Doping Sodium Tungsten Bronze-Like (Na₅W₁₄O₄₄) Near-Infrared Shielding Functional Units in Bulk Borosilicate Glasses for Energy-Saving Window Applications

Tungsten bronze is an effective near-infrared (NIR) shielding material for fabricating energy-saving smart windows. While high-performing NIR-shielding glasses can be fabricated by coating a tungsten bronze film on window glasses, these glasses suffer a short lifespan due to the adhesion and degeneration of film. In this work, we show that tungsten bronze-like material Na5W14O44 can be distributed in the bulk glass matrix during the facile melt-quenching glass fabrication process under an air atmosphere, overcoming the limitations of film-based glasses. X-ray diffraction, Raman spectroscopy, and X-ray photoelectron spectroscopy confirmed the presence of tungsten bronze-like Na5W14O44 functional units in the SiO2–B2O3–NaF glass matrix. The addition of a small amount of H2WO4 and Sb2O3 is critical for the formation of Na5W14O44 functional units because H2WO4 provides W for Na5W14O44, and Sb2O3 acts as a reducing agent that helps the formation of W5+ in Na5W14O44 under an air atmosphere. Furthermore, the NIR-shielding ability can be tuned by adjusting the concentration of Sb2O3 in the range of 0–1.5 mol % and of H2WO4 in the range of 4–6 mol %. The optimized composition containing 1.25 mol % of Sb2O3 and 5 mol % of H2WO4 exhibits excellent NIR-shielding ability (ΔT = 62.8%), high visible light transmittance (Tmax = 67.7%), and excellent thermal insulation. This performance is comparable to cesium tungsten bronze film-based glasses and much better than soda lime glass and ITO glass under sun irradiation. This study sheds light on fabricating energy-saving windows in a simple and cost-effective way with flexibility for fine tuning of NIR-shielding performance.