We report the synthesis of three out-of-plane chemically ordered quaternary transition metal borides (o-MAB phases) of the chemical formula M4CrSiB2 (M = Mo, W, Nb). The addition of these phases to ...the recently discovered o-MAB phase Ti4MoSiB2 shows that this is indeed a new family of chemically ordered atomic laminates. Furthermore, our results expand the attainable chemistry of the traditional M5SiB2 MAB phases to also include Cr. The crystal structure and chemical ordering of the produced materials were investigated using high-resolution scanning transmission electron microscopy and X-ray diffraction by applying Rietveld refinement. Additionally, calculations based on density functional theory were performed to investigate the Cr preference for occupying the minority 4c Wyckoff site, thereby inducing chemical order.
Structural design on the atomic level can provide novel chemistries of hybrid MAX phases and their MXenes. Herein, density functional theory is used to predict phase stability of quaternary i‐MAX ...phases with in‐plane chemical order and a general chemistry (W2/3M21/3)2AC, where M2 = Sc, Y (W), and A = Al, Si, Ga, Ge, In, and Sn. Of over 18 compositions probed, only two—with a monoclinic C2/c structure—are predicted to be stable: (W2/3Sc1/3)2AlC and (W2/3Y1/3)2AlC and indeed found to exist. Selectively etching the Al and Sc/Y atoms from these 3D laminates results in W1.33C‐based MXene sheets with ordered metal divacancies. Using electrochemical experiments, this MXene is shown to be a new, promising catalyst for the hydrogen evolution reaction. The addition of yet one more element, W, to the stable of M elements known to form MAX phases, and the synthesis of a pure W‐based MXene establishes that the etching of i‐MAX phases is a fruitful path for creating new MXene chemistries that has hitherto been not possible, a fact that perforce increases the potential of tuning MXene properties for myriad applications.
A new 2D material (MXene) based on W and C is developed through selective etching of the new parent atomic laminates (W2/3Sc1/3)2AlC and (W2/3Y1/3)2AlC. The latter i‐MAX phases are discovered through theoretical predictions combined with experimental verification. The W1.33C MXene displays vacancy ordering, and is shown to be a promising catalyst for the hydrogen evolution reaction.
Refractory transition-metal diborides (TMB2) are candidates for extreme environments due to melting points above 3000 °C, excellent hardness, good chemical stability, and thermal and electrical ...conductivity. However, they typically suffer from rapid high-temperature oxidation. Here, we study the effect of Al addition on the oxidation properties of sputter-deposited TiB2-rich Ti1-xAlxBy thin films and demonstrate that alloying the films with Al significantly increases the oxidation resistance with a slight decrease in hardness. TiB2.4 layers are deposited by dc magnetron sputtering (DCMS) from a TiB2 target, while Ti1-xAlxBy alloy films are grown by hybrid high-power impulse and dc magnetron co-sputtering (Al-HiPIMS/TiB2-DCMS). All as-deposited films exhibit columnar structure. The column boundaries of TiB2.4 are B-rich, while Ti0.68Al0.32B1.35 alloys have Ti-rich columns surrounded by a Ti1-xAlxBy tissue phase which is predominantly Al rich. Air-annealing TiB2.4 at temperatures above 500 °C leads to the formation of oxide scales that do not contain B and mostly consist of a rutile-TiO2 (s) phase. The resulting oxidation products are highly porous due to the evaporation of B2O3 (g) phase as well as the coarsening of TiO2 crystallites. This poor oxidation resistance is significantly improved by alloying with Al. While air-annealing at 800 °C for 0.5 h results in the formation of an ~1900-nm oxide scale on TiB2.4, the thickness of the scale formed on the Ti0.68Al0.32B1.35 alloys is ~470 nm. The enhanced oxidation resistance is attributed to the formation of a dense, protective Al-containing oxide scale that considerably decreases the oxygen diffusion rate by suppressing the oxide-crystallites coarsening.
