(205g) Flame Aerosol Synthesis of High-Entropy Ceramic Nanoparticles

Tailoring properties by creating mixtures is an age-old method to improve material performance. The main mixing strategy is addition of a small amount of second component into a primary component, like addition of a small amount of carbon into iron to create steel, which greatly increases strength and corrosion resistance. However, this conventional strategy limits the range of possible material types and available composition space. In 2004, scientists discovered that an alloy containing more than five elements in near-equiatomic concentrations can be stabilized in a single phase by its high configurational entropy. This insight opens up a vast compositional space for new materials discovery. It was not until 2015 that the concept of “high-entropy ceramics” was first proposed with the rock-salt structured (MgCoNiCuZn)O_x solid solution oxide reported. 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 elemental composition and ratio. Features ranging from catalytic activity to lithium-ion uptake to low thermal conductivity at high temperature can be exploited in energy and environmental applications such as catalysis, sensors, supercapacitors, insulation, and electrode materials.

To date, the brute-force ball milling method has been the most common strategy to fabricate high-entropy ceramics, but this approach has limited ability to control particle size and prevent phase-separation. More recently, bottom-up synthesis methods have been developed, including carbo-thermal shock, laser ablation, and spark discharge, but these methods often face barriers of scalability due to the complex processes, low yield, and highly energy required. Here, we reported a continuous, low-cost, and scalable flame aerosol process to synthesize high-entropy ceramic nanomaterials. In a general synthesis, an aqueous precursor containing multiple metal salts is delivered to a flame reactor and atomized into microdroplets within which evaporation and reaction drives formation of the solid nanoparticles. The reactor resistance time (~0.05 s) is much shorter than the time required for phase separation by solid-state diffusion, so the initial high-entropy well-mixed state produced at high temperature in the reactor can be retained in the product. The rapid quenching with diluting nitrogen prevents phase separation so a single ceramic phase can be obtained. The closed reactor chamber provides a controllable oxidizing or reducing environment that allows further tuning of, for example, oxygen vacancy concentration. We demonstrated the generality of above mechanism by mixing various elements, including transition metals, noble metals, alkaline-earth elements and others. Many different phases, each of uniform well-mixed composition, were produced, such as amorphous, rock-salt, and fluorite structures. In addition, many products form a hollow nanoshell structure directly in the droplet-to-particle process by which the nanostructures form. Explorations of this class of high-entropy ceramic nanoparticles for catalysis and Li-battery applications are ongoing.