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Λιόλιος Ξερολιβαδιώτης : ένας αγνοημένος Μακεδόνας αγωνιστής του '21 και λίγα για την ιστορία του Ξερολιβάδου/ Γιώργου Χ. Χιονίδη

Dynamic relaxations and relaxation-property relationships in metallic glasses

Electrochemical water splitting offers an attractive approach for hydrogen production. However, the lack of high-performance cost-effective electrocatalyst severely hinders its applications. Here, a multinary high-entropy intermetallic (HEI) that possesses an unusual periodically ordered structure containing multiple non-noble elements is reported, which can serve as a highly efficient electrocatalyst for hydrogen evolution. This HEI exhibits excellent activities in alkalinity with an overpotential of 88.2 mV at a current density of 10 mA cm-2 and a Tafel slope of 40.1 mV dec-1 , which are comparable to those of noble catalysts. Theoretical calculations reveal that the chemical complexity and surprising atomic configurations provide a strong synergistic function to alter the electronic structure. Furthermore, the unique L12 -type ordered structure enables a specific site-isolation effect to further stabilize the H2 O/H* adsorption/desorption, which dramatically optimizes the energy barrier of hydrogen evolution. Such an HEI strategy uncovers a new paradigm to develop novel electrocatalyst with superior reaction activities.

A Novel Multinary Intermetallic as an Active Electrocatalyst for Hydrogen Evolution.

Iridium-based nanomaterials for electrochemical water splitting

As a potential energy carrier, hydrogen has surged up the priority list as part of broader decarbonization efforts and strategies to build or acquire clean energy economies. Driven by renewable electricity, electrochemical water splitting (WS) promises an ideal long-term, low-carbon way to produce hydrogen, with the ability to tackle various critical energy challenges. To improve the efficiency of electrocatalytic water splitting, electrocatalysts with enhanced conductivity, more exposed active sites, and high intrinsic activity are crucial for decreasing the energy gap for the rate-determining step (RDS) and subsequently improving the conversion efficiency. The incorporation of multidimensional imperfections has been demonstrated to be efficient for modulating the electron distribution and speeding up the electrocatalysis kinetics during electrocatalytic processes and this is now attracting ever-increasing attention. Herein, in this review, we summarize recent progress relating to the regulation of electrical behavior and electron distributions for the optimization of electrocatalytic water-splitting performance via defect engineering. With an emphasis on the beneficial aspects of the hydrogen economy and an in-depth understanding of electron redistribution caused by defect effects, we offer a comprehensive summary of the progress made in the last three to five years. Finally, we also offer future perspectives on the challenges and opportunities relating to water-splitting electrocatalysts in this attractive field.

Perfecting electrocatalysts via imperfections: towards the large-scale deployment of water electrolysis technology

Since their discovery in 19601, metallic glasses based on a wide range of elements have been developed2. However, the theoretical prediction of glass-forming compositions is challenging and the discovery of alloys with specific properties has so far largely been the result of trial and error3–8. Bulk metallic glasses can exhibit strength and elasticity surpassing those of conventional structural alloys9–11, but the mechanical properties of these glasses are critically dependent on the glass transition temperature. At temperatures approaching the glass transition, bulk metallic glasses undergo plastic flow, resulting in a substantial decrease in quasi-static strength. Bulk metallic glasses with glass transition temperatures greater than 1,000 kelvin have been developed, but the supercooled liquid region (between the glass transition and the crystallization temperature) is narrow, resulting in very little thermoplastic formability, which limits their practical applicability. Here we report the design of iridium/nickel/tantalum metallic glasses (and others also containing boron) with a glass transition temperature of up to 1,162 kelvin and a supercooled liquid region of 136 kelvin that is wider than that of most existing metallic glasses12. Our Ir–Ni–Ta–(B) glasses exhibit high strength at high temperatures compared to existing alloys: 3.7 gigapascals at 1,000 kelvin9,13. Their glass-forming ability is characterized by a critical casting thickness of three millimetres, suggesting that small-scale components for applications at high temperatures or in harsh environments can readily be obtained by thermoplastic forming14. To identify alloys of interest, we used a simplified combinatorial approach6–8 harnessing a previously reported correlation between glass-forming ability and electrical resistivity15–17. This method is non-destructive, allowing subsequent testing of a range of physical properties on the same library of samples. The practicality of our design and discovery approach, exemplified by the identification of high-strength, high-temperature bulk metallic glasses, bodes well for enabling the discovery of other glassy alloys with exciting properties. Bulk metallic glasses made from alloys of iridium, nickel, tantalum and boron are developed by combinatorial methods, with higher strength at high temperature than those previously produced.

