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Herein we topotactically reacted sulphur, S, with Mo2CTx MXene powders to synthesize Mo2CTx sheets covered by MoS2 nanosheets and tested the resulting material as a hydrogen evolution reaction, HER, electrocatalyst. As a result, a high electrochemical active surface area catalyst was obtained. It allows driving a current density of 10 mA cm−2 at a low overpotential of only 150 mV. Stabilization of the water activated complex as well as interfacial charge transfer accompanied by changes in the adsorption energy hydrogen are most likely responsible for the high electrochemical activity. Moreover, no efficiency loss was observed under working conditions during 30 h in a 1 mol l−1 KOH electrolyte, confirming the remarkable electrochemical stability of this composite catalyst.
Synthesizing high-performance, multifunctional composites often entails poor filler dispersion, complex processes, or limited processing conditions. Hence, the advent of versatile syntheses employing simple conditions, inexpensive fillers, standard equipment, and robust reactions could expand attainable composites having enhanced performance, functionality, and customizability. We report on the synthesis of a novel multifunctional elastomeric composite from a new, reinforcing titania-based filler and a repairable, dynamic-covalent thiol-yne network or dissociative covalent adaptable network. Composites were processed by in situ polymerization via thiol-yne "click" chemistry initiated with a trialkylborane. Composite processing yields quantitative monomer conversion and uniform filler dispersion. The filler was a titania-based mesoparticle, self-assembled from 1D lepidocrocite nanofilaments through a recently discovered, scalable, and nearly universal route to wide-ranging nanostructures. A 47-fold enhancement in the modulus was achieved with 60 wt % filler versus the neat polymer, agreeing with theory. The reinforced, cross-linked composites were multifunctional, providing reprocessability over multiple damage-repair cycles with restoration of their mechanical properties.
Since the discovery of the first MXene, Ti3C2Tz, in 2011, more than 30 new MXene chemistries have been reported. A universal method via which all MXenes can be synthesized is by etching the "A" layer from the MAX phase using hydrofluoric acid, HF. After etching, to delaminate the resulting MXene multilayers, MLs, they are typically treated with a surfactant like tetrabutylammonium hydroxide, TBAOH, tetramethylammonium hydroxide, TMAOH, dimethyl sulfoxide, DMSO, etc. In our earlier work, we found that treating HF-etched Ti3C2Tz MLs with a base like NaOH renders them ion-exchangeable, similar to the earlier reported treatments using expensive and more toxic bases like TMAOH or TBAOH. In this work, we study the effects of sodium hydroxide, NaOH, treatment on Nb2CTz and Mo2Ti2C3Tz MXene MLs. We show that, similar to Ti3C2Tz, Na ions also intercalate in these two MXenes. This was clearly confirmed by X-ray diffraction analysis, which showed an increase in the interlayer spacing post treatment of HF-etched MXenes with NaOH. This increase in the interlayer space reduces the van der Waals interaction between the flakes, allowing for their facile delamination to single or few layers. It was also observed that NaOH treatment reduces the flakes' surface oxidation during processing, compared to when TBAOH is used. The surface chemistry and morphology of the MXenes post NaOH and TBAOH treatments were also analyzed using X-ray photoelectron spectroscopy and scanning electron microscopy, respectively, and compared. Finally, we conclude that inexpensive NaOH can be used to substitute for TBAOH or TMAOH, conventionally used for MXene delamination.
Reactive single-step hot-pressing at 1473 K and 35 MPa for 4 h produces dense, bulk, near single-phase, low-cost and low-criticality Fe$_2$Al$_{1.15}$B$_2$ and Fe$_2$Al$_{1.1}$B$_2$Ge$_{0.05}$Ga$_{0.05}$ MAB samples, showing a second-order magnetic phase transition with favorable magnetocaloric properties around room temperature. The magnetic as well as magnetocaloric properties can be tailored upon Ge and Ga doping, leading to an increase of Curie temperature $T_C$ and spontaneous magnetization $m_S$. The maximum isothermal entropy change $\Delta s_{T,max}$ of hot-pressed Fe$_2$Al$_{1.15}$B$_2$ in magnetic field changes of 2 and 5 T amounts to 2.5 and 5 J(kgK)$^{-1}$ at 287.5 K and increases by Ge and Ga addition to 3.1 and 6.2 J(kgK)$^{-1}$ at 306.5 K, respectively. The directly measured maximum adiabatic temperature change $\Delta T_{ad,max}$ is improved by the composition modification from 0.9 to 1.1 K in magnetic field changes of 1.93 T. Overall, we demonstrate that hot-pressing provides a much faster, more scalable and processing cost reducing alternative compared to conventional synthesis routes to produce heat exchangers for magnetic cooling devices. Therefore, our criticality assessment shows that hot-pressed Fe-based MAB phases provide a promising compromise of material and processing cost, criticality and magnetocaloric performance, demonstrating the potential for low-cost and low-criticality magnetocaloric applications around room temperature.
Abstract Two-dimensional, 2D, niobium carbide MXene, Nb 2 CT x , has attracted attention due to its extraordinarily high photothermal conversion efficiency that has applications ranging from medicine, for tumor ablation, to solar energy conversion. Here, we characterize its electronic properties and investigate the ultrafast dynamics of its photoexcitations with a goal of shedding light onto the origins of its unique properties. Through density functional theory, DFT, calculations, we find that Nb 2 CT x is metallic, with a small but finite DOS at the Fermi level for all experimentally relevant terminations that can be achieved using HF or molten salt etching of the parent MAX phase, including –OH, –O, –F, –Cl, –Br, –I. In agreement with this prediction, THz spectroscopy reveals an intrinsic long-range conductivity of ∼60 Ω −1 cm −1 , with significant charge carrier localization and a charge carrier density (∼10 20 cm −3 ) comparable to Mo-based MXenes. Excitation with 800 nm pulses results in a rapid enhancement in photoconductivity, which decays to less than 25% of its peak value within several picoseconds, underlying efficient photothermal conversion. At the same time, a small fraction of photoinjected excess carriers persists for hundreds of picoseconds, and can potentially be utilized in photocatalysis or other energy conversion applications.
