Hydrothermal Preparation Of Tungsten Oxides And Its Application In Transparent Heat Insulation Coating

Reasonable regulation of the amount of solar energy into the building can reduce the energy consumption of air conditioning and decrease carbon dioxide emissions. Transparent solar heat-shielding materials can selectively sheild the near-infrared (NIR)/heat rays (wavelength of780-2500nm) radiation, while ensuring the day lighting and meeting the requirements of visual sense. They can be used in energy-saving building glass in the hot summer areas. Currently, the most widely used inorganic materials with transparent thermal insulation property are mainly transparent conductive oxides, such as antimony-doped tin oxide (ATO) and tin-doped indium oxide (ITO). Howerver, both of ATO and ITO contain toxic element, antimony and indium, respectively. Besides, the indium is a rare noble metal, thus is scarce in reserves. So the cost is expensive in pratical applicaiton. Tungsten trioxides (WO3-x)and tungsten bronze (MxWO3) which contain mixed valence tungsten ions possess an excellent transparent thermal insulation property. And their low-cost and nontoxic features make them a potential substitute for ATO and ITO materials.In this paper, tungsten oxides were chosen as research objective under the consideration of their transparent thermal insulation property. The influence of parameters (reaction temperature, dosage of reducing agent and different additives) on phase, microstructure and optical property of hydrothermally prepared tungsten oxides have been systematically addressed. The samples were characterized by X-ray diffracion, scanning electron microscope, transmission electron microscope, Fourier Transform Infrared spectoscopy and thermogravimetry. The optical property is tested by UV-Vis-NIR spectrometer. The main research results are listed as follows:(1) Through single factor experiments, effects of reaction temperature on phase phase, microstructure and optical property of hydrothermally prepared tungsten oxides have been studied. The results show that the sample prepared at100℃is hexagonal (NH4)xWO3-y (h-(NH4)xWO3-y) with a [001] growth direction (c axis). With higher reaction temperatures of140and180℃, the hexagonal (NH4)xWO3-y partially grow through oriented attachment and gradually transforms into orthorhombic WO3·0.33H2O (o-WO3·0.33H2O) grown along c axis, resulting in a mixed phase. The phases of crystallites seem to be strongly related with crystallite sizes; nano-and micro-sized crystallites adopt h-(NH4)xWO3-y and o-WO3·0.33H2O, respectively. Optical measurements indicate that h-(NH4)xWO3-y nano-rods exhibite a decent selective shielding ability (74.5%) of near infrared (NIR) light, while still maintaining a high luminous transmittance (67.6%). But with the temperature elevating, the transparent thermal insulation property of samples deteriorates. (2) Effects of the dosage of reducing agent on phase phase, microstructure and optical property of hydrothermally prepared tungsten oxides have been studied. Experimental results show that the dosage of reducing agent shows no significant effect on the phase and the morphology of the products. All of the prepared samples is h-(NH4)xWO3-y. Besides, high Zeta potential of nanocrystals plays a vital role in forming (NH4)xWO3-y nanorods. If the Zeta potential of sample is low, then the mutural repulsion of nanorods decreases, resulting in a poor dispersibility of the samples. In the range of0-3.1mg·mL-1, with the dosage of reducing agent increasing, the oxygen vacancy concentration of the products increases; however, if the dosage is increased to3.1mg·mL-1or more, the stability of the products becomes too bad to carry out optical experiments.