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Transition-metal diboride thin films, which have high melting points, excellent hardness, and good chemical and thermal conductivity, severely suffer from rapid oxidation in air. Here, we explore the ...influence of varying B content and resulting nanostructure change on the oxidation properties of TiBx thin films, with x = 1.43, 2.20, and 2.70. Results show that all as-deposited layers have columnar structure. The column boundaries of as-deposited TiB2.20 and TiB2.70 films grown by direct current magnetron sputtering (DCMS) are B-rich, while the as-deposited TiB1.43 films grown by high-power impulse magnetron sputtering (HiPIMS) show no apparent grain boundary phases and contain Ti-rich planar defects. The oxidation rate of TiB1.43 air-annealed at 400 °C up to 48 h is significantly lower than that of TiB2.20 and TiB2.70 films. The oxidation rate of TiB1.43, TiB2.20, and TiB2.70 films was measured at 2.9 ± 1.5, 7.1 ± 1.0, and 20.0 ± 5.0 nm/h, respectively, with no spallation of even as thick oxide scales as 0.5 μm in any of the films. The improved oxidation resistance can be explained by the absence of B-rich tissue phase at the column boundaries of understoichiometric TiBx films, a phase that interlaces the nanocolumnar TiB2 structures in the corresponding overstoichiometric films. An easy oxidation pathway is thus eliminated.
•Improved oxidation resistance in TiBx thin films with reduced boron content•Microstructural differences in over- and understoichiometric TiBx dictates oxidation kinetics.•Understoichiometric TiBx thin films exhibit superior oxidation resistance and mechanical properties compared to overstoichiometric TiBx.
TiBx thin films with a B content of 1.43 ≤ x ≤ 2.70 were synthesized using high-power impulse magnetron sputtering (HiPIMS) and direct-current magnetron sputtering (DCMS). HiPIMS allows compositions ...ranging from understoichiometric to overstoichiometric dense TiBx thin films with a B/Ti ratio between 1.43 and 2.06, while DCMS yields overstoichiometric TiBx films with a B/Ti ratio ranging from 2.20 to 2.70. Excess B in overstoichiometric TiBx thin films from DCMS results in a hardness up to 37.7 ± 0.8 GPa, attributed to the formation of an amorphous B-rich tissue phase interlacing stoichiometric TiB2 columnar structures. We furthermore show that understoichiometric TiB1.43 thin films synthesized by HiPIMS, where the deficiency of B is found to be accommodated by Ti-rich planar defects, exhibit a superior hardness of 43.9 ± 0.9 GPa. The apparent fracture toughness and thermal conductivity of understoichiometric TiB1.43 HiPIMS films are 4.2 ± 0.1 MPa√m and 2.46 ± 0.22 W/(m·K), respectively, as compared to corresponding values for overstoichiometric TiB2.70 DCMS film samples of 3.1 ± 0.1 MPa√m and 4.52 ± 0.45 W/(m·K). This work increases the fundamental understanding of understoichiometric TiBx thin films and their materials properties, and shows that understoichiometric films have properties matching or going beyond those with excess B.
•B deficiency accommodated by Ti planar defects in understoichiometric TiBx.•Superior mechanical properties for understoichiometric TiBx•Deposition fluxes in HiPIMS and DCMS gives rise to widely different compositions.
Synthesis of delaminated 2D W1.33C (MXene) has been performed by selectively etching Al as well as Sc/Y from the recently discovered nanolaminated i-MAX phases (W2/3Sc1/3)2AlC and (W2/3Y1/3)2AlC. ...Both quaternary phases produce MXenes with similar flake morphology and with a skeletal structure due to formation of ordered vacancies. The measured O, OH, and F terminations, however, differ in amount as well as in relative ratios, depending on parent material, evident from X-ray photoelectron spectroscopy. These findings are correlated to theoretical simulations based on first-principles, investigating the W1.33C, and the effect of termination configurations on structure, formation energy, stability, and electronic structure. The theoretical results indicate a favored F-rich surface composition, though with a system going from insulating/semiconducting to metallic for different termination configurations, suggesting a high tuning potential of these materials. Additionally, free-standing W1.33C films of 2–4 μm thickness and with up to 10 wt % polymer (PEDOT:PSS) were tested as electrodes in supercapacitors, showing capacitances up to 600 F cm–3 in 1 M H2SO4 and high capacitance retention for at least 10000 cycles at 10 A g–1. This is highly promising results compared to other W-based materials to date.
The atomic structure and local composition of high quality epitaxial substoichiometric titanium diboride (TiB1.9) thin film, deposited by unbalanced magnetron sputtering, were studied using ...analytical high-resolution scanning transmission electron microscopy, density functional theory, and image simulations. The unpaired Ti is pinpointed to inclusion of Ti-based stacking faults within a few atomic layers, which terminates the {11¯00} prismatic planes of the crystal structure and attributed to the absence of B between Ti planes that locally relaxes the structure. This mechanism allows the line compound to accommodate off-stoichiometry and remain a line compound between defects. The planar defects are embedded in otherwise stoichiometric TiB2 and are delineated by insertion of dislocations. An accompanied decrease in Ti-Ti bond lengths along and across the faults is observed.
