MAB Phases Research Articles

Trends in the Behavior of Point Defects in MB and MAB Phases

Binary and ternary transition metal borides exhibit many outstanding properties, such as high melting temperature, high temperature strength, and electrical and thermal conductivities. However, defect properties have been rarely studied so far. Here, using first-principles calculations, we investigate structural effects on defect recovery processes in MB and MAB phases focusing on how Al layers and the type of transition metal affect annihilation of defects. First, we find that metal interstitials cannot be accommodated in MAB phases that have two Al layers, but instead they form antisites, which are largely immobile and therefore detrimental for defect recovery processes. We also find that the absence of Al layers in MB phases that have B chains leads to faster migration of metal interstitials and lower recombination barriers of B Frenkel pairs and hence contributes to efficient defect recovery processes of those MB phases as compared to the corresponding MAB phases. In the other types of MB and MAB phases that have B rings, we find that the role of Al layers varies depending on the type of metal elements. Lastly, we find that a larger M–B bond strength, which depends on the type of transition metal in MAB phases, correlates with a higher recombination barrier of B Frenkel pairs. Based on the discovered trends, we provide insights into the selection of materials to help design diverse applications where defect recovery processes are important.

Control of multiphase evolution and Al-deficiency in reactive-sintered MoAlB ceramics with excessive Al

MoAlB is a ternary boride of MAB phases with strong resistance to oxidation and ablation at service temperature, thanks to preferential Al diffusion to form a protective oxide scale. However, excessive Al were often necessary in sintering of MoAlB ceramics, suggesting that a liquid-phase densification might occur thus leading to complex microstructures. By quantitative SEM analysis, ~17 mol.% Al2O3 phase was found common in MoAlB ceramics. The liquid-phase of Al-Mo-B-O facilitates the direct conversion from MoB to MoAlB before in situ formation of Al2O3. An intermediate Mo3Al8 phase competes with layered conversion, which limits the insertion rate of Al into B-B layers of MoB to form abundant Al-deficient stacking-faults. Intermetallic Mo3Al8 phase precipitates further in the intergranular regions parallel to the crystallization of Al2O3, leaving the reprecipitation of remaining Al. Both layered-conversion and intergranular oxides can improve ablation behavior for MoAlB ceramics, as well as fracture toughness and compressive strength.

Defect recovery processes in Cr-B binary and Cr-Al-B MAB phases: structure-dependent radiation tolerance

We have investigated the radiation tolerance of Cr-B binaries and Cr-Al-B ternary MAB phases using a combination of ab initio calculations and experiments. One key discovery is that Cr3AlB4 has excellent resistance to radiation-induced amorphization, and therefore it is a promising material for applications in extreme environments. We also demonstrate that both the type of B network and the presence of Al layer in the structure have important implications for defect kinetics. We find that the order of the tolerance to radiation-induced amorphization is, from high to low, CrB, Cr3AlB4, Cr3B4, Cr4AlB6, and Cr2AlB2 at 150 °C and 0.5 dpa, and Cr3AlB4, CrB, Cr3B4, Cr4AlB6, and Cr2AlB2 at 300 °C and 1.0 dpa. The results are explained in terms of defect properties determined from ab initio calculations.

Insights of electronic structures, mechanical properties and thermal conductivities of TM5SiB2 (TM=V, Nb, and Ta) MAB phases

ABSTRACT The electronic, elastic, and thermal properties of TM5SiB2 (TM = V, Nb, and Ta) MAB phases are calculated herein using first-principles calculations. The obtained electronic structures show that there are p-d hybridizations between B, Si, and TM atoms, and TM-B and TM-Si bonds are formed by the electrons transfer between TM and B, TM and Si. Elastic Modulus and elastic Anisotropy Index, three-dimensional surface construction, and two-dimensional projection are used to discuss the elastic anisotropy, and the results show that these TM5SiB2 MAB phases are anisotropic in elastic modulus with the order of V5SiB2 > Nb5SiB2 > Ta5SiB2. Besides, the Debye temperature and sound velocity of these TM5SiB2 are also analyzed. The thermal conductivities are obtained, and the thermal conductivity is also anisotropic and in the same order as elastic modulus.

