Abstract:

The world faces numerous societal issues, including the global warming crisis driven by increased energy consumption and associated greenhouse gas emissions from human activities. A new generation of advanced materials with superior multifunctional properties, such as multicomponent alloy systems (otherwise known as high-entropy alloys—HEAs), is required for lightweight structural applications and clean energy production to meet environmental quality demands. HEA-based catalysts, which offer alternatives to rare/expensive catalysts (e.g., Pt, Pd, Ru, and Ir) for electrochemical/thermal gas-phase reactions, require particle-size tuning to enable direct manipulation of surfaces and interfaces and to create more potential active sites on the catalyst surface. Among the various HEA synthesis methods, mechanical alloying via ball milling has emerged as a scalable and environmentally friendly approach for producing nanocrystalline solid-solution alloy microparticles in solid state. Microparticle HEAs produced via ball milling are now actively used for azo dye degradation and have further potential for electrocatalytic reactions, owing to unique electronic structures that facilitate faster reaction kinetics. It is, however, unclear how elemental composition and the starting particle size of the constituent affect the mechanical alloying process. Although ball milling is often reported to provide limited control over powder microstructure and morphology, careful optimization of key parameters—such as process control agent (PCA) type and concentration, milling speed and duration, ball-to-powder ratio (BPR), and milling media—can, to some extent, govern the microstructural evolution, phase formation, and morphology of alloy particles. Despite significant progress in parameter optimization, relatively little attention has been given to the effect of the initial precursor particle size on the alloying process. This oversight may stem from the common assumption that “...the powder particle size is not very critical, except that it should be smaller than the grinding ball size”. We challenge this line of thought: the initial particle size is expected to influence the diffusion rate and onset of alloying, as the surface-to-volume ratio strongly—and thus the available contact area for diffusion—is inherently size-dependent. To investigate particle size effects on alloying onset, powder morphology, and microstructure, three HEAs were studied across four distinct particle-size ranges. We found that, for all mechanically-alloyed HEAs, smaller particle sizes promote solid-solution formation at significantly shorter milling times. Additionally, these results provide new insight into how elemental composition governs solid-solution formation: onset of alloying is not solely controlled by the “melting temperature” of the HEA. The performance of the selected HEAs on azo dye degradation will also be discussed. This study demonstrates that selecting an appropriate initial particle size and elemental composition can significantly accelerate the onset of alloying, thereby optimizing the synthesis route and making ball milling a more efficient method for producing HEA-catalysts, substantially reducing synthesis times compared to other methods.

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Ms. Dilnaz Yuldasheva