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CrB x thin films with 1.90 < x < 2.08 have been deposited by direct-current magnetron sputtering (DCMS) from a stoichiometric CrB 2 target at 5 and 20 mTorr (0.67 and 2.67 Pa) Ar ...pressure onto sapphire (0 0 01) substrates. All films, irrespective of deposition conditions, exhibit a (0 0 01) texture. Attesting to the achievement of close-to-stoichiometric composition, epitaxial film growth is observed at 900 ?C, while film growth at 500 ?C yields (0001) fiber texture. Film composition does not depend on substrate temperature but exhibits slightly reduced B content with increasing pressure for samples deposited at 900 ?C. Excess B in the overstoichiometric epitaxial CrB 2.08 films segregates to form B-rich inclusions. Understoichiometry in CrB 1.90 films is accommodated by Cr-rich stacking faults on { 1 1? 00 } prismatic planes. ? 2021 The Author(s). Published by Elsevier Ltd on behalf of Acta Materialia Inc. This is an open access article under the CC BY license ( http://creativecommons.org/licenses/by/4.0/ )
Exploratory theoretical predictions in uncharted structural and compositional space are integral to materials discoveries. Inspired by M5SiB2 (T2) phases, the finding of a family of laminated ...quaternary metal borides, M′4M″SiB2, with out‐of‐plane chemical order is reported here. 11 chemically ordered phases as well as 40 solid solutions, introducing four elements previously not observed in these borides are predicted. The predictions are experimentally verified for Ti4MoSiB2, establishing Ti as part of the T2 boride compositional space. Chemical exfoliation of Ti4MoSiB2 and select removal of Si and MoB2 sub‐layers is validated by derivation of a 2D material, TiOxCly, of high yield and in the form of delaminated sheets. These sheets have an experimentally determined direct band gap of ≈4.1 eV, and display characteristics suitable for supercapacitor applications. The results take the concept of chemical exfoliation beyond currently available 2D materials, and expands the envelope of 3D and 2D candidates, and their applications.
Out‐of‐plane chemical order in layered T2 phase Ti4MoSiB2 (o‐MAB) is theoretically predicted and experimentally verified. Chemical exfoliation of the compound is shown by derivation of a 2D material, TiOxCly. The 2D sheets have a direct bandgap of ≈4.1 eV and display characteristics suitable for supercapacitor applications.
Direct-current magnetron sputtering (DCMS) and high-power impulse magnetron sputtering (HiPIMS) were used to deposit understoichiometric Ti1-xAlxB2-y diboride coatings by sputtering from a segmented ...TiB2-AlB2 target using Ar and Kr as sputtering gas. For films with a fixed Al/(Ti + Al) ratio of x = 0.1 (Ti0.9Al0.1B2-y), the B content was varied with y ∈ (0.1, 0.6 and 0.7). For films with a fixed y = 0.7 (Ti1-xAlxB1.3), the Al content was varied with x ∈ (0.1, 0.4 and 0.7). Evaluation of the mechanical properties of the Ti1-xAlxB1.3 samples showed a reduction in both hardness and elastic modulus with increasing Al concentration, while the Ti0.9Al0.1B2-y samples showed a hardness increase with decreasing B content. Thus, Ti0.9Al0.1B1.3 films exhibited a superior hardness of 46.2 ± 1.1 GPa and an elastic modulus of 523 ± 7 GPa, compared to the values for Ti0.9Al0.1B1.4 and Ti0.9Al0.1B1.9, showing a hardness of 44 ± 1 GPa and 36 ± 1 GPa, and an elastic modulus of 569 ± 7 GPa and 493 ± 6 GPa, respectively. The oxidation behavior of the mechanically most promising Ti0.9Al0.1B2-y sample series was investigated through air-annealing at 600 °C for durations from 1 h to 10 h. All films formed a mixed non-conformal Al2O3-TiO2 oxide scale which acts as an inward and outward diffusion barrier, significantly reducing the oxidation rate compared to TiBz films, which form an oxide scale consisting of porous TiO2. The thinnest oxide scale after 10 h was found in the B-deficient samples, Ti0.9Al0.1B1.3 and Ti0.9Al0.1B1.4, at ~200 nm, which is significantly below that for Ti0.9Al0.1B1.9 at 320 nm. The enhanced oxidation resistance of highly understoichiometric films is due to the elimination of the B-rich tissue phase that is present at the grain boundaries for higher B content, where the latter has been shown to enhance the rate of oxidation in borides.
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