In desert environment, wind turbines blades undergo severe erosion process caused by air-borne sand particles. The erosion damage on blade surface is sensitive to particles velocity, mass flux and impingement angle. The objective of the present work is to get insight into the underlying mechanics of damage evolution by erodent particles in coated Glass Fiber Reinforced Polymer (GFRP) at different impingement angles within the framework of Discrete Element Method and Finite Element (DEM-FE). This paper presents a novel experimental technique to measure sand particles velocity which is then compared to Computational Fluid Dynamics (CFD) simulations based on Eulerian-Eulerian multiphase flow model. The computed sand solid phase velocity and mass flux were used into the DEM-FE analysis to investigate the erosion damage on the coated GFRP surface at multiple impingement angles. Primary findings of CFD show strong dependence between sand particles velocity and its volume fraction. DEM-FE results showed that, the evolution of eroded surface is strongly dependent on the particles impingement angle; in normal impact, the maximum material removal occurs initially, and in oblique impact there is a gradual removal of material along the erosion process.
Garnering attention for high conductivity, nonlinear optical properties, and more, MXenes are water-processable 2D materials that are considered candidates for applications in electromagnetic interference shielding, optoelectronic and photonic devices among others. Herein we investigate the intrinsic and photoexcited conductivity in Nb <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</inf> CT <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x</inf> , a MXene with reported high photothermal conversion efficiency. DFT calculations show that hydroxyl and/or fluorine-terminated $\mathrm{Nb}_{2} \mathrm{CT}_{x}\left(\mathrm{~T}_{x}=\mathrm{OH}\right.$ or $\left.\mathrm{F}\right)$ is metallic, in agreement with THz spectroscopy, which reveals the presence of free charge carriers that are highly localized over mesoscopic length scales. Photoexcitation of Nb <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</inf> CT <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x</inf> , known to result in rapid heating of the crystal lattice, is found to produce additional free carriers and a transient enhancement of photoconductivity. Most photoexcited carriers decay over the sub-picosecond time scales while a small fraction remain for much longer, sub-nanoseconds, times.
Reactive single-step hot-pressing at 1473 K and 35 MPa for 4 h produces dense, bulk, near single-phase, low-cost and low-criticality Fe$_2$Al$_{1.15}$B$_2$ and Fe$_2$Al$_{1.1}$B$_2$Ge$_{0.05}$Ga$_{0.05}$ MAB samples, showing a second-order magnetic phase transition with favorable magnetocaloric properties around room temperature. The magnetic as well as magnetocaloric properties can be tailored upon Ge and Ga doping, leading to an increase of Curie temperature $T_C$ and spontaneous magnetization $m_S$. The maximum isothermal entropy change $Δs_{T,max}$ of hot-pressed Fe$_2$Al$_{1.15}$B$_2$ in magnetic field changes of 2 and 5 T amounts to 2.5 and 5 J(kgK)$^{-1}$ at 287.5 K and increases by Ge and Ga addition to 3.1 and 6.2 J(kgK)$^{-1}$ at 306.5 K, respectively. The directly measured maximum adiabatic temperature change $ΔT_{ad,max}$ is improved by the composition modification from 0.9 to 1.1 K in magnetic field changes of 1.93 T. Overall, we demonstrate that hot-pressing provides a much faster, more scalable and processing cost reducing alternative compared to conventional synthesis routes to produce heat exchangers for magnetic cooling devices. Therefore, our criticality assessment shows that hot-pressed Fe-based MAB phases provide a promising compromise of material and processing cost, criticality and magnetocaloric performance, demonstrating the potential for low-cost and low-criticality magnetocaloric applications around room temperature.
Reactive single-step hot-pressing at 1473 K and 35 MPa for 4 h produces dense, bulk, near single-phase, low-cost and low-criticality Fe <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</inf> Al <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1.15</inf> B <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</inf> and Fe <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</inf> Al <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1.1</inf> B <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</inf> Ge <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.05</inf> Ga <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.05</inf> MAB samples, showing a second-order magnetic phase transition with favorable magnetocaloric properties around room temperature. The magnetic as well as magnetocaloric properties can be tailored upon Ge and Ga doping, leading to an increase of Curie temperature and spontaneous magnetization. The maximum isothermal entropy change of hot-pressed Fe <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</inf> Al <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1.15</inf> B <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</inf> in magnetic field changes of 2 and 5 T amounts to 2.5 and 5 J(kgK) <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-1</sup> at 287.5 K and increases by Ge and Ga addition to 3.1 and 6.2 J(kgK) <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-1</sup> at 306.5 K, respectively. The directly measured maximum adiabatic temperature change is improved by the composition modification from 0.9 to 1.1 K in magnetic field changes of 1.93 T. Overall, we demonstrate that hot-pressing provides a much faster, more scalable and processing cost reducing alternative compared to conventional synthesis routes to produce heat exchangers for magnetic cooling devices. Therefore, our criticality assessment shows that hot-pressed Fe-based MAB phases provide a promising compromise of material and processing cost, criticality and magnetocaloric performance, demonstrating the potential for low-cost and low-criticality magnetocaloric applications around room temperature.
