Investigation On The Cs_xWO₃ Based Transparent And Thermal Insulating Coatings

Tibet is located in the southwest border area of China,with annual sunshine time of about 2600?3400h and total annual solar radiation of about 6680?8400MJ/m~2,which is the most abundant area of solar energy resources.However,the temperature difference between day and night in Tibet is large,and a large amount of heat enters the room through the building windows during the day,while the heat radiates outward through the window glass at night,resulting in an uncomfortable environment where it is hot during the day and cold at night.For this reason,additional cooling and heating are needed,generating huge energy consumption.In order to reduce energy consumption,a transparent and thermal insulating coating used on building glass is particularly of importance.These properties will help to reduce the heat gain and undiminished luminosity in buildings,respectively.So far,many particles with transparent thermal insulation properties have been reported.However,these particles have some drawbacks that limit their commercial application in the field of energy-efficient materials to some extent.For example,the raw materials used to prepare ITO are expensive and the cut-off wavelength is higher than 1200 nm,which will result in low NIR shielding in the short wavelength region(780-1200 nm).In addition,the visible transparency of vanadium dioxide is still not suitable for room lighting.Tungsten oxide has attracted a lot of attention due to its excellent photoelectric properties,and tungsten oxide powder doped with cesium element has extremely strong absorption properties in the IR region,while the visible light transmission is high.Cesium-doped tungsten oxide-cesium tungsten bronze(Cs_xWO₃)is currently an inorganic nano-powder with the best absorption capacity in the near infrared due to its low resistivity and low-temperature superconductivity,making it a transparent heat insulator and a green energy-saving and environmentally friendly material with good prospects for blocking infrared,glass and automotive and other glazing windows.In this work,Cs_xWO₃ nanoparticles were prepared by using two different types of reaction precursors,tungsten hexachloride(WCl₆ as W source)and cesium hydroxide(Cs OH as Cs source)and tungsten hexachloride(WCl₆ as W source)and cesium carbonate(Cs₂CO₃ as Cs source),respectively.The simplest solvothermal reaction method was used in the experimental scheme,and samples with different properties were obtained by varying the process conditions such as temperature,precursor concentration,reaction pressure,and time.The samples were characterized and tested using X-ray diffractometer(XRD),scanning electron microscope(SEM),transmission electron microscope(TEM),X-ray photoelectron spectroscopy(XPS),and ultraviolet-visible near-infrared spectrometer(UV-Vis-NIR).After using 1.6 g/ml of WCl6 and 0.225 g/ml and 0.245 g/ml of cesium hydroxide and cesium carbonate,respectively,40 ml of ethanol as well as 10 ml of acetic acid solution were added,and Cs_xWO₃ nanoparticles with hexagonal crystal system structure were successfully synthesized at230°C.The lattice constants were obtained as:a=b=7.41?,c=7.60?and the crystallinity of the prepared materials was high.In the Cs-doped system,the best visible light transmittance and near-infrared light absorption were obtained when the atomic ratio of Cs to W reached 1:3.To further improve the visible transmittance and near-infrared absorption of Cs_xWO₃,Li-doped tungsten oxide(Li_xWO₃),and Li and Cs co-doped WO₃((Li,Cs)_xWO₃)were also prepared in this work.Li_xWO₃was successfully prepared by using Cs OH,WCl₆,lithium fluoride(Li F)and ethanolacetic acid as precursors.Li_xWO₃ showed significantly higher transmittance in the visible region compared with Cs_xWO₃,but relatively weaker absorption in the near-infrared region,especially in the 780 nm-1500 nm wavelength band.This provides a theoretical and experimental basis for the preparation of(Li,Cs)_xWO₃.The conditions of Cs_xWO₃ were fixed at a Cs/W atomic ratio of 1:3,and the content of Li was gradually increased.The experimental results showed that the prepared samples did have enhanced transmittance in the visible band when the concentration of Li F reached 2 mmol/ml,and the absorption in the near-infrared region was also significantly increased,reaching the maximum in the near-infrared absorption.However,the NIR absorption decreased when the content of Li F continued to increase,which indicated that the precursors remained in the samples after doping saturation,affecting the electrical properties of the samples and thus the IR absorption of the samples.By comparing the absorption values of(Li,Cs)_xWO₃,Cs_xWO₃ and WO₃,it is clear that the prepared Li and Cs co-doped WO₃ samples have significantly higher transmittance in the visible region and significantly higher absorption in the NIR region,which is a transparent and thermal insulating material with excellent performance..