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

Abstract

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 (Al 2 TiO 5 )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 Al 2 TiO 5 phase. Based on classical nucleation theory, we deduce the significant influence of the configurational entropy (S config ) 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.

Role of Oxygen in Vacancy-Induced Phase Formation and Crystallization of Al2TiO5-Based Chemical Vapor-Deposited Coatings

“…72,73 This is also corroborated by the performed peak fitting, shown in Figure S9 in the Supporting Information, where any potential contribution of Ti 3+ is practically negligible. Since we have previously established the co-formation and crystallization of Al 6 Ti 2 O 13 and Al 16 Ti 5 O 34 in the annealed samples by X-ray diffraction, 21,23 we can conclude that Ti 3+ is not likely a structural entity belonging to any of these phases. Thus, Al 6 Ti 2 O 13 and Al 16 Ti 5 O 34 are independent phases with unique Al−Ti stoichiometries.…”

“…, density, structure, composition). , Therefore, given a uniform (homogeneous) chemical composition of our amorphous coatings, a single crystalline phase should be expected to arise during the phase transition; namely, the phase whose stoichiometry is closest to the overall composition of the amorphous layer. , Nonetheless, previous examinations have revealed that a significant co-formation of Al 6 Ti 2 O 13 and Al 16 Ti 5 O 34 also happens even when the Al–Ti ratio in the coating is close-to-ideal (1.93) with respect to the Al 2 TiO 5 ’s stoichiometry ( i.e. , 2–1). , This infers a possibility for compositional fluctuations to be the main reason for the co-formation of Al 6 Ti 2 O 13 and Al 16 Ti 5 O 34 . Indeed, such compositional variations and elemental enrichments are a typical deviation from CNT and have frequently been reported in the crystallization of many glass-forming materials. − …”

“…20,57 Nonetheless, previous examinations have revealed that a significant coformation of Al 6 Ti 2 O 13 and Al 16 Ti 5 O 34 also happens even when the Al−Ti ratio in the coating is close-to-ideal (1.93) with respect to the Al 2 TiO 5 's stoichiometry (i.e., 2−1). 21,23 This infers a possibility for compositional fluctuations to be the main reason for the co-formation of Al 6 Ti 2 O 13 and Al 16 Ti 5 O 34 . Indeed, such compositional variations and elemental enrichments are a typical deviation from CNT and have frequently been reported in the crystallization of many glass-forming materials.…”

“…By this approach, we were able to exploit the 3037 keV O resonance 62 for depth profiling of the oxygen content in the film with high accuracy. Measurements were made before and after annealing at 800 °C, a temperature where the coatings readily crystallize, 21,23 for 1 h. A typical result is shown in Figure 1, and additional spectra can be viewed in the Supporting Information (Figures S1 and S2), essentially showing similar results. Moreover, diffractograms showing the phase formations and crystallinity after annealing are provided in Figures S3 and S4.…”

“…Recently, we have demonstrated this route in fabricating aluminum titanate-based (Al 2 TiO 5 ) coatings. , Al 2 TiO 5 is a refractory ceramic that is challenging to make from conventional diffusional-based techniques. The properties of this material include, among many, a low-to-negative thermal expansion, , high thermal shock resistance, good corrosion resistance, , and a self-healing of cracks at elevated temperatures. , These properties make the phase a potential candidate for many high-temperature and metallurgical applications .…”