Synthesis And Properties Of Zinc Selenosulfide-based Color Pigments And Coating Composites With High Near-Infrared Reflectance

Various inorganic pigments with a high solar reflectivity are developed as‘cool materials'due to the emergence of urban heat island effects and the increased energy consumption for cooling interior environment.Some color tunable inorganic pigments with a high near-infrared?NIR?reflectance have promising applications as efficient solar-reflective paints and coatings due to their heat-reflective properties and various controllable color hues.In this thesis,the synthesis and properties of zinc selenosulfide(i.e.,ZnS_xSe_1-x)-based color pigments and coating composites with a high NIR reflectance were investigated.The main research contents and the obtained results are given as follows:?1?A series of ZnS_xSe_1-x-based pigments with various colors and tunable bandgaps were synthesized via co-precipitation reaction and subsequent calcination,i.e.,ZnS_x Se_1-x and Cu-/In-co-doped ZnS_xSe_1-x(ZnS_xSe_1-x:Cu/In)pigments.The color and optical properties of ZnS_xSe_1-x-based pigments were analyzed based on the Kubelka-Munk model.The results show that the color of ZnS_xSe_1-x-based pigments can vary at different[S]/?[S]+[Se]?anion compositions or Cu-/In-doping ratios due to the change of the tunable band-gaps range from 3.21 eV to 2.13 eV.The absorption edge of as-synthesized pigments continuously shifts to a longer wavelength?i.e.,394-615 nm?when Se content or Cu-/In-doping ratio increases.The color of ZnS_xSe_1-x-based pigments varies from white to yellow,bright yellow to orange,red orange to red,and then reddish brown.The band structure and electronic property of ZnS_xSe_1-x-x were investigated by Vegard's law and the first-principle calculation based on the density function theory?DFT?.The calculated results show that ZnSe is compatible with ZnS and ZnS_xSe_1-x as direct band gap semiconductors.Based on the Hirshfeld analysis,the contributions of Se's and S's electrons on the valence and conduction bands of ZnS_xSe_1-x are nearly equivalent.The similarity of S and Se thus makes the band gaps of ZnS_xSe_1-x-x tunable,and the color and optical properties of ZnS_xSe_1-x-based pigments can be adjusted at different anion compositions x or Cu-/In-doping ratios.The pigment with a yellow color exhibits the maximum NIR solar reflectance?i.e.,R^*=86.65%?in a wavelength range of 700-2500 nm.It is also indicated that the surface temperature on aluminum plate with a solar reflective coating colored by the as-synthesized pigments with yellow and red colors can be reduced by 4.2-7.9?.?2?To improve the NIR solar reflectance of ZnS_x Se_1-x-based pigments with various colors,HGMs@ZnS_xSe_1-x or ZnS_xSe_1-x:Cu/In composite pigments with a litchis-like core-shell structure were prepared with HGMs dispersed and surface-corroded in an alkali-solution under ultrasound and ZnS_xSe_1-x or ZnS_xSe_1-x:Cu/In particles deposited on HGMs surface via in-situ co-precipitation and subsequent sintering.The results show that ultrasound can promote the corrosion of HGMs surface in alkali solution and form a mesh-pore structure,thus providing some heterogeneous nucleation sites on HGMs surface and favoring the nucleation of pigment precursors on HGMs surface in subsequent in-situ co-precipitation reaction.The mechanism of enhanced NIR reflection of composite pigments were discussed.The results show that the retro-reflection of HGMs core layer,the constructive or destructive interferences of HGMs/ZnS_xSe_1-x interface,and the grain boundary scattering effect of ZnS_xSe_1-x nanoparticles synergistically enhance the NIR reflectance and thermal insulation effect of HGMs@ZnS_xSe_1-x-based composite pigments.The NIR solar reflectance of HGMs@ZnS_x Se_1-x or ZnS_xSe_1-x:Cu/In composite pigments is 90.61%or 73.13%.The surface temperature of aluminum plate coated with HGMs@ZnS_xSe_1-x composite yellow pigment is 3.4?or 9.6?lower than that of aluminum plate uncoated or coated with ZnS_xSe_1-x pigment.This approach may shed a light on a promising composite structure design for high-performance solar reflective‘cool materials'.?3?Low-temperature sol-gel synthesis of zircon?i.e.,ZrSiO₄?with respect to soft MC activation effect was investigated.The reaction mechanism and the kinetics of ZrSiO₄ synthesis were discussed.The results show that soft MC activation can accelerate the dehydration/dehydroxylation reactions,reduce the bonding degree of Si-O bonds,and break a certain extent of[SiO₄]^4-tetrahedral three-dimensional network that is formed in the sol-gel reaction,thus enhancing the reactivity of precursors and facilitating the formation of ZrSiO₄from Zr⁴⁺ions with[SiO₄]^4-tetrahedra during the subsequent thermal treatment at a lower temperature?i.e.,700??.Meanwhile,to improve the thermal stability of ZnS_xSe_1-x-based pigments,zircon-based in-situ encapsulated pigments(i.e.,ZnS_xSe_1-x@ZrSiO₄ and ZnS_xSe_1-x:Cu/In)were prepared with the assistance of soft mechano-chemical?MC?activation of ZrSiO₄precursors.The crystal formation of ZrSiO₄ precursors pretreated by soft MC activation occurs simultaneously with the synthesis of ZnS_xSe_1-x-based pigments,and ZrSiO₄ grains are formed in-situ on the pigment particle surfaces.ZrSiO₄ grains gradually grow up and densely encapsulate ZnS_x Se_1-x-based pigments during subsequent sintering at 700?.The as-synthesized ZnS_x Se_1-x@ZrSiO₄ or ZnS_xSe_1-x:Cu/In encapsulated pigments possess superior chromatic performance,high-temperature thermal stability and chemical stability as well.?4?The application and energy saving of ZrSiO₄ encapsulated ZnS_xSe_1-x-based pigments on ceramic tiles were evaluated.The annual building energy performance of applying ZnS_xSe_1-x-based NIR reflective ceramic tiles was simulated by a software named DesignBuilder based on the dynamic energy consumption simulation.The simulations indicate that substituting ZnS_xSe_1-x-based NIR reflective ceramic tiles?solar albedo 0.700?for cement walls?solar albedo0.300?can yield positive annual load,source energy,energy cost,CO₂,NO_x and SO₂ savings in subtropical regions?e.g.,Guangzhou,China?.The‘cool roof'solution for ZnS_x Se_1-x-based NIR reflective ceramic tiles has excellent energy saving and economic benefits and great popularization and application value in energy efficiency standards of public and residential buildings.Finally,the major conclusions of this work were summerized,and some prospects for the future research and improvement of environmentally-friendly color tunable NIR reflective inorganic pigments were given.