Synthesis and characterisation of MAB phase ceramics via in-situ and ex-situ techniques

The MAB phases are an emerging family of nanolaminated advanced ceramics with potential applications in increasingly relevant industries. Theoretical and lab-scale research into their properties and applications is well established, but the lack of synthesis methods viable for large-scale industrial production are preventing their application. This project investigated the formation of MAB phase ceramics using in-situ and ex-situ techniques to characterise their formation pathways, and the feasibility of two alternative synthesis methods. The key objective of this project was to improve the understanding of the formation of MAB phases and the effect of preparation and synthesis parameters on their reaction mechanisms. Understanding the effect of sample preparation parameters and synthesis methods on the formation of the MAB phases is critical for the improvement of these methods to allow large-scale production of high-purity material that is industrially applicable. The MAB phases Fe₂AlB₂, Mn₂AlB₂, Cr₂AlB₂, and MoAlB were found to form using both alternative synthesis methods investigated in this project: induction assisted self-propagating high-temperature synthesis (SHS) and aluminothermic exchange reactions. These methods have advantages over traditional pressureless synthesis methods. These include significantly shorter reaction times, less required energy input due to the self-propagating reactions occurring in induction-assisted SHS. The ability to use metal oxide reactants, which are generally much cheaper than pure refined metal powders, for aluminothermic exchange reactions. This project also included the first high-intensity in-situ neutron diffraction study of the MAB phases to characterise the formation pathways of Fe₂AlB₂, Mn₂AlB₂, Cr₂AlB₂, MoAlB, and WAlB. Synthesis via traditional pressureless synthesis and induction-assisted SHS, as well as the effect of varying reactants and preparation parameters, were investigated. Notable findings include complex reaction pathways with multiple stages and intermediate phases in multiple systems, including two formation temperature windows for MoAlB. Previously, hypotheses on the formation mechanisms of the MAB phases were based on ex-situ techniques and differential scanning calorimetry studies which suggested formation via a simple, direct intercalation reaction. This project has revealed the reaction pathways of most MAB phases to be significantly more complex, and based on in-situ temperature measurements, highly dependent on some sample preparation parameters. The information discovered in this project can be applied to improve synthesis methods, through the adjustment of preparation and heating parameters to improve efficiency, and phase purity of the product MAB phases.