(403e) A General Flame Aerosol Process to Create High-Entropy Nano-Ceramics

High-entropy materials, stabilizing more than five elements in a single phase by maximizing the configurational entropy, provide ultrahigh compositional diversity and many fascinating characteristics such lattice distortion, synergistic interactions among multiple elements, and altered kinetics of phase change and sintering processes. These unique properties of high-entropy materials, as well as the disorder in their configuration, can significantly enhance properties including strength, hardness, corrosion resistance, or thermal stability, which in turn make them promising for use in various applications. For example, in catalysis, highly dispersed active sites, high defect density, and synergistic effect among components of high-entropy catalysts often lead to significantly improved catalytic activity. Typically, due to the complexity of crystal structure, high-entropy ceramics are more difficult to fabricate than high-entropy alloys. To date, the most common method for fabricating high-entropy ceramics is solid-state reaction. However, this method relies only on thermodynamic control to generate the equilibrium phase with lowest Gibbs energy. It does not harness the kinetics of material formation and cooling processes. When the entropy of mixing is not sufficient to overcome the (positive) enthalpy of mixing, macroscopic phase-separation will occur. During slow cooling, into a temperature regime where a single phase is no longer thermodynamically stable, local phase segregation and clustering may occur, leading to poor homogeneity. Recently, some non-equilibrium synthesis methods such as carbo-thermal shock and laser ablation, in which materials are formed in milliseconds or less, have been developed. In this case, even when formation of a single phase is unfavorable, the atoms don’t have enough time to diffuse to form separated phases but are kinetically trapped into a single metastable phase. Access to metastable phases greatly expands the space of possible high-entropy ceramics. However, these methods often face barriers of scalability due to complex processes, low yield, and high energy requirements.

Flame aerosol processing has been the most common technology for industrial production of nanoparticles due to its intrinsic scalability, one-step and continuous operation, and low production cost. Here, we present a general flame aerosol route to fabricate high-entropy nanoceramics, utilizing a modified flame reactor. In this process, an aqueous solution of metal salts is atomized into precursor droplets and the high-entropy nano-ceramic forms via a droplet-to-particle conversion in milliseconds. Thus, the fast reaction kinetics greatly limits diffusion during the material formation process, which incorporates multiple elements into a single crystal lattice and provides access to many metastable phases that are not thermodynamically favorable. Meanwhile, fast N₂-quenching downstream of the particle formation zone maintains the initial high-entropy configurationally disordered state. Furthermore, our method uses inorganic salt aqueous instead of metal organic salts as precursors in traditional flame aerosol process, greatly increasing the variety of elements that can be incorporated. These advantages provide great flexibility to combine elements in the different regions of periodic table. We have successfully synthesized high-entropy nano-ceramics with various crystal structures and mixed more than 20 elements into a single crystal lattice. We demonstrate improved sintering resistance in these high-entropy nanoparticles, which is important for producing durable catalysts. We also investigate the entropy effect in improving dispersion of atomically active sites. As a model application, we show that high-entropy catalysts with the rock salt crystal structure exhibit ultrahigh activity and long-term stability for CO₂ hydrogenation to value-added chemicals.