Electronic, elastic, and thermal properties, fracture toughness, and damage tolerance of TM5Si3B (TM = V and Nb) MAB phases

The first-principles calculations are employed herein to predict the electronic, elastic, and thermal properties, fracture toughness, and damage tolerance of TM5Si3B (TM = V and Nb) MAB phases. According to formation enthalpy, phonon dispersion and single-crystal elastic constants, V5Si3B and Nb5Si3B are thermodynamically, dynamically, and mechanically stable. Pugh's ratio, Cauchy pressure, hardness, fracture toughness, and elastic anisotropic indexes are calculated, and V5Si3B and Nb5Si3B are brittle, tolerant to damage, and elastic anisotropic. The possible cleavage plane and slip systems are the (0001) plane and < 11¯00 > |{112¯0} and < 112¯0 > |{0001}, respectively, which is determined by the strong TMII-Si, TM-B, and TMI-Si bonds parallel to the (0001) plane. Finally, lattice thermal conductivity, minimum lattice thermal conductivity, and dependence of heat capacity on temperature are predicted.

Theoretical predictions of phase stability for orthorhombic and hexagonal ternary MAB phases.

In the quest for finding novel thermodynamically stable, layered, MAB phases promising for synthesis, we herein explore the phase stability of ternary MAB phases by considering both orthorhombic and hexagonal crystal symmetries for various compositions (MAB, M2AB2, M3AB4, M4AB4, and M4AB6 where M = Sc, Y, Ti, Zr, Hf, V, Nb, Ta, Cr, Mo, W, Mn, Fe, and Co, A = Al, Ga, and In, and B is boron). The thermodynamic stability of seven previously synthesized MAB phases is confirmed, three additional phases are predicted to be stable, and 23 phases are found to be close to stable. Furthermore, the crystal symmetry preference for forming orthorhombic or hexagonal crystal structures is investigated where the considered Al-based MAB phases tend to favour orthorhombic structures whereas Ga- and In-based phases in general prefer hexagonal structures. The theoretically predicted stable MAB phases along with the structural preference is intended to both guide experimental efforts and to give an insight into the stability for different crystal symmetries of MAB phases.

Synthesis of nanolayered ternary borides powders (MAB phases) by sustainable molten salt shielded synthesis/sintering (MS3) process

Synthesis of nanolayered ternary borides powders (MAB phases) by sustainable molten salt shielded synthesis/sintering (MS3) process

Chemical order or disorder – a theoretical stability expose for expanding the compositional space of quaternary metal borides

Theoretical predictions for finding thermodynamically stable quaternary MAB phases upon metal alloying.

Pushing the limit of thermal conductivity of MAX borides and MABs

The emergence of MAX borides as well as MAB phases attracted great attention because of the renewable developments of ternary ceramics and offering great opportunities in potential applications. However, the number of borides remains limited, and further fundamental descriptions and detailed investigations on various properties are still lacking. In this report, we employ an integrated computational scheme that combines density functional theory with the evolutional algorithm to search for the favorable structures of P- and S-glued ternary borides terminated by Nb metal. We discover that the structures of 212-type, as e.g. Nb2PB2 and Nb2SB2, belong to the P6¯m2 space group, while those of 211-type, as e.g. Nb2PB and Nb2SB, prefer to crystallize in the P63/mmc space group, and the corresponding carbides Nb2PC and Nb2SC are also considered for the sake of completeness and comparative analsys. The predicted Nb2PB2, Nb2PB, Nb2SB, Nb2PC and Nb2SC are energetically stable, as revealed by the negative formation energies and by the proposed reaction paths with respect to the most competing phases, as well as dynamically stable, as suggested by the non-imaginary phonon spectra. The thermal conductivities of the six materials show unusual behaviors, particularly for the acoustic and optical contributions, and are accompanied by a strong anisotropy. Most importantly, Nb2PB2 is found to be an excellent thermal conductor with a total thermal conductivity of ~65 W/(m K), while Nb2SC is found to be an ultra-low thermal conductor, with a total thermal conductivity of ~5 W/(m K). These values are clearly outside the currently reported range of thermal conductivities, which makes Nb2PB2 and Nb2SC extremely interesting for fundamental research as well as prospective applications with the aid of artificial tunings on the almost independent MB block and the A layer. The discovery of these novel materials is expected to contribute substantially to the rapid development of ternary ceramics and to accelerate attempts in the applicability of MAX phases for heat conduction.

