Bismuth and Vanadium-Substituted Yttrium Phosphates for Cool Coating Applications

Luminescent yttrium phosphate is engineered into an environmentally benign near infrared (NIR) reflective yellow pigment by the substitution of bismuth and vanadium metals in the host lattice. A series of YP_(1-V O₄ (x = 0, 0.05, 0.1, 0.15, 0.2, and 0.4), Y_(1-Bi PO₄ (y = 0.1 and 0.3), and Y_(1-Bi P_(1-V O₄ (x = y = 0.2, 0.4, and 0.6) were prepared by the precipitation method. Secondary phase was noticed at x = 0.2 and y = 0.2 while substituting vanadium and bismuth, respectively, due to high ionic radii of the dopant ions. Co substitution of vanadium and bismuth in the YPO₄ lattice enhanced both NIR reflectance and yellow color of all the fabricated materials. XPS spectra proved the presence of trivalent bismuth and pentavalent vanadium in Y_0.4Bi_0.6P_0.4V_0.6O₄. Due to the substitution effect, a more defined morphology was noticed, which enhanced the scattering co-efficient of the fabricated materials; hence, the NIR reflectance of the materials was increased from 68% (YPO₄) to 83% (Y_0.4Bi_0.6P_0.4V_0.6O₄). Chemical and thermal stability test of Y_0.4Bi_0.6P_0.4V_0.6O₄ confirmed the color and strength of the designed pigment. With good yellow hue (b* = +56.06), high NIR solar reflectance (R* = 83%), and good stability, Y_0.4Bi_0.6P_0.4V_0.6O₄ can act as an environmentally benign cool yellow pigment.