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Role of Oxygen in Vacancy-Induced Phase Formation and Crystallization of Al2TiO5-Based Chemical Vapor-Deposited Coatings
Uppsala University, Disciplinary Domain of Science and Technology, Chemistry, Department of Chemistry - Ångström, Inorganic Chemistry.ORCID iD: 0000-0002-7071-3917
KTH Royal Inst Technol, Dept Mat Sci & Engn, Unit Struct, SE-10044 Stockholm, Sweden..
Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy. (Röntgenfysik)ORCID iD: 0000-0001-6162-1167
Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Applied Nuclear Physics.ORCID iD: 0000-0003-3172-5736
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2023 (English)In: The Journal of Physical Chemistry C, ISSN 1932-7447, E-ISSN 1932-7455, Vol. 127, no 13, p. 6456-6465Article in journal (Refereed) Published
Abstract [en]

Oxygen is a commonly overlooked element influencing the properties of many metal oxides. By combining several analytical in situ techniques and theoretical calculations, we demonstrate that oxygen plays a vital part in the phase formation and crystallization of Al2TiO5-based chemical vapor-deposited coatings. Rutherford backscattering spectrometry (RBS) corroborates a polymorphic transformation during crystallization. Subsequent hard X-ray photoelectron spectroscopy (HAXPES) shows that crystallization occurs through a displacive (diffusionless) mechanism. Coupled with theoretical calculations, the crystallization and co-formation of Al2TiO5, Al6Ti2O13, and Al16Ti5O34 are suggested to be driven by the migration of oxygen ions and their corresponding vacancies.

Place, publisher, year, edition, pages
American Chemical Society (ACS), 2023. Vol. 127, no 13, p. 6456-6465
National Category
Condensed Matter Physics Materials Chemistry Inorganic Chemistry
Identifiers
URN: urn:nbn:se:uu:diva-502112DOI: 10.1021/acs.jpcc.2c08570ISI: 000959751400001OAI: oai:DiVA.org:uu-502112DiVA, id: diva2:1759298
Funder
Swedish Foundation for Strategic Research, RMA15-0048Swedish Research Council, 2019-00191Swedish Research Council, 2020-06409eSSENCE - An eScience CollaborationSwedish National Infrastructure for Computing (SNIC)Available from: 2023-05-25 Created: 2023-05-25 Last updated: 2024-06-05Bibliographically approved
In thesis
1. Reaching Kinetic Selectivities: In Pursuing Novel Ternary Oxide Coatings, and Beyond
Open this publication in new window or tab >>Reaching Kinetic Selectivities: In Pursuing Novel Ternary Oxide Coatings, and Beyond
2023 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

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.

Place, publisher, year, edition, pages
Uppsala: Acta Universitatis Upsaliensis, 2023. p. 127
Series
Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology, ISSN 1651-6214 ; 2242
Keywords
CVD, coatings, kinetics, crystallization, nucleation, diffusionless, Al2TiO5, oxides
National Category
Materials Chemistry Inorganic Chemistry
Research subject
Chemistry with specialization in Inorganic Chemistry; Chemistry with specialization in Materials Chemistry
Identifiers
urn:nbn:se:uu:diva-496824 (URN)978-91-513-1721-2 (ISBN)
Public defence
2023-04-14, Eva von Bahr, 100195, Ångström Laboratory, Lägerhyddsvägen 1, Uppsala, 09:15 (English)
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Supervisors
Funder
Swedish Foundation for Strategic Research, RMA15-0048
Available from: 2023-03-16 Created: 2023-02-21 Last updated: 2023-10-24

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Öhman, SebastianLindblad, RebeckaNagy, GyulaBroqvist, PeterBerggren, ElinJohansson, FredrikTörndahl, TobiasPrimetzhofer, DanielBoman, Mats

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