Microstructural Evolution of Single-Grain-like TiO2 Microspheres Prepared Through Spray Drying and Multi-Step Sintering

The preparation of TiO2 microspheres by spray drying followed by controlled sintering was investigated. Starting from commercial rutile TiO2 nanoparticles dispersed in water, spherical microspheres with broad size distributions were produced by adjusting key spray-drying parameters, including solid concentration, gas flow, and suspension composition. The results showed that controlling the spray-drying conditions and the subsequent sintering cycle influenced the resulting microsphere size and morphology, grain evolution, and porosity. A tailored multi-step sintering approach was used to control the microstructural evolution of individual TiO2 microspheres while limiting inter-microsphere coalescence. SEM and three-dimensional nano-tomography revealed pronounced grain growth and progressive pore elimination during sintering. The grain size increased from approximately 2–5 µm at 1200 °C to 20–40 µm at 1400 °C, while ex situ nano-tomography showed a decrease in pore volume fraction from 15.9% to 1.3% over the same temperature range. In situ nano-holotomography further confirmed the progressive reduction in the connected pore network during heating. Overall, sintering at 1400 °C provided the best compromise between densification, grain growth, microsphere isolation, and preservation of the spherical morphology.