Reaching Kinetic Selectivities : In Pursuing Novel Ternary Oxide Coatings, and Beyond

Abstract: Kinetically driven synthesis pathways have the potential to allow new ways to develop materials and phases with much-improved properties. This particularly concerns metastable and multicomponent phases that require a selective kinetic targeting during the synthesis to circumvent the formation of thermodynamically stable products. Thin film deposition techniques, including chemical vapour deposition (CVD), can offer this selectivity. However, conventional CVD relies heavily on halide-based precursors, which are corrosive, toxic, and typically require high deposition temperatures. Moreover, the significant variations in their displayed vapour pressures, reaction routes, and reaction rates impede their chemical compatibilities, thus limiting the prospects of making novel multicomponent coatings, especially mixed-metal ones. Therefore, there is a need to find new types of precursors that may mitigate the drawbacks of halides, which also can strengthen CVD as a technique for both existing and future-emerging technologies.In light of this, this thesis explores the simultaneous use of metal-organic precursors in synthesising and designing novel chemically vapour-deposited coatings. The ternary Al2TiO5 phase, renowned for its good refractory properties and low-to-negative thermal expansion, has been synthesised, which is typically challenging by other approaches. By combining aluminium and titanium isopropoxide in an in-house built CVD reactor, a selective targeting of the phase can be made that avoids stable binary phase formations. The role of local coordination in the phase formations is expressed by discovering unconventional phases in the Al–Ti–O system, such as Al6Ti2O13 and Al16Ti5O34. All coatings were amorphous as-deposited and readily crystallised at lower temperatures than those typically suggested by pseudobinary phase diagrams. In situ X-ray diffraction studies revealed that the crystallisation process was predominantly nucleation-controlled rather than governed by diffusion. The diminishing role of diffusion was also corroborated by subsequent studies using additional in situ analytical techniques, including hard X-ray photoelectron spectroscopy (HAXPES), Rutherford backscattering spectrometry (RBS), and heating in a transmission electron microscope (TEM). Combined, these techniques show that the amorphous-to-crystalline transformation occurs through a displacive (diffusionless) transformation. Based on the results, it can be inferred that short-range structural displacements by oxygen are essentially required to spur nucleation and subsequent crystallisation. Performed theoretical calculations and molecular dynamics simulations support this notion, which also highlights the potential involvement of oxygen vacancies during the crystallisation and co-formation of Al6Ti2O13 and Al16Ti5O34 beyond Al2TiO5.The coherent results of this thesis emphasise situations where kinetics – rather than thermodynamics – may control the phase selection and microstructural evolution of CVD coatings. It is proposed that the findings of this doctoral work may contribute to expanding the capabilities of CVD as a technique and the rational synthesis of inorganic materials in general, especially in terms of new functional oxides.