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Synthesis and Characterization of an Alumina Forming Nanolaminated Boride: MoAlB.

ABSTRACT: The 'MAlB' phases are nanolaminated, ternary transition metal borides that consist of a transition metal boride sublattice interleaved by monolayers or bilayers of pure aluminum. However, their synthesis and properties remain largely unexplored. Herein, we synthesized dense, predominantly single-phase samples of one such compound, MoAlB, using a reactive hot pressing method. High-resolution scanning transmission electron microscopy confirmed the presence of two Al layers in between a Mo-B sublattice. Unique among the transition metal borides, MoAlB forms a dense, mostly amorphous, alumina scale when heated in air. Like other alumina formers, the oxidation kinetics follow a cubic time-dependence. At room temperature, its resistivity is low (0.36-0.49 ??m) and - like a metal - drops linearly with decreasing temperatures. It is also a good thermal conductor (35?Wm(-1)K(-1) at 26?°C). In the 25-1300?°C temperature range, its thermal expansion coefficient is 9.5?×?10(-6?)K(-1). Preliminary results suggest the compound is stable to at least 1400?°C in inert atmospheres. Moderately low Vickers hardness values of 10.6?±?0.3 GPa, compared to other transition metal borides, and ultimate compressive strengths up to 1940?±?103 MPa were measured at room temperature. These results are encouraging and warrant further study of this compound for potential use at high temperatures.

SUBMITTER: Kota S

PROVIDER: S-EPMC4879536 | biostudies-literature | 2016 May

REPOSITORIES: biostudies-literature

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Synthesis and Characterization of an Alumina Forming Nanolaminated Boride: MoAlB.

Kota Sankalp S Zapata-Solvas Eugenio E Ly Alexander A Lu Jun J Elkassabany Omar O Huon Amanda A Lee William E WE Hultman Lars L May Steve J SJ Barsoum Michel W MW

Scientific reports 20160525

The 'MAlB' phases are nanolaminated, ternary transition metal borides that consist of a transition metal boride sublattice interleaved by monolayers or bilayers of pure aluminum. However, their synthesis and properties remain largely unexplored. Herein, we synthesized dense, predominantly single-phase samples of one such compound, MoAlB, using a reactive hot pressing method. High-resolution scanning transmission electron microscopy confirmed the presence of two Al layers in between a Mo-B sublat ...[more]

PMID: 27220751

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Project description:Recently, two-dimensional transition metal borides (2D MBenes) have demonstrated advanced physical, mechanical, and electrochemical performance; however, there are fewer studies in the literature. In particular, 2D molybdenum boride (2D MoB), created from molybdenum aluminum boride (MoAlB, MAB phase), is one of the important materials introduced due to its thermodynamically stable structure. Herein, we present the preparation of MoAlB and its chemical exfoliation by employing NaOH and LiF-HCl etching solutions to synthesize 2D MoB MBene nanostructures. It is found that the etching conditions significantly affect the selective extraction of the interlayer Al from the MAB phase in solution, forming 2D MoB MBene nanostructures. The resulting 2D MoB MBene illustrates sheet-like morphology, abundant redox active sites, and moderate textural features, which are critical for electrolyte accumulation and transfer. Powder X-ray Diffraction (PXRD) analysis confirmed the crystallinity and phase purity, while Scanning Electron Microscopy (SEM) and Transmission Electron Microscopy (TEM) revealed the sheet-like morphology and structural integrity. X-ray Photoelectron Spectroscopy (XPS) was employed to analyze the elemental composition and oxidation states, confirming the successful removal of Al and the formation of MoB MBene. Additionally, Brunauer-Emmett-Teller (BET) surface area analysis provided insights into the porous nature and surface area, which are crucial for electrolyte interactions. The all-solid-state flexible interdigitated supercapacitor (ISSC) device prepared using 10/MoAl1-x B electrode exhibited a good areal capacitance of 20.3 mF/cm2, outstanding cyclic capability with 92% capacitance retention after 1300 cycles, and remarkable flexibility. Last but not least, the Density Functional Theory (DFT) of the optimal MoB sample was constructed in terms of structural, conductivity, and thermodynamic stability perspectives, which is well-aligned with experimental results. Applying these 2D MoB MBene electrodes as all-solid-state flexible interdigitated electrodes opens a new avenue in supercapacitor technology.

Project description:Coating system(s) will be required for Nb-silicide based alloys. Alumina forming alloys that are chemically compatible with the Nb-silicide based alloy substrate could be components of such systems. The intermetallic alloys Nb1.45Si2.7Ti2.25Al3.25Hf0.35 (MG5) and Nb1.35Si2.3Ti2.3Al3.7Hf0.35 (MG6) were studied in the cast, heat treated and isothermally oxidised conditions at 800 and 1200 °C to find out if they are αAl₂O₃ scale formers. A (Al/Si)alloy versus Nb/(Ti + Hf)alloy map, which can be considered to be a map for Multi-Principle Element or Complex Concentrated Nb-Ti-Si-Al-Hf alloys, and a [Nb/(Ti + Hf)]Nb5Si3 versus [Nb/(Ti + Hf)]alloy map were constructed making use of the alloy design methodology NICE and data from a previously studied alloy, and were used to select the alloys MG5 and MG6 that were expected (i) not to pest, (ii) to form αAl₂O₃ scale at 1200 °C, (iii) to have no solid solution, (iv) to form only hexagonal Nb₅Si₃ and (v) to have microstructures consisting of hexagonal Nb₅Si₃, Ti₅Si₃, Ti₅Si₄, TiSi silicides, and tri-aluminides and Al rich TiAl. Both alloys met the requirements (i) to (v). The alumina scale was able to self-heal at 1200 °C. Liquation in the alloy MG6 at 1200 °C was linked with the formation of a eutectic like structure and the TiAl aluminide in the cast alloy. Key to the oxidation of the alloys was the formation (i) of "composite" silicide grains in which the Nb₅Si₃ core was surrounded by the Ti₅Si₄ and TiSi silicides, and (ii) of tri-aluminides with high Al/Si ratio, particularly at 1200 °C and very low Nb/Ti ratio forming in-between the "composite" silicide grains. Both alloys met the "standard definition" of high entropy alloys (HEAs). Compared with HEAs with bcc solid solution and intermetallics, the VEC values of both the alloys were outside the range of reported values. The parameters VEC,  and  of Nb-Ti-Si-Al-Hf coating alloys and non-pesting Nb-silicide based alloys were compared and trends were established. Selection of coating alloys with possible "layered" structures was discussed and alloy compositions were proposed.

Dataset Information

Synthesis and Characterization of an Alumina Forming Nanolaminated Boride: MoAlB.

Publications

Synthesis and Characterization of an Alumina Forming Nanolaminated Boride: MoAlB.

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