Visible-light photoactive thermally sprayed coatings deposited from spray-dried (Na0.5Bi0.5)TiO3 microspheres

Abstract (Na0.5Bi0.5)TiO3 (NBT) ceramics are candidates to be used as coarse coatings in several applications including environmental remediation, alternative energies, electromechanical devices, etc. In this work, visible-light photoactive thermally sprayed coatings deposited from spray-dried NBT microspheres are reported. NBT powder was synthesized and processed by mixed oxides solid state reaction and milling. The spray drying process and sintering treatments to produce NBT microspheres were optimized to obtain a size distribution suitable to be used as a feedstock for flame spray deposition. The effect of spray drying and sintering parameters on the characteristics of NBT microspheres are presented and discussed. NBT microspheres with size distribution described by D10 = 25 μm and D90 = 83 μm were prepared in a design of experiments using a two-fluid nozzle spray dryer equipment varying nozzle size, atomizing pressure, feeding rate of the spray drying equipment, and the solid content (vol%) of the slurry formulation. NBT spray-dried agglomerates showed predominantly dense spheres with D50 sizes in the range of 17–55 μm. NBT Perovskite was confirmed by XRD in the synthesized powders and sintered microspheres. NBT powder was fed into a flame spray gun and a thick coating of approx. 100 μm was deposited. Rapid solidification and high temperature of the flame spray process allowed to couple a heterojunction composed by NBT with small amounts of TiO2 in both, anatase and rutile phases. The heterojunction showed a direct band gap of 1.93 eV, which makes the NBT + flame spray process an excellent combination to couple visible light photoactive NBT + TiO2 heterojunctions.