Coalescence rate of high-entropy materials (HEMs) by molecular dynamics (MD) simulations. (T)

ΕΠΙΒΛΕΠΩΝ : ΕΙΡΗΝΗ ΓΟΥΔΕΛΗ, Αναπλ. Καθ. Χημ. Μηχ. Παν/μιο Πατρών (πληροφορίες: eirini.goudeli@unimelb.edu.au)

The high reaction temperatures (>3000 K) accessible in aerosol synthesis routes, such as flame spray pyrolysis, allow for rapid nucleation and nanoparticle growth, while downstream in the aerosol reactor rapid quenching (cooling rates of >10⁴ K/s) takes place. This delicate control of high-temperature particle residence time in the flame allows capturing non-equilibrium nanostructures that are not in their lowest energy state but are stable under specific conditions, kinetically trapping them upon rapid cooling. This has led to the successful manufacture of high entropy materials (HEMs), metastable structures [1] and, nanomaterials with engineered defects containing more than 20 components [2].

Recently, FSP has been used to engineer an array of thermodynamically unstable nanomaterials, such as photocatalysts for pollutant removal [3], and molecular sensing of volatile organic compounds [1], circumventing the thermodynamic immiscibility limitations of solids. Thus, fundamental understanding of the thermodynamics and kinetics of defects and phase transitions during flame aerosol synthesis is at the heart of modern material design and innovation. 

Coalescence is dominant at high temperatures of the flame, while sintering (partial coalescence) may still take place even at low temperatures further downstream in the flame, especially for small (sub-5 nm) nanoparticles (e.g., see Fig. 2) [4] that exhibit melting points far below the bulk one [5]. Coalescence and sintering can alter the configurational order of HEMs by providing diffusion pathways that allow partial or full reorganization, potentially leading towards ordering, segregation or phase separation. Thus, the system can depart from high-entropy configurations even if the composition remains multicomponent. In this project, the effect of coalescence on short-range order would be elucidated as a function of the nanoparticle size, initial crystalline state, and process temperature, following the approach of Goudeli and Pratsinis [6]. Easy-to-use expressions for the HEMs coalescence rate will be proposed as a function of flame process parameters that can be readily used in particle dynamics models for process design.