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Atomic-scale investigation of structure, diffusions, and kinetics of Al2O3/MgO reaction interfaces during spinel growth

Periodic Reporting for period 1 - INTERFACIAL REACTIONS (Atomic-scale investigation of structure, diffusions, and kinetics of Al2O3/MgO reaction interfaces during spinel growth)

Reporting period: 2015-05-01 to 2017-04-30

Much of our understanding of the evolution of the earth and other planets comes from the analysis of minerals and rocks, which contain abundant information about the formation conditions in their structure and composition. A typical case of mineral formation is reaction rim growth in which a new phase forms at the interface between two types of minerals, which is also referred to as solid-state reactions. Such a solid-state reaction not only happens during natural mineral evolution, but can also be applied to synthesize functional materials such as oxides and alloys. During the growth of the interlayers, the arrangement of atoms at the interfaces between the reactant and the product phases plays a crucial role. Typically the interface behaves differently during the initial and late growth stages. Therefore in order to figure out the fundamental mechanism of the interlayer growth, it is essential to understand the atomic structures of the reaction interfaces and their development during growth.

In this research, the spinel forming reaction, MgO+Al2O3=MgAl2O4 is used as an ideal model system for studying solid-state reactions, as it is feasible for laboratorial growth, and the structure of these oxides can represent many other common metal oxides in nature and in functional application. MgAl2O4 layers with different thicknesses have been synthesized and the structure representing different growth stages have been selected for the microscopic study using a state-of-the-art aberration-corrected scanning transmission electron microscope (STEM) with sub-Å resolution.

The main objective of this project is to understand the motion of interfaces during the growth of an MgAl2O4 interlayer between MgO and Al2O3 reactants. This has been fully achieved at the end of the project period. Furthermore, we have extended our research to natural volcanic rocks featuring an Mg(Fe)Al2O4 spinel rim around corundum (Al2O3), and obtained some preliminary results which might lead to further research.
The interface structures at both MgAl2O4/MgO and MgAl2O4/Al2O3 interfaces have been understood at the atomic scale, and the interaction between the interface reaction and the long-range inter-diffusion of cations has been explained from initial to late growth stages. It is found that the interface morphology in micron-scale is strongly linked to the structure in atomic scale.

The detailed workflow is listed below.

1. Selecting MgAl2O4 interlayers grown in different stages, examining their crystallographic orientation relationship with electron backscatter diffraction (EBSD) on a scanning electron microscope (SEM), and then use site-selective focused ion beam (FIB) and low voltage Argon ion-milling to prepare specimen for further atomic study via atomic resolution STEM.

2. Investigating both MgAl2O4/MgO and MgAl2O4/Al2O3 interfaces via the state-of-the-art atomic resolution STEM, from micron-scale down to atomic scale, especially on the misfit dislocation structure at the interfaces. The interface configurations at initial and late growth stages have also been compared.

3. Basing on the microscopic observations, structure models have been built to understand the interface reaction mechanism.

4. At the end, all the results from micron-scale morphology and atomic structure, as well as modelling have been combined for analysis and discussion: the atomic structure and micron-scale morphology have been linked; the interaction between the interface reaction and the long-range inter-diffusion of Al3+ and Mg2+ from the reaction interfaces across the MgAl2O4 interlayers has been discussed; and the interface motion during the initial and late growth stages have been compared. All these lead to a multi-dimensional understanding of the interface motion.

5. At the end of the project, we extended our study to natural minerals, and achieved some preliminary results: The crystalline orientation relationship at the natural spinel/corundum interface is similar to the experimentally grown materials, despite that the natural mineral has a 3-dimentional curved interface. The inclusions in the natural mineral show an interesting structure change across the corundum/spinel interface, which might lead to a new project.

Exploitation and dissemination:

This research directly leads to 8 papers including 1 article published in ‘Philosophical Magazine’ at 2016, 2 conference proceedings, 2 articles to be submitted, and 3 in preparation. During the Marie Curie Period, Dr. Li has published 14 papers in peer-reviewed journals and conference proceedings, including 5 first author papers. 10 of them were from previous projects. Dr. Li also delivered 12 Oral Presentations for Conferences and seminar talks, including 6 invited lectures by Universities and research institutes, 4 conference talks, and 2 internal department seminars. And 2 more conference talks are upcoming in August 2017. Besides, several presentations from Dr. Li’s collaborators also include her results.
This study has pushed our understanding of interface reaction into atomic scale using the MgO/MgAl2O4/Al2O3 system, which has potential impact for understanding a wide range of materials with similar structure in minerals as well as functional materials. This research also shows that the advanced ability of state-of-the-art STEM on understanding mineralogical materials, which are traditionally more studied by optical microscopy and scanning electron microscopy. Bridging geoscience and material physics leads to a cutting edge multidisciplinary research, which opens new possibilities for understanding complex materials irrespective of a natural or synthetic origin.
Periodic scalloped-shaped MgAl2O4/MgO interface