Hexagonal MBene (Hf2BO2): A Promising Platform for the Electrocatalysis of Hydrogen Evolution Reaction.

Hexagonal MAB (h-MAB) phases and their two-dimensional (2-D) derivatives (h-MBenes) have emerged as promising materials since the discovery of Ti2InB2. Herein, we identified that a possible h-MBene, 2-D Hf2BO2, can be an excellent platform for the electrocatalysis of hydrogen evolution reaction (HER) by density functional theory calculations. We proposed two approaches of transition metal (TM) modifications by atom deposition and implanting to optimize the HER performance of 2-D Hf2BO2. It is revealed that a moderate charge reduction of surface O, which is induced by the introduction of TM atoms, is conductive to a higher catalytic performance. The synergistic effect between implanted TM atoms and Hf2BO2 matrix can efficiently activate the surface by broadening O-p orbitals and shifting up p-band center, especially for V, Cr, and Mo as dopants, which can reduce the Gibbs free energy (ΔGH*) from 0.939 to -0.04, 0.05 and -0.04 eV, respectively. Interestingly, this effect works within a local region and the activity can also be evaluated by bond length of Hf-O, in addition to ΔGH*. This work suggests that due to its excellent electrocatalysis properties, h-MBenes can open up a new area for 2-D materials and will stimulate researchers to explore the synthesis of h-MAB phases and the stripping of h-MBenes.

Radiation damage of MoAlB at elevated temperatures: Investigating MAB phases as potential neutron shielding materials

A new family of ternary nano-laminated compounds, MAB phases, are studied as a promising class of neutron shielding materials for applications within fusion reactors. The shielding capacity against high-energy neutrons was evaluated, and the damage tolerance of MoAlB against Si+ irradiation was investigated over the temperature range of RT- 600 °C. The linear attenuation coefficients of these materials over wide neutron-energy ranges imply that Mo(W)AlB have a high neutron shielding capacity. MoAlB shows a strong resistance to crack formation and excellent tolerance to amorphization under higher temperatures. The detailed thermal dynamic behaviors (reaction barrier and migration barrier) associated with the defects in MoAlB were studied through DFT calculations. Also, the lattice parameter changes are related to the formation of various point defects and the defect evolution evidenced by Rietveld refinement of the GIXRD and DFT calculations. MoAlB is confirmed to be a great candidate as a neutron shielding material.

High-pressure monoclinic phase of MoAlB

The pressure-induced phase transitions of MoAlB have been fully studied to provide insights into the typical MAB phases under high pressure. A novel high-pressure monoclinic structure (C2/m, Z = 4) was firstly identified for the MoAlB above 234 GPa, using the first principles crystal structure prediction approach. First-order structure transition character of MoAlB from the ambient-pressure orthorhombic Cmcm phase to this high-pressure monoclinic C2/m phase was demonstrated by a volume drop of 1.28%. Inspections of crystal structures showed that the occurrence of C2/m phase follows the slip in the metallic Al-Al layers and the fracture of linear zigzag B-B chains in the Cmcm phase under compression, confirming the earlier reports. Our argument has been also supported by the softening of elastic constant under high pressure. Phonon curves calculations suggest the high-pressure C2/m phase can be quenchable to ambient pressure. The electronic and mechanical properties of this novel C2/m phase have thus been systematically investigated.

