The interplay between precursor-solvent-mixture phase, droplet disruption, and powder properties in the aerosol synthesis of metal-oxide nanoparticles

• Applicant: Dr. Niklas Jüngst
• Subject Area: Energy Process Engineering ; Mechanical Process Engineering
• Term: since 2025
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 570910141

Metal-oxide nanoparticles are, among others, applied as catalysts, battery-electrode material, or sensors. While wet-chemical and solid-phase synthesis require extensive pre- and post-processing, spray-flame synthesis, also known as flame spray pyrolysis, enables the synthesis in a single step. In contrast to most aerosol synthesis techniques, it uses low-volatile (and thus much less toxic than gaseous precursors) and inexpensive precursors which are available for a broad range of elements. The metal precursors are dissolved in combustible liquids and the solution is dispersed into a premixed flat flame. The high-temperature environment ideally brings the precursors into the gas phase where they efficiently mix, react, and eventually condense into (mixed) metal-oxide nanoparticles via a gas-to-particle route. The precursor-laden droplets are known to undergo frequent puffing and micro-explosion, a thermally-induced breakup (disruption), which is thought to aid in bringing precursor into the gas phase and thus in producing homogeneous particles with a narrow size distribution. However, even for well-studied materials such as iron oxide it remains largely unknown how the extremely low-volatile metal-containing species evaporate from the droplets and do not precipitate in the liquid phase via the unwanted droplet-to-particle process. In this project, we identify potential precipitation and liquid-liquid phase separation in the droplets in spray-flame synthesis and how it affects droplet disruption and the powder properties. Thermodynamic considerations and experimental evidence suggest that precipitation in the droplets might be rather the rule than an exception. To answer these questions, various optical in situ diagnostics are developed and applied in the spray flame. Suspensions and emulsions are fed into the flame and the droplets are microscopically imaged. From conventional shadowgraphy images, their disruption statistics are extracted with available methods and from images acquired with diffuse back illumination dispersed particles, droplets, and bubbles can be identified. This knowledge then serves as a benchmark to find dispersed phases in droplets containing precursor solutions, from which also the powder is sampled and analyzed. Laser- and non-laser-based diagnostics are then used to image aerosolized particles near the disrupting droplets. These can be, potentially micrometer-sized, precipitates ejected from the droplets during disruption or nanoparticles formed either directly from the gas phase or from the decomposing precipitates. Beyond the cause-and-effect relationship between precipitation, phase separation droplet disruption, and the powder properties, the project yields a substantially enhanced understanding of the particle-formation processes in spray-flame synthesis.

• DFG Programme: Research Grants