Research progress on control technologies for flame synthesis of condensed-phase nanomaterials

Flame synthesis is a method for the preparation of condensed-phase nanomaterials. It is energy efficient, cost effective and has the potential for large scale industrial applications.

This review examined the control technologies for the morphology, composition and microstructure of flame synthesized condensed-phase nanomaterials from four aspects, i.e., flame stability, flame temperature and species profiles, product particle size and morphology, and product physicochemical properties. The flame stability control part mainly introduced the swirl stabilization method, including both strong swirl stabilization (S_{g, tan} > 5 or S > 0.6) and weak swirl stabilization (S≤0.6). The calculation and design criteria of different swirl numbers for different types of swirlers were evaluated. Methods such as the addition of pilot flame, the increase of active component concentrations at the flame nozzle, the use of high-enthalpy precursor solution, and the addition of sheath gas were also summarized. For the control of flame temperature and species profiles, adjustments might be made to the fuel-air equivalence ratio, oxygen flow rate for atomization, or precursor liquid flow rate and concentration. The installation of the cooling meshes and quenching rings, water addition to precursor liquid and high-enthalpy solvent replacement were also good strategies. The particle size and morphology of the product might be controlled indirectly by adjusting the flame temperature and species profiles which could affect the particle dynamics process. From another perspective, it might also be regulated by substrate materials, droplet microexplosion, precursor ultrafine atomization, inclusion of high boiling point active agent, and plasma discharge, etc. These were realized by controlling the droplet size before particle formation and regulating particle attributes for precipitation, collision, and condensation, thereby changing its size and shape. Four primary approaches for controlling physicochemical properties were summarized, including crystal structure control, element doping, core-shell structure design and post-heat treatment. The crystal structure control mainly included temperature-induced phase transitions and doping-induced phase transitions. The element doping was mainly achieved by regulating precursor liquid with different components and different proportions. The core-shell structure design was mainly realized by using the different precipitation characteristics of different solutes or by the installation of auxiliary devices to stagger the time and space distributions of the two different kinds of materials. The post-heat treatment process primarily consisted of the annealing procedures at different atmospheres, temperatures and time durations to remove unwanted combustion residues and defects, and to induce phase transformation, etc.

The first two aspects of this review mainly focus on the control of the external parameters such as flame temperatures and species profiles, while the latter two on the control of the internal properties of the materials such as particle size and physicochemical properties. In actual experiments or production, the control technologies will need to be comprehensively used according to specific situations. Considering the complexity of the composition of the precursor liquid, the stability, volatility, and precipitation characteristics had better be comprehensively analyzed before flame synthesis, so that the mechanism identification and active regulation of the material nucleation and other processes will be enabled from the initial stage of particle formation.