High-temperature bulk metallic glasses developed by combinatorial methods

Although various catalytic materials have emerged for hydrogen evolution reaction (HER), it remains crucial to develop intrinsically effective catalysts with minimum uses of expensive and scarce precious metals. Metallic glasses (MGs) or amorphous alloys show up as attractive HER catalysts, but have so far limited to material forms and compositions that result in high precious-metal loadings. Here, an Ir25 Ni33 Ta42 MG nanofilm exhibiting high intrinsic activity and superior stability at an ultralow Ir loading of 8.14 µg cm-2 for HER in 0.5 m H2 SO4 is reported. With an overpotential of 99 mV for a current density of 10 mA cm-2 , a small Tafel slope of 35 mV dec-1 , and high turnover frequencies of 1.76 and 19.3 H2 s-1 at 50 and 100 mV overpotentials, the glassy film is among the most intrinsically active HER catalysts, outcompetes any reported MG, representative sulfides, and phosphides, and compares favorably with other precious-metal-containing catalysts. The outstanding HER performance of the Ir25 Ni33 Ta42 MG film is attributed to the synergistic effect of the novel alloy system and amorphous structure, which may inspire the development of multicomponent alloys for heterogeneous catalysis.

Low-Iridium-Content IrNiTa Metallic Glass Films as Intrinsically Active Catalysts for Hydrogen Evolution Reaction

Despite the importance of glass forming ability as a major alloy characteristic, it is poorly understood and its quantification has been experimentally laborious and computationally challenging. Here, we uncover that the glass forming ability of an alloy is represented in its amorphous structure far away from equilibrium, which can be exposed by conventional X-ray diffraction. Specifically, we fabricated roughly 5,700 alloys from 12 alloy systems and characterized the full-width at half-maximum, Δq, of the first diffraction peak in the X-ray diffraction pattern. A strong correlation between high glass forming ability and a large Δq was found. This correlation indicates that a large dispersion of structural units comprising the amorphous structure is the universal indicator for high metallic glass formation. When paired with combinatorial synthesis, the correlation enhances throughput by up to 100 times compared to today’s state-of-the-art combinatorial methods and will facilitate the discovery of bulk metallic glasses. The glass forming ability of alloys is found to be strongly correlated with the full-width at half-maximum of the first diffraction peak in the X-ray diffraction pattern, which facilitates the discovery of bulk metallic glass compositions.

Data-driven discovery of a universal indicator for metallic glass forming ability

Refractory high-entropy alloys (HEAs) exhibit remarkable mechanical properties desired for high-temperature applications. However, their oxidation resistance at high temperatures received less attention. Recent work indicates that MoTaTiCrAl alloy exhibits excellent oxidation resistance, but the effects of Al and Cr remain unclear. In this work, we demonstrate that the addition of Al is unnecessary for preventing oxidation of the MoTaTiCrAl alloy and the removal of Al leads to a more oxidation resistant MoTaTiCr medium entropy alloy. Structural and chemical analyses indicate that the excellent oxidation resistance of MoTaTiCr is mainly associated with the formation of continuous CrTaO4 oxide layer. The results indicate that complex oxides can be adopted as effective candidates for enhancement of oxidation resistance, in addition to typical strategy of forming Al2O3, Cr2O3 or SiO2 barrier layer.

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Enhanced oxidation resistance of MoTaTiCrAl high entropy alloys by removal of Al

Peculiar temporal intermittent microscopic dynamics has been recently reported during structural relaxation in metallic glasses, though its origin is poorly understood. By using x-ray photon correlation spectroscopy, we have investigated the atomic motions in a model system of metallic glass with distinct stability and in situ stress states. We find that the microscopic dynamics can be tuned from dramatically nonmonotonous to monotonous. The intermittent dynamics is always accompanied by the activation of a secondary relaxation process, as indicated by the drop of the initial nonergodic plateau of the intensity-intensity correlation function. Both phenomena depend on the material state and can be suppressed by simultaneously lowering the fictive temperature and external loading, leading the glass in a stationary metastable state which appears independent on the thermal history of the system.

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Li Mingxing

Papers

High-temperature bulk metallic glasses developed by combinatorial methods

Low-Iridium-Content IrNiTa Metallic Glass Films as Intrinsically Active Catalysts for Hydrogen Evolution Reaction

Data-driven discovery of a universal indicator for metallic glass forming ability

Enhanced oxidation resistance of MoTaTiCrAl high entropy alloys by removal of Al

Nonmonotonous atomic motions in metallic glasses