Advanced Synthesis of High-Entropy Alloy Nanoparticles (HEA-NPs) Through Fast Moving Bed Pyrolysis (FMBP)

High-entropy alloys (HEAs) are single-phase metallic materials composed of multiple elements in nearly equimolar ratios, gaining significant attention due to their unique chemical and physical complexity. One of their most impressive characteristics is the stability of their chemical and mechanical properties at high temperatures. HEAs composed of refractory elements like Cr, Mo, Nb, V, Ta, W, Zr, Ti, and Re exhibit good strength at elevated temperatures. For example, NbMoTaW and NbMoTaWV HEAs have been reported to achieve yield strengths over 400 MPa at 1600 °C, far surpassing the ~ 900 °C limit of conventional Ni-based superalloys [1]. One of the most common processing routes to obtain this material is through mechanical alloying. However, this method introduces several contaminants into the HEA microstructure, resulting in the formation of undesirable oxides, nitrides, or carbides [2]. Meanwhile, several works have been developed in chemical synthesis of HEA nanoparticles (HEA-NPs) [3,4], demonstrating that HEA-NPs retain the same advantages as bulk HEAs at the nanoscale, while exhibiting increased specific surface area and higher surface energy [5]. A new Fast Moving Bed Pyrolysis (FMBP) strategy has been developed to prepare ultrasmall, highly dispersed, and contaminant-free HEA-NPs, comprising up to ten elements, ensuring high supersaturation of monomers and preventing phase separation [6]. The FMBP strategy offers promising avenues for industrial application, enhancing our understanding and utilization of these complex materials. In this work, we synthesized HEA-NPs based on Mo, Nb, Ta, Ti, Zr and Ru via FMBP. Graphene oxide was used as the support, along with 1,10-Phenanthroline (C12H8N2, 97.0%) and the metal precursors. The GO was suspended in ultrapure Milli-Q water and ethanol (in a ratio of 5:1) and kept under ultrasound for 12 hours at 55 °C to achieve a homogeneous mixture. Then, 1,10-Phenanthroline was added in a ratio of 3:1 relative to the HEA-NPs. The different metal precursors were sequentially added according to their activity, in a ratio of 5:1 on GO. The solution was kept under ultrasound at 55 °C until dry. Once dried, the solution was poured into an alumina boat that had been previously rinsed and dried. For the FMBP, a sliding tube furnace (MTI Corporation OTF-1200X) was used. The alumina boat containing the HEA-NP solution was placed in a region outside the furnace heating zone. A vacuum pump extracted the gas for 30 minutes, followed by purging the tube with Ar gas for 30 minutes, keeping flowing Ar (100 sccm). The furnace was then heated to 650 °C, and the alumina boat was slid into the heating zone. After annealing at 650 °C for 2 hours, the furnace was naturally cooled to room temperature. Finally, the HEA-NPs were obtained and the agglomerated nanoparticles were broken down using a mortar and pestle. Transmission Electron Microscopy (TEM) analysis was conducted using JEOL 2100 and JEOL ARM 200CF microscopes, both operated at 200 kV. The nanoparticles were dispersed in high-purity isopropanol and sonicated for 15 minutes to achieve a homogeneous suspension. A droplet of this suspension was then placed onto Cu grids. X-Ray Diffraction (XRD) analysis was carried out using a Panalytical Empyrean X-Ray diffractometer with CuKα radiation (λ = 1.5418 Å). The FMBP strategy presents a significant advancement in the synthesis of high-entropy alloy nanoparticles, holding great promise for various applications in materials science. The successful synthesis and characterization of HEA-NPs with diverse elemental compositions underscore the potential of this method, paving the way for new innovations and industrial applications.<br/><br/>[1] Mo, J. Y. et al. Rare Metals, 2022.<br/>[2] Moravcik, I. et al. Metals (Basel) 10, 1–15, 2020.<br/>[3] Al Zoubi, W. et al. Nano Energy vol. 110, 2023.<br/>[4] Wan, W. et al. J Mater Sci Technol 178, 226–246, 2024.<br/>[5] Hashimoto, N. et al. Nano Lett, 2024.<br/>[6] Gao, S. et al. Nat Commun 11, 2020.