MSc.Thesis Defense: Yasemin Akyol

DYNAMICS OF INTERFACE REACTIONS AND METASTABLE PHASES IN MIXED ALUMINATE STRUCTURES

Yasemin Akyol
Materials Science and Nano Engineering, MSc. Thesis, 2024

Thesis Jury

Prof. Dr. Mehmet Ali Gülgün (Thesis Advisor)

Prof. Dr. Cleva W. Ow-Yang (Co-Advisor)

Prof. Dr. Selmiye Alkan Gürsel

Assoc. Prof. Aligül Büyükaksoy

Date & Time: 9th July, 2024 – 3:30 PM

Place: FENS 2019

Zoom link: https://sabanciuniv.zoom.us/j/92588588320?pwd=F8MN61wbTGWeGaGfuTiViXz6tq6BRW.1

Keywords : Aluminate Systems, Metastable Phases, Interface Reactions, Interphase, Sapphire (α-Al₂O₃), TEM, EELS

Abstract

Aluminate structures exhibit phase formation anomalies irrespective of the initial stoichiometry of the oxide powder mixtures. In metal oxide systems (Me²⁺ or Me³⁺), multiphases form at different cation to alumina (Me^x+/Al₂O₃) ratios. These systems contain tetrahedral ((AlO₄)^5-) and/or octahedral ((AlO₆)^9-) polyhedra in the aluminate crystallography. The first forming phase in strontium, calcium and yttrium aluminate systems is 1:1 compound SA (SrAl₂O₄), CA (CaAl₂O₄), and YAP (YAlO₃). Large cation size of Me²⁺ in aluminate systems was claimed to be the reason of the difficulties in diffusion.

In the presented work, first, a sol-gel Pechini method was applied to synthesize 14 compounds present in the three binary equilibrium phase diagrams of SrO-Al₂O₃, CaO-Al₂O₃, and Y₂O₃-Al₂O₃. Phase formation pathways of calcium and strontium aluminates were presented with X-ray diffractograms and quantitative Rietveld analysis. Synthesis conditions were determined to produce X-ray phase pure aluminate compounds. Characteristic fingerprints of strontium aluminates were demonstrated with Al-L_2,3 Electron Energy-Loss Near Edge Structure (ELNES) edges. Secondly, Solid-state reactions were performed to investigate the effect of having tetrahedra or octahedra Al-O network by mixing carbonate particles and γ- or α-Al₂O₃ particles. Strontium or calcium carbonate mixture with γ-Al₂O₃ particles formed hexaluminate stoichiometry at 1200 °C and 1400 °C, respectively. Calcium carbonate and α-Al₂O₃ mixtures resulted in the formation of CA and CaAl₄O₇ (CA₂) phases above 950 °C. When α-Al₂O₃ powders mixed with strontium carbonate, Sr₃Al₂O₆ (S₃A) and SA compounds were formed at 900 °C.