Yidong Hu,Fan Yang,Guoliang Ren,Jianguo Li,Qiaodan Hu
Preparation of dense Bi 2 Ti 2 O 7 ceramics with single-phase pyrochlore structure is challenging using the conventional sintering method due to its thermal instability at ≥ 650 °C. Here we synthesized dense and phase-pure Bi 2 Ti 2 O 7 ceramics from a mixture of Bi 2 O 3 and TiO 2 by a novel deep undercooling rapid solidification method using the aerodynamic levitation technique. We reported a surprising discovery that the pyrochlore phase can exist with Bi/Ti ratio varying between 0.9 and 1.1. By a comprehensive investigation on the phase purity, crystal structure, oxidation states and electrical properties, we revealed that the pyrochlore structure is primarily stabilized by the solidification process rather than presence of a large amount of Ti 3+ . With the advantages of being containerless, one-step and fast processing, deep undercooling rapid solidification is an effective and efficient approach to prepare dense Bi 2 Ti 2 O 7 pyrochlores, and can be expanded to other thermally unstable compounds to obtain bulk materials.
Extreme solidification processes in advanced manufacturing (e.g., laser additive manufacturing and in space fabrication) amplify challenges in predicting phase selection under severe thermodynamic non-equilibrium. Using aerodynamic levitation containerless processing, we investigate the solidification behavior of pseudo-binary MgTiO 3 -TiO 2 system. Our experiments reveal an anomalous trans-phase-regime selection phenomenon in which the thermodynamically stable MgTi 2 O 5 phase is kinetically suppressed from precipitation. This selection arises from nucleation kinetic barriers of MgTi 2 O 5 , where coupled undercooling and compositional fluctuations destabilize critical nuclei formation. By integrating composition- and temperature-dependent interfacial energy dynamics into classical nucleation theory , we develop a predictive framework that quantitatively maps anomalous phase selection regions. This approach establishes the first non-equilibrium kinetic phase diagram of this system with experimental validation, resolving long-standing discrepancies between equilibrium predictions and non-equilibrium experimental outcomes. The proposed methodology establishes a generalized paradigm for phase selection prediction, offering transformative insights into microstructure and phase control under extreme non-equilibrium solidification conditions.
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