Defect chemistry of Cr-B binary and Cr-Al-B MAB phases: Effects of covalently bonded B networks

Transition metal borides, which are three-dimensional (3D) layered materials containing covalently bonded B networks, have shown a number of excellent properties, such as radiation resistance and the ability to act as a diffusion barrier in integrated circuits. However, defect behavior, which controls many of the materials' properties, has remained unknown in these materials. Here, we investigate the effects of the B networks on the defect chemistry in both binary borides (CrB, ${\mathrm{Cr}}_{3}{\mathrm{B}}_{4}$, ${\mathrm{Cr}}_{2}{\mathrm{B}}_{3}$) and ternary MAB phases (${\mathrm{Cr}}_{2}\mathrm{Al}{\mathrm{B}}_{2}$, ${\mathrm{Cr}}_{3}\mathrm{Al}{\mathrm{B}}_{4}$, ${\mathrm{Cr}}_{4}\mathrm{Al}{\mathrm{B}}_{6}$) using first-principles calculations. We find that increasing the number of B rings in the structure leads to lower formation energies and higher concentrations of Frenkel pairs. The results can be explained by the fact that the strongest Cr-B bond is weakened when borides have more B rings, leading to a reduction in the formation energy of Cr and B vacancies. Also, the bonds associated with Cr atoms bonded within B rings are softer in structures containing more B rings, which allows Cr interstitials to form with a lower energy cost and contributes to an increase in the concentration of Cr interstitials.

MBenes: Two-dimensional transition-metal borides with ordered metal vacancies

2D MBenes have been theoretically predicted to possess unique electronic structures and physicochemical properties, and thus shown great promise in various applications. However, the synthesis of individual single-layer MBene remains a grand challenge due to its orthorhombic structure of MAB phases. Recently, scientists from Linköping University have fabricated 2D monolayer Mo4/3B2-xTz with ordered metal vacancies. Their results demonstrated the feasibility of top-down approach by chemical exfoliation of laminated compounds and provided the principle for further preparation of a wealth of MBenes.

Highly Active and Abundant MAB Phases Nin+1ZnBn (n = 1, 2) toward Hydrogen Evolution

Whilst MXenes (2D carbides and nitrides) have become highly popular in several research fields including the hydrogen evolution reaction (HER), unfortunately they are not competitive HER electrocatalysts in their bulk form (MAX phases). The related MAB (2D‐like bulk borides) phases and the derived 2D MBenes, however, are less studied but show better HER properties. Herein, two highly HER‐active and abundant MAB phases, Nin+1ZnBn (n = 1, 2), are studied experimentally and computationally. The pressed pellet electrodes from bulk polycrystalline powders of these phases drive a current density of 10 mA cm−2 at impressive overpotentials of η10 = −0.171 V (n = 1) and η10= −0.145 V (n = 2) to efficiently produce hydrogen. Density functional theory (DFT) calculations prove that the most active site is the hollow site on the nickel basal plane, showing a free energy value comparable to that of the hollow site of Pt (111). This study paves the way for further development of bulk and nanoscale MAB phases for clean energy applications.

Effect of temperature on the initial reaction behavior of MAB phases (MoAlB powders) at 700–1000 °C in air

MoAlB suffers from degradation below 1000 °C owing to the formation of volatile oxides, which is critical for its application and has been less studied so far. In this work, the initial evolution of phase and morphology of MoAlB powders with the uniform size of about 50 μm reacted at 700-1000 °C for 5 h in air has been systematically investigated using thermogravimetric analysis in combination with XRD, SEM and HRTEM. The results show that the oxidation of MoAlB powders does not occur until 690 °C. The oxidation of MoAlB powders predominates at 700 °C, while the evaporation of rod-shape MoO3 and amorphous B2O3 gradually intensifies with the temperature increasing up to 950 °C. At 1000 °C, the well-developed plate-shape Al4B2O9 can hinder the further reaction to some extent. Based on this, the reaction mechanism of MoAlB powders at the initial stage is presented.

