Flame Synthesis of Nanoparticles, Nanotubes, 2-D and Porous 3-D Materials

Flame synthesis has a demonstrated history of scalability and produces several million metric tons of metal-oxide and carbon nanoparticles, valued at over $10 billion, annually [peb6]. In the laboratory, it produces functional nanomaterials, from a myriad of compositions and crystallinities of nanopowders, 1-D carbon nanotubes and metal-oxide nanowires, 2-D graphene, 3-D mesoporous metal-oxide films and bulk nanomaterials [peb7, peb8]. These flame-synthesized nanomaterials can exhibit superior performance for various applications, including energy conversion devices and sensors, due to their tunable morphologies, high purity, and crystallinity. In the examples of as-synthesized morphologies (e.g. Fig. peb5(a,b)) described below, flame synthesis is characterized by ultrafast growth rate, high purity, and high crystallinity of the synthesized material, as flames can operate uniquely at atmospheric and high temperature (1000-2400ยบC) conditions. In our setups, premixed and non-premixed flames in axisymmetric stagnation-point flows (Fig. 1(c)) and counterflow are utilized, along with electric-field control. Routinely, ceramic nanopowders such as SiO2, TiO2, Al2O3, and ZrO2 can be synthesized, in various phases, using liquid precursors. Recently, a high-rate production of nanopowders, nanowires, and large-area deposition of nanostructured coatings has been achieved using a multiple inverse-diffusion flame (m-IDF) process, in which many tiny flames contain the oxidizer in the center of the surrounding fuel. This flame configuration eliminates the need for a chambered synthesis, so it can run in the open atmosphere, and over large area contoured surfaces. They can readily synthesize mesoporous and dense films on heated substrates. Finally, flame synthesis can produce bulk materials by solid-phase reactions through self-propagating high temperature synthesis (SHS).