Flame-based Aerosol Synthesis of High Entropy Oxide Nanoparticles

Shuo Liu, MARK SWIHART, University at Buffalo - SUNY

     Abstract Number: 146
     Working Group: Nanoparticles and Materials Synthesis

Abstract
Recent decades have witnessed great advances in the ability of aerosol synthesis processes to tune nanomaterial properties, such as particle size, structure, and morphology, but the ability of aerosol processing to access new regions of composition space is less well-explored. In 2015, the concept of “high-entropy ceramic” materials was presented in a report of a (MgCoNiCuZn)O_x solid solution oxide with more than 5 elements in near-equiatomic concentrations in a single rock salt crystal phase. As a highly disordered, multicomponent, and often metastable system, high-entropy ceramics can exhibit interesting and useful new properties, like high-temperature stablity, extreme lattice distortions and abundant oxygen vacancies. New and unexpected properties may emerge upon tuning the cation composition. Several methods have been developed to fabricate nano-sized high entropy nanomaterials, including carbo-thermal shock, laser ablation, and spark discharge, but these routes often face barriers of scalability due to the complex processes, low yield, and high energy requirements. Here, we reported a continuous, low-cost, and scalable flame aerosol process to synthesize high-entropy oxide aerosols. For this, an aqueous precursor containing multiple metal salts is delivered to the flame reactor and atomized into microdroplets within which evaporation and reaction drive formation of solid nanoparticles. The reactor resistance time (~0.05 s) is much shorter than to the time required for phase separation by solid diffusion, so the initial high-entropy state in the droplet precursor can be retained in the product. Rapid quenching with diluting nitrogen prevents phase separation, so a single ceramic phase can be obtained. The reactor environment can be tuned to oxidizing or reducing conditions to influence stoichiometry and oxygen vacancy concentration. We demonstrate the generality of this method by mixing various elements to produce high entropy nanomaterials of several different crystal structures.