres
Degree: Ph.D.
Year: 2016
Subject: Glass -- Microstructure.
Insulation (Heat)
Building materials -- Thermal properties.
Hong Kong Polytechnic University -- Dissertations
Department: Dept. of Building Services Engineering
Pages: xxxi, 173 pages : color illustrations
Language: English
InnoPac Record:
Abstract: Hollow glass microspheres (HGM) form an inorganic material with a hollow structure. Because of the special structure, the HGM has many favorable properties such as excellent dispersion, high flow ability, low density and superior thermal insulation property. Therefore, the material could be widely used in many areas, such as aerospace, deep-sea exploration, hydrogen storage, etc. Especially, it can be used as a building heat conservation material because it is also not flammable. However, for radiant heat transfer, heat radiation can pass through the HGM to heat the building directly. The main objectives of this project are to study the fundamental mechanisms of the HGM material and to develop a novel insulation material for application in buildings. There has been little researches in this area. Among them, limited attention focused on enhancing the reflectivity of HGM with a coating of TiO₂. However, it was made mainly by the calcination. The product was not uniform. In this project, hydrothermal method was applied and the TiO₂ coated HGM with uniform coating was prepared. TiO₂, nevertheless, is not fully effective in blocking radiative heat transfer. Thus, ATO (antimony doped tin oxide) which shows excellent IR insulation property was also used. This novel method has barely been applied in research. ATO was firstly finely dispersed in an aqueous phase before coated on HGM. By this method, ATO makes a much more uniform HGM coating. In addition, since the IR-blocking property derives from the surface coating, the fouling of the surface must negatively affect the optical properties. Thus, self-cleaning technologies were studied so as to prevent the fouling of the surface. To adapt to different climates properties, both super-hydrophilic and super-hydrophobic were developed for the material. First of all, the TiO₂ coated HGM was developed using a novel soft chemistry method. Tetrabutyl titanate (TBOT) was selected as the Ti-source and transferred to anatase TiO₂ via hydrothermal method. XRD, SEM and EDS were conducted, which demonstrated that the TiO₂ was coated on the HGM surface. The coating did not much influence the thermal conductivity. It changed from the 0.0475 W/(m·K) of original HGM to 0.0546 W/(m·K) of TiO₂ coated HGM. It is reasonable that the thermal conductivity increases after the coating of TiO₂ because it can be considered that the wall thickness was increased. Then they were both bonded on the glass sheet for the Vis-NIR characterizations The thickness was 0.2 mm. Compared with the original HGM, the reflectivity of TiO₂ coated HGM increased by 19.6 %. The transmittance was still about 50 %. With this reflectivity and transmittance, part of the IR is reflected but the result was not quite successful enough. Thus, another material, antimony-doped tin oxide (ATO), was developed. Secondly, a novel dispersion method was proposed to finely disperse ATO nano particles in an aqueous phase before it was coated on HGM surface. It did not need any organic solution or dispersion agent. The whole process was environmental friendly. A series of experiments demonstrated the Cl⁻ and H⁺, which were mutually responsible for the fine dispersion of ATO particles in the aqueous phase. For those samples with excellent dispersion results, their Zeta potential values were all over 40 mV, which meant good dispersion stability. The TEM also demonstrated their primary sizes varied from 5~12 nm. The dispersion works because ATO particles first adsorb the Cl⁻ on the surface forming a negative charged colloid core. The H⁺ in the solution is then adsorbed on the core forming the stern layer and the slipping layer. In this way, the nano ATO particles become stable. With higher Cl⁻ and H⁺ concentration, the ATO aqueous dispersion was more stable.
Then, with the help of finely dispersed ATO nano particles, the original ATO coated HGM was developed. The coated HGM was bonded on glass sheet for the Vis-NIR tests. The thickness was also 0.2 mm. The reflectivity and transmittance from 400 to 2500 nm was measured, both relatively low. The transmittance at the band above 1500 nm was only about 3.8 % and about 85.7 % of incident light was absorbed by the ATO coating. This barely influenced the thermal conductivity which changed from the 0.0475 W/(m·K) of original HGM to 0.564 W/(m·K) for the ATO coated HGM. Even though part of the absorbed light may transfer to heat, because of the extremely low thermal conductivity of HGM, the heat cannot go through the HGM, thus avoiding any direct heating of the building. Additionally, self-cleaning materials (including super-hydrophilic material and super-hydrophobic material) were introduced on the coating. The super-hydrophobic TiO₂ coated HGM was developed via the novel one-step hydrothermal method, since the anatase TiO₂ already possessed the super-hydrophilicity. A super-hydrophobic agent was grafted on the surface. PFOTES (1H,1H,2H,2H-perfluorooctyltriethoxysilane) was selected as the super-hydrophobic agent. It was mixed with TBOT (Tetrabutyl titanate) and coated on HGM in one step. In the final products, the super-hydrophobicity was displayed (contact angle:153°, sliding angle: 16°). The thermal conductivity barely changed. It was 0.0543 W/(m·K) and that of TiO2 coated HGM was 0.0546 W/(m·K). Following that, super-hydrophilic ATO/TiO₂ composite nano particles were developed by choosing a novel self-dispersed anatase TiO₂ as the dispersion agent of ATO. TEM images showed that the primary size of those particles was about 5~12 nm and for the original ATO particles, the primary particle size was about 2 μm. The Zeta potential of dispersed samples ranged from 37.01 to 41.76 mV, but for the original ATO particles, it was merely 6.7 mV. Vis-NIR transmittance was an important parameter for the transparent IR insulation property. It demonstrated that with increasing ATO concentration, the IR blocking property increases too. When concentration was 83.33 % wt., the transmittance in the IR band was only 1 %. All of the results reflected the excellent dispersity and IR insulation property of the as-synthesized ATO particles. In addition, the TiO2 not only helped to disperse the ATO particles, but also provided the super-hydrophilicity. The samples with best IR blocking property was coated on HGM. The contact angle was only 3° and the coated HGM was bonded on the glass sheet. The thickness was 0.2 mm. The transmittance was slightly higher (6.8 %) than the ATO coated HGM at the band above 1500 nm. The reason was the using of TiO₂ decreased the concentration of ATO. However, the super-hydrophilicity was provided. The thermal conductivity (0.553 W/(m·K)) barely changed if compared with the ATO coated HGM (0.564 W/(m·K)). Finally, a novel super-hydrophobic ATO coated HGM was developed by the PFOTES modification. The contact angle was 153.4° and the sliding angle was 15°. The IR transmittance and thermal conductivity remained unchanged compared with the ATO coated HGM. Above all, this project lead to the development of a series of novel inorganic building heat conservation materials based on HGM. Such materials not only block heat conduction but also cut the radiative thermal transfer. The project is novel with little previous research. Further, self-cleaning properties was also developed creatively for those materials. Both super-hydrophilic and super-hydrophobic properties were studied in relation to different climates. In this way, the coating is protected from fouling and the its useful life prolonged.

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