Circumventing Thermodynamic Constraints in Nucleation-Controlled Crystallization of Al2TiO5-Based Chemical Vapor Deposition Coatings

Nucleation is a fundamental part in most syntheses of ceramic materials. Yet, few techniques enable control of this step, which would offer possibilities to attain full-scale kinetic selectivity of the syntheses to reach novel compounds with unique properties. Herein, we present a nucleationcontrolled crystallization pathway to synthesize coatings of aluminum titanate (Al2TiO5)-renowned for its low-to-negative thermal expansion-at significantly reduced temperatures than conventional solid-state techniques. Based on a kinetic study using in situ X-ray diffraction, detailed mechanistic insights into the crystallization process and phase evolutions within the Al- Ti-O system are obtained. The lowest activation energies for crystallization are given when the Al-Ti ratio is close-to-stoichiometric or Ti-enriched. Along with these compositions' similar kinetics at the earliest stages of the transformation, a joint nucleation behavior is discovered, revealing the elemental role of titanium in nucleating the main Al2TiO5 phase. Based on classical nucleation theory, we deduce the significant influence of the configurational entropy (Sconfig) when crystallization occurs in the nucleation-controlled domain. Finally, peculiar transition features are observed in the Al-enriched regime during annealing at intermediate temperatures, whose causes are ascribed to the presence of secondary nucleation events and possibilities of structural relaxations in the amorphous matrixes when crystallizing.