S. Liu, M.T. Swihart
University at Buffalo (SUNY),
Keywords: nanoparticle, flame, ceramic
Summary:Ceramic solid solutions containing two or more cations in a homogeneous phase may have composition-tunable properties that enable better performance than unary oxides in many fields, such as catalysis, fuel cells, sensors, and various other functional materials. Most reported ceramic solid solution materials are limited to elements of similar character (similar preferred oxidation state and coordination, atom diameter, and electronegativity), such as the alkaline-earth elements. This requirement greatly limits the space for new materials discovery and applications. For example, Ni-based ceramic solid solutions with the oxides typically used as catalyst support are rarely reported due to the immiscibility between NiO and these support materials. Conventional wet-chemistry approaches such as co-precipitation performed at low temperature with long reaction time often cannot produce solutions of immiscible elements (those whose oxides form separate phases at equilibrium under ambient conditions). In recent years, some nonequilibrium synthesis methods have been developed to incorporate immiscible elements into a single alloy or ceramic phase, such as carbothermal shock, laser ablation, and spark discharge. Generally, these nonequilibrium synthesis methods provide an instant high temperature material formation process to maintain a uniform elemental distribution in the product, followed by a rapid quenching step to prevent phase separation. Here, we report a scalable, continuous, and low-cost flame aerosol process to mix immiscible elements into a single ceramic solid solution phase. As shown below, an aqueous solution of metal salts is atomized into droplets by hot combustion products. The whole droplet-to-particle conversion (~0.05s) is faster than the timescale for elemental diffusion, so the initial uniform composition can be retained in the product. A larger flow of nitrogen rapidly quenches the product and prevents phase separation. We demonstrate that this approach is a generic method to incorporate immiscible elements, from binary and ternary oxides to high-entropy ceramic materials, from transition metal elements to alkaline-earth elements, and many crystal structures can be achieved.