Incipient stages of inorganic nanoparticle formation in flames: The role of thermal nonequilibrium

• Applicants: Professor Dr. Christof Schulz; Dr.-Ing. Irenäus Wlokas
• Subject Area: Energy Process Engineering
• Term: since 2024
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 543853173

Flame-assisted synthesis of nanomaterials is receiving rapidly growing attention for its capability to produce high-purity nanoparticles of almost all elements in continuous, scalable single-step processes and at moderate cost. A key to predictive, model-based flame synthesis of nanomaterials is detailed knowledge of the temperature and species concentration history experienced by the nascent particle aerosol, affecting composition, crystallinity, and morphology of the resulting nanomaterial. In this project, we focus on nanoparticle formation and evolution in iron-doped flames – a system prototypical for many other transition metals, both in strong coupling of precursor chemistry and flame kinetics affecting local reaction conditions as well as in technological relevance. Despite the progress reached in understanding the mechanism of iron-oxide nanoparticle formation, numerical simulations of nanoparticle flame synthesis processes so far rely on isothermal models that assume particles fully thermalized with the surrounding gas, simply to avoid an additional internal coordinate in the population balance model and because of not sufficiently well-understood details of the thermalization process. Assuming that particles are isothermal with the gas phase is at best a simplification, aiming to reduce the modeling effort, but it is often insufficient and can lead to erroneous analysis of experimental observations. Nanoparticle growth proceeds via condensation and coagulation in parallel to heterogeneous exothermic oxidation, reduction, and surface-mediated recombination that all heat up the nascent particles. At the same time, particles cool down by evaporation, heat conduction, radiation, and thermionic emission. As a result of the balance of these processes, the particle temperature can thus be below, equal, or above that of the gas phase, depending on the stage of particle formation/consumption. Very recently, we provided first quantitative evidence of significant elevation of the nascent particle internal temperature with respect to the gas-phase based on optical emission spectroscopy and stochastic simulation of discrete collisions. These findings for several materials systems indicate that non-equilibrium particle temperatures are a rather ubiquitous phenomenon that can significantly affect precursor reactions and particle growth. Such effects are, however, still largely overlooked and poorly understood. This project aims to explore – via experiment and modeling – the correlation between flame parameters (fuel/oxygen ratio, flowrate, fuel type, precursor load, and composition) and the non-equilibrium temperature of nascent iron(oxide) particles, which in turn impacts the synthesis flame structure and the properties of the target material.

• DFG Programme: Research Grants
• International Connection: Israel
• Co-Investigator: Professor Dr.-Ing. Andreas Kempf
• International Co-Applicant: Professor Igor Rahinov, Ph.D.