Synthesis and characterization of two-dimensional nano-layered ternary transition metal carbide and boride (MAX/MAB) phases

Abstract

In recent years, with the discovery of graphene, which is a carbon derivative, layered structures have emerged as an important material group for materials science due to their significant potential. Among these layered material groups, three-dimensional carbide (MAX) and boride (MAB) phases have been quite popular material types since the 2010s, and this interest has continued to the present day. MAX phases are defined as ternary carbide or nitride phases with the chemical formula of Mn+1AXn, where M denotes a transition metal, A refers to a Group IIIA or IVA metal, and X represents C or N. MAB phases are also called ternary structures where the X element was replaced by boron (B). In this thesis, the synthesis of MAX and MAB phases of different transition metals by mechanical activation-assisted annealing has been aimed. For this purpose, the synthesis of Cr2AlC from Cr-Al-C powders and Ti3AlC2 from Ti-Al-C powder mixtures has been completed. During the synthesis of these phases, the standard molar ratios of 2:1:1 for Cr:Al:C and 3:1:2 for Ti:Al:C were used. Additionally, to investigate the effect of the Process Control Agent (PCA) during the synthesis of the MAX phase from Cr-Al-C powders, 1 wt. % stearic acid (SA) was added to the powder mixtures. Fe2AlB2 and Mn2AlB2 structures were produced from MAB phases by activating Fe:Al:B and Mn:Al:B powder mixtures prepared in a 2:x:2 (x = 1, 1.5, and 2) stoichiometric ratio. The first step in the synthesis phase involves mechanical activation, which is applied for different periods of time. Prepared powder mixtures were mechanically activated using a high-energy ball mill at 1200 rpm for 1, 3, and 5 hours, and then prepared for subsequent synthesis steps. As a result of the mechanical activation process, the particle sizes of all powder mixtures have been reduced without causing any side reaction between the starting materials. Mechanically activated powder mixtures were subjected to annealing during the subsequent synthesis process. MAX phase starting powders were annealed in a tube furnace under a constant Ar gas flow at 1100, 1300, and 1500 °C for 3 hours, while MAB phase starting powders were annealed at 900, 1100, and 1300 °C for 3 hours, thereby completing the synthesis of the MAX and MAB phases. In addition, MAX phase starting powders were annealed at 700 and 900 °C, and MAB phase starting powders were annealed at 500 and 700 °C to determine the phase transformations and reactions occurring during the synthesis of MAX and MAB phases. In the first stage of characterization, phase analyses of the MAX and MAB phases were completed, and the optimal conditions for synthesizing these phases were determined. Generally, the corresponding MAX and MAB phases were successfully synthesized at nearly all annealing temperatures and mechanical activation durations. During the synthesis of the Cr2AlC MAX phase, 99.7% Cr2AlC phase was obtained in a sample annealed at 1100°C for 3 hours after 3 hours of mechanical activation, making it the most successful experimental result for this phase. For the synthesis of the Ti3AlC2 MAX phase, 97.7% Ti3AlC2 MAX phase was achieved in powders annealed at 1300°C following 3 hours of mechanical activation, establishing these parameters as optimal conditions. Conversely, during the synthesis of the MAB phase, it was necessary to determine the most suitable stoichiometry in addition to the mechanical activation time and annealing temperature. The synthesis of Mn2AlB2 revealed that the optimal Mn-Al-B molar ratio is 2:1.5:2, and powders prepared at this ratio, subjected to 3 hours of mechanical activation and then annealed at 900°C, resulted in products containing 100% Mn2AlB2 phase. Similarly, for the synthesis of Fe2AlB2 MAB phase, the optimal Fe:Al:B molar ratio was found to be 2:1.5:2, and 100% Fe2AlB2 phase was achieved in powders activated for 3 hours and annealed at 1100°C. Following microstructural characterization, the layered structures of the MAX and MAB phases were observed. A layered structure with an average thickness of 36.3 nm was obtained in the Cr2AlC sample annealed at 1100 °C after 3 hours of MAc. However, with the addition of stearic acid as a process control agent, the layer thickness of the Cr2AlC sample synthesized at 1300 °C after 1 hour of MAc decreased to 20.81 nm. TEM analysis results have shown that the Cr2AlC phase has formed a layered structure with different planes growing in various directions, such as the (101) and (103) planes. In the Ti3AlC2 sample synthesized by annealing at 1300 °C for 3 hours after MAc, a MAX phase structure with an average layer thickness of 91 nm was obtained in a stacked arrangement. In addition, TEM analyses revealed (101) and (105) planes growing in different directions in the structure, as well as MAX phase layers that were both stacked and separated as flakes. In the SEM images of the Mn2AlB2 phase, a structure formed by the embedding of broken layers within the overall structure is observed, along with a stacked layered structure averaging 110 nm in thickness. Furthermore, TEM images of the Mn2AlB2 phase show that the layers of the synthesized MAB phase have grown in various directions on different planes, and even within the same grain, three different planes oriented in three distinct directions have been observed. Fe2AlB2 exhibits the same tendency as Mn2AlB2, and a MAB phase structure with an average layer thickness of 110 nm was achieved, with different planes oriented in various directions. The oxidation behaviour of the synthesised MAX phases and the structures they form after oxidation have been investigated. According to the oxidation kinetics of Cr2AlC MAX phase, this phase showed oxidation within the cubic law at both 700 and 900 °C. Additionally, while no significant oxide layer was detected on the surface of the Cr2AlC phase at 700 °C, an oxide layer with a thickness of 6.424 µm formed on the surface as the temperature increased to 900 °C. On the other hand, a different oxidation mechanism takes place for the Ti3AlC2 MAX phase. The oxidation mechanism, which acts according to the cubic law during the first 6 hours at 700 °C, slows down during the subsequent process, causing the mechanism to become irregular. At 900 °C, it was determined that the Ti3AlC2 phase oxidised completely according to the parabolic law and that the oxidation rate also decreased. In addition, while a very thin TiO2-Al2O3 layer formed after oxidation at 700 °C, at 900 °C this structure was replaced by a much thicker layer consisting of three different layers and TiO2 and Al2O3 phases.