Investigating the effect of WO3 on the crystallization behavior of SiO2–B2O3 – Al2O3–Na2O – CaO–ZnO high VIS-NIR reflecting glazes

Abstract

White glass enamels with high solar reflectance and containing different WO3 concentrations have been prepared and characterized with regard to their optical, mechanical and microstructural characteristics. Upon addition of WO3 to a glass containing SiO2, B2O3, Al2O3, Na2O, CaO, and ZnO, the crystallization of scheelite follows a crystallization mechanism of bulk type where scheelite grows in one-dimension in a patterned morphology dominated by the heating rate and the concentration of WO3. Octahedral bipyramids and arrow-like crystals appeared in enamels containing WO3 concentration above 6%. The presence of scheelite crystals with different orientations also leads to slight variations in hardness and Young modulus thus obtaining Hv values between 8 and 8.8 GPa and E values between 72 and 83 GPa. Similarly, the optical properties such as whiteness, brightness and solar reflectance increase with the presence of scheelite, and the highest solar reflectance occurs for the enamel containing arrow-like and bipyramidal crystals.