Synthesis and Characterization of the MAB Phases
Authors:   , Kota Sankalp S. , Barsoum Michel W. 
Published: July 2021

The MAB phases are atomically laminated, ternary transition metal borides whose crystal structures are comprised of stacked M-B layers (M = transition metal) interleaved by monolayers of Al or Zn, or bilayers of Al atoms. This name stems from the fact that their nanolaminated atomic structures resemble the nanolaminated MAX phases. Structurally, the MAB phases are highly anisotropic because the M-B layers contain parallel, covalently bonded B-B chains and the M-B layers themselves are separated by relatively weaker M-Al bonds. Though nearly all the MAB phase compounds were discovered prior to the 1990s, their properties had remained largely unknown, in part, because of the unavailability of bulk, high purity, polycrystalline samples. In this thesis, three MAB phases are synthesized as high-purity, fully dense ceramics and five as powders using reactive powder metallurgy with the goal of systematically characterizing their fundamental physical properties. Some of these compounds were synthesized in bulk for the first time, while many of these properties were never characterized for polycrystalline samples. The MAB phases MoAlB, Mn₂AlB₂, and Fe₂AlB₂, like their binary boride counterparts, are found to be good conductors of electricity (0.3-5 [mu][omega]m) and highly dependent on the transition metal. From low-temperature heat capacity studies, the high conductivities are found to be related to high densities of states at the Fermi level. In turn, the high electrical conductivity of MoAlB translates to good thermal conductivities (28 Wm⁻¹K⁻¹ at room temperature), wherein a majority of the heat is transported by electrons. In terms of mechanical properties, the Young's and shear moduli ranged from 250-370 GPa and 100-150 GPa, respectively, and showed a weak temperature dependence up to 1000 °C. Among these three MAB phases, the hardness ranged between 8-11 GPa, and illustrate that they are significantly lower than the 16-23 GPa range found for the corresponding MoB, MnB, and FeB compounds. From an in-depth study of the oxidation resistance of MoAlB in ambient air, it was clear that it is the first transition metal compound to be highly oxidation resistant in air and performs comparably to state-of-the-art alumina-forming alloys and compounds. Because the unidirectional B-B chains and the face-sharing BM₆ trigonal prism building blocks in the MAB phases are also characteristic of the binary transition metal monoborides, this work endeavored to gain insights on how the Al layers distinguish these two families of solids. The relatively weakly bonded nature of the Al layers was evidenced in several ways: (1) the thermal decomposition of the MAB phases into the corresponding MB phase and elemental Al at high temperatures, (2) the formation of protective aluminum oxide scales on MoAlB when heated in ambient air to high temperatures, (3) considerably lower Vickers hardness and elastic moduli than the corresponding binary borides, and (4) the occurrence of intralayer delamination during indentation. However, there are similarities between the two as well. Both the binary monoborides and MAB phases of the 3d transition metals were found to have their lowest thermal expansion coefficients parallel to the B-B chains, which reflects the high strength of the covalent B-B bonds. Moreover, the insertion of Al into CrB and Cr₃B₄, to form Cr₂AlB₂ and Cr₃AlB₄, respectively, did not alter the magnetic ground state from paramagnetism.