Optimization of High-Entropy Alloy Nanoparticles Synthesis via Thermal Plasma Using a Supersonic Nozzle

Abstract

High-entropy alloy nanoparticles (HEA NPs) have attracted significant interest in catalytic applications due to their compositional tunability and structural stability. A scalable inductively coupled plasma jet (ICPJ) process has recently been demonstrated for the continuous synthesis of HEA NPs. However, limited control over thermal history during cooling and solidification can lead to relatively large particle sizes, broad size distributions, phase separation, and compositional non-uniformity.

This thesis focuses on optimizing HEA NP synthesis via thermal plasma through the integration of a supersonic converging-diverging nozzle and a systematic control of synthesis parameters. The study first establishes the generality of synthesizing multiple HEA compositions using the baseline ICPJ process and defines reference conditions for comparison. Compared to this baseline, the nozzle is demonstrated to enhance quenching and reduce residence time in the liquid zone during solidification, which kinetically limits diffusion-driven segregation, enabling improved control of particle size, microstructure, and compositional uniformity.

Across the studied HEA systems, including those with poor elemental miscibility, the nozzle-assisted process produces reduced particle sizes and narrower size distributions with FCC as the principal crystalline phase under optimized conditions. However, oxide-related or residual-feedstock features occur in some samples. Modeling studies correlate the predicted changes in jet dynamics to estimated thermal histories and residence times, providing a mechanistic insight into nucleation and solidification behavior. As such, this study establishes a nozzle-enabled kinetic-control framework for scalable thermal-plasma synthesis of HEA NPs and provides a pathway toward controlled morphology, structure, and composition for potential catalytic applications, particularly for the hydrogen evolution reaction (HER).