Exploring structural, electronic, and mechanical properties of 2D hexagonal MBenes

A family of two-dimensional (2D) transition metal borides, referred to as MBenes, is recently emerging as novel materials with great potentials in electronic and energy harvesting applications to the field of materials science and technology. Transition metal borides can be synthesized from chemical exfoliation of ternary-layered transition metal borides, known as MAB phases. Previously it has been predicted that thin pristine 2D Sc-, Ti-, Zr-, Hf-, V-, Nb-, Ta-, Mo-, and W-based transition metal borides with hexagonal phase are more stable than their corresponding orthorhombic phase. Here, using a set of first-principles calculations (at absolute zero temperature), we have examined the geometric, dynamic stability, electronic structures, work function, bond strength, and mechanical properties of the hexagonal monolayer of transition metal borides (M = Sc, Ti, Zr, Hf, V, Nb, Ta, Mo, and W) chemically terminated with F, O, and OH. The results of the formation energies of terminated structures imply that the surface terminations could make a strong bond to the surface transition metals and provide the possibility of the development of transition metal borides with those surface terminations. Except for ScBO, which is an indirect bandgap semiconductor, the other transition metal borides are metallic or semimetal. Particularly, TiBF, ZrBF, and HfBF are metallic systems whose band dispersions close to the Fermi level indicate the coexistence of type-I and type-II nodal lines. Our calculated work functions indicate that 2D transition metal borides with OH (O) functionalization obtain the lowest (highest) work functions. The results of the mechanical properties of the considered structures imply that oxygen functionalized transition metal borides exhibit the stiffest mechanical strength with 248 < E (N m−1) < 348 while non-terminated transition metal borides are generally the weakest systems with 206 < E (N m−1) < 283.

Predictions of attainable compositions of layered quaternary i-MAB phases and solid solution MAB phases.

MAB phases are layered materials combining metallic and ceramic attributes. Their ternary compositions, however, have been limited to a few elemental combinations which makes controlled and tailored properties challenging. Inspired by the recent discovery of Mo4/3Y2/3AlB2 and Mo4/3Sc2/3AlB2i-MAB phases, i.e., quaternary layered MAB phases with in-plane chemical order, we perform an extensive first-principles study to explore formation of chemical order and solid-solutions upon metal alloying of M2AB2 phases of 1092 compositions (M from group 3 to 9 and A = Al, Ga, In, Si, Ge, Sn). This large dataset provides 39 chemically ordered (i-MAB) and 52 solid solution (MAB) phases that are predicted to be thermodynamically stable at typical synthesis temperatures, of which a majority have not yet been experimentally reported. The possibility for realizing both i-MAB and solid solution MAB phases, combined with the multiple elemental combinations previously not observed in these boride-based materials, allows for an increased potential for property tuning and potential chemical exfoliation into 2D derivatives.

Designing of magnetic MAB phases for energy applications

Based on high-throughput density functional theory calculations, we performed screening for stable magnetic MAB compounds and predicted potential strong magnets for permanent magnet and magnetocaloric applications.

Theoretical exploration of quaternary hexagonal MAB phases and two-dimensional derivatives.

The newly discovered hexagonal MAB (h-MAB) phases have shown great promise in synthesizing 2-D borides, but wide application is hindered by the limited kinds of transition metals that can be involved. In this work, an extensive structure search was performed to identify stable quaternary h-MAB phases by alloying ternary h-MAB phases with a fourth component. The predicted 22 stable quaternary h-MAB phases reveal that there is plenty of room for further exploration of new materials by element alloying. Moreover, we theoretically proved the possibility of exfoliating h-MBenes from the predicted quaternary phases through the selective removal of A components. The simulations for the hydrogen evolution reaction (HER) revealed that the bi-metal combination demonstrates a great advantage to enrich the application perspectives of h-MBenes. The discovery of quaternary h-MAB phases and two-dimensional derivatives offers new insights and understanding of boride-based materials.