Kinetics of phase transformation and optical property of pink coral zirconia powders

Highlights: • The single phase of tetragonal ZrO₂ formed when calcined at 1223 K for 1 h. • The tetragonal ZrO₂ fully converted to ZrSiO₄ when calcined at 1323–1473 K for 1 h. • The activation energy of t-ZrO₂ formed is 399.9 kJ/mol when 5 mol% Fe₂O₃ added. • The activation energy of the Fe/ZrSiO₄ formed is 257.7 kJ/mol when 5 mol% Fe₂O₃ added. • The growth morphology parameter and crystallization index are about 2.0 and 1.0. - Abstract: The kinetics of phase transformation and optical property of pink coral zircon powders have been studied. The ZrO₂–SiO₂–Fe₂O₃ precursor powders were synthesized using Zr(NO₃)₄⋅4H₂O, Si(C₂H₅O)₄ and Fe(NO₃)₃⋅9H₂O as initial materials via the hot–wet routes. The kinetics of phase transformation of the ZrO₂–SiO₂–Fe₂O₃ precursor powders was characterized by thermo-gravimetric (TG)/differential scanning calorimeter (DSC), X-ray diffraction (XRD), transmission electron microscopy (TEM), nano-beam electron diffraction (NBED), and spectrophotometry. The crystallization temperatures of tetragonal zirconia (t-ZrO₂) and zircon (ZrSiO₄) of ZrO₂–SiO₂ precursor powders with 1 mol% Fe₂O₃ were estimated to be approximately 1204 K and 1496 K, respectively, based on a DSC analysis conducted at a heating rate of 20 K/min. The activation energies of t-ZrO₂ formulation are 428.2, 403.2, and 399.9 kJ/mol, respectively, for ZrO₂–SiO₂ precursor powders containing 1, 3, and 5 mol% Fe₂O₃, respectively, whereas the activation energies of the Fe/ZrSiO₄ formulation are 271.9, 261.9, and 257.7 kJ/mol, respectively. The parameter of growth morphology (n) and index of crystallization (m) were approximated as 2.0 and 1.0, respectively, meaning that two-dimensional growth with plate-like morphology was the primary mechanism of ZrO₂ crystallization from ZrO₂–SiO₂–Fe₂O₃ precursor powders. The XRD results show that when the precursor powders of ZrO₂–SiO₂–1 mol% Fe₂O₃ and ZrO₂–SiO₂–5 mol% Fe₂O₃ after calcination at 1323–1473 K for 1 h, the ZrO₂ phase of the product powders was fully converted to zircon (ZrSiO₄). On the other hand, no reflection peak of Fe₂O₃ appeared in the XRD patterns for the ZrO₂–SiO₂–Fe₂O₃ precursor powders after calcination at 1223–1473 K for 1 h. The dominant wavelength of the zircon increased from 591.4 nm to 630.1 nm when the precursor powders after calcination at 1473 K for 1 h