Thermal Flash Annealing Stabilizes the Tetragonal Phase in Zirconia Microarchitectures as Revealed by Europium Optical Probing

Controlling polymorphism in ceramic microarchitectures is essential for linking processing, structure, and function yet remains difficult for ceramics produced by two-photon lithography (TPL). During thermal conversion, crystallite growth typically promotes the thermodynamically stable monoclinic (m) phase, restricting access to tetragonal (t)-ZrO2. Here, we show that infrared (IR) flash annealing stabilizes t-ZrO2 microarchitectures over a broad 600–1000 °C treatment window. Rapid IR heating (20 °C s–1) limits crystallite growth and suppresses the t-to-m transformation commonly observed near 750 °C under slower annealing. Scanning transmission electron microscopy (STEM) confirms crystallite-size-dependent t-stabilization during rapid IR heating, while Europium (Eu3+) serves as an embedded optical probe of local symmetry, providing distinct photoluminescence signatures without destructive preparation. Flash thermal processing controls polymorphism, while luminescent symmetry probing enables its non-destructive diagnosis in 3D ceramic microarchitectures, providing a transferable approach for studying phase evolution in lanthanide-doped ceramics.