The gas synthesis
of nanoparticles has gained major interest by
different industries and research groups for the development of new
materials and their subsequent implementation in numerous devices
and applications. Actually, there are numerous research and application
activities in flame-based nanoparticle production, mainly focusing
on the process and its development, the in situ and ex situ analyses in experiments and simulations and new
material and material combination developments. This review provides
an overview on the flame synthesis of metal oxide nanoparticles and
focuses in particular on the process design of flame spray pyrolysis
(FSP). The first part of this review describes the historical background
of flame-based particle production and further summarizes the emerging
developments, achievements, and trends during the past 2 decades.
The second part covers a general process overview of the flame aerosol-based
manufacturing by explaining the key aspects of synthesis (chemical
raw materials, metal precursors, solvents, and reactor configurations), in situ conditioning approaches, and powder and byproduct
collection, along with additional post-processing steps (storage,
handling, and modification). Applications of flame-made nanoparticles
involve catalysts, gas sensors, energy storage materials, and nanotoxicity
studies, among others, which will not be discussed in detail but are
overviewed by listing the latest reviews of the respective field.
The third part focuses on the growth mechanisms of solid particle
products in liquid droplet/spray pyrolysis (liquid-to-particle and
gas-to-particle conversion). The concept of characteristic process
times is then exemplified for a lab-scaled FSP reactor, while afterward,
particle dynamics and continuum models for reactor simulation are
reviewed. The fourth part is a survey of more than 20 different diagnostic
techniques that are commonly employed for characterization of reactive
sprays and flame-made powders. We briefly explain the metrology basics
of each method, indicate advantages and limitations, and present exemplary
measurement results that have been acquired on flame reactor facilities
for particle synthesis. Finally, we summarize the review and address
some current challenges that might potentially shape the near future
of nanoparticle synthesis in flame sprays.